Need information for use in designing a variable-pitch propeller

[piolenc]

16 minutes ago

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I've been exercising my neurons about variable-pitch propellers for some time, because it turns out that a VP prop is essential to several projects that I plan to undertake. After a fair amount of skull sweat I have come up with a mechanism that I think will work, but now I need solid numbers to use in sizing its components.

The centrifugal force will be taken by the propeller mount bearings, and all bearings will be low-friction rolling element bearings. The remaining friction forces should be calculable from manufacturer's data. There is however another force which I know exists, but don't know how to calculate. This is the force that tends to force an unrestrained propeller to take the full flat pitch position. It is caused by the fact that, in that position, the propeller has the lowest potential energy with respect to centrifugal force (i.e. all mass elements are at their furthest outward point). Unfortunately, I haven't been able to figure out how to calculate the force driving the prop to that position, and I need to know it to know the actuation force for each blade and for the operating shaft. If it is too great, it is possible to partially compensate for it using counterweights, and those too need to be calculated.

If anybody knows of a book or a technical report that can help, I would be grateful to hear about it

 

[WaspAir]

I don't have a technical contribution on this one, but merely a comment that constant speed props on acrobatic airplanes fail to coarse pitch to prevent overspeed rather than flat pitch, and that might be something to look at.

 

[bryancobb]

piolenc,

GIVENS FOR BOTH 1 and 2
RPM is 100
Distance (r)1 = (r)2

Let's see if I can shed some light on the topic. What I was taught is the phenomena is called "centrifugal feathering." Look at the two figures I quickly sketched and uploaded below. In illustration 1, the blue barbell with substantial mass at it's tips is being slung around AXIS A/distance (r) at 100 RPM creating most significant centrifugal force at green distances (d). In illustration 2, the blue barbell with substantial mass at it's tips is being slung around A AXIS A/distance (r) at 100 RPM creating most significant centrifugal force at green distances (D).

It's easy to see that distance (D) is farther than distance (d), so the barbell in Illustration 2 is not in equilibrium because wants naturally to move to the position of equilibrium shown in illustration 1 (where retention centripetal force along (r) = escape centrifugal force sum along (d)+(d) ). That natural movement seeking equilibrium is called centrifugal feathering.





Now propeller designers use things like engine oil pressure increase/decrease that occurs with crankshaft RPM changes to create hydraulic pressure for use to vary prop pitch and they install "flyweights" sometimes to mechanically vary prop pitch automatically with RPM changes. That info is easier to find on the web than info on centrifugal feathering. The FAA Handbooks have great explanations.