> Is there a common percentage that the front wood (or box) spar is placed > from the leading edge, say on high wing airplanes like the Piper Cub, or > Aeronica.? >
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It depends largely upon the designer and the mission or role of the aircraft.
Col. Virginius Clark (U.S.Army Air Corps) created a defacto light-plane standard for his family of airfoils (USA-35b, as used by Taylor in the design of the Cub [USA = U.S.Army], plus the entire Clark-x series, upon which the NACA Four-Number Series (ie, 4415, etc) is based.
Clark found that for strut-braced, fabric covered wings using wooden spars, positioning the spars at approximately 15% & 65% of the chord (measured from the leading edge) provided the best ratio of strength to weight.
This ratio (and Clark's airfoils) would apply to the two samples you've cited. But Clark's spar spacing also proved to scale rather well. Many earlier designs (ie, prior to about 1923) when re-engineered with a 'Clark wing' usually shed quite a few pounds and enjoyed significant gains, not only in climb but cruise performance. (Lindy hopped the pond under a 'Clark wing.')
-R.S.Hoover
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