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Wings 59

A thicker wing will have a structural advantage, since the The Fokker D-VII was the first
wing’s structure can be contained inside the wing itself. The airplane to exploit thick wings.
thin wings used by early airplanes required external bracing As a result it was the only
and thus higher parasitic drag. Before aerodynamic drag was airplane specifically mentioned in
understood toward the end of WWI, emphasis was placed on the Treaty of Versailles. The
thin wings, with many wires and struts to give the wing Germans were required to hand
strength. Biplanes were used most frequently because they over every airplane to the Allies.
made for a nice boxlike structure. There is a natural
competition between the aerodynamicist who wishes to have a thin
wing and the structural engineer who wants the wing to look
internally like a nice fat box.

Leading Edge

Another design characteristic to be considered in designing a wing is
the shape of the leading edge. A wing with a sharp leading edge will
go abruptly into a stall. A blunt leading edge will have a much less
abrupt stall entry, with the stall starting at the trailing edge with the
separation point progressing forward as the stall becomes deeper. Fig-
ure 3.1 shows the early stall development of a wing with a well-
rounded leading edge as well as the stall of a wing with a sharp
leading edge. The wing with the sharp leading edge goes directly into
a full stall. Another way of looking at stall entry is shown in Figure
3.2, which shows the lift as a function of effective angle of attack for
both wings. It is clear that the sharp wing goes from maximum lift to
full stall with a very small change in angle of attack.
Airplanes that are designed to operate at lower speeds or to be used
as trainers have fairly round leading edges. Fast jet fighters have
sharper leading edges. A difficult problem for pilots transitioning from
lower-speed trainers to high-speed fighters is appreciating the more
abrupt stall. Many new pilots have been “surprised” when their high-
speed airplane stalled without warning. A classic use of a sharp
leading edge wing section is on the F-104 Star Fighter, shown in Figure
3.3. The leading edge is so sharp it has been known to cut the hands
of mechanics who inadvertently rub against it.
STOL (short takeoff and landing) aircraft have very fat, round
leading edges. This will hurt the airplanes’ top cruise speed, but top