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Direct drive or reduction?

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lake_harley

Well-Known Member
Supporting Member
Joined
Jan 4, 2007
Messages
1,071
I have a Zenoah G-25B that I'll be using on my under-construction ultralight. I found a thrust estimation calculator somewhere and I don't remember the particulars, but I was suprised that the theoretical thrust in direct drive was higher than through a 2.25:1 belt reduction system that I have for the engine. Again, based on prop recommendation info from some source, I was using "what if" prop sizes of 37-39" diameter with 14 pitch on the direct drive, and 52-54" diameter and 24-27 pitch on the reduction. Maybe the props were all wrong for the engine.

The engine is rated at 20-22 HP at 6200-6500 RPM. The variations of those specs are due to differing specs depending on where I've found info. Can anyone speak from personal experience or from an engineered standpoint to weed out the best way to go? I'd go direct drive in a heartbeat if it can be effective. Basic info on what I'm building is a high-wing, aluminum tubing frame ultralight, tractor configuration, with a wingspan of 24' plus rounded wingtips. I've been very weight conscious in the build and estimate empty weight may be as low as 200# +/-, and with me at 155# and fuel the gross weight should be about 400#. I'd like to cruise at 45-48 MPH with gusts up to 50-52 MPH possible.

Thanks, in advance, for input and direction. Let me know if there's other information I'd need to supply to help with advice.

Lynn
 
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