Hi All,

Normally I don't go around playing "salesman" touting a particular product. This will be an exception, as I'll give credit where it's due (I have no affiliation to the product / person).

Anyone designing model aircraft, or otherwise interested in airfoil selection and performance comparisons, should look at Profili 2.0 (and its price ==> $10 ).

The author, Stefano Duranti, has released a new version (latest is 2.03d as of this writing) that will generate polars for any of the 2200+ airfoils in its database (which can all be modified, e.g. change camber, thickness, etc), or for any custom airfoil per user-entered coordinates.

Now, priced at $10, you're probably thinking "The analysis capability can't be very sophisticated," however consider that the analysis engine built into Profili is the well-known XFoil program by Dr. Mark Drela, professor of Aero / Fluids at MIT, whose research specialty is computational aerodynamics. Basically, Profili delivers the power of XFoil in a user-friendly format, coupled to an extensive airfoil database. In addition, XFoil's processing parameters can be modified via a Windows menu.

Just a few mouseclicks generates not only standard polars, but multi-airfoil comparison plots, or plots of various combinations of Cl, Cd, Cl/Cd, Cm, AOA, Re.

Given the complexity of CFD analyses, even if not highly accurate in terms of actual numbers, the results can be insightful for comparative purposes, or to see effects of modified airfoil geometries, different Re, etc.

No doubt Profili will continue to evolve, but this version 2 marks a huge refinement over the previous version dealing with airfoil geometry alone.

Get Profili here: http://www.profili2.com

Below are a couple Profili outputs (I pasted the legend; it actually appears above the charts).

Here's a standard polar of 5 airfoils at Re = 70000, predicting low-Re breakdown of the E387 & Zagi 12:

And here's airfoil L/D for the same five sections at Re = 120,000

Your post raises a question I have. If you look at Cl/Cd the 7037 foil beats the MH32 across the range. Why is the MH32 the popular foil of the day?

Yes, Profili is good. Something you didn't point out is that it works as a basic plotting and modifying tool for FREE. The payment of the paltry $10 let's you unlock the advanced features that are well worth the price.

But I've been lazy and have yet to get the unlock code from him ....

Anyway, another great tool is Tracfoil. I find that the slight differences in the utilities makes having them both worthwhile. And, once again being lazy, I still have to register for the $20 US (IIRC) and open up my trial version.

Don't know specifically why, but l/d is just one among several considerations depending on type of plane and its designed purpose, flying style, etc.

I've heard the 7037 has a slightly nastier, more abrupt stall characteristic. Also that the MH32 is very responsive to flap input, efficient over broader speed range, generally better at high speed, may be a better multi-tasker.

Another thread w/related comments: http://www.rcuniverse.com/showthread...hreadid=311728]

The 70K polar does not cover the conditions of most interest on a 3m glider. You'd have to look at polars at 250-300K or so. The MH32 in this case does much better at low CL, like in a fast penetration glide.

This is interesting stuff, thanks for the link. Can we arrange a test? The issue of simulators came up as a byproduct of an innocent question originally posed at:

http://www.rcuniverse.com/showthread...744&forumid=19

Im no expert that this stuff, but now Im curious. I ran the MH32 using Dreescode/Snack at various Re to try & encompass the range you gave as an example. is it possible to determine how the results compare??

Yes, and the best measure of validity would be comparison to actual (wind tunnel) test data.

Consider the following 3 charts of MH32 polars. This one is at Re = 70 / 100 / 150K in Profili/XFoil, for comparison to DesignFoil:

This is actual test data at nominal Re = 60 / 100 / 200 / 300K:

This is at Re = 60 / 100 / 200 / 300K in Profili / XFoil, for comparison to actual test data.

At least from this MH32 example, and being mindful of the different Re values, I think it's clear which is the better software, especially when considering the depiction of low-Re separation / breakdown.

Mark,

Tangent question - Can the transition / separation points predicted by XFoil be used to properly place turbulators? Is it known whether XFoil tends to predict these points fore or aft of the actual locations? Any general guidelines on turbulator placement, e.g. as a % of the predicted x/c location?

Wind tunnel test data's validity at low reynolds numbers depends on the accuracy of the test model, the low turbulence level of the tunnel, the instrumentation, etc. I heard Dr. Selig talk at one of the MARCS National Sailplane Symposiums about 1989. He expressed the idea that people who have done wind tunnel testing tend to question the results but, others tend to take the data more seriously than they should.

Martin Heperle was complaining on his web site about some wind tunnel test data for the MH 32. He felt that inaccuracies in the test model made the MH32 look worse than it should in some series of wind tunnel tests.

Another case in point is shown in Fig. 2.14 of Soartech 8 on page 34. The figure compares FX60-100 tests at Stutgart in 1980 with Princeton tests in 1989. The drag coefficient results at the same lift coefficient and reynolds number disagree by as much as 60%!!!!

It sure makes you wonder.

Forgive me if this is a silly question but I'd like to clear up somenthng. Wildblueyawner, you're quoting profili / Xfoil. Can I take this to mean that the advanced features unlocked when I pay uses an embedded Xfoil to calculate the polars?

Can someone explain to the uninitiated (me), whats actually happening behind the scenes in these programs:

Is there generic 'code' available that takes airfoil shape + Re information & spits out resultant polars? If so, is this reliable? What are the limitations?

If the program spits out polars based on actual tunnel tests at measured Re's, smoothed or otherwise, isnt this basically a glorified graphical database? Doesnt this then pose a limitation if you modify an airfoil or request a different Re because it is then forced into some sort of intrepolation routine?

Ollie - Yes. Although wind tunnel data is obtained physically, it still isn't the real (practical) world, and results from various sources would, to be meaningful, have to be obtained under conditions that are matching and proper, fat chance. Conversely, real aircraft have aspect ratio, surface defects, appendages, and aren't flown in air that's been "straightened."

BMatthews Yes. Profili simply melds a graphical front and back end onto XFoil to ease data entry and the output of plots - Basically turns XFoil keystrokes into mouse clicks. The analysis and computation has already been performed by XFoil when the databased airfoils were pre-processed by Stefano, or will be performed by XFoil when considering a modified or custom airfoil (see below re pre-processing).

ptxman XFoil is anything but generic, based on its ability to handle low-Re conditions and the associated separation bubbles. It's capabilities are summarized here, plus links for downloading the actual XFoil files http://raphael.mit.edu/xfoil/

Whoa! Careful here - That plot of wind tunnel data I posted was NOT from Profili / XFoil. That came from here: http://www.nasg.com/afdb/index-e.phtml

XFoil doesn't simply interpolate chart data - It performs a real CFD analysis on the airfoil in question. A simple interpolation routine wouldn't handle the discontinuous function presented by phenomena such as separation.

A limitation when modifying an airfoil (or picking some weird Re other than the standard values in Profili) relates more to convenience - The time to generate a custom polar. This is addressed with a pre-processing option that can run XFoil on modified or custom airfoils unattended, then saves the polar data. Depending on the number of new airfoils, pre-processing can take considerable time (Stefano states that it took over 500 hours of pc run time to pre-process the 2200+ airfoils ==> call it 15 minutes per airfoil) but, again, it can be done unattended, i.e. click mouse, come back later. Generation of the polar charts for the included airfoils only takes a few seconds.

Which tool to use depends on what you are trying to do. If, for example you want a low drag airfoil for a speed model you would be well advised to look at the coordinates of the wind tunnel test model of the S6063 rather than the true S6063 airfoil.

One thing that wind tunnel data is best at is showing the stall and post stall recovery behavior of airfoils.

If you are trying to select an airfoil for a particular application, Profili or X-foil will give you consistent comparisons that may not be perfecty accurate but the inaccuracies tend to cancel out making them excelent tools for comparisons.

If you are not prepared to strive for airfoil contour accuracy during the design and building process then you probably shouldn't be bothered with any of the above mentioned tools. The spanwise flow of very low aspect ratio wings tends to render the tools mentioned above of little value for that application.

For aerobatic aircraft applications, stall and stall recovery characteristics may be way more important than the drag characteristics.

For load carrying competitions, lift characteristics may be dominant. For sailplanes and long range aircraft L/D is the dominant priority.

Which tool, if any, depends on the priorities of the particular application.

I think that's a really key point, as many would-be Profili / XFoil users might discount the utility of such a tool, based on the numeric values being off by X%.

It's the shape of the generated curves that reflects whether the relevant physical principles are addressed and, to that end, whether the program algorithms are correct or sufficiently deep. The obvious exhibit here is the smoothly continuous profile of the DesignFoil low-Re MH32 polars posted above, indicating a lack of Profili / XFoil's sophistication in this flow regime.

For comparative purposes, the fact that the curves may be shifted in some direction away from the real values is entirely inconsequential.

wildblueyawner, You have me confused here. Did you mean to say the lack of DesignFoil's sophisication in this regard? It appears from the posted polar's that XFoil did a good job.

I have 2 Mantis sailplanes, one with a 7037 wing and one with an MH32 wing. Both weigh 60 ozs. I like them both and find they have very similar characteristics. To my insensitive abilities there isn't much to choose between them. For overall flying I like the MH32 a little better.

Sorry for the confusing wording - Wanted to say XFoil is more sophisticated and does a better job, because the polars it generates are NOT smoothly continuous at low Re (closer to reality).

FYI - A new version (2.10) of Profili / XFoil has just been released. One of the new features that caught my attention is the ability to analyze a turbulated airfoil. Haven't upgraded yet; should be interesting.

Hope the analysis of flapped airfoils has been cleaned-up (crashed every time I tried it in ver 2.03).

Does Profili allow you to consruct the foam templates for the upper and lower airfoil surfaces? While playing around with my registered version I've only had success with the upper surface.

Don't know yet - I've only tried Profili's analysis functions; will try playing with its geometric / template features. Will let you know if it handles upper & lower surfaces (I'd say a program of this type has got to handle both sides, otherwise the feature is pretty much useless).

The upper foam template can be obtained by specifying a negative number for "h" in the template editing screen:

Thanks Wild Blue! Worked like a charm--why didn't I think of that?