Concur on the "be careful."
It is hard even for traditional, full scale prop manufacturers like Sensenich. They tried to make an aluminum prop for homebuilt aircraft a few years back. Just like a Cessna or Piper prop, right? No. Even though the same engine and same horsepower, the speed ranges of the homebuilts needed more pitch. No problem, either add twist to the existing forging or make a new forging that starts with more so that you don't have to tweak (stress) so much. Good now, right? No, not so much. Problem was that a slightly smaller diameter and greater pitch had different stiffness and moved the resonances (you have to account for a large number of harmonics for the blade to avoid fatigue issues, not just the first two or three...or even half dozen) and the harmonics were excited and sitting right in the cruise power band of the Lycoming. I think that project finally resulted in a restriction that prohibited operation between such and such rpms. That kept it from being very popular, I believe. I didn't order one, even though I wanted one from a maintenance and reliability standpoint over wood.
To Brett's point about the power pulses and not the rpm--the rpm if constant is not an alternating stress and does not induce fatigue. The power pulses do. And there is likely a combined alternating load case that might be important. Not just the bending mode from lift, but in-disc bending from the once per cycle firing (angular accelerations) plus the torsion on the blade since it is unlikely that you have the blade shape perfectly balanced to achieve zero moment about the blade axis. In fact, this would only be possible at one design point. All the blades I have built I tried to put excess area behind the axis of the blade. Thus it flattens out with load. If the blade is not stiff, then at least it is not divergent. Etc.
There is a reason that AMA banned metal props on models. No hobbiest can do the analysis and testing needed to ensure it will be safe. (Unless you do it all empirically such as HALT/HASS.) So they decided that we should just not go there. I just saw a metal prop from the 1940's(?) in someone's collection that was just made out of thick sheet metal, twisted and airfoiled. On anything more powerful than something like an O&R it would be seriously scary. Also remember that wood has very good damping which is an important feature in fatigue situations. Wood is often considered fatigue resistant. Not sure I ever saw anyone analyzing wood for six million cycles.... For composites, depending on what you are using for resin, a carbon prop may or may not have much damping if I recall my comparison sheet correctly. I'd bet that a resin hard enough to work well in the prop hub (avoid thermal creep due to mounting compressive forces) might have very low damping. But the fatigue life should be very good if it is not operating at too high an alternating stress level. See the rough graphical comparison I borrowed off the internet.
Interesting project. Good luck, Motorman
Dave
Topic: blank (Read 1650 times)
