Jay Dub
Well-Known Member
Reading on many different aircraft and Rotax forums, many people say to adjust a ground adjustable prop on a Rotax 912 for either 5500-5650 RPM WOT in level flight or others say adjust for 5800 RPM WOT level flight.
The Rotax manuals and specs state the max takeoff RPM is 5800 and max power is also generated there. If a ground adjustable prop is WOT 5800 RPM in level flight then takeoff and climb RPM is around the 5300-5500 RPM meaning this engine is never making rated HP unless full throttle in level flight for which there is no need as the specs also state to not run above 5500 RPM for more than 3 minutes (I believe this is a heat rejection issue more than an engine stress issue). If set for WOT less than 5800 the engine is never making rated HP, ever, and is constantly over pitched.
I know there is a balance between cruise and climb performance hence the in flight adjustable and the constant speed props were developed so you can take off making rated power then add more pitch for cruise. For the sake of trying to get decent cruise speeds and decent climbs with a ground adjustable or fixed pitch prop you have to compromise both. It just seems crazy the consensus in the Rotax world is to lug the engines never making their rated HP. That just strikes me as odd that most everyone agrees to never let their engines make their rated power.
I would guess those in the STOL world, like Hal Stockman and others running Rotaxes probably flatten their props to make 5800-6000 RPM on their takeoff roll then throttle back once off the ground. Of course cruise speed would suffer at such a flat pitch. Does anyone set their props so they make 5800 RPM in takeoff and climb so they get rated power? How much does this affect cruise? The cruise RPMs would be higher for the same airspeed for sure but since loading would be less, fuel burn should be less for the same RPM.
Just trying to find balance but it seems odd to never make rated power.
Thanks.
The Rotax manuals and specs state the max takeoff RPM is 5800 and max power is also generated there. If a ground adjustable prop is WOT 5800 RPM in level flight then takeoff and climb RPM is around the 5300-5500 RPM meaning this engine is never making rated HP unless full throttle in level flight for which there is no need as the specs also state to not run above 5500 RPM for more than 3 minutes (I believe this is a heat rejection issue more than an engine stress issue). If set for WOT less than 5800 the engine is never making rated HP, ever, and is constantly over pitched.
I know there is a balance between cruise and climb performance hence the in flight adjustable and the constant speed props were developed so you can take off making rated power then add more pitch for cruise. For the sake of trying to get decent cruise speeds and decent climbs with a ground adjustable or fixed pitch prop you have to compromise both. It just seems crazy the consensus in the Rotax world is to lug the engines never making their rated HP. That just strikes me as odd that most everyone agrees to never let their engines make their rated power.
I would guess those in the STOL world, like Hal Stockman and others running Rotaxes probably flatten their props to make 5800-6000 RPM on their takeoff roll then throttle back once off the ground. Of course cruise speed would suffer at such a flat pitch. Does anyone set their props so they make 5800 RPM in takeoff and climb so they get rated power? How much does this affect cruise? The cruise RPMs would be higher for the same airspeed for sure but since loading would be less, fuel burn should be less for the same RPM.
Just trying to find balance but it seems odd to never make rated power.
Thanks.
