Can Plane Take Off

xeg_IS said:

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.

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Indeed, the trick of the question is that the plane will move forward, relative to you, to the earth, the the bystander. Why? Because the propeller will push the plane forward through the air, while the wheels are just a free rotating agent. Assuming no friction, the belt cant go a million miles an hour backwards, wheels would just spin fast.

As long as the plane stands still though, there cannot be a draft to generate vertical lift.

Okay, screw it. I"ll seek out some help from much more experienced physicists drawing up the free body diagrams for this, that way I can have it solved by sometime next weekend. Then, I"ll post the results.

Time to knock out my Discreet homework and go to bed.

GaliemVaelant said:

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.

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If it were a car, that would apply, but the forward momentum is based off the thrust of the air, not the ground. The first explanation explains this. Might want to go back and re-read it.

The belt does not effect the speed of the plane, because the wheels are not pushing it. Like the skateboard example, even if the treadmill doubled its speed, all it would mean is your wheels would spin 2x as fast, but you could still pull yourself forward, because your strength is much greater then the friction of the wheels.

Angrier said:

Indeed, the trick of the question is that the plane will move forward, relative to you, to the earth, the the bystander. Why? Because the propeller will push the plane forward through the air, while the wheels are just a free rotating agent. Assuming no friction, the belt cant go a million miles an hour backwards, wheels would just spin fast.

As long as the plane stands still though, there cannot be a draft to generate vertical lift.

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Wooo you got it! =P

So I just posted a bunch of shit for nothing because the question is not about wether the plane will lift off if it"s relative speed is zero, but rather its a trick to say that the plane is still moving forward, creating air draft, and all that good shit that I posted about earlier.

Basically. =)

...so the conveyor belt isn"t supposed to match the horizontal velocity of the plane?

The landing gear is attached too, right?

I think I followed y"all"s line of thought earlier, but I"m still going to take a look at the math now.

You don"t get it, it"s not a scientific question. It"s a trick question, and the trick is: to make youthinkit"s scientific. It"s not about whether the plane will take off if the relative speed to you is zero. It"s about the plane moving ahead just like on a normal runway. Of course it will take off.

I guess it"s a good question, but not for interesting thought, but rather for the trick it plays. Sort of like that "How hot can your stove heat water to?" question. 100 degrees celcius, then it evaporates.

GaliemVaelant said:

...so the conveyor belt isn"t supposed to match the horizontal velocity of the plane?

The landing gear is attached too, right?

I think I followed y"all"s line of thought earlier, but I"m still going to take a look at the math now.

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The convayer belt matches the speed of the plane, but the plane is not connected to the belt, so the wheels just spin and it goes forward on its thrusters. Yes the landing gear is touching the belt, but again, its not connected, it just spins its wheels at whatever speed the belt is moving at + the speed of the plane. So when the plane is moving forward 100mph, and the belt is going backwards 100mph, the wheels are going 200 mph.

I think i see what your problem is though. What determines the speed of the plane? If its the wheels, then the plane cannot move forward, because the wheels would be spinning faster then the convayer belt, which is not allowed. So the plane must stay still.

I think you would have to judge the speed of the plane by how it does it in the air. Through the pressure thingy, which I have no idea how it works. So that way, it will register wind speeds of 100mph, (assuming no natural wind, only the movement of the plane wind) so the belt will move 100mph backwards, and the wheels will now be moving at 200mph. But the wheels dont count, so the rule still holds.

No, I get that the wheels are not the source of the plane"s thrust.

Source or not, if the plane accelerates, so do the wheels. Take a toy car, and push it.

It is absolutely impossible that the conveyor belt would not hold the plane still if it matches the tangent acceleration. I"ve *just* finished an exam on rotations, from rigid body dynamics, to physical pendulums. What you are describing is not physically possible.

The wheels always count, unless they are not attached to the plane.




Now, if there were no wheels, but rather if the plane just sat belly down on the conveyor belt, and was somehow kept from rolling over, then exactly what you describe is what would occur.


This would be akin to a hydroplane taking off from the surface of water.

edit: This would also occur if the wheels did not spin, which goes back to the idea that a plane"s breaks can not withstand that much force.

I think i see what your problem is though. What determines the speed of the plane? If its the wheels, then the plane cannot move forward, because the wheels would be spinning faster then the convayer belt, which is not allowed. So the plane must stay still.

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Indeed, but for the question to have it"s intended effect you must assume airspeed.

GaliemVaelant said:

No, I get that the wheels are not the source of the plane"s thrust.

Source or not, if the plane accelerates, so do the wheels. Take a toy car, and push it.

It is absolutely impossible that the conveyor belt would not hold the plane still if it matches the tangent acceleration. I"ve *just* finished an exam on rotations, from rigid body dynamics, to physical pendulums. What you are describing is not physically possible.

The wheels always count, unless they are not attached to the plane.

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I dont understand why you think the wheels matter. Al that will happen is they will spin faster. For the plane to have 0 forward motion, something must be acting against the planes forward momentum. In this case, you say its the wheels. But how can the wheels, which are spinning, offer equal resistance to the jets/propellers on the plane to keep the planes velocity at 0?

If you have a toy car close by, grab it. Put it on a peice of paper. Push the toy, and pull the paper. Then, push the toy a little faster, and pull the paper in the opposite direction real fast. Did the car go backwards? No, it still went forwards, just the wheelsl spun faster. Thats all thats happening.

Zuuljin said:

If you have a toy car close by, grab it. Put it on a peice of paper. Push the toy, and pull the paper. Then, push the toy a little faster, and pull the paper in the opposite direction real fast. Did the car go backwards? No, it still went forwards, just the wheelsl spun faster. Thats all thats happening.

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When I manage to pull the paper with the same force with which I push the car, the car does not move.

Yes, but the force of the plane going forward is about a million times stronger then the force of the wheels pulling it back. The onlything holding it back is friction. And if the tires are spinning, that = less friction.

Zuuljin said:

Yes, but the force of the plane going forward is about a million times stronger then the force of the wheels pulling it back. The onlything holding it back is friction. And if the tires are spinning, that = less friction.

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Negative. The forward momentum of anything on wheels is transfered to the wheels as angular momentum, regardless of where the push comes from.

edit: Physicists learn about systems and motion using ideal systems, that is, systems with no friction and often times no mass to prove that universal laws are absolute. This holds even when there is absolutely no friction (which never occurs in reality, even on the surface of an object moving through the vacuum of space).

If the conveyor belt matches the movement of the wheels, then the plane can not move.

There is one other thing that would allow for it, and that is if there were an initial lift on the plane, provided by nothing more than the movement of the air due to the props or jets.

If this were the case, then the conveyor belt would match the velocity of the wheels, but it could not match the angular momentum of the wheels. The reason for this is that the plane"s entire mass would not be resting on the wheels. Some would rest on the wings, and in turn, on the air.

This is why I initially said that it would lift off.

If the folks here who say that there is no lift without forward momentum are correct, then the plane does not move.

This is a cool mental experiment, and I appreciate it, so I certainly won"t argue with you. I can say that the only way that you could convince me of what you are saying is to prove it, either mathematically or physically.


If it weren"t for the principles behind my saying this, the conveyor belt wouldn"t be able to move. As a matter of fact, your car wouldn"t run because the belts in it would spin freely with no grip, and likewise the plane itself would be nothing more than a pile of scrap metal.

The same goes for every single other machine that employs any mechanism of any kind that depends on the transference of angular momentum as an operational principle.


edit: If there is a video out there of the plane taking off, then there can only be two possible reasons for why it would happen:

1) The conveyor belt did not match the momentum of the wheels.

2) The folks who say that planes have no vertical acceleration without forward momentum are wrong.

Well i dont know physics well enough to prove it. But somewhere you are incorrect. If it were a car, then yes it would stay still because the car is propelled by the wheels. The belt is propelled by the rotating of its gears. The plane however is NOT propelled by its wheels, they simply spin.

What exactly is slowing the plane down then? Angular momentum?

Fromt he first explanation:
"Manfred relaxes a bit because the conveyor cannot stop him from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it"s under 40 pounds, and the engine has overcome that for years; plus the drag doesn"t increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates."

Yes, but the wheels support the plane"s mass, and are therefore attached to the plane"s center of mass. If the plane moves, the wheels move with equal momentum. If the wheels move, the plane moves with equal momentum.

Yes, this is the same case with the car. No, the car"s wheels do not provide its thrust. An axle connected to the wheels do it.

In the same way, whatever part of the plane is connected to the wheels cause the things I describe to happen.


Also, nothing is slowing the plane down, relative to the conveyor belt. As a matter of fact, relative to the belt, the plane has twice the momentum. The plane is still accelerating relative to the belt.

The thing is that, when wheels hit something and make it go (like in a car), or when something goes and makes the wheels spin (like a shopping cart), the reason that the object moves is because the wheels apply a force to the surface they"re on, and that surface in turn returns the force to the object"s center of mass.

In the case of the plane, the force applied to the wheels is taken with no returned force. The conveyor belt spins, and does not return the force to the plane as per Newton"s Third Law. This is not to say that there is no reactive force, but just that the reactive force is not returned to the plane"s center of mass.

Most likely, since the plane"s forward momentum triggers the belt"s movement, the reactive force is sent to a mechanism that drives the belt"s reactive acceleration.

edit:
I should add that you would be correct also if the belt only spun under whatever force the wheels apply to it. This would be exactly like the tires of a car spinning out before it goes. That"s not the case with this problem, however, as the belt is self-powered, and it matches the momentum of the plane.


edit2:

Also, the jets do not alone provide the basis of movement for a plane, independantly of the wheels, until it is actually in the air, because it starts off with its entire mass resting on the wheels.

A plane on a belt and a car on a belt are 2 completely different things. This may be where your getting mixed up. The wheels of the car cause the car to move. The wheels of the plane do not cause the plane to move. You state yourself that the force that the wheels are pushing on is not returned to the mass, so then how does it affect the acceleration of the plane?

Also, the forward momentum of the plane does NOT cause the belt to move. The wheels are not pushing off of the belt to make the plane move. There is a subtle difference there. The wheels are there to support the plane so it doesnt crash. Thats it.

I thought of a good example. Lets say you have a bike riding down vertical wall. Are you saying that if the wall starts accelerating up, at the speed of gravity, that the bike will stop falling?

In response to edit#2, the planes do in fact do 100% of the acceleration. The wheels do nothing but keep the plane up.

Its late and I think im headed off to bed lol. When you get the answer you let me know. Was fun. =)

You"re thinking three dimensionally, not four dimensionally.

I"ll use a shopping cart as an example. When you push a shopping cart, you are what is moving the cart, right? Wrong.

You push on the cart, and the wheels turn with a momentum equal to the mass of the cart times the velocity with which it moves.

The point of the wheel that touches the ground pushes against the ground, transfering that momentum. This occurs through a force equal to the mass of the cart times the tangent acceleration of the wheel.

The ground, as per Newton"s Third Law, pushes back. It doesn"t push back on the wheel. It pushes back on the center of the cart"s mass, thus moving the cart.

Thus, you are not moving the cart. The Earth is.

heh There is no spoon.

The plane"s drive works the same way through the wheels of its landing gear, until it is in the air. Once it is airborn, only then does the force driving its center of mass come from the air.

Have a good night, man.

I thought of a good example. Lets say you have a bike riding down vertical wall. Are you saying that if the wall starts accelerating up, at the speed of gravity, that the bike will stop falling?

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That example is completely unrelated.. One of the most important facts of the airplane thing is that it is completely horizontal to the ground (gravity always goes the same direction on earth, straight down, perpendicular to the plane/conveyor belt. Other forces, however, are relative to the situation).

A bike "riding down a vertical wall" would be pushed away from wall due to natural force.


These are my own conclusions and should not be considered as fact