STALL PATTERN AND WING DESIGN
The most desirable stall pattern on a wing is one that begins at the root. The primary reason for a root first stall pattern is to maintain aileron effectiveness until the wing is fully stalled. Additionally, turbulent airflow from the wing root may buffet the empennage, providing an aerodynamic warning of impending stall. Different planforms have characteristic stall patterns.
The lift distribution on the rectangular wing (λ = 1.0) is due to low lift coefficients at the tip and high lift coefficients at the root. Since the area of the highest lift coefficient will stall first, the rectangular wing has a strong root stall tendency. This pattern provides adequate stall warning and aileron effectiveness. This planform is limited to low speed, light-weight airplanes where simplicity of construction and favorable stall characteristics are the predominating requirements.
A highly tapered wing (λ = 0.25) is desirable from the standpoint of structural weight, stiffness, and wingtip vortices. Tapered wings produce most of the lift toward the tip and have a strong tip stall tendency.
Swept wings are used on high speed aircraft because they reduce drag and allow the airplane to fly at higher Mach numbers. They have a similar lift distribution to a tapered wing, and therefore stall easily and have a strong tip stall tendency. When the wingtip stalls, the stall rapidly progresses over the remainder of the wing.
The elliptical wing has an even distribution of lift from the root to the tip and produces mini- mum induced drag. An even lift distribution means that all sections stall at the same angle of attack. There is little advanced warning and aileron effectiveness may be lost near a stall. It is also more difficult to manufacture than other planforms, but is considered the ideal subsonic wing due to its lift to drag ratio.
Moderate taper wings (λ = 0.5) have a lift distribution and stall pattern that is similar to the elliptical wing. The T-34C uses tapered wings because they reduce weight, improve stiffness, and reduce wingtip vortices. However, the even stall progression is undesirable for the same reasons as with the elliptical wing. As a stall progresses, the pilot will lose lateral control of the airplane.
WING TAILORING
Although stalls cannot be eliminated, they can be made more predictable by having the wing stall gradually. Since most airplanes do not have rectangular wings, they tend to stall with little or no warning. Wing tailoring techniques are used to create a root to tip stall progression and give the pilot some stall warning while ensuring that the ailerons remain effective up to a com- plete stall. Trailing edge flaps decrease the stalling angles of attack in their vicinity, causing initial stall in the flap area.
Geometric twist is a decrease in angle of
incidence from wing root to wingtip. The root
section is mounted at some angle to the lon-
gitudinal axis, and the leading edge of the
remainder of the wing is gradually twisted
downward. This results in a decreased AOA
at the wingtip due to its lower angle of inci-
dence. The root stalls first because of its
higher AOA.
Aerodynamic twist, also called section variation, is a gradual change in airfoil shape that increases CLmax AOA to a higher value near the tip than at the root. This can be accomplished by a decrease in camber from the root to the tip and/or by a decrease in the relative thickness of the wing (as compared to chord) from the root to the tip. Since thicker and more positively cambered airfoils stall at lower angles of attack, the wing root stalls before the wingtip.
Stall Fences -A sharply angled piece of metal called a stall strip is mounted on the leading edge of the root section to induce a
stall at the wing root. Since subsonic airflow cannot flow easily around sharp corners, it separates the boundary layer at higher angles of attack, ensuring that the root section stalls first.
