CTMI Posted May 8, 2018 Report Share Posted May 8, 2018 Ok here’s a brain bender. Engine 1: O-320 160hp and 311 ft/lbs of torque at the prop when: Engine RPM: 2700 Prop RPM: 2700 Engine 2: ROTAX 915 141 hp and 310.4 ft/lbs of torque at the prop when: Engine RPM: 5800 Prop RPM: 2386 Which will create more takeoff thrust assuming both have CS props with blades optimized for their rpm? this is really messing with my head, because engine 1 has more horsepower, they both have the same torque, and engine 2 has lower rpm which should be more efficient transfer of energy.... Thoughts? Which produces more thrust at takeoff? At cruise? Link to comment Share on other sites More sharing options...
procharger Posted May 8, 2018 Report Share Posted May 8, 2018 Torque is torque biggest difference would be at certain RPM where the best torque would be. Should be a graph somewhere for Rotax. Link to comment Share on other sites More sharing options...
Anticept Posted May 8, 2018 Report Share Posted May 8, 2018 Slower turning props being more efficient is a rule of thumb, not a hard fast rule. http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf Great explanation on propellers. Another factor is the cruise speed of the airplane. Forward motion through the air contributes to how close the tips are approaching the speed of sound. Even slow turning props on a plane that goes really fast will still suffer ill effects. Sum the vector of the prop rotation and the forward movement and you'll see. This is when you arrive at something called the advance ratio, and props have a changing efficiency based on how they are moving through the air. Weight of the airplane and drag is also quite a factor. Link to comment Share on other sites More sharing options...
Roger Lee Posted May 9, 2018 Report Share Posted May 9, 2018 According to Kitfox that just put a 915 in their plane that had an 0-360 180 hp the 915 out performs it easily. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted May 10, 2018 Report Share Posted May 10, 2018 Don't we need DA numbers to get the answer, since one is turbo and the other's not? Link to comment Share on other sites More sharing options...
FlyingMonkey Posted May 10, 2018 Report Share Posted May 10, 2018 On 5/8/2018 at 8:55 PM, Roger Lee said: According to Kitfox that just put a 915 in their plane that had an 0-360 180 hp the 915 out performs it easily. I'm guessing that's do to lower weight, not more thrust. But again, DA dependent. Link to comment Share on other sites More sharing options...
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