Planform and handling, wing-tip stalling

When a wing stalls, the pressure becomes almost constant over the upper sur­face, and the centre of lift moves aft towards the centre of the section. Thus, the

Flight

velocity

Fig. 2.14 Wing-tip stall

As the wing tip descends, the effective angle of attack is increased deepening the stall on that tip. On the opposite rising tip, the effective angle of attack is decreased, inhibiting stalling of that tip. The aircraft therefore starts to roll and yaw

nose will tend to drop, the angle of attack will decrease, and stall recovery will be almost automatic. If the tips of the wing stall before the inboard section, however, one tip will invariably start to drop before the other, and as it drops, its effective angle of attack will be increased, as illustrated in Fig. 2.14. The stall deepens on that tip, and it continues to fall. The aircraft thus starts to roll, and the opposite wing tip rises. On the rising tip, the relative flow direction reduces the stalling tendency, and the tip still generates lift. The rolling moment is thus sustained. The stalled tip produces more drag than the unstalled one, and therefore, the aircraft also starts to turn or yaw. This combination of rolling and yawing can lead to the classic dangerous spin condition described in Chapter 12. Wing-tip stalling is, therefore, a condition that we normally wish to avoid.

On rectangular planform wings, the downwash is greatest near the tips. The effective angle of attack of the tips is thus less than inboard, and the tips will stall last. This relatively safe stalling characteristic of the rectan­gular planform wing makes it attractive for the amateur pilot flying a small aircraft. Rectangular-planform wings are also generally cheaper and easier to construct. When performance is the overriding consideration, a tapered wing giving a closer approximation to the low-drag elliptical lift distribution, is preferred.

Highly tapered wings not only produce poor stall characteristics, but if the taper is excessive, the approximation to an elliptical lift distribution is inferior. It is unusual to find aircraft where the tip chord is less than one third of the root chord, despite the structural advantages of a high taper.

The shape of the wing tip also influences its stalling characteristics. The use of rounded or chamfered tips, as seen in Fig. 2.15, produces stable separated conical vortex flow at high angles of attack, inhibiting tip stall. This effect is also employed on the so-called BERP tip developed by Westlands for the tips

of helicopter rotor blades, as illustrated in Fig. 2.16. The Lynx helicopter shown in Fig. 1.29 uses this type of rotor blade tip.