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Which has more thrust?

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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? 

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Slower turning props being more efficient is a rule of thumb, not a hard fast rule.


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.

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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.

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