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Note: Forward slips with wing flaps extended should not be done in gliders wherein the manufacturer’s operating instructions prohibit such operation. 

## Sideslip 

A sideslip, as distinguished from a forward slip, is one during which the glider’s longitudinal axis remains parallel to the original flightpath, but in which the flightpath changes direction according to the steepness of the bank. To perform a sideslip, the upwind wing is lowered, and simultaneously the opposite rudder is applied to maintain the landing area alignment. The sideslip is important in counteracting wind drift during crosswind landings and is discussed in a later chapter. 

The dihedral angle of the wings works to add lateral stability to the airframe and ease the pilot’s tasking to correct for upsets. As the glider flies along, turbulence may upset the balance and raise one wing and roll the glider about the longitudinal axis. As the wing rises, the vertical lift vector decreases while the horizontal component of the wing’s lifting force increases. As the other wing descends, the lifting force vertical component increases while the horizontal component decreases. This imbalance is designed so the airframe returns to level without pilot input. Depending on the airflows, the AOA on the wings may or may not be a factor. If the air on one wing is descending (sink) and the air on the other wing is ascending (lift) both wings will have different relative winds, thus different AOAs and developed lift. 

## Stalls 

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. 

## Spins 

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] 

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