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Propeller efficiency vs hybrid... (theoretical)

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tspear

Well-Known Member
Joined
Feb 12, 2014
Messages
1,874
Location
Outside Boston
Not sure what prompted the following question in my head.
Anyway, a propeller is basically an airfoil, as such the propeller tips have significant drag. Now some of the more modern designs like the "Q-Tips" are supposed to reduce the amount of drag. However, there is a practical limit to how efficient you can make the design due to the changing blade angles required in a constant speed prop.

This can be seen in some of the comparisons between the fixed pitch and constant speed. I was reading a while back on Van's forums about how a high speed cruise prop can run away from the constant speed prop planes in level flight. But considering the amount of climb and descent time, such a fixed pitch cruise prop ends up being slower block to block.

Now the reason for the constant speed prop is to allow the engine to run at a optimal speed to produce the most sustained power, often because the engine cannot produce enough power to spin the propeller at a higher speed.

However, an electric motor produces the same power regardless of RPM. Therefore, a fixed pitch prop optimized for max cruise seems like a more efficient choice, even if only from an aerodynamic perspective. As the plane goes faster, the prop increases in speed and the engine can deliver a constant amount of torque to the prop.
A Hybrid engine has efficiency loss by converting mechanical energy to electrical.

So how would you determine which has the greater total efficiency? A generator set to an electric motor to a fixed pitch propeller or an engine with a constant speed prop?

Tim (dreaming of planes while paying for kids in college)
 
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