Radio Control Soaring
Lee Estingoy [[email protected]]
Dr. Mark Drela shares his airfoil-design theory
Q&A
LE: It seems as though every few years there’s a new “in” airfoil. Could you perhaps describe the performance differences between a contemporary airfoil, such as the one on the Supra, with those from the past, including the ubiquitous 7037?
MD: The Supra AG4x airfoil series are significantly thinner and have less camber than the SD7037. The SD7037 section was designed when some 120-inch gliders pushed 80 ounces or more.
The AG4x series is geared for lower Reynolds numbers, which made them a better match for the lighter gliders that started to come into vogue in the early 2000s. It turns out that this newer “light + low camber” approach has resulted in a wider overall speed range than the older “heavy + high camber” approach.
Another major step was using a significant variation of airfoils across the span. The Supra wing-root AG40d airfoil is 8% thick and is similar to the MH32. The Supra wingtip AG43d airfoil is 6.5% thick and is very nearly the same as a DLG (discus-launch glider) airfoil. This makes the wing perform significantly better than if one airfoil is used across the entire span.
Previous airfoils were also typically designed for some neutral flap setting, and a camber/reflex flap was then typically added by the wing designer almost as an afterthought. In contrast, the AG4x series was a priori designed for use with a camber-changing flap, with the contours designed for both the high- and low-camber flap positions.
Incidentally, this latter approach has been the standard practice in the design of full-scale glider airfoils since the 1980s, so it’s not really new.
LE: I have a loose grip on Reynolds number and wing-chord concerns. Smaller wing chords offer unique design concerns. I’m curious to know your thoughts about Scale sailplanes. Some of the smaller models of high-performance gliders, such as the Nimbus, wind up with some terribly small chords at the tips. Is there a sweet spot for chord for Scale models, given their usual operating speeds?
MD: Airfoils for scale sailplanes are tough to design, for several reasons.
- The high aspect ratios make the structural demands severe, so the airfoils naturally want to be thicker than what the aerodynamics alone dictates.
- It’s not clear what the peak loads are, unlike in a TD (thermal-duration) glider, where the peak load is simply the known winch load. So on a scale glider it’s difficult to make a rational aero/structural tradeoff.
- Suitable airfoils do not “look right” on an RC scale glider, at least to me. If matched well to the Reynolds number, they are noticeably thinner and have their maximum thickness point farther forward than on the full-scale glider. I don’t know if this really matters to most RC scale glider fliers, and it may not be a big deal.
On the issue of small-chord tips, that’s actually less of a problem than one might think at first. The trick is to make the airfoil progressively thinner towards the tip, as on the Supra. Then one can add washout to quash any tip-stall tendencies. The resulting airfoil will especially “look wrong” as noted above, but such a wing will perform well and have good stall characteristics.
The Supra wing uses a touch of washout to get the same effect. The amount of washout needs to be carefully chosen via a vortex-lattice method like AVL (a program for the aerodynamic and flight-dynamic analysis of rigid aircraft) or something similar. If overdone, the high-speed performance will suffer, as is well known.
LE: Your model designs all seem to stress lightweight building techniques. However, many modelers seek to put ballast in their models to modify performance on windy days. Could you shed some light on the compromises that are involved in adding weight to sailplanes? What exactly does adding ballast achieve?
MD: I'm not sure it's possible to define an "optimum ballast" for XXX mph wind that is best in all situations. It also depends on what type of lift is present. I very rarely use ballast in either DLGs or TD gliders, even in serious wind. I instead fly more conservatively and simply don't allow myself to get too low when flying downwind of the field.
I take the penalty in penetration performance to still be able to react to and work very tight and fleeting puffs of lift, which are common in windy weather. Other people prefer to ballast up and live with the increased turning radius. Both extremes and anything in between can be made to work, so there's no magic ballast formula.
LE: The conventional wisdom out there is that some airfoils "like" to carry weight more than others. For instance, the 7037 airfoil seems to do very well if the sailplane is a bit heavier, such as an electric. This probably has something to do with the speed at which the model is flown—back to the Reynolds number again, right?
MD: Yep. Compared to the AG40d, the SD7037 likes more weight largely because it’s designed for a larger Reynolds number.
LE: Where are we headed with airfoil design for models? What can we look forward to in the way of improvements, if anything?
MD: Tough to say. One big issue is construction accuracy and variability, especially with built-up or vacuum-bagged models. This influences transition and especially high-speed performance in ways we don't really know how to quantify. And even if such variability could be quantified, it's not clear how one could make better airfoils with the information.
So I guess it's fair to say that I don't know what the best next step in airfoil design is.
LE: Competition sailplanes seem to have grown in the past years. Two-meter and 100-inch Open-class sailplanes gave way to 3.2-meter and now 3.7-meter gliders. What is the benefit of having the larger wings? What are the costs of having such larger wings?
MD: Pros:
- More range (in terms of actual glide range and visual range).
- Higher launch (up to a point—assuming the winch is not maxed out).
- Can handle stronger winds better.
Cons:
- They are less maneuverable, especially in small low-level thermals.
- There's more airplane to haul around.
- They crash harder.
LE: What's on your building table now?
MD: Nothing right now. In the past year I've been far too busy with work. I'm also at the point in my RC “career” where early-Christmas gliders occasionally show up on my doorstep. (Thank you, RC Builder and Maple Leaf Design.)
It's hard to get motivated to build from scratch when you get such really first-class hardware with no building work required.
LE: If you were a sailplane or soaring coach, what suggestions would you have for new pilots?
MD: When coaching relatively new pilots, the most frequent suggestions I make are:
- Detect lift or sink and watch carefully for uncommanded motions of the glider, both in roll and pitch. The glider not only rolls away from lift, but also pitches up and balloons when flying into lift.
- Follow the lift as it drifts with the wind.
- When struggling against the wind to get back to the field, always maintain a good positive ground speed—at least half of the wind speed, say. Never, ever let your ground speed drop to zero—then you're just losing altitude for nothing.
LE: What's your favorite model of all time? Is there any one sailplane you thoroughly enjoy taking to the field for an afternoon?
MD: Either the SuperGee II or the Bubble Dancer, depending on the conditions. The SG2 is most fun in tight and puffy thermals, with lots of wind shifts.
The BD is most fun for floating around in low wind with really weak high up, such as late afternoon or at sunset. When there's lots of strong lift everywhere, and staying up gets easy enough to get boring, I really like to fly the BD from hand throws.
About Mark Drela
Professor Mark Drela obtained his Bachelor of Science (1982), Master of Science (1983), and Doctorate (1985) degrees from MIT. He is currently the Terry J. Kohler Professor of Fluid Dynamics in the MIT Department of Aeronautics and Astronautics. He joined the faculty there in January 1986.
Mark's primary research interests are in low-speed and transonic aerodynamics and computational aerodynamic-design methodology. He has developed a number of computational aerodynamic design and analysis codes that are currently being used in the aircraft and gas-turbine industry.
He has also developed tools for analysis and design of control systems for highly aeroelastic aircraft. Mark teaches aircraft-design fundamentals, external aerodynamics, and fluid mechanics of boundary layers at the undergraduate and graduate levels.
Transcribed from original scans by AI. Minor OCR errors may remain.
