Physics Question for you.....

[crokett]

Too lazy to read 13 pages and see if I replied before so....


If I answered no previously, changing my answer to yes. I thought of an example that works that we can relate to.

Take your belt sander and flip it upside down. Turn it on. Take your kid's R/C car and fire that sucker up. Put it on the belt sander and pretend for a minute that the belt sander RPMs match the wheels. The car will stay in place on the belt sander.

Now doctor the car so the wheels are disconnected from the motor and spin freely. Strap a model rocket engine to the roof - this is the same as a jet. Turn on the belt sander. Drop the car on the belt sander and hold it in place while you ignite the egine. Let it go and the car will go flying across your shop. The thrust from the engine is not against the belt sander, rather the hot gases shooting out of the engine are pushing it. It doesn;t give a hoot that the ground is moving under it.

[TheRic]

Mythbuster test

Mythbuster did the plane on a conveyor belt test tonight.

They even explained why in the "Science Content" section.

The found that on the small scale and the full size scale........

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the plane took off.

I guess this resolves this question that has been plaguing mankind.

[BigguyZ]

I was going to find this thread too and mention the Mythbusters episode. Too bad you beat me to it. I found it funny that the very pilot they hired to do the test thought the plane would "sit there like a brick".

[Mr__Bill]

Yes, the plane uses air to push, not friction with the ground. Let's say it needs a speed of 100 to take off. At the moment of takeoff it is moving 100. The converyor is moving at 100 backwards. The wheels are effectively spinning at 200. Assuming the wheels can handle double speed without blowing out, should be able to take off.

The fact that the runway moves is irrelevant. Offthemark has it right!

oops, somehow I missed that there were a gazillin replies to this, but the plane sill takes off

[guycox]

Yes, the plane uses air to push, not friction with the ground. Let's say it needs a speed of 100 to take off. At the moment of takeoff it is moving 100. The converyor is moving at 100 backwards. The wheels are effectively spinning at 200. Assuming the wheels can handle double speed without blowing out, should be able to take off.

I don't think the wheels will be doing 200 mph. If it were a car and being driven by the wheels. If the wheels didn't spin (0 mph) the plane would move 100 mph in reverse -- but since the plane is stationary they'll be moving at 100mph. If the plane moves forward at 1 mph then the wheels would be spiing at 101 mph..

[Russianwolf]

I don't think the wheels will be doing 200 mph. If it were a car and being driven by the wheels. If the wheels didn't spin (0 mph) the plane would move 100 mph in reverse -- but since the plane is stationary they'll be moving at 100mph. If the plane moves forward at 1 mph then the wheels would be spiing at 101 mph..

The 100 mph needed for takeoff in the above example would be the plane's body in relation to the ground. Not in relation to the conveyor surface. So the wheel would indeed spin at an rpm equivalent to twice the speed of the body to the ground (200 mph in the example).

Or as you said. The plane is moving forward at 100 mph, so the wheels would be spinning at 200mph.

I can't believe this topic has gone 13 pages.

[TheRic]

..I can't believe this topic has gone 13 pages.

They did say it was the #1 fan requested thing... or something like that.

Even the pilot (used in this test result) thought it would sit there. SO I can see why many people are 100% sure their theory is correct.

[Alex Franke]

Wow... and I always thought the answer was 42. Time for another pan galactic gargle blaster, I guess....

[TheRic]

Wow... and I always thought the answer was 42. Time for another pan galactic gargle blaster, I guess....

Wrong question, we will send some mice over to make sure you get it correct.

Have a few more pan galactic gargle blaster until they arrive.

[BobSch]

Wrong question, we will send some mice over to make sure you get it correct.

Have a few more pan galactic gargle blaster until they arrive.

And thanks for the fish.

[Ed62]

Obviously, whether or not the plane takes off will depend on whether or not the chicken crossed the runway. And if he did, we want his name.

Ed

[crokett]

The plane will fly.. period...

Yep. Coming late to the discussion but I was formerly confused and thought it would not fly. It is real simple. The plane's props/jet exhaust is pushing against the AIR NOT THE conveyor. Therefore, the plane will move. Therefore, there is airspeed over the wings and when it gets fast enough the plane will fly.

[Wizzett]

Harold Pendell PE, Design Engineer

The plane will move and will be able to take off.



Reason for this is because the planes engines work by applying thrust against the planes mass and do not apply physical drive to the wheels that are in direct contact with the runway.



This means that Newton's Third Law Of Motion applies which states that for any action, there is an equal and opposite reaction. (i.e. thrust from the engines exhaust in the aft (backward) direction will cause a proportional reaction in the forward direction, thus moving the plane forward).



Because the plane is reliant on airspeed to take-off (approx 150 knots for a commercial type passenger aircraft) the plane will move forward regardless of what speed or indeed in what direction the conveyor moves.



If the same was done to a car which applies physical drive to the road wheels, the car would remain static regardless of how fast the wheels are traveling.

[annunaki]

Plane will Fly

PLANE ON THE TREADMILL

Mythbusters did the airplane on the treadmill bit last night. The airplane took off with a normal takeoff roll.

http://boortz.com/more/video/mythbusters_plane_conveyor_belt.html

[JimD]

I cannot tell what mythbusters did but the only thing that matters is the velocity of the air across the wing. I did not look at all the posts but I did not see anybody say this correctly. The reason there is lift is that the product of the pressure and velocity of ideal gasses is constant. The ideal gas law is normally stated at PV=nRT but what that means is that if the moles of material and it's temperature stays the same, PV stays constant. Air is not an ideal gas but is close enough that this is still true for most purposes. Because the air moves further to go over the wing than it does going under it, the pressure under the wing is higher and there is lift generated. As has been stated, the lift is a function of the square of the velocity (regardless of what units the velocity is stated in, all that changes is the units of the pressure change).

If you believe the airplane moves relative to the air, then lift will be generated and if the velocity of the air relative to the wing is high enough, the plane will take off. If the airplane is stuck in one place due to the conveyor, it will not fly.

The function of jet or propeller engine(s) is to force gas in the opposite direction of what the desired plane direction is. The result is forward motion of the airplane. If the engine also moves air over the wing, it could increase lift but that is not the primary purpose of the engine and it will not make a stationary plane lift off.

Hover craft can lift off just from thrust because they direct the engines at the ground. Regular aircarft direct the thrust the direction the plant wants to move away from and will not lift the plane other than by moving it thorugh the air (increasing it's velocity).

I am a mechanical engineer is anybody cares. I can dig out the books and give you all the equations but what I've just said will not change.

Jim