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Old 12-04-2005, 02:07 AM
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Location: Over da rainbow, KS
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Default RE: Transitional Pitched Prop???

This thread is like describing an elephant.

The aerodynamics of propellers is very complex, and there are lots of considerations to the design and manufacturing of a propeller. I worked for Chris Machin 30 years ago making Rev-Up props while in college, so I had a chance to learn a bit about it from the inside.

There are a lot of different pitch profiles out there. The most simplistic is just using a constant pitch for the entire length of the blade. And since most modeler don't own a pitch gage, but do own tach's, there has been a trend to fudge a bit on the actual pitch. For instance, on the large wood props that you were asking about, it take a very thick piece of wood to hold the same pitch down to the hub if the blade width is to remain about the same. And of course, the hub of the prop needs plenty of meat just to hold together. So one way around this problem is to lower the pitch at the hub. This works to our advantage anyway, because the air at the fuselage moves slower than the free stream air further away, so de-pitching at the hub unloads the prop when the aircraft is in flight.

Now a lot of props are also washed out towards the tips as well. This is a bit like wash-out in a wing, moving the lift inboard on the wing or prop to give an elliptical lift distribution for lower drag. With the lower drag, then you end up with more rpm. This works well with most sport models where the pitch is much lower than the diameter, and they fly over a larger range of speeds.

For racing models where the pitch and diameter are about the same, most are increasing pitch as they approach the tip, and the only area that is lower pitch is closer to the hub for the reason in the first paragraph.

Regardless of what brand of props you like, it takes experimenting to find the one that performs best with your engine and airframe. One size does not fit all, and the recommendation that comes with the engine may not work well at all. Some very creative marketing out there with heavy loads (high pitch and diameter) as well as hp ratings that you would only see with very small light props. Brand X must be better than Brand Y because they use a bigger prop. Experiment, buy a dozen sizes and fly them all.
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