Propeller efficiency versus efficiency losses.

[Onemorechance]

When you say that a propeller has an efficiency of 51%, does it then mean that the efficiency losses with respect to the ideal propeller are 49%?
My answer would be NO!
Why? Because the ideal propeller will have an efficiency lower than 100%! Therefore the losses with respect to the ideal propeller will be less than 49%.

Right?

 

[CESSNA1]

ONEMORECHANCE: An ideal propeller has an efficiency of 100%. The efficiency of a propeller is the same as other mechanical devices. PowerOut / PowerIn. If there is an actual propeller absorbing 100 hp from an engine then the propeller will generate less that 100 hp worth of thrust. The propeller loses include, but not limited to: drag, compressibility, tip losses, etc.

Regards
Dave

 

[Onemorechance]

Yes, it only depends on how you define it.
If defined as follows:
Overall Propeller Efficiency = Useful Power / PowerIn = (Useful Power / PowerOut)* (PowerOut / PowerIn)

Then an ideal propeller has efficiency < 100%, because Useful Power < PowerOut.
(Useful power is the Thrust * Airspeed)

 

[JohnAck]

What effect does diameter have on speed... say from 78 to 84 inches but in the same blade shape... What effect does altitude have on all this? Horsepower remains the same nad rpm remains the same

 

[rb1957]

the larger diameter creates a larger disc (D^2) and so produces more thrust (as you've described it ... same section, same rpm) since you've specified same Hp then the power will be absorbed (by the propeller disc) at a lower velocity (P = T*V). the caution is that as you increase the D and maintain the rpm, then you're increasing the tip speed (rpm*(2pi/60)*D/2) which could go supersonic and in any case may invalidate the assumption that the propellor is creating the same Ct.

altitude affects things in a really complicated manner (write that in your answer and see how far you get !). altitude typically reduces the power output of the engine (as you reduce the amount of O2 in the charge air volume). supercharging helps maintain sealevel performance at altitude. the V in the power equation above is true airspeed; the Thrust generated by the propeller (T = Ct*q*A) is, i think, relatively independent of altitude.

 

[JohnAck]

Eighty two inch diameter Hartzell Super Scimitar prop turning 2500 rpm and six and eight thousand feet 280 HP on a 3800 lb plane