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I modified the prop tips on my friend Oscar's Lancair
235/320. It was a Great American two-blade fixed-pitch prop, and we got 214
mph-215 mph TAS at 8500' dalt at 2600 rpm, totally stock O-320. I made the tips
into a slashed tip, with the tip shape sorta' elliptical in plan-form from the
LE to the TE, but with no change in diameter at the TE. The lower surface of the
tip was slanted up to the top surface to form a sharp edge all along it. The top
speed increased from 214-215 mph TAS to 219-220 mph mph TAS at 2660 rpm,
the rpm increase due to the increased thrust from the prop giving higher speed
and rpm, not from increased power. Since horsepower is roughly a cubic function
of speed ratio, going from 214 at 2600 to 219 at 2660 is an efficiency
(horsepower) increase of (219/214)^3 x 2600/2660 = 4.8%; this from some simple
tip shaping. BTW, these speed numbers were carefully arrived at from GPS, not
IAS! My three-blade ELIPPSE prop on my 235 with a 125 HP O-235 gets me off the
ground in 1000' at 1350 lb, 500' dalt according to the tower personnel. I
typically climb at 110 mph IAS, 2410 rpm, and get a ROC of 1400-1550 fpm at
1000' dalt. I've modified several props by increasing the root chord and pitch
with fiberglass and modifying the tip shape and have gotten more speed with less
rpm. The tip shape is the biggest contributor to prop in-efficiency, with
the other being the poor helix angle and the un-aerodynamic shape of the
root airfoil.
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