Parasite Drag

Parasite drag is easy to visualize: Think of the force you feel on your hand when you stick it out the car window at highway speed. The same force acts on the airplane as it flies. The structure of the airplane is sleek and aerodynamic, but it still creates a lot of wind resistance.

We call this component of parasite drag “flat-plate drag.” To arrive at an airplane’s flat-plate drag ratio, airplane designers look at all the surface area on an airplane and make some allowances for the drag-reducing qualities of its design. By arriving at a flat-plate drag ratio, designers are saying that the surface area of an airplane is equal to the drag of an imaginary flat plate of a certain size.

For example, a small, two-seat Cessna 152 has a flat-plate area of slightly over 6 square feet. That’s pretty small considering how much total surface area the airplane has. But by comparison to other planes, the Cessna 152 is a “dirty,” or drag-intensive, airplane. The Beechcraft Bonanza sports a flat-plate area of only 3.5 square feet, and the sleekest of all mass-produced general aviation airplanes, the Mooneys, have a flat-plate area of around 2.8 square feet.

Some flight instructors dust part of the airplane’s skin with talcum powder to demonstrate the fact that the jagged surface of the airplane’s skin holds air still at a microscopic level. Because talc is so light, is should blow off the airplane at high speed. But because the powder is so fine that it settles into the micro­scopic nooks in the wing, it is sheltered from the main flow of air, just as air molecules are. At the end of the flight no matter how fast the plane moved, the talc will still be where it was before the flight.

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Another component of parasite drag is the wind resistance caused by skin friction. If you examine the skin of an airplane at a microscopic level, you’ll see a jagged surface with lots of nooks and cavities that are too small for us to feel, but more than large enough for air molecules to hide in. Like small eddies along a riverbank that hold water stationary while the stream nearby flows rapidly, the jagged irregularities on a plane’s surface hold a thin layer of air perfectly still, even though the airplane might be moving at a very high speed. As you move a little farther from the plane, but still at a microscopic distance, the air moves a little faster, and at a few millimeters from the skin, the wind is moving at full speed. The viscosity of the air, or its resistance to flowing smoothly, is to blame. That viscosity adds to parasite drag.