EAAAirVenture OshkoshShopJoin

How do I pick an airfoil design

Posted By:
Ried Jacobsen
#1 Posted: 1/15/2010 08:34:29

 There have been many airfoil designers who have design many different airfoils.  Many books have compiled these airfoils.

My question is, out of these thousands of different airfoils, how should I pick one for the motorglider I am trying to design?

I expect my aircraft will have a wingspan of 40 -45 feet.

Wing area will be betwee 130 and 150 SF, with an aspect ration of 9-12:1.

Wing loading expected to be between 7 and 10 pounds per SF.

I want a stall speed between 35 to 40 knots.

I would like a cruise speed around 110 knots.

Which airfoil should I select?

Do I randomly flip open an airfoil book and point to the "lucky" selection?  Do I use an Ouiji board?


Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
#2 Posted: 1/15/2010 09:59:28

Go to http://www.actechbooks.com/products/act750/ and buy Barnaby Wainfan's Book.  Just about all you need to know.

Todd Parker
Homebuilder or Craftsman
#3 Posted: 1/16/2010 15:57:19

Selecting an airfoil is both a complex and simple thing.

Almost any airfoil will make an airplane fly at some speed, so in the simplest case, almost anything you pick will work. On the other hand if you looking for certain criteria to be met such as you mentioned above then the selection becomes more complicated and you need to decide what things are important to your design and what things are not.

If stall speed is most important then you need an airfoil with very high lift at low Reynolds numbers.

If maximum L/D is important at some speed, then you need to find an airfoil that will give the maximum L/D and lowest pitching moment for the design speed (reynolds number).

If pitching moment is important, then you need to select one with low moments or at least moments that are within the abilities of your tail surface to balance.

If maximum speed is important, then you need an airfoil with minimum drag at the design speed you would like to go regardless of the L/D ratio.

Now usually more than one of these is important and that is where the ART of selecting an airfoil comes into play. You need to prioritize what you want the aircraft and hence the airfoil to do for you. Then decide how much you are willing to compromise from your needs or ideals to get the best overall compromise in performance you can.

If you want to try different airfoils to see which you like better, you can model these along with the entire aircraft using X-Plane simulation software and then try the various airfoils until you get one that flies the best. Getting the airfoils into X-Plane is a bit of a chore, but I have done it many times.

Todd Parker

Always thinking about airplanes
Theodore Fails
Homebuilder or Craftsman
#4 Posted: 1/25/2010 13:55:57

Hi Ried:


I am working on a similar problem, and there is a lot of good information available.  As another poster has suggested, Wainfans's book is indeed very good.  I like von Mises "Theory of Flight" and Abbott & von Doenhoff "Theory of Wing Section."  There is quite a bit of neat design software also.  Search online for Xfoil, QFLR5, and Profili 2. 

Best of all, for free and available for immediate download take a look at the archive section of "Sport Aviation" on this site.  Try September 1975 for the article "Airplane Stability, Control and Trim" by Robert K. Wattson, Jr. (EAA 75616) and August 1975 for "To Choose an Airfoil" by Capt. Will D. Mitchell (EAA 85466).  Lots of great design info in these archives.


Good luck and keep us posted.




Jon Harris
Vintage Aircraft Association MemberYoung Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
#5 Posted: 1/25/2010 23:43:44

You might start by looking at other aircraft that are similar.  That will at least give you a place to start comparing the trade-offs.

I suggest the UIUC site   http://www.ae.illinois.edu/m-selig/ads/aircraft.html.  The UIUC site also has a very large catalog of airfoil section. http://www.ae.illinois.edu/m-selig/ads/coord_database.html


From there you can start to look at stall angles, stall characteristics, drag buckets... and the list goes on.

As other have said there are a lot of good books that will help you understand the trade offs.




Lincoln Ross
#6 Posted: 4/25/2010 21:35:06

As someone else recommended, Wainfan's book is a useful introduction. If you get intensely into it, you may find Xfoil or Profili useful. (Profili is an easier way to use Xfoil, plus it does other neat stuff, but for the full version you have to pay a fairly small fee.) This way you can get info for exactly the Reynolds numbers you're interested in. Of course, your results may vary a little, or perhaps a lot unless the wing is very precisely made. I'm not sure how much those two programs will help with stall characteristics either.


An important issue is how accurately you can build and how far back the smooth, non-wavy part extends. Also how much bug wiping you're willing to do. Otherwise, the laminar foils won't be worth it. Another important factor is how draggy the rest of the glider is. An ultra low drag airfoil won't help all that much more if you have a large, awkward fuselage.


From the two loadings you're giving and the desired stall speed, the wing Cl you'll need is between maybe 2.4 (for 10 lbs and 35 knots) to 1.3 (for 7 lbs and 40 knots). You're not going to get 2.4 with a low drag wing, especially since the max airfoil Cl should be somewhat higher. 1.3 would be no problem and you could probably get quite a bit more. You probably shouldn't use one airfoil across the whole wing, either, although it may be ok if it makes building easier.


Oh, and one more thing, which is covered, I think, in the Wainfan book. Do you want to use flaps? Flaperons?