New Airfoils for R/C Sailplanes

Michael S. Selig and Ashok Gopalarathnam
UIUC Applied Aerodynamics Group
Department of Aerospace Engineering
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

At the 1996 AMA Nats, we performed an airfoil survey [Ref 1] by asking those competing in the Unlimited Thermal Soaring Competition what airfoil, sailplane, and radio they used. Of the approximately 110 flyers, we were able to reach 101 of them for the survey. In the airfoil category, the most popular by far was the SD7037 flown on 40 of the gliders, followed by the S3021 and RG15 each with 8, and then the SD7080 with 6. The remainder of the pack included over 24 different airfoils. In contrast, there were over 60 different sailplanes flown, the most popular were the Peregrine and Thermal Eagle, which numbered 5 each, followed by the Esteem and Super V, 4 of each. More details of the survey can be in the Camberline newsletter (No. 4) found at the UIUC Low Speed Airfoil Tests (LSATs) web site. The popularity of the SD7037 has led to the current airfoils reported here.

It is clear that of those flying the SD7037 some would like to have the airfoil a little faster, others perhaps slower, some thicker and others thinner. One approach to the problem would be to simply change the thickness and camber of the SD7037 as desired. But by how much should the camber and thickness be changed? And what will happen to the aerodynamics in the process? To avoid these questions, we took a different approach. Using the inverse airfoil design code PROFOIL [Ref 2], we systematically designed the airfoils to share common aerodynamic characteristics from the outset.

We have recently finished the design of the first three in the series - the Selig/Ashok Gopalarathnam SA7035, SA7036, and SA7038 shown in Fig. 1 together with the baseline SD7037. The corresponding velocity distributions are shown in Fig. 2. Each airfoil is 9.2% thick, and for reference the camber values for the SA7035, SA7036, SD7037, and SA7038 are 2.6%, 2.8%, 3.0% and 3.3%, respectively.

Figure 1 Overlay of the SA7035, SA7036, SD7037 and SA7038 on an expanded scale.

Figure 2 Inviscid velocity distributions for the SA7035, SA7036, SD7037 and SA7038 airfoils at 2, 4, 6, 8, and 10 deg relative to the zero lift line.

The performance predicted by Mark Drela's XFOIL program [Ref 3] is shown in Fig. 3 for the new airfoils and compared with that of the SD7037. The data is plotted for a reduced Reynolds number of 125,000. When the data is plotted in this way, the airfoil polars can be compared without having to interpolate on the Reynolds number as the sailplane speed changes with lift coefficient. Put simply, the drag coefficient plotted is that experienced by the aircraft in flight over its entire speed range. As seen the first two (SA7035 and SA7036) are lower lift versions of the SD7037 and the third (SA7038) is a higher lift version. For comparison, the SA7035 is shown in Fig. 4 together with the SD7080. As seen, the SA7035 is an improvement over the SD7080 over most of the performance range. In particular, the low-lift/high-speed and high-lift/low-speed ends of the polar are extended at the price of slightly higher drag through the middle range.

Figure 3 SA7035, SA7036, SD7037 and SA7038 predicted performance for a reduced Reynolds number of 125,000.

Figure 4 SA7035 and SD7080 predicted performance for a reduced Reynolds number of 125,000.

Our plans are to have some of these new airfoils wind tunnel tested during the current test series, which has started as of this writing (January 97). We are also planning to design two similar series, one 7.7% thick and another 6.2% thick. The final family of airfoils will therefore consist of 12 airfoils, one of which will be the SD7037. When this airfoil family is complete, sailplane designers will be able to make fine adjustments to the performance of a new design. Specifically, from one sailplane design to the next, designers will be able to migrate through the airfoil family until the best airfoil combination (root-to-tip) for a particular configuration/weather conditions is selected.

An airfoil family like this has been needed for quite some time, although now it is especially important since the majority of designs have evolved to using a single airfoil, which is not likely to be the optimum over a range of sailplane sizes and weather conditions. Expect to see more variety at this years Nats! Finally, coordinates and performance data (a mix of experimental and predicted data) for all of the airfoils will be included in the next Summary of Low-Speed Airfoils book. For coordinates, click here



  1. Ninham, C. and Selig, M.S.
  2. Selig, M.S., Guglielmo, J.J., Broeren, A.P. and Giguere, P., Summary of Low-Speed Airfoil Data - Vol. 1, SoarTech Publications, 1995.
  3. Selig, M.S., Lyon, C.A., Giguere, P. Ninham, C.N. and Guglielmo, J.J., Summary of Low-Speed Airfoil Data - Vol. 2, SoarTech Publications, 1996.
This article first appeared in Sailplane and Electric Modeler, Spring - Vol. 2, No. 3, p. 14-17

Copyright 1997 by Michael S. Selig and Ashok Gopalarathnam
All rights reserved.

Postscript 12/28/97: Comments on the similarities in the SD7080 and SA7035 airfoils shapes is here