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Does these propeller numbers sound right? I'm studing the effects of diameter.

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HumanPoweredDesigner

Well-Known Member
Joined
Sep 6, 2009
Messages
1,030
Location
Arizona
I put some numbers into a computer program to see just how much better a 72 inch propeller is than a 36 inch propeller. Both have a maximum cord of 3 inches and are constant pitch.

What I noticed is the 72" propeller does better, but not by a long shot. It outputs cruise thrust with 20% less power, and outputs 30% more thrust during taxiing. I suppose that more than pays for the weight of the redrive and propeller. But is it worth the monetary cost of these, or the extra difficulty of fitting the bigger propeller on? And how much louder is mach 0.65 than mach 0.47 in this situation?

72 inch propeller:
rpm: 1526
tip mach number: 0.47
Low advance ratio so the efficiency was in the low 70's.
@ 35 mph, 36# thrust, 4.7 hp
@ 27 mph, 60# thrust, 6 hp
@ 9 mph, 88# thrust, 7 hp

36 inch propeller:
rpm: 4185
tip mach number: 0.65
efficiency at cruise is in the high 60's.
@ 35 mph, 36# thrust, 5.7 hp
@ 27 mph, 33# thrust (I don't trust that number), 5.3 hp
@ 9 mph, 65# thrust, 7.2 hp.
I tried 35 mph in the off design tab, and it gave me a value that did not match the design tab value of 35 mph, so I know the program has bugs.

Maybe I should redo the numbers with a lot more thrust for climbing fast, and see if the 72" does a lot better there at 20 hp.

@ 35 mph, the 72" prop puts out 67# of thrust with 9 hp, compared to 12 hp for the 36". So at higher thrust loadings, the difference grows. And after total aircraft drag is subtracted, the excess thrust multiplied by the speed gives power for climbing. Excess thrust is where the big prop prevails, though the engine would have to be smaller to make up for the extra weight of the redrive and larger prop.
 
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