I have TurboCAD and obtained a number of airfoils in .dxf format. I superimposed the E193 airfoil on top of the S3021 airfoil and see that the rear upper section of the E193 has more height with a 'gentler' curve and that the high point on the E193 is further back on the airfoil. They are both 10 inch airfoils. I sort of feel that I would not really notice much of a difference between the two if flown on the same model but balanced appropriately (for each of the airfoils). Maybe the difference would be noticed in high speed flight between thermals (which would require faster flight with more high speed efficiency) and when working a thermal (slow, tighter circles).

My guess (and ONLY a guess) is that the S3021 would fly faster (with more efficiency) because of the thinner upper surface contour and that the E193 would work thermals better (light thermals for example) because of the gentler curve on the upper surface. Both have similar leading edges.

I am wondering how much of a visual difference would I have to SEE in the different airfoils before I would be able to DETECT a difference in performance in the air. And how much difference would there be in 'dead calm' cool air between the two airfoils mentioned.

I also overlayed the SD7037 and the S3021. They are very similar. Yet the SD7037 has become very popular in contest circles. It is hard to believe the very small difference I see in the airfoils can sway so many people away from one (S3021) and toward the other (SD7037).

I see the NACA2411 has a rather blunt leading edge and rather thick section but the E374 has a sharper leading edge. Both are almost the same thickness at the thickest point. I wonder where the E374 would excel and where the NACA2411 would excel.

Helpful, informative comments welcome.

What is also informative is to overlay similar airfoils, with the 'y' scale expanded 3:1, and look at the differences in contour.
Such as these.... Clark-Y, 3021 and 7037.
And contemplate how precisely you are capable as a builder to build any profile to the exact contour selected.
And then contemplate how your typical wing deviates from perfect, and flies acceptably.
Which leads to... could you detect -any- difference in performance between say the 3021 and 7037, when built to your standards?

.
http://www.angelfire.com/indie/aeros...Airfoils01.jpg

Precisely my point ... On top of the actual construction (sanding, wing twist and surface material, weight) is the variation in balance, pilot capability and chosen control settings for any particular condition and at various distances from the pilot and various temperatures and wind conditions (assume all are well made foam core wings). I really wonder if even a better than average pilot could tell the difference in performance between the popular SD7037 and the lesser attention grabbing S3021 (which look very much alike), or the S3021 and the E193 (which are by simple glance, different airfoils). However there is the psychological advantage that one airfoil is more popular than another.

I sometimes wonder if the SD7037 seems to rate high in contests because of the large number of people that utilize that airfoil in contests and thus the airfoil 'self perpetuates' the use of it. This is not a BAD thing, just interesting to contemplate.

I chose eleven airfoils and each has been given a ten inch cord. Some are suggested sloper airfoils and others more contest or thermal oriented. I am trying to eliminate the need to build all eleven! Thus I am looking for obvious differences between them. Some as mentioned above are visually different and others are very close in one way or another. I wish I had the ability to compare airfoils in this way twenty years ago. CAD makes it easy. Here are the various airfoils grouped to be somewhat visually similar.

S3016
RG15

E374
NACA2411

E387
SD6080
S4061

E193
E205
S3021
SD7037

I believe that with some combinations such as the S3021 and SD7037 I would be hard pressed to notice any consistant differences in actual use.

The group that starts with the E387 (above) is another set that I believe I would not be able to notice a difference in during actual use. But the E193 => IS <= visually different in both the top back section and the CG point. Here I would think there would be SOME type of handling difference.

On another note it would be interesting to be able to 'inflate' an elastic outer membrane (rubber or whatever) and be able to change between a clean, fast moving airfoil such as the RG15 and the more thermal oriented airfoils (maybe the E193). Or at least vary the form of the leading edge. This would be something like a 'Rhom Air Variable Airfoil System' ('RAVAS') using a lightweight canister of compressed air. This is one area (variable airfoil) that would be of interest in the future. There were discussions in the past using mechanical 'cams' but being able to inflate and deflate an exterior membrane cut specifically to transition between a fast penetrating airfoil and thermal oriented airfoil would be of interest and would be rather simple in design once the ability to retain proper contour was achieved. In one situation the membrane would hug the 'high speed thinner contour' and in the other it would INFLATE to the more 'thermal efficient' contour. Or changing just the leading edge entry point such that normally one would have a 'sloper' (mid airfoil) type entry point that could be inflated to have a 'lower' entry point found in thermal oriented leading edges. I wonder what industries utilize changes of this type (via inflation of a membrane) that could be 'appropriated' for use on our planes.

If what you want to build is a sailplane that itself behavior well to high and low velocities.
For it I suggest you any of these three profiles.
S 2046
HN 1069
HQ 2,5-9,0 SE

Picking up on the point about why one airfoil is so "better because...", I have often wondered how much "fashion" has to do with this.

Because a particular model is used in winning world champs, and because that model uses section "X", then the world (or a goodly part of it...) assumes that "X" must be a dang-fine-numberone type airfoil. So it gets used on the latest scratch-builds, then kits and arfs, and all of the factors that have been listed here in previous posts are ignored.

For someone like me, the side of a gumboot might be just about as good .

Oh, BTW I just found out that the airfoil "X" used in the WC winning model had an error in the parameters for the foam cutter, and consequently the wing looks nothing like it should by about 2mm on a 250mm chord.

The "halo effect".
Joe Always1stPlace says he uses the Thom McCan #11 wingtip for his super plane.
So everyone within earshot finds that shape and uses it.. and finds it is exactly what Joe Al... says it is.
Which brings up this.... way back when, famous desert motorcycle racer J Roberts (JR) botched a weld on his tuned pipe... rather than dress it out and redo it, he painted the area around the botch flat black.
At the next race, all the JR wanna-bes saw this difference, and asked about it...
Which brings up this... Dogbert says, "Beware the advice of successful people. They do NOT seek company!"..
So JR winged it.. about mid-range torque, horsepower, and other techno-stuff..
Anyhoo, the next race.. lots of tuned pipes with black bands just where JR said to put them, and ALL of the riders swore they could detect the "mid-range torque.... etc.."
.
Now, JR, like JoeAlways1stPlace could take your (or my) best equipment, set up the way you (or I) set it up. and BEAT YOU (and me) with it!
.
With the obvious difficulties in generating a true-to-contour airfoil, without access to precision CNC machinery that can manufacture a true-to-contour female mold for the wing. about all anyone can expect is to get close to a desired profile. What with the building inaccuracies that creep in, no one's wing will be much better or worse, when you try hard to do it right.
Any performance difference will be due to the PILOT!
Practice, in other words, makes JoeAlways1stPlace the flier he (or she) is!

Most of the models we test in the UIUC wind tunnel are built by modelers. Construction methods used include glassed foam cores (most common) and a few built up models. Recently we have also been using professionally built CNC models, but these have been used for more specialized applications such as airfoils designed here for specific UAV applications. Most of our models still come from "normal" model builders, though.

Accuracies that these modelers attain (as measured on a coordinate measuring machine), vary from an average distance error of 0.0160 inches on the "worst" models (about 1/64") on a 12" chord, to an average difference of 0.004" or sometimes better on the more accurate foam core or built up models. The CNC models are usually slightly more accurate than that, but of course few modelers have access to that type of equipment. It seems therefore that 0.004" average error is about the best you can get when building by hand. With these accuracies, the models behave very similar to the predicted characteristics in almost all cases.

If you have an overpowered sports plane, I doubt you would feel the difference between a "good" and "average" airfoil. However, if you are a competition level glider or pylon racing pilot and know what you are doing, you certainly will see the difference in performance between the different airfoils. At low Reynolds numbers you can often see a 50% or more difference in minimum drag coefficient between a "good" and an "average" airfoil.

Of course, if you cannot build as accurate as this (lets say within 1/64" accuracy or better on a 12" chord), then your airfoil selection is probably not going to always give you what you want. However, I think most experienced modelers can build to that type of accuracy when they really take care (such as building a model for competition purposes), and therefore I do not see any reason why you should not spend the extra time to carefully select your airfoil(s) for a particular design. Even if you cannot build it that accurate, your airplane performance and flying characteristics will still be more predictable if you build as closely as possible to a known airfoil than just using a random shape. Of course, the more accurate you can build, the bigger the chance of actually achieving the predicted airfoil characteristics. Also, some airfoils are less sensitive to building accuracy - and if you know you cannot build that accurate, it may be a good idea to choose one of these airfoils. An example is the type of airfoils used on many trainer and "first low wing" type aircraft. The NACA 4-digit airfoils also seem to work relatively predictable when slightly off on accuracy.

When you see how much wind tunnel data differ between different airfoils (or similarly, if you start studying results from a program such as XFOIL or Profili), you start realizing how much improvement you can get by a) selecting a good airfoil and b) building accurately. How critical this perofrmance difference is, all depends on what you want to get out of a particular design...

Very interesting. This makes me think that what I do not have when I select airfoils of interest is some type of index that gives me an idea HOW critical the shape actually is. Maybe this is obvious from some type of plot I do not have.

That is, if I select two airfoils 'A' and 'B' and airfoil 'A' allows XX amount of variation before the airfoil begins to loose 25% of its predicted flight characteristics and airfoil 'B' has far less tolerence than XX, it may be in my best interests to stay with airfoil 'A'.

There is something to be said for consistancy in manufacturing. One big advantage of using a CNC based wing is that once the flight characteristics become 'second nature' to the pilot and he destroys the plane he has a high probability of obtaining a replacement that will behave the same way. This is a big plus assuming the CNC airfoil was appropriate for the purpose in the first place. Thus all the practice with the first wing can be transfered to the second and subsequent wings. And this becomes even more important when that wing has proven to be 'just right' for the application.

Probably what would convince me of the difference in airfoils will be that point when we have a number fo CNC based wings with various airfoils available for comparison, all within the same tolerances, especially if your statement is accurate about the sometimes 50% difference in minimum drag between the same airfoil, but with different tolerances. That amount of difference seems significant.

I agree with you that this information would be extremely useful. Unfortunately there is no plot that can quickly give you that information (at least not that I know of). Some airfoils really "push the limits" on boundary layer development - in other words careful tuning of laminar boundary layer growth, then a transition ramp that minimizes the separation bubble size, and a recovery region that keeps the flow close to separation to minimize the turbulent skin friction drag. These types of airfoils are usually very sensitive to inaccuracies in manufacture. The older early series NACA airfoils (4 and 5-digit) in contrast were more conservative, and are therefore usually less sensitive to inaccuracies (which of course still doesn't mean you can just use a shape that more or less looks like a NACA airfoil).

Also, most airfoils are more sensitive to inaccuracies on one area of the surface than on another and since most airfoils are published with very little information on how the designer went about designing the airfoil, it is also difficult to know just how much the designer pushed the limits.

What I would suggest is to use a program such as XFOIL, or Profili (which is basically a very userfriendly interface to XFOIL), and then make small changes to the airfoil geometry. Let XFOIL then re-analyze it and see how much the performance differ between the "exact" and the slightly modified airfoil. You can also let Profili help you select multiple "similar" airfoils (there is a filter option somewhere) and then compare the data between these geometrically similar airfoils. If the performance data differ a lot between them, even though they are geometrically very similar, then you know that this type of airfoil is going to be sensitive to inaccuracies in manufacture.

You can find Profili here: www.profili2.com

ciao
Stefano