Further to the recent "Dropped Wing" correspondence, supposing that (for the sake of argument) the left wing tip stalls and drops, and the pilot responds with right rudder (and nothing else), at least two effects come into play.

Firstly, the left wing tip obtains some additional lift due to the local airflow speed increasing as a result of the yaw rate. This restorative extra lift depends on the square of the increase in tip speed, and so is not insubstantial, albeit brief. The angle of attack is not altered by this effect, and so the degree of stalling is unaffected.

Secondly, the yaw rate will quickly result in a yaw angle with the nose pointing to the right of the original flight path, and a consequent side flow from the left. This is when the aircraft's lateral stability might be expected to kick in and roll the aircraft back to wings level. Let us suppose that the aircraft obtains its lateral stability from wing dihedral, which depends for its operation on an increased effective angle of attack on the advanced wing (the left wing in our case), resulting from the side-flow. Since the left wing tip has already stalled, it is likely to stall more deeply, losing some lift and dropping again. Thus the rudder technique is unlikely to work.

However, the laterally stabilising effects of sweep-back and a high fin or keel surface do not depend on changing the advanced wing's angle of attack, but on other principles that will remain effective even when a wing is at its stalling angle of attack. Furthermore, lateral stability from a high wing depends on an increase in angle of attack on the advanced wing close to the fuselage, leaving the wing tip largely unaffected. If the aircraft depends on any of these three features for its lateral stability, the rudder technique is more likely to work effectively.

Consequently, the efficacy of using "top" rudder to restrain a dropping wing is very much dependent on the design of the aircraft in question. The only generally acceptable solution is to recover conventionally from the stall before the problem escalates.Chris Carpenter Sleaford, UK

Source: Flight International