Different reduction ratio between 80 and 100 hp

by nicola di biase in 912 / 914 Technical Questions 13 years ago

I'd like if someone can explain why gearbox propeller reduction rate is different - 1:2,43 versus 1:2,27 - between rotax 912 uls and ul ( 100 hp and 80 hp versions of rotax 912)

The two different versions of this engine have the same max and cruise rpm (5800 and 5500 ) and the rpm operative envelope is more or less the same ( with different level of power and consumption). One reason can be 912 uls was designed dome yeras after 80 hp version within the meantime planes gaining speed performance.

Thanks

Nicola di Biase

Re: Different reduction ratio between 80 and 100 hp

by Rob Seaton » 13 years ago

Rob Seaton

3 1.32k 251

I am sure there is a more technical answers but briefly most aircraft owners want to lower the prop tip speed so you can use a longer prop (great for floatplanes) or pitch your existing prop more heavily for more performance. The 100hp and 914 115hp have more power/torque so this lower gearing is possible.
You can fit an optional narrow gear 2.43:1 ratio to the 80hp but you need to be cautious of detonation and increased wear from overloading the pistons (nice, slow/quiet prop but itâ€™s a bit like driving a Porsche around town in forth gear)

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Different reduction ratio between 80 and 100 hp

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