Stall Performance

Introduction:

Geometric Twist/Washout
  • Stalls are an aerodynamic condition whereby air can no longer smoothly flow over an airfoil, resulting in a rapid loss of lift
  • A stall is, therefore, 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): the 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
  • The angle of attack is measured in arbitrary units
  • Approaching the critical angle of attack, several indications of a stall will be present, warning pilots of pending danger
  • Despite an abstract concept in abstract units, several stall speed considerations are considered in aircraft design and performance envelopes to keep aircraft inside of the normal flight envelope
  • Still, stall avoidance practices are critical
  • Of note, compressor stalls are a result of the same airfoil stall principles but are specific to turbine aircraft and generally not a factor when aircraft operations inside of their normal flight envelope

Angle of Attack:

  • Instrument Flying Handbook, Angle of Attack and Relative Wind
    Instrument Flying Handbook, Angle of Attack and Relative Wind
  • The angle of attack is fundamental to understanding many aspects of airplane performance, stability, and control
  • AoA is the acute angle measured between the relative wind or flight path and the chord of the airfoil [Figure 1]
    • 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
    • Angle of Incidence: Chord line of the wing is angled up when attached to the fuselage
    • Pitch Attitude: Angle between an airplane's longitudinal axis and the horizon
      • Equal in magnitude and opposite in direction to the flight path
      • Note that flightpath, relative wind, and angle of attack should never be inferred from pitch attitude
  • Any time the control yoke or stick moves fore or aft, the AOA is changed
    • As the AOA increases, lift increases (all other factors being equal)
    • When the aircraft reaches the maximum AOA, lift begins to diminish rapidly
      • This is the stalling AOA, known as the maximum critical lift ("CL-MAX") or the critical AOA
    • The CL increases until reaching the critical AOA, then decreases rapidly with any further increase in the AOA [Figure 2/3]
  • The lift created (or reduced in the case of negative AoA) is measured with the coefficient of lift, which relates to the AoA
  • Every airplane has a specific angle of attack where maximum lift occurs
    • This is called the critical angle of attack, at which point, regardless of airspeed, flight attitude, or weight, the airfoil will stall
  • With an understanding that angle of attack is the wing's relative "bite" out of the air, this can put airflow principles in perspective
  • Instrument Flying Handbook, Angle of Attack and Relative Wind
    Instrument Flying Handbook, Angle of Attack and Relative Wind

Airflow Over an Airfoil:

  • CL-Max
    CL-Max
  • Coefficient of Lift Curve
    Coefficient of Lift Curve
  • 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 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
    • The total distance between the aircraft surface and the free stream velocity 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 slows
    • Succeeding streamlines slow less until eventually, some outer streamline reaches the free airstream 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:

      • A Favorable Pressure Gradient (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:

      • An Adverse Pressure Gradient (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 reverses
      • 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

Indications of a Stall:

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

Stall Speed Considerations:

  • As AoA increases up to CL MAX AoA, True Airspeed (TAS) decreases to a point where it cannot be any slower than stall speed (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 as high AoA
  • 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 fewer air molecules, so a higher TAS is required; however, indicated airspeed 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 [Figure 4]
    • 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
    • Also, with the power, you will have induced airflow over the wings
  • Wing Tailoring:

    • 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:

    • Geometric Twist/Washout
      Geometric Twist/Washout
    • A decrease in angle of incidence from wing root to wingtip [Figure 1]
      • The wing gradually twists downward, decreasing its AoA
  • Aerodynamic Twist (Section Variation):

    • The 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)
  • Vortex Generators:

    • Vortex generators increase turbulent flow over the wings to delay separation
  • Stall Strips:

    • Stall Strips
      Stall Strips
    • Sharply angled piece of metal at the root section to induce a stall at the root [Figure 6]
    • Subsonic air cannot make sharp angles
    • Stall Strips
      Stall Strips
  • Flaps/Slats:

    • Lowering flaps decreases stall speed and increases drag
    • Raising flaps increases stall speed back to Vs speed while also decreasing drag
    • Consider the impacts of configuration changes (and more importantly, the stall speed) when in a low, slow, and potentially go-around situation
    • The same for flaps is true for slats, although slat deployment is generally automatic

Stall Recognition:

  • Feel:

    • The pilot will feel control pressures change as speed reduces
      • 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 aircraft can stall 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
      • This sensitivity 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

Stall Avoidance:

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

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

Common Training Aircraft Stall Warning System Characteristics:

  • Stalls, Spins, and Safety Revised Edition
    Stalls, Spins, and
    Safety Revised Edition
  • 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
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References: