error in the "will the plane take off"

Cecil's Columns/Staff Reports
1

a friend put this the will the plane take off question up on our forums and basically it turned into a debate about how was right and in the end this info of nugget popped up from me:

Quote “Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground–and more importantly the air–regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off.”

true the air moving backwards over and under the wings generates lift. but where is this air coming from? while the plane is moving it is also STATIONARY ON THE SAME FIXED POINT IN SPACE (note: arguments about how its actually and technically not, are irrelevant given that although the earth rotates at aprox 650mph we dont have 650mph winds gusting around the planet and a nutral wind speed) RELAVTIVE TO THE AIR AROUND IT. LIFT IS NOT GENERATED BY AIR MOVING INTO THE ENGINES - IT IS GENERATED BY THE DIFFERENT AIR PRESSURES ABOVE AND BELOW THE WING. (ILL DRAW YOU THE DIAGRAMS OF HOW IT WORKS ON PAINT OR COPY THEM FROM SOMEWHERE ELSE ON THE NET IF YOU WANT POPE) Just because the conveyer belt is moving it doesnt mean all the air around it is going backwards as well.
the person was right in what he complared to the cars movement but a car is different because it can only go along surfaces whereas a plane can fly. it should be something along the lines of: lets imagine a car has wings and accelerates, then when it reaches to correct speed it will take off because of air pressures above and below the wing and their differences. but in this situation the car is being held in the same point by a conveyer belt (speed of tyres and conveyer belt actually is irrelevant - my mistake from earlier I ADMIT IT) (and it is) because othewsie it would speed off the belt forwards. so while this is stuck in the same placeit would be the equivelent of having this winged car right up against some steel blast doors and then starting to accelerate, then even put this car on a conveyer belt if you want with the same circumstances as said THE CAR WILL STILL REMAIN IN THE SAME POSITION, THE ONLY CHANGE IS THAT THE FORCE OF THE CAR ON THE BLAST DOORS IS GREATLY REDUCED AND EVEN BECOMES NON-EXISTENT EVEN THOUGH THE FRONT OF IT REMAINS IN CONTACT WITH THEM.

so yeah, while the plane is being held in the same place there is no air moving over the wings.

oh and another point: IT DOES MATTER ON THE TYPE OF AEROPLANE and i shall explain: a propeller powered aeroplane (SUCH AS THE ONE SHOWN IN THE PICTURE AND WE WERNT TOLD ABOUT) actually CAN TAKE OFF. because of the propellers shape it forces air backwards over the wings (like a fan blows air forwards when the air is still (i.e no wind) in a room) and this air generates lift. THE POINT I WAS MAKING WAS FOR THAT OF A TYPICAL MORDERN DAY COMMERCIAL AIR LINER THAT USES JET ENGINES WHICH DONT BLOW AIR OVER THE WINGS - THEY JUST SUCK AIR INTO THE ENGINES AND OUT OF THEM TO GENERATE SPEED THROUGH THE WHEELS WHICH ACT (in normal circumstances) ON TARMAC AND PUSH THE PLANE FORWARD UNTILL IT GENERATES ENOUGH SPEED THAT THE WINGS PUSH THE AIR OVER WINGS INSTEAD OF THE AIR BEING PUSH OVER THEM. kinda like 1 of newtons laws “every action has an equal and opposite reaction” using this law proves how the wings can push air over itself as the opposite reaction of air being pushed over the wings.
sorry its so long but it proves the quote that “it is irrelevant of what sort of plane it is, all planes work in the same way” is incorrect, and that it does matter what sort of plane it is. after a short while my friend linked me to the question which had a picture of a plane powered by a propellor which can take off - though having not been told it was powered by one i jumped at taking your average air liner which uses jet engines and has a different take of style as i have explained. overall i hope that this will lead to a change it the question to state that it is a propeller powered craft.

thanks again
Dr tk maxX

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oh there are some bits in that which may not make snese to you as i just copied it from our forums. but the whole thing is still relevant.

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Welcome the the Straight Dope, Dr tk maxX. In case you didn’t know, Cecil has two columns on this subject: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off? (03-Feb-2006) and “A plane is standing on a runway. . .” No, it’s not. Here’s why. (03-Mar-2006)

With this particular problem, you have to be aware of a couple things. First is the physics: In order to take off, the plane requires an airflow over the wing, which means the problem reduces to: Does the plane move with respect to the air around it? Next, in order to prevent the plane from taking off, the treadmill had to stop the plane from moving with respect to the air around it. Preventing motion takes force. That means the problem reduces to: Can and does the treadmill transmit enough force to the place to keep it from moving?

Second is the semantics: there are different wordings of the problem and different assumptions that can be made about how it’s set up. You, yourself, demonsatrate that in your post: Even though Cecil says, “the plane moves forward at pretty much the usual speed relative to the ground,” you say “the plane is… STATIONARY ON THE SAME FIXED POINT IN SPACE.” Not the same assumptions at all…which doesn’t mean wither is wrong, but does mean you’re using different assumptions, or a different problem wording.

Since this has been discussed before, let me repost, as a pre-emptive strike, something that I’ve posted a few times before:

There are plenty of answers to this question, because the key to the question is the wording and your interpretation and what you assume from the beginning. And these answers can all be correct, but the assumptions are the key. Let’s start off at the top:

A. Suppose we actually built a treadmill and put a 747 on it, and had the treadmill match the speed of the plane. Would the 747 take off? If the treadmill matches the plane fuselage speed, then yes. The treadmill simply accelerates in the opposite direction that the plane does; the plane moves in one direction relative to the ground and the treadmill moves in the other. The wheels wind up rotating twice as fast as they normally would, but the plane will take off, leaving a treadmill behind that’s rotating in the opposite direction.

B. Let’s reword the question. Suppose we actually built a treadmill and put a 747 on it, and had the treadmill match the speed of the wheels. Would the 747 take off? Depends. If “exactly matching the speed of the wheels” means that the treadmill matches the hub speed of the wheels (the speed of the wheel center, which is the same as the fuselage speed), then yes. Just like in the last scenario, the treadmill accelerates in the opposite direction that the plane does, the wheels rotate twice as fast as they normally would, and the plane takes off.

C. But that problem is trivial. Let’s assume that “exactly matching the speed of the wheels” means “matching the outer diameter surface velocity”–the velocity with respect to the hub, or the “speedometer” speed. Would the 747 take off? Almost certainly it would, but only because we can’t build a treadmill capable of keeping up with the thrust transmitted to the plane by the engines, and not enough force is transmitted to the plane to keep the speeds matched–in other words, we violate the spirit of the question, because the treadmill isn’t matching the wheel velocity.

D. OK, that’s stupid. It’s a thought experiment. Posit a magic treadmill that can accelerate as fast as desired. And it doesn’t break. I imagine the wheels will skid on the treadmill, because the friction won’t be able to transmit the necessary force. In that case, we again violate the spirit of the question, and–

E. It’s a thought experiment, smart guy. Assume there’s enough friction to rotate the tires. All right. When the engine lights off, the treadmill will accelerate until the force transmitted through the wheel hub to the plane exactly balances the thrust. The plane would stay stationary as the thrust power was dissipated in the wheel bearings (as friction), tires (hysteresis), and in accelerating the wheel to ever-increasing speeds. Since all the power is dissipated in the wheels, eventually either the bearings would overheat, the tires would blow, or the wheel would rip itself apart due to inertial forces. After that, the plane crashes and burns. Then you’ve destroyed a rather expensive magic treadmill.

F. Thought experiment, I said! Let’s posit ultra-strong and heat resistant tires. All right. It turns out the real world is rather complicated. If the treadmill is a long, runway-sized treadmill, it will eventually, running thousands of miles an hour, pull in air at high enough velocity that the plane will lift off at zero ground speed (but substantial air speed). However, now you’re running into trans-sonic compressibility effects…

G. No speed of sound effects! And assume magic air that doesn’t become entrained with the treadmill motion. And don’t throw in any other crazy stuff, either. In that case, the treadmill speeds up (still balancing the plane’s thrust force) and the plane stays in place until the engines run out of fuel. I imagine the treadmill goes pretty fast at that point. The plane stays put until the fuel’s gone, at which point the magic treadmill whips it backwards.

H. Backwards, shmackwards. Now we’re getting somewhere. What if we had infinite fuel? Then the wheels keep going until they’re running near light speed, and relativistic effects take over. The wheels get smaller, I suppose…

I. None of that! No relativity-- Hey, wait a minute. Back up. Suppose we have zero friction bearings and tires. That doesn’t seem so unreasonable for a thought experiment. Well, zero friction tires would mean they just skid on the runway, since nothing turns them. So the plane will take off, tires motionless, and the treadmill won’t move.

J. Hey! Quit it! I already said the tires don’t skid! Sorry. Just friction on the tire/treadmill interface, then, but none in the bearing or sidewall. With zero friction in the bearing, you lose the friction coupling between the treadmill and the jet. But you still have inertial coupling. The wheels accelerate, and that acceleration takes force. Now you have the same case as you do with friction. The jet stays stationary as the wheel accelerates; the wheel just accelerates faster.

K. Well, how about the other way around? Massless wheels, but you still have friction? Here it starts to get complex. As you accelerate the wheels, the bearings will change shape and heat up and so forth, so it’s reasonable to guess that the “friction coefficient” goes up with increasing speed. If that’s the case, then when the engines start, the treadmill accelerates up to whatever speed will give enough friction to balance the thrust. The plane stays stationary, wheels rotating at some reasonably constant (but large) velocity, dissapating the engine power through friction.

L. But I want massless wheels and a constant coefficient of friction. Indestructable wheels, remember? None of this hand-waving “it’s gonna get bigger” crap. OK. It is a thought experiment. With a limited “friction coefficient,” only a limited amount of energy can be absorbed by the friction. When the engine lights off, the treadmill instantly accelerates to infinite speed. It’s never able to counteract the thrust force, and thus plane takes off, leaving the infinite-speed treadmill behind.

M. Ah. OK, one last step. What if we had no bearing friction and massless tires? What happens then? Pretty much the same thing. There’s now no energy losses in the wheels and tires, no coupling between the treadmill and the plane–no bearing friction, no inertial effects, no air resistance, and no way for the treadmill to affect the plane’s motion. The same thing would happen as above, with the plane taking off, leaving the infinite-speed treadmill behind. However, there’s one added interesting thing: This is now an unstable runaway system. There’s no resistance to treadmill motion, and a positive feedback circuit. Imagine the poor mechanic who bumps a wheel, setting it in motion. A very slight roll by the tire is sensed, and the treadmill luches forward. The tire goes faster, the treadmill goes faster, the tire goes faster… Since we’ve posited an instantly-accelerating treadmill and no relativity and no air resistance and no wheel inertia, the treadmill goes from zero to infinity in no time flat. Try to keep your balance on that.

Pick your scenario–they’re all correct.

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Just a note - lift is generated by more than just the Bernoulli Principle - from what I understand, planes have to deflect enough air downwards to provide an upward lift force, and lift can be generated from Bernoulli, the Coanda effect, and in some other ways.

http://www.straightdope.com/mailbag/mairplanesfly.html

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Thorough answer, zut.

I think the OP was looking for something like the Boeing YC-14. The engines were mounted ahead of and above the wing, with the engine nozzles blowing across the upper surface of the wing. Even standing still, all that air over the wing will generate some lift. It’s not enough to take off, but the YC-14 could use very short runways. Presumably, blow enough air over the wings and you could take off straight up (and blow enough air over the tail so you have control), treadmill or no.

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Note this this is incorrect - an airplane’s wheels do not “act on tarmac and push the plane forward.” All the pushing is done by moving air with the engines; the wheels simply roll along the ground. This is the key to understanding the simplest form of the problem, which Zut lists as “A”, above.

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And we all know how painful that can be.

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All Hail the magical treadmill!! The mighty force that keeps planes stationary in the face of all physics. Hail!

9

Dr tk maxX why why why are you bringing this up again?

I’m sure your ideas about this subject are new and exciting to you, but trust me, it has been discussed. All arguments on every possible aspect of this problem have been discussed. Let it go.

10

If that is true, how do you explain the Predator UAV which is an aircraft whose only propeller is actually behind the wing? The propeller certainly does not force any air over the wings.

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A pusher propellor would certainly draw some air over the wing, by causing a partial vacuum. However, as to the original question, it’s neither here nor there.

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Right. I take it that the OP simply didn’t understand that the answer stipulates that the plane will move in respect to the ground, regardless of the treadmill’s actions.

I think zuma’s standard boilerplate response covers all possibilities, though. (And is better written than the OP, anyway, IMHO)

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Then zut will have to add an N answer.

N. But let’s say there is no ground to move in respect to, that the magic treadmill is suspended in a magical place with no ground…

14

Funny. Apparently the OP came, saw, answered and left for good.

Still, I like the “nth” concept you started. But… wouldn’t a plane on a treadmill that has no ground beneath, by definition, already be flying?

15

And if I just pull on my boots hard enough, I will levitate, and fly around the room.

SLK

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Invalid analogy. When you pull on your boots, your boots are pulling back on your arms. Zero net force. In the case of an airplane, this is not true.

Actually, I don’t think a normal prop airplane will take off while not moving relative to the atmosphere, but that is because a normal takeoff depends on the ability of the aircraft to accelerate to rotation speed; the props alone won’t send enough air over the wings. But an overpowered plane could, assuming that the weight of the additional engines and additional frame and struts doesn’t spoil it.

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Or falling. Perhaps even Intelligent Falling.

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You know, I read that whole article without realizing it was from the Onion and thought, “Yeah, I can see how some crazy fundies would think this.” Then I saw the article captions at the bottom and then scrolled back up. Sure enough, The Onion.
…but for a minute…

19

Then why don’t they put big fans on sailing ships, and go anywhere they want, independent of wind?

Intuitively, I think we have a situation like the ‘trying to to loop the loop on the swings’ other factors come into effect before it happens. Imagine a plane chained by the tail, straining to take off. Do you imagine it rising from the ground, just from prop backwash? Cuz I don’t.

My analogy, like most, fell apart under scrutiny, but it’s just as absurd.

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Such a plane isn’t actually impossible, just massively impractical. Certainly no normal plane is capable of this (though RobotArm has linked to one that uses this principle to make short takeoffs possible).