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What is the nature and extent to the benefit of covering up the bottom of an airfoil?
It seems like all the early airplanes just had ribs and spars that were wrapped only on top, but then pretty soon wings started getting wrapped underneath too.
Despite the added weight of covered wing bottoms, I'm assuming that the planes flew much better this way.
Are there any resources about experiments on covered vs exposed wing bottoms? How much less aerodynamic is it to leave them exposed?
Lift is a function of velocity squared. As more powerful engines became available, higher airspeeds allowed designers to choose the airfoil shape that provided the least amount of drag for a given amount of lift at that airspeed.
Undercambered wings, as seen on the Bleriot XI which flew at around 40 knots were replaced by "filled in" or covered wing bottoms, as seen on the Fokker D7, which flew at around 100 knots.
A little undercamber is still there, and is also seen on the famous Clark Y airfoil.
As things got faster, up to 200 knots, the bottom was filled in completely, becoming convex, as seen on the Davis airfoil.
These wings served well until even faster speeds demanded consideration of Mach factors, especially in areas over the wing where airflow is accelerated, which lead to these "supercritical" wings, in use today on jet airliners.
There most certainly is, early hang gliders we’re single-surface, then more advanced designs adopted a small under-surface just behind the leading edge, and the extend of the lower surface increased with higher-performing designs. That’s not exactly experimental per se but a reflection of the development work done as that aircraft type evolved.
