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Jun 23, 2021, 10:22 PM
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# Reynolds numbers for models

this is dedicated to those who are interested in improving their knowledge on aerodynamics. it is a science that has been applied also to model airplanes.
but is has been discovered that there are some laws that dont apply so well. and took some time and research to learn that indeed, there are laws that are specific for models.
and this is easier to understand by following the next:

Reynolds Number (Re)-A simplified approach.

There is a difference in performance of a wing, depending mainly of its size (chord) and speed, and Osborne Reynolds managed to put that together in an equation that lets us predict its behavior.
In charts that show the lift and drag, the curve changes depending of the Re. The stall, the max lift and drag happen at different angles of attack, and the efficiency changes, too.
There are several ways to determine the Reynolds number. A simplified way is if we multiply the wing chord (in feet) by the speed of the plane (in ft/sec), and then divide it by 0.000157
If we know the speed at which certain airplane is flying (and its chord), we can calculate its Re with this equation (and that speed can be measured with a device called ‘How Fast’). seems that still is available here (and the see how too): http://nemhobby.com/how-fast-p32614.html
Now we are ready for this:

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.

28 aug 2012
more on turbulators:
in his book 'aerodynamics', martin simons discusses low reynolds, and when gets into turbulent flow (pages 79-86), analizes turbulators and shows that with 1 turbulator ahead or at the leading edge the flow becomes turbulent, and this makes the airfoil more efficient,but then a separation bubble forms afterwards. where he fails is that he does not realize that this could be avoided by placing another turbulator, and then another, and another, at intervals. where they have to be placed is hinted by the birds, especially those that are good at soaring. if we look at the lines where feathers are located, they are around 20, 40, 60 and 80% of the chord. i have tested planes with 2 identical wings, 1 with, and another without several turbulators and measured the speed with the 'how fast' from winged shadow, and to my surprise the plane with the turbulators flies faster (40%), the glide is more flat, the flight lasts longer, and is more stable, so the turbulators keep the airflow turbulent and the efficiency improves-at low reynolds. another device that (at low reynolds) improves the stability (although creating lots of drag) by creating a turbulent flow, is the step, that has been tried extensively. this, perhaps due to that it is very thick, keeps the flow turbulent for a large % of the chord so does not need more than 1.
for those inclined to deal with this, you all are welcome to participate with comments polite, positive and to the point.
Last edited by phil alvirez; Aug 26, 2021 at 06:00 PM.
 Jun 24, 2021, 12:36 AM Registered User Can you give more details on the ‘How Fast’ device? tks
Jun 24, 2021, 09:27 AM
Registered User

# here

here you can search for instructions and details. sadly, all of his products are discontinued, but you may search at the thread where sell things and even ask for it. you may find some1 willing to sell it after not using it anymore.