Samuel wrote-

>has anyone used thinner than 15%. Would thinner mean more speed?

I don't know about the speed, but I can tell you that there was a considerable 
amount of credible and documented (and flight-proven) study and research done 
to arrive at the AS50xx-series of airfoils that are specifically tailored to 
the KR2/2S.  Although you may get higher speed by tweaking the airfoil to get 
lower induced drag, you'll be giving up something else in the exchange, either 
in stall speed/sharpness, higher approach and landing speed as a result, or 
something else.  It sounds like you're already familiar with the development of 
the AS50xx airfoils, but you may want to revisit the design goals and 
achievements here- .

As to using a thinner wing, I believe one of the objectives of the AS airfoil 
development was to utilize the stock KR spars so that there would not need to 
be a whole new structural analysis done and so that the airfoil could be 
retrofitted to existing KRs without modifying the spars.  Bear in mind that the 
RAF48, the original KR/KR2S airfoil, is a 15% (actually closer to 14.9%, but 
you get the idea), and that the strength of a spar in bending generally goes as 
the square of its height (tallness).  As an example using numbers that are easy 
to deal with, let's say that a box beam spar is 5" tall and 1" wide and it's 
constructed in such a way as to result in its having a section modulus of 4 
in^3.  If we reduce its height by 1", all else being equal, the section modulus 
is reduced to 2.56 in^3 or a reduction of 36%... more than a third.  Just to 
continue down this theoretical line, let's say the 15% airfoil spar was 
designed for Normal category, +3.8G, with a 50% margin of safety (5.7G 
ultimate).  If you reduce that by 36% you now have a spar that is at ultimate 
loading at just about 3.7G and your margin of safety is gone.  You'll want to 
redesign the shallower spar to try to restore the margin of safety.

Disclaimer: the numbers above are simplified, they are not specific to the KR 
box spar, but I hope I've made the point.

Oscar Zuniga

Medford, OR


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