Subject: [FlyRotary] Re: 20B

 

Al,

 

Same here on my 20b. Out of a 94 Cosmo.  It has the counter weight under the

accessory cover, no harmonic damper in the front or back.

 

Barny

 

The reason I had asked this question is because in doing some development testing on a 20B, Powersport determined the need for a harmonic damper on the shaft at the front to reduce a torsional vibration.  The test were done with the Powersport re-drive, which, if you are not familiar with it, is a tight tolerance spur gear arrangement which is designed to be “stiff” in order to keep any resonance frequencies above the top end of the operating range.

 

I have been looking (again) at the torsional vibration issue, and wondered in Mazda had determined there may be a problem with the 3-rotor, either regarding the engine internals, or the driven accessories.  Upon further thought, I realized that Mazda only used this engine with a torque converter coupling to an auto trans; and the torque converter is about the best vibration damper you could find.  Putting a stiff drive on the output end changes the picture entirely.

 

Tracy’s re-drive design takes the opposite approach; i.e., a flexible coupling with some play in the gears which lowers the resonance frequency to keep it below the low end of the operating range.  This obviously has been quite successful in using with the 2 rotor rotary.  Use with the 3-rotor is not yet proven. Going to a 6-planet system obviously can handle more continuous torque, but if you do have a resonance in the operating range; 6 vs 4 doesn’t really make much difference, because you can get torque amplifications of 10 or more.

 

On first cut, it seems to me likely that picture with the RD-1B on the 3-rotor may be even better than on the 2-rotor because of a further separation of the driving frequency (higher because of more pulses/rev) and the natural frequency of the re-drive system (which stays roughly the same).  Unfortunately, until you test, you don’t know, because the entire (geared) rotational system from front pulley to prop is extremely complex analytically.

 

Now; if you are not asleep yet, I will tell you that I was concerned about the amount of belt vibration (alt and pump drive) I observed on my 20B during the dyno runs.  This could be caused by some non-uniformity in the pulleys, but there seems to be no measurable runout.  It is likely that it was the result of torsional vibration.  This could result in reduced belt life, or a failed alternator mount, or nothing.  One thing is pretty certain, a harmonic damper on the front would reduce the risk of a problem.

 

The torsional vibration characteristics of the system will be different for the engine, with the re-drive and the prop, than it was on the dyno.  It won’t be long (crossed fingers and toes) before I can observe what happens on the airplane.

 

If I could find a harmonic damper that will mount to the front end of the 20B e-shaft, I’d look at putting it on (especially a viscous damper).  It could reduce the torsional vibration for the entire system.  (Anybody know of one?)

 

BTW; I’m not trying to be an alarmist, it’s just that torsional vibration in pulsed drive engines is a fact of life – auto engines, airplane engines; whatever.  Any torsional vibration resonance, which is what can get you, is something manufactures have always had to deal with.  People have in the past wrecked dynos and broken drive shafts when unknowingly running into a torsional vibrations resonce, and one topic-du-jour example is the MT prop on 4-cylinder Lycomings.  Seems that there is mysterious cracking of prop blades, and MT has restrictions on ceratin RPM ranges.  I find evidence (although limited and may just be my interpretation) of problems with a 3-rotor and the Ross drive.  So we need to be observant when going into uncharted territory.

 

Al