# Is this a good or a bad propeller?

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?

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%.

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

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.

• 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. – Robert Werner Oct 28 '15 at 17:05