Airfoils

Airfoil Design

An airfoil is a structure designed to generate a force by interacting with the air stream around it. Air behaves differently under varying pressures and velocities, and airfoils are shaped to take advantage of these principles to produce lift efficiently. Your wings use the principles listed below to generate lift. Airfoils, however, are more than just wings. Any component on your aircraft that produces lift is an airfoil. For example, your horizontal stabilizer produces a down force, by producing lift downwards. Your propeller also produces "lift", although we generally call this thrust, in the forward direction. This section explores airfoil geometry, lift generation, aerodynamic principles, and airfoil performance.

Bernoulli's principle 

According to Bernoulli’s Principle, as the velocity of a fluid increases, its pressure decreases. In the case of airplanes, the we're interacting with fluid is air. The camber (curvature) of the upper surface of the wing accelerates the air moving over it. This creates an area of low pressure on the top of the wing and an area of  higher pressure on the bottom of the wing. Because of this pressure differential, the higher pressure air on the bottom of the wing pushes the underside of the wing up into the area of lower pressure on the top of the wing. This is because the area of high pressure wants to move to the area of low pressure to equalize the pressure differential. This upward force generated on the wing is lift. When enough air is moving over the wing at sufficient velocity lift is greater than weight and the aircraft becomes airborne. 

Figure 1 Air pressure decreases in a venturi tube (Pilots Handbook of Aeronautical Knowledge)

Bernoulli's principle is not only used to produce lift but also in carburetors. In a carburetor, there is a venturi that accelerates the air which creates an area of low pressure. This creates an area of relatively higher pressure outside of the carburetor which causes the air outside the carburetor to be "sucked" in and mix with fuel.

Figure 2 Basic Carburetor Design (Cross Section) (New World Encyclopedia)

This is a very useful video that dissects a carburetor to better understand how it works

and how Bernoulli's principle applies:

Newton's Third Law

Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction. In the context of aerodynamics, as air hits the bottom of our wings (particularly at high angles of attack) an equal opposite force is applied up on our wing.

Basic Airfoil Design

A typical airfoil has distinct features:

• Leading Edge: The front of the airfoil that first encounters the relative airflow.

• Trailing Edge: The narrow, tapered rear portion where airflow from both surfaces meets.

• Chord Line: An imaginary straight line connecting the leading and trailing edges.

• Camber: The curvature of the airfoil.

Figure 2 Typical airfoil section (Pilots Handbook of Aeronautical Knowledge)

Lift and Drag

Lift results from the pressure differences between the upper and lower surfaces of the airfoil. Two primary aerodynamic phenomena contribute to this:

• Bernoulli’s Principle: Higher velocity airflow over the curved upper surface leads to lower pressure, generating lift. (refer to the section on Bernoulli's principle above)

• Newton’s Third Law: Air is deflected downward by the airfoil, producing an equal and opposite upward force. (refer to the section on newtons third law above)

Drag is the aerodynamic resistance encountered by the airfoil and is classified as:

• Parasitic Drag: Includes skin friction, form drag, and interference drag.

• Induced Drag: Related to the generation of lift and the formation of wingtip vortices.

Angle of Attack

The angle of attack (AoA) is the angle between the chord line and the relative wind. As AoA increases, lift increases. Every airfoil has a critical angle of attack, beyond which the airflow separates from the upper surface, causing a stall. When producing lift, a wing has a Center of Pressure (CP) which is located on the top of the wing. It is the location where the sum of all aerodynamic forces acting on the airfoil exist. As angle of attack increases, the center of pressure moves towards the leading edge of the wing. Once the center of pressure gets too close to the leading edge, the airflow over the top of the wing separates and the area of low pressure is disturbed. When this occurs, the pressure differential between the bottom and top of the wing decreases significantly and the wing is no longer able to produce sufficient lift. This is called a stall.

Figure 3 Pressure distribution on an airfoil and CP changes
with AOA. (Pilots Handbook of Aeronautical Knowledge)