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.

Al