The Angle of Attack for an Airfoil

While an airplane wing is one of the most popular examples of the Bernoulli effect, many discussions allege that the Bernoulli lift is actually a small part of the lift force which allows the aircraft to fly. You can argue that the main lift comes from the fact that the wing is angled slightly upward so that air striking the underside of the wing is forced downward. The Newton's 3rd law reaction force upward on the wing provides the lift. Increasing the angle of attack can increase the lift, but it also increases drag so that you have to provide more thrust with the aircraft engines.

Some pilots have been known to get a bit testy about their lift being attributed to the Bernoulli effect, and reply "Then how do you suppose we can fly the plane upside down?". It looks a bit tricky, but you can adjust the attitude of the aircraft when upside down to give the proper angle of attack to get lift.

The discussions of "Bernoulli vs Newton" continue, but aerodynamicists such as Eastlake take the point of view that they are ultimately equivalent models and that neither is incorrect. In his wind tunnel testing at the Department of Aerospace Engineering, Embry-Riddle Aeronautical University, the Bernoulli approach is preferred because it can be tested more readily with the type of measurements which can be made in a wind tunnel. Making numerous point measurements around the airfoil and summing (integrating) them in the context of a Bernoulli model gives consistent modeling of observed lift forces.

Boundary Layers in Fluids

It is part of the nature of viscosity and viscous flow that the part of the fluid at a surface such as the walls of a tube is essentially at rest. This nearly stationary fluid layer at the surface is often called a "boundary layer", and this boundary layer has important implications in fluid phenomena.

Spinning balls carry a boundary layer around with them, and the nature of that boundary layer can affect their trajectories. The nature of the boundary layer around a spinning baseball allows it to significantly interact with the air, speeding up the air relative to the ball on one side and slowing it down on the other. This boundary layer interaction coupled with the Bernoulli effect is responsible for curving baseballs, and can produce a drop ball if the ball is given topspin.

Golf balls are given underspin and as a result of the boundary layer interaction with the surrounding air and Bernoulli effect, they experience aerodynamic lift. Much of this lift is lost if the dimples on the ball are filled in. The smooth golf ball has a much shorter range.

The nature of the boundary layer makes it difficult for you to get all the dust off your car by just using a hose; the thin layer of water closest to the dust is hardly moving! It is always surprising to find a layer of fine dust on a fan blade when it has been spinning rapidly, but the boundary layer closest to the blade surface is almost at rest.

A boundary layer is a complex phenomenon; only simple descriptive statements are attempted here. It seems evident that boundary layers play a role in the redirection of flow around spinning surfaces, and any redirection of flow involves forces and therefore reactive forces in the opposite direction. Another kind of description of such forces is invoked in the Kutta-Joukowski theorem to characterize the lift on a spinning cylinder in an airstream.