Hello Paul et al,

I am now starting with my B.Eng thesis work, which I decided to be:
A Rotary PSRU for constant-speed propellers.

Because of availability, i think it is best to design this unit first
for Renesis engine, and for FAA commuter class req's. Aerobatic, 13B and
20B can come later.

First, I need to find valid parameters to design the unit.

1) Requirements for external forces, as laid on FAR 23.371:

##########
§ 23.371 - Gyroscopic and aerodynamic loads

(a) Each engine mount and its supporting structure must be designed for
the gyroscopic, inertial, and aerodynamic loads that result, with the
engine(s) and propeller(s), if applicable, at maximum continuous r.p.m.,
under either:

(1) The conditions prescribed in Sec. 23.351 and Sec. 23.423; or

(2) All possible combinations of the following--

(i) A yaw velocity of 2.5 radians per second;

(ii) A pitch velocity of 1.0 radian per second;

(iii) A normal load factor of 2.5; and

(iv) Maximum continuous thrust.

(b) For airplanes approved for aerobatic maneuvers, each engine mount
and its supporting structure must meet the requirements of paragraph (a)
of this section and be designed to withstand the load factors expected
during combined maximum yaw and pitch velocities.

(c) For airplanes certificated in the commuter category, each engine
mount and its supporting structure must meet the requirements of
paragraph (a) of this section and the gust conditions specified in Sec.
23.341 of this part.
###########

That sets the requirement for PSRU case and engine mount.


2) Second requirement set is the static values, like engine power,
torque, rev range, and intended prop rpm on cruise.

Max power and torque for stock 6-port renesis is stated as:
- 177 kW / 231 SAE hp @ 8200 RPM
- 211 Nm / 300 ft/lb @ 5500 RPM
This will change with cruise-tuned induction manifold. 6000-6400 is a
likely choice for 75% cruise.

Idle RPM is set to abt. 2000 RPM on current 13B's, so we are likely to
follow suit there.

Prop cruise RPM that Mistral chose was 2000rpm @ 6000 rpm, ie. ~3:1
reduction ratio. Their statement was that it gives -10 dB reduction on
prop noise. That's not bad, can be a starting point.


3) Third requirement set is the dynamic properties of the engine,
absorber, gears, and prop.

Without absorbers, power pulses can beat away the gear teeth, and props
don't like beating either. It's true that current crop of LyContisaurs
are direct drive, but the props are accordingly designed that much
heavier. So, there's also a possibility for saving on prop weight here.

13B 180 deg. combustion pulses are on 2nd harmonic - so on 6400 rpm
cruise we see 12800/60 = 213 Hz excitation on the end of the engine
shaft. That can be most effectively dampened by harmonic damper, that
sticks to 2nd harmonic, no matter of rpm. The remaining pulse spectrum
(ie. 3nd, 5th etc) could be handled with an elastic absorber, or
something more intelligent.

If idle is set to 2000 RPM, system critical frequency should be 3-4x
lower, ie 660-400 rpm. That could be a starting point.


So, the biggest unknowns for a good design are:

- 13B-MSP torque spectrum: x=RPM, y=amplitude, z=x.nd harmonic - 3d map.
- Engine inertia and spring values
- Prop inertia and spring values

All these can be measured on a proper rig, or got from valid
manufacturer's data (which i don't yet have...)

But reasonably accurate estimates would serve just fine as a starting
point.

Anyone? Please.

Cheers,

Petri

Are you sure about that 300 foot pounds on the RX8 engine. I think
that is in error. More like less than 200 foot pounds.

Paul Lamar


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Yes, that's right. I forgot to translate Nm to ftlb. Should be 156 ft/lb