> Turbulant boundary layers can have a higher lift to drag ratio than
> laminar. The problem is that the manufacturing tolerances and initial air
> flow are more critical.

For Laminar flow.

> I think some of the first human powered aircraft had wings
> with lift/drag ratios of 50 to 100 but stability was lacking.

The stability problems are not entirely linked to the L/D (efficiency) of the wing, but as you configure for performance in one area you do have to sacrifice somewhere else. In hangliding, many of the highest performance gliders have an adjustment which lets you tighten the cloth on the wing for higher L/D performance, with contrllability and some stability being sacrificed. You can then loosen the cloth later (close to ground) and gain controllability and stability when you need it. It's referred to as variable geometry, though it's quite different from the same term as used on the F-14, F-111, et. al.