Airplane physics question

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under18carbon

under18carbon

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I stumbled upon this over another forum the other day, and apparently it's been making rounds over the internet but I haven't seen it posted here yet. If it's a repost, then mods please delete the thread!

Question :

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.There is no wind.

Can the plane take off?

Discuss.

...

(I don't know if this should be in here or the Opinions forum :guilty: )

Moderator Edit
For the avoidance of doubt, yes, the plane will take off. The plane's propulsion acts on the air, not on the wheels, therefore the wheel speed is irrelevant, it's the speed at which the wings move through the air that determines the lift.

This is demonstrated in this video:

 
This has to be one of those things where it's "obvoiusly" one way, then when you hear the opposite explanation, that becomes "obvious". :lol:

Anyways, if the plane is to take off it must get moving relative to the air. Any sort of prop/jet plane gets moving by pusing against the air (with it's engines), so the airplane will move forward no matter which way the wheels/conveyor rotate. So the plane can take off.
 
No. Because there will be no air pushing under the wings because the plane is stationary and it should not take off. But I might be wrong....
 
I think, the engines will push air out the back, and the plane will get moving on the ground like a rocket does, from the momentum of the backward moving air. When there is enough air over the wings, it takes off.

Think of how those water-planes take off.
 
gt_masta
No. Because there will be no air pushing under the wings because the plane is stationary and he should not take off. But I might be wrong....
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Quite right.... Almost.

The air movement over and under the wings is what is needed, not just under the wings. Lift - is what you're looking for. Without movement of air, the wing desgin cannot create lift, therefore the plan will sit stationary.
 
Of course it could take off. An aircraft is not driven by it's wheels, so the conveyer would have absolutely no way of countering the craft's movement. In fact, it would be impossible for the conveyer to match the speed of the wheels, because the plane would be moving.
 
In theory, it wouldn't take off if the wheel speed matched the conveyor speed at every exact moment. Where the idea goes wrong, however, is that the plane gains forward momentum from pushing the air behind it, not by pushing the wheels. In reality, the conveyor would never be able to match the speed of the plane at the wheels. The conveyor would have to be set to move backwards with the same force as the plane is exerting on the air, slightly faster than the wheels. Then the plane wouldn't take off.

The idea would work for an F1 with its aero set for front end lift. The car wouldn't leave the ground, but could attain about 400-500 km/h at the wheels[b/] due to the lack of air resistance.
 
Liek Der Alta said, it wouldn't take off.

However, if enough air could be pushed over the aerofoils (by a good few turbo-props) then it would be able to create enough lift to get off the conveyer and fly. But it would take a hell of alot of propulsion and a very light aeroframe.
 
under18carbon
...The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation...
Click to expand...

Sounds like this might be a trick question. How do you define "the opposite direction of rotation"? It's talking about wheel rotation, not movement of the plane itself.

Does it mean that the conveyor belt moves with the plane and keeps the wheels from rotating as the engine thrust accelerates it? That is "the opposite direction of rotation", isn't it?

In that case, the conveyor is simply keeping the wheels from rotating as the plane gathers speed. The plane lifts off with its wheels not rotating.

Note that it only says "exactly match the speed of the wheels", not "exactly match the rotational speed of the wheels". The conveyor belt has "exactly matched" the speed the wheels are travelling (the speed of the plane itself), and keeps them from turning.

This is either a trick question, or whoever wrote it blew the wording, and it is mis-stated.
 
Zardoz
Sounds like this might be a trick question. How do you define "opposite direction of rotation"? It's talking about wheel rotation, not movement of the plane itself.

Does it mean that the conveyor belt moves with the plane and keeps the wheels from rotating as the engine thrust accelerates it? That is "opposite direction of rotation", isn't it?

In that case, the conveyor is simply keeping the wheels from rotating as the plane gathers speed. The plane lifts off with its wheels not rotating.

Note that it only says "exactly match the speed of the wheels at any given time", not "exactly match the rotation of the wheels". The conveyor belt has "exactly matched" the speed the wheels are going (the speed of the plane itself), and keeps them from turning.
Click to expand...
One problem. The plane needs to roll on it's wheels in order to get to take off speed. The plain won't suddenly attain take off speed.
 
ExigeExcel
One problem. The plane needs to roll on it's wheels in order to get to take off speed. The plain won't suddenly attain take off speed.
Click to expand...

(EDIT: I edited my above post a little for clarification.)

Instead of a conveyor belt, envision an extremely high-performance aircraft carrier that would accelerate with the plane. The ship matches the plane's speed, the plane attains takeoff speed (airspeed) and lifts off, even though it is still sitting in the same spot on the deck of the carrier.

Some ancient biplanes could take off at about 35 MPH. Carriers will do that now. An old kite like that could lift right off the deck of a carrier without its wheels ever rotating, and "hover" over one spot on the deck.

I really think this is a trick question.
 
Are you saying the conveyor is a dyno to the plane, and if there is no wind then will the plane take off. No..
 
Jedi2016
Of course it could take off. An aircraft is not driven by it's wheels, so the conveyer would have absolutely no way of countering the craft's movement. In fact, it would be impossible for the conveyer to match the speed of the wheels, because the plane would be moving.
Click to expand...
That sounds right to me.

It doesn't matter whether the wheels are turning. The plane uses the thrust from the engines to gather speed.
 
Lamboracer
...Are you saying the conveyor is a dyno to the plane...
Click to expand...

What do you mean by "dyno"?

Assume the plane is pointed north. If the "conveyor belt" moves north with the plane, the wheels do not rotate as the plane accelerates. The plane and the conveyor belt accelerate to the plane's takeoff airspeed and the plane lifts off, even though the plane's wheels are not rotating.

Wheel rotation does not make a plane lift off. Only airspeed matters.
 
If the plane is stationary relative to the Earth (or more importantly, the air, which is usually moving at about 10-30mph relative to the Earth) then it will not take off. Ever. If the plane is moving relative to the Earth (ditto above) then it will takeoff when it reaches takeoff speed. This is because planes need air to be moving over the wings in order to generate the lift that they need. No air = no lift. This is why planes always take off and land into the wind, and never with the wind behind them.


The above situation has (ignoring technicalities) a conveyer belt moving so that it matches whatever speed the wheels attain but counter to the motion of the aircraft. This means that the aircraft is stationary relative to the Earth, so it will not take off.


In certain extreme situations, an aircraft which is stationary relative to the ground can take off. Like nose-first into a Category 5 hurricane. In this case the aircraft is stationary relative to the ground but moving relative to the air (even though it is the air itself which is moving).
 
It applies to helicopters too.
 
How so?

I understand that a helicopters use the basic principles of aerofoils to create lift, but a helicopter can take off vertically without moving relative to the earth.
 
As I think this over, we're assuming that the direction of the wheels or the converyor belt need to be figured upon. If this plane was propelled by Jet engines, the tires do nothing more than sit upon the ground because of the planes weight. Assuming that the plane inches forward, the converyor belt would spin in reverse to counteract the movement of the wheels. The wheels and the converyor belt would have no effect on the Jet engines (or props for that matter). Thus the movement of the wheels and conveyor would not stop the plane from gaining enough forward moment to leave the ground.

However, the forward movement of the wheels would cause the belt to move in every attempt to stop the tires. One can not move without moving the other. Thus the belt/tire interaction would reach infinite proportions in a matter of moments.

LIkely fragmenting the tires or belt and turning the event into a Concorde scale of destruction.
 
What Famine said is correct (as usual). If the plane remains stationary relative to the ground, it won't take off, not matter how much thrust it is producing.

As for helicopters, they CAN take off while stationary, but they do it by effectively spinning their "wings"through the air to a speed where the lift under the blades can pick the aircraft off of the ground. Prop-powered planes and seaplanes also use this principle to move the craft forward, and to attain take-off speed.
 
ExigeExcel
How so?

I understand that a helicopters use the basic principles of aerofoils to create lift, but a helicopter can take off vertically without moving relative to the earth.
Click to expand...

Yes - but they can also take off vertically without being turned on, with a strong enough wind.

I very carefully chose to have only "plane" (not "aircraft") in the first part and only mention the ability to take off into a sufficiently strong wind (and not inability to do anything else) in the second part...


DA - If you glued a plane to a runway, it wouldn't go anywhere because, although the power is being provided, the wheels aren't turning so the plane cannot move relative to the air around it. The conveyer belt is acting as the glue - the wheels are effectively stationary, so the plane is not moving relative to the air around it. The mechanics are a little screwy though. If the conveyer belt moved in the direction of travel of the plane (the wheels rotating clockwise when viewed from the right, the conveyer belt moving similarly clockwise) then the plane would take off as both it and the ground it is standing on are moving relative to the air. If the conveyer belt moved against the direction of travel of the plane (the wheels moving clockwise, the belt moving anticlockwise) then it wouldn't take off as, though it is moving relative to the ground, the ground's motion relative to the air is the same magnitude but in the opposite direction to the ground's motion relative to the plane.
 
Very true, Famine. My line of thought was the tires are not anchored and can turn freely. The engines are thrusting against the mass of the plane to counteract the friction of the air in front of the body, not against the friction of the wheels on the ground. Because the wheels and the converyor belt can move freely, they have no effect on the mass of the planes body. That is what the engines are forcing forward.

The wheels and the conveyor belt only support the weight of the plane, they do not work in opposition to any movement. The air around the plane is what is being pushed upon.

Because of this, the plane will move, and the conveyor belt is only attempting to counteract the motion of the wheels. Motion of the wheels is inevitable based upon the plane is not using either to incur forward motion.
 
Yeah, I think this problem was worded to be ambiguous, as to lead to this type of debate. My interpretation is that the conveyor works in such a way to keep the plane in the same spot, in which case it can't take off.
 
under18carbon
...apparently it's been making rounds over the internet...
Click to expand...

Sure has:

http://forum.physorg.com/index.php?showtopic=2417&st=0&#entry19967

http://www.airliners.net/discussions/tech_ops/read.main/136068/

http://forums.sportbikes.net/forums/showthread.php?t=297891&page=1&pp=15

http://www.gtaforums.com/index.php?showtopic=224348


kylehnat
...worded to be ambiguous...
Click to expand...

...and, ultimately, pointless.

"If a tree falls in the forest..."; "How many angels can dance on the head of a pin..."

*yawn*
 
ExigeExcel
One problem. The plane needs to roll on it's wheels in order to get to take off speed.
Click to expand...

Actually, it doesn't. If, as Zardoz pointed out, if the conveyor is travelling in the same direction as the plane, the plane will move forward, just it's wheels won't be turning. Therefore the wings are cutting through the air, creating lift.

For the plane to actually be able to take off, the conveyor (and plane) need to reach normal take-off velocity.

:)
 
It won't take off if the conveyor belt is moving in the opposite direction of where the plane is facing. If the conveyor belt is moving (at take off speed) in the same direction of where the plane is facing, the plane will lift off.
 
under18carbon
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.There is no wind.

Can the plane take off?

Discuss.
Click to expand...
I have not read any of the discussion above.

Of course the plane can take off. It's driven by the propellor and the wheels are irrelevant. In fact, it will take off without making the wheels rotate at all; the conveyor belt will simply move forward under the plane until it reaches airspeed, and then the plane will lift off.

[edit] The thing is, The Indigo One has got it wrong.

He's correct in saying that it takes an extremely strong wind to lift a plane off the ground if it is not moving relative to the earth.

But the plane on the conveyor belt is moving relative to the earth (and the atmospheric mass of air); therefore it will take off with no problem. If you choose to view the conveyor belt as "earth", then the "earth" is in fact moving through stationary air with enough velocity to lift the plane.

Whoever envisioned the high performance aircraft carrier had it right.

The wheels on a plane freewheel and so it is only the plane's speed relative to the air, which is generated against the air by the propellor/jet, that matters.

WorldsSmallestAirport.jpg
]

SmallestAirport.jpg


Bonus points for whoever can tell me what the aircraft and the cars are.
 
If the conveyor belt forward speed matched the forward rotation of the airplane wheels, then yes, the plane would take off. The belt is moving the plane, thus moving air over and undrer the wing creating lift.

If the conveyor belt is moving in the opposite direction of the wheels, then no, the plane would therefore be standing still relative to the Earth, and therefore no wind is flowing over and under the wings whcih means no lift.
 
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