REDUCING DRAG

-by Clay Ramskill

This subject is tough, assuming we want to stay clear of complexity. To get into the nitty-gritty of drag reduction, we need a wind tunnel, some heavy computations, and a whole bunch of witchcraft!

So we'll stick to some more basic principles, and leave the name dropping and number crunching to someone more learned than we are!! We do, however, have to make one distinction -- drag due to lift. That is pretty much separate from the rest, because it's strictly a function of lift -- the more lift we need, the higher the angle of attack our wing must operate at, the more lift drag we have. And once our wing area, shape, and airfoil are established, there's really only one control we have, and that is the weight of the plane.

Put simply, the heavier the plane, the more this form of drag will degrade performance, throughout the speed range!

Having gotten past that, there are several other drag components to look at -- skin friction, form drag, and interference drag, as well as cross-sectional area.

Cross-sectional area is easy. The more air you have to push aside as you go through it, the more drag. So we need to keep fuselages reasonably slender, airfoils reasonably thin. But the size is not nearly as important as shape.

Form Drag: Good "streamlining" is an area where we can really see some results. What we'd like to see is every component of the plane shaped like a good symetrical airfoil -- or like a drop tank as seen on jet aircraft. At the speeds we're interested in, a really sharp point in the front is not necessary (that's what you see on supersonic planes!). What is desirable is a nice smooth curvature.

Where we DO want the "pointiness" is at the rear. A good, smooth, continually tapering curve ending at a relatively sharp trailing edge or point. The main thing to avoid is abrupt or angular changes in the airflow.

Retracts: Easily the worst contributor to drag is the landing gear. Fixed gear drag can be reduced by wheel pants and cuffs on struts -- but retracting gear is the obvious solution. There are, however, weight, complexity and expense penalties.

Now, let's look at skin friction. First, the less skin, the less friction! Rounding corners not only cuts form drag, it cuts the skin area. Round forms enclose the most interior volume with the least skin area. A smooth skin cuts drag -- dirt, rough covering overlaps, and covering wrinkles all increase drag. You won't do much better than good sanding and Monocote! We should point out that sharp corners, even when aligned with the airflow, will tend to increase turbulence and produce more drag. A rounded fuselage is less draggy than square -- the same goes for wingtips.

Interference Drag: We did a nice little wind tunnel experiment in school: we measured the drag of a fuselage, and then the wing. Then we put in the wing and fuselage attached together. The combination had extra drag beyond the sum of the components!

The interference caused by projecting objects (like wings, landing gear, gear struts, stabs, etc.) can be reduced, usually by the use of fillets. These were quite pronounced on WWII fighter wings, as on the Spitfire and P-40, and just rounded off the interior square corners, carrying the rounding well aft of the wing. You'll see these on pattern and racing planes.

Projections: The best solution to projections is -- get rid of them! Retract the landing gear, hide the control horns, enclose the radio antenna, countersink the bolt heads, etc. Cowl in the engine, use an enclosed muffler. Look at a competitive pattern plane -- you'll see all of these features.

Like most things aerodynamic, drag reduction involves many details, all of which add up in achieving your goal. "If you want to go fast, get out the sandpaper"? Yep, but remember, we need both a smooth skin AND a smooth form!

Draggies.GIF - The graphic for this article.

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Last Update: 10/12/97