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Old Jan 11, 2004, 07:35 PM
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Delta, BC, Canada
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Very low Re airfoil wind tunnel data?

Does anyone know where I could find some airfoil wind tunnel data for very low Reynold's numbers? (E.g. 5000 - 40,000)

Even flat-plate data at these Re's would be good.

Most data I've found on the web is not available below Re=40,000.

Thanks
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Old Jan 12, 2004, 01:23 AM
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Soartech #8, Airfoils at Low-Speeds reported wind tunnel measurements at reynolds numbers of 60K, 100K, 200K and 300K for some of the same airfoils tested at NASA, Deft, Stutgart and Princeton. The agreement in drag data from facility to facility was generally within 10% at the higher reynolds numbers but ranged from 50 to over 100% differences at a reynolds number of 60 K. The conclusion is that wind tunnel results at reynolds numbers below 60K are likely to be of uncertain accuracy and if they were available, would be of questionable utility.
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Old Jan 12, 2004, 06:36 PM
B for Bruce
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There was some indoor model specific wind tunnel work done on single surface arced airfoils back in the early days. I seem to remember that Frank Zaic had something to do with that. There was a write up in one of the old magazines. That's all I can tell you though. It was too many years ago and the magazine wasn't mine so I can't fill in any more details than that.

It's not wind tunnel work but if you copy what the free flighters use you won't go too far wrong. Their models have evolved and mutated all within that Rn=15,000 to 40,000 soup since the dawn of model aviation.
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Old Jan 14, 2004, 12:19 AM
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Ollie: Why did the low Re measurements vary so much? Is it a flaw with their testing method or do 2D results become meaningless and full 3D wing measurements are needed? Odd

BMatthews: Do you know of a good free flight resource/book that demostrates the state of the art?
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Old Jan 14, 2004, 01:18 AM
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From Soartech 8, page 17,"The discrepancies found in these comparisons are primarily due to differences in (1) flow quality, (2) accuracy of measurements. (3) methods of measurement, and (4) model accuracy."

I attended Selig, Donnovan and Fraser's lecture about their Princeton wind tunnel tests at the 1989 National Sailplane Symposium in Madison Wisconsin. As I recall, one of the remarks by Selig was that people that use the wind tunnel data have more faith in it than the people who do the wind tunnel experiments. He also mentioned that it was extremely difficult to get repeatable measurements at a reynolds number of 60,000. Residual turbulence in the flow, acoustical noise, mechanical vibration and even the wind outside the building greatly affected the results. They had to schedule the 60K reynolds number tests for the wee hours of the morning when the air was calm outside the building to get fairly repeatable results.

Until resently there have only been model applications for low reynolds number wind tunnel data. Not a lot of effort or money has been expended world wide to overcome the very difficult problems associated with accurate measurements even as low as a reynolds number of 60K. it might interest you to know that NASA spent $50,000 dollars for a wind tunnel test model of the E387 airfoil. Wind tunnel testing doesn't come cheap.

I get the impression that computational aerodynamic simulation of airfoils is displacing physical measurement in wind tunnels. There is a Yahoo discussion group lead by Dr. Mark Drela on the application of his XFoil program. You might want to look into that source of low reynolds number airfoil information.
http://groups.yahoo.com/group/xfoil/
http://raphael.mit.edu/xfoil/
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Old Jan 14, 2004, 03:05 AM
B for Bruce
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Quote:
Originally posted by groundfx
....BMatthews: Do you know of a good free flight resource/book that demostrates the state of the art?
Because of the problems presented by Selig et al as related by Ollie I have no doubt that the material set forth in Zaic's work would be of the same questionable value. Although the National Free Flight Society (NFFS) puts out annual journals geared to the currect state of the art for free flight I do not know of any articles that specifically deal with these super low reynolds number airfoils. What has replaced that class of work is a set of Rules of Thumb for free flight airfoils. Namely, keep them thin, keep the leading edge sharp or very small radius to promote earlier turbulent flow and select camber to suit the model's mission. The wind tunnel for such design work is then the great outdoors and the test results are reflected in the scores of the model. Hardly the neat and scientific answer you are looking for but that's how it works in the Reynold's number basement.

If you wish to persue this line you may be able to obtain back issues of the NFFS digest. These compilations are chock full of interesting low speed aerodynamics and drawings of models chosen for that year that excelled. Using airfoils from such models that closely match your requirements is probably the best way to proceed if you are trying to achieve something in particular. Sadly I do not have any of these digests but I have seen them from time to time with friends.

PS: I should have mentioned that thanks to free flight becoming an endangered form of model flight these NFFS Symposia reports are pretty much the final word on the aerodynamics of very slow speed flight of this nature.
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Old Jan 14, 2004, 01:00 PM
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groundfx,
I agree with Ollie and BMatthews about Rn data below 50k to 60k. Your best bet at finding data below Rn=60k is through NFFS sympo reports. The NFFS web site is http://freeflight.org/ . I have a set of CD-ROMs that contains the Sympos from 1968 through 2000. These can be ordered on line at www.F1D.biz. As I remember a lot of low Rn work was done under rubber props. Also, Jean Wantzenriether, "Airfoil Correlation" NFFS symposium 1983 addressed the low Rn problem as I recall.
Hope this helps.
Dick Huang
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Old Jan 14, 2004, 08:33 PM
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Groundfx:
Indoor News and Views (INAV) had some very good articles on low Rn and low speed areodynamics sometime within the last 2 years. Get in touch with Tim Goldstein at www.F1D.biz I'm sure (hopefully) he will be able to send you a reprint of the articles.
Sincerely,
Jon B. Shereshaw
Gladstone, NJ
PS: Regards - to Dick Huang from Ben
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Old Jan 16, 2004, 04:04 PM
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Quote:
Originally posted by groundfx
Ollie: Why did the low Re measurements vary so much? Is it a flaw with their testing method or do 2D results become meaningless and full 3D wing measurements are needed? Odd

BMatthews: Do you know of a good free flight resource/book that demostrates the state of the art?
groundfx,

I'm not a wind tunnel guy, but I will hazard a guess as to why results at low Reynold's numbers are less consistent between different facilities. One thing that you have to 'get right' in order to get valid results from wind tunnel tests is the location of the transition from laminar to turbulent flow. At high Reynold's numbers, the transition is much more likely to occur early, unless special care is taken. At low Reynold's numbers, it is more likely that the wind tunnel models which have very smooth surfaces will not see transition until close to the trailing edge, while rougher or wavier ones might transition much earlier. This could make a big difference, because early transition can prevent a laminar separation bubble, or even a laminar separation. Even if the surface quality is about the same, the nature of the flow in the wind tunnel upstream of the model can have similar effects, and different tunnels have different ways of 'conditioning' the flow in the test section.

I'll do a little Google search and see whether I can find some better explanations than this. There may not be much information out there, because most of the money that pays for wind tunnel work is aimed at aircraft that operate at high Reynold's numbers.

banktoturn
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Old Jan 16, 2004, 05:02 PM
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Consider that microfilm covered indoor models are so light and fragile that they can be easily destroyed by a sneeze, literally. The lift force involved is tiny indeed. The drag force is an order of magnitude less. Even with the air conditioning system, heat and ventilation off the effects of convection currents in the atmosphere of the building where these models fly are noticable. Now picture the difficulty in getting air stable enough and instrumentation sensitive enough to accurately measure what is happening at the reynolds numbers involved. It is a formidable problem requiring an approach and technology quite different from a typical wind tunnel and its instrumentation.
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Old Feb 17, 2004, 05:34 PM
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consider doing a google serach on micro air vehicles...these tiny air vehicles fly in these ranges and more and better wind tunnel work is starting to be done in this area....

heres one such report:
Aerodynamic Measurements at Low Reynolds Numbers for Fixed Wing Micro-Air Vehicles

http://www.nd.edu/~mav/belgium.pdf
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Old Feb 19, 2004, 11:00 PM
Marion
USA, NC, Hillsborough
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Does Profili II have anything to offer on this subject ?? I undersdtand it is based on XFOIL. I think the web site is profili.com - I think...
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Old Feb 22, 2004, 11:30 AM
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The right address is www.profili2.com

ciao
Stefano
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Old Feb 22, 2004, 02:01 PM
B for Bruce
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The 'Wack, BC, Canada
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There's an Xfoil discussion group at Yahoo groups that would be able to answer that question.

The big problem that has been already identified is that as the Reynolds number falls into the low 5 digit range the air behaves in odd ways that are very difficult to predict. At indoor Rn's in the 4 digit low thousands range it's even harder. Add to that the fact that the measurement instruments need to be very sensitive and accurate as Ollie described and it becomes even harder to use something like Xfoil to predict such behaviour.

But if you seek out and join the Xfoil discussion group I'm sure that Dr Mark Drela can fill you in on these points.
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Old Feb 22, 2004, 04:24 PM
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Quote:
Originally posted by BMatthews

The big problem that has been already identified is that as the Reynolds number falls into the low 5 digit range the air behaves in odd ways that are very difficult to predict. At indoor Rn's in the 4 digit low thousands range it's even harder.
Not quite. The 4-digit Re's are much easier. The big uncertainty comes from uncertainty in predicting and modeling transition, not the low Re itself. The hard range is 100K - 20K or so. Once Re falls below 20K, the flow becomes fully laminar, and the prediction uncertainties disappear.

The result is that Xfoil appears to be very reliable if Re < 20K, such as on indoor FF models. This is based on comparisons with drag measurements via glide tests, and comparisons with "exact" Navier-Stokes computations.
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