A slip is a descent with one wing lowered and the glider’s longitudinal axis at an angle to the flightpath. It may be used for one or both of two purposes: to steepen the approach path without increasing the airspeed, as would be the case if a dive were used, or used to make the glider move sideways through the air to counteract the drift that results from a crosswind.
Formerly, slips were used as a normal means of controlling landing descents to short or obstructed fields, but they are now primarily used in the performance of crosswind and short-field landings. With the installation of wing flaps and effective spoilers on modern gliders, the use of slips to steepen or control the angle of descent is no longer the only procedure available. However, pilots still need skill in the performance of forward slips to correct for possible errors in judgment of the landing approach.
The shape of the glider’s wing planform can greatly affect the slip. If the glider has a rectangular wing planform, the slip has little effect on the lift production of the wing other than the wing area being obscured by the fuselage vortices. The direction of the relative wind to the wing has the same effect on both wings so no inequalities of lift form. However, if the wing is tapered or has leading edge aft sweep, then the relative wind has a large effect on the production of lift.
If a glider with tapered wings, as shown in Figure 3-14, were to begin a slip to the left with the left wing lower, the left wing will have a relative wind more aligned with its chord line and effectively higher airflow (airspeed) that generates more lift as compared to the higher right wing with angled relative wind, resulting in lower effective airflow (airspeed) over that wing. This differential in airflow or relative airspeed of the wings when taken to the extremes of the flight envelope results in the higher wing stalling and often an inverted spin.
Depending on the exact wing shape, an elliptical wing can have characteristics more like a tapered wing. [Figure 3-14] Pilots should always consult the GFM and know what the gliders limitations are concerning slips.
The use of slips has limitations. Some pilots may try to lose altitude by violent slipping, rather than by smoothly maneuvering, exercising good judgment, and using only a slight or moderate slip. In short-field landings, this erratic practice invariably leads to trouble since enough excess speed may prevent touching down anywhere near the proper point, and very often results in overshooting the entire field.
If a slip is used during the last portion of a final approach, the longitudinal axis of the glider must be aligned with the runway just prior to touchdown so that the glider touches down headed in the direction in which it is moving over the runway. This requires timely action to modify the slip and align the glider’s longitudinal axis with its direction of travel over the ground at the instant of touchdown. Failure to accomplish this imposes severe sideloads on the landing gear and imparts violent ground looping tendencies.
Discontinuing the slip is accomplished by leveling the wings and simultaneously releasing the rudder pressure, while readjusting the pitch attitude to the normal glide attitude. If the pressure on the rudder is released abruptly, the nose swings too quickly into line and the glider tends to acquire excess speed.
Because of the location of the pitot tube and static vents, airspeed indicators in some gliders may have considerable error when the glider is in a slip. The pilot must be aware of this possibility and recognize a properly performed slip by the attitude of the glider, the sound of the airflow, and the feel of the flight controls.
The forward slip is a slip in which the glider’s direction of motion is the same as before the slip was begun. [Figure 3-30] The primary purpose of a forward slip is to dissipate altitude without increasing the glider’s speed, particularly in gliders not equipped with flaps, or if the spoilers are inoperative. There are many circumstances requiring the use of forward slips, such as a landing approach over obstacles and shortfield landings, in which it is always wise to allow an extra margin of altitude for safety in the original estimate of the approach. In the latter case, if the inaccuracy of the approach is confirmed by excess altitude when nearing the boundary of the selected field, slipping can dissipate the excess altitude. If there is any crosswind, the slip is much more effective if made toward the wind.
Assuming the glider is originally in straight flight, the wing on the side toward which the slip is to be made should be lowered by use of the ailerons. Simultaneously, the airplane’s nose must be yawed in the opposite direction by applying opposite rudder so that the glider’s longitudinal axis is at an angle to its original flightpath. The degree to which the nose is yawed in the opposite direction from the bank should be such that the original ground track is maintained. The nose should also be raised as necessary to prevent the airspeed from increasing.
Note: Forward slips with wing flaps extended should not be done in gliders wherein the manufacturer’s operating instructions prohibit such operation.
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.