Stall

Stall

A stall is a loss of lift and increase in drag that occurs when an aircraft is flown at an angle of attack greater than the angle for maximum lift.

Objectives

  • To recognize the symptoms of an approaching stall.
  • The entry to the stall.
  • Recognition of the stall itself.
  • The correct recovery for minimum altitude loss.

Why

  • when we practice slow flight a stall can occur, and slow flight is a part of landing and take off, so it is really important to recognize the symptoms of the stalls, how to avoid it and how to recovery it as soon as possible.

Link

  • Slow flight.

Review questions

  • What action must be taken to maintain altitude, if you notice airspeed is decreasing ?
  • Explain why, when the aircraft is set up for slow flight, a higher power setting is required than for best endurance ?
  • When slow flying, why should you be particularly alert , when checking engine temperature gauge ?
  • how to place the aircraft in slow flight ?

Angle of attack

  • The angle of attack is the angle between the chord line of the wing and the relative wind (parallel to flight patch).

Stalling angle

  • An airplane can be stalled at any airspeed and in any flight attitude.
  • A stall results when the critical angle of attack is exceeded and the smooth airflow over the airplanes wing is disrupted.
  • The critical angle of attack at which an airplane stalls will be the same regardless of airspeed, weight, or configuration.

Why does a wing stall ?

  • The lift generated by a wing is dependent upon a smooth accelerated airflow over the the wing.
  • At moderate angle of attack the airflow near the trailing edge of the wing become mildly turbulent.
  • As the angle of attack increases, the turbulent air progresses forward toward the leading edge of the wing until the stalling angle is reached.
  • At that point, the downwash is and the pressure differential are greatly reduced, and a loss of lift result.
  • Due to the loss of lift and increase in drag, the remaining lift is insufficient to support the aeroplane, and the wings stalls.

The relation between lift and angle of attack for a particular aerofoil can be shown on the following diagram:

  • The curve is linear at the beginning to a certain point when it begins to fall off. Eventually, the lift force reaches a maximum and begins to decrease.
  • The angle at which it does so is called the stalling angle of attack (or critical angle of attack), and the corresponding magnitude of lift reaches its maximum.
  • A typical stalling angle is about 16 degrees.

Symptoms of the approaching stall

  • Stall warning horn.
  • Airspeed just slightly above the stall.
  • Pitch attitude and power setting.
  • Excessive stick backpressure.
  • “Mushy” controls.
  • Buffet.

Types of Stalls

  • Departure Stalls (power-on)
    • Simulate takeoff and climb out or overshoot.
  • Arrival Stalls (power-off or reduced power)
    • Simulates normal approach-to-landing.
  • Accelerated Stalls
    • Higher-than-normal speeds due to abrupt and/or excessive control applications.
Departure Stalls (power-on)
  • After leaving the ground and accelerating to climbing airspeed, the aircraft passes through a period of low airspeed and low altitude. Any abrupt pull-up or reduction in engine power could cause the aircraft to stall.
  • Departure stalls will have more buffeting, and a sharper break when the plane stalls.
  • The elevator may be in the full up position when the plane stalls.
  • Airspeed may be below the green arc, most likely due to the installation error of the pitot static system.
  • Stall recover will require a more pronounced release of back pressure, and larger pitch change.
Acceleration stalls
  • At the same gross weight, configuration, and power setting, an aircraft will consistently stall at the same indicated airspeed, provided no additional load factor is incurred by a manoeuvre or abrupt use of the controls.
  • The aircraft will, however, stall at a higher airspeed when manoeuvring loads are imposed by sudden turns, pull-up, or abrupt changes in its flight patch.
  • Stalls entered from such flight situations are called acceleration stalls.

Stall: Effect of flaps, thrust and weight

Flaps:

  • Lowering of flaps increases lift (wing shape).
  • Lower stall speed
  • Nose attitude is lower at stall.
  • Stall is more abrupt.

Thrust:

  • Because of the additional upward thrust and other lift contributing factors of a power-on stall, the stalling speed will be lower than with power-off.

Weight:

  • More speed required to gain extra lift.
  • Stall speed is proportional with the aircraft weight. Stall speed increases, as the weight increases, and decreases as the weight decreases.

Stall entry

  • Hasel check
  • Look out (90 degrees right/left then return to original heading or 180 degrees only)
  • Cut power (power off stall) or apply chosen power setting (power on stall, check POH).
  • When the speed is in the white arc of the airspeed indicator down the flap if necessary (depends on the type of stall chose).
  • Back pressure to maintain altitude.
  • Maintain aileron neutral with yoke.
  • Control the yaw and roll with rudder.
  • When the Stall warning horn full back pressure and stall.
Power on entry:
  • Because of the additional thrust the nose must be raised higher to accomplish a stall.
  • Yaw due to slipstream and asymmetric thrust will make directional control more difficult.
Power-on Stall.

Recognition of Stalls

  • Low Airspeed.
  • High angle of attack.
  • Loss off altitude with back pressure applied.
  • Stall horn.
  • Mushy controls.
  • Buffeting.

Stall Recovery

  • Push forward to decrease angle of attack
  • Full power
  • Carburetor off
  • Use rudder to control direction and keep aileron neutral
  • Leave the plane accelerate above stall speed.
  • Raise the nose to come back at straight-and-level flight

Turning Stall

Level and Descending turns:
  • The lower wing (inside wing) will stall first.  
  • The lower wing meet the relative airflow at a greater angle of attack.
  • Moves slower.
  • Produces less lift.
  • Be careful when you turn from base to final.
Climbing turns:
  • The higher wing (outside wing) will stall first.
  • The higher wing meet the relative airflow at a greater angle of attack.
  • Moves faster.
  • produces more lift.

Review questions

  • What happen when the critical angle of attack is exceed ?
  • When setting up for a stall entry, why is it necessary to maintain wing level and the ball centered ?
  • What speed are you at just prior to stalling the aircraft ?
  • What are the symptom of imminent stall ?
  • What is the entry procedure for a power off stall ?
  • How do we recover from any stall ?