[notch4u]

A plane is standing on a movable runway( something like a conveyor). As the plane moves the conveyor moves but in the opposite direction. The conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

The question is, will the plane take off or not?

http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

Quote:
The crucial point is that the plane engines will accelerate the plane towards 100 knots relative to the surrounding air (not relative to the ground) - and that’s the only speed that matters here. The fact that the “ground” is going backwards is irrelevant - the wheels just spin twice as fast as they would normally and it makes no appreciable difference to anything.
The plane flies.




Quote:
Airplane on a Conveyor Belt

A riddle was proposed on the Neal Boortz show today:

If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?

Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God… this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?

Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal’s position… that the plane would not be able to take off.

Their argument is this, to quote one poster:

Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.

The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.

You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.

So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!

When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.

Gah… people are freakin’ stupid.

Update: There is a variation on this riddle that says that the conveyor belt matches the speed of the plane. It doesn’t matter… the plane still takes off. The conveyor belt could be going 5 times as fast as the plane, and the plane would still take off. You’d get into issues about tires blowing out, but assuming that the wheels can take the strain, the airplane would still take off.

AND

http://www.straightdope.com/columns/060203.html

Quote:
First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily

[notch4u]

i say it will as long as

1)the converyer is still as long as a regular runway
2)the wheels/tires have no issues with the extra speed

but it will take a little to overcome the initial start

but i am also thinking that it might take a longer runway to do it

[Awshux]

Here we go again...

[notch4u]

woops? posted allready?

[Awshux]

Originally Posted by notch4u

woops? posted allready?

Yeah, you could say that...

[notch4u]

shoot..i didnt mean to be that guy

mod delete fast!

[Cause]

Don't think so...

The fixed wing aircraft requires the air pressure differential above and below the 'airfoil' of the wings to give it lift. If the plane is NOT moving there is no differential seen by the wing. The engine 'thrust' is there to create the air pressure differential at the wing. If the plane remains relative stationary and the belt below is moving there is no pressure differential on the wings 'airfoil'.



I think...

[Awshux]

And so it begins...

[LandRover]

Did you guys hear they're coming out with a 4 door wrangler!!!???

[notch4u]

lol..it will take off

thusters have no relation with ground speed if the wheels are doing thier job..it will overcome the intial then will take off but i think it might have to be a little bit of a longer runway

[mmasuperman12]

haha i saw this thread and was like oh boy not again, i think some one on here needs to build this experiment to put this whole damn thing to rest.

[Cause]

Originally Posted by LandRover

Did you guys hear they're coming out with a 4 door wrangler!!!???

Will they have nails in the tires?

[FlaJeepin]

Please someone make this topic go away!

[FlaJeepin]

Originally Posted by LandRover

Did you guys hear they're coming out with a 4 door wrangler!!!???

Really, will it have a Desial option?

[RollerHman]

Originally Posted by LandRover

Did you guys hear they're coming out with a 4 door wrangler!!!???

oh noes, what is the world coming too. i doubt it will ever go offroad........