Can the Plane Take-Off?

[Andrew Blair]

Originally Posted by Elgin Clock

Ok.. I'm not gonna give in on my answer, but I see the way it could potentially work now.. Still kind of mind boggling.

So.. this leads to the real world applications and possibilities:

Why not build this huge conveyer belt system as an alternative to making short runways on a mountain where real estate is a prime commodity???

Think of the potential a system like that could have if it worked.
This way larger planes could take off in a small space, and cargo can be brought in by air to these reletively remote places.

Uh oh though.. will this same problem work in reverse, yet be to my thought advantage this time, and make the plane land and stay still so as not to leave the area of the huge conveyer belt and still use that same small real estate???

Or will the speeds (plane and treadmill type device) have to be non equal and perfectly harmonized for this to work, and this is just a pipe dream???

Yeah..

-------> ( Me - "Outside of box")
BOX

As Ken said, I'm pretty sure this would be the opposite of what you want. An airplane, unless you do something like manipulate the speed of the earth/atmosphere, needs to have a certain space of air to fly through to achieve a certain velocity through the air. So, the x-distance traveled, if the plane's speed is constant, can also only be constant. The conveyer would only be limiting the velocity of the plane ( not by much, but still). The only practical way to shorten takeoff is to increase the acceleration of the plane via a catapult device.

Or you could turn the atmosphere's rotation speed in relation to the plane...

[sanddrag]

One thing you could do (theoretically) to shorten a takeoff or landing distance is to create a nice strong headwind along the runway. The airplane would maintain enough of a speed difference with relation to the air to maintain lift, but you could theoretically have it with zero speed in relation to the ground. You could takeoff and land while not moving in relation to the ground. You could even do it going backwards in relation to the ground. (I've done this on my RC flight simulator program and it is great fun).

However, flying a plane in any sort of strong wind is tricky and it would be difficult to construct an apparatus to move such a large volume of air at such a high velocity to create this artificial headwind.

[greencactus3]

Originally Posted by artdutra04

Ken and all the others are correct. If you want proof, see this video clip. They mounted a fan on top of a skateboard, and as the skateboard began to gain velocity, the paper underneath the skateboard was pulled backwards at the same velocity in the other direction. The skateboard continued to accelerate, just as normal.

Here is another good way to think of this problem, as it was posted by an unregistered user on another forum:

hmm.. well although this wont mean anything, there is no proof that the paper was moving at the same speed... actually, it wasnt.

i have no idea how i should word this, but well...
you are all saying that the belt is moving at the same speed as the plane but the plane moves forwards. wheels or not, if Vplane=-Vground(in this case belt), Vplane+Vground=0. which means the plane does not move. ignore the wheels completely. the plane is on blocks attached to the plane.. the plane has unlimited thrutst so this does not affect anything.. (the belt also has unlimited thrust... or torque, if its a conventional conveyorbelt. so that also doesnt change anything)
so at first the plane is standing still. so Vplane=0.... which means Vbelt=0 also

now the throttle is pushed on the plane.
so Vplane relative to the air = 1
except since Vplane =1, Vbelt=-1... except the blocks and belt have enough friction ............................
or waaaaaaaaaait a second...... Friction is changed only by normal force...(since the coefficients dont change)....
Crap.. a BIG point everyone including me has failed to point out.... Force of Kinetic friction is always constant no matter how fast the two planes are rubbing right?darn.... so basically the wheel's bearings.... no matter if they are spinning at 1rpm or 287346821736498723649876rpm, the force of friction is the same right?... so the conveyor belt... no matter how fast it spins, it can not stop the plane...
aha. and no, noone mentioned that before, yes people have said that the friction was negligible but noone backed that up. i think im not the only one that was under the impression that the faster you......
or.... darn it. lost it.... anyone continue on that proof?
till then..

but yea. well i guess this shouldnt be called a physics problem cuz so much is ignored.... and yea. the plane does go up i guess. or does it..
all these people have examples of experiments.... even the video.. but noone has created a "true" conveyor belt... so those experiments are flawed. SO confusing... but this problem is a neverending pit of trying to prove the other wrong.

[ahecht]

Originally Posted by greencactus3

you are all saying that the belt is moving at the same speed as the plane but the plane moves forwards. wheels or not, if Vplane=-Vground(in this case belt), Vplane+Vground=0. which means the plane does not move.

Woah, I just realized that this problem is a whole lot more interesting than I originally thought.

We can all agree that the real question here is "does the plane move forwards?". There is a lot of talk about how planes are different from cars, but let's look closely at what would happen if we put a car on the conveyor. I will restate the problem thusly:

A car is standing on a street that can move (some sort of band conveyer). The car moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the car speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

The question is:

Will the car move or not?

I am going to argue that it does!

Imagine that you are in the car on the conveyor. You start the car moving forwards at 1mph, and the belt begins to move backwards at 1mph. However, when the belt reaches 1mph, your car is now essentially going at zero speed, so the belt stops. With the belt stopped, you are now going at 1mph, so the belt goes to 1mph, and so on. If you take the average speed of the belt and your car, you will see that each is half the time going 1mph and half the time stopped, so the average of each is .5mph.

Of course, you have to take acceleration into account here, since the belt cannot start and stop instantly. If you do, you will see that between the belt speeding up and the car slowing down as a result, you will soon reach the point where, as stated above, the belt's speed is 0.5mph and your car's speed is 1mph-0.5mph=0.5mph. That means that with everything working, you will always move forward at half the speed showing on your spedometer.

Therefore, my conclusion is that this is a poorly worded question since it fails to identify how speed is measured (speed of the wheel's rotation, speed of the plane relative to the ground, or speed of the plane relative to the belt).

Originally Posted by greencactus3

now the throttle is pushed on the plane.
so Vplane relative to the air = 1
except since Vplane =1, Vbelt=-1... except the blocks and belt have enough friction ............................
or waaaaaaaaaait a second...... Friction is changed only by normal force...(since the coefficients dont change)....
Crap.. a BIG point everyone including me has failed to point out.... Force of Kinetic friction is always constant no matter how fast the two planes are rubbing right?darn.... so basically the wheel's bearings.... no matter if they are spinning at 1rpm or 287346821736498723649876rpm, the force of friction is the same right?... so the conveyor belt... no matter how fast it spins, it can not stop the plane...

One of my pet peeves about physics education is that teachers make sweeping generalizations about things like friction, which people (myself included) take to be fact.

What you said about friction is only true for dry friction. However, when you have lubricated bearings, you must take viscosity into account. When you have "friction" against a fluid, the resisting force is actually a function of the speed you are moving through that fluid. This is why falling objects reach a terminal velocity: as the speed increases, the resisting force increases until it is equal to the force of gravity, and which point the object stops accelerating since the net force on it is zero. However, as Ken below me pointed out, this effect is very small compared to the rolling friction in the wheel itself.

[KenWittlief]

Originally Posted by greencactus3

Friction is changed only by normal force...(since the coefficients dont change)....
Crap.. a BIG point everyone including me has failed to point out.... Force of Kinetic friction is always constant no matter how fast the two planes are rubbing right?

aha. and no, noone mentioned that before, yes people have said that the friction was negligible but noone backed that up.

um... no. Kinetic friction being a constant has been brought up - go back and read the thread again

FROM PAGE 3: the other thing that is counter-intuitive in this problem is this: the rolling friction of wheels is independant of the speed at which they are rolling

you simply ignored what we have been saying until now, because you jumped to your conclusion first, then tried to backfill your reasoning. *

This is a common debug problem that engineers face everyday: we think we know the answer, but then when things dont work out we cant figure out why. Its always something you assumed to be correct, and very often its stareing you right in the face.

This plus the logical error that planes spin their wheels the same way cars do. If the plane is not moving forward relative to the earth, then what is making the wheels rotate? Nothing. If the wheels are not spinning, and the conveyer is not moving, then what is stopping the plane from moving? Nothing!

and BTW, the rolling friction of a wheel is not fluid friction. The bearings are oiled or greased but they roll over the races, they do not slide through a pool of oil or grease. The major component of rolling friction on a tire is the compression of the rubber. This is what causes a tire to heat up on straight and level pavement, the rubber is constantly flexing. But this amount of energy dissapation is small.

I have to say, this problem does demonstrate how difficult it can be to convey the laws of physics into a frame of reference that the average person can grasp. I was so temped to draw detailed vector force diagrams, but keeping this discussion in text made it more challenging.

* wanted to add: this is human nature. When we get an idea in our heads it becomes 'our idea' and for some reason we feel the need to defend it. Part of it is ego and part is pride. Its something we have to deal with as engineers all the time: jumping to a conclusion, then feeling like you have to stand behind it, no matter what.

In fact, this is a very interesting aspect of engineering. A Jeckel and Hyde situation. In the early stages of the engineering design cycle we try to figure out everything, and avoid mistakes at all costs. We dont want to design something that will have bugs.

But when you reach the point where something is fabricated, or manufactured, or code is written, then you WANT to find the errors - then errors are golden, you have to discover your errors and embrace them, discover the root cause of every mistake in the design.

The reason is, if you dont find the errors in your system (through testing and debug) then your customers will! Once you start shipping product or delivering systems its very expensive to recall them and make updates or corrections.

So at first mistakes and errors are a bad thing and you shun them, then they are golden and you must embrace them and understand them. Engineers very often cant make the jump to the second part. When errors or bugs show up in a product, some want to sweep them under the rug, or patch them up as quickly as possible, instead of understanding how they happened, how to fix them correctly, and how to keep them from happening on your next project.

[greencactus3]

Originally Posted by KenWittlief

um... no. Kinetic friction being a constant has been brought up - go back and read the thread again

whoops. sorry. mustve missed that. posts pile up so fast i cant keep up.

Originally Posted by KenWittlief

you simply ignored what we have been saying until now, because you jumped to your conclusion first, then tried to backfill your reasoning. *

This is a common debug problem that engineers face everyday: we think we know the answer, but then when things dont work out we cant figure out why. Its always something you assumed to be correct, and very often its stareing you right in the face.

wait a second. from the moment i posted this problem up i said i agree with both sides. i just kept trying to find ways to unbalance the arguements. kinda ran out of ideas tho,..

Originally Posted by KenWittlief

and BTW, the rolling friction of a wheel is not fluid friction.

good point. didnt notice that detail.

Originally Posted by KenWittlief

* wanted to add: this is human nature. When we get an idea in our heads it becomes 'our idea' and for some reason we feel the need to defend it. Part of it is ego and part is pride. Its something we have to deal with as engineers all the time: jumping to a conclusion, then feeling like you have to stand behind it, no matter what.

i totally agree. i just hope you arent directing that towards me.

[Justin Montois]

I tried to look at this as simply as possible. When a person is running on a tread mill, the only reason you dont move forward or backward is because you are moving at a constant speed with the treadmill. If the treadmill sped up, you would fly backwards. If you sped up, you would run into the control panel. But if the treadmill could read how fast you were increasing your speed, as you did it, then you would remain still. Because the speeds would be constant. And you say you need wind for an airplane to take off, if you were running on a treadmill at 10 mph, then you increased to 100mph, if the control system kept up with you, you wouldnt feel wind hitting your face because in essance, your not moving. therefore I feel the plane will not take off.

Granted people are saying that the enignes are producing thrust against air and therefore you dont need the wheels, yet in this case, the engine is producing X amount of thrust and is in turn causing movement, which, when pushed against something that is not moving(a paved runway) it causes motion which can be read in MPH. So the engine thrust is causing motion(MPH) So if the runway can match the MPH of the plane, no matter what air the engines are pushing against, the planes "movement" is still in MPH and that can be matched by this runway so the runway would cancel the thrust by the engine and the plane would remain still. Granted the engines would be in full throttle and the runway would by whipping underneath but the plane would be motionless.

When all esle fails, read my orignal paragraph

[sciguy125]

Originally Posted by 340x4xLife

So the engine thrust is causing motion(MPH) So if the runway can match the MPH of the plane, no matter what air the engines are pushing against, the planes "movement" is still in MPH and that can be matched by this runway so the runway would cancel the thrust by the engine and the plane would remain still.

What you're missing is that thrust is a force, not a velocity. Go back to Newton's 2nd Law: F=ma. Velocity is the result of a force acting on a mass. Actually, I think that if thrust was a velocity, it still wouldn't matter.

Let's assume an ideal case: infinite traction between the wheels and the tredmill, 0 friction between the wheels and the axles (through the bearings). In this situation, how does the tredmill stop the plane from moving?

Now let's assume a practical case: some finite but large amount of traction, some very small but non-zero friction in the bearings. What effect does this have on the plane's movement?

[Justin Montois]

Originally Posted by sciguy125

What you're missing is that thrust is a force, not a velocity. Go back to Newton's 2nd Law: F=ma. Velocity is the result of a force acting on a mass. Actually, I think that if thrust was a velocity, it still wouldn't matter.

Let's assume an ideal case: infinite traction between the wheels and the tredmill, 0 friction between the wheels and the axles (through the bearings). In this situation, how does the tredmill stop the plane from moving?

Now let's assume a practical case: some finite but large amount of traction, some very small but non-zero friction in the bearings. What effect does this have on the plane's movement?

Now I see what your saying. The wheels have no connection with the engine so if they are forced backwards,theres not enough friction reated to act against the power of the engine. I think this should have a poll attatched with it. Be intersting to see where everyone stands

The Debate Rages on

[Cody Carey]

The plane would most certainly take off.
What are we not getting here?
If the plane was in a wind tunnel, then it would not take off, but since it is on the ground, and the wheels aren't powered, it will.Thats for the same reason that a car can drive into a stiff wind, because while the wind is pressing against the car with force, its not enough to overcome the power of the wheels.

Think of a toy car on a treadmill, the treadmill could be going 100mph, and if you were holding the car on the treadmill, the wheels would be spinnng at that speed, but the car would be still. Now imagine that you pushed the car foward on the treadmill at 5mph. The body of the car would be moving foward at 5mph, and the wheels would be moving at 105mph. The planes engines would be like your hand, not efected by the treadmill because it's not touching the treadmill.

If that didn't help you visualize it, I'm sorry, that was the best I could do and I tried

[Ian Curtis]

Originally Posted by Cody C

The plane would most certainly take off.
What are we not getting here?
If the plane was in a wind tunnel, then it would not take off, but since it is on the ground, and the wheels aren't powered, it will.

Well I don't want to muddy this debate with my lack of appropriate formulas, but I think the plane would still take off in a wind tunnel.

A plane is not a big brick wall, it is aerodynamic to reduce wind friction. Therefore in a wind tunnel it would still take off if the wind was blowing at the same speed as the plane was moving, because the wind is slipping around the plane, not pressing against it.

[greencactus3]

Originally Posted by iCurtis

Well I don't want to muddy this debate with my lack of appropriate formulas, but I think the plane would still take off in a wind tunnel.

A plane is not a big brick wall, it is aerodynamic to reduce wind friction. Therefore in a wind tunnel it would still take off if the wind was blowing at the same speed as the plane was moving, because the wind is slipping around the plane, not pressing against it.

yes. in a windtunnel it will take off. technically not because the wind is "slipping" around the plane but because the plane's aerodynamics do not choose how fast the plane is moving to the ground but only with how fast the air over and under the wings are flowing.

[Arkorobotics]

NO! This is not possible because the speed of the trake and wheels (which usually are not powered, it's the engine that pulls the plane if I am not wrong. But lets go with the example) will cancel out and the airplane will stay in place. Airplanes have airfoils, where high pressure builds on the bottom and low pressure on the top as air passes by. This creates lift, but how can the airplane lift if there is no air moving across the wings? The answer is it can't! In this sitution it can't.

[EricH]

As many people have said, there is no connection between engines and wheels in an airplane. The treadmill could be going backward at lightspeed, but if that plane's engine(s) is (are) going full blast forwards, or close to it, then the plane will move forward, thus getting the lift it needs. Wheel drag will not be enough to stop the plane from taking off unless the pilot is standing on the brakes, in which case he wouldn't take off anyway.

Now, people have used an F-18 as an example, but I'll try a Harrier for an example. Harrier won't go forward at all, but it will take off in its VTOL manner (that's Vertical TakeOff and Landing) and take off. No question about that, right? (Yeah, I'm cheating a bit, but if it's any airplane, I'll just go with a VTOL and take off without the debate. )

[greencactus3]

Originally Posted by EricH

Now, people have used an F-18 as an example, but I'll try a Harrier for an example. Harrier won't go forward at all, but it will take off in its VTOL manner (that's Vertical TakeOff and Landing) and take off. No question about that, right? (Yeah, I'm cheating a bit, but if it's any airplane, I'll just go with a VTOL and take off without the debate. )

The plane moves in one direction, while the conveyer moves in the opposite direction.

vtol is not allowed. the plane's thrust is angled down so the conveyor belt can't move in the opposite direction anymore. illegal move