APC propeller data - what's new ?
As far as I know, APC is the only brand that provides data of their propellers. The database has been recently updated, with some explanations about how it was produced.
You can have a look here.
Some may think the propellers were tested in a wind tunnel, but this is not the case. APC propellers data are based on the vortex theory (which one ?). Here's a paper I've just found, thanks to google, about propeller modelling techniques (A Review of Propeller Modelling Techniques Based on Euler Methods by G.J.D Zondervan). I must admit I haven't read it all, but it may be interesting !
Such methods are known to have difficulties in predicting post stall behavior, as noted by Daniel V. Uhlig and Michael S. Selig (Post Stall Propeller Behavior at Low Reynolds Numbers). This means that the thrust and power predictions are inaccurate in static and at low speeds. APC seems to try solving this issue with an interpolation from static measurements.
I'm trying here to compare the APC performance data with some wind tunnel measurements.
Also, I have to say that APC propellers are my favorite propellers ! I feel a bit guilty to criticize this initiative, which I approve and encourage.
The sources of the wind tunnel tests are :
- BART (Basic Aerodynamics Research Tunnel) = approximation from Analytical - Experimental Comparison for Small Electric Unmanned Air Vehicle Propellers by Michael Ol, Cale Zeune and Michael Logan,
- WSU = Measurement of Static and Dynamic Performance Characteristics of Small Electric Propulsion Systems by Aron J. Brezina and Scott K. Thomas.
My preference definitely goes to the UIUC data. One of my favorite games is to predict the in flight performance of my models and the best results are obtained using the UIUC data.
So what's new ?
The old data were very strange, they had nothing natural. The new are more credible, with the qualities and defaults of the vortex theory.
Below, you can find graphs showing the thrust and power coefficients and efficiency versus advance ratio of various APC props, according to wind tunnel tests and APC data.
Thrust coefficient : Ct = T / ( rho*n2*D4 ),
Power coefficient : Cp = P / ( rho*n3*D5 ),
Efficiency : eta = J*Ct/Cp,
Advance ratio : J = v/nD.
T = thrust (N),
P = power (W)
rho = air density (kg/m3),
n = rev/s,
D = diameter (m),
v = airspeed (m/s).
Ct and Cp vs J curves reflects thrust and power behavior versus airspeed for constant rpm.
APC 8x4 Thin Electric :
Here, we see that APC over estimates both thrust and power coefficients. Is it because of a recent variation in blade shape ?
Peak efficiency is over estimated, and occurs at a higher advance ratio according to APC data (so at a higher speed if we consider constant rpm).
APC 8x6 Thin Electric :
Here we can see that the BART and APC data match pretty well, while UIUC data under estimate both coefficients (again, blade shape variation ?).
The decrease in Cp value near static (J = 0) is typical of the vortex theories as applied to small propellers.
Personally, I don't believe the high peak efficiency shown by the BART and APC data. In flight logs tends to prove that this propeller isn't producing any thrust beyond J = 0.9.
APC 8x8 Thin Electric :
The decrease of the Cp value at low speed is even more pronounced here with this high pitch propeller. This may lead to a wrong prediction of the power consumption of course, but also of the thrust since the rpm will be lower than expected.
APC 11x7 Thin Electric :
Here, all the data seem to be in good agreement. This may be the case for the large propellers in general.
Efficiency is still over estimated by the vortex theory used by APC, but this is not surprising.
APC 19x12 Thin Electric :
This large propeller tends to prove that large propeller performance are better predicted.
APC 9x6 Thin Electric :
Now this is interesting. The same propeller at various rpm. It looks like APC hasn't taken into account any Reynolds effects. This is surprising since we know the heavy influence of the Reynolds number on the performance of our models' propellers.
And we can notice the big difference of the value of both coefficients for this propeller (again, blade shape variations ?).
APC 4.1x4.1 Thin Electric :
What about small propellers ? Ouch !!!
The performances of such small propellers are probably heavily impacted by the Reynolds numbers and radial flow effects. The thrust coefficient seems correct in static, but only in static (remember, it has been measured, not computed).
Still the same trend : over estimated peak efficiency, at an over estimated speed.
APC 9x6 Slow Fly :
APC 9x6 Sport :
APC 14x13 Sport :
Static power coefficient !! See how the post stall behavior is hard to predict.
APC 8x10 Sport :
Computation and experimentation are in perfect contradiction here with this more than square propeller !
In conclusion :
Well, sometimes there are big differences, but also some good estimates. Most of the differences were expected.