From
Index of Aerodynamics Slides
Fallacy Number 1. The "Longer Path" theory, or the "Equal Transit Time" theory. This theory is one of the most widely circulated, incorrect explanations. The theory states that airfoils are shaped with the upper surface longer than the bottom. The air molecules have farther to travel over the top of the airfoil than along the bottom. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from Bernoulli's equation, the pressure is lower. The difference in pressure across the airfoil produces the lift.
Fallacy Number 2. The "Skipping Stone" theory is often seen on web sites and in popular literature. The theory is based on the idea that lift is the reaction force to air molecules striking the bottom of the airfoil as it moves through the air. Because this is similar to the way in which a flat rock thrown at a shallow angle skips across a body of water, it is called the "Skipping Stone" theory of lift. It is sometimes called a Newtonian theory of lift, since it involves Newton's third law.
Fallacy Number 3. The “Venturi” theory is based on the idea that the airfoil upper surface is shaped to act as a nozzle which accelerates the flow. Such a nozzle configuration is called a Venturi nozzle and it can be analyzed classically. Considering the conservation of mass, the mass flowing past any point in the nozzle is a constant; the mass flow rate of a Venturi nozzle is a constant. The mass flow rate m dot is equal to the density r times the velocity V times the flow area A:
m dot = r * V * A = constant
For a constant density, decreasing the area increases the velocity.
Turning to the incorrect airfoil theory, the top of the airfoil is curved, which constricts the flow. Since the area is decreased, the velocity over the top of the foil is increased. Then from Bernoulli's equation, higher velocity produces a lower pressure on the upper surface. The low pressure over the upper surface of the airfoil produces the lift.
The Real Story. There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning
From a Newtonian perspective,
lift is generated by turning a flow of air. The flow turning creates a downwash from the wing.
Ground Effect. The proximity of the ground reduces the downwash. At a height equal to the span the induced drag is reduced by 2%. At a height equal to 1/10th the span induced drag is reduced by 50%. The reduction in drag will only occur where the induced drag is predominant. At high speeds where parasite drag is predominant ground effect will not result in a drag reduction. Ground effect will only be significant during take off and landing. As the ground is approached during landing the induced drag progressively decreases and a constant angle of attack will produce an increasing C
L, and the reduced downwash produces a change in longitudinal stability and a change in trim. The reduction in induced drag and increasing C
L will cause the aircraft to “float”.
Am I naïve to think that Dr. Dick Whitcomb, inventor of area rule, the supercritical wing and winglets, and his colleagues may know what they are talking about?
