Special Phenomena of Stall

Crossed-control Stall

A crossed-control stall can occur when flying at high angles of attack while applying rudder in the opposite direction to aileron, or too much rudder in the same direction as aileron. This will be displayed by the ball in the slip indicator being displaced from neutral.

Crossed-control stalls can occur with little or no warning; one wing will stall a long time before the other and a quite violent wing drop can occur. The “instinctive” reaction to stop the wing drop with aileron must be resisted. The rudder should be used to keep the aircraft in balanced, co-ordinated flight at all times (ball in the middle), especially at low airspeeds/high angles of attack.

Accelerated Stall

An accelerated stall is caused by abrupt or excessive control movement. An accelerated stall can occur during a sudden change in the flight path, during manoeuvres such as steep turns or a rapid recovery from a dive. It is called an “accelerated stall” because it occurs at a load factor greater than 1g. An accelerated stall is usually more violent than a 1g stall and is often unexpected because of the relatively high airspeed.

Secondary Stall

A secondary stall may be triggered while attempting to recover from a stall. This usually happens as a result of trying to hasten the stall recovery: either by not decreasing the angle of attack enough at stall warning or by not allowing sufficient time for the aircraft to begin flying again before attempting to regain lost altitude. With full power still applied, relax the back pressure and allow the aeroplane to fly before reapplying moderate back pressure to regain lost height.

Large Aircraft

During airline “type” conversion training on large aircraft, full stalls are not practised. To familiarize pilots with the characteristics of their aircraft, only the approach to stall (stick shaker activation) is carried out.

(a)Jet Aircraft (swept wing): there are no special considerations during the approach to the stall.

(i)Power-off stall: at stick shaker, smoothly lower the nose to the horizon, or just below, to un-stall the wing; simultaneously increase power to the maximum recommended to minimize height loss, prevent wing drop with roll control, raise the gear and select take-off flaps.

(ii)Power-on stall: as with power-off.

Stalling 7

Stalling 7

(b)Multi-engine propeller.

(i)Power-off stall: at stick shaker, smoothly lower the nose to the horizon, or just below, to un-stall the wing; simultaneously increase power to the maximum recommended to minimize height loss, prevent wing drop with rudder and aileron control, raise the gear and select take-off flaps.

(ii)Power-on stall: as with power-off.

The primary difference between jet and propeller aircraft is the rapidly changing propeller torque and slipstream that will be evident during power application. It is essential for the pilot to maintain co-ordination between rudder and aileron while applying the control inputs required to counter the changing rolling and yawing moments generated by the propeller when the engine is at high power settings or during rapid applications of power. Yaw must be prevented during a stall and recovery.

Small Aircraft

(c)Single-engine propeller

(i)Power-off stall: at stall warning, smoothly lower the nose to the horizon, or just below, to un-stall the wing; simultaneously increase power to the maximum recommended to minimize height loss, prevent wing drop with rudder and raise the gear if applicable.

(ii)Power-on stall and recovery in a single-engine propeller aircraft has additional complications. At the high nose attitude and low airspeed associated with a power-on stall, there will be considerable “turning effects” from the propeller. (These are fully detailed in Chapter 16).

To maintain co-ordinated flight during the approach to, and recovery from, a power-on stall, the pilot of a single-engine propeller aircraft must compensate for the turning effects of the propeller with the correct combination of rudder and aileron. It is essential to maintain coordinated flight (ball in the middle) when close to the stall AND during recovery. Any yawing tendency could easily develop into a spin. When the aircraft nose drops at the stall, gyroscopic effect will also be apparent, increasing the nose left yawing moment - with a clockwise rotating propeller.

An accidental power-on stall, during take-off or go-around, when a pilot’s attention is diverted, could easily turn into a spin. It is essential that correct stall recovery action is taken at the first indication of a stall. (Forward movement of the pitch control; neutralize the roll control; and prevent wing drop with the rudder).

In a climbing turn, airspeed will be lower and in a single-engine propeller aircraft, the rolling and yawing forces generated by the propeller and its slipstream will add their own requirements for unusual rudder and aileron inputs. E.g. for an aircraft with a clockwise rotating propeller in a climbing turn to the left at low speed it may be necessary for the pilot to be holding a lot of right roll aileron and right rudder. If an aircraft in this situation were to stall, the gross control deflections could make the aircraft yaw or roll violently. Correct co-ordination of the controls is essential, in all phases of flight, to prevent the possibility of an accidental spin.

Conclusions

In whatever configuration, attitude or power setting a stall warning occurs, the correct pilot action is to decrease the angle of attack below the stall angle to un-stall the wing, apply maximum allowable power to minimize altitude loss and prevent any yaw from developing to minimize the possibility of spinning (pretty much, in that order). “Keep the ball in the middle”.

High Speed Buffet (Shock Stall)

When explaining the basic Principles of Flight, we consider air to be incompressible at speeds less than four tenths the speed of sound (M 0.4). That is, pressure is considered to have no effect on air density. At speeds higher than M 0.4 it is no longer practical to make that assumption because density changes in the airflow around the aircraft begin to make differences to the behaviour of the aircraft.

SHOCKWAVE

SEPARATED AIRFLOW

Figure 7.33

Stalling

Stalling 7

At high altitude, a large high speed jet transport aircraft will be cruising at a speed marginally above its critical Mach number, and it will have a small shock wave on the wing. If such an aircraft overspeeds, the shock wave will rapidly grow larger, causing the static pressure to increase sharply in the immediate vicinity of the shock wave. The locally increased adverse pressure gradient will cause the boundary layer to separate immediately behind the shock wave, Figure 7.33. This is called a ‘shock stall’. The separated airflow will engulf the tail area in a very active turbulent wake and cause severe airframe buffeting - a very undesirable phenomenon.

High speed buffet (shock stall) can seriously damage the aircraft structure, so an artificial warning device is installed that will alert the pilot if the aircraft exceeds its maximum operational speed limit (VMO /MMO)* by even a small margin. The high speed warning is aural (“clacker”, horn or siren) and is easily distinguishable from the “low speed” high angle of attack “stick shaker” warning.

We have seen that approaching the critical angle of attack can cause airframe buffeting (“low speed” buffet) and we have now shown that flying too fast will also cause airframe buffeting (“high speed” buffet). ANY airframe buffeting is undesirable and can quickly lead to structural damage, besides upsetting the passengers.

It will be shown that at high cruising altitudes (36 000 to 42 000 ft), the margin between the high angle of attack stall warning and the high speed warning may be as little as 15 kt.

*VMO is the maximum operating Indicated Airspeed, MMO is the maximum operating Mach number. (These will be fully discussed in Chapter 14).

Note: It is operationally necessary to fly as fast as economically possible and designers are constantly trying to increase the maximum speed at which aircraft can fly, without experiencing any undesirable characteristics. During certification flight testing, the projected maximum speeds are investigated and maximum operating speeds are established. The maximum operating speed limit (VMO /MMO) gives a speed margin into which the aircraft can momentarily overspeed and be recovered by the pilot before any undesirable characteristics occur. (Tuck, loss of control effectiveness and several stability problems - these will all be detailed in later chapters).