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Aerofoil analysis help
For several years I've had a Mini Graphite wing (MH32 airfoil) on top of a cupboard gathering dust. My plan back then was to fill it full of carbon and turn it into a DS hack. But then I got Opus MCTs and forgot about it. Recently I started playing with XFLR5 airfoil analysis software and ran some results..
What surprised me is that according to the software, the Mini Graphite's MH32 section is much lower drag than the Opus' RG14 at >300mph (Re 2.5e06 to 3.0e06 and Mach 0.4 to 0.5). Lower Cd even than Rk40 and DS19, popular sections for DS. Am I running XFLR5 too far from its design? Using the software wrong? Are there other aerofoil properties that also need to be considered? 




Hobster, I'm not sure what you did. I just ran 2 identical wings (3D analysis), one with the MH32 and one with the DS19 at 300 mph. The CL(CD) is different than your findings. The DS19 definitely has lower drag at low to moderate CL. In fact the MH32 wouldn't converge above 5* at this speed. I normalized and refined the airfoil before processing.
When you defined the polar did you use the same definition for each airfoil? 



Here is the CL(CD) polar with the RK40 added. Both the DS19 and the RK40 have better performance with this wing at 300mph. This again is a 3D analysis.




Here is a 2D analysis at Re 1,000,000 and 2,000,000. Polar shows DS19, RK40, an MH32.




Hang on a second...
Oooops. I just realized that I was comparing modified airfoils.
Here is a 3D analysis of the original 3 foils on an average 2 meter wing at 300 mph. The MH32 has higher drag than both the DS19 and the RK40 at low to moderate CL. 



Many thanks wyowindworks for replies. Your results look more like what I would expect.
I originally tried 'Normalise' and 'Derotate' the foil but had little effect. This time I tried Normalise and 'Refineglobally' which appears to interpolate more points on the airfoil (so hopefully more accurate calculations) The results are more bunched up but still show mh32 as low drag (at Re 2.0e06, Mach 0.4) Guess I will have to read the manual! 


Sydney
Joined Apr 2009
719 Posts

Hobster I also did a quick check as a 2D comparison on XFOIL at around 300mph on an average 200mm chord which was 1.8 to 2,000,000 Re.
It supports Wyowind's comments both the DS19 and the RK40 have lower drag especially at < 0.5CL. The MH32 is not too bad and would be easier to land perhaps although it would require lots more ballast to fly with similar L/D to the DS19 and the RK40. Give it a try if you have it. John 







Quote:
Your graph appears to shows that mh32 is lowest Cd between about Cl 0.4 and 0.8. According to my alpha/cl graph, that equates to AoA of about 0.5 to 4 degrees. So it might be quick if I dont yank the turns too hard. Perhaps I should just give it a try as you suggest. I was originally most interested in mh32 vs rg14 (9% ish) to compare against the Opus wing, so interested in any comparison against that if any one has the time Thanks again, Steve 



Sydney
Joined Apr 2009
719 Posts

Steve, these should help.Note the RG14 appears to have an edge on the DS 19 at CL's >0.3.
Girsberger certainly designed some classic foils. The DS19 indicates however why it is so popular. A very fast and forgiving airfoil from these plots and would carry more weight. John PM Just discovered this on a reread.I Stumbled a bit here Steve wrt to the drag of the RG14. The RG14 has less drag(should have shown< instead of >) than the DS19 below approx 0.3CL  which may not mean much as Adam points out below. My guess would be that the DS19 would be the better overall choice. 



BTW, a 2M with 480 square inches flying 300 mph, weighing 80 ounces, and pulling 50 G, requires a CL of .326. If you hit 80G you need a CL of .521. Flying the MH32 tighter may be the faster way to fly it...if it can actually get up to 300.




Quote:
Flying the same weight and wing area plane at 300mph in a circle at a more human lap time of say 4s would require a CL of approx 0.14. If you inherited SL's flying skills and flew 3.3s laps, a CL of approx 0.17 would be required. The point I hope to illustrate is that, despite the tremendous 'G' forces, because of the high speed the actual CL or alpha required to generate the necessary lift force is surprisingly low. L= CL 1/2 p v^2 S. Because the value of v^2 is very large, in order to provide the required lift force the value of CL is low. Therefore, provided you can launch and land, an aerofoil with lower drag at low CL should be better. I worked it out thus: 300mph = 134 m/s. Circuit time = 4s, therefore circumference is 134 x 4 = 536m. radius = c/2pi so 536/2pi = 85m Centripetal acceleration = v^2 / r so 134^2 / 85 = 211m/s^2 Force = mass x acceleration (80oz = 2.27Kg) so F = 2.27 x 211 = 479N. This is close as dammit to the lift force required to fly the plane in the assumed circle. You can work out the total lift vector required to account for the vertical component to overcome gravity but you will find the difference it makes is only about 1N so can be considered negligible during these extremely hard turns. Rearranging the lift formula we get CL = L / .5 p v^2 S (480sq in = 0.31m^2) so CL = 479 / .5 x 1.225 x 134^2 x .31 = 0.14 If we be more realistic and use a racetrack pattern to simulate our DS path, this does require tighter radius turns. If we approximate a racetrack using straight edges, say 30m each, we end up with a 4s circuit requiring a CL of 0.16, still a low value. I hope this is of interest. Best regards Steve (another Steve, there are plenty of us in the DS world!) 

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