Aerodynamic stalls are inherently dangerous unless recovery is well managed. Revising what causes the wing to stall is something that too many pilots seem to learn in their training but forget very quickly....yet it is one scenario that can cause serious problems for a pilot who doesn't manage the recovery proficiently.

This months newsletter discusses the reasons why one wing may stall before the other, resulting in the stall commonly known as a wing-drop stall, as well as the consequences. The recovery technique must be addressed in conjunction with a qualified training instructor.

By wing-drop stall we mean a stall where one wing stalls before the other. The wing that reaches the critical angle of attack first (at about 15-17 degrees) will stall first, losing lift and causing a roll at the stall. This often happens because of poor pilot technique where the aircraft is out of balance at the stall, or aileron is being used.

Once the wing stalls, aileron will not stop the roll, it will worsen the situation. If the wing-drop is not promptly recovered, a spin may develop. The action must be to stop the natural tendency to pick the wing up with aileron and to practice the correct method of recovery.

 

Revisit Principles of flight

Out of balance

If the aircraft is permitted to yaw at or near the stall there will be a tendency for it to roll (further effect of rudder), which will increase the angle of attack on the down-going wing. In addition, if an attempt is made to maintain wings level with aileron, the down-going aileron will increase the angle of attack on that wing. This usually results in that wing reaching the critical angle first.

Ice or damage

If ice forms on the wings, or one wing is damaged, by bird strike or 'hangar rash', the smooth airflow over the wing will be disturbed, and may break away sooner than the flow over the other wing – resulting in that wing stalling earlier than the other.

Weight imbalance

If all the passengers or fuel are on one side of the aircraft, some aileron may be required to maintain wings level.

Turbulence

When operating near the critical angle, a gust or turbulence may result in aileron being used to maintain wings level, or the modified airflow as a result of the gust may cause one wing to exceed the critical angle.

Power

Slipstream modifies the angle of attack on each wing because of its rotational nature. In clockwise rotating engines (as viewed by the pilot), the angle of attack is decreased on the starboard wing and increased on the port. Therefore, the aircraft may drop a wing more readily when partial power is used.

Flaps

It is possible for flap to extend at slightly different angles. In addition, when flap is extended the aircraft is less laterally stable, as the centre's of pressure on each wing move in toward the wing root. This increases the tendency for the aircraft to be easily disturbed in roll, which may cause one wing to exceed the critical angle. However, there is also a greater need to use aileron to maintain wings level in this configuration. Therefore, the aircraft may drop a wing more readily when flap is selected.

The wing that stalls first has a reduction in lift, causing roll. The roll increases the angle of attack on the down-going wing and may delay the stall of the up-going wing. Increasing the angle of attack past the critical angle will result in a decrease in lift but a substantial increase in drag.

The increase in drag yaws the aircraft toward the down-going wing, which may further delay the stall of the up-going wing as a result of increased airspeed. This process, where yaw causes roll, which causes yaw, is known as autorotation and the beginning of a spin..

By using aileron to stop the roll (a natural tendency), the angle of attack increases on the down-going wing. The lift continues to decrease with an increase in angle of attack (past the critical angle), while the drag continues to increase rapidly with any small increase in angle of attack.

The use of aileron adversely affects the roll and favours auto-rotation. This is the reason for maintaining ailerons neutral in the initial stall recovery.

The correct method of stopping auto-rotation is to break the yaw-roll-yaw cycle, and since aileron cannot be used effectively to stop the roll, top rudder is used to prevent further yaw. The nose is lowered simultaneously (back-pressure relaxed) with the application of rudder, and this will stop the roll immediately.

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Well another year has slipped by and notably Covid has had a significant impact on all our lives. That said I would like to wish all the readers of this newsletter a wonderful Christmas and look forward to sharing more things aviation in 2021.