With a 2.85 psru and a "small" diameter prop, you are turning the prop
slower than if you had a 2.17, so the tip speeds would be
relatively low
- good for noise reduction. To go fast with a "slow" turning prop will
require a lot of pitch, which might tend to stall more at the start of
takeoff roll. But you have lots of power so a prop can be built that
will work. What is max RPM you want to operate the engine?
Perry
Perry,
I do not want to go much beyond 7500 RPM and would go lowest at 5000
I have 48 " from the thrustline to the ground and the prop flang is
41.5" to
42" from the firewall. Right now this is theoretical as I have not
purchased the PSRU because I am trying to decide on a ratio. Thanks
for any
input.
Joe Berki
I think mine measures about 48" from thrustline to ground at the prop,
but I lowered the thrustline a couple inches because of the ducted fan.
When I build the new engine mount I plan to raise it back up where it
should be. And your prop is farther aft than mine and probably most
Long-EZs. So I don't think you can go larger in diameter than the
recommended 62 to 64 inches on the prop. Max. RPM on a 62" prop is about
3000, exceed that and you start to go above 800 fps at the blade tips.
3000 prop RPM is 6510 engine RPM with the 2.17 drive, which is where I
run mine. My engine redlines at 7000. If you want to go higher than 6500
engine RPM you would need the 2.85 drive. 7500 / 2.85 = 2630 prop RPM -
reasonable. But you will still be stuck with the small prop diameter to
avoid prop strike with nose high attitude on landing or takeoff.
To go 220 mph with your 2630 RPM prop and your engine running at 7500
RPM you will need 88" of pitch, which might give you some grief on
takeoff.
Perry
Perry,
14" is a lot of clearance, is that because it's a pusher? I'm working
on 10"
for my tractor application with a minimum of 6". The more the better
is an
'understood', provided you have the capability to extend the
undercarriage
legs. Which is something I'm considering.
George (down under)
Yes it's 14" while rolling but will be less when you raise the nose on a
pusher. It will be over 10 degrees pitch to actually get the prop to hit
the ground. I land pretty hot so as not to get the nose too high. You
can calculate the angle that your prop will hit: measure the horizontal
distance between the main wheels and prop plane (x) and the distance
between prop tip and ground with the aircraft level (y). max. pitch
angle = inv-tan y/x
Perry
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