derek840378

my focus is on the wheels. lets say the plane produces enough thrust to momentarily advance the plane. the conveyor belt will then speed up to match the wheel speed and kill forward progress.

hotdram

Bernoullie's Principle Illustrated

~Rob

derek840378

this is because the plane is overcoming the flow

derek840378

im sorry, i fail to see how that fits.

gman07

Well, I have gone back and forth a few times on this one, but my brain now seems to agree with derek84... The conveyor belt would have to travel really fast, fast enough that the rolling resistance of the wheels equals the thrust provided by the propeller/jet. Thats just my take on it right now though...

CJSamples

If there is no wind moving over the wing area, then the plane will not take off. The wheels are basically non-exsistant in this quesiton as they are cancelled out by the conveyor belt. The belt is killing all forward progress thru the atmosphere so no low pressure area above the airfoil, which is also known as lift.
A wing works by creating a low pressure area on the upper surface. The upper surface of a wing is actually a longer distance across, causing the air to move faster and causing a lower pressure. The airfoil reacts to the lower pressure by being sucked into the lower pressure area, therefore lifting.

Now if the conveyor belt was also speeding up the wind coming across the wings, then it might lift off. Imagine a string tied to the front of the airplane holding it in place on the belt. If it doesn't physically move forward, then it will not fly.

You can throw all this out the window if it is a Harrier jet which actually pushes jet engine exhaust downward to lift off.

Chris (Yes, I work in commercial aviation)

12valve@heart

Wow! Talk about a difference of opinions.

This question requires several assumptions.
1) Assume by the statement "intends to take off" that the jets or prop are running normally. I'm going to assume jets for the purposes of this discussion.
2)Assume that "moving in the opposite direction of rotation" means in the opposite direction of the planes wheels.

Based on these assumptions, let's start the plane. As we accelerate, the jet's suck in air and expel exhaust forcing the plane forward in relationship to the surrounding air (the hair dryer effect). As the wheels start to roll, the conveyor starts causing the wheels to speed up. At point-in-time X, the jets are pushing the plane forward at 5 MPH in relationship to the surrounding air. The wheels are turning 10 MPH backwards as the conveyor matches the wheel speed in an attempt to keep the plane from moving (but failing because the jets push against the air, not the ground).

Question: When there is no wind but you are being forced forward by compressed air at a speed of 5 MPH, do you feel a breeze in your face?

Answer: Yes

As the throttle is increased, the plane WILL contine to accelerate in relationship to the surrounding air and will take off because the forward motion will cause a low pressure above the wing. Takeoff distance should be normal but the wheels will be spinning backwards at twice the planes forward speed.

Now, about that fly . . . .

John Rodriguez

Plane will take off. Planes don't use wheels to accelerate. If you had a truck on the belt, it wouldn't go anywhere because it's getting pushed back at the same speed of the tires. Now imagine a tow-rope from the front of that truck to a tow truck NOT on the conveyor belt. The truck will accelerate down the treadmill reguardless of wheel speed because OUTSIDE forces are working on it. It's not powering itself through the wheels. The tires will be doing 100mph if the truck is only rolling fowards at 50mph. The plane will take off because it will accelerate THROUGH the friction of the tires/axles because it's powering itself by PULLING WIND and not PULLING TREADMILL.

hotdram

Here is a better example for Bernoulli:
Click Here
Click on the "Airfoil Lift" link from the above page.

~Rob

derek840378

a plane cant get lift if its not moving

derek840378

the tow-rope is preventing it from going backwards, the air doesnt, therefore the plane still stays stationary

Begle1

The engines are holding on to the air. They are pushing against the air with thrust. It is just like pushing a rolling pin against a conveyer belt that's moving against you.

derek840378

im thinking about calling up the guys from mythbusters!! this is getting ridiculous!
i dont like the idea of rolling pins and tow-ropes. to me it seems different than jet engines....
after a little thought consider this... if your holding a rolling pin in your hand and turn on the machine, the rolling pin stays in place, same for the tow-rope. the plane, on the other hand, will move backwards with the conveyor.

derek840378

if the wheels are moving 300 mph, the belt will move 300mph in the opposite direction, the plane will remain stationary, no wind, no lift.

no matter what, the plane can not move!

gman07

The plane will never move. In order for it to accelerate relative to the ground (and therefore the air around it), the wheels must turn faster than the conveyor belt. The belt must move exactly the same speed as the wheels at all times, which in all reality is not possible, but for this we will assume it is. Thus, if the plane cannot accelerate, there will be no air moving over the wings of the plane so it can't take off. The conveyor will accelerate extremely rapidly until a point is reached at which the rolling resistance is equal to the thrust provided by the jets.