Hey Mark, I'm not an engineer at all so this may be a dumb question. What are your thoughts on using a dual mass flywheel with centrifugal pendulum vibration absorbers to 1) add mass moment of inertia on the engine side as you said and 2) dampen/absorb torsional vibration?
I am unsure about the effectiveness of a DMF/CPVA at higher RPMs since they have mostly been used to help isolate the torsional vibrations of a 3rd order torque impulse signature of 3 cylinder engines at lower revs.
But do you think a combination of a DMF with one of your gearboxes with a low MMOI propellor could be a reliable unit?
I understand that you asked for Mark's opinion, but if I may, I have professional experience in these very areas
Dual Mass Flywheels were invented to help with isolation of engine from transmission. The issue in automotive applications is that the soft spring in such a system is normally in the clutch hub, and thus downstream of the large inertia turning and oscillating with the crankshaft. Big mass - spring - little mass. Achieving good isolation requires very low spring rates and then a lot of spring to both have low rates and enough torque capacity. This can get expensive and take a lot of volume that simply is not there. An alternative is to split the engine side inertia and put the soft isolation spring between the inertia, effectively moving a bunch of inertia downstream without adding much mass or total inertia. It also makes a lot more volume available at the OD of the flywheel for packaging the springs. The large operating radius of the arc springs used in DMF also become subject to large hysteresis - lots of friction from centrifugal forces - and so their spring rates rise rapidly with rpm. This has two undesirable tails: Transmitting much of the firing pulses at high rpm to the downstream components, and; raising natural frequency of the system as rpm goes up. Also, the springs for these devices are typically designed for the engine and transmission combination, and they are arc springs for minimum drag in their tracks.
So, does this rather highly optimized for a car gadget work for an engine-PSRU? Depends upon what you are trying to do. The original post is about a "stiff" system. You keep the stiffness of everything way up and keep the inertia way down. You can not have any soft elements in this system. If instead, you are trying to run a "soft" system, your spring rate will rise quickly with engine speed, making your natural frequency go up with engine speed, and maybe faster than the firing frequency does. If the approach each other, the system will try to tear itself apart. It would have to be carefully checked out to make sure that you do not bring resonance in as rpm comes to flight levels, and then checked again to make sure that the transmitted firing pulses are within the long term capacity of the downstream parts to survive reliably. If your needs do not match the available pieces from the dealer's part counter, you then have to design new springs and see if you can get them made. I do not even want to think about how many thousands of dollars your set up cost will be for each spring you scheme out.
They might be made to work, but I would sure not want to try to make any money doing it for a little airplane PSRU.
Now to centrifugal pendulums. These gadgets have been around since the 1930's in big piston engines. Generically, they are order tuned absorbers. They were introduced in cars over a decade ago, with high end diesels being the first applications. They are tuned to specific number of oscillations per engine turn and for the fluid they are operating in. Air is different from automatic transmission oil. If you find a two-per-rev gadget for a manual transmission and your engine has two firing pulses per rev it could work. But if you swipe the two-per-rev pendulum damper from a torque converter, well, you will have to run it in ATF if you want it to pull off firing pulses. Everitt Hatch built a pendulum damper for the Powersport conversions, and it was expensive to make, so he backed out and came up with the stiff system being revived by the OP of this thread. Ev's pendulum damper appeared to have more than one tuned order (looked like three to me). Besides their being expensive, in airplanes they have had a tendency to be sensitive to rapid movements of the prop lever.
The OP is not working in either of these areas. And Mark Kettering has figured out where the sweet spots are for his engines, PSRU, and likely props are with his elastomeric spring isolators.
I like the sound of your 3 gear gearbox with the 9.8" offset. From what I have seen from other rotary conversions, most builders are forced to custom build intake manifolds to be able to fit under cowling due to the zero or low offsets of their redrive. Therefore they give up the opportunity to utilize the many well tested manifold options out there for rotaries in the automotive world because of the height issue.
True. Wankels have been sometimes dicey to make work for turning props. If I were Mark, I would stay away until I ran out of other engines to develop. He has some very good packages in the works. As to making room for appropriate intake manifolding, Ev Hatch laid his Mazda on its side for pacjaging of engine, manifolds, etc, and it worked fine. Too bad we lost that fine gentleman so early.
Billski
