A physical description of flight; revisited

A Physical Description of Flight; Revisited
David Anderson
Fermi National Accelerator Laboratory Ret. dfa180@aol.com

This material can be found in more detail in Understanding Flight 1st and 2nd editions by David Anderson and Scott Eberhardt, McGraw-Hill, 2001, and 2009 ©

& Scott Eberhardt
Boeing Aircraft Company Seattle WA 91895-2400 clipper-108@comcast.net Almost everyone today hasflown in an airplane. Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong. As an example, most descriptions of the physics of lift fixate on the shape of the wing (i.e. airfoil) as the key factor in understanding lift. The wings in these descriptions have a bulge on the top so that the air must travel farther over the topthan under the wing. Yet we all know that wings fly quite well upside down where the shape of the wing is inverted. To cover for this paradox we sometimes see a description for inverted flight that is different than for normal flight. In reality the shape of the wing has little to do with how lift is generated and everything to do with efficiency in cruise and stall characteristics. Anydescription that relies on the shape of the wing is wrong. Let us look at two examples of successful wings that clearly violate the descriptions that rely on the shape of the wing. The first example is a very old design. Figure 1 shows a photograph of the Curtis 1911 model D type IV pusher. Clearly the air travels the same distance over the top and the bottom of the wing. Yet this airplane flew and was thesecond airplane purchased by the US Army in 1911. The second example of a wing that violates the idea that lift is dependent on the shape of the wing is of a very modern wing. Figure 2 shows the profile of the Whitcomb Supercritical Airfoil (NASA/Langley SC(2)-0714). This wing is basically flat on top with the curvature on the bottom. Though its shape may seem contrary to the popular view of theshape of wings, this airfoil is the foundation of the wings modern airliners.

Figure 1. Curtis 1911 model D type IV pusher

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Figure 2. Whitcomb Supercritical Airfoil The emphasis on the wing shape in many explanations of lift is based on the Principle of Equal Transit Times. This assertion mistakenly states the air going around a wing must take the same length of time, whether going overor under, to get to the trailing edge. The argument goes that since the air goes farther over the top of the wing it has to go faster, and with Bernoulli’s principle we have lift. Knowing that equal transit times is not defendable the statement is often softened to say that since the air going over the top must go farther it must to faster. But, this is again just a variation on the idea of equaltransit times. In reality, equal transit times holds only for a wing without lift. Figure 3 shows a simulation of the airflow around a wing with lift.

Figure 3 Air over a wing with lift. The Bernoulli equation is a statement of the conservation of energy. It is correct, but not applicable to the description of lift on a real wing. The wings of an 800,000 pound airplane are doing a great deal ofwork to keep the airplane in the air. They are adding a large amount of energy to the air. One of the requirements of the application of the Bernoulli principle is that no energy is added to the system. Thus, the speed and pressure of the air above a real wing in flight are not related by the Bernoulli principle. Also, descriptions of lift that evoke the Bernoulli principle depend on the shape ofthe wing. As already stated, the shape of the wing affects the efficiency and stall characteristics of the wing but not the lift. That is left to the angle of attack and speed.

Newton’s laws and lift
So, how does a wing generate lift? To begin to understand lift we must review Newton’s first and third laws. (We will introduce Newton’s second law a little later.) Newton’s first law states: A...