A plane on a runway, how smart is r3vlimited?

[Ray Smoodiver]

Have any of you ever acutally been in a plane?

[trent]

Ding ding, we have smarties in the house.

Half of being smart is doing research.

[Ray Smoodiver]

Thrust alone doesn't make planes fly - if that were the case - the wings on a propellor driven plane would be exactly the same size as the prop.

[Ray Smoodiver]

In Bernoulli's theorem - velocity increases as pressure descreases. I'm not seeing any pressure decrease on your stationary plane. The conveyor belt isn't moving a gigantic chunk of air at the plane at the same time its turning the wheels a zillion mph in the opposite direction.

[Borat]

dam ray, the plane IS moving forward- hence wings will generate lift.
as someone stated the plane is moving forward at 200mph, then conveyor will be moving back @ 200mph then wheels will be spinning at 400mph.

If people don't get it after reading those sites then I don't think any amount of arguing will convince them.

[Borat]

right, someone with a motorized treadmill...
get a toy car (or plane if you must), attach string to it so you can move it forward or back. Turn on treadmill, place car on treadmill. now believe it or not, using the string you will be able to pull the car forward or back! the wheels will spin faster or slower depending on whether you are pulling the car forward or letting it roll back.
Exact same thing with the plane, except string has been replaced with jet engines.

[Rigmaster]

I can't believe all you guys. Yes, the plane will take off.

Airplanes use jet engines that propel the plane forward. They are not powered by the wheels like cars.
Once you start the engines and try to take off, the speed of the ground has no effect. The Jet engines will still propell the plane forward enough to gain enough speed to fly.

Look at it this way, Let's say that the jet engines have the ability to push the plane 200mph upon take-off (I have no idea how fast they accually go, but that's irrellevant). The conveyor belt is also moving 200mph backward.
The plane is still traveling at 200mph, while the wheels are spinningh at 400mph.

-Erik

Of course they don't use the wheels to propel forward, BUT in this case, the wheels allow the plane to roll on the conveyer- it doesn't really matter what is propelling (or attempting to propel) the plane forward, IF the conveyor is keeping the plane in the same position, relative to the ground (and the air), then it's not going to generate lift and it's not gonna take off.

My answer to this question is based on MY interpretation of the set up- that the conveyor is magically keeping the plane from moving forward.

No forward motion = No take off.




Bret

[Ray Smoodiver]

Assuming calm wind (or any wind with a headwind component less than that required for takeoff), the aircraft will NOT take off.



There are four forces acting on an aircraft: lift, weight, thrust, and drag. Lift counteracts weight and thrust counteracts drag.

If the aircraft is set up as in the initial post:

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

then there are only two forces being imparted on the airplane (assuming zero wind) = lift and weight. The weight of the airplane is acting vertically toward the center of the Earth, as it always does. Thrust is being provided by the aircraft's engines, which is being matched exactly by the thrust imparted by the conveyor belt.

To visualize this, imagine a game of tug-of-war between twins who are precisely equal in strength. The rope wouldn't move.

Mathematically, we can illustrate it this way:

Assume the aircraft is producing 2000 N of thrust in a forward direction. The conveyor belt would then be producing 2000 N of thrust in the exact opposite direction. The net thrust would be [2000 + (-2000)] = 0. Therefore the aircraft is stationary relative to the Earth.

The amount of lift produced by a typical aircraft varies directly with the square of the increase in velocity of air flowing over an aircraft's wings. As the aircraft is stationary relative to the Earth, and assuming zero wind, the velocity of the air flowing over the plane's wings would match the speed of the wind: zero. If you were conducting this grand experiment outdoors and the winds were blowing, say, 15 knots straight toward the nose of the plane, the speed of the air over the wings would be 15 knots. Yes, you'd be creating SOME lift, but you'd be creating the exact same amount of lift whether the engines/conveyor belt were producing 1 N in opposite directions or 1,000,000 N in opposite directions. So you could strap on about 48 GE90's to a Cessna 152, pour the coal into 'em, and have the conveyor belt running at about Mach 2--but as long as the net thrust is zero, the plane won't be going anywhere.

[Ray Smoodiver]

Someone call mythbusters

[Axxe]

But it is moving forward, because of the thrust from the motors. The conveyer does NOTHING but increase the speed of the wheels and as thus, increase drag due to friction in the bearings, however it isn't enough to even worry about when compared to teh amount of thrust the motors produce.

[JasonC]

Someone call mythbusters

+1

[Borat]

my last post in this, i swear.
if the wheels brakes were on then the plane wouldn't move, but we are assuming the wheels are free spinning. The conveyor simply can't stop the plane from moving.

ray said:
Assume the aircraft is producing 2000 N of thrust in a forward direction. The conveyor belt would then be producing 2000 N of thrust in the exact opposite direction. The net thrust would be [2000 + (-2000)] = 0. Therefore the aircraft is stationary relative to the Earth.

this is incorrect, the conveyor is providing thrust, but has no effect on the plane becuase the wheels will simply spin.

[Axxe]

Ray, the conveyer belt does NOT exert any force directly on the plane, it all happens through the wheels, which are the point of interface between the plane and the conveyer belt.

[erik325i]

Of course they don't use the wheels to propel forward, BUT in this case, the wheels allow the plane to roll on the conveyer- it doesn't really matter what is propelling (or attempting to propel) the plane forward, IF the conveyor is keeping the plane in the same position, relative to the ground (and the air), then it's not going to generate lift and it's not gonna take off.

My answer to this question is based on MY interpretation of the set up- that the conveyor is magically keeping the plane from moving forward.

No forward motion = No take off.




Bret

Sorry, but you are wrong. The conveyor belt is not keeping the plane stationary.
The wheels on a plane are simply there to roll. No power is going though the wheels. All the conveyor belt is doing is spinning the wheels really fast. It has no effect on the speed of the plane.

Imaging a toy car sitting on a dinner table. If you yank on the table cloth, the wheels will spin, but the car will remain on the table. That is exactly the same as the airplane on the conveyor belt.

Even if you turn the conveyor belt to 1000mph, the plane will still be able to take off. You may need to replace the wheel bearings because of the rediculous speed the wheels are spinning, but the plane is still being thrust forward by the jet engines.

-Erik

[Ray Smoodiver]

right, someone with a motorized treadmill...
get a toy car (or plane if you must), attach string to it so you can move it forward or back. Turn on treadmill, place car on treadmill. now believe it or not, using the string you will be able to pull the car forward or back! the wheels will spin faster or slower depending on whether you are pulling the car forward or letting it roll back.
Exact same thing with the plane, except string has been replaced with jet engines.

The treadmill isn't accelerating and decelerating everytime you tug on the string is it - unlike the question stated. Nice try Mike.