Objective: the student will understand the aerodynamics of stalls, and conditions that produce stalls. The student will become proficient in recognizing an approaching stall, and develop the habits of prompt preventive or corrective action. ### Purpose In-flight loss of control (I-LOC) resulting from an inadvertent stall or spin is consistently one of the most significant contributors to fatal accidents in ALL types of aircraft. Preventing this type of accident is contingent upon a thorough understanding of the aerodynamics of stalls and conditions that produce stalls, as well as becoming proficient in recognizing an approaching stall, and developing the habits of prompt preventive or corrective action. ### Aerodynamics Recall the lift equation: $ L = C_L1/2ρv^2S $ The two variables that the pilot has direct control over are velocity and coefficient of lift. Coefficient of lift is a dimensionless coefficient that simply relates lift to dynamic pressure, and is determined by airfoil design and **angle of attack**. Increasing angle of attack increases coefficient of lift, but only up to a point; any further increase in angle of attack results in a **decrease** in coefficient of lift. This angle of attack is called the **critical angle of attack**, and the loss in lift experienced as a result of exceeding this critical AoA is a **stall**. ![[AoA vs Cl.png]] Coefficient of lift decreases beyond the critical AoA due to separation of airflow on the upper surface of the wing. As AoA increases, the upper surface of the wing develops a more **adverse pressure gradient**. As the wing reaches the critical AoA, the energy of the airflow on the upper surface of the wing can no longer stay attached, and separation occurs. ![[adverse pressure gradient.png]] Airflow separation begins at the trailing edge of the wing, and moves forward as the AoA increases and the stall deepens. Wing planform will influence stall progression; most aircraft have a planform design and/or **washout** so that the stall progresses from the wing root to the wing tip. ![[stall progression.png]] ![[A4NA planform vs stall progression.png]] ### Conditions associated with stalls A wing will **always** stall when the **critical angle of attack** is exceeded. How and when this happens will be dependent on a number of conditions: - Weight: increasing weight increases stall speed; at a given speed, more lift is generated by flying at a higher AoA - Center of Gravity: moving CG forward increases stall speed; more tail-down force is required for aerodynamic balance, resulting in higher wing loading and a higher AoA - Configuration: deploying spoilers/dive brakes will decrease lift and increase stall speed; deploying flaps will increase lift and decrease stall speed - [[Load Factor and Stall Speed]]: stall speed increases with the square root of the load factor. The aircraft experiences increased [[Load Factor in Turns]], so increasing bank angle increases stall speed ### Stall Recognition A stall can occur at any attitude, airspeed, or configuration; a wing will **always** stall when the **critical angle of attack** is exceeded. Unintentional stalls may occur during maneuvering flight due to increased load factor, however, this accounts for a minority of stall/spin accidents. Unintentional stalls during launching and landing operations, in the traffic pattern, and low altitude emergencies (ie [[Towline Break]]), account for the majority of accidents, so developing proficiency in recognizing and preventing stalls in these scenarios are critical. Cues of an approaching stall include: - Pitch attitude: the glider is unpowered, and cannot **sustain** flight at high pitch attitudes - Low airspeed: in unaccelerated flight, decreasing speed must be offset by increasing AoA to maintain lift; cues should be familiar from [[Maneuvering at Minimum Controllable Airspeed]], such as decreased control effectiveness, reduced volume of airflow over the canopy, etc - Buffeting: as airflow begins to separate from the wing, turbulent eddies will cause airframe buffeting Indications of a full stall: - Loss of pitch control: as the critical AoA is exceeded, lift decreases, and the pitch attitude will decrease, even as the pilot maintains aft stick pressure ### Stall Recovery If exceeding the critical angle of attack causes a stall, reducing angle of attack below the critical angle of attack will recover from a stall. Relieving the aft stick pressure, or applying forward pressure (depending on aircraft design, trim, and loading) will reduce the AoA. This will result in a decreasing pitch attitude, and usually a loss of altitude as the glider returns to normal flight. ### Execution Stalls can be practiced in straight or turning flight, in a variety of configurations (flaps/spoilers opened or closed, etc), with recovery performed at the first indication of a stall, or after a full stall has occurred - Select an altitude that will allow for the maneuver and recovery to be completed above 1,500ft AGL while remaining within gliding distance of an airport - Visually clear the area and perform clearing turns - Align the glider with a visual heading reference - Begin with a [[Glider Fundamentals of Flight#Straight Glides]] at best glide or minimum sink speed - Establish the desired bank angle and configuration for the stall - During turning stalls, [[Overbanking Tendency]] and [[Adverse Yaw]] will need to be managed to avoid a [[Slip]] or [[Skid]]. - Smoothly and progressively increase the pitch attitude of the glider to an unsustainable attitude - The target pitch attitude will vary with the specific model of glider, and the conditions associated with that flight; for dual flights in the SGS 2-33, the front seat pilot's feet should be on the horizon - Acknowledge cues at the first indication of a stall (ie buffet) - Recover at the first indication of a stall, or after a full stall has occurred, as instructed by the instructor/evaluator - Reduce angle of attack, **then** return to a wings level attitude and clean configuration - Establish a normal glide attitude and return to normal flight ### Common Errors - Improper pitch and bank control - Rough or uncoordinated controls - Failure to recognize the first indications of a stall - Failure to achieve a full stall - Improper recovery procedures - Secondary stall during recovery - Excessive altitude loss or excessive airspeed during recovery ### Completion Standards - For initial attempts: understands the aerodynamics of stalls, and conditions that produce stalls. Recognizes an approaching stall, and applies the correct series of actions to avoid or recover from a stall, with instructor assistance. - For solo: understands the aerodynamics of stalls, and conditions that produce stalls. Proficiency in stall recognition, stall entry, stalls, and stall recovery. - Sport/Private Pilot: selects an altitude that will allow for the maneuver and recovery to be completed above 1,500ft AGL; performs a stall in straight and turning flight in various configurations, recovering at the stall, maintaining up to 15° bank ±10° during turns - Commercial Pilot: selects an altitude that will allow for the maneuver and recovery to be completed above 1,500ft AGL; performs an imminent stall in straight and turning flight in various configurations, recovering at the first indication of a stall, maintaining up to 15° bank ±5° during turns ### Additional Resources [<iframe width="560" height="315" src="https://www.youtube.com/embed/aa2kBZAoXg0?si=j9dKl1EZHOT0IeN2" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>] ### References - FAA, *Glider Flying Handbook* - FAA, *Airplane Flying Handbook* - FAA, *Pilot's Handbook of Aeronautical Knowledge* - Hurt, H. *Aerodynamics for Naval Aviators* - Fidkowski, Krzysztof. *How Planes Fly* - FAA, *Practical Test Standards for Glider Category*