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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