Stalling — Page 158, Lesson 190

Let's pick up with the tapered wing, because that's where the real design problem lives. On a rectangular wing, separation starts at the root and spreads outward. That's the good behaviour. But a tapered wing is different. Look at Figure 7.10. On a tapered wing, separation tends to occur first in the region of the wing tips. The tip is unable to support the tip vortices, so those vortices form closer to the root. That gives a decreased effective angle of attack at the wing root, which delays the root stall. So the tip stalls first. Now, why is that bad? If an actual wing were allowed to stall this way, stalling would give aileron buffet and perhaps violent wing drop. And wing drop at the stall gives an increased tendency for an aircraft to enter a spin. There would be no buffet on the tail, no strong nose-down pitching moment, and very little, if any, aileron effectiveness. So the tapered wing, left alone, gives you the worst possible stall behaviour. To fix it, we modify the tapered wing using one or more of three methods. The first is geometric twist, also called washout. That's a decrease in incidence from root to tip. Incidence is the angle at which the wing is set relative to the fuselage. By reducing it at the tip, we decrease the angle of attack at the tip, so the root will tend to stall first. We've forced the good behaviour back. The second method is varying the aerofoil section throughout the span. We put sections with greater thickness and camber near the tip. Camber is the curvature of the aerofoil. Those sections have a higher CLMAX — that's the maximum lift coefficient, the highest lift the section can produce before it stalls. Because that's higher at the tip, the tip stalls later, so again the root stalls first. The third method is leading edge slots, shown in Figure 7.11. A slot is a gap in the leading edge, towards the tip. It re-energizes the boundary layer — that means it increases the kinetic energy of the boundary layer. That raises the local CLMAX. It's useful both for delaying separation at the tip and for retaining aileron effectiveness. The full function of slats and slots is described in Chapter 8, but for now, know that a slot is a deliberate opening that feeds high-energy air into the boundary layer to keep it attached. So the rectangular wing gives you the ideal stall response — aileron effectiveness, nose drop, aerodynamic buffet, and no violent wing drop. But it has unacceptable wing bending characteristics and isn't very aerodynamically efficient. That's why most modern aircraft use a tapered and/or swept planform, and they use these three fixes — washout, varied aerofoil sections, and leading edge slots — to recover the good stall behaviour.