I played around with some airfoil analysis and by reducing the leading
edge diameter from 10% to 5% of chord, the maximum L/D ratio of the
airfoil increases from 115 to 126 (induced drag is not included) and the
maximum Lift Coefficient increases from 1.57 to 1.79.
So it's around 10% to 15% increase in airfoil performance for both
maximum L/D and Lift by reducing the LE diameter in half.
I also increased the LE diameter to 15% and the result was not very
desirable so you definitely don't want to go there.
P.S., These numbers should be taken as an approximate.
P.P.S, The airfoil moment (tendency to flip over the nose) is slightly
more stable with smaller LE diameter
P.P.P.S., Most modern LE diameter on market is around 9 or 10% of chord.
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