The forward slip is a flight maneuver. It is used when a pilot
wishes to increase drag on the airplane. It is typically used when landing - if the pilot finds that he has arrived on final
too high, it will be necessary to either go around or to descend sharply. Sometimes, if it is not desirable to go around (say, during an emergency landing
) or if the altitude
discrepancy is not too severe, the pilot may choose to simply increase the angle of descent in order to get back onto the proper glidepath
. The problem with this is that increasing the angle of descent will increase airspeed
- and if you simply descend sharply and then pull the nose back up to the proper angle of attack
just before touchdown, the airplane will have too much airspeed and will 'float on' rather than properly stalling to a landing. If the speed is too high, the airplane will not stall until it is too far down the runway
to safely come to a stop.
In order to counteract the increase in airspeed, the pilot can choose to enter a forward slip. To do so, he will apply rudder to one side or another along with opposite aileron. The rudder input will cause the plane to yaw to one side; however, the opposite wing will be lowered by the aileron inputs. Since the plane will always attempt to move towards the low wing, the two inputs will cancel each other out. The airplane, however, will no longer be pointed along its heading; the nose will be yawed towards the side selected by the rudder. As a result, the side of the airplane will be presented to the oncoming air, and the drag experienced will go up sharply. This will cause the airplane to descend as the efficiency of lift is reduced due to the yaw displacing the wings from their optimum angle, and thus the airplane will begin to sink, with the energy lost from altitude going into turbulence from the increased drag rather than into increased airspeed.
In short, if the pilot applies opposite inputs to rudder and aileron, the airplane will lose lift and undergo increased drag. This will cause it to sink without gaining speed.
The reason this is called a forward slip is that when done properly, the airplane will continue to follow the same ground track after it enters the slip, even though the nose is no longer pointed in the same direction; it will continue to move forward with no sideways vector.
Exiting a forward slip is simple; the pilot lifts the low wing using aileron inputs and releases the opposite rudder. However, although the steps to exiting are simple, the pilot should be experienced enough with his airplane to be able to remove these inputs simultaneously and, if necessary, add power if the airplane exits with too little energy. As always, the risk of a stall increases when the airplane's configuration is changed at low speeds, and since this maneuver is usually used during landing, it will mean little to no time or altitude to recover.
There are additional reasons a pilot might use a forward slip. Some airplanes, notably biplanes and aerobatic craft, do not have flaps. As a result, the forward slip is their best option for increasing drag on final approach. Alternatively, a pilot who is unable to clearly see the runway on final might choose to enter a forward slip in order to view the oncoming runway out of the side window(s). Their windscreen might be obstructed with oil, or plain old icing, or they might be flying a plane like the Supermarine Spitfire. High-performance piston-engined fighters in World War Two had long noses, in order to house their enormous engines, and in addition were taildraggers, landing with their noses high and a tailwheel. Slipped approaches were standard landing procedure.
NOTE: Do not use this writeup to fly your airplane. Find your instructor and ask them to train you on slips - they are part of any light aircraft training program.