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
  • 1
    $\begingroup$ 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
  • 2
    $\begingroup$ Possible duplicate of Is there any equation to bind velocity, thrust and power? $\endgroup$
    – user11933
    Oct 25, 2015 at 22:12
  • 3
    $\begingroup$ "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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.