# KR> Airfoil and Speeds

```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- http://www.krnet.org/as504x/ .

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
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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