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Gary: I haven't flown Oscar's plane for a while, but as I
remember, it started off somewhere around 2300-2400, dropped about 75-125 rpm
during the takeoff roll, then was back up to 2400 during a climb at 110 mph
IAS. The replacement, highly modified Great American prop on my
Lancair, to replace my beautiful three-blade ELIPPSE prop which I destroyed in a
gear-up landing at Santa Paula, before an open-house crowd, displays the same
high-low-high rpm thingy! My three-blade didin't; it started at 2230 rpm and
just continued to increase rpm during the takeoff roll, reaching 2400 at
lift-off at about 70-75 mph IAS, then continuing at 2410 while climbing at 110
mph IAS. With my 125HP O-235, it gave me an ROC of about 1450-1550 fpm at
1000 dalt at 1350 lb. Oscar and I did a lot of calibration flights on his plane
and on different props we tried on it. Initially his GA prop gave 214-215 mph
TAS at 2600 rpm. I did a slight mod to the prop tips, which boosted speed to 219
mph at 2750-2760 rpm. The rpm increase was due to the greater efficiency of the
prop converting HP to thrust which gave more speed. That 2.1% increase in rpm
was hardly sufficient in increasing the engine horsepower the 5% necessary for
that speed increase. My GA and another of Oscar's props was modified by
increasing root chord and pitch and reducing tip chord, and we were able to get
more speed on less rpm, generally increasing efficiency 5%-10%. The reason many
fixed-pitch props have that high-low-high rpm characteristic is that different
parts of the prop come out of stall at different loads/rpm, whereas my design
shows the same CL and lift all along the blade vs speed. It typically doesn't
have the static and low speed thrust of the typical FP prop, but once it gets up
to about 40-50 mph IAS, it really comes into its own. Tom Aberle describes the
effect as having the same effect as cutting in an AB at about 70 mph on his
winning biplane.
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