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Stalls

Introduction:

  • The Pilot Handbook of Aeronautical Knowledge defines a stall as "a rapid decrease in lift caused by the separation of airflow from the wing’s surface, brought on by exceeding the critical angle of attack"
  • A stall is an aerodynamic condition in which the Angle of Attack (AoA) becomes so steep that air can no longer flow smoothly over the airfoil
  • Said another way, a stall is a condition of flight in which an increase in AoA results in a decrease in lift
    • Angle of Attack (AoA/Alpha;): angle between the relative wind and the chord-line of an airfoil
    • Critical AoA: the angle of attack whereby any further increase will result in a separation of airflow which results in a stall
  • Upon airflow separation from the wing the airfoil will no longer produce lift
  • Angle of attacks are usually measured as arbitrary units
CL-Max
CL-Max
Coefficient of Lift Curve
Coefficient of Lift Curve

Airflow:

  • To better understand how air flows over a wing, you must first understand its characteristics
  • Airflow can either be laminar or turbulent and referenced to in layers
  • Boundary Layer:

    • The boundary layer is defined as the layer of airflow over a surface that demonstrates local airflow retarding due to viscosity (as it gives up kinetic energy to friction)
    • The air molecules in the boundary (surface) layer have zero velocity in relation to the surface; however, the layer just above moves over the stagnant molecules below because it is pulled along by a third layer close to the free stream of air
    • The velocities of the layers increase as the distance from the surface increases until free stream velocity is reached, but all are affected by the free stream
    • The distance (total) between the skin surface and where free stream velocity is reached is called the boundary layer
      • At subsonic levels the cumulative layers are about the thickness of a playing card, increasing in thickness as it moves aft
    • When air flows across any surface, friction develops
    • As a viscous fluid resists flow or shearing, the adjacent layer of air is also slowed
    • Succeeding streamlines are slowed less, until eventually, some outer streamline reaches the free air stream velocity
    • Laminar Flow:

      • The air moves smoothly along in streamline
    • Turbulent Flow:

      • Streamlines that break up causing the flow to be disorganized and irregular
      • Produces higher friction than laminar
      • Adheres better to the surface of the airflow, delaying separation
  • Pressure Gradients:

    • Favorable Pressure Gradient (FPG):

      • A FPG assists the boundary layer in adhering to the surface by maintaining its high kinetic energy
      • As air flows aft from the point of maximum thickness toward the trailing edge (low to high static pressure), it encounters adverse pressure gradient
    • Adverse Pressure Gradient (APG):

      • An APG impedes the flow of the boundary layer
      • Strongest during high lift conditions and at high AoAs in particular
      • If the boundary layer does not have sufficient kinetic energy to overcome the APG, then the lower levels of the boundary layer will stagnate and separate as airflow is reversed
      • As separation moves forward, the net suction decreases and CL decreases, resulting in a stall
      • Even at low angles of attack, there will be a small APG behind the point of maximum thickness
        • As the separation moves forward with increasing AoA, eventually the air cannot conform to the sharp turn

Definitions:

  • Flight Path: path described by its center of gravity as it moves through an air mass
  • Relative Wind: airflow the airplane experiences as it moves through the air
    • Equal in magnitude and opposite in direction to the flight path
  • Angle of Attack: the angle between the chord of an airfoil and the direction of the surrounding undisturbed flow
    • Note that Flight path, relative wind, and angle of attack should never be inferred from pitch attitude
  • Angle of Incidence: Chord line of the wing is angled up when attached to fuselage
  • Pitch Attitude: angle between an airplane's longitudinal axis and the horizon
  • Critical Angle of Attack: stalls occur at the same angle of attack regardless of airspeed, flight attitude, or weight

Indications of a Stall:

  • Rudder pedal shakers
  • Stick shakers
  • Horns
  • Buzzers
  • Warning lights

Stall Speed:

  • As AoA increases up to CLMAX AoA, TAS decreases to a point where it cannot be any slower (Vs)
  • Airspeeds may change based on weight and configuration, but units of AoA remains the same
    • You can stall at any airspeed
    • Going too slow causes high AoA while going too fast causes shock waves on aircraft not designed for supersonic or even transonic flight, causing the same disruption has high AoA
  • Stall Speed Considerations:

    • Weight:

      • As weight decreases, so does stall speed, due to less lift required
      • Dropping a bomb or just using fuel decreases stall speed and thus, approach speed (AoA approaches)
    • Altitude:

      • Higher altitude results in less air molecules, so a higher TAS is required however, IAS remains the same
      • Increase altitude results in increased stall speed
    • Angle of Bank vs. Stall Speed
      Angle of Bank vs. Stall Speed
    • Angle of Bank:

      • As you increase your angle of bank, stall speed increases
      • See turns
    • Power Off/On:

      • Power on conditions will have lower stall speeds as the aircraft is supported partially by the vertical component of thrust
      • In addition, with power you will have induced airflow over the wings
    • Wing Tailoring:

      • Makes stalling characteristics more predictable by attempting to stall the root first
      • Power-on stalls may tend to stall at the tip first, due to induced lift
      • With the wings stalling at the root first, the aircraft maintains some aileron authority
    • Geometric Twist/Washout:

      • A decrease in angle of incidence from wing root to wingtip
        • The wing is gradually twisted downward, decreasing its AoA
    • Geometric Twist/Washout
      Geometric Twist/Washout
    • Aerodynamic Twist (Section Variation):

      • Gradual change in airfoil shape accomplished by a decrease in camber from root to tip and/or reducing the chord
    • Stall Fences:

      • Redirect the airflow along the chord
      • Allows the wing to achieve a higher AoA without stalling (delaying tip stall)
    • Stall Strip:

      • Sharply angled piece of metal at the root section to induce a stall at the root
      • Stall Strips
        Stall Strips
        Stall Strips
        Stall Strips
      • Subsonic air cannot make sharp angles
    • Flaps:

      • Lowering flaps decreases stall speed and increases drag
      • Raising flaps increases stall speed back to Vs speed while also decreasing drag
      • Consideration must be given to adjusting flaps (and more importantly, the stall speed) when in a low, slow, and potentially go-around situation

Stall Recognition:

  • Feel:

    • The pilot will feel control pressures change as speed is reduced
      • With progressively less resistance on the control surfaces, the pilot must use larger control movements to get the desired airplane response
      • The pilot will notice the airplane’s reaction time to control movement increases
      • Just before the stall occurs, buffeting, uncommanded rolling, or vibrations may begin to occur
  • Vision:

    • Since the airplane can be stalled in any attitude, vision is not a foolproof indicator of an impending stall
    • However, maintaining pitch awareness is important
  • Hearing:

    • As speed decreases, the pilot should notice a change in sound made by the air flowing along the airplane structure
  • Kinesthesia:

    • The physical sensation (sometimes referred to as "seat of the pants" sensations) of changes in direction or speed is an important indicator to the trained and experienced pilot in visual flight
    • If this sensitivity is properly developed, it can warn the pilot of an impending stall
  • Stall Recovery:
    • Stall recoveries are fundamentally the same if you remember "Max - Relax - Level"
      1. Apply maximum power (increases lift)
      2. Relax the nose (decreases the AoA)
      3. Level the wings (reduces the stall velocity to allow all available lift to break the descent

Compressor Stalls:

  • Compressor stalls, while related in their cause, have nothing to do with the wing
  • To learn more about compressor stalls, visit the Powerplant page

Stall Avoidance

  • Avoid flying at minimum airspeeds
  • Remain in the normal flight envelope
  • Avoid abrupt maneuvers

Common Training Aircraft Stall Warning System Characteristics:

  • Piper Arrow:

    • Activated by a lift detector on the left wing
    • Activates 5 to 10 knots before stall
    • Warning horn sounds at 90Hz

Conclusion:

  • There is no question why it is important to understand stalls
  • Stalls are dependent on AoA only and the only way to recover is to reduce the AoA
  • A stall can occur at any pitch attitude or airspeed
  • To learn more check out Stalls, Spins, and Safety
  • To learn more about stalls and airflow check out NASA's FOILSIM III below

References: