@L8-X

As with the Vauxhall forum and a few others people have stumbled upon, it would seem the jury is still out. People seem to be 90% sure they know the answer, and it seems the split between yes it will/no it won't is about 50%.

Perhaps we shall have a poll to decide and forget physics, let democracy decide:

Voting thread.

Funny how all of us, reasonably intelligent people can't seem to truly work this out. We need to ask a real physicist. If someone knows a university professor or suchlike who is expert in this field, perhaps you can direct him to this thread as a guest speaker?

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@A1200

You guys are making this harder then it should be. The plane won't fly.

1. The plane is sitting on the conveyer belt so the runway can not make the wheels turn. Only the plane moving forward will cause the wheels to rotate. The wheels will rotate at the same speed as on a fixed runway.

2. The plane is stationary to the ground, therefore no air is moving over the wings to give lift.

Lets just say if the plane did manage to rise off the belt, its air speed would be zero knots relative to the ground. I know of no planes that will stay airborne for long at that speed. With thrusters at max, it would just launch itself forward into the ground.

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@sundown

You sound really sure of that. Yet I have had people talking to me until they are blue in the face, saying the contrary.

I think that a guy being pushed on roller skates at 10MPH is a good example of why it would work. Stick a guy on on roller skates a treadmill and set the treadmill to 10MPH. Now push the guy at 10MPH, because the power is not at the wheels (i.e. they are free-wheeling), the guy will still move forward on the treadmill at 10MPH, it's just the effective speed on the wheel would be double.

I have heard this question is open to interpretation, if the plane and treadmill ARE both traveling at the same speed, then no, if taken literally it cannot take off. If however you consider how the wheels are powered is factored in, then you can see the argument for the plane taking off. Whoever posed this question is probably evil and wants to take over the world by making everyone mad, Mad, MAD!

I voted it probably would take off, but I might be wrong. A real practical test would be nice. If we had a word with a rich guy to get us a plane and construct a giant conveyer belt, perhaps we could do the test for real. Richard Branson would be a good person to ask first, as he already has the planes and is a sucker for adventure.

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@A1200

Hey 2 more posts and I will be a Super Member! I think I will save post 1000 until my attendance at the 3cag meeting tomorrow.

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@A1200

Quote:

tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)

With a system like this, the plane will be absolutely stationary on the runway with reference to the ground. The movement of the belt would cause the plane to have some added forward thrust, but the belt would compensate for that as it's a perfect system. The airspeed is still going to be zero knots & would fly as well as a brick. We need someone to write a program to test this.


Hey, I was the first super member of '06. Did #1000 30 sec after midnight, Jan 1st.

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@sundown

Quote:

sundown wrote: @A1200 You guys are making this harder then it should be. The plane won't fly. 1. The plane is sitting on the conveyer belt so the runway can not make the wheels turn. Only the plane moving forward will cause the wheels to rotate. The wheels will rotate at the same speed as on a fixed runway.

But what do wheels have to do with anything, they are free to rotate at any speed they like since they're not directly driven by anything.
Look at seaplanes, they don't need wheels to take off.

The question said the conveyor only matched the planes speed, not momentum, so unless the extra friction from the wheels/bearings being rotated twice as fast is equal to the force generated by the engines the plane will move forward relative to ground.

If the plane has enough power to overcome the extra friction, and the conveyor belt is long enough for the plane to reach take off speed (relative to air, not the belt), then it should fly.

Quote:

2. The plane is stationary to the ground, therefore no air is moving over the wings to give lift.

Only if there's no wind. Ground speed != air speed.
Why do you think gliders are tethered when left outside?

Last edited by Doobrey on 26-Feb-2006 at 04:04 AM.

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@sundown


Quote:

sundown wrote: @A1200 You guys are making this harder then it should be. The plane won't fly. 1. The plane is sitting on the conveyer belt so the runway can not make the wheels turn. Only the plane moving forward will cause the wheels to rotate. The wheels will rotate at the same speed as on a fixed runway.

Not necessarilly! You are using imprecise terminology. What do YOU mean by the term "forward"? There are situations when aircraft can 'fly backward' relative to ground co-ords- Storch
Quote:

quote from site: The Storch was used as a liaison plane by the artillery divisions. Similar to the British Westland Lysander or the Stinson L-1 at the time, it had amazing short field capabilities. As the story goes, the Storch is capable of flying backwards, proven by witnesses seeing a pigeon sitting on its wing while it approached an airfield on final.

Quote:

2. The plane is stationary to the ground, therefore no air is moving over the wings to give lift. Lets just say if the plane did manage to rise off the belt, its air speed would be zero knots relative to the ground. I know of no planes that will stay airborne for long at that speed. With thrusters at max, it would just launch itself forward into the ground.

On your second point, consider the diagram below:


Say, a conveyor belt beneath THAT sailplane terminates immediately prior to the stationary winch -- can YOU advance any reason to show why the sailplane should not become airborne?

The accelerating-force provided by the winch mimics the thrust-force provided by the engines of a powered-aircraft! Recall that the engine-thrust acts on the airframe via the engine mountings!

Where the aspect of what is happening with the undercarriage wheels intrudes -- consider the aircraft shown below:


This aircraft uses an aircushion for undercarriage, so there are no wheels to turn!

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@thread

It's a trick question. The conveyor belt is irrelevant. The thrust of an aircraft, whether jet or prop, only reacts on the aircraft, not its surroundings. The aircraft will fly, because movement of the ground is not part of the flight profile, only movement of the aircraft against the air. Notice that aircraft have no problem flying over rivers or ocean currents.

To put this in a perspective most of us are familiar with, let's invert the scenario. Will a car sitting on solid ground move if the wind is blowing the other way? Of course it will, since movement of the air is irrelevant to a cars motion, up to the point that drag exceeds thrust. Drag is not a problem with the aircraft, since there's no (appreciable) drag from the ground/conveyor belt on the plane.

As for any argument about the friction of the wheel bearings, bear in mind that the motor for the conveyor belt will burn out long before the wheel bearings will. IOW, friction is also irrelevant.

Note: Parked airplanes are required to have wheel chocks to keep them from moving, otherwise, if the parking brake fails or is off, the first breeze that comes along would start them rolling. (Yes, even the big A340s and 747s have wheel chocks)

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People who generalize are always wrong.


1989 - 500 / 1991 - 3000 / 1997 - Genesis Flyer 1200T / 2003 - A1XE

@Doobrey

Quote:

The question said the conveyor only matched the planes speed, not momentum, so unless the extra friction from the wheels/bearings being rotated twice as fast is equal to the force generated by the engines the plane will move forward relative to ground.

I don't see where there would be much forward momentum unless the conveyor belt were to suddenly quit moving & my reference to the wheels was that they would rotate no faster then they would on a normal runway during take off. The engines would have to work a lot harder to keep the plane moving forward & who's to say it's not moving the plane backwards.

Not all planes are created equal, so what kind of a plane are we talking about here?

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@vortexau

Different types of planes will react differently to the situation. Very small light planes would most likely fly, a loaded 747 would still be a brick.

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@sundown


Come on man think, The conveyor has NO lateral mechanical coupling to the
Airplane (if you ignore a little bit of friction).

It does not matter which way the conveyor is moving; forward or backward,
it has next to NO effect because the wheels of the airplane rotate freely.

You are assuming that an airplane is driven by its wheels; like a car,
IT IS NOT !


An airplane pushes its way through the air. the air is stationary, so the
plane WILL still move forward and the wings will generate lift.





Wol.

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If you consider the wheels as a bearing part in the mechanical model, inverted speed of the runway won't have an effect on the forward motion of the airplane. However, it isn't a bearing until a certain threshold of thrust ( or rather traction ) is reached.

Do the math, and stop telling people to think. It is rude.

They are thinking but the question is so poorly worded they are not modelling it in terms of thrust, force, gravity and resistance. Instead modelling it in terms of speed, and that leads you to ignore that the speed of the plane is measured relative to the earths surface once it has achieved traction and the conveyer is measured relative to a fixed point on the conveyer surface.

This is a similar philosophical question to the arrow and the tortoise.

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@syrtran

Quote:

The aircraft will fly, because movement of the ground is not part of the flight profile, only movement of the aircraft against the air.

Lets say there's a pole in the ground next to the plane & lets also say that there is zero wind at the time. In theory, the plane will stay next to the pole no matter how fast it tries to accelerate forward. The planes forward speed is nullified by the conveyor speed so no air moves. Same thing with wind speed. If a plane requires a ground speed of 50 knots to become airborne & theres a 50 knot tail wind, will the plane get off the ground when it reaches 50 knots? I think not, as planes are positioned to take off into the wind to aid in take offs. In my opinion, the conveyor belt would have the same effect as a variable & matching tail wind where the plane can never reach take off speed.

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@Wol

Just because I don't agree with you doesn't make me a fool. I never once said that the wheels moved the plane. I certainly can't see how you can say a plane is moving forward when the conveyor is moving backwards at 50 knots & the plane is moving forward a 50 knots. The 2 motions cancel each other out, or did I read the scenario wrong?

Guess I should just step aside & let the really smart ppl figure this out.

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@A1200

Given that the conveyer tracks the plane speed, the plain will develop the speed a (imho little) bit smaller than it would normally (the thrust comes form the engines, not the wheels, extra friction from the wheels and not stationary air speed near the engines, jet/prop/whatever, will cause the opposite force to smaller thrust respectively). The average velocity magnitude under the wing will be higher (air close to either the wings and the conveyer moves due to viscocity effects, it'll take some time to develop) so the pressure under the wing while the conveyer is moving will be smaller, thus the lift force will be smaller too, but eventually the plane will take off.

Jack

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Quote:

sundown wrote: The 2 motions cancel each other out, or did I read the scenario wrong?

Only if the conveyor belt exerts a force equal to that generated by the planes engines.
Since the wheels are essentially bearings between the airframe and the belt, how does the belt exert enough force on the airframe to stop it ?

Last edited by Doobrey on 26-Feb-2006 at 01:43 PM.

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@sundown

Quote:

Lets say there's a pole in the ground next to the plane & lets also say that there is zero wind at the time. In theory, the plane will stay next to the pole no matter how fast it tries to accelerate forward.

No, the plane will move forward.

Quote:

The planes forward speed is nullified by the conveyor speed so no air moves.

No it isn't. The only things in the problem affecting the plane's forward movement are the force of the engine's thrust, and the mass of the plane.

Quote:

Same thing with wind speed. If a plane requires a ground speed of 50 knots to become airborne & theres a 50 knot tail wind, will the plane get off the ground when it reaches 50 knots?

(emphasis mine) Here is the whole source of the confusion. A plane's takeoff speed is not a ground speed. It is an air speed. The ground speed has no bearing, whatsoever, on the plane's ability to take off. You even point this out here:

Quote:

I think not, as planes are positioned to take off into the wind to aid in take offs.

A plane is pointed into the wind on takeoff to aid its airspeed. The ground is not relevant.

Quote:

In my opinion, the conveyor belt would have the same effect as a variable & matching tail wind where the plane can never reach take off speed.

Yet again you mention an airspeed - "tail wind". The ground has no effect on an airplane's takeoff.

Let's try another example:
You get up in the morning and have breakfast. You use up the bread, so you put bread on your grocery list. Later in the day you are out and about and decide to buy groceries, so you stop at the supermarket. While you are inside, it starts to rain - a heavy, continuing downpour. It's still raining heavy when you come out with a shopping cart. There's water streaming across the parking lot from your car towards you (It's only about an inch deep). Will the water (moving at a good 5-10 mph) prevent you from getting to your car? No, because you still have traction on the ground.

Now, in this analogy the cart is the plane, the water is the conveyor belt, and you (and the ground) are the engine.

The conveyor belt has no bearing on the physics of the plane taking off. It does not push in any significant way against the plane, it only makes the wheels spin. The wheels spin, anyway, on a normal takeoff. In fact, the wheels' only reason for existence is to prevent the ground from causing drag on the plane.

Like I posted before: It's a trick question.

_________________
Tony T.

People who generalize are always wrong.


1989 - 500 / 1991 - 3000 / 1997 - Genesis Flyer 1200T / 2003 - A1XE

@Thread..



I went to see a friend yesterday , who just happens to be a PILOT.

I presented him with the this problem.

He looked down at the floor for 3 secs while he was thinking, then
said "The plane would still take off, the wheels would turn twice as
fast".


From a Pilots mouth..



Wol.

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@All

The problem is worded perfectly adequately, if one assumes a frame of reference of a point which is static at v=0:

The problem states that the runway moves with the same speed as the plane - if the plane is stationary the runway is also stationary.

The problem DOES NOT state that the runway applies an equal opposing force to the plane as that exerted by the plane engines on the surrounding air.

In the absence of a paint program and the use of algebraic symbols I'm not gonna start drawing free body diagrams, scanning them in, and writing the force-balance equations, but the upshot is:

There is an imbalance of forces - the plane accelerates, the plane takes off.

Step-by-step:

1) The plane begins to move - let's assume it moves at v=30m/s

2) The ground moves in an opposite direction relative to the plane at v=-30m/s.

3) The wheels are free to rotate about their bearings.

4) The wheels rotate as if the plane were moving at 60m/s, ie the planes velocity relative to the moving runway.

5) The plane doesn't give a crap how fast either the ground is moving or the wheels are rotating (Neglecting friction at the bearings - which, because they are bearings is negligible) - the only thing which concerns the plane is its own velocity relative to the air around it which (in simplified terms) creates lift.

6) The only thing the motion of the ground will accomplish is making the wheels rotate faster, and the bearings produce twice as much heat as normal due to friction.

7) The plane will take off.

Phewww! 4 years of studying mechanical engineering, and it seems some of it sticks after all.

Any arguenment and I get the algebra out......

Last edited by Boot_WB on 26-Feb-2006 at 04:42 PM.

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opp., the vast majority who voted silently with their feet.

Just been catching up with the views on A.org and the Vauxhall Owners Forum. There is another link mentioned:

http://forum.physorg.com/index.php?showtopic=2417&st=0

296 Pages?!?!

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