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
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
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
Airflow Over an Airfoil:
CL-Max
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
Traction pads and even bugs can disrupt laminar flow and affect aircraft performance
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 payload 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:
As you increase your angle of bank, stall speed increases [Figure 4]
This is because the vertical component of lift has decreased, resulting in the pilot raising the angle of attack to produce more lift until stalling (at a lower speed)
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
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
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
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
Stalls Relating to Icing:
Stalls due to icing are particularly insiduous as the indications may not be present, or are quite different
Icing causes the aircraft to stall at a lower than normal angle of attack, potentially before the pilot can recognize an abnormal condition
Icing related stalls may not have an accompanying stall horn, due to the frozen position of the indicator, or becaues aircraft will stall at a lower than normal angle of attack
Stall speed may decrease by as much as 20 knots
Pilots may experience lightness in the controls, difficulty trimming, or PIO
Certification information (if certified) can be found in FAA Notice (8900.267) Focused Review of Flightcrew Member Training for Ice-Contaminated Tailplane Stall or on the Type Certificate Data Sheet (TCDS)
Slow usually indicates wing stall whereas fast usually indicates tailstall
Stall Recovery:
Stall recoveries are fundamentally the same if you remember "Max - Relax - Level"
Apply maximum power (increases lift)
Relax the nose (decreases the AoA)
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
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, despite common discussions regarding stall "speed"
