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Old Apr 29, 2006, 11:43 PM
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Joined May 2004
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Originally Posted by theKM
It's only more prop disc because the gearbox prolly had a reduction gearbox at the same time as combining the three motor's power.
Yes indeed! The original setup used a belt-drive and I think about 4:1 reduction ratio (I forget the exact number). Even so, the original propeller was absurdly small when compared to the wingspan of this 50' brute.

Thanks for the clarification about multiple propellers having more disc area if the reduction ratio is taken out of the equation. I had wondered about this for a second or two, but didn't take the time to think it through earlier.

I assume this area increase comes from the (diameter^4) term in the formula for power absorbed by a propeller. Going to a twin engine setup with half the power absorbed by each propeller at the same RPM results in only about a 16% reduction in prop diameter (reduces to 84% of original), and consequently about a 29% reduction in each prop disc area. With two props that means the new total disc area is about 41% bigger than the original single prop.

The numbers are even better with a trimotor setup: 1/3 the power to each prop would result in a 24% reduction in diameter of each one, or a 42% reduction in the disc area of each prop. With three props, the total disc area would then be 73% bigger than the original single prop.

Before Flybike came by those three giant AXI outrunners, I remember someone suggesting simply mounting a couple dozen Speed 600 motors all over Big Floyd. With appropriate gearing on each one, that might not have been an entirely bad idea!

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