Can Plane Take Off

Lucane Darkseraphim said:

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.

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Here"s the original poster:

the plane is not a harrier, or V-take off .. just a normal plane, can range from small to large, or even have a propellor or jet engine.
not giving answer till more polls are in.

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No it can"t take off with a single propeller on it"s nose. Where are the wings going to get the airflow from?

Angrier said:

Here"s the original poster:

the plane is not a harrier, or V-take off .. just a normal plane, can range from small to large, or even have a propellor or jet engine.
not giving answer till more polls are in.

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No it can"t take off with a single propeller on it"s nose. Where are the wings going to get the airflow from?

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Yes it can still take off with a single propeller on its nose. The propeller"s purpose is never to push air over the wings but rather to pull it through the air. The pull through the air provides the air to rush over the wings resulting in lift.

Lucane Darkseraphim said:

Yes it can still take off with a single propeller on its nose. The propeller"s purpose is never to push air over the wings but rather to pull it through the air. The pull through the air provides the air to rush over the wings resulting in lift.

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I think you just owned yourself there. You said "pull through air"... air around the plane is standingstillif the plane is on a conveyor. Why would the air around the plane be moving? Answer me that.

Lucane Darkseraphim said:

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.

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That all clears up a good bit, thank you

I think that I had missed the point of the car on the treadmill example before.


Are there any planes that do operate by pulling air over the wings, such that they require no horizontal momentum for liftoff?

Ok I"m no scientist, so tell me which step is wrong in my reasoning:

1. Propeller"s primary function is to push the plane through air. (So yeah, I don"t give a shit about the wheels, I know they are free moving.)

2. As the airplane moves through air, it creates draft around the wings, creating vertical lift.

3. The conveyor prevents the airplane moving through the air, as the air around the plane is still.

4. And if the air around the airplane is moving at the same speed as the plane relative to the ground (0 mph).. lol where is the draft? From the propeller? Don"t think so, you just said that the propeller"s function is not to provide draft for wings, but to move the airplane through air, and it is the moving through air which provides the draft.

Angrier said:

I think you just owned yourself there. You said "pull through air"... air around the plane is standingstillif the plane is on a conveyor. Why would the air around the plane be moving? Answer me that.

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No, I fully grasp this scenario.

You improperly assume that just because a conveyor moves at 250mph backwards that a plane moving 250mph forwards results in the plane sitting still. The conveyor only has one means of holding the plane on it, and that is with friction. Between the wheel and conveyor is static friction, as is between the axle and bearings. The friction holding the plane back will be caused by the kinetic friction between the bearings and its holding. Simply put, that amount of friction results to almost no loss of movement against the plane"s forward progress. If you don"t believe that, then grab a sheet of paper and a pencil. Place the pencil on the paper and pull the pencil forward at the same speed that you pull the sheet backwards. Did the pencil stay still, or did it move forwards?

GaliemVaelant said:

Are there any planes that do operate by pulling air over the wings, such that they require no horizontal momentum for liftoff?

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Yah. Called helicopters. ; -)

Lucane Darkseraphim said:

No, I fully grasp this scenario.

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Apparently not. Let me try again. Refer to my post above yours. The airplane requires air drift against its wings to create vertical lift, does it not? Where does that drift come from, if the airplane is not moving through air?

Hell I"ll make it real simple for you Lucane Darkseraphim:

Air no move, plane no fly.

Angrier said:

Ok I"m no scientist, so tell me which step is wrong in my reasoning:

1. Propeller"s primary function is to push the plane through air. (So yeah, I don"t give a shit about the wheels, I know they are free moving.)

2. As the airplane moves through air, it creates draft around the wings, creating vertical lift.

3. The conveyor prevents the airplane moving through the air, as the air around the plane is still.

4. And if the air around the airplane is moving at the same speed as the plane relative to the ground (0 mph).. lol where is the draft? From the propeller? Don"t think so, you just said that the propeller"s function is not to provide draft for wings, but to move the airplane through air, and it is the moving through air which provides the draft.

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I already explained this. Number 3 is where you make your error. You assume that the conveyor belt cancels out the motion of the plane but it simply does not. Even if the conveyor belt moves backwards at 200mph and the plane only goes 100mph, the plane will still be moving forwards. This is due to rotation and the little affect that friction plays in holding the plane back.

Lucane Darkseraphim said:

I already explained this. Number 3 is where you make your error. You assume that the conveyor belt cancels out the motion of the plane but it simply does not. Even if the conveyor belt moves backwards at 200mph and the plane only goes 100mph, the plane will still be moving forwards. This is due to rotation and the little affect that friction plays in holding the plane back.

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You just said step 3 is wrong but you didn"t say why. Rather, you thought you did, but what you said has nothing to do with step 3 in my post.

3. The conveyor prevents the airplane moving through the air, as the air around the plane is still.

Why is the air not still around the plane?Tell me. What is pushing the air to move around the plane, to create airspeed around the plane?

Hell let"s break it down some more.

3A. The air around the plane starts out at zero mph, relative to the bystander, to the ground.

3B. The propeller starts turning, the conveyor starts moving. There is no magical wind that encompasses the whole plane, wings and all. Yes, wind is being pushed by the propeller. No, it"s not creating a draft around the whole plane.

Hell, to help you out, imagine this:

Increase the wingspan to 100 miles. Just because the propeller at the nose of the plane is spinning, does that magically create a wind 100 miles away, at the tip of the wing?

You didn"t say that my Step 2 was wrong, which was:

2. As the airplane moves through air, it creates draft around the wings, creating vertical lift.

That means airplane, wings and all. You are standing next to the plane, is it moving relative to you? No. The air around the plane is not being moved by anything either, so the air is still, relative to you, and relative to the plane, and it"s wings.

Can"t fly if it"s still relative to a bystander.

The plane IS moving throught he air angrier. The plane is NOT standing still. The treadmill has no effect on the plane whatsoever.

Here is yet another way to think about it. Pretend there is no gravity. And there are no wheels. The plane is floating above the treadmill by a few feet. Now the treadmill starts, and the plane starts its engine. Since the treadmill does not act on the air, which the plane is pulling itself on, it does nothing! The treadmill could be moving at 57.3 times the speed of light and it wont mean anything, because the plane is pushing off the air.

Now add gravity, and put wheels there so the plane doesnt scrape the ground, and make the wheels able to spin infinitly fast, and you have the same scenario. The friction of the wheels is by far outwieghed by the thrust of the engine.

Ok ok, help me understand this then.

Airplane is on conveyor, it has a propeller, which is turned on. Magically, the whole airplane is encompassed in a wind... right? Because that"s what is needed for the airplane to take off. Just how localized is this wind phenomenon? Does it extend beyond the wingspan? Or is there some magical force that"s only creating wind around the plane and nothing else?

Remember, you are standing still next to the plane. Remember, the plane is standing still next to you.

This is all under the assumption that the plane"s speedrelative to you(the earth) is zero.

This is stated in the original question.

Relative to you, the plane"s speed is zero. Relative to you, the air around the tip of the wing is zero. Wasn"t that the whole point of the question? That next to a bystander, the plane is not moving forward. So where"s that draft...?

Original question.. is it about the airplane standing still relative to a bystander, or is it about the plane still moving forward regardless of the conveyor?

Ok lol. First, the planes speed relateve to you is NOT 0. Read it is NOT 0. The plane is actually moving foward, creating the wind that will lift it. What your getting caught up on is the phrase "the convayer belt moves as fast as the plane, in the opposite direction." But this is meaningless, because the planes wheels are freely spinning. If the plane is going forward at 100 mph, the treadmill is spinning at 100mph backwards, but the plane is still going 100mph forward. To the wheels, they are spinning at 200mph, but again, it doesnt matter.

What is causing the plane to go forward is NOT the wheels. It is the engine "pulling" the plane forward on air. LIke when you swim, you push against the water and pull yourself forward. That is what the plane is doing, but in the air. Now if the bottom of the pool your swimming in suddenly starts moving backwards, you can still swim forward, because your pushing off the water, not the bottom of the pool. ( Assuming the moving pool bottom does not affect the water at all.)

Ok I realize that if there is no friction the belt can go at a million miles an hour backwards and the plane can still be pushed forward by the propellers. My whole argument is trying to say that as long as the plane"s relative speed to you is zero, there cannot be a draft to push it up, which is what I thought the question is all about.

Angrier said:

Relative to you, the plane"s speed is zero. Relative to you, the air around the tip of the wing is zero. Wasn"t that the whole point of the question? That next to a bystander, the plane is not moving forward. So where"s that draft...?

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That"s what I got from both the question and the first posted explanation.

The fact that the same folks who posted all that say that it moves (which makes perfect sense the way that they say it) is why I"ll be looking at the math.

The problem with the idea that the plane does move forward is that any translational motion of the aircraft is immediately transfered to rotational motion in the tires of the landing gear.

The conveyor belt, as per the original question and first answer, is designed to respond to the rotation of the wheels by matching the tangent acceleration.

If that is the case, then the plane does not move forward. Period.

If the plane is to become airborn, it then has to happen with no horizontal velocity. Thus my previous mental model of the problem in which the plane is subjected to vertical force, but not horizontal. There is no horizontal momentum on the body of the aircraft, relative to the ground.

A few folks imply that horizontal translations relative to the belt are the same as translations relative to the ground. As per the universal laws of gravitation, this is not the case.

Folks who know a bit about aircraft here say that there is no vertical lift without horizontal translation because the engines do not cause compression of the air above the wings, thus the plane does not become airborn.

Nonetheless, the engines do provide a force against the air, and by Newton"s Third Law, the air returns this force. Since the conveyor belt takes time to react, and the force is ever present and increasing, then the plane is lurching forward at small increments, while the belt adjusts to meet the tangent acceleration.

Eventually, these lurches will be powerful enough to provide enough forward momentum that the plane will lift off. The air velocity will be much, much higher than that for a normal take off.

Thus, the plane both lifts off and does not lift off, going by the consensus of those who know more about aircraft, the laws of physics, and those who have had more time with the problem, but ignoring peoples" specific answers as to whether the plane lifts off.

Therefore, we have a paradox that can only be resolved through math or experimentation, even if all components are ideal with the exception that the plane and belt both have a mass (they would have to for this to make any sense).

Could it be that something in the original question was misquoted?

You guys are making this waaaay to complicated... there"s no need to know all fancy laws of newton and shit just common insight.

Just imagine yourself standing on a skatebord on top of a conveyor belt. You"re holding a rope that is tied to the wall in front of you while the conveyor belt is spinning to the back. Will you move ? NO, you won"t... as long as you hold the rope that is !

Now imagine that you start pulling yourself forward using that same rope. Result is you moving FORWARD, nomatter how fast the belt is spinning backwards !

In case of the plane this means that the force of you pulling yourself forward using the rope is similar to the propellor pulling the plane forward. Nomatter what the plane is on... ground, water, sand or even a conveyor belt ! As long as the plane is not locked in place it WILL move, and it WILL take off as soon as its speed is high enough !

Angrier said:

Ok I realize that if there is no friction the belt can go at a million miles an hour backwards and the plane can still be pushed forward by the propellers. My whole argument is trying to say that as long as the plane"s relative speed to you is zero, there cannot be a draft to push it up, which is what I thought the question is all about.

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You are correct in that if the plane"s relative speed to you or the air is 0, then no, it cannot take off. But the question is a trick, making you think the plane is standing still, when it never directly says that. Just that the belt moves backwards as fast as the plane moves forward. You implied it in your head that the 2 would cancel each other out, when thats not the case.

This question isn"t hard.

Lift is generated by air flowing over and under the wings of an airplane. It is called relative wind. Go drive a car and hold your arm out the window. That is relative wind. If the plane is not moving forward and there is no wind passing over or under the wing. The plane will not take off.

The only instance where the airspeed gauge will show movement is in the case of a propeller or other engine that is situated ahead of the pitot tube (where airspeed is measured). The pitot tube measures your airspeed by pressure. So, depending on how fast air is flowing into the tube, your airspeed gauge will be affected.

The plane does not take off.

Xtacaz said:

You guys are making this waaaay to complicated... there"s no need to know all fancy laws of newton and shit just common insight.

Just imagine yourself standing on a skatebord on top of a conveyor belt. You"re holding a rope that is tied to the wall in front of you while the conveyor belt is spinning to the back. Will you move ? NO, you won"t... as long as you hold the rope that is !

Now imagine that you start pulling yourself forward using that same rope. Result is you moving FORWARD, nomatter how fast the belt is spinning backwards !

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The problem with this model:

An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast.

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Thus, when you pull the rope, the conveyor belt just goes faster, so that you don"t move.




Thus, the escence of the question, phrased more simply, if this:

Is enough vertical thrust supplied by the motion of the air due to the engines alone that a plane will lift off with no horizontal motion?

Those who know about planes here say no, when phrased this way. Thus, the airplane does not take off.


All of that energy has to go someplace, though. The laws of conservation of energy then lead to the plane"s eventually taking flight.


Thus, it"s a paradox.


Without further, deeper analysis, the plane both does and does not take flight.