Hi All,

Hoping you can help with the choice of an airfoil for my new project which is a RES glider.

First, the macro environment info. Altitude above sea level 1650 metres. Average daytime temperature 22 celcius. Humidity varies according to season and wind speeds from 5 to 15 knots. Lift is variable from small elusive bubbles to the occasional boomer. Also have a little ridge lift when the wing is in the right direction.

The plane. Span 3.2 metre, wing area 75,9 dm2. AR 13.5:1. Root chord 270mm, tip chord 120. Polyhedral 5 panel wing with flat centre section which will house the spoiler. EDA 9.9. Conventional tail configuration. Built up wing.

Power system is Kontronik Fun 400 with a 4.3:1 gearbox swinging a Graupner folding 12 x 6 prop. Kontronic ESC. Batteries are 1700 mAH Ni-Mh. All up flying weight is expected to be under 1400 grams which will give a wing loading of around 18 to 19 grams per dm2.

Having got this far, I now need to select an appropriate airfoil or airfoils which will maximise the thermalling capability of such a lightly loaded wing, and also provide a bit of speed for decents and thermal hopping.

Thanks in anticipation.

Soarbird

You would probably do well to use mark Drela's Aegea series of airfoils. In fact, there's a whole wing design available for a winch-proof wing. Anyway, the dat files for the airfoils are here:

http://groups.yahoo.com/group/Allegr...iles/Aegea-3m/

These are quite thin and were intended for use on a plane with camber control. You would have to fly a little fast or build-in a little "flap" camber.

There are also the Bubble Dancer airfoils. BD is one of the best RES designs out there. These airfoils are a bit thicker and "floatier". Airfoils here:

http://groups.yahoo.com/group/Allegr...Bubble_Dancer/

Another Drela fan here. But I'd suggest the "purpose designed for balsa construction" airfoils off the Allegro-Lite and Bubbledancer.

What they said already PLUS -

Take a long and hard look at the construction methods used by Drela for Allegro and BD. If you also take a look at what the freeflight (F1A ) builders have been doing over the past 10 years or more you will see where the ideas come from The most important part of all is the main spar construction. [I} Flying Models International[/i] is a goldmine for current thinking in ff glider. The technology does transfer well...Drela is witness to that.[sm=thumbup.gif]

Second up is to listen to what Drela says about airfoil selection. How effective one particular airfoil is depends upon accurate construction. I would add to that pilot ability as a statement of the obvious rather than any implied criticism. (I can't fly my models.. I rely upon them flying themselves )

ooopppss

Thanks for the responses so far. One of the guys here has a Bubble Dancer but does not seem to place too well at the altitude we fly at here which is 5,700 ft above sea level. This together with the high summer ambiant temperatures gives it's own density altitude problems in terms of thin air.

The top three local competition airfoils seem to be MH32, RG15 and SD7036. The former two are fitted to F3B type glass ships but the SD7036 is fitted to a wide variety of glider types and sizes from 2 metre to 3.2 metre span. I have an own design 2.0 metre glider which uses the SD 7036 at 11.8% which appears reasonably versatile but not as good as a comparable plane at our club which uses the FX60-100. I have another own design 3.1 metre glider which also uses the FX60-100 and is a sedate flyer but at twice the wing loading of my new design.

So confusion continues to reign. Any comments??

Best wishes,
Soarbird

Probligo thanks for your comments and suggestions.

Fortunately we have laser cutting available to us for the ribs etc so with a lot of care, shold be able to produce a wing with a reasonably accurate conformance to the selected airfoil.

Would you possibly have the link to Flying Models International?

Agree 100% with your comments on pilot ability. When I first started flying I was afraid of getting the nickname of 'pilot error'

Regards,
Soarbird

OK, so with the one bubbledancer you've seen, are you sure the pilot is getting the most out of it? What about the competition? What it beating it? You altitude difference makes less than 15% difference in reynolds number - which means you should be flying 5-10% faster. If the guy is trying to fly too slow it'll be bad.

The RG15 is a fast flying airfoil so it's not really for a floater RES. The MH32 is fine, and so is the SD7037. You can use those and your glider will be very nice. However, you asked what's best with a wide speed range. The Drela airfoils have been custom designed for their operating Re and have speed range that beats anything else. Where most people build a sailpane wing with a single airfoil over the span, Drela wings typically have 4 different airfoils customized for the different Re at different chord lengths. If you think your Re is lower because of altitude, select one of the Drela airfoils that normally would be used for a 7" chord and use it for your 10" root. Adjust other stations accordingly.

Also let's not forget that a glass ship with the inherently more consistently accurate airfoil will always be a better choice. You mention the 7036 but you need to realize that this airfoil, along with the RG15 is very fussy about it's accuracy. Balsa construction with open bays will not provide that level of accuracy and may very well limit such a model to no better than or inferior performance to the Bubble Dancer.

But I aslo echo John's comments about the doubt surrounding the BD's piloting and trimming abilities. You may be trying to compare apples to oranges.

If I might add my 2 cents.


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Schpankme

That airfoil is meant for full sized sailplanes. At our Reynolds numbers it would not perform as advertised. Wortmann did some model airfoils but most of his offerings are meant for full sized stuff.

Hi there,

John and Bruce it looks like you may have another Drela convert on your hands but please check my logic - or lack of.

Would I be correct in saying that the wing is divided into four equal sections. Root would be AG35 and tip would be AG38 with transition from root to tip using AG36 and AG37 at the intermediate stations?[sm=confused.gif]

BTW I got hold of the BD pilot and after a brief chat found out that it was only his third flight with the model. As he is one of the forerunners on the local scene, the wrong assumption was made. My apologies gentlemen.

Thanks for the help.

Reagrds,
Malcolm

Yes, that is correct, except I wouldn't necessarily use equal divisions between airfoils. It will make little difference though. One more link for you. Lot's of stuff here about those airfoils and construction:

http://www.charlesriverrc.org/articl...egrolite2m.htm

Bruce, you are correct sir.

In comparing the Wortmann with an Eppler Airfoil prior to my response, I pasted the wrong airfoil in my post; my apologies.
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Schpankme

Can you clarify this? What current airfoil? What does "simplistic nature" mean. Sorry, I just didn't follow.

Our guy soarbird has a laser cutter lined up, so accurate ribs for his project shouldn't be a problem, and the airfoils we are suggesting are specially designed to have very little cuvature in the areas covered by thin film, so they maintain accuracy. They also have half the profile drag of the E231 at these Reynolds numbers.

The current airfoil looks to be a modification of an Eppler Airfoil (E205-ish), simple flat bottom. With this type of airfoil it's very easy to build a wing fairly accurate. However, using the Eppler 231 (as shown) would require a different building technique or skill set to jig the wing. My thoughts were if built incorrectly there would be zero benifit. Snice my last post, I've had a chance to read through the Allegro-Lite 2m website completely; I like the current design and EZ of build.
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Schpankme