Originally posted by Xar Ke Zeth
I'll add my analogy to the plenty already listed.


You are on a bike without a gear chain in a corridor, and there are handrails on the walls of the corridor. The bike is the conveyor belt, the handrails are the air, your legs are the wheels, and your arms are the jet engines.

In a car, your wheels move you forward. So in this case, your legs would be pedalling, but not getting anywhere, due to the conveyor belt cancelling out your motion with your wheels.

In a plane, your jet engines move you forward. So in this case, your arms are pulling on the handrails, and are moving you forward. Your legs don't do anything - they just sit there and keep you stable.

The jet engine provides thrust through the air, so you move forward. Your wheels do nothing but spin like crazy lepers.


For initial start up you might roll back a bit, but that's to be expected. Your engines will move you through the air, regardless of what the wheels are doing on the ground.

I have to admit this question tricked me... But after reading the answer, it makes a lot of sense.


Edit:

To add another thing, while swimming it's like using your arms to drag you along the lane rope, where it doesn't matter if your legs provide any thrust at all.

[edit on 15/2/06 by Xar Ke Zeth]

Another way of doing it might even be simpler.

Similar scenario ... you're in a wagon (like a Radio Flyer) in a corridor with handrails ... only this time the floor IS a conveyor belt.

So:

floor = conveyor belt
you + wagon = plane
wagon wheels = plane wheels
arms = engines
handrails = air

If the belt is not moving, everyone can agree that you can pull yourself along down the hallway with your arms right (assuming you are strong enough ... since your arms are engines we have to assume that they are)?

Also, everyone can agree that if you hold on to the guardrails you could turn the belt on and still remain stationary right? Might take some effort at higher speeds, but that's due to too much friction in the wheels of the wagon anyways. At any rate you can do it.

The next logical step is that you could move your arms up the guardrails one by one and "drag" yourself up the corridor with the belt moving.

This is all a plane is doing ... the arms (engines) are pulling air and creating thrust (like you pull on the handrails).

Another similar scenario is wearing rollerblades on a treadmill. You can easily hold on to the sides of the treadmill while it spins your wheels. Pulling yourself closer to the front of the treadmill can be done with some effort as well.


The kicker here is that people assume (as above) that the belt is flying along already. But it was stated in the problem that the conveyor only moves as fast as the plane moves along. Initially you are at rest, as is the belt ... so there would be no additional pulling strength required on the belt than there would be if the floor was instead tile. The wheels will just spin twice as fast at every given point of acceleration.

Any clearer?

Originally posted by ShatteredSkies
Beer_Guy's post did it, I understand why the plane is moving forward, all of this, and all someone had to say was "The engines are pushing against the air", I was thinking too much into it, which is why I couldn't understand it, but it is in fact simple once you get it, the plane isn't being pulled by means of the wheels, but of the engines, so no matter what, the plane will move because it is pushing against the air, not the ground. And it is the basis on which thrust aids lift when taking off, the plane must be pushed forward through the air, not over the ground.

Shattered OUT...

no. you guys just dont get it (and i dont mean that offensively). the engines are pushing the aircraft along the ground, or in this case, the conveyor belt. the aircraft is still motionless in relation to the airmass around it. therefore, no lift is being generated and the aircraft cannot fly. the poster who made the comparison to carrying a kite on a treadmill is absolutely correct. does the kite fly if you have the treadmill on the highest setting and are running with the kite in hand? no! because you are stationary to the airmass around you, no matter how fast you are going on the treadmill.

there are four forces that act upon an aircraft:

weight, thrust, drag, and lift. thrust and drag are equal, because the treadmill is keeping the aircraft stationary in regards to the surrounding airmass. therefore, no lift is being generated. how can you fly with no lift?

Personally I believe it would take off the only thing that matters is the speed no matter what the wind is doing etc the plane is still travelling fast therefore must have wind going over it.

But I put this to all of you why argue when you can get someone esle to do it......................
..................................MYTH BUSTERS

snafu, you are missing it completely, but you are telling us we are wrong lol. Thrust and drag are not equal because there is no mechanical link between the wheels and the propeller, the wheels are completely independant and thats why they don't cancel out the thrust. The kite analogy is wrong because when you run with the kite your legs are pushing against the belt for motion, thus when the belt moves in the opposite direction at an equal rate it keeps you stationary and the kite will not fly. Up to here you are right. However the plane is NOT pushing against the belt at all. the propeller is acting on the air around it alone and so the plane will still move forwards, whatever the belt tries to do, the wheels and belt will just get faster and faster but once the engine is fully revved up the plane will take off virtually as normal.

shattered; what was it "somebody only had to say that the engine was pushing against the air" or something? Thats what people have been saying for at least a couple of pages, hence the attempts to find different ways of explaining it for you. You might have been a little more gracious than to sound like our efforts were a waste of time

[edit on 15-2-2006 by waynos]

This is interesting, the tides have turned, non-believers become believers ^.^.

Come Anakin Skywalker, join the darkside.

Shattered OUT...

Originally posted by waynos
However the plane is NOT pushing against the belt at all. the propeller is acting on the air around it alone and so the plane will still move forwards, whatever the belt tries to do, the wheels and belt will just get faster and faster but once the engine is fully revved up the plane will take off virtually as normal.

one question waynos:

is the aircraft moving in regards to the airmass around it? is it stationary to the ground? as i understand the question, the belt keeps the aircraft in one spot, and is not moving in regards to the air around it, therefore no lift is generated.



[edit on 16-2-2006 by snafu7700]

Originally posted by snafu7700
is the aircraft moving in regards to the airmass around it?

Yes.

Originally posted by snafu7700
the belt keeps the aircraft in one spot

It can't because your wheels are not your driving force like they are in a car, they just spin freely.

I'm a convert that was under the same impression you have also, so I understand where you're coming from.

The best example is the wagon/handrail example.

If you're in a car on a treadmill, your wheels are your driving force so a treadmill can cancel your momentum out because the wheels "pull" against the ground/treadmill to move you forward.

However, if you are in a wagon(plane), and put the treadmill in a hallway with a handrail, your wheels are not your driving force, aka the wheels spin freely like on a plane.

Then your arms are the "prop" and the handrail is the air that you pull on to move you forward.

Your wheels can spin like crazy, but, it's your arms/prop and the handrail/air that pull you forward, regardless of the speed (or direction) that your wheels spin.

[edit on 2/15/06 by redmage]

Of coarse it will take off, unless a couple of wheel bearings lock up from the friction. The thrust shouldnt have a problem overcomming the additional friction the wheels will be under from the conveyor trying to keep pace with the wheels.

If you are in a low power to wieght ratio craft like a J-3 cub you may not be able to overcome the added friction, also a taildragger would suck in that situation I would think.

okay, evidently we all need a refresher on what the actual question is, so from howard's first post:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

the sentence that i enboldened is the kicker. it specifically says that the conveyor is built so as to ensure that it maintains an exact opposite of the speed of the aircraft, NOT the aircraft's wheels. now, this is impossible, but it is how the question is worded. therefore, the aircraft will never at any time be moving forward in relation to its surroundings, and lift will never be generated. it makes absolutely no difference what kind of propulsion system is being used. push or pull, the machine is calibrated to maintain and equal and opposite speed, which keeps the aircraft in one spot.

Originally posted by snafu7700
okay, evidently we all need a refresher on what the actual question is, so from howard's first post:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

If we take it that literally, then we get the "slippery slope" of: if the plane's "speed" is zero, then, the conveyer belt must not be moving at all.

Admittedly, in the "real world" the wheel's bearings would likely seize up (or the tires would burst) before it took off, but, it boils down to the fact that the wheels are free-spinning and have no direct effect on the planes forward, or reverse, momentum like they do in a car.

The wheels are not the driving force on a plane like they are on a car.

The prop would still pull the plane forward through/with the air to takeoff (barring a friction seize, or burst, of the wheels).

Look at the wagon example, if you speed up the belt to however fast you are pulling yourself forward with the rail, you will still be moving forward only your wheels will be spinning faster than you're pulling.

Point: it won't stop you from moving forward, because the wheels are free-spinning and they are not what's "driving" you.

[edit on 2/16/06 by redmage]

Originally posted by redmage
The best example is the wagon/handrail example.

If you're in a car on a treadmill, your wheels are your driving force so a treadmill can cancel your momentum out because the wheels "pull" against the ground/treadmill to move you forward.

However, if you are in a wagon(plane), and put the treadmill in a hallway with a handrail, your wheels are not your driving force, aka the wheels spin freely like on a plane.

Then your arms are the "prop" and the handrail is the air that you pull on to move you forward.

Your wheels can spin like crazy, but, it's your arms/prop and the handrail/air that pull you forward, regardless of the speed (or direction) that your wheels spin.

[edit on 2/15/06 by redmage]

I think your analogy is a bit flawed.

We seem to agree that lift can only be generated if air flows over the airfoil of the wing, right?

We also all seem to agree that on a plane, the wheels serve basically two purposes; to support the weight of the plane while on the ground, and to facilitate the plane's ability to taxi efficiently.

I think we can also agree that that neither function of the plane's wheels is related to the propulsion of the plane while it taxis or once it is airborn. And neither function is especially crucial to the generation of lift, which allows the plane to become airborn.

Your analogy does a fine job of pointing out the relative irrelevance of the actions of the wheels insofar as not enhancing the forward progress of the plane through the air mass in the given scenario, while at the same time underscoring the necessity of the plane to move relative to a fixed point on the ground, as would be required to generate the needed airflow over the wing, and thus lift the plane off the runway.

However, you may have overlooked one significant factor: The handrails, which in your scenario are analogous to the air mass the plane is to fly through, are solidly fixed to the walls of the hall housing the treadmill/runway. These walls are themselves solidly fixed to a point on the ground!

This detail, rather than proving that the plane can indeed take-off, only serves to move the moment of inertia from the non-stationary contact point between the plane's tires and the "conveyor belt runway" to a Stationary contact point between your hand and the Fixed (that is to say a point fixed relative to a point on the unmoving ground) contact point on the handrail.


The analogy would only prove your point IF you could show that air is both solid and , more importantly, fixed to a specific point on the ground.

Obviously, air is not solid, nor is the mass of air above a specific point on the ground fixed to that one spot. Therefore, I propose that your analogy might be more germain to the scenario if instead of grasping the handrails and pulling yourself along, you instead "grasp the air" and attempt to propel yourself forward by making pulling motions in the air, as it were.


Another point relating to the action of an aircraft's propulsion. This may be little more than a question of semantics, but nevertheless...

I believe that it has been stated that the purpose of an aircraft engine is to "pull/push against the air" thus driving plane forward.

As I understand it, however, the function of the enigine is actually to push/pull the mass of the plane through the air with sufficient velocity to allow the wings enough lift to counter-act the weight of the aircraft, vis-a-vis the pull of gravity. Heavier than Air flight is thus made possible.

So it might be more precise to say that, in keeping with Newtonian physics, the force generated by the plane's propulsion (whatever form it may take) is employed to push/pull against the airplane itself to counter its inertia and lend it suffcient velocity to fly.

Again, it may be just a matter of semantics.

Guys,
I ain't a techie nor an aeronautical engineer. But the answer is as simple as eating pizza. (Nowadays the ones u get in India are as good as the originals from Italy!). OK. So this simple question has a simple answer. Here goes.

The crunch part of an aircraft (Or glider) that makes it fly are the wings. The lift depends on:
1. The dehedral angle.
2. Angle of attack.
3. Shape of the cross section of the wing.
4. width of the wing.
5. Sweep.
6. SPEED OF AIR FLOWING OVER THE WINGS (As related to forward thrust).

Lets concentrate on the cross section of the wings and airflow.
Notice that the cross section shows that the front is shaped thicker, curving up and tapering towards the rear. The bottom portion of the wing is more or less flat. Now the air flowing over the wing takes a longer time to reach the rear than the air flowing below the wing as the distance required to travel by the latter is less (Remember the wing is flatter at the bottom). So what happens? A partial 'vacuum' is created above the wings resulting in lesser pressure than below the wings. This results in LIFT.

So, in other words, if there is no air flow over the wing, there will be no LIFT. PERIOD. Unless of course its an antigravity machine!! (We'll ask the little Green men about this technology later!).

In this case, since the aircraft is static in relation to the air around it, there will be no air flow over the wing, no partial vacuum resulting in lesser pressure over the wing and therefore NO LIFT.

In other words you're grounded till the cows come home!

However, keeping the original question in mind, what would happen if there was a huge propller placed ahead somewhere on the runway, facing this plane and churning out the minimum wind velocity required for this plane to take off? Now scratch your heads. I don't know the answer as I use my head only to keep my ears apart!

Originally posted by mikesinghIn this case, since the aircraft is static in relation to the air around it, there will be no air flow over the wing, no partial vacuum resulting in lesser pressure over the wing and therefore NO LIFT.

BZZZZZT!
Your airplane will not be stationary relative to the air around it. The propeller serves to move it through the air.

The only ways to keep the airplane from having motion relative to a fixed point on the ground are to either tie it down so it cannot move forwards (which is what a few posters apparently believe happened here), or to put a VERY large fan in front of it to provide a wind speed equal to what the propeller can produce. (Which would result in flight, as you've provided the airspeed)

The airplane IS moving, as there is nothing to keep it from doing so. Which means it will fly. All mention of the conveyerbelt and it's speed control mechanism is irrelevent, and included in the original question only to cause confusion.

It takes off

When doing maths it's often good to look at it the other way so lets do that for this problem. Lets say the plane is landing. The conveyor is still matching the airplanes speed. Now for those of you that believe that the plane remains stationary, as soon as the plane lands it will instantly become motionless. Its speed relative to an outside observer will instantly become zero. Think about that for a minute. The plane changes its speed from say 100kph to 0kph. Remember this is all relative to an observer standing next to the conveyor. consider the reality of that. The huge number of gees the people in the plane would experience. What happens to all that momentum that the plane had? The plane has to gradually slow down. If this is true the opposite must be true. The plane will take off.

Here are the basic facts everyone seems to forget:

1. Weight, inertia and friction will act on the plane as you try to accelerate.
-if the wheels are irrelivent, try this: (A)Sit in a chair that is anchored in place and strap youself in. (B) Have someone Anchor a steel cable to a 747. (C) Have the pilot make sure the breaks are off. (D) Pull at the cable and see if you can get the plane to move.

The Plane won't go anywhere! Assuming that weight and friction are irrelivent is a flaw of logic!

2. Lift comes from airflow. No airflow=no lift! Why is this so hard to understand?

Tim

Tim, and snafu. see my post about there being an initial effect that is overcome as the engine revs up.

snafu, the belt moving at a speed equal to but opposite the speed of the aircraft does not mean the plane is stationary.

I understand the obvious point about there being no lift if the plane is not actually physically moving forwards, but I don't get how you think the belt can counteract the speed of the plane. This is the sticking point for both sides.

The plane will move forwards through the air but the effect of the belt is not enough to prevent the plane from moving, the overall net effect is that it only rotates the wheels faster. Of course if this was done for real with a fixed length of belt that was equal to the planes normal take off run from a concrete runway then there would be enough friction at the beginning of the run to prevent take off speed being reached by the time the plane reached the other end, but the plane would be moving and would be accelerating relative to a fixed point on the ground. Therefore, in this theoretical situation, there would come a point, given a longer belt, where take off speed would be reached. If the belt was moving in the opposite direction to the aeroplpane but at the same speed and take off speed was 150mph then the wheels would be rotating at 300mph.

Of course friction and weight play a part, but they only complicate the equation, they do not disprove it. For instance, using tims example, a Boeing 747 would take a very long time to run its engines up to the required speed and begin to overcome belt assisted inertia which would require an incredibly long belt, but it would get there in the end. The effect of inertia is magnified, say it required a 10 times longer take off run than a piper tripacer on concrete, that does not mean a ten times longer belty would suffice, it may be that 1000 times longer belt is required but this is just making the basic premise unnecessarily convoluted, the basic point is that given sufficient length of belt any aeroplane would overcome its decelerating effects and take off eventually.

I've been meaning to mention, twice the circular momentum must be imparted to the tires, which is a very real amount of consumed energy, more perhaps than the friction involved. On the other hand, the momentum of the tires is insignifigant next to the mass of the airplane.

People are looking too much into it, just like I was.

The Plane WILL TAKE OFF!

I am a pilot, and an aircraft mechanic, and I was fooled. The landing gear have nothing to do with the plane, they just roll along. So if you have a conveyor 2100 feet long, and a cessna attempting a takeoff, as the Cessna revs the engine, it begins to move forward, the conveyor starts moving.

Now the wheels are spinning along at twice the speed the normally would, but the airplane is moving against the airmass still, to an observer, the plane is moving as it normally would for takeoff. The wheels will never have the friction necessary to slow the plane down enough to stop takeoff, but you might have some worn out tires.