Hey guys, my crazy side visited me last night while I slept and planted a
seed.  Here is what my alter ego said-

If we are going to end up with a functioning EFI, why not use it to set the
RPM we want like jet engines do?  We could have say 3 or 4 detents or set
points, WOT, cruise, flight idle and ground idle and let the engine control
the rest of it automatically.  No more prop overspeeds, etc.  Set the RPM at
6000, start a climb and the computer would keep the 6000 by adding more
throttle.  Start a descent, same thing but backwards, the EFI would throttle
back to keep 6000.

It would require a servo motor on the throttle, and not much else. The
throttle cable could still be there for manual over ride if the servo motor
failed.  Kind of like cruise control in cars.  Turn it on and its on, tap
the brakes and its off.


Kevin Alderman


What prop overspeeds are you talking about?  As Paul has so wisely
pointed out, a prop won't normally overspeed that much because as the
RPM rises beyond a certain point, the blade tips approach
transonic/supersonic airspeed, which dramatically increases drag.  The
RPM will stop rising.

If you design a fixed pitch prop to allow (provide) 6000 RPM while
climbing (lowered airspeed), when 6000 RPM is maintained in cruise,
power will need to be significantly reduced and this will definitely
reduce cruise airspeed.

A correctly designed prop will be operated with the prop tips around
mach 0.7 or so - an efficient RPM.  It should be reasonably easy to
make the overall system (rotary engine + reduction unit + propeller)
capable of withstanding operation 25% over the best efficiency RPM.  I
would say this is mandatory.

If you're concerned about flutter, you might consider a
throttle-by-wire system that starts reducing power if a max _airspeed_
is exceeded.  Say at VNE.  That wouldn't keep you from blowing through
VNE, but you'd have to be pointed downhill much more steeply.  You
could even have a turbulence driven airspeed limit that was lower than
VNE.  One thing to keep in mind though is that many airplanes have a
noticeable pitch response to power changes.  And, I know that power
isn't what controls airspeed.. But power does (literally) add energy
to the system, and total energy balance (thrust, potential energy drop
(altitude change), and drag) is what determines airspeed.


Regards,

Matt-

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