It is important to remember that a stall can occur at any airspeed and at any flight attitude. A stall occurs when the critical AOA is exceeded. [Figure 3-31] During a stall, the wings still support some of the aircraft’s weight. If the wings did not, it would accelerate according to Newton’s Second Law. The stall speed of a glider can be affected by many factors, including weight, load factor due to maneuvering, and environmental conditions. As the weight of the glider increases, a higher AOA is required to maintain flight at the same airspeed since more lift is required to support the increase in weight. This is why a heavily loaded glider stalls at a higher airspeed than when lightly loaded. The manner in which this weight is distributed also affects stall speed. For example, a forward CG creates a situation that requires the tail to produce a greater downforce to balance the aircraft. The result of this configuration requires the wings to produce more lift than if the CG were located further aft. Therefore, a more forward CG also increases stall speed.
Environmental factors also can affect stall speed. Snow, ice, or frost accumulation on the wing’s surface can increase the weight of the wing, in addition to changing the wing shape and disrupting the airflow, all of which increase stall speed. Turbulence is another environmental factor that can affect a glider’s stall speed. The unpredictable nature of turbulence can cause a glider to stall suddenly and abruptly at a higher airspeed than it would in stable conditions. Turbulence has a strong impact on the stall speed of a glider because the vertical gusts change the direction of the relative wind and abruptly increase the AOA. During landing in gusty conditions, it is important to increase the approach airspeed by half of the gust spread value in order to maintain a wide margin above stall. For example, if the winds were 10 knots gusting to 15 knots, it would be prudent to add 2.5 knots ((15 – 10) ÷ 2 = 2.5) to the approach speed. This practice usually ensures a safe margin to guard against stalls at very low altitudes.
If the aircraft is not stalled, it cannot spin. A spin can be defined as an aggravated stall that results in the glider descending in a helical, or corkscrew, path. A spin is a complex, uncoordinated flight maneuver in which the wings are unequally stalled. Upon entering a spin, the wing that is more completely stalled drops before the other, and the nose of the aircraft yaws in the direction of the low wing. [Figure 3-32]
The cause of a spin is stalled airflow over one wing before airflow stalling over the other wing. This is a result of uncoordinated flight with unequal airflows over the wings.
Spins occur in uncoordinated slow flight and high rate turns (overbanking for airspeed). The lack of coordination is normally caused by too much or not enough rudder control for the amount of aileron being used. If the stall recovery is not promptly initiated, the glider is likely to enter a full stall that may develop into a spin. Spins that occur as the result of uncoordinated flight usually rotate in the direction of the rudder being applied, regardless of the raised wing. When entering a slipping turn, holding opposite aileron and rudder, the resultant spin usually occurs in the direction opposite of the aileron already applied. In a skidding turn in which both aileron and rudder are applied in the same direction, rotation is also in the direction of rudder application. Glider pilots should always be aware of the type of wing forms on their aircraft and the stall characteristics of that wing in various maneuvers.
Spins are normally placed in three categories, as shown in Figure 3-33. The most common is the upright, or erect, spin, which is characterized by a slightly nose-down rolling and yawing motion in the same direction. An inverted spin involves the aircraft spinning upside down with the yaw and roll occurring in opposite directions. A third type of spin, the flat spin, is the most hazardous of all spins. In a flat spin, the glider yaws around the vertical axis at a pitch attitude nearly level with the horizon. A flat spin often has a very high rate of rotation; the recovery is difficult, and sometimes impossible. If a glider is properly loaded within its CG limits, entry into a flat spin should not occur. Erect spins and flat spins can also be inverted. The entry, wing form, and CG usually determine the type of spin resulting from an uncoordinated wing stall.
Since spins normally occur when a glider is flown in an uncoordinated manner at lower airspeeds, coordinated use of the flight controls is important. It is critical that pilots learn to recognize and recover from the first sign of a stall or spin. Entering a spin near the ground, especially during the landing pattern, is usually fatal. [Figure 3-33] A pilot must learn to recognize the warning signs, especially during the approach and landing phase in a crosswind. A crosswind resulting in a tailwind on the base leg may lead the pilot to tighten the turn using rudder, or too steep a turn for the airspeed. An uncoordinated turn could lead to the upper wing exceeding its critical AOA before the lower wing, which could result in a very high rate of roll towards the upper wing as the upper wing stalls. If an excessive steep turn is attempted, the glider may roll towards the inside wing or the outside wing depending on the exact trim state at the instant of the stall. Situational awareness of position to final approach should be part of a before-landing routine.
Ground effect is a reduction in induced drag for the same amount of lift produced. Within one wingspan above the ground, the decrease in induced drag enables the glider to fly at a lower airspeed. In ground effect, a lower AOA is required to produce the same amount of lift. Ground effect enables the glider to fly near the ground at a lower airspeed and causes the glider to float as it approaches the touchdown point.
During takeoff and landing, the ground alters the three-dimensional airflow pattern around the glider. The result is a decrease in downwash and a reduction in wingtip vortices. Upwash and downwash refer to the effect an airfoil has on the free airstream. Upwash is the deflection of the oncoming airstream upward and over the wing. Downwash is the downward deflection of the airstream as it passes over the wing and past the trailing edge.
During flight, the downwash of the airstream causes the relative wind to be inclined downward in the vicinity of the wing. This is called the average relative wind. The angle between the free airstream relative wind and the average relative wind is the induced AOA. In effect, the greater the downward deflection of the airstream, the higher the induced AOA and the higher the induced drag. Ground effect restricts the downward deflection of the airstream, decreasing both induced AOA and induced drag.
Ground effect, in addition to the decrease in wind due to surface friction and other terrain features upwind of the landing area, can greater increase the landing distance of a glider. A glider pilot, especially a visiting pilot, should inquire about local effects from local pilots to enhance flight planning and safe landings.