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Took closer note of throttle position on todays
flight. At 2000 ft the throttle was only about 1/3 open (position of
throttle quadrant, but it closely mirrors butterfly position) at economy cruise
setting which was 6.0 GPH. This is about 42% power, 82 HP according
to the EM2 which is fairly close but not perfectly calibrated yet. The
same power setting with the -B drive would bave been a bit further
open.
I did a quick check of MAP at 1000 ft at full
throttle and found I was not getting any drop at 150 MPH and had .5" boost at
220 MPH (ram air recovery I assume). I did not get around to checking it
in slow climb which would more accurately compare to what you would see on a
dyno.
Finally got some reasonably calm air to do
performance comparisons of -C drive vs -B drive. Without the prop blade
cuffs it looks like the break even point is at 203 MPH. Above that -C
burned more fuel than the -B. After installation of the cuffs, the break
even point was off the scale! i.e., above top speed with -B drive. I had a
good data point on the -B drive while burning 17 GPH (209 mph during SUN
100 race). At the same speed, the C drive was burning 15.8 gph. This
was better than I had hoped for.
Tracy
Thanks,
Tracy. I was hoping
there would be someone out there flying with the same TB
diameters.
Like most things,
TB diameter is a tradeoff. My conclusion from the dyno data is that 44mm
per rotor (1 ¾) is a bit small as the MAP is dropping off over 5000 RPM.
But if you want to idle at 1500, and have a decent transition from there
to 3000; 1 ¾ is good. For a 2.85 to redrive, I’d want to increase that
flow area by 30% or so – to about 2” dia for each rotor.
My data may not be
representative because of restricted flow to the TB. The ‘airbox’ size
is restricted by the cowl, and may have restricted the flow a bit. In
hindsight, it would have been smart (and easy) to make a run with the airbox
off and see what difference it made.
Al
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