Note: Forward slips with wing flaps extended should not  be done in gliders wherein the manufacturer’s operating  instructions prohibit such operation. 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. 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. 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. 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.  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] 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  Separation starts Separation moves forward Airfoil stalls Figure 3-31.  A stall occurs when the critical angle of attack is exceeded. 3-17 Spins Stalls Sideslip