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Propellor optimization
In the past I’ve used Qblade to design optimized turbine blades for “whirligigs” and small scale turbines. (See my instructional video series on how to design 3D printed turbine blades using Qblade: link)
I’m now trying to design a small scale optimized propeller, using the PROP mode. By it seems that no matter what parameters I enter into the “optimize prop geometry” dialog box, (my desired target is a flight speed of 12.5 m/s, at 6000 RPM, in air) I get the error: “Optimization caused NaN values; change settings!”
Any suggestions for what I might be doing wrong, or a suggested set of starting parameters which might not crash the optimizer? Is this optimizer algorithm just not suited for small propellers?
I’ll attach the simple file I’m using for testing.
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Hi Todd,
nice video series! I had a quick look and probably found the issue. When optimizing a propeller you have to choose if you want to optimize for a given thrust, power consumption or torque of the propeller. The dialog uses the units kN (thrust), kW (power) and kNm (torque) for these optimization parameters, as it was laid out for a full size sports plane prop. Since your prop design is model scale (32cm diameter) you have to use relatively small values for these optimization parameters (a 32cm diameter prop wouldn’t be able to generate 1kN of thrust that why you get the NaN).
Best,
David
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I totally missed that the dialog was asking for an input on that line. Thanks a ton!
To make it a little more user friendly, you might note that the first input box applies to whichever radio button parameter is selected, but the second input box only applies to the last parameter (constant AOA). Even better would be to give an input box to each of the parameters in that first line, so it’s very clear that the number entered matches up with whichever radio button is selected.
Next question: It looks like changing airfoil sections along the blade is OK, but if you put a circular airfoil at the root, that seems to crash the optimization. Is that correct?
Hi Todd,
you are correct that the optimization is not working for circular airfoils. Actually in its current implementation the comlete optimization is based on the polar at the first station of the propeller. In my work with propellers so far the props usually only cosistent of a single foil design. You could for instance optimize a prop and then change some of the inner stations to circular foils to get around this. I’ll probably fix this issue for the next release and also consider you suggestions! Thanks!
BR,
David
Yikes. That information should definitely be in the documentation somewhere. That’s unfortunate because my use case for QBlade was specifically to design a propeller with airfoils which vary along the span. I was just exploring the usefulness of the airfoil interpolation tool for generating intermediate airfoils for blending between airfoil types.
It does explain the crazy results I was getting. I thought that maybe the optimization needed airfoil sections with a non-constant 360 degree polar, so I approximated the round root section as a NACA 0050 section. This completely changed the look of the rest of the blade.
That begs the question: Do the HAWT blade optimizations have the same limitation? Even more than propellors, HAWT turbines often have deliberately varying airfoil sections along their length. Does QBlade take this into account?
Hi Todd,
for both PROP and VAWT features not a lot of information had been documented yet. At the moment we are focussing more on the documentation of the HAWT offshore wind modeling and simulation capabilities as this work can somehow be compensated with industrial projects. The PROP tools are a side project that we can only work on when the time allows. But over time we will extend the documentation more and more…
Regarding the optimization: I am not even sure if the methodology by Adkins and Liebeck (publication link) allows to optimize a propeller with different foils. Programatically its not a big change though and the next release will have that feature. For HAWT the local foil/polar is of course taken into account during the optimization.
Large WT blades have different foils mostly for reasons of structural integrity, where you place thicker foils towards the hub – from an aerodynamic perspective using such thicker foils doesnt make sense however. As propellers are significantly smaller I am not sure whether they have similar structural limitations or if you could use the most efficient foils over the complete rotor…
Best,
David
Yes, propeller and helicopter blades have the same sorts of transitional geometry between the airfoil sections and the hub geometry. Down near the root the chunky geometry will probably be best represented as a circular airfoil of constant drag and no lift. But near the fully developed airfoil portion of the blade, there are interpolated sections which contribute to lift, and could be incorporated into the blade optimization calculations.
