View Full Version : Propeller efiiciency

06-08-2010, 11:12 AM
From a website I got this calculation procedure. What do you think ?

advance ratio (J), power coefficient (C_P):

J = 88.0 * v / (N * D),

where v is aircraft speed in mph, N is prop rotation in RPM, and D is prop diameter in feet

C_P = 52.5 * gamma* BHP / [(N / 1000)^3 * D^5 * rho / rho_0],

where gamma is fraction of full power being applied, BHP is engine brake horsepower, N is prop RPM, D is prop diameter in ft, rho is the air density, and rho_0 is the air density at standard sea level.

eta_i / (1 - eta_i)^(1/3) = (pi / 2)^(1/3) * J / C_P^(1/3),

eta is approx 0.85 to 0.87 eta_i.

06-08-2010, 11:40 AM
I think that I'd prefer metric. http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

There a couple of airscrew efficiency tests / reports around, which appear to use similar formula. For instance this one (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090014838_2009013985.pdf). Though I think most of these simply emphasize on measured data and just use some of the math.

06-08-2010, 02:53 PM
C_P increases as air density drops?

06-08-2010, 03:05 PM
You have discovered a portion of blade element theory.

Advance ratio is not propeller efficiency. It is advance ratio.

Np = (Ct*J)/Cp

Yes JtD your Cp goes up when the density goes down.

06-08-2010, 03:55 PM
I asked that, not JtD.
But now I see that perhaps Cp (is C_P?) is a/the divisor in determining prop efficiency?

06-08-2010, 11:22 PM
C_p kind of describes how loaded the prop disc is. Lower rpm, smaller diameter, less dense air and higher power increase C_p.

C_p is a/the divisor for the efficiency eta in that equation.

06-09-2010, 01:45 AM
"It was my understanding that there would be no math."

Chevy Chase

06-09-2010, 09:23 AM

You shouldn't be using a computer then. http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

06-25-2010, 01:37 PM
Originally posted by Kettenhunde:
You have discovered a portion of blade element theory.

No, I think this is a rule of thumb calculation. You do not input the number of blades, or blade width etc.

Advance ratio is not propeller efficiency. It is advance ratio.