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.
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.
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
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.