aspect ratio - Supra/Icon design concept

[benjaminr]

I have been thinking about the optimal wing planform for an F3J and an F3B/F plane for quite some time. It should have as little sweep back as possible in order not to destabilize the laminar boundary layer (the wingtips make an exaction here) and to avoid twist under high wing loads. Furthermore the lift distribution should be reasonable close to the elliptical one, but taking into account, that the outer Airfoils can not provide as much lift as the inner airfoils due to the lower Reynolds number at the wingtips. The thing I am not sure about is the right aspect ratio.

The Icon for example has a comparatively low aspect ratio, but since the Reynolds number scales linearly with the cord of the wing, airfoil drag may be low and perhaps one can build such a wing somewhat closer to the elliptical lift distribution than a wing with a high aspect ratio. By the way, does anyone know anything about the airfoils being used on the Icon?

The Supra design seems to go for a high aspect ratio to reduce lift induced drag at high Cls. Perhaps one can not fly the high Cl regime in which the induced drag can be significantly reduced by a high aspect ratio wing in steady-state flight conditions with such a light weight model.

Which other thoughts may have lead to the two very different, good working designs? As both design concepts seem to work very well, I wonder how to find the optimum for an F3J plane. Since in F3J one flies in the mid- and high lift regime, I tend to go for a higher aspect ratio (~ 16-17.5), but the Icon really made me think.

Putting the same question up for an all-round F3B plane, I do not know where to start as there are so many parameters to consider. I only know that the F3B plane impressing me the most at the moment, the Schocker designed by the Herrig brothers, has an aspect ratio of 14.3. What would be “the best solution” here?

Cheerio

Benjamin

[chlee]

You might also explicitly consider the structures, weight, construction method and cost when trying to optimize a design.

I'm finding that design (for me) is driven by the wing construction method, which is in turn driven by cost, skill and labor. For example, I don't think I have the skill, time or money to build a Supra-style spar or a molded wing. However, a bagged wing with a Barnes-style carbon/kevlar skin is within my means.

Once I choose a construction method, I can look at other planes with similar construction and get a rough estimate of the weights I can achieve for a class of sailplanes. Then, I can start thinking about wingloading, aspect ratio, choose airfoils, etc. Once I pick these, I might get a more accurate estimate of the plane's weight, and then repeat the process.

So, my opinion is that your optimal wing design heavily depends on what you can build. If you can build a 3m plane at 45 oz, then you'll likely have pretty different design constraints from me.

Btw, Prof. Drela posted somewhere (in the Yahoo Allegro-lite forum, maybe?) about the comparison of Icon and Supra, and you might find it as instructive as I did. I'll post back here if I a link.

- Chung

[mlachow]

The whole design is much more complex than what you list. Launch performance is another consideration and handling in F3j, especially at extreme distances. WIng planform is just one tiny part of the overall design tradeoffs.

[Ollie]

"Design Philosophy"

by Don Stackhouse

"There have been some rather heated discussions lately on the R/C Soaring Exchange about the relative merits of computer analysis versus old cut-and-try methods in the aerodynamic design of wings. Similar threads have argued over the shape of the "ideal" planform, the "best" airfoil, the "optimum" aspect ratio, and the validity of modified airfoils or of blending from one root airfoil into a different tip airfoil. All this controversy reminds me of one of my favorite stories:

"The Four Blind Men and the Elephant (an old Hindu parable)

"One day four blind men encountered an elephant for the first time. They approached it cautiously, but with great curiosity. The first one grabbed hold of the trunk and declared "Aha! An elephant is just like a snake!" The second found an ear and replied "No, an elephant is exactly like a tent." The third bumped into a leg and decided the elephant was just like a tree, and the fourth caught the tail and maintained that the elephant was just like a rope. They all went home arguing, each steadfastly insisting that he was right and the other three were wrong.

"The flight of a model sailplane is a complex phenomenon, each portion of the model seeing its own unique set of conditions at any given time, yet still having an influence on all of the other parts of the model at the same time. In addition, we expect our models to perform well at a wide variety of operating points within the overall flight envelope. To help us achieve this aim we have available a large database of experimental data and theoretical analysis tools, plus the insight garnered from all of our own experiences and the experiences of others, and the results of actual tests of the model. The results depend on how well we use all of this information together to reach the final design.

"If we get hung-up on one parameter, or one design technique, or one phase of the design process, we automatically give ourselves a case of "tunnel vision". There is no single airfoil, aspect ratio, planform, tail size or type, etc., that is optimum at all flight conditions for even a single model, much less a variety of models.

"While it is true that the section at the mid-span of a wing of two wildly different tip and root airfoils may have or may not have any of the characteristics of its parents, it is also just as risky to believe that the airfoil that is optimum for the conditions at the root will be equally appropriate at the tip. Ideally you should study the sections at a variety of points along the wing, as well as local chord, twist, flow characteristics, et cetera.

"The effects of what is happening at the tail, along the fuselage, along the span of the wing, all influence each other in different ways at different flight conditions, with corresponding effects on the overall control, stability and performance of the model.

"None of our design tools is perfect. None of our data is completely reliable at all conditions, and some of it isn't very reliable at any condition. In my experience the best approach is to use all of the available tools and data to the fullest extent possible, then look for the consensus forming between the different approaches. This way the strong points of the different approaches can compensate for their individual shortcomings. The first '93 Monarch hlg went through 150 hours of computer work, PLUS six fuselages, seven tails and eight wings before we froze the design.

"If you try to build an elephant with only a tree, or only a rope, a snake, or a tent, your result will almost certainly fall short of your expectations. Likewise, the model that was designed with only theoretical methods, or only past experience, will probably not be the best design possible. Only by using all of the available tools as cooperative members of a team effort can you achieve a design that is more than just the "sum of its parts".

[histarter]

"Design Philosophy"

by Don Stackhouse

"There have been some rather heated discussions lately on the R/C Soaring Exchange about the relative merits of computer analysis versus old cut-and-try methods in the aerodynamic design of wings. Similar threads have argued over the shape of the "ideal" planform, the "best" airfoil, the "optimum" aspect ratio, and the validity of modified airfoils or of blending from one root airfoil into a different tip airfoil. All this controversy reminds me of one of my favorite stories:

"The Four Blind Men and the Elephant (an old Hindu parable)

"One day four blind men encountered an elephant for the first time. They approached it cautiously, but with great curiosity. The first one grabbed hold of the trunk and declared "Aha! An elephant is just like a snake!" ....

Only by using all of the available tools as cooperative members of a team effort can you achieve a design that is more than just the "sum of its parts".

This is the truth, and is difficult for zero tolerence trained individuals to comprehend. :rolleyes:

Each individual has gifts endowed - with great variance. A man should know his limitations, a quote by Dirty Harry; :eek: however a 'winner' accents his attribute gifts - instead of fighting his limitations, and can thus have greater success doing it his way - without being a blind follower! :D

[oakman7004]

" I only know that the F3B plane impressing me the most at the moment, the Schocker designed by the Herrig brothers, has an aspect ratio of 14.3. What would be “the best solution” here?"

The best solution comes in a package for F3B, as mlachow are saying. Meaning that the plane AND equipment must be opimized together. Lines, Winch, plane weight must be tried out carefully when you buy and fly a plane which concept are towards the extreme in design, not to forget the tactics.

A small plane, like Schocker, Racemachine (small version), Caldera (3d plans here http://www.aerodesign.de/) can have a disadvantage in those windy days(+7m/s) since they are not carrying the same total weight in total as a "bigger" plane can. Also the distance performance are suffering, some, compared to other designs. This was seen on the small Racemachine(weighing around 1700g empty) and yes the guys (Pasi Vaisanen, Joakim Stahl) made the wingspan bigger, (2,95 to 3,18 m).

You can also see the opposite for very high aspect ratio F3B plane like Martin Weberschock First Caracho 3,33m with 17,3A/R. Martin often suffered in speed and weren't 100% satisfied in that aspect. But hey that plane is a major distance plane. :)

So In my opinion you cannot get one plane solving everyhting, you will end up with compromizing in some aspects. And ONE should find the package that carries least dissadvantage for your owns pilot skills.

BUT, during this years F3B Worlds there were ONE plane who shined the most, no offence to Crossfire and Andreas Bohlen, but the performance of the Schocker were outstanding and Andreas H pilot skills together with tactics made it a joy to see the Schocker fly. Unfortunately the Worlds lacked this windy skandinavian weather were we prevously had seen the small Racemachine struggeling in so, I still not commenting Schockers performance there.

And guys maby we can buy the Schocker in the future. They thinking of offering some version to the public..I know I wouldlike to have one. :) Check this page regurlary, http://www.tud-modelltechnik.de/

A long reply but this is my .02 and remeber the pilot skills are probably the area were each and every one can improve the most. There is a reason the same guys always end up at the top...There are during the years significant flight hours for people like Vaisanen, Stahl, Wurts, Perkins, Kolb, Jolly, Liese...

Cheers

Jonas Ekman, Sweden

[Ollie]

I agree histarter up to a point.
"Each individual has gifts endowed - with great variance. A man should know his limitations ---"

The attitude between histarter and me is different. Histarter "knows" his limitations. I'm still learning my limitations. I am still, at 74 years old, learning from Mark Drela and Don Stackhouse. Too bad for histarter.

[histarter]

I agree histarter up to a point.
"Each individual has gifts endowed - with great variance. A man should know his limitations ---"
The attitude between histarter and me is different. Histarter "knows" his limitations. I'm still learning my limitations. I am still, at 74 years old, learning from Mark Drela and Don Stackhouse. Too bad for histarter.

Interesting but confusing, I don't feel different than Ollie? :confused: I am a supporter of Stackhouse and Mark Drela, however knowing my limitations I tend to fly a bit simpler, because I can handle it more effectively. Flat out perfection has never been me in anything I do, (I have over 2000 business machines I designed out there - that are total compromises for efficiency). ;)
My only concern is doing the best I can, and if it takes ingenuity/originality to improve my 'scoring', I don't concern myself with other opinions. I use statistical analysis to replace peer judgement, which gives me the confidence to apply my different technology - that makes me stronger than following the herd in certain areas. Comp flying full house, I was at the bottom of the list with my Ellipse and similar equipment, whereas handicapped with my 2 channel Oly III, or Shuttle reMark, I would reside in the upper 1/2 of the pilots. Not having good 3D hand eye coordination (as demonstrated by 20 years of flying RC) I needed all the embelishments I could muster. I am very satisfied with my accomplishments; and believe there are more modelers like myself (majority), that could have more fun if the peer pressure of "seeking perfection" were lifted! :D I guess at 70 I am a 'young upstart' in Ollie's view.
[Prominance for leadership is great for wild animals, but is not totally effective for thinking humans.]

[benjaminr]

Hi there,

thank you very much for your answers so far!

@Chung: That is a good hint. If you could find Prof. Drela’s post on the comparison of the Icon and the Supra and if you could copy and past it here, that would be very interesting! Thank you very much! Perhaps the designers of both of the planes will find the time to comment on their work here as well.

@mlachow: I know that the whole design process is much more complex than what I have listed, but we need to start somewhere. I think the engineering approach would be to tackle one problem at a time and not the whole set of problems. I would like to start with the wing planform afterwards get the airfoils done before I would start to design the fuselage and the fins to get the desired handling- and flying qualities.

@Jonas: Thank you very much for you thoughts! I agree, that in F3B the whole package of pilot, model, winch, team etc. is important, but I think again you have to start somewhere. As you can adopt the diameter of the winch drum and the line you use to a model, you have some freedom in the design of the model. For the reason you mentioned, I would not build it too small with an aspect ratio of about 15, which sadly is just a guess.
On the one hand your statement about optimizing the personal flying skills holds definitely true; on the other hand I got the impression that a good model really helps a lot in F3B, especially in the speed task.

Thinking of the Supra and the Icon, I am still wondering what design ideas lead to such very different airplanes that seem to do equally well in F3J. I guess it would be very educating to hear about that.

Cheers

Benjamin

[BMatthews]

....Thinking of the Supra and the Icon, I am still wondering what design ideas lead to such very different airplanes that seem to do equally well in F3J. I guess it would be very educating to hear about that....

One thing that is missing here is the observation that model design at this level is a bell curve sort of deal. There's a range of models that fit over the top of the curve within, say, 5 or 10% of the peak and then things fall off from there. If you're within that upper 5 to 10% then it's all good and the advantage may come from how the model fits your flying methods. One pilot may favour a wider wing with lower aspect ratio in order to thicken the wing for a harder launch and then use that extra altitude to fly the way he likes. The other may be more comfy with finessing a superior, but slightly weaker and lower lauching, design that slighty stresses the other end of the bell curve peak.

A compromise is not neccesarily a bad thing if it strengthens another area that the pilot can utilize to a higher degree.

I know this is not answering the question about what aspect ratio to use but it may help you to realize that the Supra and Icon are BOTH the ultimate in the right hands that can use the slight special advantage that each posseses.

[histarter]

Well said!
I fly machines that are more in the 20% area off peak bell - because they respond outside the Supra/Icon performance Universe (in order to match my practice launching regimine, and my skill level). I am content with my slice of soaring, and appear competitively stronger because I am not overloaded.

[markdrela]

From Joe's description he sent me some time ago, the Icon was designed considering the sink rate, L/D, and launch height in some combination. Joe can probably fill in here.

The problem with talking about "optimum" Aspect Ratio is that it's almost impossible to define in a realistic design problem. You can optimize AR for min sink rate or max L/D in straight and level flight, but RC gliders do not spend their life in straight and level flight at the optimum airspeed. There's also launch, zoom, circling slowly, running back, landing, etc, etc. The end result strongly depends in how these factors are weighted against each other.

The main contributions to total drag are
1) Profile drag, varies as 1 / Re^0.5 or so
2) Induced drag, varies as 1 / AR
Most people are probably not aware of this, but there is a third source during maneuvers
3) Unsteady-lift drag, varies as 1 / AR
This becomes important when "pulling G's" during slow flight, such as when working a spotty thermal. Since this 3rd component also varies 1/AR, its presence favors a larger AR than if it were ignored.

So the bottom line is that the Supra's large AR is better geared towards working small spotty thermals. You can crank it around harder than usual and it won't lose energy as much as a lower-AR glider. The combination of narrow tips and slight washout also helps here.

[benjaminr]

Hello Mark,

thank you very much for your answer! According to your first paragraph it seems to be okay not to run any calculations on the aspect ratio problem, but to decide on the basis of experience.

Just for my clarification I have some questions on the three main contributions to total drag you put up. I think I understood your second point and plotted CD,i=CL^2/(pi*e*AR) for some const. values of CL and the aspect ratio varying from 10…20 to get an idea of the influence.
I did not really understand your first point on airfoil drag. Does this assume we have 100% laminar flow and a flat plate (I think it was CD=2*1.328/sqrtRe or so)? Does this mean it is okay not to take pressure drag into account? I thought that in our Reynolds number regime the bubble drag is a major factor.
How do I have to treat the unsteady-lift drag? Do I just use the equation for CD,i but with higher values of CL that can be reached under unsteady flight conditions? Are these dynamic stall effects?

Sorry for all the questions and thanks a lot for sharing your thoughts.

Benjamin