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Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 2:11 am UTC
by Algrokoz
So I am well aware that an airplane will indeed take off on a conveyor belt, but I think most people's refusal to accept this has to do with their misunderstanding of what the specifics of the experiment are actually implying. Almost all the "refutations" I see have to do with the plane not moving. So my question then is this: If we were to slightly change the setup of this problem, and force the plane to not move by placing it inside a giant clamp while we revved up the engine, could the air the engine forces over the wings provide enough lift to IMMEDIATELY cause the plane to fly when released from the clamp. In this variation, I really don't think they will take off. Think about an aircraft carrier for a moment. Why would the Navy use the pneumatic catapults if the plane could simply power up its engines to flight levels and then take off from standstill at the end of the carrier? Seems like that would be a huge space saver for the Navy if it were possible.

Any thoughts on this one?

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 2:24 am UTC
by firechicago
Obviously the plane would not take off immediately, since it's only just starting to accelerate. The critical quantity here is the speed of the plane in relation to the air. People who think the plane will not take off think so because they believe that the treadmill will keep the plane from moving forward. If the plane isn't moving forward fast enough it won't take off. What they're missing is not the ability of the plane to take off without moving forward, they're missing the fact that a plane is perfectly capable of moving forward without any traction on the ground.

An interesting wrinkle: Would it be possible for a propeller powered plane (which generally pushes a stream of air from the propeller back over its own wings) to theoretically be able to take off without moving forward while a jet plane (where the exhaust of the jet is ejected behind the wing) would not?

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 2:25 am UTC
by Dopefish
I was under the impression that the reason an airplane on a converyor belt will be able to take off is because the wheels are essentially independant of the motion of the body of the plane (I don't remember the proper term, 'free spinning wheels' perhaps?), and thus when the plane revs it's engine to take off, it will indeed move forward regardless of the speed of the treadmill, and then that motion allows it to take off.

As such, in the situation where the body of the plane is clamped down, then it wouldn't be able to take off immediately after, since it would still need the same amount of distance to get up to take off speed.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 2:58 am UTC
by jjfortherear
Think he means having propellers in front of the wings which actually push enough air over the wings to cause lift even when plane is not moving relative to air, which, I suppose, is possible, but you'd be a lot better off with vertical thrust if that's what you're trying to do.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 3:01 am UTC
by ++$_
firechicago wrote:An interesting wrinkle: Would it be possible for a propeller powered plane (which generally pushes a stream of air from the propeller back over its own wings) to theoretically be able to take off without moving forward while a jet plane (where the exhaust of the jet is ejected behind the wing) would not?
Could you theoretically? Yeah, sure.

Could you practically? Well, maybe, but it would be a lot easier to just orient the propeller vertically and call it a "helicopter", rather than having a horizontal propeller and wings to deflect the air downwards.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 3:18 am UTC
by Korrente
Moving air over an airfoil using a fan is no different than moving it by using forward velocity. I've wondered this myself and I think the answer is fairly obvious but I've never seen it in practice. Now obviously aircraft can fly in wind-tunnels that move air, so why would it be any different if you attached the fan to the aircraft? A real airplane propeller causes some other issues like slipstreams and vorticies that would make the airflow very turbulent but I still think the overall concept is possible.

firechicago wrote: (which generally pushes a stream of air from the propeller back over its own wings)


If you're implying that a propeller pushes air over the airfoil to cause lift, you're about 95% incorrect, most prop-moved air doesn't cause any lift. If you're just saying the propeller pulls the plane, then you're correct but a plane's propeller acts in the same manner of a boat's propeller or even a wood screw: it pulls the aircraft along through the medium (air in this case). And in practicality a jet probably wouldn't be able to do this because of the way it intakes air, but there should be little difference between blowing the air over the wing from the front and sucking over it from behind.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 3:53 am UTC
by jmorgan3
The channel wing: a propeller plane designed to take off vertically.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 4:33 am UTC
by Korrente
jmorgan3 wrote:The channel wing: a propeller plane designed to take off vertically.


Interesting, I've never seen that before. I wonder if that would be any more feasible with a turbofan since you'd get the good thrust but likely less of the oscillation the article talked about.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 5:34 am UTC
by Algrokoz
jmorgan3 wrote:The channel wing: a propeller plane designed to take off vertically.

i think this answered my question. a common plane would not take off but that does not mean that you couldn't come up with a experimental design that could. thanks

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 7:09 am UTC
by EdgarJPublius
so... basically Zero Length Launch but without the rocket assistance?

I know some fighters (such as the F-16, F-15 and Eurofighter Typhoon) have thrust-to-weight ratios greater than one and should be capable un-assisted ZLL under the right circumstances, though there's still the problem that sufficient volumes of air need to be moving over the control surfaces for the aircraft to be controllable.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 8:37 am UTC
by MolBio
Most of the wing won't be in the propwash.
Unless you have really stubby wings, and a pig propellor.
In which case, you can probably take off like a helicopter:
Image
Image


What you propose sounds similar to the concept of the XFV-15
http://en.wikipedia.org/wiki/Rockwell_XFV-12

Basically, the bottom of the wing opened like venetian blinds, and the jet exhaust was blown against/ducted through these.

It didn't quite work out.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 8:47 am UTC
by Sockmonkey
Korrente wrote:And in practicality a jet probably wouldn't be able to do this because of the way it intakes air, but there should be little difference between blowing the air over the wing from the front and sucking over it from behind.
No, there is a huge difference since air sucked over a stationary wing is limited by the force of external air pressure pushing it in while blown air is pushed by the engine.
It's helpful to remember that suction doesn't pull. Everything is pushed, and suction happens when you make one side push less than the other.
The majority of planes don't blow air directly over the wing anyhow, and of those that do, only those designed for STOL do so enough to significantly affect the amount of lift produced.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 10:40 am UTC
by Carnildo
Algrokoz wrote:So I am well aware that an airplane will indeed take off on a conveyor belt, but I think most people's refusal to accept this has to do with their misunderstanding of what the specifics of the experiment are actually implying.

There are two airplane-on-a-conveyor-belt problems, differing in how they define the relationships between the speed of the airplane, the speed of the conveyor belt, and the ground. The arguements arise because the normal problem statement doesn't distinguish between the two.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 1:56 pm UTC
by Korrente
Sockmonkey wrote:
Korrente wrote:And in practicality a jet probably wouldn't be able to do this because of the way it intakes air, but there should be little difference between blowing the air over the wing from the front and sucking over it from behind.
No, there is a huge difference since air sucked over a stationary wing is limited by the force of external air pressure pushing it in while blown air is pushed by the engine.
It's helpful to remember that suction doesn't pull. Everything is pushed, and suction happens when you make one side push less than the other.
The majority of planes don't blow air directly over the wing anyhow, and of those that do, only those designed for STOL do so enough to significantly affect the amount of lift produced.


That's what I meant by the way it intakes air, it can't create enough pressure difference to move enough air in front of it. But if one were to duct the air over the wing surface (like a very extended intake cowling butting up to the wing kind of how the channel-wing moved through the wing root) wouldn't the air at the intake have to move at almost the same speed as the air going into the compressor?

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 3:23 pm UTC
by jaap
Korrente wrote:That's what I meant by the way it intakes air, it can't create enough pressure difference to move enough air in front of it. But if one were to duct the air over the wing surface (like a very extended intake cowling butting up to the wing kind of how the channel-wing moved through the wing root) wouldn't the air at the intake have to move at almost the same speed as the air going into the compressor?


Inflowing air comes from all free directions, whereas outflowing air is directional.
A simple way to experimentally verify this is using a bendy straw. If you blow through a bent straw (preferably from the long end), the straw will shoot off sideways very easily. If you suck through the same straw, it will hardly move.

I don't think ducting will help much. Most of the air still comes from all directions, apart from the duct itself.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 3:58 pm UTC
by Zamfir
jaap wrote:Inflowing air comes from all free directions, whereas outflowing air is directional.
A simple way to experimentally verify this is using a bendy straw. If you blow through a bent straw (preferably from the long end), the straw will shoot off sideways very easily. If you suck through the same straw, it will hardly move.

That's not entirely true. You can easily see the problem if you define a cut through the straw, slightly befor the free end. The force on the straw is equal to the momentum transfer through this cut. If you blow and suck the same amount of air, the momentum transfer will be opposite but equal. Inflowing air is directional too.

The effect of a propeller (or jet) on the leading or trailing side of the wing is a very tricky thing to predict. The flow pattern around a wing is of course far from symmetric (except for the left vs right wing), so putting a fan on either side will have different effects.

But as a first approximation, both a front propeller and an aft propeller increase the lift, compared to the same plane gliding at the same speed with its engines turned off. I think this increase in lift is usually accompanied by a disproportional amount of drag in cruise, so most aircraft designs don't try to increase this effect.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Tue Mar 22, 2011 4:22 pm UTC
by EdgarJPublius
You can re-direct compressed air from a jet over the the wing to generate greatly increased lift at low speed without massive control-ability issues. It's called a Blown Flap and is used on many different aircraft.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Thu Mar 24, 2011 6:16 am UTC
by Algrokoz
Carnildo wrote:
Algrokoz wrote:So I am well aware that an airplane will indeed take off on a conveyor belt, but I think most people's refusal to accept this has to do with their misunderstanding of what the specifics of the experiment are actually implying.

There are two airplane-on-a-conveyor-belt problems, differing in how they define the relationships between the speed of the airplane, the speed of the conveyor belt, and the ground. The arguements arise because the normal problem statement doesn't distinguish between the two.

What do you mean? The reason people don't believe that the plane will take off is that they assume that the conveyor belt will prevent the plane from moving forwards when all it really means is that the wheels turn twice as fast.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Fri Mar 25, 2011 2:37 am UTC
by Carnildo
Algrokoz wrote:
Carnildo wrote:
Algrokoz wrote:So I am well aware that an airplane will indeed take off on a conveyor belt, but I think most people's refusal to accept this has to do with their misunderstanding of what the specifics of the experiment are actually implying.

There are two airplane-on-a-conveyor-belt problems, differing in how they define the relationships between the speed of the airplane, the speed of the conveyor belt, and the ground. The arguements arise because the normal problem statement doesn't distinguish between the two.

What do you mean? The reason people don't believe that the plane will take off is that they assume that the conveyor belt will prevent the plane from moving forwards when all it really means is that the wheels turn twice as fast.

Thank you for proving my point. You've picked one interpretation, and are assuming that the other one is inherently wrong.

A plane is standing on a conveyer-belt runway. The plane moves in one direction, while the belt moves in the opposite direction. The belt runs at a speed that exactly opposes the plane's forward motion


Interpretation #1: The conveyor belt's velocity relative to the ground is opposite that of the airplane's velocity relative to the ground. In this case, it is intuitively obvious that the airplane will take off.

Interpretation #2: The conveyor belt can run at any velocity needed to keep the plane's forward motion 0. Assuming the belt has some means to apply force to the airplane (tires with rotational inertia, bearings with friction, or as in a real-world airplane, both), it is inherent in the problem statement that the airplane cannot take off.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Fri Mar 25, 2011 3:47 am UTC
by phlip
It's not two interpretations, it's two distinct puzzles, presented with different wording. And I'll happily maintain that one is "wrong".

It's like... consider the Monty Hall problem. This is a puzzle with a simple, easy, obvious, wrong answer... that is, everyone at first guesses the answer is 1/2, but the real answer is 1/3. However, if you don't truly understand why the answer is 1/3, then when you repeat the puzzle, it's easy to mess up and misquote the puzzle... and end up with a different puzzle where the answer isn't 1/3, but rather is 1/2, or any other number, or even "there's not enough information".
For instance, the real puzzle says that you choose a door, and then the host always opens a door with a goat behind it, and then offers for you to switch. However, it's easy to misquote the puzzle as saying that in this particular instance Monty has shown a goat and offered for you to switch... with no information on Monty's general behavior. And without knowing the host's general behavior, you can't answer the question (maybe the host always offers for you to switch if your initial guess is right, but never if it's wrong... in which case you should never switch if it's offered. Or maybe it's vice-versa, in which case you should always switch if it's offered).
Now, luckily, the Monty Hall problem is popular enough that it's possible to call the former puzzle the "real" one, and the latter one "wrong"... but that's not the case with the aeroplane/treadmill puzzle.

With the aeroplane/treadmill has two versions - in one, which I call the "right" one, the treadmill matches the speed that the plane would be moving if the treadmill was absent. Note that this would hold the plane stationary if it moved by applying force to the ground, like a car, or a person running. A car running fast enough to move at 60kph, and put on a treadmill running backwards at 60kph, would be stationary. So the simple, easy, obvious, wrong answer is that the same happens to a plane... which isn't the case. The plane moves forward, and takes off. This is an interesting puzzle, and follows the usual logic-puzzle pattern of obvious wrong answer, clever revelation, right answer.
The other variation, which I'll call the "wrong" version, the treadmill's speed matches the speed read by a speedometer attached to the plane's wheels, or the plane's groundspeed indicator, or suchlike. This is a boring puzzle, as it mathematically devolves immediately to "due to an undisclosed mechanism, the plane can't move; can the plane move?" to which the answer is "no, duh", and all useful thinking stops. The puzzle doesn't say why the plane can't move, it just says in a roundabout way a statement logically equivalent to "the plane's speed is 0".
It's also worth noting that the "right" version of the puzzle is in the form of an experiment - "if you do x to set up a situation, what will happen?". But the "wrong" version is in the form of a deductive logic puzzle - "a situation happens, during which you observe x... what is happening?". Because you can't go in and set it up such that the treadmill's speed matches the wheels' speedo, because that doesn't make physical sense... but you can observe a situation where the two match, and deduce that something is stopping the plane from moving. Though you can't deduce what is stopping the plane from moving. And if you're assuming ideal-physics-universe, as you normally do for this puzzle (ie infinite friction between wheel and treadmill, zero friction between wheel and axle, massless wheel) then it's certainly not the treadmill. Maybe it's a giant clamp, like the OP suggested.
Unfortunately, unlike the Monty Hall example, the "wrong" version of this puzzle has been repeated heaps, possibly even more than the "right" version, probably because the "right" version needs to take that extra care in defining the treadmill's speed. And then after that, people just say "the aeroplane/treadmill puzzle" without clarification, and different people assume different versions. Vis: people who see the Mythbusters episode where they tested the "right" version, and copped flack from people who assumed the "wrong" version, and said that the Mythbusters were doing it wrong because they allowed the plane to move. That's the whole point of the ("right" version of the) puzzle! That the plane does, counterintuitively, move!

Note that this is also why many people who've heard the "right" version, when confronted by people who've heard the "wrong" version, often point out that the plane moves by pushing on the air, not the ground... because that's the "clever revelation" that causes the penny-drop that leads to the answer to the "right" version. Because they see the other person not as "person who's talking about a different, inferior, puzzle" but "person who is stuck in the 'obvious wrong answer' stage of solving my puzzle". Unfortunately this point is irrelevant to solving the "wrong" version... it doesn't matter how a plane normally moves, the puzzle says the plane isn't moving. So all this serves to do is convince the person using the "wrong" version that the person using the "right" version is missing the point.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Fri Mar 25, 2011 6:14 am UTC
by jaap
I find most versions of the problem similar to "What happens when an immovable object meets an unstoppable force?". An inherently impossible situation is described, and to resolve it one thing or the other has to give.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sat Mar 26, 2011 12:01 am UTC
by Algrokoz
Carnildo wrote:Interpretation #2: The conveyor belt can run at any velocity needed to keep the plane's forward motion 0. Assuming the belt has some means to apply force to the airplane (tires with rotational inertia, bearings with friction, or as in a real-world airplane, both), it is inherent in the problem statement that the airplane cannot take off.

I am pretty sure that it would be technologically unfeasible to even create such a conveyor belt. Besides, worst case scenario in this setup is that the wheels can't take the heat, fall off, and then you scrape the plane up to a speed where it could take off. (The fast moving ground in this case would probably help as it would lower the fricative force on the underside of the plane.)

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sat Mar 26, 2011 2:07 am UTC
by Carnildo
Algrokoz wrote:
Carnildo wrote:Interpretation #2: The conveyor belt can run at any velocity needed to keep the plane's forward motion 0. Assuming the belt has some means to apply force to the airplane (tires with rotational inertia, bearings with friction, or as in a real-world airplane, both), it is inherent in the problem statement that the airplane cannot take off.

I am pretty sure that it would be technologically unfeasible to even create such a conveyor belt. Besides, worst case scenario in this setup is that the wheels can't take the heat, fall off, and then you scrape the plane up to a speed where it could take off. (The fast moving ground in this case would probably help as it would lower the fricative force on the underside of the plane.)

You've just demonstrated the other reason why people tend to argue over this problem: some of them are reasoning about spherical airplanes, while others are reasoning about real-world airplanes, without specifying which they're talking about.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sat May 14, 2011 2:46 am UTC
by wtspman
Steady level flight requires the balance of four forces: lift vs gravity, and thrust vs drag. In the scenario outlined in the OP the plane is generating enough lift to counter gravity, and except for the clamps holding the plane the engine is generating thrust; since the plane isn't moving forward there is no drag. So I predict that when you release the clamps the plane will accelerate forward and upward until the drag has increased to the point to match the thrust, at which point it will level out and fly at constant velocity.

Here is a way of visualizing the situation: hold a sheet of paper by two adjacent corners; bring it to your mouth and blow across it. The airflow over the paper will generate lift. Let go of the paper and it will fly away (and fall). In this case you are creating drag by blowing on the paper, and since it has no source of thrust it accelerates backwards. Attach the source of airflow to the wing and and it becomes a source of thrust not drag.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sun May 15, 2011 3:20 pm UTC
by johnny_7713
wtspman wrote:Steady level flight requires the balance of four forces: lift vs gravity, and thrust vs drag. In the scenario outlined in the OP the plane is generating enough lift to counter gravity, and except for the clamps holding the plane the engine is generating thrust; since the plane isn't moving forward there is no drag. So I predict that when you release the clamps the plane will accelerate forward and upward until the drag has increased to the point to match the thrust, at which point it will level out and fly at constant velocity.

Here is a way of visualizing the situation: hold a sheet of paper by two adjacent corners; bring it to your mouth and blow across it. The airflow over the paper will generate lift. Let go of the paper and it will fly away (and fall). In this case you are creating drag by blowing on the paper, and since it has no source of thrust it accelerates backwards. Attach the source of airflow to the wing and and it becomes a source of thrust not drag.


I'm not quite sure how you assume the plane is generating lift? Lift will only be generated once the aircraft develops a speed relative to the air, either due to forward motion (w.r.t an inertial frame) or due an engine pushing the air over the wings. The first is impossible due to the clamps, the second is possible in theory but may require an infeasibly large engine (or multiples thereof).

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sun May 15, 2011 4:56 pm UTC
by Meteorswarm
johnny_7713 wrote:I'm not quite sure how you assume the plane is generating lift? Lift will only be generated once the aircraft develops a speed relative to the air, either due to forward motion (w.r.t an inertial frame) or due an engine pushing the air over the wings. The first is impossible due to the clamps, the second is possible in theory but may require an infeasibly large engine (or multiples thereof).


This is the problem stated in the OP:
OP wrote:could the air the engine forces over the wings provide enough lift to IMMEDIATELY cause the plane to fly when released from the clamp?


Edit for quote formatting.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sun May 15, 2011 5:19 pm UTC
by Soralin
Meteorswarm wrote:
johnny_7713 wrote:I'm not quite sure how you assume the plane is generating lift? Lift will only be generated once the aircraft develops a speed relative to the air, either due to forward motion (w.r.t an inertial frame) or due an engine pushing the air over the wings. The first is impossible due to the clamps, the second is possible in theory but may require an infeasibly large engine (or multiples thereof).


This is the problem stated in the OP:
[quote=OP]could the air the engine forces over the wings provide enough lift to IMMEDIATELY cause the plane to fly when released from the clamp?
[/quote]
Typically engines aren't forcing much air over the wings at all, they're often under the wings, or in the main body of the plane or such, not in front of the wings or behind them. A plane would only be able to take off like this if it could take off without a runway (VTOL). I mean, it doesn't take much time to move the engines to full power, and yet after the engines are at full power, the plane still needs a runway to accelerate on.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Sun May 15, 2011 8:46 pm UTC
by Hawknc
Twin props typically sit right in front of the wing, and so force air over it. But even at full throttle there wouldn't be enough air moving over the wings to generate sufficient lift to take off, simply because the propwash is only passing over a small percentage of the total wingspan. So the answer to the OP's question would have to be categorically no.

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Fri Dec 08, 2017 12:59 am UTC
by Oxilite
First let me say, I am so sorry. I know how much the topic is loathed by everyone, but I have no where else as intelligent with which to turn... Tonight I was able to try first hand a Good way Curv treadmill, and though my gymmates were not impressed, all I could think of was how (if at all) this type of treadmill might alter the airplane problem...
If you've never seen one of these, a picture does little to do it justice, it's quite a different sensation, but basically, the slight curve at each end and low friction mean your weight is what keeps you centered. So despite understanding and agreeing with the documented solution, my confusion is this...

Because the airplane doesn't instantly generate 100% needed lift, any amount it rolls forward, it's own weight is what sends it backwards... Does this affect it like I think it must whether or not it ultimately changes the outcome?

Re: Variation of the dreaded airplane/conveyor belt...

Posted: Fri Dec 08, 2017 8:58 am UTC
by p1t1o
Well if you extrapolate the treadmill up to runway-size, the curvature is next to nothing, so the solution remains about the same.

You are essentially just introducing extra drag to the aircraft by making it roll "uphill", this is countered to an extent by some extra acceleration caused by rolling "downhill" for the first half, but there will naturally be friction losses. So if the plane generates a bit more thrust, it will counter this and the problem proceeds as normal.