This briefing 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 and correct recovery technique.
Stalling in the turn may produce the same consequences and requires the same recovery technique. If the turn is to be maintained rather than level flight regained, only the entry and the last item in the recovery are different. Therefore, stalling in the turn may be incorporated in this briefing. However, at the PPL level, the CFI may prefer a separate briefing for stalling in the turn (refer CFI).
By wing-drop stall we mean a stall where one wing stalls before the other. The wing that reaches the critical angle first (at about 15 degrees) will stall first, losing lift and causing a roll at the stall. This often happens because of poor pilot technique where the aeroplane 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 purpose of this exercise is to stop the natural tendency to pick the wing up with aileron and to practise the correct method of recovery.
To revise stalling with power and flap.
To carry out a stall from straight and level flight (and the turn) recovering from a wing drop with minimum altitude loss.
Revise the cause of the stall – exceeding the critical angle of attack, regardless of the observed airspeed.
There are many reasons why aileron may be in use at the stall.
During the turn, angle of bank is maintained with aileron.
If the aeroplane is permitted to yaw at or near the stall there will be a tendency for the aeroplane 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 mean angle of attack on that wing. This usually results in that wing reaching the critical angle first.
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.
If all the passengers or fuel are on one side of the aeroplane, some aileron will be required to maintain wings level.
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.
One wing can stall before the other due to incorrect rigging.
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. Again, due to aileron use, or an increased angle of attack, the aeroplane may drop a wing more readily when partial power is used.
It's possible for flap to extend at slightly different angles. In addition, when flap is extended, the aeroplane is less laterally stable, because the centres of pressure on each wing move in toward the wing root. This increases the tendency for the aeroplane 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 aeroplane may drop a wing more readily when flap is selected.
The consequences of one wing exceeding the critical angle before the other are discussed.
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 – use CL (coefficient of lift) and CD (coefficient of drag) against angle of attack graph (see Figure 1).
The increase in drag yaws the aeroplane 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'.
By using aileron to stop the roll (a natural tendency), the mean 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. Show the effect of aileron on the CL and CD curves on the graph.
The use of aileron adversely affects the roll and favours autorotation. This is the reason for maintaining ailerons neutral in the initial stall recovery.
The correct method of stopping autorotation is to break the yaw-roll-yaw cycle, and since aileron cannot be used effectively to stop the roll, rudder is used to prevent further yaw. The nose is lowered simultaneously by the control column checked centrally forward (no aileron input) with the application of rudder, and this will stop the roll immediately.
Revise the requirement to carry out all stalling practice in a safe environment.
Revise the HASELL and HELL checks.
Emphasise symptom recognition for avoidance.
The student should strive to improve situational awareness by integrating the attitude and airspeed with the aeroplane's configuration, phase of flight, and symptoms of the approaching stall.
As the objective is to carry out a stall with a wing-drop, a configuration most likely to induce a wing-drop is used, commonly 1700 RPM and full flap. The combination of these two factors will often lead to a wing-drop occurring at the stall.
Some aeroplane types, eg PA38, will perform good wing-drop stalls in the basic configuration (power idle, flap up) (Refer CFI).
Revise the airspeed and RPM limits.
Overlearning is used to improve information processing to recognise the situation and consciously ignore the roll while responding with the correct recovery technique.
Start by revising stalling in various configurations. This will help make the student more comfortable before tackling the wing-drop stalls.
When satisfied that the student is ready to progress, you should begin the exercise with the demonstration and patter of a wing-drop stall (see Airborne sequence).
HASELL checks are completed, and a prominent outside reference point (backed by the DI) on which to keep straight is nominated.
From level flight, carburettor heat is selected HOT and the power smoothly reduced to ____ RPM. As the nose will want to yaw and pitch down, keep straight with rudder and hold the altitude with increasing backpressure.
Below ____ knots (in the white arc) select flap gradually, if applicable to the aeroplane type. During the application of flap, check forward to prevent any gain in altitude due to the increase in lift, before reapplying backpressure to maintain altitude.
Through ____ knots, or when the aural stall warning is heard, select carburettor heat COLD, as full power will shortly be applied.
At the stall, altitude is lost, the nose pitches down, and one wing may drop.
If the aeroplane is reluctant to drop a wing at the stall, alter the power and flap combination (refer CFI) and relax rudder pressure to simulate the pilot's failure to maintain directional control. Alternatively, a gentle turn may be required (5 degrees angle of bank).
There is nothing underhand about these techniques, as permitting the aeroplane to yaw or stall in the turn are possible causes of a wing-drop stall.
Avoid an accelerated stall (by zooming the entry) which may produce a rapid roll. The student should see a rapid stall at some point in their training, but the first stall is not the time for it. If a pronounced wing-drop occurs, the application of full power may need to be delayed to avoid exceeding flap limiting speeds, or VNE.
The recovery may be discussed in three parts, but the ultimate objective is to coordinate all three actions.
Keep the ailerons neutral.
Simultaneously decrease the back pressure (check forward) and apply sufficient appropriate rudder to prevent further yaw.
Excessive rudder should not be applied (to level the wings through the secondary effect of rudder) as this may cause a stall and flick manoeuvre in the opposite direction to the initial roll (wing drop).
Full power is smoothly but positively applied. At the same time, level the wings with aileron (as the aeroplane is now unstalled), centralise the rudder, and raise the nose smoothly to the horizon to arrest the sink and minimise altitude loss.
Hold the nose at the level attitude, and reduce the flap setting (as appropriate to aeroplane type) immediately.
At a safe height, safe airspeed, and with a positive rate of climb, raise remaining flap (counter the pitch change). The aeroplane will continue to accelerate, and at the nominated climb speed select the climb attitude.
Straight and level flight should be regained at the starting altitude and the reference point or heading regained.
The student should be capable of positioning the aeroplane within the training area at a suitable altitude, completing the necessary checks, and possibly carrying out the advanced stall and recovery. Instructor assistance is given only as required.
For the purposes of demonstration and patter, the recovery may be broken down into three separate phases (refer CFI). Alternatively the three phases may be condensed into two or even one phase, depending on your assessment of the student's ability.
It is recommended that all stalling exercises finish with a reminder that outside of the training environment the student would recover at the onset of the stall at the latest.
At the completion of this exercise, there may be time to practise Maximum rate turns, if previously covered.