UIUC Low Speed Airfoil Test Program
Bulletin #3


Airfoil Testing for Model Aircraft
Michael Selig, Chris Lyon, Cameron Ninham, Philippe Giguere, Andy Broeren and Ashok Gopalarathnam
December 17, 1995

What's New

Our First Book of Results.

Test series #1 is over, and the results are published in Summary of Low-Speed Airfoil Data - Volume 1, which contains 317 pages of narrative and wind-tunnel test data on over 30 airfoils for a wide variety of model aircraft. You can purchase the book by sending $25 to SoarTech Aero Publications, 1504 N. Horseshoe Circle, Virginia Beach, VA 23451. Add $4 for international surface mail. For Air Mail to the Western Hemisphere add $6, to Europe add $13, and for other parts of the world add $17. Disks containing all the airfoil polar and coordinate data is also available.

A significant portion of the proceeds from all book and data disk sales will be returned to UIUC to provide part of the continuing support for the airfoil tests.

As compared with SoarTech 8, the format has changed, we believe for the better. In SoarTech 8, to get a good understanding of an airfoil required running through a number of pages, first to see the polar, then the airfoil shape, then the accuracy of the model, and back to the discussion. The new format simplifies this task. A single chapter contains all the polars, accuracy plots, and also the airfoil velocity distributions, a feature not included in SoarTech 8. In this same chapter, the airfoils are listed in alphabetical order, and all the data for one airfoil is included in a single group of pages. So there is no need to run back and forth through several pages. To make matters simpler (than SoarTech 8), the airfoil names are listed in the margin so that one can quickly thumb through to find a given airfoil.

The discussion of the airfoils is also different. Instead of discussing the airfoils in alphabetical order, the airfoils are categorized and discussed as a group. This makes the process of comparing the airfoils a simpler matter and again alleviates the need to flip back and forth to compare one airfoil discussion with another as required with SoarTech 8.

Among the 30 or so airfoils tested, several were previously tested at Princeton. Re-testing these airfoils has allowed us to validate (or "calibrate") the tunnel by providing data for comparison with the Princeton tests. Without going into any further details (since that's what the book is for), all of the airfoil tested during test series #1 are shown by category below for reference.

Our Progress Continues.

We have just finished Test Series #2 of the UIUC Low-Speed Airfoil Tests and will now begin the process of putting all the data into Summary of Low-Speed Airfoil Data - Volume 2. Airfoils tested include the SD7037, RG15, S7012 and S7075 (all with flaps at 0, 5 and 10 deg), the S4083 and CR-001 (for R/C hand launch airfoil), S8025 (new tail airfoil), LD79 and Davis 3R (F1B), NACA 2415 and NACA 2414 (sport), S5010 (flying wing), S1223RTL and E423 (high lift) and others. In all, over 20 models were tested. We will have more to say once we begin to study the data and make comparisons with the previous tests. We hope to have the book finished and published by SoarTech Aero in late January.

After two years of preparation and testing, we are beginning to settle into a routine in which we will test two times a year - during Sept/Oct and then Feb/March. We have found that taking the data, reducing it, interpreting it, writing the book, and starting for the next tests takes about 6 months.

Some Plans for Test Series #3.

In the past, the majority of work done within the UIUC LSATs program has revolved around R/C sailplane airfoils. While limited data has been published regarding sport airfoils (see Vol. 1), the variety of airfoils tested was by no means comprehensive. This problem is being addressed by designating individual themes for upcoming books. We have selected the theme for our next book to be sport-powered airfoils with a great deal of focus on trainers. This will concentrate the airfoil data pertaining to this class of aircraft into a single volume thereby making the data more attainable. It will also allow us to expand our base into the powered area of R/C aircraft and will hopefully generate more support and interest in what the UIUC LSATs project can do for the R/C aircraft modelers and the industry.

For those of you who are hard-core sailplane pilots, don't worry. The backbone of the UIUC LSATs effort has always been R/C sailplane airfoils, and we intend to keep it that way by continuing to include sailplane airfoils in each book.

You may be asking yourself why we chose to focus our attention on trainer aircraft. Many R/C pilots have at least once in their career flown a trainer that appeared to have a mind of its own. The plane tended to trudge through the air like a pound of lead was hanging from it, jump around the sky like it was on a trampoline, and stall like it just fell off a cliff. These poor characteristics are often due to inadequate airfoil performance, and while the experienced pilots may find them an inconvenience, to the beginner it's the difference between landing successfully and using a crowbar to remove the plane from the ground. This is where the UIUC LSATs program can help. We intend to examine existing airfoils from a variety of sport-trainer aircraft, and then design a family of airfoils to be used on the next generation of trainers, which will lead to much gentler flying qualities and more forgiving aircraft.

Focusing on trainers also gives us a natural starting point from which to expand. Just as there are numerous classes of aircraft in sailplane competition (e.g., thermal duration, F3B, hand-launched, etc.), there are also several classes of powered aircraft (pattern, fun-fly, scale, etc.). It is our intention to address as many classes of powered flight as possible. This is where you can help. While we already have a smattering of airfoils in the pattern (DH4009 and E472) and scale (S8036 and S8037) classes, we would definitely like to have more. To help advance the sport, the reader can "spread the word" about our plans to local hobby shops and flying buddies. In doing so, we hope to receive a large influx of ideas from a large cross-section of people with the ability to provide building skills, airfoil designs, and financial support. Remember, this project stays alive because of your support. Without you and support from manufacturers, we're grounded.

As a beginning, the S8036 and S8037 airfoils were designed for the newly released Top Flite P-47D. You may have seen this advertised in the recent (January 1996) issue of Model Aviation. Dave Ribbe at Top Flight was the force behind the new model. He gave us the design constraints (intended wing loading, stall speed, airfoil thickness and more) from which the new airfoils were designed. Off the boards the aircraft flew like a dream (the ad is not all hype). It handles surprisingly well for a scale warbird. We hope to repeat the success that we had with the Top Flite model. The S8036 and S8037 airfoils will be wind tunnel tested and compared with predictions to help guide us further.

Jim Guglielmo Graduates!

Jim Guglielmo, the UIUC LSATs co-founder and first coordinator, has graduated and taken employment at McDonnell Douglas Aerospace in Huntington Beach, CA. Jim is currently working on the X-33 program, specifically in the areas of experimental aerodynamics and flight mechanics. Special thanks go to Don Edberg for the lead on the job opening. If you wish to reach Jim, his new e-mail address is: guglielmo@dlvd.mdc.com.

Low-Speed Airfoil Aerodynamics.

In the first bulletin, we listed twenty things a knowledgeable modeler interested in airfoils ought to know something about. In the second bulletin, we answered one. That pace was too slow, so here we answer three.

"Why new airfoils should be designed when the old ones still work."

Many people wonder why such great effort is spent designing new airfoils when the old ones seem to work just fine. Perhaps the best way to answer this question is with an analogy. Many readers own a computer. Some of you probably started years ago with one of the original Apples or PCs. Today, I doubt you still have the same old machine. You've upgraded to a faster and more powerful computer. Why did you upgrade when the old one worked just fine? Well, with the new one, you are able to do more things. It's more user friendly (most of the time), and overall more fun to use. The same goes with airfoils. Sure, the older airfoils were OK, but the new ones have better performance, lower drag, more lift and stall more gently. But perhaps beyond these reasons lies one a bit closer to the heart. It's fun and interesting to use new airfoils. Think of how bland the sport would be if everyone was still using the Clark Y.

"Why laminar flow is not necessarily good for models."

One of the more important aspects of low-Reynolds number aerodynamics is the effect laminar and turbulent flow have on an airfoil's performance. Contrary to full scale aircraft, laminar flow often degrades performance on R/C models. Why is this? The difference lies in laminar and turbulent flow's tendency to separate from the airfoil. When an aircraft flies at low-Reynolds numbers (small models at low speeds) as compared to high-Reynolds numbers, laminar flow tends to be prolonged further aft along the airfoil. If laminar flow is prolonged far enough aft that it begins to enter a region of increasing pressure (decreasing speed), it will separate more readily than turbulent flow. This is because laminar flow has lower momentum closer to the surface of the airfoil as compared to turbulent flow, and this lower momentum is less capable of overcoming the increasing pressure. Therefore, the turbulent boundary layer stays attached longer, which means less drag due to separation and, therefore, less overall drag.

"The relation between Eppler, Wortmann and Althaus."

Drs. Eppler, Wortmann and Althaus are all from the Stuttgart University.

Dr. Richard Eppler pioneered a practical method for multipoint inverse design of airfoils. His airfoil design and analysis code is used by airfoil designers worldwide. He has also designed airfoils for a variety of applications including general aviation, sailplanes, model airplanes and ship propellers. His recent book Airfoil Design and Data (published by Springer-Verlag, ISBN 0-387-52505-X, 1990) is an excellent reference for any person interested in airfoil design.

Dr. F.X. Wortmann is famous for his FX-series airfoils for low-speed applications ranging from sailplanes (full-scale), man-powered aircraft and model aircraft. He was among the pioneers responsible for the wide-spread use of low-drag, natural laminar flow airfoils in sailplanes. His death was a big loss to the aviation community.

Dr. D. Althaus is well-known for his extensive wind-tunnel measurements of airfoil characteristics and his investigations on wing-fuselage intersection drag. The characteristics of airfoils for full-size sailplanes are compiled in Stuttgarter Profilkatalog (with Dr. Wortmann) and the airfoil data for model-airplanes are compiled in Profilpolaren fur den Modellflug.

What We Test and Why.

Due to the considerable interest in the test program, requests to have specific airfoils tested exceeds our current testing capacity. We have found a single test series, which invovles taking the data, reducing it, interpreting it, writing the book and starting for the next tests, takes about 6 months. In this 6 month period, we can test 30 airfoils (or 60 in a year). Since we are limited, we must be selective. We are, therefore, focusing our work on airfoils that have in some way proven to be better than the competition. Also, new airfoil designs thought to offer even better performance will also be tested. In this respect the airfoil design and analysis expertise at UIUC will be applied. It should also be mentioned, in those cases where an airfoil falls outside of the scope of the current effort, the wind tunnel time can be rented for a standard fee. Efforts such as these can provide income to continue the test program.

Existing Capability of the UIUC LSATs.

Up to this point, we are only able to test two-dimensional objects (i.e., airfoils). Our equipment is configured in such a manner that it is not possible to test wing planform shapes, half-span ailerons, tip geometry, fuselage, etc. Regarding our two-dimensional capability, a new lift balance and a pitching moment balance have been designed (as mentioned in the last news bulletin) and more recently built thanks to the efforts of Lynn King (see below). The new setup will be operational for the upcoming test series # 3. Therefore, experimental pitching moment data should be available in the third volume of Summary of Low-Speed Airfoil Data.

Our Thanks to ...

We should mention that in addition to those listed above, many other people have contributed to the project. Their names are listed in Summary of Low-Speed Airfoil Data, Vol. 1.

Correspondence in other languages than English.

The UIUC LSATs is an internationally recognized airfoil testing program in the modeling community. In order to better serve the modelers whose mother tongue is not English, we now offer the possibility to handle correspondence in three other languages: Africans, French and Urdu. For those of you who would rather communicate with us in these three languages, you are welcome to do so. Please address your correspondence in Africans to Cameron Ninham, in French to Philippe Giguere and in Urdu to Farooq Saeed. Of course, this special service will be offered as long as the above mentioned graduate students are present at the University of Illinois.

Illinois Aerospace Institute.

This is not part of the UIUC LSATs program but it might be of interest to some of those involved. The Department of Aeronautical and Astronautical Engineering at the University of Illinois of Urbana-Champaign is offering its annual summer aerospace program for high school students. The Illinois Aerospace Institute, July 14-20, 1996, is a residential program for students entering grades 9-12 in Fall 1996. The institute provides a multi-faceted introduction to aerospace science and engineering through classroom studies, laboratory work, field trips and workshops. Some institute activities include: (1) talking with aerospace engineering specialists and a NASA space-shuttle astronaut!, (2) visiting an FAA control tower to learn about aircraft navigation during flights at the UIUC Institute of Aviation and (3) designing, building and flying model airplanes and rockets. Participants will stay at a university-approved campus residence hall. The registration fee for the week is $700 (deadline April 15, 1996). The program is sponsored by the department and also Aerospace Illinois Space Grant Consortium. For further information and registration forms, contact Diane Jeffers, 306 Talbot Lab, 104 S. Wright St., Urbana, IL 61801 or call (217) 244-8048.

What's Old, But Still Important

The Mailing List.

All those who have made a donation to the program are automatically on the snail mail list for these bulletins. If you would like us to save trees and receive the e-mail version, please let us know. This same information can be obtained from the web at the UIUC Applied Aerodynamics homepage. It is also sent out to several newsgroups on the Internet. If you are on the Internet, you will probably see it.

Airfoil Data on the Web.

If you are on the Internet and have access to the World-Wide Web (WWW) through a browser like Netscape, then you can:

The address on the Web is:

http://uxh.cso.uiuc.edu/~selig

Once you find this site, if you have problems linking to some of the data it is probably because the server is a PC that runs Windows and Linux (a Unix operating system for the PC). When the PC is running in Windows mode (which is rare), you will not get to the data. Be patient, eventually (in hours) it will be back up in Linux.

Coordinates for the New Airfoils.

Until the airfoils are published through SoarTech Aero, they can be obtained from the Web site or by sending a self-addressed stamped envelope with your request to:

Chris Lyon, Coordinator
Dept. of Aeronautical and Astronautical Eng.
University of Illinois at Urbana-Champaign
306 Talbot Laboratory, 104 S. Wright St.
Urbana, IL 61801-2935

Club Presentation Available.

If you are interested in presenting a discussion of the airfoil test program at a club meeting, a 25-page+ presentation with an updated and detailed narrative is available for free. To receive the presentation, please send your request to Chris Lyon.

Offer to Build a Wind Tunnel Model.

If you have an interest in building a wind tunnel model, please contact Chris Lyon. 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. 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 are 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 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 K&S brass tubing and collars for the models are supplied along with full-scale plots.

The airfoils are 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 has been modified for the UIUC tests. Lift and drag measurements for each airfoil are taken at Reynolds numbers of 60k, 100k, 200k and 300k; however, sometimes data is taken down to 40k and up to 500k.

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 Tests. You can receive this white short-sleeve shirt for a suggested donation of $18 - $15 for the shirt and $3 for mailing in US, Canada, and Mexico. Other countries should include $7 for mailing. Proceeds will be used 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 Beefy-T brand and 100% cotton. Shirts can be obtained from the UIUC LSATs coordinator:

Chris Lyon, 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: c-lyon@uiuc.edu

What is SoarTech 8?

Airfoils at Low Speeds by M.S. Selig, J.F. Donovan and D.B. Fraser - a book with results on over 60 airfoil models tested over the Reynolds number range 60k - 300k at Princeton University. It has become a popular source of airfoil data for R/C sailplanes. It's almost 400 pages and a bargain at $20 in the US ($22 in Canada and Mexico, $25/$35 in US dollars for Surface/Airmail in other countries). The book is only available direct from:

SoarTech Aero Publications
H.A.(Herk) Stokely
1504 N. Horseshoe Circle
Virginia Beach, VA 23451
http://www.soartech-aero.com/

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 percent sales tax to the above rates. Sorry no credit card or COD orders at this time.

Your Help

The UIUC LSATs is now beginning its 3rd year, and despite the doubts of many the program is succeeding largely through the support of individual modelers, especially those receiving this bulletin. If you have made a donation already, we offer you our sincere thanks. It has allowed us to complete two test series and plan for two more, and we have been able to continue and maintain the testing capabilities through the purchase of new equipment and instrumentation. Of course, donations play a key role in supporting the UIUC LSATs Coordinator (now Chris Lyon). Also, the interest that the program generates has attracted more students (both graduate and undergraduate students), and several people outside of the university have become involved. The leverage provided by donations has been significant.

To keep the UIUC LSATs program going, we need the involvement of more modelers and the continued support of those who have already made a contribution. "Why me?" Large gifts (such as a recent anonymous one for the purchase of equipment) will obviously make a valuable impact on the program, but they cannot do the job alone. The cumulative financial impact of a great many thoughtful gifts - and the encouragement they provide to major contributors - is immense. For the most part, this program depends financially on the support of many individuals (over 300 so far). You make a difference. Donations can be made payable to the "University of Illinois, AAE Dept" and sent to the address below. Please write on the check "Selig - Wind Tunnel Testing/AAE Unrestricted Funds."

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.
\
(217) 244-5757 work
(217) 244-0720 fax
WWW: http://www.uiuc.edu/ph/www/m-selig

If you would like to wire money directly to the University of Illinois in support of the wind tunnel testing, you will need the following

Bank: First National Bank of Chicago
ABA#: 0710 000 13
Account#: 11-12201 in the name of University of Illinois

In order for the university accounting department to identify where the funds should be deposited once it has arrived, please reference "Selig - Wind Tunnel Testing, 1-6-41784." Also, send a fax to Michael Selig indicating your support. If you need further information, please feel free to contact Ms. Sheryl Netherton in the Aero/Astro business office at (217) 244-7139.

Any portion of this bulletin can be reproduced.

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