Types of slip vary in degree from inadvertently flying cross-controlled in the cruise i.e. one wing slightly low and compensating with opposite rudder, to a fully-fledged cross-controlled turn where the aircraft is steeply banked in a descending turn with full opposite rudder applied. All slips result in increased drag. This is a manoeuvre only for the pilot who has a very good feel for his/her aircraft because, among other things, the ASI will probably be giving a false airspeed readout. There seem to be as many definitions of the types of slip as there are exponents of slip techniques but the safe execution of all sideslips requires adequate instruction and continuing practice.
The straight sideslip approach to landing
The ‘helmet and goggles’ crowd who, very sensibly, like to fly biplanes and other aeroplanes not equipped with flaps, need a manoeuvre for use on the landing approach to a short strip which enables them to lose height quickly without increasing airspeed and which provides a good view of the landing area. The answer is the cross-controlled straight sideslip which is a manoeuvre designed to lose height over a short distance, dumping the potential energy of height by converting it to drag turbulence rather than kinetic energy. Such sideslips may also be a requirement when executing a forced landing.
Once established on the approach descent path at the correct airspeed, the aircraft is banked with sufficient opposite (top) rudder applied to stop the directional stability yawing the nose into the relative airflow and thus turning. Slight backward pressure on the control column is probably needed to keep the nose from dropping. The aircraft sideslips in a moderate to steep bank with the fuselage angled across the flight path, giving the pilot a very good view of the landing area. The greatly increased drag from the exposure of the fuselage side surfaces to the oncoming airflow enables an increased angle of descent without an increase in the approach airspeed.
The sink rate is controlled by aileron and power is held constant – usually at idle/low power and the sideslip must be halted well before the round-out and touchdown. When recovering care must be taken to coordinate the relaxation of the back pressure, the levelling of the wings and the straightening of the rudder otherwise the aircraft may stall – particularly in turbulent conditions.
The straight sideslip is limited by the maximum rudder authority available, there will be a bank angle beyond which full opposite rudder will not stop the aircraft from turning.
The sideslipping turn
This is a very useful manoeuvre if it is necessary to increase the sink rate during a turn – such as the turn onto final approach in a forced landing when an overshoot of the landing site is apparent. It is just a sideslip where the bank applied exceeds the opposite rudder applied and the aircraft enters a sideslipping turn. The rate of turn and the rate of sink are controlled by the amount of bank and the amount of rudder, very high descent rates are achieved if the bank angle applied exceeds the full rudder authority.
Fish tailing is a series of sideslips where the wings are held level in the approach attitude while the aircraft is repeatedly yawed from side to side by applying full alternate rudder. The increased drag increases the sink rate.
Sideslip to a crosswind landing
In a sideslip to a crosswind landing the aircraft is always banked with the into-wind wing down so that the sideslip can be smoothly decreased to a forward slip [see below] before the roundout. Most aircraft tend to be slower in the slip so the nose will need to be a bit lower than that needed to maintain the normal approach speed. A smoothly executed sideslip approach requires much practise but displays considerable finesse to a ground observer.