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Old May 25, 2011, 09:16 AM
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Windsor, Canada, near Detroit
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aerodynamics for small model airplanes

Making sense of it all.

I am presenting this to be analyzed by everybody, and then each one to decide if it makes sense to him.
From the beginning of the 20th Century, there was great interest in flying, and in Germany there was a scientist who believed that aeromodeling should be taught in grade schools. For that, he began making studies that led him to believe that small model airplanes behaved differently from full size airplanes, and even from large models. He even performed wind tunnel tests (at Gottingen) and reached some interesting conclusions.
His major discovery was that at certain (low) Re, things changed suddenly, and below that, all the known laws of aerodynamics were no longer effective. It seems as if everything reverses. Things like thick airfoils, rounded leading edges, smooth surfaces (especially the upper), that are good at high Re, become bad stuff, and the opposite becomes true (Could it be that’s why the turbulators and the steps work so well?).
The name of this fellow is Frederick W. Schmitz, and his work can be found in the archives of the Redstone Scientific Information Center in the Redstone Arsenal, Alabama.
It is entitled ‘Aerodynamics of the Model Airplane’, accession number N70-39001, (201 pages); NASA CR or TMX or AD number TMX 60976.
(By the way, in Germany he was awarded the Ludwig Prandtl Prize for 1941 for this work)
There is also another NASA Technical Memorandum from him, entitled:
‘The Aerodynamics of Small Reynolds Numbers’ NASA TM-75816 (45pages)
(This is not a condensed issue of the other one above)
With this scientific evidence at hand, the advocates of steps and turbulators may have what is needed to substantiate their claims, and also explains why these devices do not work on full size airplanes or large models, just on small models.
There have been some articles in magazines that briefly discussed the issue (and that do not give Schmitz any credit), another that got into great detail ( and that even if mentions him in the references at the end, he is lost among others, without explaining what his participation was). On the other hand, there is another (Model Builder, Feb ’75), by Ted Off, that gets into great detail and gives Schmitz all the credit, and even provides the sources to get the documents that substantiate this claim.
But the fact is that suddenly below Re 100,000, things deteriorate and don’t stabilize until reaching 40,000, when everything seems to be upside down. Lift has dropped, and drag has increased dramatically. So if you find your plane flying below Re 100,000, better consider the advice provided by Schmitz (it is when all these turbulators and steps work).
I grew up learning all that modelers had to know about aerodynamics, in order to be able to make a model airplane fly (those were the days when there was free flight only), even if I was well aware that between small and large models there were big differences that could not be explained by those rules, and it was not until I had Schmitz evidence, that I could understand why.
If you want, you can get all the details shown on those 2 studies, or just consider Schmitz conclusions.
On page 31 of his 200 pages study he wrote:
Comparative Conclusions for Model Airplane Wings
1.-Round nosed, thick wings are sensitive to Reynolds number and turbulence;
Thin, sharp nosed airfoils are insensitive to Reynolds number and turbulence;
2.-The lower the Reynolds number, that is, the smaller the model airplane or its speed, the thinner the profile must be, to achieve the supercritical flight state.
3.-To achieve the supercritical flight state it is sufficient if the upper surface flow is turbulent.
4.-The critical Reynolds number of an airfoil sensitive to Reynolds number and turbulence can be reduced by artificial creation of turbulence in the upper surface boundary layer, through:
a.-pointing the wing nose (knife edge);
b.-by a rough surfaced wing nose;
c.-most effectively, by stretching a turbulence wire or thread parallel to the wing’s leading edge.
He also advices the use of a constant chord (page 142) to reduce or avoid wing tip stall.
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Old May 25, 2011, 12:31 PM
B for Bruce
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The 'Wack, BC, Canada
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I don't know about all those books but the points you made at the end of your post were and are still used by free flight modelers on their contest models. I can attest to the fact that a 6 to 7% thick airfoil with all the factors you listed flies a lot nicer on a 3.5 foot span rubber power model than the rather thick and dumpy rounded nose RAF32 does. The RAF32 airfoil was used on the old Korda world champ model. The glide of the one I built and flew for many years was fairly steep but still slow compared to the thin airfoils with sharp leading edges I used on some other models.

And at the super low Reynolds numbers side of things you find single surface wings such as found on insects and indoor duration models. Again real life recognition of the findings.

I suspect that a lot of this write up could have come from studying what was already well known in model building and flying circles. So it's hard to say if he just observed what was working well and reported it or if he used some experimenting and study to arrive at the findings you listed in your 4 points.
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Old May 25, 2011, 03:26 PM
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Quote:
Originally Posted by BMatthews View Post
I don't know about all those books but the points you made at the end of your post were and are still used by free flight modelers on their contest models. I can attest to the fact that a 6 to 7% thick airfoil with all the factors you listed flies a lot nicer on a 3.5 foot span rubber power model than the rather thick and dumpy rounded nose RAF32 does. The RAF32 airfoil was used on the old Korda world champ model. The glide of the one I built and flew for many years was fairly steep but still slow compared to the thin airfoils with sharp leading edges I used on some other models.

And at the super low Reynolds numbers side of things you find single surface wings such as found on insects and indoor duration models. Again real life recognition of the findings.

I suspect that a lot of this write up could have come from studying what was already well known in model building and flying circles. So it's hard to say if he just observed what was working well and reported it or if he used some experimenting and study to arrive at the findings you listed in your 4 points.
you say you don't know about those books. it would be very convenient if you read what he did before you dismiss his work and emmit a judgment. he did wind tunnel tests at gottingen nothing less, and got the Prandtl award for 1941 for his outstanding work. it's all shown on his work. and if you give a look at my blog, you will learn that some of my models are from 18" wingspan, with airfoils as thin as 1mm.
those are the reynolds about which am talking.
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Old May 25, 2011, 11:10 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Maybe read what I typed? I was agreeing with his findings as posted by youself, not dismissing them.

The only question is was the knowledge already fairly well known in the model airplane world and he collected and published it or was his work the source of the knowledge. Some of this is hard to say since the movement towards using thin airfoils was well underway by the late 30's. But the use of sharp leading edges and truly thin airfoils and turbulator spars and other methods of turbulating only really were used for aerodynamic reasons in the time after the war. So on that count his work may well have been the original source. Either way is work was spot on accurate for very low speed flight.

Books of that sort are just not commonly found in most city libraries. Hence not many of us will be familiar with them.
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Old May 26, 2011, 04:53 AM
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making it clear

i agree that it is a matter of interpretation. thanks for your clarification. communication is an art and a science.
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