Brain Teaser

Randy · Dec 14, 2005

I've seen this discussed on other forums, but, don't believe I've seen it here yet.

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's wheel speed and tunes the speed of the conveyer to be exactly the same in opposite direction.

The question is:

Will the plane take off or not?

radman · Dec 14, 2005

Yes. It would just have some incredible wheel speeds when it did. The thrust of the engines is unrelated to wheel speed.

sparky3008 · Dec 14, 2005

Yes. It would just have some incredible wheel speeds when it did. The thrust of the engines is unrelated to wheel speed.

While this may be true since the plane is essentially remaining still do the wing create lift since there would be no air movement under the wings?

Just Plain Dave · Dec 14, 2005

OK, I'll take a shot at it. I think that if the runway is wide enough and things moved fast enough, the runway moving at an extremely fast speed would create a sufficient force of wind for the plane to leave the ground, but it wouldn't get very high.

sparky3008 · Dec 14, 2005

OK, I'll take a shot at it. I think that if the runway is wide enough and things moved fast enough, the runway moving at an extremely fast speed would create sufficient air force for the plane to leave the ground, but it wouldn't get very high.

Ok so if I understand the movement of the runway in the opposite direction is creating a current? Hummm

radman · Dec 14, 2005

I repeat-thrust has nothing to do with the wheels. They are independent of each other. The thrust moves the plane regardless of what the wheels are doing-the engines are not driving the plane via the wheels.

ionbeam · Dec 14, 2005

the plane is essentially remaining still do the wing create lift since there would be no air movement under the wings

No take off because as Randy pointed out, there needs to be air movement over the airfoil for lift.

[Assume small single engine aircraft.] If an airplane were standing still, without the wheels turning and you blew 110 mph air over the wings the airplane would take off (leave the ground) with zero forward speed. Even with the engine off. :blink: :wacko: True!

--Technically, with the engine off, the airplane would actually start moving backwards at an increasing rate until lift stalled due to drag.--

Alan

sparky3008 · Dec 14, 2005

Bernoulli principle?

sparky3008 · Dec 14, 2005

the plane is essentially remaining still do the wing create lift since there would be no air movement under the wings

Click to expand...

No take off because as Randy pointed out, there needs to be air movement over the airfoil for lift.

[Assume small single engine aircraft.] If an airplane were standing still, without the wheels turning and you blew 110 mph air over the wings the airplane would take off (leave the ground) with zero forward speed. Even with the engine off. :blink: :wacko: True!

Alan

Actually I pointed it out Randy asked the question...

sparky3008 · Dec 14, 2005

I repeat-thrust has nothing to do with the wheels. They are independent of each other. The thrust moves the plane regardless of what the wheels are doing-the engines are not driving the plane via the wheels.

While the engine and wheels are independent of each other (not connected by a belt, chain, shaft etc) The engine creates thrust to move the plane forward with its end result to be air flow under & over the wings for lift.

If the conveyor is preventing the plane from any forward movement then no airflow no lift...

Toecutter · Dec 14, 2005

A plane would take off (maybe with a burned wheel bearing or three), but an FJR would just sit in one spot. The question is, would it still hurt just as much to fall off? :haha:

radman · Dec 14, 2005

I quit. :blink: You guys just aren't gettin it. Classic teaser though, and it's working.

sparky3008 · Dec 14, 2005

I quit. :blink: You guys just aren't gettin it. Classic teaser though, and it's working.

maybe you could just say it differently?

ionbeam · Dec 14, 2005

One true thing --

I said:

as Randy pointed out

To which Sparky3008 replied:

Actually I pointed it out Randy asked the question...

Sorry Sparky! :blushing:

Alan

ionbeam · Dec 14, 2005

Reading the original post a little closer --

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

Well, damn, if the plane [SIZE=14pt]*is moving*[/SIZE] and it is moving fast enough to have airflow that will cause lift the plane WILL take off.

:stunned:

radman · Dec 14, 2005

I quit. :blink: You guys just aren't gettin it. Classic teaser though, and it's working.

Click to expand...

maybe you could just say it differently?

The engines are moving the fuselage. The fuselage doesn't care what the wheels are doing, they just prevent intimate contact with the runway. When an aircraft lands, the wheels are static until they contact the runway, instantly coming up to speed-hence the puffs of smoke upon landing. If the runway were moving opposite the planes direction at a trillion miles per hour, the wheels would just have to match that much higher speed. It would not affect the speed of the aircraft itself. Another teaser is what would happen if, while driving 100mph, you were to drive up the ramp of a car transporter ahead of you doing 50, a popular movie scene?

Just Plain Dave · Dec 14, 2005

I repeat-thrust has nothing to do with the wheels. They are independent of each other. The thrust moves the plane regardless of what the wheels are doing-the engines are not driving the plane via the wheels.

Thrust will not get a plane off the ground. The only thing that can raise a plane is lift. Something must create lift or that plane ain't going nowhere. (Yeah, I know, double negative.) If thrust was all that is required, I saw a huge truck on TV the other night that would be in the clouds.

sparky3008 · Dec 14, 2005

I quit. :blink: You guys just aren't gettin it. Classic teaser though, and it's working.

Click to expand...

maybe you could just say it differently?

Click to expand...

The engines are moving the fuselage. The fuselage doesn't care what the wheels are doing, they just prevent intimate contact with the runway. When an aircraft lands, the wheels are static until they contact the runway, instantly coming up to speed-hence the puffs of smoke upon landing. If the runway were moving opposite the planes direction at a trillion miles per hour, the wheels would just have to match that much higher speed. It would not affect the speed of the aircraft itself. Another teaser is what would happen if, while driving 100mph, you were to drive up the ramp of a car transporter ahead of you doing 50, a popular movie scene?

One of the engineers here at work made the scenario as this.

Conveyor one direction Wheels going the opposite with a winch pulling the plane forward. With the winch being the thrust of the engine it might be a little and I repeat a little easier to see that it would still move forward.

Here is what probably gets me.

Boat, water running one direction and you paddling or a motor trying to push you in the other direction. With the water continuing to increase as you increase motor you would never move. With the case of the plane the plane doesn't use the ground to push forward it uses another median the air and hence my minds problem of comprehending it but still willing to accept...

yanktar · Dec 14, 2005

Yikes.

Don't you guys get it? The wheels are free-wheeling. They don't generate thrust the way the rear wheel of the FJR does. That's why sand sailboats can go over sand that will bog down a jeep or other 4x4--thrust is from the wind.

Radman is perfectly right--no effect on takeoff.

ToeCutter is right too--you may burn up the wheel bearings if the ramp going backwards causes them to turn faster than the max RPM they are spec'ed at.

Sometimes what seems to make sense, upon closer examination, makes NO sense.

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Like White House policy and Bush's speeches.

(Just HAD to toss in some raw meat for the carnivores...

:lol:

)

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