Ponder this one


#1

Read the question and really think about it before you answer.

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

The question is:

Will the plane take off or not?


#2

it wouldnt move cuz its staying in one place and there is no wind flow for it to catch to be lifted off the ground


#3

Re: RE: Ponder this one

What he said. It’s all about lift, and a stationary wing doesn’t make much.


#4

it is possible. the plane had to get on the runway, it can get off :banana :booze :smiley:


#5

It will take off. Have the plane fly full throttle then shut all engines off causing the plane to be thrown backwards therefore taking off backwards.

:dunno What? You never said which direction.


#6

yes


#7

Re: RE: Ponder this one

[quote=StangMang]It will take off. Have the plane fly full throttle then shut all engines off causing the plane to be thrown backwards therefore taking off backwards.

:dunno What? You never said which direction.[/quote]

wings are generally meant to work in one direction…

and no, it wont take off. theres the same thead on MM and i think its been discussed to death there already :smiley:


#8

Yes the plane will fly.

Consider the following:
A plane is powered by jet propulsion or by propeller not by the wheels, a car is, so a car with wings would just sit there and never gain foward movement. But, as the plane pushes the air behind it, it will gain foward momentum regardless of what speed the belt or wheels are doing beneath it. This will cause the foward speed of the plane increases until lift is achieved. The speed of the belt driven runway would increase exponentially until the plane left the surface trying to match the speed of the unfettered plane.

A plane only requires air speed to maintain lift and regardless of the relative speed of the motorized runway beneath it, it will reach that designated air speed via rear thrust as long as there are no other forces holding it in place (ie. cable or chain)


#9

:agree


#10

Re: RE: Ponder this one

[quote=RecklessTim]Yes the plane will fly.

Consider the following:
A plane is powered by jet propulsion or by propeller not by the wheels, a car is, so a car with wings would just sit there and never gain foward movement. But, as the plane pushes the air behind it, it will gain foward momentum regardless of what speed the belt or wheels are doing beneath it. This will cause the foward speed of the plane increases until lift is achieved. The speed of the belt driven runway would increase exponentially until the plane left the surface trying to match the speed of the unfettered plane.

A plane only requires air speed to maintain lift and regardless of the relative speed of the motorized runway beneath it, it will reach that designated air speed via rear thrust as long as there are no other forces holding it in place (ie. cable or chain)[/quote]

be honest, did you just copy and paste that :rofl


#11

interesting point tim. i dont think its that cut and dry though. i think you opened up a whole new issue though.

this brings in the issue of the friction of the bearings in the wheels ( i am assuming the friction against the ground is very high, since they are rubber tires anyhow).

imagine two cases:

  1. the wheels are really loose and oiled up, and when the treadmill thing starts moving, the wheels just turn under the plain but it doesnt actually move.

  2. the wheels have more resistance than that, and therefore the entire plain starts moiving backwards on the treadmill.

in each case, if the engines come on,

  1. here the plain is sitting still so it can just start forward when the jets come up, assuming the wheel bearings can take the extra stress (since if the plane is miving at speed X and the mill is going backwards at speed Y, the wheels see an effective speed of X+Y) and assuming this isnt an issue then the plane can go as normal.

  2. here, if the plane is already movingbackwards, it will take more power than usual since the engines first have to get the plane to ‘0’ speed, and then more so to get it going forward.

i may be overanalyzing this, but the bottom line is i think tim has a valid point that none of us really mentioned


#12

Re: RE: Ponder this one

[quote=artisan00]interesting point tim. i dont think its that cut and dry though. i think you opened up a whole new issue though.

this brings in the issue of the friction of the bearings in the wheels ( i am assuming the friction against the ground is very high, since they are rubber tires anyhow).

imagine two cases:

  1. the wheels are really loose and oiled up, and when the treadmill thing starts moving, the wheels just turn under the plain but it doesnt actually move.

  2. the wheels have more resistance than that, and therefore the entire plain starts moiving backwards on the treadmill.

in each case, if the engines come on,

  1. here the plain is sitting still so it can just start forward when the jets come up, assuming the wheel bearings can take the extra stress (since if the plane is miving at speed X and the mill is going backwards at speed Y, the wheels see an effective speed of X+Y) and assuming this isnt an issue then the plane can go as normal.

  2. here, if the plane is already movingbackwards, it will take more power than usual since the engines first have to get the plane to ‘0’ speed, and then more so to get it going forward.

i may be overanalyzing this, but the bottom line is i think tim has a valid point that none of us really mentioned[/quote]

yes, you are overanalyzing it. There is no way in hell the wheels couldn’t be “oiled up” enough to cause such a drag that the thousands of horspower turbine engines couldn;t over power them


#13

You are right about the friction on the wheels, but it would have to be a hell of a lot of friction to slow down some of these larger planes.

I mean, if you want to get real crazy here… That would be one very large belt and it will pull the air along it creating a very fast wind low to the ground. And as everyone knows… a plane derives lift by the parabolic shape of the wing. The air travels across the top of the wing faster than the underside of the wing creating low pressure above the wing generating lift. So if the moving belt on the runway is too rough and drags enough air along with it to create enough wind low (relative to the plane) and the air is moving across the bottom of the wing faster than over the wing, it will pin the plane to the ground with no chance of it ever taking off.


#14

This is not a logic problem. This is a simple physics problem, once you understand the forces at work here (pun definately intended).

First of all, we need to conisider the problem… lets assume we’re talking about a Jet, though a plane with prop would yield the same results:

  1. Jet sits on a conveyer belt
  2. The conveyer belt will match the jet wheel speed at all times
  3. Will it fly?

Lets look at item #2… I drew a diagram.

Friction… friction occurs when there is slipping between two points. Lets look at the point where the wheel touches the conveyer. The point where the wheel touches the conveyer and the conveyer both have the same velocity and in the same direction. This means that there is no slipping between them, and thus, there are no friction forces that eminate from the wheel/conveyer interation.

http://www.mustangmods.com/data/18033/airplane.jpg

So, this illustrates that because the conveyer matches the speed of the plane’s wheels, there is no friction force eminating from the wheel/conveyer interaction.

This right here is the key to understanding this solution. Re-read if necessary.

If you’ve managed to understand the friction conclusion from above… we move on.

So now we look at what forces are left:

  1. Thrust force from the jet engines, T
  2. Gravity force (mass*g), N

When force T is applied to the plane, it will accelerate forward at a rate of accel = T/mass.

Now, here is where the understanding of friction forces from above come into play…

So, yes… when the plane accelerates forward, the conveyer accelerates also to match the wheel speed. BUT, since the wheel/conveyer interaction yields NO FORCES, there is nothing to counteract the “acceleration = T/mass” of the plane.

So, if the only acceleration is due to thrust, T, then the plane will move forward and take off.

http://www.mustangmods.com/data/18033/airplane2.jpg

We can ignore the Normal force due to gravity because that’s beyond the scope of the question.

So, in conclusion, the plane will take off


#15

im glad you wasted all that time to explain it when it was common sense! :rofl


#16

Re: RE: Ponder this one

Copy and past does wonders :slight_smile:


#17

haha, i was hoping that was the case


#18

maybe a future episode of mythbusters this will be on it. :dunno


#19

Not all of that is common sense, OUZ. Not everybody has a degree in uh… whatever that’s about. lol


#20

Well I have to disagree with PhillyBlunt474 on the friction part.

According to Newton’s first law (inertia: an object in motion will stay in motion unless acted upon by and outside force)
If a wheel touching the ground had no friction, then you could start a car rolling 10 mph on a flat plain and it will continue to roll at 10mph until the friction from the air stopped it. But this isn’t true, rubber wheels contacting the ground create a lot of friction. There are two main types of friction concerning your tires, Static friction and Rolling resistance. Static friction is the friction between the tire and the road, without it, the tire would just slide across the road and never turn when pushed. Rolling resistance is the friction of the sidewall compression, and tread compression of the tire. In your car “rolling resistance” accounts for about 15% of the total stopping resistance. (overcoming inertia is responsible for about 35% of the vehicle’s resistance. Driveline friction is about 45%; air drag is about 5%) That my boy is a hell of a lot of resistance (friction) there. And it gets greater as the load increases (ie. a jet)

Now back to the original problem of the moving runway. The faster the plane moves, the faster the belt moves to compensate. This would create speeds on the tires much greater than the tires and wheel bearings were designed to handle. If they didn’t fail under the extreme temperatures of heat generated by the friction of the rotating tires, they would at least put a hell of a strain on the foward momentum of the plane.

I still believe the plane would lift off once it accelerated enough, but I think it will take much more power than normal to reach that speed and you may end up without functioning tires to land with.