Re: Need a Sparrow Strainer alternative


Do a web search on "Gurney flap". Dan Gurney put the first one on Bobby Unser's Eagle Indy car in 1971; it's now widely used on aircraft, especially helicopter tail surfaces. It's a length of (usually) aluminum angle at the trailing edge of an airfoil that alters the airflow in ways that increase the pressure on the Gurney-equipped side of the surface and decrease it on the other side with little or no increase in drag; i.e., it improves the L/D while also increasing lift or downforce.

I'm going to try Gurney flaps as a substitute for sparrow strainers once I get my Dragonfly flying. I'll remove one strainer and replace it with, say, two feet of Gurney; if high-speed runway testing goes okay I'll try a flight. I'll change the span of the Gurney flap until its effect matches that of the sparrow strainer, then put an identical one on the other side. Careful before/after testing *may* reveal a speed improvement, but I'll settle for no loss -- the robustness of a Gurney alone would make it worth using.

The keys to making Gurney flaps work efficiently are to keep them within the boundary layer -- they seem never to be more than about three-eighths of an inch tall -- and not to mount them so that they form a ledge on the opposite side of the trailing edge since this destroys the smooth flow out from under. It also might help, and might even prove essential, to put fences on the ends -- on racing cars they're always configured that way, while the aircraft applications all seem to be full-span, possibly obviating the need for end-fences. Full-span Gurneys would seem to be too much for this application, though only flight-testing will tell. Worth a try, I think.

Rick Nordgarden
Council Bluffs IA
Dragonfly Mk. II-H, installing systems...

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