Page 83 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 70

70 CHAPTER THREE

theory became widely known to designers in the 1930s and resulted
in many aircraft with elliptical wings. Two notable WWII-era fighters
with an elliptic wing planform are the British Supermarine Spitfire
and the American P-47 Thunderbolt shown in Figure 3.12. Today one
does not see elliptic wings on airplanes because they are expensive to
build and there are other ways to create approximate elliptical lift
distributions.
Figure 3.13 shows the wing loading for a rectangular wing, an
elliptic wing, and a linearly tapered wing. The rectangular wing has
the highest loading at the tip, while the linear taper “unloads” the tip.
A linearly tapered wing loses only 7 percent of the lift of an elliptic
wing. Many airplanes use linear-tapered wings because of the reduced
construction costs with only a small performance penalty.

Twist
One method of tailoring the lift distribution on the wing is to twist the
wing, with the angle of attack greater at the root than at the tip.
Another term for this type of twist is washout. The lower angle of
attack at the tip unloads the tip and thus approaches the elliptical lift
distribution. There is another advantage of using washout. Because
the wing root is at a higher angle of attack, the wing will stall at the
root first. Since the ailerons, which control roll, are usually on the out-
board portion of the wing, the ailerons can still be effective after the

Tapered

Elliptical

Lift
Rectangular

Root Tip
Span
Fig. 3.13. The effect of wing shape on the distribution
of lift.