Friday, January 04, 2008

A Lesson In Fluid Mechanics & Aerodynamics For Neal Boortz

Here's the myth;
A plane is sitting on a conveyor belt pointing This way ---->

The conveyor belt begins to move the other way. <-----

The myth states that the plane will take off, and Neal Boortz adamently agrees.

I very much disagree. The myth states that the wheels of the plane and the conveyor belt are going the same speed in opposite directions. The plane itself will be stationary compared to the ground, and the wind that it needs in order to take off.

The net velocity of the plane and the wind must be the take off velocity in order for the plane to lift off. For example, if the take off velocity of a plane is 100 miles per hour, the velocity of the plane minus the velocity of the wind (Positive if the wind is a tailwind, and negative if it's a head wind, hence why planes usually take off INTO the wind) has to equal the take off velocity. Therefore a plane traveling 95 miles per hour into a 5 mph head wind is going to take off.

Flight is possible because of a concept explained by the Bernoulli Principle. It states this;
"An increase in velocity is associated with a decrease in pressure, and a decrease in velocity is associated with an increase in pressure." The concept of flight is that an airfoil has a longer distance over the top side of the wing than it does on the bottom. The air flowing over the top has to flow faster then the air on the bottom side of the wing. Since the air has the same density, same static pressure, and a negligable change in potential energy due to height change, the pressure on the top of the wing is lower than the pressure on the top side of the wing. Then you multiply that pressure difference by the surface area of the wing to get the amount of lift.

If the plane is not moving, there will be no air flow over the wings, and no lift.

The only way a plane could take off from a conveyor belt is if it were stationary (relative to the conveyor belt) or if it were bein g propelled in the same direction as the belt.

Look at these pictures to see what I mean.

Neal, I will be laughing at you all day on Jan 31st.