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wrjjrs@aol.com wrote:
Rusty,
You may be getting something that was studied with dirt bike singles and narrow angle v-twins (Harleys). These engines worked well in getting traction on dirt tracks due to their firing order. If I recall it was called climb and retreat by the guys actually studying it. It would apply to a prop very well. Your prop has at the very least some flexibility in the blades. The power pulse hits and forces the blade forward violently, followed by a lull. Think of it like driving a long screw into wood. Every 1/2 turn you must re-position your grip. Any twist in the shaft of the screwdriver is released until you start turning again. The fore and aft movement might be caused by the large power pulse of the single rotor. This may be a totally natural event. OF course I've been wrong before, YMMV etc...
Bill Jepson
Another dynamic factor will be that when you hit the prop with that pulse, will it make the blade twist? That will be wholly dependent on the mass distribution around the blade chord at each station. There's no way to make the blade 100% stiff, but I think composite blades would be more flexible than other types. So, the pulse hits (hard) and produces a slight twist in the blade which increases the angle of attack. Producing more lift on the blade, which is already being stressed by the power pulse, causing it to flex forward. The pulse goes away, or even negative, and the blade snaps back like a loaded spring.
You might want to consider finding a way to extend the prop further out away from your trailer before you try any more experiments. Then see if maybe you can borrow a few different props and see what sort of reaction they give you. You might find that a different prop will have a different mass distribution that doesn't exhibit the problem. Or maybe you can make one yourself that has extra material to resist warping.
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