### Question

Aircraft are able to climb and maintain altitude by utilizing a combination of the Bernoulli effect and simple momentum transfer in the air flow to generate lift. Lift is generated primarily by the wing of the aircraft.

To generate lift using the Bernoulli effect, the cross-section of the wing is shaped in such a way that the distance from the leading edge to the trailing edge is greater along a path that runs over the wing than under the wing. As the aircraft is propelled forward, air necessarily flows both over and under the wing. Because of the greater distance that air must travel over the wing, the air pressure over the wing is less than the air pressure under the wing. This difference in pressure between the top and bottom of the wing results in a net upward force, or lift, on the wing. This is the Bernoulli effect.

One way of understanding the physical basis of the Bernoulli effect is to consider that, since the path of the air flow over the wing is longer, the air over the wing is forced to expand into a greater volume than that flowing under the wing. Consequently, the air pressure over the wing is necessarily lower than that under the wing.

Lift can also be generated by dynamic momentum transfer from the airstream. In this method, the tilt, or angle of attack, of the wing relative to the overall direction of motion causes the flow of air striking the under surface of the wing to be turned, or deflected, downward. That is, a downward momentum is imparted to the airflow by the deflection induced by the wing. Newton's law dictates that a compensating reaction force (lift) acts in the upward direction on the wing according to F = dp/dt, where p is the momentum (F and p are vectors). This is the same principle that generates sufficient upward force on water skis to keep a skier from sinking below the surface.

In practice, the aerodynamics of flight are complicated by many other effects. Obviously, opposing the upward lift is the downward force of gravity. Likewise, opposing the forward thrust of the aircraft is the backward force of drag created by resistance to passage through the air. Also, both the Bernoulli and momentum transfer effects depend upon the smooth, or laminar (streamline), flow of air around the wing. Various factors can induce turbulence in the air flow that (generally) spoils lift, increases drag and decreases the overall efficiency of the aircraft in flight.

For more details, see the web site:

http://www.aviation-history.com/theory/index-theory.html

Or, use one of the search engines, such as AltaVista or Yahoo, to search for phrases such as 'theory of flight' and 'aerodynamics.' Greater detail can also be found in the article 'Aerodynamics' in any edition of Van Nostrand's Scientific Encyclopedia.
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA

Airplanes fly by making use of conservation of energy.

The cross-sectional shape of the wing (and often the body) of an airplane is called an air foil. An air foil looks like a tear drop sawed in half so that one side is flat and the other is curved. The curved side is on the top of the wing and the flat side is on the bottom. As a wing is pushed through the air, the air is divided. Some of it goes above the wing and some of it below. As the wing moves foward, the air moves apart and then back together again filling the void where the wing was a moment earlier.

The air that goes over the top must move faster than the air that goes below, because the top path is longer. (The shorteset distance between two points is a straight line.)

The air has a certain amount of kinetic (motion) energy based on its temperature. In stationary air, this motion enegy is aimed in all directions. There are millions of molecules going up, millions going down, millions going to the left and millions going to the right. They bounce off each other and everything around them. We interpret all of these tiny collisions as pressure.

If we assume the temperature of the air isn't changed much by the passage of a wing, then the air which goes over the top must use more of its kinetic energy to get out of the way of the wing then the air which goes underneath. In other words, the air on top is more organized than the air on bottom. The air on bottom is using its kinetic energy in all directions. Some of it is colliding into the bottom of the wing and pushing it upwards. The air on top however can't spend as much of its energy pushing downwards, because it is using it to move up and over the wing. This imbalance of forces (pressures) pushes the wing up. If the wing sits still, the air is as unorganized on top as it is on bottom, so you have no lift. In fact the faster the wing moves the more exaggerated the pressure imbalance is; this is why the airplane has to race down the runway to lift off.

An interesting experiment to illustrate the fact that fast moving air exerts less tangental pressure than stationary air is the following. Put a stick pin through the center of an index card. Take an old thread spool, (the paper coverings should be intact) and put the pin into the center hole with the card pushed flush against the spool. Now blow hard on the other side of the spool as though it was a trumpet. The card will stick tightly contrary to common sense.

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