Subject: [FlyRotary] FW: [VAF Mailing List] Engine Choice

Guys,

Forgive me for = beating a dead horse about torsionals. I have been reading another list for = (VAF and RV-7) ~2 years, and have anxiously anticipated installing a 13B, and = now instead a Renesis engine, in my soon-to-start RV-7. Now, I am = hearing things that cause me to question my ability to make a sound decision in = this regard, as this is the first discussion of torsionals is the first I = have heard. I am beginning to question my ability to follow through on = what I had taken to be the best alternative engine choice.

I am therefore = forwarding the most seemingly competent post (I am not enough of an engineer to = evaluate this) regarding this issue. My first impressions were that = rotaries were much less susceptible to torisionals that regular engines, but recently = that view has come into apparently serious question, based on the statements = below.

Appreciate your = thoughts and comments.

Greg;

There is no question that there is a = concern of torsional vibration resonance with any reduction drive on an engine with = pulsed output, piston or rotary. Bill makes it sound as though it is more difficult with a rotary than with piston. I’d disagree = – it is just different. The issues inherent in the problem of vibrational = resonance are complex and difficult to identify; so certainly in early experience = it seemed mysterious. And the natural response to things breaking was = to make them stronger, and they still broke because the resonant frequency = of the system was still in the operating range, and the energy buildup in a = resonance can be huge.

Having the resonance occur at higher rpm = (stronger, stiffer system) puts them into the region of higher power, and makes = them very difficult to solve. Adding energy absorbing dampening into a less = rigid system (hopefully) gets the resonant frequencies to the lower rpm and = power end of the operating range, or out of the operating range altogether. = So; as Bill points out:

There are two ways to address this issue but both involve addressing =
the natural frequency of the entire drive system, …….<snip>

· The = second method is to develop a drivetrain that makes use of a
strong and durable coupling, which reduces the natural frequency =
below the operating range of the engine. Furthermore, this coupling =
should also incorporate a damping mechanism, that would allow the =
system to absorb any possible feedback from any variation of =
harmonics the drive might see.

I would say that this = describes Tracy’s drive = quite adequately; and at least as it applies to a 2-rotor, it has been proven = to be reliable.

I am going to be using the RD-1B = on a 3-rotor, with a three-blade prop. I’m optimistic that it = will work just fine; although there is the possibility of an excitation of = resonance at a higher rpm and higher power. Not likely, since the damped coupling = should keep the higher frequencies at bay; but I can’t know for sure = until I get there. It won’t be too long now as I expect powered runs on = the airplane in a few months.

I have run the engine on a dyno, = direct coupled, up to 7000 rpm and 285 hp with no problems whatever. = Putting in a undamped spur gear reduction would be asking for a potential problem, = whether a rotary or a piston.