UIUC Low Speed Airfoil Test Program
Bulletin #2

Airfoil Testing for Model Aircraft
Michael Selig, Jim Guglielmo, Andy Broeren, Philippe Giguere and Cameron Ninham
March 23, 1995

What's New

At last, success! And lots of it. During the first major test phase of the UIUC Low Speed Airfoils Tests, over 300 hours of wind tunnel test time was logged and over 80MB of data were taken. Airfoil performance data were gathered on over 30 airfoils for free flight models, R/C sailplanes, sport planes and more. Here's a snapshot of some of the results:

All of this and more will be published in an upcoming volume of SoarTech Aero, which we are now in the process of writing. In a worst case scenario (see below regarding the coordinate measurement machine), we should have the page proofs ready within 2 months. When the book becomes available, it will be announced in the upcoming Bulletin No. 3 and you can send your request to:

SoarTech Aero Publications
H.A.(Herk) Stokely
1504 N. Horseshoe Circle
Virginia Beach, VA 23451

Photos of the Wind Tunnel

Appear here. These gifs are a little bit too dark, and we are trying to fix that...

A Follow-Up on Bulletin No. 1

In reference to the last bulletin:

Airfoil Data on the Web

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

The address on the Web is:


Next Test Phase and New Airfoils

Currently, our plans are to be in the tunnel in August 1995. Some of the airfoils now being designed include: (3) thermal duration/F3J airfoils, (2) R/C hand launch airfoils, (2) power airfoils (sport and Quickee 500). Part of the focus of the next test series is to look at flap effects in detail on the SD7037 and RG15. The idea about using trips on airfoils as discussed in Bulletin No. 1 is still being explored. It still looks promising.

Low Speed Airfoil Aerodynamics

In the last bulletin, a list of 20 topics in low speed airfoils aerodynamics were posed as "things a knowledgeable modeler interested in airfoils ought to know something about." The obvious happened. We received requests to answer them! So we will. The list started with:

"What a bubble ramp is."

This discussion must first begin with an explanation of what a bubble is. Airfoils at low Reynolds numbers (short chords flying at model like speeds) do not behave like airfoils at higher Reynolds numbers such as those typical of general aviation aircraft. As adapted from SoarTech 8 (with a few edits):

The flow behavior over an airfoil at high Reynolds numbers -- greater than, say, 1-3 million -- is well known. The boundary layer is laminar from the leading edge to a point typically near mid-chord where it makes a transition to turbulent flow. This transition, as well as the flow behind it, is generally well behaved.

Unlike full-size airplanes, model sailplanes typically operate at chord Reynolds numbers between 50k and 500k, often called the low Reynolds number regime. At these low Reynolds numbers the flow is fundamentally different. The transition process is neither abrupt nor does it usually take place while the boundary layer is attached to the airfoil. Instead the laminar boundary layer separates, that is, it physically detaches from the airfoil surface. The flow then becomes unstable while separated, and makes a transition to turbulent flow in `mid-air.' Only then does the flow reattach to the airfoil. If the laminar separation point is sufficiently far aft or if the Reynolds number is very low, the flow sometimes entirely fails to return to the airfoil surface. In either case large energy losses are associated with this process. This laminar separation, transition to turbulence, and turbulent reattachment enclose a region of recirculation flow aptly called the `laminar separation bubble' or `bubble' for short.

Problems with the bubble are greatest on the upper surface where the flow is accelerated the most. Typically, the laminar separation bubble can extend over 10-30% of the airfoil upper surface. The bubble on the lower surface (if present at all) is much smaller under normal operating conditions. The presence of the bubble is the principal reason for the degradation in performance at low Reynolds numbers. Efforts towards drag reduction, therefore, largely concentrate on reducing the size and extent of the bubble.

The bubble can be reduced in size by destabilizing the laminar boundary layer to promote a transition as early as possible. One method of promoting transition is through the use of turbulators affixed to the upper surface of the airfoil before the laminar separation point. (If turbulators are immersed in the bubble downstream of the separation point they have little effect.) Another approach used to destabilize the laminar boundary is to employ a gradual upper surface pressure recovery so as to gradually slow the flow down from its highest value. This gradual pressure recovery which shortens the length of the bubble and thereby reduces the bubble drag is called the "bubble ramp."

One down, 19 to go. (They are not all this hard.)

A Word from Selig on the S9000 Airfoil Series

Several people have asked for coordinates (and performance data) for the S9037 airfoil used on the OPUS 750 standard class and the S9000 on the BLACKHAWK open class sailplanes. Both of these gliders and their airfoils were designed while I was still at Penn State -- after the Princeton wind tunnel tests and before the UIUC tests were even envisioned. The idea was, of course, to apply my experience in aero engineering to generate some income in case I found myself unemployed (like many of my fellow graduate students and colleagues at the time). Fortunately, I did not have to depend on that income. The upshot of all this is (1) the airfoils are proprietary to the current manufacturers, (2) I am not able to release them, (3) the airfoils will not be wind tunnel tested, and (4) your donations did not subsidize that work done at Penn State. People are now becoming aware of this work since the gliders are only recently being produced -- some 3-4 years after they were designed. But for those patient enough and should I decide to exercise a clause in the agreements, the airfoils can be released after 10 years from the date that the sailplanes were first made available. I will be very surprised if by that time someone else has not figured out how to clone the S9000 (BLACKHAWK) and S9037 (OPUS 750) airfoils.

Finally, all of the results generated in the UIUC wind tunnel studies will be publically available.

Publications: S1223 Airfoil and More

The high-lift S1223 (was S1214) airfoil was published as AIAA Paper 94-1866: M.S. Selig and J.J. Guglielmo, "High-Lift Low Reynolds Number Airfoil Design," presented at the AIAA Applied Aerodynamics Conference, Colorado Springs, CO, June 1994. Several of the SAE Aero Design Teams have requested information on this airfoil. For now, this paper is the source. Only predicted performance is included in the paper. To date no wind tunnel test data have been released. SAE Teams: Watch the Web for more data. We will announce the availability of the data under the What's New heading.

Also, the AIAA Paper 95-1783: J.J. Guglielmo and M.S. Selig, "Large Spanwise Drag Variations in Wake Profiles for Airfoils at Low Reynolds Numbers: Issues and Observations" will be presented at the Applied Aerodynamics Conference this summer.

E-Mail Mailing List Instead

If you are currently receiving this bulletin and would rather receive an electronic version, please let us know and we can help save some trees.

Your Snail Mail and E-Mail Received

No doubt about it, interest in the airfoil tests is pretty high. We have received at least 1000 letters and e-mail messages combined since the start of the program in December 1993. Now, on an average day there's a phone call or two related to the airfoil tests and we receive between 3-6 pieces of mail. These numbers go up when hot airfoils appear in Popular Science (see February issue, p. 18) or when the model magazines publicize the tests. We appreciate the attention, and we definitely appreciate your comments, donations, and other forms of support. Unfortunately, it's almost impossible to respond to everything, so please don't be offended if we are slow to respond (if at all) or if we merely send a perfunctory thank-you note. Rest assured, the airfoil tests go on -- that's the prime directive.

Points of Contact

You can write, e-mail (preferred), fax or call any one of us. Use the fax number and address later listed for Jim Guglielmo. To help you decide who to write to, you might want to consider our profiles:

Your Letters Received

We like your mail. Besides a lot of juicy technical stuff, we get some praise and here's a sample:

I received the T-shirt today. Thanks so much!

I Love the airfoil data on the net. I jumped on today and downloaded the graphs from the S7012 and RG15 wind tunnel tests. What a way to publish data! ...

Kevin McKiou Naperville, Illinois

Thanks ...

Special thanks to Mr. and Mrs. Hermann Andresen, who provided food, lodging, and hospitality in their home for Jim and Phil during a recent visit to California. The purpose of the trip was to meet with Hermann Andresen and design new lift and pitching moment balances for the wind tunnel. Thanks to Mr. Andresen's expertise, Mrs. Andresen's excellent cooking, and some wonderful weather, the trip was both a success and very enjoyable. As a result, both the lift and pitching moment balances will be built and installed for the next test phase in August.

We would also like to thank Roger Morell and Norm Furutani, who helped organize a talk on the UIUC Low-Speed Airfoil Tests. The talk was sponsored by the Southern California Aero Team, and members of SULA were also present. Overall, we received some excellent feedback concerning the testing program and were extremely happy with the turnout. Thanks to everyone who came.


To all old and new builders, please take a few minutes to fill out the enclosed "Model Builder Information Form." The purpose of this form is to provide us with a complete record on each builder involved with the testing program.


If anyone has any comments or suggestions, please fill out the enclosed "General Information Form."

What's Old, But Still Important

Airfoil Coordinates for S1210, S1223, S4083, and S7012

Until the new airfoils are published through SoarTech Aero, the airfoil coordinates and a brief description of the performance characteristics for these designs can be obtained from the Web site or by sending a self-addressed stamped envelope 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.

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

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 and $3 for mailing in US, Canada, and Mexico or $7 in other countries) 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 graduate student coordinator:

James J. Guglielmo, Coordinator
c/o 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) 244-0684
fax: (217) 244-0720
e-mail: jjgug@uxh.cso.uiuc.edu

Offer 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 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.

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). Click here for more information. The book is only available direct from:

SoarTech Aero Publications
H.A.(Herk) Stokely
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 percent sales tax to the above rates. Sorry no credit card or COD orders at this time.

Get on the Mailing List

To receive the next bulletin, send a self-addressed stamped envelope to Michael Selig at the address listed previously. If you have made a donation, you are automatically placed on the mailing list.

Your Help

The airfoil testing (and the book that is now in the works) would not exist without your financial support. If you like what we are doing, your contributions are welcome. Here is a sampling of what some people have done to support the airfoil testing.

Thanks everyone!

How to Wire Us Money

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: Bank One N.A.
Address: One Bank One Plaza, Chicago, IL 60670
ABA Number: 0710 000 13
University Account Number: 11-12201
Account Title: Univ of Illinois Operating Account
Reference: "Selig -- Wind Tunnel Testing, 1-6-41784"

Banking info updated 8-4-00

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 the Aero/Astro business office (Ms. Lori Ballinger ) at (217) 244-7139.

Any portion of this bulletin can be reproduced.

List of Contributors

(as of March 1, 1995: % -- percent of total support received)

[Top] [UIUC Applied Aerodynamics Group]