AG airfoil question for Dr. Drela

[Bernie Wolfard]

Are the AG airfoils more tolerant of LE inaccuracies than other current airfoil sections? Other sections are hypercritical such as the recommendation that the MH32 sections should be created in a CNC cut mold. Yet your AG sections are seen on built up or bagged wings where it is impossible to maintain molded accuracy. What are the tolerances for LE shape with your sections?

[Ollie]

It is not impossible to achieve a very high degree of accuracy with balsa built up construction. Laser cut ribs are of CNC accuracy. Dr. Drela's tangent sanding methods give very precise results. With some skill and attention to detail the results are close to those achievable with CNC produced molds.

The CNC controlled, laser cut rib sets for the Bubble Dancer come with female leading edge templates to check every rib station along the taperd wing. Under these circumstances accuracy is limited mainly by the skill, patience and attention to detail by the craftsman.

Since you posted the question on the open forum I assume it is appropriate to chime in.

[Sparky Paul]

You might ask yourself if any performance degradation due to an "inaccurate" leading edge is noticeable when you are flying.
There's such a thing as worrying too much about things you can't control.

[Tony D.]

If I may throw in my .02.

I have built 5 bagged type wings using Dr. Drelas' airfoils and while I strive for the most accurate airfoil reproduction possible close inspection will show that all 5 of the wings have slightly different leading edges because of the sanding required to finish off even the most carefully bagged wing.

But all 5 wings perform exactly as I would expect them to, no one of them stands out for better or worse so I would say work as accurately as you can, try to keep it light and you will be rewarded with a highly performant wing.

And on the subject of the "hypercritical" MH32 one might think that a bagged wing using this airfoil would be a waste of time untill you fly an Addiction or any of the Phil Barnes built examples so I think that there is a lot of talk going around that may not be based on experience.

The bottom line is IMO build it and fly it and worry about all this other stuff later.



Tony D.

[Bernie Wolfard]

The reason I posted this question is that Dr Drela has quoted the MH site’s statement that for best results the MH 32 needed to be built in a mold to get it accurate enough to take full advantage of the shape. He seemed to implying that his AG series didn’t require this degree of accuracy. I just want to know if this is what he meant and if the AG series is more forgiving.

We all try hard to produce the most accurate airfoils we can, but it is simple not possible to consistently create a degree or accuracy < 100th of on an inch by hand, and the spec for the hypercriticals is < 1000th of an inch. Dr Selig noted this when he had experienced modelers produce sections of the SD7037 for wind tunnel testing. They all produced different results in the tunnel because of varying levels of inaccuracy. This does not mean that a hand made airfoil won’t produce great results, they just cannot be optimal. I also don’t believe that most molded airfoils meet this spec. Therefore, it seems to me that if some airfoils that do what you want are more tolerant of surface imperfections than others it would be a good thing to know.

[Ollie]

The big advantage of the AG3- series airfoils is that they allow open bay wing structures without the degradation of covering sag between ribs. This allows a wing structure of better strength to weight ratio than any hollow molded wing. The resultant decrease in sinking speed, bank angle in turns and mass at the wing tips are more than justification enough for choosing this series of airfoils and the associated structure for planes that have thermalling ability as the highest priority. For an F3B model, it may be that a molded wing is a better choice. A wing using CNC molds is beyond the means of virtually all home builders. Built up and vacuum bagged, foam cored wings are within the capabilities of many home builders.

Forgive me for stating the obvious but, the finer points of airfoil leading edge accuracy may be important in a general sense but may not be a very important consideration in which airfoil to choose.

[markdrela]

It is almost impossible to specify a building tolerance for an airfoil, as a machinist might understand the term. A 10 mil bump just under the LE will probably cause unacceptable degradation in speed performance. But if a uniform 10 mil thick layer is added everywhere, there will probably be little effect. So just saying that the error must be "within 10 mils" does not suffice. How the error is distributed is also crucial. On full size sailplanes people sometimes quote a maximum tolerable waviness, or curvature error, but that isn't perfect either.

One generalization one can make is that airfoils which are "spiky", due to a combination of small camber and small LE radius, tend to be more sensitive to LE inaccuracies (this is based on Xfoil simulations rather than building experience). The MH32 does indeed fall into this category, as do some of the Selig slope sections like the S6062 and S6063. In contrast, the SA7035 is far more forgiving. I didn't try to make the AG airfoils any more or any less sensitive -- they just came out the way they did. The only exception is the AG3x series for built-up wings, where I intentionally increased the LE radius a bit. I can't say with 100% certainty whether this made them less sensitive.

[Bernie Wolfard]

Dr. Drela, thank you for the explanation. I especially appreciate the generalization about “spikey” airfoils as it helps me evaluate airfoils for building. I am trying to maximize performance with the type of gliders I can afford and I cannot afford molded gliders unless I buy used. I also like to build instead of buy. I don’t think there any examples molded gliders being with AG airfoils but I know in the real world they perform well either built up or bagged. If I am going to build it seems to maximize performance I need to accommodate some building inaccuracy and inconsistency.

Specifically, I am would like to build a Mantis with the AG airfoils. Because I believe form follows function and have a strong sense of irony I plan to build it with original or ugly fuselage. I would love to beat planes with something that challenges the idea that pretty is as important as functional.