UIUC Low Speed Airfoil Test Program
Bulletin #1


Airfoil Testing for Model Aircraft
Michael Selig
July 26, 1994
The UIUC Low Speed Airfoil Test Program is off to an excellent start. More and more, modelers are choosing to support the airfoil wind tunnel tests. Over 70 modelers to date have offered to build airfoil wind tunnel models. As of August 1994, the program will be supported through the generous donations of many modelers, clubs, and the model industry. Fred Burgdorf of Landing Products (makers of APC propellers), Academy of Model Aeronautics, Airtronics, National Free Flight Society, Jerry Robertson, SoarTech Aero Journal, Tri County Aero Club of Vincennes, IN, D'Anne Thompson, and Northeast Sailplane Products have all made large contributions. Several other donations have been received - over 100 (yet a very small fraction of modelers who might benefit from this work). We have also had a lot of valuable input from individuals, and coverage in the model press (and elsewhere) always helps to keep the airfoil tests in the spotlight. A complete list of those who have made donations, built wind tunnel models, and offered to build or help in any other way thus far is included with this update. If your name should be on the list but isn't, please contact us!

Just think, if support came from everyone who has taken advantage of the Princeton airfoil tests (not to mention those who will benefit from the new effort), then the future of the UIUC Low Speed Airfoil Test Program would be assured. I am confident that the program will thrive with continued support.

The idea of supporting the airfoil tests through donations is, I believe, new, but to my surprise (and that of the other faculty in the department) it's going to be possible, at least for the first year! Jim Guglielmo will be working full time on the airfoil tests. In addition, Andy Broeren and Phil Giguere (both graduate students) have volunteered to help with the wind tunnel testing. We will be testing again in September and October, and then again around December. Time in between is used to reduce the data, interpret it, design what we hope to be better airfoils, and plan for the next phase. For the next test phase, we expect to contact 5-10 modelers who have offered to build wind tunnel models. No plans have been made yet for 1995, although as mentioned the testing program will continue at least until May 1995.

Of course, ongoing financial support is needed to continue these wind tunnel tests. Maintaining the airfoil design and test program is expensive and does not occur in a vacuum. We need your input (comments, time and effort) and monetary contributions.

Preliminary Testing in May

Some preliminary calibration tests were performed in May during the initial setup of the test apparatus. Prior to the tests, two new airfoils were designed and built. The first airfoil (S1210 built by Mark Allen) was designed for the SAE Aero Design competition, and the second (S7012 built by Mike Lachowski) was designed to be an improvement over the RG15 (built by Bob Champine) and SD7003 (built by Mark Allen) for R/C sailplanes. The wind tunnel results are quite encouraging. The new S1210 airfoil outperforms the FX 63-137 airfoil by a large margin, which to date has been favored for the SAE heavy lift competition. In particular, the S1210 generates 15% more lift, and the next-generation airfoil (S1214) to be tested is predicted to generate 30% more lift than the FX 63-137. The S7012 appears to be faster than the RG15 (but slower than the SD7003) for F3B competition. Mike Lachowski reports that it out-launches the RG15 and SD8000. Also, in thermal performance, the new airfoil should be better than both the SD7003 and RG15. Both airfoils will be tested again in September after further refinements are made to the test setup and data reduction codes. Additional models built by Charles Fox, William Friedlander, Leon Kincaid, Harley Michaelis, and D'Anne Thompson were received, but there was not enough time to test them in May.

Airfoil Coordinates for S7012 and S1210 and Others

Until the new airfoils are published through SoarTech Aero, the airfoil coordinates and a brief description of the performance characteristics for these designs (or any of the other new thermal airfoils) can be obtained by sending a SASE with your request to:

Prof. Michael Selig
Dept. of Aeronautical and Astronautical Eng.
University of Illinois at Urbana-Champaign
306 Talbot Laboratory, 104 S. Wright St.
Urbana, IL 61801-2935.
voice: (217) 244-5757
fax: (217) 244-0720

WWW: http://www.uiuc.edu/ph/www/m-selig

New Airfoils for Sept/Oct Test Phase

During the summer, several new airfoils will be designed for the next test phase. A new airfoil (S4083 - 8.5%) for hand launched gliders has already been designed. For a typical hand launched glider, the lift-to-drag ratio (L/D) is 16% better than the E387 (at the same airspeed) As compared with the SD7037, the lift-to-drag ratio is 7% better and occurs at an airspeed 22% faster. In thermal performance, the new airfoil is equal to the E387 but 5% worse than the SD7037. It's a tradeoff, but the improved L/D might be worth it. Another airfoil (S7055 - 10.5%) has been designed primarily for those who prefer built-up construction. This airfoil is flat bottomed past the D-tube for easy building on flat surfaces. One airfoil for thermal duration/F3J competition will be designed as well as a couple of airfoils for R/C power sport flying. If you have an airfoil request, get your order in!

Test Plan for Sept/Oct Test Phase

In addition to testing the new airfoils, tests on the S1210, S7012, RG15, and SD7003 will be repeated. Several of the airfoil models from the Princeton Tests will also be tested as a check on the current data. Members of the National Free Flight Society, acting through Gil Morris, will be providing two models for tests. Some preliminary tests will be performed on a new airfoil/trip design philosophy. We are also planning to take detailed spanwise measurements to better quantify the degree of spanwise three-dimensionality of the wake behind the airfoil. At Princeton, four spanwise drag measurements were taken, and in some cases the drag polar was not as smooth as expected. In such cases, four spanwise drag stations may not be enough. The new tests may help determine a priori when more than four spanwise drag stations are needed to provide a " sufficiently good" average drag. Prior to the testing, we will be upgrading the data aquisition board from a 12-bit A/D converter to a 16-bit for better resolution. Plans are currently underway to replace the lift force balance, and we are also planning to install a moment balance to measure the pitching moment on the airfoils. These two new changes may not be installed for the Sept-Oct test phase, but will probably be ready for the December tests.

At Princeton, the wind tunnel speed was set manually at the start of the run and could change slightly throughout the entire run, which sometimes lasted 2 1/2 hours. For the current tests, we will be using a speed controller and software to set and hold the tunnel speed constant throughout the run. Once this is in place, the entire wind tunnel run and data taking will proceed automatically.

Currents Needs

We are looking for someone who has access to a coordinate measuring machine (CMM) so that the model coordinates can be measured as described in SoarTech 8. Also, for cutting foam core models, we would like to have EDM templates made (or templates made on some other numerically controlled milling machine) so that the models can be more accurately reproduced.

Making Best Use of Boundary Layer Turbulators (Trips)

I have developed a new design philosophy for making optimal use of trips. One of the problems with trips is that in cruise flight between thermals when the bubble is relatively thin, trips handicap performance. But when there is a thick bubble in thermal mode, a trip usually reduces drag. That's the popular story to explain why trips are not used - it's a tradeoff. On the less technical side: " There's no such thing as a free lunch." The scenario described above takes place when a trip is placed on an airfoil that originally was not designed to make use of a trip. To take advantage of a trip, however, it must be integrated into the airfoil design from the outset. When this is properly done, an optimum airfoil/trip combination will probably outperform a clean optimum airfoil designed to the same requirements. If the current hypothesis proves valid, it will usher in the next generation of airfoils.

Need Interest from Power Flyers Acting as Technical Advisors

We want to design some new airfoils for power (e.g., pylon, aerobatics, sport, etc.), but ideally we would like to start with an existing airfoil and try to improve on it. The plan would be to first test what is known to be a superior airfoil (could be a french curve airfoil, but must have a proven track record as attested by many modelers). After the tests, an attempt will be made to improve the design, that is, to re-design the airfoil. The new airfoil will then be tested to see if indeed it out-performs it's predecessor. We are looking for someone with first hand experience with an existing high-performance airfoil. This person would act in an advisory role, and would not necessarily be involved with the building of the wind tunnel models. Can you help?

(Generally this approach through the design loop applies to any class of model aircraft. Any new airfoil design must have a starting point, and we need someone involved to help guide the development effort.)

Support the Test Program: Make a Donation for a T-Shirt

Cody Robertson of Flagstaff, AZ has designed a T-shirt exclusively for the UIUC Low Speed Airfoil Test Program. You can receive this white short-sleeve shirt (for a suggested donation of $18 - $15 for the shirt + $3 for mailing) to help support the project by sending your check payable to " University of Illinois, AAE Dept." Please write on the check " Selig - Wind Tunnel Testing/AAE Unrestricted Funds." The shirts are Hanes brand and 100% cotton. Shirts can be obtained from Michael Selig or the graduate student coordinator:

James J. Guglielmo, Coordinator
Dept. of Aeronautical and Astronautical Eng.
University of Illinois at Urbana-Champaign
306 Talbot Laboratory, 104 S. Wright St.
Urbana, IL 61801-2935
voice: (217) 244-0684
fax: (217) 244-0720
e-mail: jjgug@uxh.cso.uiuc.edu

Presentation Available for Promotion Purposes

If you are interested in presenting a discussion of the airfoil test program at a club meeting, a 25 page presentation with a brief narrative is available. To receive the presentation, please send a LSASE with $1.21 postage to Jim Guglielmo.

You Would Like to Build a Wind Tunnel Model

If you have an interest in building a wind tunnel model, please contact Jim Guglielmo. Please give us some idea of your interests (sailplanes, power, helicopters, etc.), and your method of construction (foam core or built-up and full sheeted), your building skills - we dream about the perfect airfoil model, but don't expect to ever see one.

The wind tunnel models should be 33 5/8 inch in span with a 12 inch 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 inch 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. K&S tubing is installed in the model to adapt to the wing rods. Details of the mounting system and airfoil model dimensions are available from Jim Guglielmo (please include a SASE). Standard model construction techniques should provide the necessary strength (supporting 15-20 lb of lift when pinned at both ends). The K&S brass tubing and collars for the models will be supplied along with full-scale plots and/or coordinates of the airfoil, if requested.

New Composite Material for Airfoil Models

Jim Boxmeyer of Boxmeyer Composites has generously offered to work with a few model builders who would be interested in using a new formable composite material for covering built-up or foam core wing sections. The new material is called Tufflite, and is very smooth. Also, there is no need to sand or vacuum bag the model, promising to cut building time down significantly. Jim Boxmeyer will be providing the composite material free of charge, and is also willing to work closely with the model builder, supplying special building instructions and drawings for this new construction technique. Any interested model builders should contact Jim Guglielmo.

Where to Get Wind Tunnel Test Results

The wind tunnel tests will be published in SoarTech Aero by Herk Stokely. Hopefully, results will appear in SoarTech Aero at least twice yearly, however, new airfoils designed as an outcome of this work can be obtained directly from Michael Selig at the address previously listed. Announcement will be made through this bulletin when new airfoils are designed and when the results will be published in SoarTech Aero.

How to Get Your Name on the Mailing List to Receive this Bulletin

To receive the next bulletin directly from the University of Illinois, send a SASE to Michael Selig at the address listed above.

Photos of the Wind Tunnel

... will appear in the next bulletin and on the web!

Low Speed Airfoil Aerodynamics

Twenty things a knowledgeable modeler interested in airfoils ought to know something about.

List of Contributors

(as of July 15, 1994: by category, then in order of receipt, % -- percent of total support received)

Recommended Reading List

(mostly related to the UIUC Low Speed Airfoil Tests)

How to Get SoarTech 8

Airfoils at Low Speeds, M.S. Selig, J.F. Donovan and D.B. Fraser

The book is only available direct from:

H.A. Stokely, publisher
1504 N. Horseshoe Circle
Virginia Beach, VA 23451
When ordering, please provide a check or money order in US Dollars which can be paid at a US bank. US cash is also accepted. Residents of Virginia should add the state 4-1/2 % sales tax to the above rates. Sorry no credit card or COD orders at this time.

Logo Design Credits

The UIUC Low Speed Airfoil Test Program logo was co-designed by Karen Evans (University of Illinois undergraduate interested in advertising and graphic design) and Jim Guglielmo.

Distribution

Any portion of this bulletin can be reproduced.


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