Airflow over an airfoil
Flight is one of the most important achievements of mankind. We owe this achievement to the invention of the airfoil and understanding the physics that allow it to lift enormous weights into the sky.
All flight is the result of forces acting upon the wings of an airplane that allow it to counteract gravity. Contrary to popular belief, the Bernoulli principle is not responsible for most of the lift generated by an airplanes wings. Rather, the lift is created by air being deflected off the wings and transferring an upward force to those wings.
The most important factor in determining the lift generated by an airplane is the angle of attack. The angle of attack is the degree measure from the horizontal that a wing is elevated or declined. When the angle of attack is between 1 and 20 degrees, the most lift is generated. To find the lift generated by a particular area of wing in a standard airfoil shape, a teardrop with the fat end facing forward, the equation L=Cl 1/2 (pV2)S. Cl is the lift coeficent, which is determined by the shape of the airfoil and the angle of attack. P stands for the air mass density, V for the velocity of the air passing over the wing, and S for the area of the wing when viewed from above or below.
As the air flows over the wing producing lift, it grabs onto the wings surface and causes drag. Drag can be measured by the equation D=Cd 1/2 (pV2)S, much like the lift equation. The drag coeficent Cd is found, again, by determining the shape of the airfoil and then finding the angle of attack. Drag is less than lift up to a certain angle of attack. After that, the air encountering the surface of the wing is either pushed off or deflected in a way to cause turbulance.
So, what does lift and drag have to do with airflow? As hinted in the above paragraph lift and drag are directly related to the shape of the airfoil. A special property of liquids is this, they will tend to follow a surface that is gently curved. When air rushes over an airfoil at a 10 degree angle, the air passing over the surface of the wing follows the curvature. When the air finally leaves the trailing edge of the wing it now has a downward velocity. The air on the bottom of the wing is also deflected downwards, however, not at such a steep angle. Since every action has to have an equal and opposite reaction, the change in direction and velocity of the air is transferred to the airplane as lift.
The major forces that interact on an airplane are simple. There is forward velocity provided by the engines. An angled upward vector produced by the wings. A directly vertical vector produced by the differences in pressure on the wing surfaces. Drag caused by the wing and the airplane body, which is acting in a direction opposite that of the engines. And the force of gravity pulling directly down on the plane. From these vectors the flight of a plane, and what position it needs to be in to stay level can be calculated and understood.
This is how airflow affects modern flight. The interaction of air with the wing, and not atmospheric pressure, is the main generator of lift. Drag is the consequence of the physics of liquids and is inseparable from, and important to, lift.