I want to see if a propeller is bad or good for generating static thrust. This is what I know about the propeller and the engine used to turn it during a test:

Engine shaft power, $P$ = 0.58 hp

Propeller diameter, $D$ = 8.5 ft

Revolutions per minute, $RPM$ = 245

Static thrust, $T$ = 18.75 lbf

Can I calculate the efficiency of the propeller? What is the maximum achievable efficiency?

Additional explanations

The formula I was suggested to use could be exactly what I need. However, using it, I do not get some reported experimental results like:

Engine shaft power, $P$ = 6 hp

Propeller diameter, $D$ = 8.5 ft

Static thrust, $T$ = 66 lbf

Propeller efficiency, $\eta_{prop} \geq 66\%$

Gear efficiency, $\eta_{gears} \geq 85\%$

Doing the calculations: $\eta_{prop} = \sqrt{\frac{T^3}{(P^2 \cdot \pi \cdot \eta_{gears} \cdot \frac{D^2}{2} \cdot \rho )}} = 37.2\%$ which is well below 66%.

  • 2
    $\begingroup$ Homework question? $\endgroup$
    – Ralph J
    Oct 25, 2015 at 12:50
  • 3
    $\begingroup$ @Ralph Looks like another question about the Wright brothers. $\endgroup$
    – user11933
    Oct 25, 2015 at 13:00
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    $\begingroup$ Why the downvotes with no comments? Seems like a perfectly reasonable aviation question to me. +1 to help restore CAVOK. $\endgroup$
    – Simon
    Oct 25, 2015 at 15:34
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    $\begingroup$ Possible duplicate of Is there any equation to bind velocity, thrust and power? $\endgroup$
    – user11933
    Oct 25, 2015 at 22:12
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    $\begingroup$ "Bad or good" is opinion. $\endgroup$ Oct 26, 2015 at 10:14

1 Answer 1


Engine shaft power: P = 0.58 hp, Propeller diameter: D = 8.5 ft, RPM = 245, Static thrust: T = 18.75 lbf

$\eta_{prop} = \sqrt{\frac{T^3}{(P^2 \cdot \pi \cdot \frac{D^2}{2} \cdot \rho )}} = 49.5\%$

where $\rho=1.2 \frac{kg}{m^3}$

So at an efficiency of 49.5% that propeller is rather poor.

Note: I used the formula that relates static thrust to efficiency, power and propeller diameter from here.

  • 1
    $\begingroup$ It is known that the efficiency of a plane propeller is low at small forward velocities, so just because eff_prop =~50%, when the plane does not move, it does not mean the propeller efficiency will remain the same as the plane speeds up. $\endgroup$ Oct 28, 2015 at 17:05

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