Originally Posted by
leonard17F
"At stall, to counter a wing-drop phenomenon, on an airplane with straight wings you use rudder, and for an airplane with stretch wings (or delta), you use ailerons" ?
Leo
This is not a simple subject! I consider that stalling is, perhaps, the most complex and potentially ambiguous aspect of flight testing that there is. One interesting aspect is how you define the 'stall' this being considerably different between what is taught in training, military specifications and part 23/25 specification definitions. Also, are we considering recovering at the point of aerodynamic stall (perhaps Cl max, perhaps maximum usable lift, however defined) or manoeuvring an aeroplane close to maximum usable AoA?
Let us consider consider two cases. First, a wing drop at the point of aerodynamic stall with a straight wing and, typically, this will be a rapid/sudden event. Using aileron to try to correct the wing drop may exacerbate it by increasing local AoA. However, part 23/25 certification states a maximum permissible wing drop and lateral and directional control may be used to contain it. Therefore, most, if not all, such aircraft will have ailerons that are still effective at the stall. However, one of the problems may be significant adverse aileron yaw such that co-ordinating rudder may also be needed in order to generate a rolling moment. Attempting to use rudder to generate a rolling moment with sideslip may well be effective if only small rudder inputs are used. However, if large rudder inputs are used then a departure and spin may result.
Secondly, swept wing aircraft. The Cl vs AoA relationship inevitably will not exhibit a clear break as seen with most straight wings and there are far greater variations in 'stall' characteristics than with a straight wing. Typically, aileron power will reduce markedly as AoA increases and lateral stability will increase. Therefore, the use of lateral stick may give a weak rolling moment in the conventional sense but the combination of adverse yaw and strong lateral stability may result in a roll rate in the opposite direction to the applied stick. However, a rudder input may result in a strong rolling moment in the same direction due to the strong lateral stability. The F4 and the F-14 are two types that demonstrated this well. To roll the F4 above 17 units AoA you kept the stick in the middle laterally and used the rudder.
Returning to straight wing aeroplanes, if a wing drop occurs at the stall then the recovery procedure must be to reduce AoA to less than the stall value with a forward stick input and then roll wings level using normal lateral stick inputs (with rudder co-ordination if appropriate). For safety of flight recoveries I would not recommend using the CS-23 certification technique of controlling the bank angle with ailerons and, if required, rudder before reducing the AoA. With respect to the use of power during recovery, consider carefully whether you are recovering at the full stall or at stall warning onset. If the latter case then the use of power to minimise altitude loss is most certainly appropriate.