My Lion King Forum

This is a fun thing I brought to discussion at some other forums.

first of all, just look at the question and don't go google it or something. Just post what seems logical to you and why.

So here it is: If an airplane is put on a conveyor belt which is going the exact same speed as the aircraft but then backwards, could the plane take off?

By the exact same speed you mean an aircrafts take-off speed? And you mean the belt is going the opposite direction to the plane? Then no. It cancels out the plane's speed and although the wheels are going the speed needed for take off, there is no wind lift to get the plane in the air because it's not actually moving.

Sorry, that's not it

I'd say...maybe! Might just tip over forwards and break though XD

I'm thinking Mythbusters did an episode on this. I can't remember the outcome though. I would guess it wouldn't because it would not have enough momentum? Maybe?

It could take off, simpletons

The airplane can take off because the engine actually sends power to the propeller and not the wheels. I can't really explain all that clearly (blame my lack of physics knowledge =P) but it has something to do with where the forces are exerted and friction. Let's just say that the conveyor belt won't affect the plane's thrust force because the propeller is pushing on the air. The thrust could only be affected in the case where air would be moving (wind pushing in the opposite direction), not if ground was moving. Or something like that... x)

I know that the physics teacher I had last year explained something about that but I can't remember clearly. ^^"

Let me explain it (I had to solve this problem in physics classes). I will try to over-include the basics so that everyone will understand this.

An airplane has mass. And to move mass, you apply a force. What is a force? Force = Mass x Acceleration. No acceleration, no force. No mass, no force either (you can't apply a force on a mass-less object, such as light).

So in order to get an airplane to move, you need to have a non-zero net force. This means that the airplane will experience an acceleration in a certain direction and start to move there.

So what are the forces acting on it once it is in full throttle? Keep in mind that Y is up/down and X is left/right (let's keep in in 2 dimensions for simplicity)

1.Gravity (negative Y direction)
2. lift (positive Y direction) - is directly related to the velocity of the airplane in the X direction
3. Propeller thrust (positive X Direction)
4. Drag (negative X direction)
5. friction (negative X direction) - smaller when mass is in motion than stationary.

In order to fly, the airplane needs to be moving in the positive X direction at a certain minimum speed where the lift force is greater than or equal to gravity. The only force that accelerates the airplane in this direction is the propeller. And where does it act upon? The air around the blades - the nose of the airplane (or wings/back). The air is pushed backwards, and therefore, the airplane accelerates forwards. Then flies off.

It doesn't matter what the wheels are doing. Spawn an airplane in the air, keep it horizontal, and it will still accelerate, because the propeller thrust force doesn't work on the ground, but the air.

I hope that this is clear.

Here's the situation in practice

[youtube]YORCk1BN7QY[/youtube]

It's no different from an airplane taking off in a strong wind.

An airplane flies because wind hits the airfoil, and is deflected downward. By Newton's laws of motion, this force pushes the airplane upward. Propellers, or jets, push air around the wings, generating this lift.

(I know, I know. This is not what they teach you in physics class. While Bernoulli's principle is correct, it is mainly used by engineers, and not pilots. This is actually why airplanes can fly upside down. If an airplane flies upside down parallel to the ground, it will get sucked downward. But, by adjusting the angle of attack, lift can still be produced. In other words, if nose of the airplane is facing the sky as it's flying parallel to the ground, as a result of the angle of the wings and the direction of the wind relative to the wings, the plane will still fly. An easy way to think of it is like sticking your hand out of the window of a moving car. If you tilt your hand one way, it will be pushed upward, and if you tilt it the opposite way, it will be pushed downward. Whether the airfoil is upside down or not, this still applies.)

If an airplane can take off at a speed of 180 mph, and it is flying at 30 mph directly into the wind, it will only need to travel 150 mph relative to the runway. If the wind speed is 180mph, then the airplane can take off without even starting the engine at all.

Likewise, if an airplane is taking off with the wind pushing it, the takeoff speed would be greater than 180 mph. This is why pilots always take off into the wind, and not with it.

As DGF explained, all that matters is the speed of the air relative to the wings, or the true airspeed. Speed relative to the ground has absolutely no significance in this.