# why are airplane propellers always undersized?

While calculating propeller static thrust by using a famous Prop-power-calculator.xls (by Pé Reivers), I've noticed that no matter what "stock" configuration I put into the calculator, I do always get the propeller power required half or third of the engine power available.

I.e.:

• Zivko Edge 540: Lycoming AEIO-540-EXP 300HP@2700rpm engine, HARTZELL 76" prop @ 11.5" pitch
Calculator given performance: 51HP, 154Kg thrust

• Piper Sport: Rotax 912ULS 100HP@2400rpm engine, Woodcompo 67" prop @ 10" pitch
Calculator given performance: 17HP, 88Kg thrust

!NOTE - calculator is for 2-blade props, while above planes have 3-blade props. However, that doesn't change the mismaching in numbers a lot.

Above engine RPM's are with reduction applied (i.e. 1:2.43). That means the actual HP available is two-fold!.

I would imagine everything shouldbe the other-way around(?)

• Reduction rates of 1:0.5, RPM's * 2, engine power / 2
• Or propeller driven directly by the shaft
• Or prop diameter increased (not to practical perhaps)
• Or pitch inreased
• Or number of blades increased?
• Or all of above?

What am I missing here?

• 300hp = 223,710 watts, where did you get 38,216 from? It should also be noted that the worksheet you are using is meant for use on R/C airplanes... – Ron Beyer Oct 26 '16 at 14:42
• Another possible explanation is that the engine are less effective with altitude, so the power at sea level may be greater than the power at the ceiling altitude. – Quentin Oct 26 '16 at 14:44
• Ron, that's exactly my point. Particular prop @ particular pitch @ particular RPM gives that much of a static thrust. Why are the engines scaled up if prop is technically unable to push for more? I dare to disagree that the math behind the static thrust calculation differs for RC or True airplanes. – FlegmatoidZoid Oct 26 '16 at 14:49
• I'm not talking about thrust, at the point you enter the engine power in watts, you are off by a factor of almost 6. Edit: Ok, I see you can't enter the engine power there, but where do you enter engine details at all? I can't find that. – Ron Beyer Oct 26 '16 at 14:50
• This site calculates the 11.5 inch pitch, 2800 RPM, 300hp engine combo to deliver about 3690lbs of thrust, which is much more realistic. – Ron Beyer Oct 26 '16 at 14:59