Can the Plane Take-Off?

[EricH]

Originally Posted by greencactus3

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

I was thinking outside the box. And if the conveyor belt isn't supported well enough, it will move--downwards--until it can't stretch anymore--and then it won't move anymore.

If someone has a powered model airplane with landing gear (improvised or included), they should put it on a treadmill (in a big open area), run both, take a video, and post it here. I think that's the only sure way to answer the question.

[Mike Schroeder]

That wont work, you will never ever get the type of precision that you would need to get a computer to say okay plane is moving 60mph make conveyor move 60mph even a nano-second delay means that the plane will move forward, and with a treadmill that makes things that much more difficult because there is no way to get close to the syncronization needed for this to work

[Ryan F.]

Originally Posted by KenWittlief

in fact, when you are flying it is impossible to measure your ground speed, unless you know the wind speed and direction. Aircraft do not have speedometers attached to their wheels :^)

In jumps a beautiful aircraft device we like to call theDME .

Watches your change in distance from a VOR point, and gives you your airspeed. It only works when the pilot is flying directly too or from the VOR (though that is fairly common).

[KenWittlief]

I guess I should have been more specific. In an aircraft its impossible to measure your ground speed directly. GPS also reports ground speed, but that is measured relative to satellites in orbit, with known positions and velocities - its not measured relative to the ground. If the satellites fail (or if your VOR points on the ground fail) then the aircraft cannot know its ground speed.

There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.

By contrast, a car measure ground speed directly, by the rotation of its wheels.

[KenWittlief]

Originally Posted by EricH

If someone has a powered model airplane with landing gear (improvised or included), they should put it on a treadmill (in a big open area), run both, take a video, and post it here. I think that's the only sure way to answer the question.

It would be an interesting test. With the plane at a fixed throttle position you could measure the airspeed of the plane, with the treadmill still, and with it running, and see what effect it has on the planes airspeed.

It doesnt match the original problem/question, but you shoud be able to calculate the force the moving threadmill is applying to the plane (by the amount its airspeed is reduced) and extrapolate from there.

I think the answer you will get will be the same we have been saying since post #2 on page 1. The plane will take slightly longer to lift off than normal.

[Cody Carey]

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.

It wouldn't take off and go FORWARD, sorry for the muddy-ness


EDIT: foreward relative to the ground, not the surrounding air, Yet more muddyness, sorry.

[Cuog]

this has been going on for a while since my last post but i will venture to hypothesize that the treadmill's speed makes almost no difference to the way the plan behaves, as long as it is still pointed so that the wheels can freespin.

Now i will attempt to explain myself again since i am so terrible at explanations:

The planes engines while create thrust pushing on the AIR the AIR will then follow Newton's third law and push the plane FORWARD

Now the wheels in contact with the ground will spin at whatever speed they need to because the ground is just there, when the ground accelerates BACKWARD the wheels will spin faster and faster until the plane has TAKEN OFF then, the wheels will slow to a stop in the AIR,

OK i think i got it this time,
Cuog

[Elgin Clock]

I sent this thread to a team 237 Mentor who is a Aeronautical Engineer by Degree, and this is what he sent me back.

It's a doozy.

OK, so you have a lot of people not understanding the question or the answer and inferring too many of their own thoughts and experiences into it.
Because the question does not say specifically whether the plane's speed (that the conveyor is matching) is relative to the ground or to the air, you could argue both sides.
I believe the intent of the question is to imply that the plane never makes ground speed because the conveyor inhibits it. If this is the case, the plane will not fly since it sees no air movement over the wings (think of a glider being towed on the same conveyor- it cannot create air movement over
the wings as the conveyor matches the forward speed of the towing vehicle).
To further muddy the waters: Add Boundary Layer theory to the
equation.....
The speed of the air that the conveyor sees at it's intersection with
the air (basically the surface of the treadmill) is different than the
speed of the air the plane's wing sees (a few feet off the ground). This is
because relative air speed is slower the closer you get to the ground. If taken
to the extreme, there is no air movement at the physical intersection of
the ground and air regardless of the wind speed measured any distance away
from the surface, mainly due to frictional forces.
Now pull that friction into the equation..........
The frictional forces (acting on the air from the surface of the treadmill) will eventually begin to create air movement in the boundary layer. The boundary between still air and moving air will be higher off the surface of the treadmill as it's speed increases until eventually the plane's wing experiences air movement. Again taken to the extreme, once the air movement (call it wind) created by the friction between the conveyor & air, reaches the speed at which the wing will create enough lift, the plane will "hover" over the treadmill.
Of course, once the wheels are not in contact with the treadmill won't
the treadmill stop? Which begs the question: Does the Plane then
immediately fall to the ground and the treadmill start again?

I'll leave the theory of lift to another day (but I did see some discussion of it in the thread you showed me). I can only point out that "lift" is a misnomer. Wings do not lift a plane off the ground. They cause a condition ("adverse pressure gradient") where they are sucked off the ground.
Therefore, in future posts all references to "lift" should be changed to "suck".

Just kidding. Now you know why I don't post on CD or our forum. I tend to ramble.

Sorry about the bad formatting of the text. I tried to fix it up as much as possible.

Elgin

[KenWittlief]

thats cool! Even an aerospace engineer was confused by the question.

its really a trick question. If you think the airplane will stay in one place, because the treadmill is 'matching its speed' - thats impossble.

If the plane is staying in one place then its speed is zero. If the planes speed is zero then the treadmill speed is zero. So whats stopping the plane from taking off? where does the force come from?

The only way the treadmill CAN move is if the plane is moving - and if the plane IS moving the treadmill is not holding it in one place.

The only logical (possible) answer is that the plane takes off normally, with the wheels spinning twice as fast. Any other model of the systems behavior is self-contradictory, and therefore cannot possibly happen.

[Dylan]

Ok, here's my two cents:

The treadmill control is measuring the plane's speed relative to the rest of the world (stationary objects like trees), right? so when the plane begins to move, at say, 1 MPH, the treadmill will move in the opposite direction at 1 MPH. As stated above, a plane is propelled by it's engines reacting with the air. The wheels are completely free-rolling, except for a braking system. Therefore, except for a little increased drag from the wheels spinning twice as fast as normal, the treadmill would have no effect on the plane.
My conclusion: the plane might have to use a few extra HP to compensate for the extra drag from the wheels, but it would definitely take off.

[dlavery]

Originally Posted by KenWittlief

There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.

By contrast, a car measure ground speed directly, by the rotation of its wheels.

Not to make too fine a point of it, but that is incorrect. A car measures the rotational rate of the wheels and from that determines the surface velocity of the tires. It does not measure ground speed, ever. We rely unpon the presumption that as long as the tires are firmly in contact with the ground, the tire surface velocity and the true ground speed of the vehicle will usualy be the same. For most cases, this is a safe presumption, but it is not always true. E.g. drive slowly on a slick road and hit the gas hard. The wheels spin, the speedometer will slam the needle around to the right, but your actual ground speed will remain unchanged.

The distinction becomes important if we try to put details on the original problem statement. Depnding on how the "plane speed" is determined (e.g. speed relative to the ground as determined by a ground-based observer, speed relative to the belt as determine by a belt-based observer, speed relative to the belt as determined by an airplane-based observer, speed relative to the ground as determined by an airplane-based observer, etc etc etc), you are going to come up with a different answer to the question.

-dave

[KenWittlief]

Originally Posted by dlavery

...
The distinction becomes important if we try to put details on the original problem statement. Depnding on how the "plane speed" is determined (e.g. speed relative to the ground as determined by a ground-based observer, speed relative to the belt as determine by a belt-based observer, speed relative to the belt as determined by an airplane-based observer, speed relative to the ground as determined by an airplane-based observer, etc etc etc), you are going to come up with a different answer to the question.

-dave

but there is no equipment on any aircraft that measures ground speed through the tires - they dont use the same speedometers that a car uses.

so if you ADD one to an aircraft for the purpose of this experiment, then you are altering the way that aircraft measure their ground speed, and you are rigging the test to get the answer you want.

[EricH]

Apologies for resurrecting this ancient thread, but THIS JUST IN:

The Mythbusters will be going for this one next week at 10 E/P!

[Molten]

On wednesday? I am going to see that.

[Pat McCarthy]

Originally Posted by EricH

Apologies for resurrecting this ancient thread, but THIS JUST IN:

The Mythbusters will be going for this one next week at 10 E/P!

Isn't Mythbusters usually on at 9 PM Eastern? I saw the ad, and thought of this thread, but I didn't notice the different time.