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Old Oct 15, 2003, 01:50 PM
mkirsch1 is offline
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United States, NY, Spencerport
Joined Oct 2001
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Something like 80% of a propeller's "thrust" comes from the outer 20% of the propeller. Those aren't the exact proportions, but you get the idea, I hope...

Propellers also don't work by "blowing air backwards" as is commonly thought. Like any wing, they work by creating a pressure differential; the airplane is literally SUCKED forward by the low pressure area the propeller creates ahead of the plane.

The extreme example I've seen of your perceived problem is a Gee Bee, the R1 or R2 I believe. Most models of this fat, stubby-winged bird have propellers that barely stick out beyond the edge of the cowl, yet they fly fine. That's because the propeller is still doing its job, creating a low pressure area ahead, and a high-pressure area behind. All that really matters is the area immediately ahead of, and immediately behind the propeller. Is it more efficient to have the propeller clear of the cowl. Probably, but it still works this way.

As far as putting a larger propeller on the motor and using a large spinner to "cover up" some of the effective area, it won't work... The prop disc area increases exponentially as the prop diameter increases. If you went to an 8" from a 6" prop, you'd need a 5-1/4" spinner to cover up the extra disc area. Plus, the outer section of prop is moving at a higher linear speed, and is therefore moving more air, creating more load on the motor.

If you want to run a bigger prop, a gearbox makes much more sense.
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