Anybody up for a good aerodynamic debate?

[SoarNeck]

If so, please check out the "D-99 AeroStudy" in the High-Performance forum. I'd like to get a bunch of opinions, and I'm not sure that everyone from here checks that forum.

Link: http://www.rcgroups.com/forums/showthread.php?p=2093016#post2093016

[Ollie]

A very interesting debate in the High Performance forum. Some interesting comparisons of software modeling methods versus windtunnel measurements versus real aircraft performance are available at:
http://www.cartage.org.lb/en/themes/Sciences/Physics/FluidDynamics/FlyingDynamics/Aerodynamics/SelectedTopics/Designing/Comparison/Comparison.htm

[banktoturn]

SoarNeck,

I have an a opinion on the washin issue. Flap deployment (assuming inboard flaps), without question, tends to result in washout. In addition to increasing Cl for a particular AoA, it increases AoA, for the portion of the span where flaps are deployed. For this reason, inboard flaps give washout when deployed, by causing geometric twist. Looking at that wing, one could make the observation that the flaps cover an unusually large fraction of the span, which might mitigate the effective washout. Having said that, look at the taper ratio for the flapped (or flaperoned) portion of the span. It is very modest. I would expect that, even if the unflapped tip were too small to give a strong washout effect when flaps are deployed, the flapped portion of the span has such a mild taper that I would not expect a tip stall problem.

I'm not an expert on the subject of Xfoil, but I would be very cautious in drawing conclusions from Xfoil results. Simplifying assumptions have been made in the construction of Xfoil (as in all computational methods). This is necessary, so that the computations are affordable to run on typical computers. For flow situations which truly conform to the underlying assumptions, the results should be pretty good. Reynold's number per se is not the main issue. The real issue is separation. Xfoil cannot calculate the flow in a separated region, because the assumptions made in its construction are not valid there. A 'model' is included to try to predict where separation is likely to occur, and to correct for its effects, but it is not perfect. Using Xfoil results to draw strong conclusions about any flow which is likely to have separated regions is not a good idea. Attached flows at low Reynold's number should be very well calculated. The reason that Xfoil might get a bad rap for low Reynold's number flows is that separation, particularly in the form of laminar separation bubbles, is more likely at low Reynold's numbers, in general.

banktoturn

[Ollie]

What are tools like the Eppler code, X-foil, wind tunnels, etc. good for? At low reynolds number they are all better for assisting in making design choices than for making absolute performance predictions. They are not much good for anything at reynolds number below around 50 or 60 K. As reynolds numbers increase above 100k these tools get better and better.

[banktoturn]

What are tools like the Eppler code, X-foil, wind tunnels, etc. good for? At low reynolds number they are all better for assisting in making design choices than for making absolute performance predictions. They are not much good for anything at reynolds number below around 50 or 60 K. As reynolds numbers increase above 100k these tools get better and better.

Ollie,

I very much agree that comparison is a better use of these tools than quantitative evaluation, for our purposes. Frankly, all of these tools can be difficult for laymen to use, for various reasons, under the best of circumstances. I personally suspect that the decision making of many in the model aircraft hobby is not very well served by them. In the case of software like the Eppler code and Xfoil, I think it is more valid to say that they work better when there are no separation regions than to say that they are better at high Reynold's numbers. Of course, this is more likely to be the situation as Reynold's number increases, so it may be nitpicking to make the distinction.

banktoturn.

[Ollie]

For people like me it is an ego wrenching thing to objectively assess what you don't know and publish it. Privately one might want to plot lift distribution along the span with a wide band of uncertainty at the tips for the lower reynolds number cases and narrower bands of uncertainty near the middle of the span. One might want to plot an additional band of uncertainty due to airfoil construction inaccuracies. Yet an additional band of uncertainty for parameters not directly measured like mean camber. There are probably additional uncertainties associated with the vibration of the air (sound) and turbulence effects on the boundry layer and particularly the behavior of the laminar separation bubble. Thinking about all the uncertainty is a humbling exercise. Even so, it shouldn't be used as an excuse for avoiding technical analysis. Without a technical analysis, all you have left is an inefficient trial and error appeoach to design and we already have too much of that.

The scientific method is to treat differences of opinion as conjecture and settle the question by experimental measurements.

[banktoturn]

For people like me it is an ego wrenching thing to objectively assess what you don't know and publish it. Privately one might want to plot lift distribution along the span with a wide band of uncertainty at the tips for the lower reynolds number cases and narrower bands of uncertainty near the middle of the span. One might want to plot an additional band of uncertainty due to airfoil construction inaccuracies. Yet an additional band of uncertainty for parameters not directly measured like mean camber. There are probably additional uncertainties associated with the vibration of the air (sound) and turbulence effects on the boundry layer and particularly the behavior of the laminar separation bubble. Thinking about all the uncertainty is a humbling exercise. Even so, it shouldn't be used as an excuse for avoiding technical analysis. Without a technical analysis, all you have left is an inefficient trial and error appeoach to design and we already have too much of that.

The scientific method is to treat differences of opinion as conjecture and settle the question by experimental measurements.

Ollie,

I very much agree. I am a big fan of using whatever analytical tools are available, instead of pure trial and error. I just think that you need to understand what the tools are capable of, and expect no more from them.

banktoturn

[Antonsoarer]

Interesting debate, some observations: Theory, tools and subjective testing when combined can build up what feels like an intuitive perspective to the evaluation of performance. I think what I am saying is that measurement and scientific evaluation in the world of modelling will never enjoy the resources of the full-size high-Reynolds world. But we still have masses of subjective data (some of dubious value) which when combined with measurements and extrapolations (some of dubious value) provide intellectual stimulation, enjoyment and energy for further exploration.

The old adage 'what looks right probably is right' is often applied to model aircraft design (including choice of airfoils) but I believe that the media and conditioning mean that aesthetics will often override/cloud logic. However the modelling world has a major advantage in the area of testing and evaluation, low cost, low risk and a large number of participants and experimenters.

If we are talking performance aerodynamics at low Re (below about 80k) I don't think many of the tools can make sense of what's going on due to the increase in transient and complex conditions associated with the perfect airfoil alone. Combine this uncertainty with other factors and the tools look at best a minor point of reference for inclusion in any evaluation/investigation exercise. From my somewhat simplistic perspective it looks like a different world with different principles. I strongly believe that this low Re world is best served primarily by experimentation, observation, debate and pooling of knowledge.

SoarNeck, your proposal that a hysteresis effect may be at work makes a lot of sense; Observation: Dynamic-Soaring turns sometimes look ludicrous and intuitively wrong.

I think some foils and planforms may lend themselves to a hysteresis effect more than others. What I might call high-Q foils/conditions could be at play. This terminology will be familiar if you have spent any time in the world of analogue electronics, put simply high-q equates to a high potential for resonance for a given input at a given frequency. Hysteresis plays a major role in resonance. Nature is full of resonance, so why not aerodynamics?

When I look at foils in Profili/Xfoil and in particular the Cl/Cd(alpha) curves I think I can classify them into two categories at a given Re. Firstly the smooth convex curve types (low-q) represented by an SD7037 and secondly the single spike with concave sides (high-q) for example an Epler374 and some other Epler sections. The 374 looks like an airfoil with high-q characteristics with perhaps the potential for ringing or resonance. It may be that all foils have the potential to cause/precipitate resonance to varying degrees under certain conditions. I am not familiar with the foils in the other thread or subjective impressions of the low Re's (except maybe on some of my early chuck-glider test-beds).

Tony.