



Help!
XFLR5 Wing Design Help Needed
Greetings All,
I am attempting to design a scale sailplane wing with XFLR5 and am having some issues that I could use some help with. The wing is for a 1:3 scale SchemppHirth Discus 2c. The wing is flapless which, counterintuitively, makes the design a little more challenging. I am using the tutorial written by Francesco Mescia (http://www.rcsoaringdigest.com/pdfs/...SD200802.pdf), plugging in my values where appropriate. The problem I'm having is with the spanwise lift distribution. Can someone explain the downward "spikes" I'm getting around the quarter span? Thanks in advance, Mike 




I'm not an expert but I will play around with it if I have any time to goof off tomorrow.




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About the airfoil choice: I don't think you will get away with a foil that only has 8% thickness. I'd recommend 11% at the root and between 9 an 10% for the rest of the wing. Friedmar 




My XC wing was done with 11 y panels per division.
Norbert Habe has a large collection of thicker airfoils for large wing planes. I looked at them for XC and they did not apply but maybe for this they would be ideal. http://tracfoil.free.fr/airfoils/h.htm 



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Structural concerns aside, what is it specifically about the choice of airfoil that you think won't work? I am looking for a high L/D at cruising speeds, sacrificing some performance in climb. Thanks, Mike 




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Mike 




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While on the tracfoil website take a look at Thierry Platons TP210 and TP202 series. They seem well suited for your application. Friedmar 




Perpetual motion machine?
I did a little playing around with the TP210 airfoil that Greg and Friedmar suggested and I'm getting some pretty strange results. Perhaps someone can point out the error of my ways.
Using the published planform from the f/s Discus 2c (18m), I created a 1:3 scale wing that used the 11% version of the airfoil at the root and 10% for the remainder of the wing. After a couple of minor tweaks to the twist at several stations, I was able to get what looked  at least to my untrained eye  to be a reasonable spanwise lift distribution. The GPS Triangle racing rules provide a formula for maximum wing loading so I did the math and came up with a maximum weight of 15.8 kg. and guessed at a minimum weight of 9 kg. Here is where the interesting part comes in. The L/D polar is "better" at the higher wing loading. Not just a little bit better but a whole bunch better (42:1 vs 38:1). That's almost 10% better and just can't be. Maybe I'm just flailing around without a clue (and please point that out if that's the case) but something is not quite right. Is my not accounting for the drag of the fuselage skewing the results for the wing and the tail combination? Did you guys recommend a "magic" airfoil? I have to be missing something really simple, Mike ps: Just ran the venerable MH32 section using the same wing geometry and it yields similar results 



I saw the same thing with some other designs I was looking at. Seems strange at first but what I think is happening is you are increasing the Re when you increase the weight and the performance at that point is the strongest function of the Re. What is really weird is in some cases the sink rate actually gets better at the higher wing loading. That is why I am now building a smaller XC model.



Joined Jan 2009
569 Posts

Highest L/D for a given design is at highest mass (wing loading) authorized by structural design/rules, most rearward CoG with corresponding optimized decalage, cleanest configuration (smooth, all openings, doors, joints taped), lowest altitude and temperature (highest air density). For the models the Re is an important contributing factor, but the fact that by setting the glider that way you reduce the angle of attack (therefore the total drag) is why the L/D improves. On full scale because of the configuration and mass limits the L/D increase is around 1 to 2 points from "more standard" settings. On a model glider, depending on the mass variation, you may get as much as 4 points (or more) improvements. Remember, your calculations are depending on the software used for the analysis and you are only looking at the wing, not the full glider. The gain will be much less in reality due to the fuselage and all the interferences drag. The design of the fuse at this level becomes critical, this includes the design setting for the wing incidence at the optimal speed and the fuselage shape/drag at this AoA.




That doesn't makes sense!?
If you reduce the size, than the chord and hence also Reynolds will be reduced, reducing the L/D ratio. I think you should ballast your model more, not make it smaller for the same weight. 


Switzerland, AG, Lenzburg
Joined Jan 2006
1,911 Posts

well in fact it makes a lot of sense, a smaller plane is easier and cheaper to build, it is easy to transport and you will need less ballast, in fact you can build it on the heavy side from the beginning, so instead of using lots of materials on a 4m wing you can use the same amount for an extra heavy yet smaller wing
just my two cents EZ 



Back on topic:
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Please keep in mind that it's not just the glide ratio that counts for distance flying. The lower the sink rate the more overall flight time you have for finding thermals and they can also be weaker while still allowing you to climb. Mike, I am not sure if it's wise to do all the polar calculations with the tailplane included. To make it more realistic you would have to adjust decalage and CG for each operating point on the polar. So it's easier to skip the tail while still optimizing the wing. And if you can live with a semiscale plane: Increasing the area of the horizontal tailplane by maybe 510% will help maneuverability and flying characteristics. Original tails tend to be on the small side of things. For further optimisation of the wing you might want to have a look at the spanwise plot of local c_d. If the increase in c_d towards the tip is significant at slower flying speeds, then optimizing the tip airfoils for lower Re is a good idea. I think you can reduce thickness to 9% at the tip on the outermost wing segment and maybe move the maximum thickness of the airfoil forward a little (by 2% of chord). Friedmar 




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Hahhhhhhhhhh! I'm not an idiot Thanks for the explanation, fnev. So I think it is safe to say that flying at maximum wing loading is better, at least in strong conditions, with the caveat that you are going to incur a penalty climbing because of the increased sink rate which jibes with proven, 50+ yearold soaring theory. What if you turn the problem on its head and design the wing with the thickest airfoil possible  yielding the most volume for disposable ballast (water, let's say)  that still performs well at high speed? The trick is then finding a "thick" airfoil that will also perform well at lower wing loading for weaker conditions? For the wing geometry posted earlier, a backofthenapkin calculation suggested that wing could carry ~2.5 kg of water. Intuitively, that seems like an appreciable amount of weight and variability in wing loading. On the other hand, the current "hot" ships (eg, Baudis Antares, KV 304 Shark) on the GPS Triangle circuit are taking the opposite, more F3Blike approach: thinner, lower camber airfoils. Wondering if anyone has really done the math, Mike M 




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Thanks again for the help, Friedmar. Very much appreciated. Mike 

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