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Rotary progress:
I'm making up the Aluminum housing and PSRU mounting plate out of
2024 Al. now. I found that I will have to have to make a new sun
gear combination, splined shaft, to get a robust connection to the
drive spline. I'm still not completely convinced on the method for
a torsional resonator absorber. Mistral used springs and Tracy uses
rubber. I'll have to make a choice, run it and then see what
happens. I worry about the increase cycle rate and durability that
springs would encounter. Having the prop spinning a the same rpm as
the rotor could potentially be a good or bad thing. I'm beginning
to understand there is no one-off solution. Different propeller
weights and materials will resonate at different frequencies.
Cheers
Doug
Springs give no damping.
Experience with two entirely different rotary engine and coupler
designs and with different displacements, both resonating just
below 2000 RPM more or less, acting identically, I conclude there
is no solution. Idle the engine below 2000 RPM and forget about it.
After all the prop is turning at 700.
I gave the book to Robin.
Paul Lamar
I believe there are some clutch hub springs damped with rubber for heavy
trucks. How about injecting some silicone in the springs for damping?
Dale Davies
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Trucks have springs in the clutch for taking off initially. After I
let the clutch out and get moving, I dont use it again until I need
to stop. If you have ever driven one of these things, you will
notice that if you are not soft and gentle with the application of
the clutch (letting off the pedal) the thing starts to buck like a
bronco. The springs tend to add energy to the system, which leads up
to boing boing boing and an embarassing start to the trip. I would
think that springs in the backing plate could also add energy to the
system and ultimately cause a catastrophic failure. Imagine sitting
on the taxiway at idle waiting on your turn to go. Each firing of
the rotor causes an impulse that is transferred to the springs and
then ultimately to the prop. Your engine and prop are happy, then
along comes a brisk gust from behind (you are on the taxiway pointed
downwind). This gust will load the prop and compress the springs. At
the same time the rotor impulse subsides, the gust stops, and the
energy built up in the spring gets transferred to the prop, only
this time the prop tries to outrun the engine. Now here comes
another rotor impulse, the engine quickly catches and passes the
prop since it has virtually no load at the moment, and the cycle
repeats. I can see a scenario like aileron flutter- a few short
seconds and boom its over.
The closest thing I can think of to the problem that is being
discussed is a tractor and bushhog. At idle with the PTO in gear
there is considerable back and forth when each cylinder fires. Rev
up the tractor and it goes away. We solve this by using an over-run
clutch. This little device lets the bush hog outrun the tractor and
then the tractor gets to catch up. The engineering is sound, but for
us I would be concerned with the prop overspeeding during a descent.
Another alternative would be a constant pitch prop- at idle the prop
could be set almost feathered for the most drag. At run up or
takeoff, reduce to fine pitch. Lots of performance planes go to
feather at loss of oil pressure, maybe someone with a CS prop could
test this theory?
I don't think we need to dampen the system as much as just keep the prop
behind the engine- i.e. under load.
Another possible solution would be a belt drive with what would be a
tensioner device like on your car. This little wheel keeps the belt
tight, but also takes the shocks out of the system. For this reason
I kind of like the Ross type of PSRU with the big belt. I have
bulldozers with rubber tracks, so the technology is here to transfer
the power from the engine to the prop effectively and reliably with
a belt. If you look at many of the small helicopters, they have belt
drives from the engine to the gear box.
Kevin Alderman
It is not a problem over 2000 RPM.
We have several airplanes flying with hundreds of hours.
Paul Lamar
The springs in the clutch are there to take out the firing impulses during
initial take-off at low RPM. The rubber in some of them helps life. A bigger
contributor to the "bad start" to a trip is the drive shaft and frame winding
up like a torsion bar spring. I worked on a "double breasted Yamaha once that
the engine stop solenoid had failed and the cab jack for the cabover was not
working. Tried just letting the clutch out to stop it, but did not realize to
use a high gear. Damn thing just rocked back and forth a couple of times. When
it wound up the first time and the engine stopped for a fraction of a second
and then started backwards. Had to get my head into it to hit the clutch and
change to a higher gear when running the correct direction.
Dale Davies
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