Experimenter

JAN 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

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Un d e r t h e C o w l Metal props, I've often said, are the "anti-wood." They are heavy, need to be carefully designed and manufactured to resist fatigue, are relatively expensive, and are quite finite in model availability. And—opposite the wood prop's property—if you have a significant prop strike with a metal propeller, you quite likely will have internal engine damage. However, metal props do last longer, resist weather and erosion better, and do not require as much attention as wood. "Composites," admittedly a large family, try to capture the best of both wood and metal: They are light, resistant to fatigue and erosion, have good life in the elements, can be made in more optimal shapes, require little inspection and maintenance…and are expensive. Field repairs are seldom needed but never recommended. Composite prop blades also lend themselves particularly well to use in adjustable hubs in which pitch can be changed by the airplane owner, at will. How to Pick a Prop? The easiest way to start choosing your prop is to look at similar airplane/engine combinations that fly similar missions. If you're building a kit airplane, the manufacturer will have some good ideas, based on your choice of engine and your likeliest flying scenario. Your prop's diameter must be small enough that at full rpm the tips of your prop blades won't be going supersonic. For very fast airplanes, the forward speed of the airplane is a factor; but for most of us, a simpler calculation will suffice. An easy rule of thumb that includes a safety factor: Your top rpm times your prop's diameter in inches should not exceed 212,000. (Example: A top hub rpm of 3300 will allow a prop diameter of about 64 inches--212,000 divided by 3300 = 64.24) Tis wood-core composite prop failed when it was over-revved at more than 400 mph. No propeller would survive this. More sophisticated formulas exist, but this is really all you'll need to get into the right ballpark. (Who lives that An of-ramp excursion, even into sof earth, will ofen lead to a written-of prop. Tis one might be salvageable, but only the manufacturer will be able to tell. 38 Vol.2 N o.1 / January 2013 Photography courtesy Jeffrey Lo and by Tim Kern

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