Can Plane Take Off

Grimmlokk said:

The general argument for flight seems to be that the plane *would* be moving forward regardless of the conveyor belt. Since the wheels spin freely it wouldn"t matter how fast the belt moved, the plane would still move forward until liftoff.

That"s what I"m having trouble with. Seems to me the question implies that the plane is at a stand still but the thrusters are going full blast.

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Exactly correct. In both statements. It implies that to throw you off, its a trick question.

I"ve been thinking about this in a different way.

Suppose a plane is coming in for a landing going 300 mph, and the runway is a conveyor belt moving in the opposite direction as the plane...

When the plane touches down and tries to land on one spot, what will happen? Will the plane come to an immediate stop? Or will the plane move over the surface of the conveyor?

Damn questions like this make my brain hurt.

Oh yeah, if the plane landed on the conveyor and slowed down or at least landed in a shorter distance -- we could have runways the size of football fields instead of miles and miles.

It would be like landing while going 600 mph. It would actually slow the plane down in less distance, because when landing the brakes are doing more work. Not a huge difference, but a difference.

All you need to know is that a conveyor cannot hold a plane still relative to the ground no matter how fast it turns in the opposite direction if the wheels are freely rotating. (ie they don"t have the brakes on) The amount of force it can apply to the plane through the wheels is absolutely miniscule compared to the force applied by the engines.

so i felt like reading this thread, its pretty cool so far..

i like what old man wrote...i dont know much about this kind of shit, but landing at 300mph onto a massive conveyor going opposite direction traveling at what speed? same? faster? I"d probably say the planes wheels would completly rip off and the plane would get sent wherever the conveyor is going, its like a human running ontop of a treadmill that cant keep up. although that theory is saying that the massive conveyor would be going faster i think.

Zuuljin said:

Exactly correct. In both statements. It implies that to throw you off, its a trick question.

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I should agree here, but what I"ve been getting at is that the whole point of forward momentum should be moot, because it"s not the forward movement that supplies the lift.

If breaks were ever devised that could handle the strain, a plane could sit still in one spot with the engines burning until it takes off. Although I don"t have a deep knowledge of aeronautics, I do know this much thanks to the conversation I had with the pilot after my first flight.

Of course, I could be wrong on that point, seeing as how the higher pressure air should be above the wings, and not below. This is why I"ll be studying up on this stuff over the break.

Pretty much every other question about the plane"s motion can be answered with this:

Imagine the plane and everything connected to it as one particle, and that particle should be placed spatially at the plane"s center of mass. When that particle has a momentum, it will continue in the direction of that momentum until its velocity has been depleted through acceleration in the opposite direction.

The best example of the momentum conservation of the center of mass of a system of particles is in the following:

A brown bag full of firecrackers is slicked down with oil to give it some weight, and is launched at a high velocity with the firecrackers lit.

When the bag explodes, each piece of the bag and its contents will continue in the arc of the original motion, even as they are seperated by the explosion.

This goes back to Newton"s First Law. Anything in motion will remain in motion until acted upon by an external force.

GaliemVaelant said:

If breaks were ever devised that could handle the strain, a plane could sit still in one spot with the engines burning until it takes off. Although I don"t have a deep knowledge of aeronautics, I do know this much thanks to the conversation I had with the pilot after my first flight.

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I am not nearly as fluent in this stuff as you, but im pretty sure that is impossible, or else why dont people take off like that? You could simply lock the wheels down while the engine gets into gear, once its running fast enough to fly, unlock the wheels and off you go!

The only way that would be feasible is if you generated so much force that you could instantly propell yourself from a dead stop to a speed which is needed to maintain flight. But unless its instant, there will always be sometime when your moving forward, although at an extremely high rate of speed, but you still must move forward, to go up. The plane will not just start hovering just because you have the engines running full throttle.

This is getting way to technical lol. What this boils down to now is whether or not the plane can fly without horizontal movement, in which case it would be hovering. And I dont think that that is possible with a jet engine or a prop lane.

I hope that this thread has not scrolled too much after a couple weeks. I"d like to come back when I"m 100% on the answers to all of these questions.

That is, with what I understand about physics thus far, I should have enough that I don"t need to rely on what a pilot once told me. I just need the free time and energy to work out the problems involved.

If a couple folks give me the nod to go ahead, I"ll revive this thread when I"ve looked at the math related to Bernouli"s Principle, and have drawn up a free body diagram and some solutions. That would force me to follow up on my plans to study this too, so it would be mutually beneficial.

Of course, by that point this thread will have scrolled, so I certainly won"t be bumping it unless y"all nod.

GaliemVaelant said:

Summarily, yes, this is the case.

I might suggest this as an exam question for whatever physics class ends up covering the dynamics brought about by potential differences in pressures.

I"m pretty sure that advanced mechanics will cover this. This is an awesome thought experiment

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this isnt an exam question, this is a trick question. its simple if you have ANY experiance with flight, you use stupid things like the treadmill to trick people into thinking too hard and second guessing themselves. just a fyi, a plane could be sitting still and have a nosewind of 60mph and take off. seen it happen, luckily they were tied down so it only caused light damage. the ones that werent got flipped.

What this boils down to now is whether or not the plane can fly without horizontal movement, in which case it would be hovering. And I dont think that that is possible with a jet engine or a prop lane.

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if its not possible how does a kite manage to go up without applying a horizontal movement?

Ok help me with this. Talking about a single propeller plane. Are you telling me that one single propeller would create enough wind around the whold airplane (including it"s whole wingspan) to create vertical lift on the wings?

Picture a house fan. Hold a board in front of it. The air will blow on the board only where the fan is, not around the whole board. So how exactly will this one propeller let the plane lift off?

Like I mean if the propeller in front of the plane had the diameter of the wingspan, it would blow on both the wings, moving the air around the whole plane, and yes then it would take off "while standing still".. but a regular single-propeller plane has a relatively small propeller, not nearly enough to cover the wingspan. So.. how?

I think that in that case, forward momentum is an absolute must.

Single prop planes work differently from dual props and jets in that they accelerate the entire body of the plane so that the plane moves relative to the air, instead of the air relative to the plane.

Actually, those planes with single jets mounted in the tail also would absolutely have to have forward momentum.

Angrier said:

Ok help me with this. Talking about a single propeller plane. Are you telling me that one single propeller would create enough wind around the whold airplane (including it"s whole wingspan) to create vertical lift on the wings?

Picture a house fan. Hold a board in front of it. The air will blow on the board only where the fan is, not around the whole board. So how exactly will this one propeller let the plane lift off?

Like I mean if the propeller in front of the plane had the diameter of the wingspan, it would blow on both the wings, moving the air around the whole plane, and yes then it would take off "while standing still".. but a regular single-propeller plane has a relatively small propeller, not nearly enough to cover the wingspan. So.. how?

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It wont, which is why you need to be moving. And yes if you have a strong wind you pretty much "hover" as it may be. But cars can fly in a hurricane, so I think we"re assuming 0 wind speed. =)

The plane would not take off.

The thrust generated by the engine would cause the plane move forward. But against what reference point? Normally this reference point is a stationary runway, but not in this case.

In this case, let"s assume the plane is generating enough thrust to normally move it 100 mph, with the stationary conveyer belt as the reference point. If the conveyer belt then moves 100 mph in the opposite direction it carries the plane backward (using the stationary ground as a reference) at the same speed that the plane would normally move forward. Until the plane is airborne the mass of the plane is completely supported by the conveyor.

At this point, the wheels think they are moving forward at 200 mph. The plane thinks it is moving forward at 100 mph. The conveyor belt thinks it is moving the plane backwards at 100 mph.

However, from the reference point of the stationary ground the plane is not moving. Since the plane is not moving there is no airflow over the wing. No airflow means no lift, hence the airplane cannot take off.

The key to this question is understanding the proper frames of reference.

Edit: The more I think about it, I don"t think the wheels would think they are going 200 mph. They would think, just like the pilot, that they were going 100 mph.

So you go to GaTech and you"re not interested in abstract questions involving physics?

Please tell me you"re studing Industrial Engineering or something.

GaliemVaelant said:

If the string is attached to the top of the car, and not the front, yes.

For the vertical acceleration Otherwise, the car on the treadmill would only model horizontal thrust

Also, even if the plane does not move an inch horizontally until it takes off, it will take off. To demonstrate why the air is moving over the wings without the plane actually moving, place a sheet of notebook paper on the back side of a fan. Actual horizontal velocity has nothing to do with the lifting force, from what I understand.

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First off, the string being attached to the car at the hood or on the top of the roof makes no difference. Vertical acceleration? There is no vertical acceleration in his example. For the purpose of this example there are only two forces being acted upon this car: friction to the left and tension of the string to the right. No matter where this horizontal string that exerts a horizontal force is mounted on the car, it will result in motion only in the x direction and nothing in the y.

Secondly, the type of plane that we are discussing can not takeoff without horizontal movement. The engines pull the plane through the air, they do not push air over the wings. The plane is provided lift through two main methods... the one if importance is due to pressures (Bernoulli"s Principle). For the pressure difference to be made, air must be rushing over the wings. Thus the plane must be moving forward initially to take off.

A plane however can hover, but not in the cool ufo sort of way. If the windspeed is great enough to cause enough air resistance to cancel out the plane"s forward momentum, then it would both be flying and not moving horizontally.

Dintorr said:

The plane would not take off.

The thrust generated by the engine would cause the plane move forward. But against what reference point? Normally this reference point is a stationary runway, but not in this case.

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I would assume that the whole trick to the question is that the reference point is theair around the wings, not the ground itself. But like I said, that would mean that all the air around the wings would have to be moving fast enough to generate vertical lift for the wings.

TheWand said:

Only available to the members and science guys atm. .. fortunately, im one of them

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Sorry but you are not cool.

Lucane Darkseraphim said:

Secondly, the type of plane that we are discussing can not takeoff without horizontal movement. The engines pull the plane through the air, they do not push air over the wings. The plane is provided lift through two main methods... the one if importance is due to pressures (Bernoulli"s Principle). For the pressure difference to be made, air must be rushing over the wings. Thus the plane must be moving forward initially to take off.

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Exactly, if the propeller"s purpose is to push theplane(wings) forward, it is to create the pressure, thus creating vertical lift. One propeller itself would not be nearly enough to encompass the wings (not to mention to create sufficient wind speed around them).

Dintorr said:

The plane would not take off.

The thrust generated by the engine would cause the plane move forward. But against what reference point? Normally this reference point is a stationary runway, but not in this case.

In this case, let"s assume the plane is generating enough thrust to normally move it 100 mph, with the stationary conveyer belt as the reference point. If the conveyer belt then moves 100 mph in the opposite direction it carries the plane backward (using the stationary ground as a reference) at the same speed that the plane would normally move forward. Until the plane is airborne the mass of the plane is completely supported by the conveyor.

At this point, the wheels think they are moving forward at 200 mph. The plane thinks it is moving forward at 100 mph. The conveyor belt thinks it is moving the plane backwards at 100 mph.

However, from the reference point of the stationary ground the plane is not moving. Since the plane is not moving there is no airflow over the wing. No airflow means no lift, hence the airplane cannot take off.

The key to this question is understanding the proper frames of reference.

Edit: The more I think about it, I don"t think the wheels would think they are going 200 mph. They would think, just like the pilot, that they were going 100 mph.

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As has been said before, this is the incorrect way to think about this problem.

The conveyor belt moving backwards 100mph would not result in the plane moving backwards 100mph because the wheels of the plane do not cause the plane"s movement. If the plane moved forward via wheels only then yes it would stand still. But because the turbine engines pull the plane through the air, the wheels simply spin freely.

Think about the world as a conveyor. It rotates, so in essence the ground is moving. Does that mean that planes can only take off with the direction of rotation of the earth? Certainly not.