From: mpaton@oakhill.sps.mot.com (Michael J Paton)
Subject: Re: Prop-speed reduction units and torsional vibration
Date: Mon, 16 Nov 1992 18:33:09 GMT
Reply-To: mpaton@tempest.UUCP (Michael J Paton)
Organization: Motorola Inc., Austin, Texas

In article <1992Nov16.135938.12165@dartvax.dartmouth.edu> Charles.K.Scott@dartmouth.edu (Charles K. Scott) writes:
Stuff deleted..
>....................... and in one case resulted in an engine stoppage
>so sudden that the tips of the propellor splintered.  ................

I was discussing this with a local Cirrus builder just yesterday. Specifically
should one design for failure, or build strongly enough so that failure
doesn't happen. I'm not sure there is an answer; as it's for an airplane,
I suppose it really depends on the weight penalty. My point here is that I
agree that torsional oscillations can cause shaft failures, but whether the
shaft failure causes a sudden stoppage of the magnitude you describe is
another design issue entirely.

>In my talks with Lou and Chris Ross who are also producing a planetary
>gearbox PSRU, they thought that Hatch and Beckham were "reinventing the
>wheel" with what they were doing with the rotory.  Lou's opinion was
>that the rotory was fine just as it was and no extensive modifications
>were necessary.  To counter torsional vibration, they (Ross Aero) uses
>a machined clutch plate driving the gearbox.  The springs in the
>clutchplate are supposed to counter the torsional vibrations
>transmitted by the engine.  HOWEVER, in yet another recent article in
>EAA's "Experimenter", Bill Husa says the following: "This installation
>demonstrates a clear misunderstanding of the phenomenon. The springs in
>the plate only change the natural frequency of the vibration (changes
>the system's spring constant), the configuration has no ability to
>absorb and dissipate the vibrational energy.  Eventual catastrophic
>failure can still occur if the magnitude of relative motion builds past
>the point where the springs are able to absorb it."

This is true as far as it goes. However I find the "clear misunderstanding"
remarks a bit arrogant. In any constant speed system I believe it is
common practice to simply shift resonances out of the normal operating
frequency range, by means of springs or masses. If Bill Husa is proposing
(and no, I haven't read the article) to leave resonances in the operating
range and absorb them, then he'd better give some very serious thought as
to how he is going to dissipate energy which may range from 5 to 95% of the
engine power output. In a car suspension system it is the "shock absorber"
which dissipates excess vibrational energy as heat, and race type vehicles
can easily require much higher heat dissipation rated units. 

With a suitable constant speed propeller it should be possible to get away
without any torsional vibration absorbtion IF,

	1	The system is operated at a single speed

	2	The vibrations are simple enough to be shifted away from
		this speed.

	3	Any resonant ranges which must be passed through on 
		startup or shutdown can be passed through quicly enough that
		the magnitude of the resonance remains small.

Of course, this is the real world, and nothing is that simple, mostly I think
in point 2. However you did ask for discussion.

Michael Paton
mpaton@tempest.sps.mot.com