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Mefi's own November 1, 2007 11:22 AM   Subscribe


What? That first link should go to this
posted by pantsrobot at 11:27 AM on November 1, 2007


Recheck that first link, pantsrobot
posted by nanojath at 11:27 AM on November 1, 2007


Whoops. I even previewed.
posted by nanojath at 11:28 AM on November 1, 2007


Damn it. That was a great conversational IQ test. *shakes fist*
posted by empyrean at 11:30 AM on November 1, 2007


Don't worry, it still will be. Remember, Mythbusters 'investigates', not 'proves', things.

Now your conversational IQ test can be, "You remember that episode of Mythbusters with the plane and the conveyor belt?" And if they say, "Yeah, they proved that it does/doesn't take off," then you can move right on.
posted by chrismear at 11:32 AM on November 1, 2007


Fixed.
posted by cortex (staff) at 11:34 AM on November 1, 2007


Please, please, please interview PaulSC.
posted by klangklangston at 11:37 AM on November 1, 2007 [6 favorites]


Yeah, I've been wanting to bug Adam Savage to do that myth for the past year just to shut everyone up that thinks a plane would take off. :)
posted by mathowie (staff) at 11:38 AM on November 1, 2007 [1 favorite]


The plane takes off.
You have a better chance of getting the prize if you switch doors.
posted by Plutor at 11:41 AM on November 1, 2007 [4 favorites]


And 0.999... is equal to 1.
posted by Plutor at 11:45 AM on November 1, 2007


Whaaaarrrrggghhhhh!

...let's not start the conversation again. Oh god please no.
posted by aramaic at 11:47 AM on November 1, 2007 [2 favorites]


I'm pretty sure I know where they'll get the plane in question.
posted by dersins at 11:48 AM on November 1, 2007 [1 favorite]


The conveyor belt has to be long enough, but it will take off. I should be a mythbuster!
posted by shmegegge at 11:55 AM on November 1, 2007


Chicken or egg, Savage. Chicken or egg.
posted by empyrean at 11:58 AM on November 1, 2007 [2 favorites]


"We put the plane on a quarter-mile conveyor belt and tested it out,'' says Savage about the experiment using a pilot and his Ultralight plane. "I won't tell you what the outcome was, but the pilot and his entire flight club got it wrong.''

OH WHAT NOW PAULSC?!

I mean, I guess we'll have to wait for the show, is all I'm saying.
posted by chinston at 12:01 PM on November 1, 2007


The plane takes off.

The boat sinks.
posted by blacklite at 12:03 PM on November 1, 2007


The plane will take off, and the treadmill will break.
posted by Reggie Digest at 12:09 PM on November 1, 2007


Please, please tell me it was a Cessna 172. I think Ethereal and Malor should get to split the $1000.
posted by koeselitz at 12:12 PM on November 1, 2007 [1 favorite]


The plane does not take off; it maintains it's place relative to the room/ ground around it. No lift is generated because it's not moving forward.

However IANSmart, so I'll likely be proved wrong.
posted by quin at 12:13 PM on November 1, 2007


The treadmill will take off, and the plane will break?
posted by Cranberry at 12:13 PM on November 1, 2007 [2 favorites]


Hah.

I didn't see the original question and when clicking the link, immediately rolled my eyes "Fucking nerds" I thought, "who has time for this stupid shit? Why would you put a plane on a conveyor belt, jesus christ?!" Then I started to think about it and it didn't make sense, so I grabbed a toy model of the B-wing and started acting it out and debating which way it would go and then wondered how the hell I actually got married.

Anyway...I betting the pilots thought it wouldn't work, as they're pretty ingrained that flight=taxing down the runway i.e. moving through air as opposed to standing still, so it probably did work.
posted by Brandon Blatcher at 12:14 PM on November 1, 2007 [3 favorites]


I'd like to be Mefi-mailed when the thread gets made. There's a pony request for you.
posted by empyrean at 12:18 PM on November 1, 2007


The plane takes off assuming the conveyer belt doesn't supply enough of an opposite force to prevent the plane's wings from generating lift.

I am not going to draw you guys a diagram, I have homework to do.

I see this as an excellent experiment to completely fuck up, however. I predict there will be too much friction in the wheels, causing the conveyer belt to negate the force being exerted by the plane's propellers,

And then they'll blow it up! YEAH!
posted by CitrusFreak12 at 12:18 PM on November 1, 2007


Whoa, I just read about a sixth of that original thread. What a clusterfuck! Let's not start that again. I will have to yell.

koeselitz: A 172 is not an ultralight.
posted by blacklite at 12:18 PM on November 1, 2007


However, if it turns out that all they do is put an airplane on a conveyor which does not cause the airplane to remain horizontally motionless, this is a retarded waste of time and they may as well have asked the question "will an airplane take off from ice?" A lack of friction between the wheels and ground = inability to get much ground speed. Which is totally uninteresting.
posted by blacklite at 12:21 PM on November 1, 2007


"I won't tell you what the outcome was, but the pilot and his entire flight club got it wrong.''

OMG! Adam Savage just violated the first two rules of flight club!
posted by The World Famous at 12:24 PM on November 1, 2007 [24 favorites]


And apparently it's conveyor, not conveyer. Woops.
posted by CitrusFreak12 at 12:25 PM on November 1, 2007


However, if it turns out that all they do is put an airplane on a conveyor which does not cause the airplane to remain horizontally motionless, this is a retarded waste of time

That's exactly what they're doing. Obviously if the conveyor belt was hypothetically able to keep the plane in one place, it'd be a pretty boring film. The point is that there are a large number of none-too-bright folks who still aren't convinced that a plane on a conveyor belt will take off (some of these people will continue to argue for thirty-odd superficially-authoritative comments on Metafilter...).
posted by Aloysius Bear at 12:27 PM on November 1, 2007


Metafilter: This is a retarded waste of time
posted by The World Famous at 12:29 PM on November 1, 2007 [2 favorites]


immediately rolled my eyes "Fucking nerds"..."I grabbed a toy model of the B-wing"

mm-hmm.
posted by puke & cry at 12:36 PM on November 1, 2007 [3 favorites]


SPOILER ALERT:

The airplane taxiing at takeoff speed on a counter-rotating conveyor belt represents priapism. According to Freud, if an airplane appears in a person's dream or as a subject of obsession, it generally represents a penis. A penis perpetually on the brink of actualization represents unrealized desire and frustrated anticipation. If this conversation lasts for more than 4 hours, call your doctor.
posted by It's Raining Florence Henderson at 12:47 PM on November 1, 2007 [11 favorites]


blacklite: "However, if it turns out that all they do is put an airplane on a conveyor which does not cause the airplane to remain horizontally motionless, this is a retarded waste of time"

See, I think this is the primary problem. There are too many variations on the way the question is asked. But the question the way the "it won't fly" folks interpret/learned it is a bit ridiculous. In order for a conveyor to exert enough force on wheels for it to keep the plane from moving, it would need to be going a ridiculous speed. Or the axles would have to be pretty crappy. The conveyor doesn't keep the airplane motionless. It's rigged to move at the same speed as the plane.

It'll fly. And a lot of people who think the answer is "it won't fly" will be outraged with the perceived shortcoming in the Mythbusters implementation.
posted by Plutor at 12:52 PM on November 1, 2007 [1 favorite]


I love you, asavage.
posted by Rock Steady at 1:04 PM on November 1, 2007


You guys...geez! A plane is not like a car, it doesn't generate forward motion by rotating the wheels. It generates forward motion by using thrust generated by the engines. The wheels have nothing to do with it. Of course it will take off. How could anyone not understand this? (barring the conveyer spinning fast enough to impart enough drag to prevent the forward motion..impossible, and not what the question implies).
posted by lohmannn at 1:05 PM on November 1, 2007 [2 favorites]


See, I think this is the primary problem. There are too many variations on the way the question is asked. But the question the way the "it won't fly" folks interpret/learned it is a bit ridiculous. In order for a conveyor to exert enough force on wheels for it to keep the plane from moving, it would need to be going a ridiculous speed. Or the axles would have to be pretty crappy. The conveyor doesn't keep the airplane motionless. It's rigged to move at the same speed as the plane.


It's ridiculous to do that for an actual physical experiment, but as a thought experiment, why not? You have things like "If I were in a space ship traveling at the speed of light" etc. all the time in thought experiments.
posted by juv3nal at 1:17 PM on November 1, 2007


God damn it is it ever hard to resist arguing about this.
posted by cortex (staff) at 1:18 PM on November 1, 2007


Recheck that first link, pantsrobot

My brain parsed this as "Redneck that first link, pantsrobot."

I don't know what that says about whom.
posted by BorgLove at 1:21 PM on November 1, 2007


I wanna know where they hell they got a quarter-mile conveyor belt - Acme?
posted by exogenous at 1:27 PM on November 1, 2007


Do I get to be smug about being 1) right and 2) first?

no?

Oh, okay then.

posted by dirtynumbangelboy at 1:30 PM on November 1, 2007


Let me know when Mythbusters investigates that pesky Ralph Wiggum/sleep viking thing.
posted by turaho at 1:35 PM on November 1, 2007 [6 favorites]


So, they somehow got a plane with frictionless wheels and a conveyor belt that can move at speed approaching c? Wow. I have to see this show.
posted by tehloki at 1:40 PM on November 1, 2007


It's a trick question. 'nuff said.
posted by popcassady at 1:43 PM on November 1, 2007


Chicken or egg, Savage. Chicken or egg.

I'm not asavage, nor do I play him on tv, but this one is easy:

Egg.

Dinosaurs were laying eggs long before there were chickens.
posted by ROU_Xenophobe at 1:47 PM on November 1, 2007 [4 favorites]


God damn it is it ever hard to resist arguing about this.

You're wrong about that.
posted by Brandon Blatcher at 1:48 PM on November 1, 2007


To everyone's chagrin, the plane remains motionless, but the conveyor belt takes off. (Can't wait for this episode!)
posted by steef at 1:53 PM on November 1, 2007 [2 favorites]


I'm not asavage, nor do I play him on tv, but this one is easy:

Egg.

Dinosaurs were laying eggs long before there were chickens.


Correct - but you don't even have to go that far back. "Chickens" were bred from wild fowl by humans. Those wild fowl also laid eggs, as did their birdie ancestors. The first real "chicken" came from an egg that was laid by a bird that was merely a proto-chicken. Hence egg came first. Many people get this wrong because they assume the question refers to a chicken's egg (originating from) as opposed to a chicken egg (containing), but the question as traditionally posed includes no such designation, refering only to the egg. Hence, egg is the correct answer.
posted by It's Raining Florence Henderson at 1:56 PM on November 1, 2007 [2 favorites]


There's two correct answers to this problem*:

1) The theoretical model: All components are theoretical. The world-model is Newtonian physics. The belt can go infinitely fast, the wheels can spin infinitely fast without catching fire - but the plane has finite thrust and energy, like a real plane. How the plane doesn't move forward by reacting against the air is a mystery - I can only assume that the conveyor is spinning so fast it's also creating a breeze.

Plane does not take off. Because Newtonian physics is stupid.


2) The real world model: In the real world, conveyor belts don't move fast enough to statisfy the conditions of the experiment. Conveyor reaches maximum velocity, wheels on the plane spin extra fast - plane moves forward in relation to both the ground and the conveyor, pulled through the air by it's air screw, or propeller.

Plane takes off - assuming the wheels don't catch fire or blow out first.


*I'm lying so bad it hurts. There's only one correct answer, you goddamn freaks!
posted by loquacious at 1:58 PM on November 1, 2007 [2 favorites]


"So, they somehow got a plane with frictionless wheels and a conveyor belt that can move at speed approaching c?"

So, e=mc2 parses out to energy = mass x the square of the speed of your average Cessna?


Not that I want to injure your pride or anything. I'd hate to lose your steady influx of favorites.
posted by Terminal Verbosity at 2:11 PM on November 1, 2007


This entire plane/conveyor belt issue is one of the reasons I love Mythbusters.

The fine folks at the show take subjects I have never cared about and make me want to learn and know the how/why/what of those subjects.

I wish this show had been around when I was little so that I would have been interested in science and math. Fortunately, as an adult this show has made me want to learn more about those subjects.
posted by fluffy battle kitten at 2:23 PM on November 1, 2007


Can we investigate why people STAND like goobers on the people mover. Perfectly healthy people just block your path, Immobile in thrall like they're playing statue tag, instead of walking.

God damn it. Escalators and people movers. They are not so you can rest, you lazy motherfuckers. They are there to get us all there faster.
posted by tkchrist at 2:30 PM on November 1, 2007 [7 favorites]


Escalators and people movers. They are not so you can rest, you lazy motherfuckers. They are there to get us all there faster.

No. They're not. They are there to get us all there. If faster is what floats your boat, fine, I'll be the one standing to one side so I don't have to take pain pills just to change planes.
posted by It's Raining Florence Henderson at 3:09 PM on November 1, 2007 [2 favorites]


Elevators, not escalators, may be the key to this canard.
posted by breezeway at 3:09 PM on November 1, 2007


I was gonna bring you guys some beans but I ate them.
posted by Totally Zanzibarin' Ya at 3:10 PM on November 1, 2007


I hate it when people get on the elevator and then just stand there. Jump, you lazy fuckers.
posted by cortex (staff) at 3:13 PM on November 1, 2007 [25 favorites]


Going down.
posted by It's Raining Florence Henderson at 3:13 PM on November 1, 2007


Ease the seat back.
posted by breezeway at 3:17 PM on November 1, 2007


So, to sum up, there are two schools of thought on this question:

First, can a plane's engines overcome the speed of a conveyor belt rotating in the opposite direction?

This strikes me as a stupid question that no one would really ever ask. Of course there are going to be engines that can overcome a conveyor belt, it's no different (as commented above) from a plane taking off from an icy foundation. It's going to take longer, but it can happen.

Second, if a conveyor belt is capable of completely nullifying a planes forward movement, but lets it's engine operate at full capacity, will it take off?

This is more a question that might actually get asked, because there are people who don't understand that it's the air moving over the wings generate the lift, not the power of the engine.

I am certain that this is what is meant when the question is asked, and I'm equally certain that the answer is 'no'. A plane will not lift off if it's not moving forward. (ignoring VTOL, etc)

Also, the egg came first.
posted by quin at 3:19 PM on November 1, 2007


"Can we investigate why people STAND like goobers on the people mover. Perfectly healthy people just block your path, Immobile in thrall like they're playing statue tag, instead of walking.

God damn it. Escalators and people movers. They are not so you can rest, you lazy motherfuckers. They are there to get us all there faster."

The last time I connected through Phoenix, I had ten minutes to traverse the entire breadth of the airport, carrying two bags.

I ended up on a moving sidewalk behind a family who promptly stopped dead, taking up both sides. I said "Excuse me" once, then waited briefly. I said "Excuse me" again, and pushed past the dad and one of the kid, who got all huffy.

But handily, by walking briskly and saying "Fuck you. Walk on the left," the debate was closed and I made my plane.

I only wish that I'd been able to convince my girlfriend to follow a similar plan the last time we were at LAX, and stuck behind a family of turnip-farming retards from Utah.
posted by klangklangston at 3:20 PM on November 1, 2007 [1 favorite]


turnip-farming retards from Utah

Why on earth would anyone pass up the opportunity to use the word "Utahn?"
posted by dersins at 3:24 PM on November 1, 2007


First, can a plane's engines overcome the speed of a conveyor belt rotating in the opposite direction?

This strikes me as a stupid question that no one would really ever ask.


There are some really stupid people out there. For a particularly good example, check out paulsc's thirty-odd contributions to the thread, all of which are authoritative, avuncular, knowing and yet utterly, embarrassingly, singularly wrong.
posted by Aloysius Bear at 3:25 PM on November 1, 2007 [1 favorite]


'Utahd' has a nice sound to it.
posted by Aloysius Bear at 3:26 PM on November 1, 2007 [1 favorite]


The last time I connected through Munich, I had 4 hours to traverse the entire breadth of the airport, carrying two bags.

I ended up on a moving sidewalk which was going by a long window where you could watch the airplanes landing and taking off. It was relaxing and I had time to kill.

That is until some maniac who didn't plan enough time in his layover comes barreling through like he owned the moving walkway, pushing little kids aside.

Man, I hate airports.
posted by vacapinta at 3:28 PM on November 1, 2007 [3 favorites]


Now, there's also the question of the movement of air caused by that conveyor belt whipping along. The air being moved backward in sympathy with the belt, regardless of whether or not the plane is move. Would that, in that scenario, generate sufficient movement of air to create sufficient lift to raise the plane into there air? Who knows.
posted by cortex (staff) at 3:31 PM on November 1, 2007


Dammit, there I go.
posted by cortex (staff) at 3:32 PM on November 1, 2007


I don't see what the belt has to do with anything. We have to be assuming that the belt counteracts any forward motion of the plane, otherwise it's a really silly question.

1. An engine on wheels sitting on the belt won't go anywhere, assuming the belt sucks up all its forward motion. All it does is create airflow.

2. A wing standing by itself will go up if there's enough air flowing past it.

So isn't the question just: does the engine create enough airflow by itself to lift the wing? If we cut the wheels off a plane, plop it on the ground, and run the engine, does it lift off?
posted by equalpants at 3:47 PM on November 1, 2007


Before it was announced that someone would do some tests to determine the answer to this question, it was sort of a little teeny bit reasonable to argue about it on the internet.

Now it is not.

Everyone shut up and go play outside or something.
posted by aubilenon at 4:16 PM on November 1, 2007


Since the plane doesn't get it's forward thrust from a transmission turning the wheels, but from a propellor/jet, i don't see how it matters. The plane will go backwards at first, then it'll go forward at the same speed it always did (not relative to the belt, but to the ground).

Depends how well oiled the wheels are, i guess, though.
posted by empath at 4:17 PM on November 1, 2007


I keep rereading your question equalpants and I'm not sure I'm understanding it. Everything seems ok till we get to this:

So isn't the question just: does the engine create enough airflow by itself to lift the wing?

No. The engine doesn't produce airflow for lift. The engine produces thrust which forces the vehicle through the air which, in turn, creates lower pressure above the wing and higher pressure below it, which is the lift which raises the aircraft off the ground.

The engine producing airflow over the wings has nothing to do with flight.

Unless I'm completely misunderstanding your question, and if so, I apologize.
posted by quin at 4:18 PM on November 1, 2007


The problem is that nobody knows, or can agree on, exactly what the myth is.

But every permutation of the myth leads to an obvious correct answer. If the wheels can spin infinitely fast, then it's like a plane taking off from ice, which is no problem, and no slower than normal. If the plane remains stationary, then it can't take off, because planes have to move to get lift. Etc.

So it's not worth arguing about, because the argument comes down to arguing not about physics or airplanes or conveyor belts, but about the assumptions.

And that means that, no matter what Mythbusters did, half the internet will simply say they got the myth wrong.
posted by The World Famous at 4:27 PM on November 1, 2007


I don't see what the belt has to do with anything.

It holds the plane's pants up so it doesn't trip over them as it runs down the runway.
posted by ROU_Xenophobe at 4:28 PM on November 1, 2007 [3 favorites]


Which came first, the chicken or me cracking your skull with a brick and feasting on the delicious goo inside?
posted by stavrosthewonderchicken at 4:32 PM on November 1, 2007 [1 favorite]


Put a person on roller skates on a conveyor belt. Attach a rope to his waist. Pull the rope toward you (like a tug of war) while the conveyor belt moves in the opposite direction at the same speed you're pulling the rope forward. Will he move forward on the belt? (Hint for the desperately confused: if you're pulling the rope toward you and it's attached to him, what else can happen?) The force from the conveyor belt causes the skate wheels to rotate around their axles; the force from the rope causes the person (and the axles) to move forward. Person --> jet, rope --> engine.

The description in the article is pretty dumb. It says the airplane will be moving at takeoff speed. If it's moving at takeoff speed, it takes off. Otherwise, it's not moving at takeoff speed. That's the definition of takeoff speed.

Mythbusters won't solve this argument. No matter what the outcome, people in the opposing camp will claim the postulated conditions were not interpreted correctly, i.e., they didn't define takeoff speed the right way. (On preview, what TWF said.)
posted by forrest at 4:36 PM on November 1, 2007


"That is until some maniac who didn't plan enough time in his layover comes barreling through like he owned the moving walkway, pushing little kids aside.

Man, I hate airports."

Really? Did you notice if the moving sidewalk said "Walk on the left/Stand on the right"? Because this one did. Thus, were you standing there when I was hustling (due to delayed planes), I would have said, "Fuck you. Walk on the left."
posted by klangklangston at 4:43 PM on November 1, 2007


I still say if God had meant for man to fly, He would have given them conveyor belts.
posted by Astro Zombie at 5:16 PM on November 1, 2007 [2 favorites]


testing 123
posted by St Urbain's Horseman at 5:16 PM on November 1, 2007


It is never appropriate to say 'fuck you' when pushing a kid aside.
posted by found missing at 5:25 PM on November 1, 2007


"I'll never get this alphabet down! A through T, sure, and V through Z no problem, but that one in the middle, I hate it! I hate its stinkin' guts!"

"I hate it too, kid."

"It thinks it's better than me. Thinks it's too good for me."

"That's crap, kid, and you know it. It's got nothin' on you."

"But whenever I close my eyes at night, it's there! Laughin' at me!"

"Fuck U, kid."
posted by cortex (staff) at 5:36 PM on November 1, 2007 [2 favorites]


Do not read this: SPOILER!!!1!!!


The ultralight is gonna fly. The wings, "air foil thingys" fly because air passes over and under them creating lift. The "air flow", in this case, will be caused by both thrust and airspeed-both created solely by the prop. Thrust and airspeed don't give a shit about a conveyor belt. Will the landing gear/wheels turn ridiculously fast? Hell yea, watch 'em burn, but the suckers gonna fly.
posted by snsranch at 5:50 PM on November 1, 2007


"Fuck you. Walk on the left." Two of these words are redundant. Dunno why you'd use them to a kid. Golden rule an'all that.
posted by dash_slot- at 6:05 PM on November 1, 2007


I'm very curious how you people who are giving us theories explain this bit to me:

There's a good fifteen to twenty thousand pounds of thrust coming out of each of those engines. Relative to the air around it, that airplane is going to move forward.

Where do you think that thrust goes? How does the addition of a conveyor belt change the relationship between the thrust and the aircraft?
posted by five fresh fish at 6:20 PM on November 1, 2007 [1 favorite]


"There's a good fifteen to twenty thousand pounds of thrust coming out of each of those engines."

That's one hell of an ultralight.
posted by mr_crash_davis at 6:31 PM on November 1, 2007 [1 favorite]


If you made the moving walkway wider and installed seats on one side, you could accomodate lounger and hustler alike, and get the plane off the ground in the bargain, at least until Pops falls asleep sitting up again.
posted by breezeway at 6:49 PM on November 1, 2007


This is so going to wendell.
posted by casarkos at 7:06 PM on November 1, 2007


"Fuck you. Walk on the left." Two of these words are redundant.

I see what you mean. It should be "Fuck you. Walk left."
posted by ROU_Xenophobe at 7:42 PM on November 1, 2007


"Dunno why you'd use them to a kid. Golden rule an'all that."

Because it was more to the dad who was saying "What the hell do you think you're doing?"

But were I lollygaggin' and impeding a kid from getting where he needed to go, I'd feel fine being treated to a "Fuck you, old man."
posted by klangklangston at 7:57 PM on November 1, 2007


Fuck YOU, old man!!!1

just needed to be repeated.
posted by snsranch at 8:38 PM on November 1, 2007


Fuck all a yall conveyor belt ass motherfuckers.
posted by Divine_Wino at 8:46 PM on November 1, 2007


Um, am I the only one who noticed the "MeFi" in the background of last night's episode?

Also the plane doesn't take off, because it's sitting on THE TARMAC FOR EIGHT FUCKING HOURS BECAUSE OF "WEATHER" AND "MAINTENANCE" AND OH MY GOD GET ME OFF THIS PLANE!!!
posted by dirigibleman at 8:53 PM on November 1, 2007


Um, am I the only one who noticed the "MeFi" in the background of last night's episode?

Nope. It's been on there for a number of episodes now.
posted by puke & cry at 8:56 PM on November 1, 2007


So hey, guys, is there a god or what?

Follow-up question: if there is a god, what would he say about circumcision?
posted by SassHat at 9:00 PM on November 1, 2007


“We have to be assuming that the belt counteracts any forward motion of the plane, otherwise it's a really silly question.”

It's a really silly question if we assume the belt keeps the plane from moving forward. Then it's just “can a plane take-off without moving forward” which is simply “no”. (Disregarding exotic things like the belt pushing a film of air backwards.)

So it's interesting to me that you think the forward-moving version is silly but the non-moving version is not silly. I think they're both silly. A plane takes-off because it moves forward through the air. A plane isn't going to take off unless it's moving.

There's all sorts of problems in how this problem is stated. The coupling of the wheel speed to the belt speed is very problematic and confuses the question horribly. I think by far the biggest reason this question is so controversial is because it's so badly framed. On the other hand, it's clear that a lot of people get the actual physics wrong, so it must be the case that the problem is aiming for something that is an interesting misunderstanding.

As a physics problem, it's simply not interested in what would happen in the real world with all the real world messiness. But pilots who think about this question have two things going against them: one, the paradigm in which they are accustomed to think about these things may (and seems often to, in fact) cause them to misunderstand the physics involved and thus get the problem very wrong. But the other thing going against them, which isn't really their fault, is that because they are pilots, they think about this question practically. So, for example, they realize that while they're getting a plane up to take-off speed and going down the runway, the way the plane behaves on the ground differs at different speeds. With a belt speeding under a plane making the wheelspeed be, say, twice as high as it really is, then the plane's lateral movement across the belt when the front wheel is turned or when the front wheel is briefly lifted off the ground while the rear wheels are not and the plane slightly changed its orientation so that the rear wheels are not aimed parallel to the belt will be unusually, perhaps uncontrollably, fast.

Which I don't think is insurmountable (because jets deal with higher runway speeds without trouble) but the point is that real pilots will have all these real-world things they will be worrying about when they think about this question. Some of them will cause them to misunderstand the physics involved, and some of them will just overload them with practical concerns such that they're not able to think about the question clearly.

That's the charitable interpretation of paulsc's contirbution to that thread.

But with my other argument with TeamBilly in another thread where he claims that engines have nothing to do with climb rate, I've found that my opinion of the general ability of pilots to correctly understand this stuff has sunk very low. They may be examples of people who have learned a great deal of technical information which leads them to think they know a bunch of stuff they actually don't really know or understand. It's easy for a pilot to wrongly think he really understands aerodynamics and physics for the same reasons it's easy for a programmer to wrongly think he understands computer science and mathematics or solid-state physics. It's understandable, but nevertheless really annoying.
posted by Ethereal Bligh at 9:06 PM on November 1, 2007 [1 favorite]


Your favorite Internet argument sucks.
posted by Cool Papa Bell at 9:10 PM on November 1, 2007


I want Mythbusters to investigate this conundrum.
posted by Tube at 9:38 PM on November 1, 2007


Escalators and people movers. They are not so you can rest, you lazy motherfuckers. They are there to get us all there faster.

I thought they were there so that my two-year-old daughter would have something in the airport to be terrified of.

egg shot first
posted by davejay at 9:59 PM on November 1, 2007


So it's interesting to me that you think the forward-moving version is silly but the non-moving version is not silly. I think they're both silly. A plane takes-off because it moves forward through the air. A plane isn't going to take off unless it's moving.

Well, to me, the essence of the question is "can a plane take off without moving with respect to the ground", as opposed to "can a plane take off while moving less with respect to the ground than it normally does". But, yeah, I guess they're both equally silly.
posted by equalpants at 10:11 PM on November 1, 2007


It's a really silly question if we assume the belt keeps the plane from moving forward. Then it's just “can a plane take-off without moving forward” which is simply “no”. (Disregarding exotic things like the belt pushing a film of air backwards.)

More accurately, there has to be a difference in velocity between the air and the wing. If the air is moving back over the wing, the wing has lift. If the wing is moving forward through the air, as normal, same deal.

From the previous thread I am fairly sure you know this, so I'm not trying to argue, I'm just stating it for the record.

This is just such an incredibly ridiculous question.

In college I had a roommate who, one night, totally out of the blue, said (and he wasn't high), "whoa... okay okay okay, I have this great idea." "okay." "WHAT IF, you put rockets on the equator and fired them all at once."

Then, silence. I waited for him to get to the idea. You know, the part where you have some benefit from an action.

Nothing. After minutes I said "... what.. wh.. why? what?" "That's it! That's the idea!" ".... what? but... you... what..." In retrospect I realize that I should have told him that I don't think he knows what an idea is. I was too lost to think of that at the time.

This question makes me feel the same way. I don't think the questioner understands anything about the situation. The premises are incomprehensible.
posted by blacklite at 10:42 PM on November 1, 2007 [1 favorite]


"can a plane take off while moving less with respect to the ground than it normally does"

Why would the plane be moving less simply because a treadmill is involved?

The wheels decouple the motion of the treadmill from the motion of the plane under power (less an insignificant amount of friction from the wheel bearings).

The wheels will spin faster, but the plane on the whole is going to move forward as per usual: there has been no change in the power output of the engines and their action is uninhibited by the presence of a treadmill.
posted by five fresh fish at 11:16 PM on November 1, 2007


The basic flaw is in the way the question is framed. The idea, supposedly, is that the treadmill moves backwards to compensate for the plane's forward motion and thus keep it in the same place. If that were possible, then no, the plane wouldn't take off.

The thing is, it's not possible for the treadmill to move the plane backwards once the plane has started to impart forward thrust ... unless there's some very serious rolling resistance in those wheels. As in standing on the brakes rolling resistance. If there isn't, then the treadmill fails to compensate for the plane's thrust, the plane moves forward with respect to the air and everything else except the belt surface, achieves airspeed and takes off.
posted by George_Spiggott at 11:33 PM on November 1, 2007


Er, strike the "except the belt surface" part of that last sentence.
posted by George_Spiggott at 11:38 PM on November 1, 2007


What if the conveyer belt is inside a really big airplane, and the airplane inside the airplane is moving at the speed of light? Then, what if you turn on the lights on the front of the plane? Does it implode? What if no one is there to witness it, is there a sound made?
posted by blue_beetle at 11:40 PM on November 1, 2007 [1 favorite]


blacklite writes "I just read about a sixth of that original thread. What a clusterfuck! Let's not start that again. I will have to yell."



exogenous writes "I wanna know where they hell they got a quarter-mile conveyor belt - Acme?"

That's only what, 480 metres? Seems they could roll their own. The fact they need a belt that long gives away the answer otherwise they'd only need a conveyor slightly greater than the wheel base of the plane.

Terminal Verbosity writes "So, e=mc2 parses out to energy = mass x the square of the speed of your average Cessna?"

African or European?

mr_crash_davis writes "'There's a good fifteen to twenty thousand pounds of thrust coming out of each of those engines.'

"That's one
hell of an ultralight."

Maybe they are doing the JATO car myth at the same time.
posted by Mitheral at 11:42 PM on November 1, 2007


blacklite if you didn't get down to paulsc's breakdown you need to keep reading.
posted by Mitheral at 11:43 PM on November 1, 2007


“Why would the plane be moving less simply because a treadmill is involved?”

Well, yeah, but his point is that people think the question is asking what if the plane wasn't moving.

Which, yeah, I have a hard time understanding how anyone would think the plane would take off if it weren't moving. It seems so obviously like a stupid question to ask, that you'd think the people who assume the plane musn't be moving would wonder if they misunderstood the question.

But, see, they think that's the trick of it. They think that anyone who thinks the plane would be moving are the morons. Because they think that the treadmill would stop the plane from moving forward.

Which it wouldn't because, as you say, the wheels decouple the plane's motion from the treadmill. With ideal wheels (meaning they are frictionless on their axles), it makes no damn difference what the treadmill is doing. It could be going forward, going backward, either at 10MPH or a 1,000,000MPH, or standing still, and the plane doesn't care. In all those situations, it's as if the plane were taking off on ice. And the engine's thrust works as it usually does. The plane takes off perfectly normally.

Really, that's the actual physics the question is trying to get at. Do people understand that the plane is pushed forward by the engine through the air, by the air by its engines and that this is how the plane flies. The wheels have nothing to do with anything. That's what the question is trying to get people to understand.

Sadly, many don't.

I guess the question seems to take as an assumption something that is false and it expects people to figure out that the assumption is false. But that's a really bad way to teach anyone anything because we're taught to think of these sorts of questions as self-contained and that assumptions are assumptions and are, by their nature, unquestionable in this context. So a lot of people naturally assume that the question is asking whether a plane that's not moving forward with respect to the world around it (excepting the treadmill) will take-off, or not. And of course it won't. But the assumption that the plane would be standing still, as explained in the problem statement, is false. So it will take off as normal.

Finally, the question, as stated, has a paradox in it. This also confused things. With perfect wheels, you could move the treadmill backwards and the plane would stand still (perfect wheels) and the wheels would show X speed and the treadmill would show -X speed. So the idea of the treadmill “keeping up” with the wheels makes sense and could happen. However, the phrasing really seems to imply that first the propeller spins up, starts the plane moving forward, and then the treadmill starts to move backward, eventually catching up to the plane and then keeping the two equal. Unfortunately, that's not possible with ideal wheels. With ideal wheels, if the plane has any forward motion at all, then the treadmill couldn't possibly catch up to the wheels and both of them would quickly speed up to infinity (or the speed of light, whichever you prefer).

With less than ideal wheels, eventually the treadmill would speed up enough to use the wheel's friction on the axle to cancel the plane's forward motion. But that's going to be a very, very high speed! You'd probably get some other mechanical failure before you ever reached that point. If not, though, then the question manages to match how it's been phrased, in which case the plane isn't moving forward and doesn't take off.

But that's not very interesting, is it? I mean, you might as well have just tied the plane down or put blocks under the wheels.

I don't know. One way to look at this question is that it's basically targeting stupid people. People who can't tell the difference between how a car works and how a plane works.

In the final analysis, I think this question is in a special category of question where it doesn't take much intelligence to figure out the correct answer and/or to figure out the correct answers by understanding there's several ways to interpret it. However, few of us are smart enough to understand how other people misunderstand the question well enough to easily correct their misunderstanding.

That last bit may seem unrelated, but it's not, really. Knowing how to understand how other people think, and get confused, and knowing how to explain to them how they are confused, is a skill we all have to some degree and a skill we use all the time. So it's interesting to me that this question involves so many different things which are messed-up that they make it very difficult to figure out how a specific person is misunderstanding it or to correct their misunderstanding. It manages this feat by being stated in a very awkward and misleading way, point at an actually interesting physics problem it's not directly asking, involve people's practical experience and expectations of driving automobiles, non-pilot's (mis)comprehension of how plane's fly, and, occasionally, a pilots comprehension and miscomprehension of how a plane flies. It just throws all those things into a bowl and makes a big mess.
posted by Ethereal Bligh at 11:56 PM on November 1, 2007 [1 favorite]


It's just so painful, Mitheral.

*puts on mining hat, goes back to thread*
posted by blacklite at 11:57 PM on November 1, 2007


EB: you are right about everything there (I think) except you should probably quit saying that an unmoving plane cannot take off. If the engines pulled enough air through them it still could. The indicated airspeed is the only thing that matters.

e.g. A cessna sitting on the ground, parked, not tied down, experiencing a gust of wind above 55 mph will go upward. (And then come back down, and it's just bad times all around.) Which is why you tie them down.
posted by blacklite at 12:02 AM on November 2, 2007


(However, I'm just complicating the issue, and the above does not come in to play re: conveyor-plane.)
posted by blacklite at 12:04 AM on November 2, 2007


"... That's the charitable interpretation of paulsc's contirbution to that thread. ..."
posted by Ethereal Bligh at 9:06 PM PST on November 1

My offer stands. So far, I have 0 takers, and $0 in side action, at any odds, much less those that confident Barcalounger pilots might offer, via MeFiMail. I'm looking for, oh, 1000:1, or better given the certainty of thought experiment physics. Actually, given the certainty of thought experiment physics, anything short of 10,000:1 starts to look like a sucker side bet, to me. Shoot, 1,000,000:1 looks like a lock to the Barcalounger crowd, to a stupid guy like me, who still mutters an agnostic's prayer to a Lutheran God (who hasn't ever said a word back in 56 years), every time he's shoved in the throttle of a rented 172 on a 6,000 foot, 100 foot wide concrete strip.

As for Mythbuster ultralight experiments, meh. Many ultralights are, by nature and design, STOL vehicles, and as such, have far different takeoff and flight characteristics than "normal" airplanes. A 1950s era fighter jet in its ZEL variant flew, with no conveyor, off the back of trucks, and was already discussed. That's kind of the ultimate "STO" (Short Take Off) vehicle with wings (or "airplane"), and being more rocket than airplane for the first 4 seconds of flight, is kind of outside the whole question, as are ultralights, although it flew numerous times, about 50 years ago, when I was a kid building models, and flying Cox .049 control line models, which I could throw into flight, much like the Air Force was doing with F100 fighters. As I've said "Lift + thrust = Drag + weight" if you're going to fly. Actually, most pilots want some extra on the left side, at least in the form of engine reserve, but that's the classic formulation of the simple flight equation.

What's the point of playing with ultralights, except that they are cheap (conserve Mythbuster's budget?) and usually slow (easier for Mythbuster's camera men to track)? Come on folks, real money, a real airplane, a real conveyor (if you can find it) and real coffins and funereals (which probably pop Mythbuster's liability limits), if you're badly wrong...

My offer gives you a 1200 foot conveyor, if you can find it, for a classic Cessna 172 takeoff. But please, if you can't find a 1200 footer, consider yourself free to demonstrate from a 15 footer, or anything in between. The truly convinced, including the JATO accustomed (JATO or RATO or similar assisted Cessna takeoff being outside the margins of our conveyor bet), will even consider the 15 foot conveyor excessive, so, please, on my account, don't feel constrained to fly from any specific conveyor length longer than the absolute minimum distance from the nose wheel to the main gear of a standard Cessna 172. I can see, simply in terms of conviction, that any conveyor of more than 20 feet in length, will be a major impediment to the conveyor believing brethren, and I, as a non-conveyor acolyte, would not want to impede miracles, by insisting on unnecessary conveyor lengths. Personally, I think any increase in conveyor length over the nose wheel to main gear length of Cessna 172, ought to draw greater odds, in proportion to the conveyor length, unless you think I'm lying about being able to bounce a 172 on engine run-up, at 0 roll velocity. 'Cause I'll take that side bet, at only 10:1, any day you want to make it. That would look like -->You show up at an FBO I designate --> I rent a Cessna 172 --> I get in the plane, and start the engine, and without rolling, get the nose wheel off the ground --> You pay me 10x what I've bet.

Bring your money, cash, and I'll fly you home commercial, multi-engine, gratis, in North America, if you win.

Because the Barcalounger pilots are so confident on our main bet, they won't need to plan for an accelerate-stop emergency procedure, which would go like this:
"...This is a good place to discuss accelerate-stop distance for single-engine aircraft. That's the distance it takes to get up to flying speed, decide that you've had a change of heart, and abort the takeoff with maximum braking. That's a great option to have if the takeoff isn't working. It's standard procedure for multiengine equipment, but you don't hear much about it with singles. It's not a number that you'll see in the single-engine aircraft's operating manual or pilot's operating handbook (POH), but we'd sure like to see the manufacturers start publishing it.

Here's how to make an approximate calculation: Take the takeoff ground roll under ambient conditions and add it to the ground roll for landing. It's a good idea to add in some recognition and reaction time as well, unless you're really good at split-second decisions. Consider three seconds as a minimum. That's not very much and at typical rotation speeds for a light fixed-gear single, that adds another 300 feet. Here are some hypothetical ambient conditions: sea level, dry paved runway, no wind, and a temperature of 20 degrees Celsius. For an older-model Cessna 172, this works out to 835 feet for runup to rotation speed, 300 feet for decision time to reject, and another 530 feet to smoke the brakes and bring it to a stop. That's 1,665 feet and with an additional 10-percent margin allowing for older engines and brakes, not to mention pilot technique, it works out to about 1,800 feet. ..."
One reason you don't see it published for singles, like a Cessna 172, is that, if the engine quits running, or the prop breaks, or something else really bad happens to instantaneously alter your airplane's power input, you also lose, proprotionately, much of your other control inputs. Not only does your power suddenly suck, so do, very shortly, your rudder and aileron inputs, to about those of a sub-optimal glider, traveling slower than it would like to be gliding. That's only one reason commercial airliners are generally multi-engine aircraft.

But anyway, 835 feet to rotate, on a 1200 foot conveyor, for a classic Cessna 172 takeoff, must look like a fool's bet to convinced Barcalounger pilots, for whom nothing ever goes wrong on a hypothetical conveyor opposed takeoff roll. That's enough extra roll, by design that none of you committed Barcalounger folks should have a problem going for a sightseeing ride, with your intrepid pilot. You'll have plenty of conveyor, moving at only 1x your airspeed at any given point up to and including rotation, that this ride shouldn't be a problem.

1,000,000:1, hell 10,000,000:1 is like taking candy off a stupid baby, right up until it isn't. Who wants to trade in their Barcalounger for a right hand seat in a real Cessna 172 on a 1200 foot or less long conveyor moving at 1x instantaneous airspeed in a direction opposite takeoff, at 1,000,000:1 (assuming their estate can pay, in the worst case)? Who is too chicken for that, and is staying on the ground, taking off their 120 to 200+ pounds of wing loading, but giving me 10,000,000:1, or better, up front, in cash based on sincerity and conviction, in dead Presidents and Patriots? Remember folks, I'm on the hook for funereal flowers, in the unlikely event any are needed, out of my impossible winnings.

Personally, I think the classic Cessna 172, and a 1200 foot conveyor, running at 1x airspeed in a direction opposed to takeoff, is such compelling television, with a comfortable, built-in, better-than-theoretical safety margin, that I'm surprised asavage doesn't insist upon it, for Mythbusters. Unless big insurance companies aren't believers.

So come on, EB. I think, at 10,000,000:1, I can cover any side bet you'd like to make, that you're capable of paying, or laying off. It's the market price of certainty in an uncertain world, I guess. Standard aircraft insurance will terminate the instant your 172 sets nose wheel on a real 1200 foot conveyor, but insurers aren't convinced. That shouldn't matter, at all, to those who are, and if the experiment crumples a 172, (although, as you say, it shouldn't,) I expect you'll have no problem covering its cost, assuming you we don't have lots of radio gear, or an advanced model, on the test plane.

Just before the Romans bowled with my namesake Paul's head, I hear he was giving 100,000,000:1 on Jesus returning.
posted by paulsc at 12:06 AM on November 2, 2007 [2 favorites]


(This will go well.)
posted by blacklite at 12:13 AM on November 2, 2007


“EB: you are right about everything there (I think) except you should probably quit saying that an unmoving plane cannot take off. If the engines pulled enough air through them it still could. The indicated airspeed is the only thing that matters.

e.g. A cessna sitting on the ground, parked, not tied down, experiencing a gust of wind above 55 mph will go upward. (And then come back down, and it's just bad times all around.) Which is why you tie them down.”


Well, those are two different situations. A gust of wind makes the plane go up in the air because if pushes wind across the wings, which are what makes a plane fly.

The engines pushing air only push the plane in the direction the engines are facing. That won't make a plane take off if the plane isn't moving. So, no, no matter how powerful the engines are, they aren't going to make the plane take off. (Unless they are pointed upward and not parallel to the ground.)

On Preview: paulsc, your bet is just silly. There isn't such a conveyor belt in the world that you could fly a Cessna off of. You aren't risking anything with your bet, therefore it's an entirely meaningless stunt. Even if there were such a thing and your experiment were possible, the likelihood of anyone taking you up on it is so low as to be practically zero. So just stop mentioning your silly bet, okay?

Your argument is that wheels rolling on a surface for what will seem—to the wheels and the plane's motion which is dependent upon them—a high rate of speed will keep a plane from taking off because of the increased left-and-right forces if the plane isn't pointed in the direction parallel to the conveyor belt.

Fine. Maybe it will. It depends upon how fast that treadmill is running backward, and there's no reason to assume any particular speed that it would be running. The question, as it was phrased in the link in that thread, couples the speed with the speed shown by the wheels. But that's nonsensical, since they're the same thing.

Maybe the most reasonable assumption to make about how someone would actually try to do this experiment (perhaps these Mythbusters guys), is that if the plane is eventually moving forward in airspeed (on a calm day) at 60MPH right before it takes off, then you'd run the treadmill backward at 60MPH. So the wheels would show a 120MPH groundspeed, and everything that relies upon the side-to-side motion of the plane via its wheels would happen twice as fast as it normally would. If a slight turn of the front wheel made you move 10 yards to the left in three seconds at 60MPH, it would take only 5. And so on.

Your argument is that this would make it so difficult and dangerous to take off that no pilot would willingly try it. I don't know, maybe you're right. I find it a bit difficult to believe because there's lots of planes that have their take-off speed at 120MPH, but what do I know? I'm not a pilot.

In any event, the things you are arguing about are clearly beyond the scope of the question. It's a physics question. It's not worried about these lateral motions any more than it's worried that a conveyor belt would be far bumpier than a regular runway would be. You could just as easily argue that you couldn't take off on a bumpy conveyor belt.

The only reason you're arguing such arcane things is that at the beginning of the thread you argued that the plane would stand still. Or something. You talked about weird wind speeds over the wings as a result of this, which no matter how you look at it, wouldn't happen. You misconceived the problem from the beginning and just kept revising your objections until you reached this highly refined “lateral motion is very dangerous during rotation” version. So, congratulations. You reached an argument you can actually defend. You're right about it. Too bad it really has nothing to do with the problem.
posted by Ethereal Bligh at 12:28 AM on November 2, 2007 [1 favorite]


"... You aren't risking anything with your bet, therefore it's an entirely meaningless stunt. ..."

EB, I'm risking whatever your confidence, and ability to back it, require. If you're "certain" of your result, the market can price your confidence, and construct the necessary experimental equipment. You must be able to cover your mouth, is all I ask.

And I'm posting, in this thread, any odds you offer via MeFiMail, that reflect your "certainty."

When you lay down money, at odds reflecting your "certianty," post again.
posted by paulsc at 12:37 AM on November 2, 2007


Not to interrupt the exciting no-holds-barred match or anything, but...

A gust of wind makes the plane go up in the air because if pushes wind across the wings, which are what makes a plane fly. ... The engines pushing air only push the plane in the direction the engines are facing.

The engines disturb air in some way which may send some of the air going through them across the wings. Their primary purpose is of course to propel the plane forward through air, but in doing so the disturbed air, moving backwards, has to go somewhere and come from somewhere.

In situations like paulsc's Cessna 172 where the engine turns a prop at the front of the plane, air does get pushed over the wings (and tail) a bit. Enough to cause effects. There's a point in the other thread where paulsc mentions the fact that you can hop a Cessna up if you put the brakes on and put the engine on full. You really can.

Anyway, I'll get out of the way for the sparring now. Sometimes I picture roiling Mefites, steam pouring out of their ears, refreshing madly, copying, pasting, italicizing... I suppose it's better than pistols at 30 paces.
posted by blacklite at 12:38 AM on November 2, 2007


blacklite, if you're going to cover all the sucker bets, dammit, cover them :-)
posted by paulsc at 12:43 AM on November 2, 2007


“When you lay down money, at odds reflecting your "certianty," post again.”

I think I'll post on this topic whether or not I lay down any money, thanks.

I don't really understand why you're harping on your bet. Do you really think you're making any sort of a point with it? It'd be one thing if your bet was, say, to call up a professor of aerodynamics. But what you're asking for is absurd. You know it's absurd.

It's bad-faith argument. It's not serving any purpose. I've already agreed with you that your specific argument—the one about the difficulties of doing this in the real-world because of exaggerated lateral motion—has merit. What more do you want?

What I'm “certain” about is that the backward motion of the conveyor belt won't hinder the plane's ability to move forward fast enough to take off. The conveyor belt may hinder the plane's takeoff in another way. There's all sorts of possible complications, some you've mentioned, some I've thought of, others no one yet has thought of, that might make this difficult to actually do. But all that's beside the point. The question is asking whether the belt will keep the plane from taking off in the simplest, most obvious sense of “pushing the plane backward”. The answer to that question is: it won't.
posted by Ethereal Bligh at 12:59 AM on November 2, 2007


"... Do you really think you're making any sort of a point with it? ..."

Yep. My point is simply that your wallet, apparently, can't cover your mouth. In other words, for all your "certainty," you are not certain.
posted by paulsc at 1:07 AM on November 2, 2007


“Yep. My point is simply that your wallet, apparently, can't cover your mouth. In other words, for all your ‘certainty,’ you are not certain.”

I'm making the mistake of taking you seriously here, since you and I both know that your bet is a stunt, but, no, that doesn't follow. You are offering a $1,000 return on, say, a $10,000,000 investment to design and build such a coneyor belt, the cost of the plane and various expenses related to it, and to insure the belt, the plane, and the pilot. That's a bad investment.

Not to mention, I don't have access to that kind of money. Being certain doesn't give me access to that kind of money, having all the credentials in the world wouldn't give me access to that kind of money.

And maybe I'm not interested in going to all that trouble just to win a $1,000 bet and/or prove you wrong.

So, no, that I'm not taking you up on your bet neither proves that I'm wrong, nor that I'm “uncertain”. It proves nothing except that you're being kind of a jerk.
posted by Ethereal Bligh at 1:14 AM on November 2, 2007 [1 favorite]


"... It proves nothing except that you're being kind of a jerk.
posted by Ethereal Bligh at 4:14 AM on November 2

It proves you don't believe, as you say you do, at market rates, EB. Because the whole point of odds is to let you lay off risk, to those who believe as you do, and can absorb it.

You've talked, already, past your wallet.

I'll give you another day (24 hours) to lay off your silly.
posted by paulsc at 1:23 AM on November 2, 2007


Okay, original poster here. I need someone to explain this to me because I think the plane won't take off and apparently my intelligence is something close to slime mold because of it.

Here's how I see it: the plane moves forward, and the belt magically moves backward at the same speed. Fast forward to takeoff speed, say 80 mph. The belt is moving backwards at the same speed, so if you were watching the plane from the side, it would appear motionless, aside from some side-to-side and up-and-down bumps. The plane's engines are roaring, and the belt is roaring, but the plane doesn't move forward a single inch.

So if that's the case, then no wind is going over the wings of the plane and there is zero lift. So how can it take off? You can have all the thrust you want, but the belt will compensate and match that thrust, and you still have a stationary plane. There's no wind, so no lift, so how does it take off?
posted by zardoz at 1:35 AM on November 2, 2007


Frictionless wheels: the belt can do whatever it likes and the plane won't notice.
Wheels bolted to the belt: the plane engine can do whatever it likes and the belt and plane can stay put.
Wheels with small amount of friction, magic belt that can go arbitrarily fast, but non-magic plane engine: belt can produce enough backward force to keep the plane fixed.
Ordinary belt and magic plane engine: plane takes off.
Ordinary belt and ordinary plane engine: I suspect plane engines are closer to magic than conveyor belts are. Plane takes off.
posted by edd at 1:44 AM on November 2, 2007 [1 favorite]


"... I'm making the mistake of taking you seriously here, since you and I both know that your bet is a stunt, but, no, that doesn't follow. ..."

I don't post here, EB, to "win."

I post here to remind you, that in every rush down a runway, based on Bernoulli's Principle, an act of supreme faith occurs.

A man, with 3 figures of flight hours, does a pre-flight inspection, loads his family and their luggage in a rented Cessna, calculates weight and balance, remembers the nick he saw in his rented propeller and the oil stain at the base of #4 cylinder, and the dance recital his daughter is having Monday, and hopes, that God is not the jerk you think I am, dammit, down a 4,000 foot strip of concrete, as much as you hope I'll apologize, for being the jerk you think I am.

If he flies, with the luggage of his family, and the kids in the back seats, I'm happy for him. And grateful, though he be 100 lbs. over load limit, for his takeoff parameters.

And I wish you, sincerely, smooth air, and soft landings, on any bet you ever make, or back, on behalf of professionals you've hired, on man made wings.

Good night, and good landings.
posted by paulsc at 1:49 AM on November 2, 2007


“Here's how I see it: the plane moves forward, and the belt magically moves backward at the same speed. Fast forward to takeoff speed, say 80 mph. The belt is moving backwards at the same speed, so if you were watching the plane from the side, it would appear motionless, aside from some side-to-side and up-and-down bumps. The plane's engines are roaring, and the belt is roaring, but the plane doesn't move forward a single inch.”

That's a concise way to describe how you see the problem. Let's see if I can do a better job explaining to you what's wrong with this description than edd did.

Hmm. Okay.

You say that the plane moves forward. Okay, the plane has just started to move forward at, say, 2MPH. It's just slightly rolling. It's doing that because the pilot goosed the throttle a little bit, presumably.

Now you say that the belt “magically” moves backward at the same speed. How would that happen, exactly? Think about it.

If the plane is already moving forward at 3MPH, then if you start to move the belt backwards, whatever speed the belt moves backward will add to the speed the wheels are moving. So a 3MPH backward speed of the belt will make 3MPH + 3MPH = 6MPH. So it can't move backwards at the same speed as the wheels, not even “magically”.

No matter how fast you speed up the conveyor belt, it won't ever cancel out that 3MPH of the plane.

Why? Because wheels roll. Right?

So maybe you're saying, okay, forget the plane beginning to move before you start the belt.

Okay. It doesn't matter. Because, you see, for the same reason that the belt moving backward isn't going to make that 3MPH go away in the previous example, if the belt starts exactly when the plane starts, it's still not going to keep the plane from moving forward. For the same reason: wheels roll freely, moving the conveyor belt under them just makes them roll more quickly, it doesn't keep the plane from actually moving forward.

So you see? What you're thinking would happen just to set up the problem to get to the point where the plane isn't moving couldn't happen. The plane is going to move, the conveyor belt isn't going to make any difference.

The question makes it seems as if the conveyor belt could hold the plane in place by talking about the belt canceling out the wheels. But that actually doesn't make any sense, it's not possible.

Or, alternatively, it's possible if the wheels don't work very wheel. In which case it's not that the belt would move backward at the same speed to cancel it out, but some amount much higher such that it works against that little bit of friction the wheel has against its axle.

But that's not really that interesting, is it? You could just as well talk about an elephant pulling on a rope tied to the plane just hard enough to keep it from rolling forward. Or anything else.

So the answer to your question as you've asked it is this: the belt isn't going to keep the plane from moving forward as your question assumes it will. And because the plane moves forward pretty much just the same way it always does (except with a conveyor belt moving backward underneath it) then it will take off pretty much the same way it always takes off.

The trick here is understanding that comparing this to something like a car or a person running on a treadmill is a mistake because what's pushing a plane forward is different than what's pushing a car or a person forward. Both the car and the person are pushing against the ground in order to move forward, so if you move the ground backward underneath them, they won't go forward.

But a plane is pushing against the air to move forward, so moving the ground backward underneath a plane causes the plane no trouble whatsoever in moving forward. It moves forward as usual. So the conveyor belt has no effect.
posted by Ethereal Bligh at 2:14 AM on November 2, 2007


blacklite writes "It's just so painful, Mitheral.

"*puts on mining hat, goes back to thread*"


You can stop, paulsc brought it here.
posted by Mitheral at 2:37 AM on November 2, 2007


The belt is moving backwards at the same speed, so if you were watching the plane from the side, it would appear motionless

Here's your problem. The belt may be spinning backwards at the 80mph, but because the plane's wheels are free to spin, it has little or no effect on the plane's speed.
posted by cillit bang at 2:54 AM on November 2, 2007


Yup, I'm pretty sure the plane can take off. Thinking of it force-wise, if the engines are pushing the plane forwards for all they're worth, yet it's not moving, where is the balancing force-coming from? From the conveyor belt, through even reasonably frictionless wheels? Seems highly unlikely.

(It would be interesting to observe the properties of a car in a similar situation; presumably it would cost the car very little effort to move very quickly relative to the belt, while staying still relative to the earth in general, due to discounting air resistance. Its speed would largely be limited only by how fast it was able to spin the axle and wheels.)

Still, looking forward to seeing the episode. Empirical evidence is one mighty fine kind of evidence.
posted by thoughtless at 3:04 AM on November 2, 2007


"... So the conveyor belt has no effect."
posted by Ethereal Bligh at 5:14 AM on November 2

Fine, my argumentative friend. You, and your pilot, will have no trouble, according to you, taking off from a 15 foot conveyor. Suck your 172's nose up at 55 Knots Indicated Airspeed (KIAS), and hope your 15 foot long conveyor is tracking, true. You won't actually be flying just yet, but your nose wheel will be off the conveyor, and you'll be 100% dependent on rudder control for left-right steering, which, at 55 KIAS, is notably crappy for even a 172 at sea level. Let's hope you're not taking off in Kansas, a couple thousand feet above sea level, or in Colorado, where you use an entirely different weight/balance schedule, for takeoff parameters. Because, otherwise, whatever you believed, you're in the weeds, at best, if not dead, already, about 25 seconds into your "flight."

Let's be charitable, because you're a believer in whatever you believe, and nothing I've told you. Your lack of backers, at odds, doesn't register with you.

Your mythical 172, on it's 15 foot conveyor, strains to lift, without rolling off the conveyor. Unlike this miraculous fool, your 172 never needs a runway, or varies, even 5 feet, from its hoped for target position, perhaps because you don't believe it would. (This guy yaws about 15 feet, on a normal takeoff roll. He's a good pilot. He's not on any conveyor. We don't know what he believes.)

Eventually, maybe 20 or 30 seconds later, if your engine is still running, and nothing untoward happened, you may roll down the runway, or not, and if at any point you happen, in thrust + lift to exceed, even momentarily, weight + drag, you'll hop into the air!!!!!

Yay, EB.

I hope that when you "hop," your airspeed is greater than "stall speed," because otherwise, your flight will be very short, very uncontrolled, and the stall horn will be ringing your ears, badly. Let's believe somehow, for your sake, that your prop "dug in" somehow, launched you off the conveyor, and you got to 100 feet of altitude, and 65 KIAS, on your bounce.

Bad, bad place to be, EB. 100 feet of altitude, at only 65 KIAS is just enough to spill your guts down a runway, or into an alternate emergency landing zone, in a typical 172.

You really want to wait to rotate until you hit 65 KIAS on a normal runway. I have no idea what Cessna would recommend for a 15 foot conveyor opposed takeoff.

They've never published such figures.
posted by paulsc at 3:18 AM on November 2, 2007


Please, please, please interview paulsc.
posted by cillit bang at 4:14 AM on November 2, 2007 [1 favorite]


in thrust + lift to exceed, even momentarily, weight + drag, you'll hop into the air!!!!!

This is what you believe is necessary for flight? Really?

If that were true, a plane with sufficient thrust could fly even with no lift at all. Your misstatement of basic physics implies that a plane does not even need wings to fly, as long as it has a powerful enough engine pushing it forward. According to your logic, you can reduce lift to zero by chopping the wings off a plane (putting the engine/propellers somewhere else, of course), and it would still fly as long as thrust>weight +drag.

Thrust and lift are perpundicular forces. As are weight and drag. You can't just add them together like they were scalars. That's like the first week of physics 101. If lift<weight, a plane will not fly, no matter how much thrust you have.
posted by DevilsAdvocate at 4:31 AM on November 2, 2007


  • You, and your pilot, will have no trouble, according to you, taking off from a 15 foot conveyor.

    Now, paulsc, you're just inventing things that EB has never said, nor yet implied. Two can play at that game:

  • You, and your pilot, will have no trouble, according to you, taking off from the moon.

  • You, and your pilot, will have no trouble, according to you, taking off from the back of bedraggled goose.

  • You, and your pilot, will have no trouble, according to you, taking off from the tip of a unicorn's horn.

  • posted by Aloysius Bear at 4:34 AM on November 2, 2007


    "... If that were true, a plane with sufficient thrust could fly even with no lift at all. ..."
    posted by DevilsAdvocate at 7:31 AM on November 2

    Sorry, really, DevilsAdvocate. Please, take up your objections with Neil Armstrong and Buzz Aldrin, who probably agree with you, and hate every minute of the time they spent confirming your hypothesis.
    posted by paulsc at 4:42 AM on November 2, 2007


    His objections are still valid as you don't make it clear that thrust and drag are only to be taken in the vertical component (you are saying that right?). Once you do, fine. Although I can't imagine there's any drag at all in that case at the moment of take-off, as your vertical velocity is zero.
    posted by edd at 4:45 AM on November 2, 2007


    "Now, paulsc, you're just inventing things that EB has never said, nor yet implied. Two can play at that game: ..."
    posted by Aloysius Bear at 7:34 AM on November 2

    You may be right, Aloysius Bear. I am trying, perhaps too hard, to keep Ethereal Bligh from killing himself in a stupid training accident.

    I'll shut up, while he violates the flight equation, and just pick up his pieces in a Kentucky Fried Chicken bucket. After, I empty the bucket of chicken.
    posted by paulsc at 4:55 AM on November 2, 2007


    See this is why I don't like on-site ignore lists. How many of us would have plonked paulsc already? And look at what we would have missed!
    posted by Skorgu at 5:13 AM on November 2, 2007


    Yes, as edd notes I was following the aerodynamic convention that thrust is, by definition, purely a forward force, and when the force of the engine is applied other than horizontally, the part that is applied vertically is by definition lift, and not thrust.

    However, even if we accept your apparent convention that "thrust" is whatever force is applied by the engines/propellors, regardless of whether it is applied horizontally, vertically, or at an angle, it is still not the case that thrust+lift>drag+weight is sufficient for flight: according to your model, NASA could have set Armstong, Aldrin, & Collins's Saturn V on its side--or even upside down!!--and it would have flown just as nicely as it did when pointing upwards.

    Another demonstration that vectors are not scalars, and cannot be added as such, is to take the opposite tack and reduce thrust to zero, as suggested in blacklite's scenario of a plane sitting on the ground, engines/propellors off, in a strong headwind. According to you, for the plane to rise up in the air, the windspeed would have to be sufficient to create enough lift to compensate for the drag and weight combined, whereas everyone else here would contend that it need only exceed the weight for the plane to rise, regardless of the drag, i.e. the windspeed necessary for this to happen is exactly the same as the airspeed for the same plane to maintain level flight when operating.
    posted by DevilsAdvocate at 5:27 AM on November 2, 2007 [1 favorite]


    paulsc, the 15' conveyor is something you came up with because you’re looking at the question all wrong. Of course a Cessna cannot take off from a 15' conveyor, just as it cannot take off from a 15' airstrip.

    What I mean is, the plane will still need roughly the same amount of ground distance to achieve its velocity, relative to the surrounding fluid air, to get lift.

    Imagine replacing the plane with a ship that has wheels, and the air with water. Under the ship is a conveyor. Once the ship’s propellor starts spinning, it will begin to propel itself through the fluid water. If the conveyor happens to be moving in the reverse direction, it will simply cause the wheels to spin. Eventually, the ship will propel itself off the end of the conveyor.
    posted by breaks the guidelines? at 5:46 AM on November 2, 2007


    You are offering a $1,000 return on, say, a $10,000,000

    He said 10,000,000:1. That's one dollar against ten million. After looking up some statistics on airplane crashes, I wouldn't give million-to-one odds on an ordinary Cessna launching on a normal runway in good weather, even if it wasn't completely insane to make bets with such one-sided odds for the majority of us who aren't insurance companies or the like.

    I'd gladly put up a few thousand bucks at 10:1, if only the odds were a bit better of anyone caring deeply enough about this to go to the trouble of finding a giant high-speed conveyor belt just to try another variation of what Mythbusters already did.
    posted by sfenders at 5:48 AM on November 2, 2007


    But if you don't believe me, paulsc, perhaps you'll believe Dr. Scott Eberhardt of Boeing, when he says, "Lift = weight (for straight and level flight)." (slide 58)
    posted by DevilsAdvocate at 5:52 AM on November 2, 2007


    Look, paulsc, there's a standard procedure for these things, and you're not following it. Watch out, or you could lose your license.

    Here are your options:

    1) If I'm wrong, BAN ME!

    2) I'M GOING TO CUT OFF MY RIGHT HAND!

    Please choose one of the above. As it is, you're wasting everyone's time. Thanks!
    posted by languagehat at 6:20 AM on November 2, 2007 [4 favorites]


    Sorry, really, DevilsAdvocate. Please, take up your objections with Neil Armstrong and Buzz Aldrin, who probably agree with you, and hate every minute of the time they spent confirming your hypothesis.

    Oh jesus fucking christ! WTF are you rambling about? You don't even understand the difference between a rocket and an airplane. A rocket does not have lift, the reason it flies is because the trust vector is parallel to the direction of motion and antiparallel to drag and the gravity vector. A rocket without wings will not fly horizontal to earth surface because there is nothing to generate the lift with.
    In the case of an airplane, the lift and thrust vectors are perpendicular to each other, as are drag and weight. And, importantly, the lift and drag (aerodynamic) are just vertical and horizontal components of the same force.
    They are completely different things!
    posted by c13 at 6:40 AM on November 2, 2007


    See? You can't not argue about it!
    posted by cortex (staff) at 7:02 AM on November 2, 2007


    Actually, languagehat, I think we have to give paulsc credit for a new addition to that list:

    3) If you're not willing to offer me a 10,000,000:1 wager, you don't really believe it!
    posted by DevilsAdvocate at 7:04 AM on November 2, 2007


    One thing that I learned from this thread is that only really rich people can afford to be certain.
    posted by Kwine at 7:12 AM on November 2, 2007 [2 favorites]


    The fucked part of all this is that whatever the result on Mythbusters, no one is going to change their mind. If it doesn't fly, people will say, "Well yeah, sure, it's an ultralight. A jet would take off no problem." If it does, people will say, "Well yeah, sure, get a real treadmill."

    I'm sick of that "barcalounger pilot" talk. I'm a pilot with plenty of experience in a 172 and I have no earthly idea whether the plane will take off. Nothing in my training prepares me to answer the question better than the average IQ layman. Is now the point where you criticize my training because we didn't do a lesson on Ground Effect, the Conveyor Belt, and the Hot Wind?
    posted by MarkAnd at 7:14 AM on November 2, 2007 [1 favorite]


    MarkAnd: "If it doesn't fly, people will say, "Well yeah, sure, it's an ultralight. A jet would take off no problem.""

    It will fly either way. Ultralights and jets use different methods to generate forward force, but they both do it by pushing air backwards, not the ground.
    posted by Plutor at 7:23 AM on November 2, 2007


    I AM NOT GETTING INTO THIS ARGUMENT! I DON'T KNOW! AAAAAAAAAH! AAAAAAAAAAH!

    *head explodes*
    posted by MarkAnd at 7:24 AM on November 2, 2007 [1 favorite]


    See? You can't not argue about it!

    "It?" Which one? Pretty much every thing that comes out of his mouth is so painfully idiotic, and he keeps repeating them over and over again..
    posted by c13 at 7:26 AM on November 2, 2007


    (My point was merely that no true believer will accept that the conditions of the test were perfect enough to invalidate their own opinion.)
    posted by MarkAnd at 7:26 AM on November 2, 2007


    3) If you're not willing to offer me a 10,000,000:1 wager, you don't really believe it!

    Of course, this only points out how very very little faith paulsc has in his own position, since he is not willing to offer any better than 1:10,000,000 odds in defense of it.
    posted by DevilsAdvocate at 7:31 AM on November 2, 2007 [1 favorite]


    "It?" Which one?

    The (gloriously, helplessly, hopelessly multiplex) premise, c13. I know better than to pick a horse on this'n, believe you me.
    posted by cortex (staff) at 7:47 AM on November 2, 2007


    What are the odds that paulsc was one of “the pilot and his entire flight club [who] got it wrong” from the MythBusters episode?
    posted by breaks the guidelines? at 7:49 AM on November 2, 2007


    Y'know, if there's a pilot I wouldn't want to fly with, it'd have to be paulsc. That kind of aggressive wrongness is not the kind of thing that gives me comfort on take-off.
    posted by five fresh fish at 7:55 AM on November 2, 2007


    PaulSC, what kind of a motor drives the wheel of the cessna?
    posted by shmegegge at 8:22 AM on November 2, 2007


    *wheels.
    posted by shmegegge at 8:24 AM on November 2, 2007


    As long as the plane can get to 88 mph, the flux capacitor will work just fine. Duh.
    posted by The World Famous at 8:57 AM on November 2, 2007


    The plane takes off and the "girlfriend" is actually a dude.
    posted by GuyZero at 9:14 AM on November 2, 2007


    The annoying thing about this question is that it ultimately relies on a term that it doesn't define: the rolling resistance of the wheels. If it's zero, the plane definitely takes off, exactly as if the treadmill wasn't moving. Ability to take off decreases as a function of increasing rolling resistance. With real-world wheels, it seems extremely likely to me that it takes off, but I don't have the figures to prove it.
    posted by George_Spiggott at 9:14 AM on November 2, 2007


    George_Spiggot, as the first link above points out, if the engines develop enough thrust, the plane will always take off as the friction force has an upper limit (ie the wheel berrings jam and the tires start to skid).
    posted by bonehead at 9:38 AM on November 2, 2007


    Yeah, what George_Spiggott said. Here's what I predict will happen on the Mythbusters episode:

    They say they are using an ultralight. To get as close to the problem statement as possible, because of the wheels they'll use a trike.

    I have no idea what they'll use for a conveyor. A big assembly line in an idle automobile factory? A luggage belt at an airport? (Maybe there's a really big and long outside one somewhere.)

    But whatever they use, it's going to have a few characteristics that may have an effect on the test. Conveyor belts, after all, are meant to convey things from one place to another. The stuff isn't supposed to move around on them. The surface will have a lot of friction. It'll probably be pretty soft. It'll be bumpy because of the rollers.

    All these things are going to increase the rolling resistance of the wheels to be much higher than it would be for an airplane, or a trike, on a hard runway. Also, a trike's wheels are small, which increases the initial rolling resistance.

    On the other hand, trikes weight so little they are able to take off and land on very soft surfaces, such as grass. So the rolling resistance of a conveyor belt isn't going to be that much of a hindrance.

    So they'll set the trike on the belt, start its motor but not run it high enough to make it move forward, and then increase it until it does. Then they'll turn on the belt. This will be tricky because the biggest amount of rolling resistance is at the beginning of motion—it's getting started that's the biggest problem. So it matters how quickly they start the belt. If the ultralight is already moving forward at a good clip, then starting the belt probably won't have much effect. If the ultralight has just begun to inch forward, the belt will overcome that and push the ultralight backward.

    In either case, though, the belt will add a small amount of additional rolling resistance to the wheels. I'm really not sure how this will scale with increasing speed. For a normal, hard surface, it's not going to increase very much at all as the belt's speed is increased. The slope of it on a graph will be very small. But the soft surface of a belt? I don't know. Again, though, there's the fact that ultralights are ultralight and I've seen videos of them rolling forward and taking off easily on grass—in very short distances, actually. So I don't think this will affect a trike that much.

    What will happen is that maybe when they turn the belt on, it slows the trike a bit (or, if they turn it on earlier, it pushes the trike back a bit), but almost immediately afterwards the trike will accelerate and take-off pretty much in the same distance as it would normally. If it takes sixty feet normally, it might take 65 on the belt. The trike will slightly lift into the air, they'll cut the engine and end the experiment. The conclusion: it doesn't make much difference.

    The only other possibility I can foresee is if they try somehow to make the experiment follow the problem statement. That is, somehow get the belt to keep the trike from moving forward. Maybe they'll ramp the belt's speed up really, really high in an attempt. I don't know. I doubt you can make a belt move much faster than 10MPH anyway. But then again, I have no idea how they're going to manage the belt part.
    posted by Ethereal Bligh at 9:41 AM on November 2, 2007


    Would it set a precedent if this MeTa thread was sidebarred?
    posted by roofus at 9:51 AM on November 2, 2007


    Speaking of unlikely hypotheticals...
    posted by cortex (staff) at 9:53 AM on November 2, 2007


    Ancient proverb: Chicken on conveyor belt have head first to take off.
    posted by weapons-grade pandemonium at 9:54 AM on November 2, 2007


    A rocket does not have lift, the reason it flies is because the trust vector is parallel to
    the direction of motion and antiparallel to drag and the gravity vector.
    Wow, such ignorance.
    Lift is just the name of the upward vector, regardless of it's source. A plane wing generates lift, a rocket motor generates lift, the hot air in a balloon generates lift. They all do so in different ways, but the upward vector is still called lift.
    posted by nomisxid at 9:55 AM on November 2, 2007


    Lift is just the name of the upward vector ... Wow, such ignorance.

    No.

    Lift is "a mechanical force generated by solid objects as they move through a fluid." [Wikipedia]

    If that doesn't satisfy you: lift is the "upward force acting on an aircraft or other body in the air; specifically that produced by its motion through the air; the force on an aerofoil that acts at right angles to its direction of motion through a fluid." [OED]

    As you said: Wow, such ignorance.
    posted by Aloysius Bear at 10:00 AM on November 2, 2007


    Yeah, except that if you talked about a rocket's thrust everyone would know exactly what you were talking about, but you wouldn't want to then go doing 'lift+thrust'. That'd be daft.
    posted by edd at 10:02 AM on November 2, 2007


    Or better, what AB said.
    posted by edd at 10:03 AM on November 2, 2007


    AB, you've only defined aerodynamic lift, the other two flight methods have a component of their flight equation indicating their upward vector. That vector is named lift. Selective quotation doesn't improve your grasp of basic physics.
    posted by nomisxid at 10:07 AM on November 2, 2007


    zardoz, I think your difficulty is included in this statement:

    the plane moves forward, and the belt magically moves backward at the same speed. ... if you were watching the plane from the side, it would appear motionless

    If it's moving, it isn't going to appear motionless. In the first instance, you have the plane moving relative to the surface of the Earth. In your second instance, you've magically eliminated the plane's motion relative to the earth, and somehow fixed it in place. The conveyor isn't doing that; your powerful mind is.

    Think of it this way: An ultralight is flying down an up escalator. Will it rise into the air before colliding with klangklangston as he rushes up the escalator cursing and flinging children over the side?
    posted by Kirth Gerson at 10:10 AM on November 2, 2007 [2 favorites]


    edd, lift + thrust are still named vectors in rocket flight, they just happen to be parallel because there is no aerodynamic lift, only thrust-lift.
    posted by nomisxid at 10:11 AM on November 2, 2007


    But are you saying they should be added? Because it sounds like they're the same thing, so adding something to itself would be bad. Remember the 'equation' this is all about (quite how you apply < to vector quantities in that I'm really not sure).
    posted by edd at 10:15 AM on November 2, 2007


    nomisxid, you're in the wrong here. Nomenclature is not basic physics. It's just nomenclature. Lift has a technical meaning, and it's a technical term from aerodynamics, not basic physics. “Upward vector” is, sorta, a basic physics term. And lift is an example of upward vectors. But you are not arguing against someone's misuse of upward vector, you are (wrongly) arguing against someone's use of lift.

    It's as if someone had said that a cake rises from the yeast's production of CO2 and that it's not “rising” when something increases volume when it's baking in the oven because of water vaporization...and you said that “rising” means the top of the cake is moving upwards and that they don't understand basic physics if they claim that in both cases the thing isn't “rising”. You're being educated-stupid. Egregiously.
    posted by Ethereal Bligh at 10:20 AM on November 2, 2007


    George_Spiggot, as the first link above points out, if the engines develop enough thrust, the plane will always take off

    I'm assuming an ordinary plane, with enough available thrust to take off under the conditions it was designed for and little more. With that in mind, the one term of the puzzle whose value I can't account for is the all-important one. I don't know what the rolling resistance of the landing gear is, particularly on this hypothetical conveyor belt. If we have this term we can solve this, otherwise we can't. But it seems intuitive that this resistance would have to be unreasonably high to keep an otherwise reasonably powered plane from taking off.
    posted by George_Spiggott at 10:42 AM on November 2, 2007


    So, really, the question is:

    If the something makes the plane's wheels spin infinitely fast, even if they have frictionless bearings, do the infinitely accelerating wheels have any effect on the plane's ability to reach takeoff speed?

    The frictionless bearings assumption is the key. If the plane is slowed down by the wheelspin, it is only because of bearing friction.
    posted by The World Famous at 10:45 AM on November 2, 2007


    George_Spiggott:
    You might think of the inverse situation to get a handle on it. When your jet lands on a tarmac runway the wheels spin up from zero to whateversillyfast very quickly, and the plane doesn't lose much energy at all - barely noticeable to the passengers other than the vertical change in speed from touching down. In fact, it has to break quite hard once down.

    So it does seem that the friction in landing gear bearings is really very low, and I think you'd be right to say it'd have to be unreasonably high to keep the plane from taking off.
    posted by edd at 10:47 AM on November 2, 2007


    TWF, edd, rolling resistance is much more to do with tire deformation than bearing friction, and it can be fairly significant. Compare your ability to coast on a racing bike with razor tires inflated to 120psi or more with baloon tires inflated to 20psi. You can coast for blocks and blocks on the first, and about four feet on the second. Our hypothetical conveyer belt could contribute a a good deal more, depending on its smoothness and deformation properties. But as I said, I don't think it's likely to be high enough to stop a normally-specified plane from getting off the ground.
    posted by George_Spiggott at 10:51 AM on November 2, 2007


    George, you're right. But that's not a question of the plane being slowed by wheelspin. It's a question of the plane being slowed by the wheels' reluctance to spin due to increased friction against the belt.

    As I understand the hypothetical, the wheels and belt each increase proportionately in speed to infinity, which eliminates tire deformation issues, as well, essentially reducing the wheels and belt to a frictionless surface, way slicker than ice, and no impediment to forward momentum.

    But the hypothetical is stupid. And, as I said above, the whole debate is an argument about what the hypothetical and its assumptions actually are. The answer to any permutation of the hypothetical is simple.
    posted by The World Famous at 10:58 AM on November 2, 2007


    Yup, good point.
    posted by edd at 10:58 AM on November 2, 2007


    Lift is just the name of the upward vector, regardless of it's source.

    So, genius, if a plane is in 90 degree bank executing a turn, what is the force normal to the wing called?

    a rocket motor generates lift

    Please find ONE reputable source that uses this phrase.

    Oh, and guys, we've been through this before. The question states that the conveyor belt speed is exactly matched to the speed of the plane, so the weels would spin exactly twice as fast. There is no infinitely fast rotation, friction force or anything else of the sort.
    posted by c13 at 11:05 AM on November 2, 2007


    But the hypothetical is stupid. And, as I said above, the whole debate is an argument about what the hypothetical and its assumptions actually are. The answer to any permutation of the hypothetical is simple.

    Most definitely. God help me, but I think we're really arguing about whether the question supposes a literal conveyor belt or a metaphorical one...
    posted by equalpants at 11:09 AM on November 2, 2007


    it seems intuitive that this resistance would have to be unreasonably high to keep an otherwise reasonably powered plane from taking off.

    That's probably true for planes with modern engines and reasonable bearings and decent tires (and you do need to understand all sources of friction for this to be answerable). Underpowered planes, anything that needs a long runway might have trouble taking off with high wheel friction (which is what this gedaken is really all about). I would bet a fully-loaded B52 would have trouble taking off on a conveyor belt, for example, presuming you could find one that was several miles long and a couple of hundred feet wide. In contrast, this scenario probably wouldn't change the taxi distance of an F-15 significantly.

    The problem with any consideration of friction is that it's much easier to figure this out experimentally than to try to work this out from first principles. Theoretical tribology isn't up to this sort of challenge yet, which is why test engineers are still in business.
    posted by bonehead at 11:10 AM on November 2, 2007


    The question states that the conveyor belt speed is exactly matched to the speed of the plane

    Yeah, the hypothetical is stupid. If the plane isn't moving, the conveyor belt doesn't move, either. So if it could stop the plane, it would also stop, which would allow the plane to move. And so on. Just a stupid hypothetical.

    Can a plane take off if its wheels are spinning twice as fast as they normally would? Of course it can. Duh.
    posted by The World Famous at 11:12 AM on November 2, 2007


    Regardless of whether it's safe or sane, I think that the plane/ ultralight/ prop-driven vehicle will eventually achieve enough forward motion to take off. As others have pointed out, repeatedly, even if the tires are spinning so fast they're about to fly into pieces, the prop is pulling the plane forward through the air. I feel like some of the people theorizing in this thought-experiment are somehow connecting the air around the plane and you and me with the ground or conveyor belt. They are two different things. The plane is going to move forward, regardless of what the wheels do. As EB pointed out, it may take a few feet longer, but the vehicle is going to take off.

    (someone used the (I believe mostly correct) analogy of someone on rollerskates with a rope around their waist on a conveyor-- no matter how fast the rollerskate wheels turn, or the conveyor is moving, if I'm pulling you towards me by the rope, you are going to approach me. Sorry for not giving credit where credit is due, but I'm too lazy to see who said this first. Wasn't me.) Of course, the resistance becomes an issue-- friction, et cetera. But the vehicle is still going to move forward.

    I look forward to seeing the episode.
    posted by exlotuseater at 11:13 AM on November 2, 2007


    “Oh, and guys, we've been through this before. The question states that the conveyor belt speed is exactly matched to the speed of the plane, so the wheels would spin exactly twice as fast. There is no infinitely fast rotation, friction force or anything else of the sort.”

    Well...

    Because the question stupidly assumes the plane wouldn't move, which can't happen, then we're really not sure how it intended the “speed of the plane” to be measured. Since it thinks the plane won't move, then it can only mean that it's the wheelspeed that the belt matches, which, in reality, would cause the belt to go into an escalating chase to catch up with the wheelspeed. To infinity, or whatever.

    The other way to try to make sense of the question is to do away with the “not moving forward” assumption implicit in the problem statement, and then you can measure the “speed of the plane” in a more realistic way...such as its forward motion relative to the ground beneath the conveyor belt. In which case you have your “twice the speed of the plane” example.

    It's wrong to say that one attempt to make the problem make sense is more right than the other. They are both pretty obvious and valid ways—the two most obvious and completely valid ways, in fact—of trying to make sense of a problem that is confused in its presentation.
    posted by Ethereal Bligh at 11:17 AM on November 2, 2007


    “If a person on rollerskates standing on a conveyor belt is pulled by someone on a rope, and the conveyor belt is speeded up so that it moves backward just as fast as the rollerskates roll forward, will the person on skates being pulled by a person with a rope be able to move forward, or not?”
    posted by Ethereal Bligh at 11:21 AM on November 2, 2007


    yesh.
    posted by exlotuseater at 11:23 AM on November 2, 2007


    I'm afraid to watch this Mythbusters episode now. It's taken on such a momentous importance to us all that I'm starting to think Man is not meant to know the Answer to the Conveyor Belt/Jet Taking-Off Conundrum.

    Maybe, at the exact moment the world watches the plane taking off/not taking off, the world as we know it will disappear with a huge POOF!

    Or maybe not.
    posted by misha at 11:29 AM on November 2, 2007


    When the world sees this Mythbusters episode, Schrodinger's cat will die.
    posted by The World Famous at 12:08 PM on November 2, 2007


    Maybe.
    posted by dersins at 12:28 PM on November 2, 2007


    When the universe sees this Mythbusters episode, it will immediately disappear and be replaced by something even more bizarre and inexplicable.
    posted by DevilsAdvocate at 12:30 PM on November 2, 2007 [1 favorite]


    Can someone help me, specifically, understand this? I remember when it originally came up and I never asked for clarification.

    So the plane is on a conveyor belt, wheels spinning, belt spinning.

    The body of the aircraft, relative to the surface of the earth, is motionless.

    And people expect it to take off? Like... seriously?

    I thought planes flew because the air passing over the wings creates a negative and a positive pocket of pressure.

    How can a plane take off if no air is passing over the wings?
    posted by Baby_Balrog at 12:47 PM on November 2, 2007


    So the plane is on a conveyor belt, wheels spinning, belt spinning.
    The body of the aircraft, relative to the surface of the earth, is motionless.


    Lord help me for getting into this debate, but here is what you are misunderstanding:

    The aircraft IS NOT MOTIONLESS RELATIVE TO THE EARTH.

    The conveyor belt does not, cannot, prevent the aircraft from moving forward relative to the earth (and, thus the air), because the aircraft is pushed (if jet-powered) or pulled (if propellor-driven) directly through the air by its engine.

    The wheels are not part of the drive train at all.

    Thus, while the wheels may be rotating madly, the aircraft nevertheless continues to be PULLED (OR PUSHED) FORWARD THROUGH THE AIR BY ITS ENGINE, thus creating lift.
    posted by dersins at 12:53 PM on November 2, 2007


    In other words-- there IS air passing over the wings because the plane IS moving forward, despite the best efforts of the conveyor belt.
    posted by dersins at 12:54 PM on November 2, 2007


    I think this Straight Dope column (linked in the original post to the blue) makes it pretty clear.
    posted by exogenous at 12:56 PM on November 2, 2007 [1 favorite]


    BB, the body of the aircraft is not motionless, relative to the ground. That is the crux of the problem. If the craft were an automobile, and used the friction of the wheels against the ground to propel itself, the conveyor could conceivably prevent forward motion. However, the aircraft’s wheels do nothing but roll. There is no motor attached to them. The propellor moves the aircraft by pulling against the air, and there’s nothing the conveyor can do about it.

    Picture the aircraft as a glider with a rope attached to it. Beyond the far end of the conveyor, a truck is attached to the other end of the rope. If the truck moves forward, can the conveyor’s motion in the opposite direction prevent the glider from moving forward at the same rate as the truck? Won’t the glider be then moving through the air and gain lift?
    posted by breaks the guidelines? at 12:57 PM on November 2, 2007


    It's very simple. Wait, no it's not!

    It seems though, that the...

    The thing is, if the conveyor...

    Very small rocks?!?!?

    I declare the singularity!
    posted by Mister_A at 1:05 PM on November 2, 2007


    Wait, it's a prop plane?
    posted by Mister_A at 1:06 PM on November 2, 2007


    ....twitch....

    I think you people broke me.
    posted by aramaic at 1:10 PM on November 2, 2007


    *listens to laurie anderson's "from the air"*
    posted by koeselitz at 1:25 PM on November 2, 2007


    Q: How do you know if a pilot is at your party thread?

    A: He'll tell you.
    posted by LordSludge at 1:33 PM on November 2, 2007 [1 favorite]


    Cool, got it. THANK YOU METATALK YOU HAVE PROVEN YOURSELF USEFUL! for something. other than meetups.
    posted by Baby_Balrog at 2:36 PM on November 2, 2007


    BB, please, please go explain it to paulsc...
    posted by Pantengliopoli at 2:48 PM on November 2, 2007


    I'm late to the game, but I think there is an easier way to visualize this than the rollerskate analogy.

    Let's take a Hot Wheels car and place it on conveyor belt. Turn the conveyor belt on, and the car moves along with the belt. Now if we hold the car steady against the belt, the wheels spin, and the car now has momentum relative to the conveyor. I can keep turning up the speed on that belt, and continue to hold the car in place (relative to everything but the conveyor) without much effort on my part - I'm only battling the internal friction between the axle and the car. The wheels will begin to spin faster and faster, but as long as they are relatively frictionless, I won't have to exert much more force to hold on and keep the car in place.

    Now the conveyor is moving at a pretty good clip and those little wheels are just whizzing along. I decide to add more force to my arm and push the car forward a little. The wheels will spin a little faster, and the car will move forward. I can increase the speed of the conveyor while pushing the car forward, and the car still moves forward, the only difference is that the wheels will spin even faster. The only thing that can prevent me from pushing the car forward is if the friction between the car's little body and the axle becomes too great for me to compensate. We're going to have to get that conveyor running at a pretty terrifying rate before that becomes a factor, so I think it's negligible. I can pretty much push the car along as fast as I like, as long as the wheels don't disintegrate (and as long as I don't get my fingers caught in the conveyor!).

    Replace the toy car with a plane and the force from my arm with the force from propellers/turbines. The plane will roll forward and take off with its wheels spinning faster than usual to compensate for the velocity of the conveyor belt (given that the length of the conveyor is equivalent to what the aircraft would normally require).
    posted by malocchio at 4:10 PM on November 2, 2007 [4 favorites]


    Could someone with animation skills create a graphic that demonstrates how the plane takes off? There are a number of good explanations in this and the previous thread that could be adapted. Seeing a visual explanation might convince some of the stragglers. Good luck with paulsc, though - he'll freak out because the plane's tail lights are blinking at the wrong frequency and ignore everything else...
    posted by nervestaple at 5:06 PM on November 2, 2007 [1 favorite]


    Seriously? We need a picture of a plane taking off just like normal, but with its wheels spinning twice as fast on a treadmill?
    posted by The World Famous at 5:07 PM on November 2, 2007


    It may seem like an easy-enough concept for those of us that already understand, but a different look at the solution may help those that don't.
    posted by nervestaple at 5:14 PM on November 2, 2007


    Or, they could just wait for Mythbusters, I guess.
    posted by nervestaple at 5:16 PM on November 2, 2007


    If the craft were an automobile, and used the friction of the wheels against the ground to propel itself, the conveyor could conceivably prevent forward motion.

    Not "could": Does.

    Or at least the guy at the desk there sure hopes like hell it keeps the car from moving forward!
    posted by five fresh fish at 6:56 PM on November 2, 2007


    q that is a great pic.
    posted by exlotuseater at 8:46 PM on November 2, 2007


    oops! wrong thread! heh heh. heh.
    posted by exlotuseater at 8:47 PM on November 2, 2007


    Could everyone just please favourite malocchio's comment so hard that it pushes the rest of the comments out of this thread? Thanks.
    posted by chrismear at 4:39 AM on November 3, 2007


    I've read a few other comment threads on this subject, and I spotted a common pattern that is explained by a fundamental flaw in the question:

    It is impossible for the treadmill to make the wheels stay in place relative to the ground. Yet this is the interpretation that almost all the "it won't fly" proponents focus upon.

    They are, of course, correct: if by some arcane magic in a universe other than ours the treadmill were capable of preventing the wheels from showing forward motion relative to the ground, the airplane, regardless how powerful its engines, will not take off.

    Heck, one could put a Saturn V rocket on a trolley, place it on this impossible treadmill, light the fuel and see it go nowhere!

    That's a pretty neat universe, sure.

    I think the question was trying to pertain to this universe. And in this universe it is impossible for the treadmill to prevent the wheels from doing their job and decoupling the aircraft from the ground.

    If it makes you feel better, though, I'll allow a leprechaun to be pilot in this scenario. Small magic irrelevant to the core question is okay by me!
    posted by five fresh fish at 10:18 AM on November 3, 2007 [1 favorite]


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