Discussion of airfoils for RC gliders

[fprintf]

Hi guys,

In reading Ollie's response to the replacement GL wing thread got me thinking that somewhere there should be a layman's description of what airfoils are currently good for RC gliders.

I have seen the occasional posts on RCSE that describe in great detail what makes one airfoil better than another, but to a layman with no engineering knowledge all I really need to know is how it responds to the things I am supposed to be seeking. In other words, how an airfoil indicates lift, how it penetrates, how well it thermals etc.

For instance, I have seen Mark Drela say that the AG?? airfoils are much better than the Selig airfoils for the Terminator. Or that the Clark-Y (which I am using for a two-meter fan fold delta glider) is not a very efficient airfoil etc.

So lets have a discussion on airfoils. What are your favorites, and why? What makes airfoils like the AG series so much better than the NACA or Selig airfoils for our applications? Is the airfoil best for a certain wing shape (swept leading edge, delta wing etc)?

[shoe]

I have an old NSP catalog (1993) that has exactly the type of info you're looking for. Layman's descriptions of a selection of airfoils and they're uses. It'd be nice to have a more up-to-date one though. I think the reason you see more quantitative descriptions of airfoils is that it is objective. The qualitative side of things become very subjective and can vary a bit from pilot to pilot, or designer to designer.

That said it's hard to beat the SD7037, (and now the SA7035-36 series) for plain old TD tasks. They thermal well, respond to camber and reflex, and are generally fun to fly.

[Ollie]

The question of goodness of airfoils is ambiguous. It has little meaning outside of the context of the airfoils' applications. Sailplanes can have many and even conflicting objectives such as low sink rate, high sink rate, low speed ability, high speed ability, soft stall, abrupt stall, shallow glide angle, steep glide angle, good inverted performance and on, and on. Airfoil selection directly or indirectly involves almost every aspect of a sailplanes design, construction and ability to meet its objectives. Fortunately a designer can equip a model with a set of variable airfoils. Each unique position of a flap or aileron constitutes a different airfoil, shifting the wing's lift and drag characteristics.

Mark Drela is probably the most knowledgable and creative of model sailplane designers. Studying his designs is like taking a series of courses in aeronautical engineering. This is not surprising as he teaches the subject at MIT. Each of his designs utilizes a set of airfoils that has been optimized not only for the sailplanes objectives but also for that portion of the wing on which it is used. His thermal duration designs such as the Bubble Dancer, Allegro Lite and Super Gee all have lighter wing loadings than "normal." This means that for a given wing chord, the airfoil is operating at a lower reynolds number than most other designs in their class. Lower reynolds numbers mean thicker boundary layers and higher drag. To compensate for the thicker boundary layers, Mark's airfoils are thinner and actually produce less drag even with the disadvantage of the thicker boundary layer. Less drag and lower wing loadings result in lower sinking speeds and the ability to turn tighter for a given bank angle or sinking speed, enhancing their thermalling ability. These thin airfoils and integrated wing design help to overcome the wind penetration disadvantage of the associated low wing loadings because of their lower drag at mediun and high speeds. Their one pervasive disadvantage is that their thinness puts great demands on the strength and particularly the stiffness of the spar. Mark meets this challenge with composite spar designs of outstanding strength and stiffness to weight ratios. His Allegro Lite is a 2-meter RES ship whose ready to fly weight is only 18 ounces, yet it is capable of over 100 G maneuvers without spar failure. This outstanding performance comes at the price of very demanding construction techniques.

Even so, the airfoils for these thermal duration models would be unsuitable for slope aerobatic or slope racing applications.

Mark sometimes participates in this forum. If I have misrepresented any of his work I hope he will correct me.

[ICTHRMLS]

Different airfoils excel in different areas. Some more suited to speed, others lift and there is a constant trade-off between these two. IMHO, an important aspect is experience - sticktime as it may. Flying a particular airfoil until you know all about the characteristics of that foil/palne set-up results in more confidence and better contest placement. I'm partial to the S3021 - good all around thermaler, medium wide speed range (no speed demon though) and the flat bottom {from the wing joiner to the trailing edge, anyway} helps keep your flaps trimmed properly. ;)

[fprintf]

An interesting comment I read in one of the RCSE archives is that some of the differences between airfoils are so very tiny that it takes a CNC machined mould to achieve the airfoil. I think it was Michael Selig who wrote that a .002 piece of tape had a 50% impact on one of the characteristics of one particular airfoil. That is less than one swipe of a piece of sandpaper, and way beyond my abilities to achieve that kind of accuracy.

Thanks everyone for the enlightening discussion. I think the airfoils that I can make with Dow BlueCor will look similar and inaccurate enough whether I choose a Clark-Y or S3021. :) Perhaps once I get good enough to bag a wing it'll make a difference - for now I think the only difference I'll see are between flat bottomed, semi and full symmetrical designs. I gotta get way more accurate on my balsa models with the sandpaper.

[Ollie]

Airfoil accuracy is very important. Saying that a plane has such and such airfoil is often a far cry from achieving accuracy close enough to have the lift and drag polars within 5 or 10 % of the true airfoil. The contour accuracy required for a given accuracy of performance varies greatly with the location on the airfoil surface. The trailing edge could be thickend by as much as 1/16 of an inch in many cases without introducing a degradation in lift or drag of more than perhaps 5%. A similar contour error at the leading edge or on the upper surface, to a little past the highpoint could have a very, very large effect on lift and drag.

Perhaps as important as airfoil contour accuracy is harmonizing of the wing's planform, aspect ratio, airfoils, sweep and twist to achieve an elliptical lift distribution over a wide range of angles of attack. A good airfoil applied to a bad wing design is sort of like taking your new car to the barn and hitching it to a mule.

When somone claims a certain airfoil, be skeptical. Even an accurate airfoil, in and of itself is not a guarantee of performance. What counts is the whole enchilada from design to manufacture to adjustment to piloting.

Most non-technical people judge quality by what the experts pilots are flying. However, that doesn't work for many people because those planes typically require piloting skills that those persons don't posess.