If you can get your hands on "Converting Auto Engines for Experimental Aircraft" Second Edition by Richard Finch I recommend it for good reading and great insight. FWIW
On Tue, Oct 22, 2019 at 2:56 PM Daniel Branstrom via KRnet < [email protected]> wrote: > I am not an engineer, and my comment here is based on some reading and > observations that I have made over the years in aviation. My opinion is > worth the electrons that were used to produce this post. > > In every machine that has rotating parts, torsional harmonic vibration > is present. I'll call it THV. If a vibration coincides with an operating > range of the machine, the THV can destroy the machine. If the frequency > is above or below the operating rotation, it becomes much less of a > problem. In an automobile, THV is damped by the rubber tires, and the > greater rotation mass of the flywheel, and, often, automatic > transmissions. On an airplane, the power pulses go undampened to a > propeller, which has its own harmonic vibration that can be fed back to > the engine at certain frequencies and can destroy the engine. Wooden > propellers tend to dampen this because of their composition. > > Aircraft engine manufacturers take torsional harmonic vibration into > account when designing their engines. That's why some certified direct > drive engines come with a warning not to operate them at certain rpms > with certain propellers, or they have counterweights to smooth things > out. Rotax engines, for example, have a coupling that dampens THV. > Engines are certified in aircraft with designated propellers for that > reason. > > Torsional harmonic vibration can snap crankshafts and destroy reduction > drives. It is similar in concept to aerodynamic flutter in flight > controls. The THV does not disappear where the power pulses are > overlapping, so that all of the forces going in same direction as they > are in 6 or 8 cylinder engines, but at least all of the force going to > the prop is in one direction. 4 or less cylinder engines have periods in > their rotation when there is a backward force on the power output. The > inertia in the engine, as well as flywheels or propellers keep the > engine turning. > > There are very few companies that have been successful in experimental > aviation with reduction drives. That doesn't mean that there aren't good > ones out there, but the amount of engineering and manufacturing skill > needed to build them can be immense and costly, and they have to have a > big enough market to be profitable. It's also my opinion that as the > power of the engine goes up, the forces on any reduction drive go up > exponentially, and that's why it's easier to make a drive for an engine > developing under 100 hp (which seems to be the dividing line) than one > that has more power. It's for those reasons that I don't believe you > will see a Rotax 200 hp engine. > > Years ago, there was a manufacturer of a redrive that was used on large > engine conversions. It seemed to be a good one, with few problems, but > when the drive was manufactured by someone new, the drives started > breaking. I credit the problems to manufacturing technique. > > I have flown behind a Cessna 175, which has a geared engine. It was > excellent both in climb ability and cruise speed, I found it close to > the performance of a 182, and it had a fixed pitch propeller. The 175 > got a bad reputation, I believe, because pilots tried to operate it like > an ungeard engine. The engine was designed to run at above the red line > of ungeared engines, and operating it at low ungeared rpms is very hard > on it. Also, it was not good practice to let the prop drive the engine. > > Consider the reputation of anyone making a reduction drive, and look for > reports of people that have problems with it. Google engine problems or > engine issues, and look at what other engines they have sold in the past. > > That's my 2 cents. Your mileage may vary. > > Dan Branstrom > > > _______________________________________________ > Search the KRnet Archives at > https://www.mail-archive.com/[email protected]/. > Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options. > To UNsubscribe from KRnet, send a message to [email protected] > _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/[email protected]/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to [email protected]
