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Airfoils

How Does it Fly?

A Swiss physicist, Daniel Bernoulli, discovered as the velocity of a gas or liquid increases, it’s pressure decreases. That is the basic principle of flight. An airfoil, such as an airplane wing, is designed so that molecules of air going over the top have to travel a greater distance than those molecules going underneath. If two molecules hit the front, or leading edge of a wing at the same time, the one going over the top has to go faster than the one on the bottom in order for both of them to get to the back of the wing at the same time.

Remember, what generates the pressure on the wing is the movement of air molecules. The faster the movement, the greater the pressure. Airplanes generate that movement by using propellers or jets to push the airplane through the air.  The same thing can happen on the ground with the engine off! If an airplane on the ground is facing into the wind, and the wind speed is fast enough to generate the required pressure, the airplane will want to fly.

 High Velocity, Low Pressure Air

Low Velocity, High Pressure Air

At a very low angle of attack, air flows smoothly across the top of the airfoil. There is no delamination of the boundary layer of air allowing the entire lower wing area to generate lift.

The greater the angle of attack, the more delamination of air from the airfoil. There is more burble at the trailing edge and a greater loss of lift.

When the angle of attack is raised above a certain point, thereis a large area of delaminated air and burble. The area of the airfoil that produces lift is not enough to carry the weight of the airplane, passengers, and cargo. The airfoil then stalls.

Relative Wind and Angle of Attack

Two other important factors which allow an airplane to fly is Relative Wind and Angle of Attack. Relative Wind is the air in the form of a wind that meets the leading edge of a wing. This is the movement of air that provides lift. Angle of attack is the angle formed between the direction of the wind, and the position the wing to the wind. With the angle of attack low, that is, the wing going directly into the wind, air flow over the top of the wing is smooth as it gets to the back. As the angle increases, the air begins to lift off the trailing edge the wing, causing a burbling or swirling of air. This results in a loss of lift. If the lift loss is so much the wings can’t generate enough lift to support the weight of the airplane, the airplane stops flying. This is known as a stall. This type of stall has nothing to do with the engine. In fact, an airplane can stall with the engine running full throttle. To overcome a stall, the pilot lowers the nose of the airplane. This will reduce the angle of attack, increase the speed of the relative wind over the wing, and generate enough lift to allow the airplane to flying again.

Pilots learn how to do stalls. To land an airplane, the pilot has to get it to stop flying, and in order to do that, the plane has to be stalled. When a stall is performed just as the wheels touch down on the runway, the result is a nice smooth landing.

Stall angle

Climb angle

Level Flight angle

The blue lines represent the flow of the Relative Wind. The purple line going through the longest portion of the airfoil is called the Cord Line. The angle of attack is the angle formed between the Relative Wind and Cord Line.