EAA Sport Aviation - December 2010

As we got older and learned about airfoils, we tried curving the hand like an airfoil (a thick one), but quickly discovered that hands aren’t very good at being wings. Drag devices, yes; airfoils, no. Those of us seriously interested in aviation went beyond hand experiments and learned that drag is a sophisticated subject, what with induced, parasitic, and intersection drag. But for most of us down here at the light end of aviation, drag remains pretty simple. Just like big, fast aircraft— airliners, fighters, business jets, and fast propeller-driven aircraft—the more stuff we have sticking out into the airstream, the more we are slowed down by it, if you’ll forgive me for stating the obvious.

THE JOY OF THE DRAGGY

Fortunately for us, we are moving at nothing like the airspeeds of the big guys, so it would seem that we have nothing like their problems with drag. When you’re only doing 60 mph to begin with, the significance of drag and its ability to eat up power and retard airspeed doesn’t count for much, it would seem.

In everyday terms, when we’re flying a Quicksilver MX-series aircraft (which is about as draggy to start with as a loosely crumpled sheet of paper), or a less-draggy Challenger, RANS, J- 3 Cub, or CGS Hawk, we won’t suffer much airspeed loss from adding some extra wires, an air scoop, struts, or almost any small object to the airframe. Or will we?

Some years back, for my MX-type Quicksilver, I built a rectangular mahogany box for basic instruments (tachometer, dual cylinder-head and exhaust-gas temperature gauges) and mounted it in the flying wires just outboard of the pilot’s seat. The box, about 8 inches long and 2 inches thick, would have added some drag, but so little that it went unnoticed at a typical cruise airspeed of about 40 mph. That’s because if you’re flying any tube-and-wire-braced, ultralight-style aircraft with no pilot fairing and a single-surface wing, drag is enormous to start with.

INTO THE COMFORT ZONE

This fact has a comforting side effect: It’s difficult to exceed the VNE, or never-exceed speed, of a very draggy aircraft no matter how steeply you dive and no matter how much throttle you apply. In the basic MX, that speed is listed as 65 mph.