Airfoil Testing for Model Aircraft
We are searching for a group of experienced modelers to build a variety of airfoil wind tunnel models for tests at the University of Illinois at Urbana-Champaign (UIUC). A low-speed, low-turbulence wind tunnel has been instrumented to take lift and drag measurements on airfoils at low speeds over the Reynolds number range from 60,000 to 300,000 (60k to 300k). The scope of the airfoil wind tunnel tests will be limited only by the number of wind tunnel models provided and the amount of funding received. Hopefully, the proposed modeler-supported airfoil test program will become self-sustaining. Your support and help of any kind will be acknowledged in reports on the project to be published by Herk Stokely in SoarTech Aero. We plan to publish the results through SoarTech frequently possibly twice per year.
A similar undertaking (with substantial support from modelers) was started by Michael Selig, John Donovan and the late David Fraser in 1987 at Princeton University. In a two year period, over sixty various low-speed airfoils were wind tunnel tested, involving over 1200 hours of wind tunnel test time. The results were published in SoarTech 8 in 1989, and many of the new airfoil designs produced and tested during the program are now widely used on R/C sailplanes. As of November 1993, over 2200 copies of SoarTech 8 are in circulation worldwide. (SoarTech 8 is available from: SoarTech Aero Journal, c/o Herk Stokely, 1504 N. Horseshoe Circle, Virginia Beach, VA 23451).
At the present time, there is a need for new airfoils for R/C sailplanes. For example, R/C handlaunch soaring is booming, but few good airfoils (e.g., E387 and SD7037) presently exist for such sailplanes. Sailplanes for the new F3J competition are just beginning to evolve, and new airfoils will probably be required. What will they look like? In the past, only a few airfoils (e.g., HQ 1.5/8.5, RG15 and SD7003) have been favored for F3B competition. In shape, handling and performance the SD7003 is quite different from the other airfoils mentioned. These significant differences suggest that it may be possible to design new airfoils that have better overall characteristics for F3B competition. In addition to the design and wind tunnel testing of new airfoils, several existing airfoils should be tested. The SD7037 and RG15 are quite popular and often used with flaps. The flap effectiveness of these airfoils should be quantified through wind tunnel tests, and the results should be used in the design of new airfoils.
There is also a need for new airfoils for R/C sport, aerobatic, and electric planes, as well as R/C helicopters. Often, old NACA airfoils are used for these aircraft. Compared with airfoils that could be designed today, these NACA airfoils (which were designed decades ago mostly by trial and error) are inferior. At the time the NACA airfoils were designed, little was known about the complex aerodynamics of airfoils operating at low Reynolds numbers. (Airfoils with small chords at low speeds, such as those on model aircraft, are said to operate in the low Reynolds number flight regime). In recent years, much has been learned about low Reynolds number aerodynamics, and this knowledge has successfully been applied to the design of new airfoils for R/C sailplanes, ushering in a new era in R/C soaring. Overall, R/C sailplane performance has improved dramatically. Older airfoils are no longer used. R/C power aircraft performance could likewise be dramatically improved through the use of newly designed, specially tailored airfoils.
Unique airfoil design requirements also exist for other categories of model aircraft. For example, FAI freeflight aircraft (which incorporate both a powered launch segment and gliding flight) operate over a wide range of speeds. In the past, many airfoils with good performance characteristics have been designed for FAI freeflight. These airfoils should be wind tunnel tested to quantify their performance. The results gleaned from the tests could then be applied in the design process in an effort to develop new airfoils with improved performance. Also, the Society of Automotive Engineers (SAE) sponsors an annual model airplane design competition in which university student teams design, build and fly an R/C cargo aircraft. The record cargo weight that has been carried now stands at 23 1/4 lb for a model with a 60-size engine and 1200 in2 total projected area. Conceivably, this record could be broken by an aircraft with an airfoil (or airfoils) specifically designed for the competition. Clearly, the need for new airfoils and data on existing airfoils is not limited just to R/C sailplanes, but applies to any type of model aircraft where better handling qualities and overall performance are desired.
Other topics of interest include the effects of turbulators, contour accuracy and airfoil blending. Are trips simply repairs to otherwise bad airfoils, or can trips be integrated with the airfoil and result in improvements over, say, the SD7037? The Princeton tests began to address this issue, but many questions still remain. For example, what is the best trip height for a given airfoil? Also, what is the best trip geometry, where should the trip be located for best performance, and what type of airfoils respond best to trips? The Princeton tests also shed some light on how accurate airfoils must be in order to achieve expected performance, but a more systematic effort should be made to test the best airfoils for sensitivity to contour accuracy. It is also unlikely that the best performance can be obtained from a single airfoil used along the entire wing span. Rather, airfoils should be blended from root to tip. This is especially true for flying wings. Companion airfoils for blending should be designed for use with the most popular existing airfoils, e.g., SD7037 and RG15. It is expected that blended airfoils will be the wave of the future. In an effort to maximize low Reynolds number airfoil performance for model aircraft, all of these topics should be addressed. Overall, the UIUC test objectives will be to design and wind tunnel test new airfoils for each category of aircraft listed above and also to examine the effects of flaps, turbulators, contour accuracy and airfoil blending on airfoil performance. We are especially interested in testing existing airfoils that are known to have superior performance. Wind tunnel data on such airfoils will be used during the design of new and better airfoils. If you believe that we have overlooked an important area, we would be interested in your input and may consider expanding the scope of the project. The number of airfoil models to be tested has not been predefined; rather, it will be depend on the level of interest and support from the modeling community.
The wind tunnel models should be 33 5/8" in span with a 12" chord and can either be built-up or foam core. To insure a uniform contour, the built-up models need to be fully sheeted. For the foam core models, we may be able to supply two 12" chord wing templates. The surface finish can either be fiberglass or monokote; however, we are interested in the effects of surface finish and will consider testing models with non-smooth surfaces. The models will be attached to the wind tunnel balance by standard model wing rods. Details of the mounting system and airfoil model dimensions are presented in Figure 1. Standard model construction techniques should provide the necessary strength (supporting 15 - 20 lb of lift when pinned at both ends). The brass tubing and collars for the models will be supplied along with full-scale plots and/or coordinates of the airfoil, if requested.
The airfoils will be tested in the UIUC open-circuit 3 x 4 ft subsonic wind tunnel (see Figure 2). The turbulence intensity level is minimal and more than sufficient to ensure good flow integrity at low Reynolds numbers. The experimental apparatus used at Princeton will be modified for the UIUC tests. Lift and drag measurements for each airfoil will be taken at Reynolds numbers of 60k, 100k, 200k and 300k. In some instances, it may be possible to take limited data over an expanded range (20k - 1000k). The lift characteristics will be determined through force-balance measurements, while the drag will be evaluated by the momentum method through the use of a wake-rake traversed through the wake at four spanwise locations. We are also interested in airfoil pitching moment measurements, but the current apparatus does not have such a capability.
If you are interested in building wind tunnel models for the tests, please write, call, fax or send e-mail. Correspondence and calls for information should be directed to the graduate student in charge of the project:
James J. Guglielmo, Coordinator
Prof. Michael Selig
(If you would like a hardcopy of Figure 1 and Figure 2, please send a stamped, self-addressed envelope to the graduate student in charge of the project at the address listed above. Before building, please contact us first if you would like to make a wind tunnel model for the tests.)
Michael Selig and James Guglielmo