Objective: the student will understand the factors related to aerodynamics, aircraft controllability, and aircraft performance at a minimum controllable airspeed. The student will apply attitude flying and aircraft control concepts learned during fundamental flight maneuvers to a new flight regime. ### Defining Minimum Controllable Airspeed For the purposes of this maneuver, "flight at a minimum controllable airspeed" is defined as a speed at which any further increase in angle of attack or [[Load Factor]] causes an immediate stall. ### Purpose Pilots learn this maneuver for several reasons: to develop proficiency operating the glider at very low speeds, to understand how aircraft controllability and performance change at very slow speeds, and to develop awareness of an approaching stall. ### Aerodynamics Recall the relationship between drag and airspeed in steady flight; parasite drag increases as airspeed increases, induced drag increases as airspeed decreases. The point of minimum drag corresponds to L/Dmax, or the best glide ratio with zero wind. ![[GFH drag vs airspeed.png]] Additionally, recall the relationship between **angle of attack** and airspeed in steady flight; as airspeed decreases, angle of attack must be increased to maintain the same amount of lift generated. By definition then, flight at a minimum controllable airspeed will involve flight at a very high angle of attack (just below the **critical angle of attack**, or stall), with a large amount of drag (induced drag) being produced. The exact speed achieved during this maneuver will vary with the specific model of glider used, as well as the conditions associated with that flight, such as weight and center of gravity. Configuration changes also influence minimum controllable airspeed; extending flaps decreases stall speed, while extending spoilers/dive brakes increases stall speed. [[Load Factor in Turns]] also increases stall speed as bank angle is increased. ### Aircraft Controllability At a minimum controllable airspeed, there is comparatively less airflow passing over the flight control surfaces. The flight controls will feel "mushy" and ineffective; there is very little resistance to displacing the flight controls, and larger movements of the flight controls are required to produce the desired response. Additionally, the natural stability of the aircraft is reduced. At speeds below L/Dmax, the glider experiences **speed instability**. If the aircraft flies slower, it produces more drag, which in turn slows it more - the speed will continue to decay unless the pilot intervenes. The pilot should also note the cues of a high angle of attack, low airspeed condition: high pitch attitude, reduced sound of airflow over the canopy, low indicated airspeed, loss of control effectiveness, and any indications of an impending stall. ### Aircraft Performance At speeds below L/Dmax, the glide ratio of the glider **decreases** as airspeed decreases. This means that the pilot cannot "stretch" the glide of the glider by increasing pitch attitude below L/Dmax speed; this can be counterintuitive, but is critical to performing a safe [[Stabilized Approach]] during landing. At speeds below **minimum sink speed**, the sink rate of the glider **increases** as airspeed decreases. This means that the pilot cannot decrease sink rate by increasing pitch attitude below minimum sink speed; again, this is counterintuitive, but is critical to effective soaring techniques. ### Execution - 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 - Smoothly and progressively increase the pitch attitude of the glider - 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 horizon should be approximately halfway up the pitot/static mast - Trim to relieve control forces once established at a minimum controllable airspeed - Perform turns as instructed by the instructor/examiner; increasing bank angle increases load factor, so minimum controllable airspeed will be increased; [[Adverse Yaw]] and [[Overbanking Tendency]] will also be increased - Configure the glider as instructed by the instructor/examiner; extending spoilers/dive brakes increases stall speed and minimum controllable airspeed, while extending flaps decreases stall speed and minimum controllable airspeed - When finished, return to a straight glide by reducing the pitch to a normal glide attitude (best glide or minimum sink), and retrim for this speed ### Common Errors - Failure to clear the area - Failure to establish or maintain minimum controllable airspeed - Attempting to control the aircraft by reference to the flight instruments, rather than the horizon and visual references - Improper use of trim - Rough or uncoordinated use of flight controls - Failure to recognize indications of a stall ### Completion Standards - For initial attempts: understands the factors related to aerodynamics, aircraft controllability, and aircraft performance at a minimum controllable airspeed. Applies attitude flying and aircraft control concepts learned during fundamental flight maneuvers to flight at minimum controllable airspeeds. Establishes and maintains the correct attitude for flight at a minimum controllable airspeed with instructor assistance. - For solo, sport, and private pilots: establishes and maintains the airspeed at which any further increase in angle of attack or load factor would result in a stall, in straight and turning flight with configurations specified by the evaluator; maintains heading ±10° during straight flight and the desired bank angle ±10° during turns - For commercial pilots: establishes and maintains the airspeed at which any further increase in angle of attack or load factor would result in a stall, in straight and turning flight with configurations specified by the evaluator; maintains heading ±10° during straight flight and the desired bank angle ±5° during turns ### References - FAA, *Glider Flying Handbook* - FAA, *Airplane Flying Handbook* - FAA, *Practical Test Standards for Glider Category*