The aircraft's response to
disturbance is associated with the inherent degree of stability
built in by the designer, in each of the three axes; and eventuating
without any reaction from the pilot. Another condition affecting flight is
the aircraft's state of trim – or equilibrium where the net sum of
all forces equals zero. Some aircraft can be trimmed by the pilot to fly
'hands off' for straight and level flight, for climb or for descent. If
the trim is wrong, and perhaps it flies with one wing low, inherent
stability will maintain that wing-low attitude and not restore the
aircraft to a wings-level attitude. It is desirable that longitudinal trim
doesn't change significantly with alterations in power, nor does
directional trim change significantly with alterations in airspeed.
An aircraft's stability is expressed in relation to each axis: lateral
stability – stability in roll, directional stability –
stability in yaw and longitudinal stability – stability
in pitch. The latter is the most important stability characteristic.
Lateral and directional stability are inter-dependent.
Degrees of stability
An aircraft will have
differing degrees of stability around each axis; here are a few examples:
A totally stable
aircraft will return, more or less immediately, to its trimmed state
without pilot intervention; however such an aircraft is rare – and
undesirable. We usually want an aircraft just to be reasonably stable so
it is easy to fly: if it is too stable they tend to be sluggish in
manoeuvring and heavy on the controls. If it tends toward instability
the pilot has to continually watch the aircraft's attitude and make the
restoring inputs, which becomes tiring, particularly when flying by
instruments. Some forms of instability make an aircraft unpleasant to
fly in bumpy weather.
The normally or
positively stable aircraft, when disturbed from its trimmed flight
state will – without pilot intervention – commence an initial movement
back towards the trimmed flight state but over-run it, then start a
series of diminishing damping oscillations about the original flight
state. This damping process is usually referred to as dynamic
stability and the initial movement back towards the flight state is
called static stability. The magnitude of the oscillation and the
time taken for the oscillations to completely damp out is another aspect
of stability. Unfortunately a statically stable aircraft can be
dynamically unstable in that plane i.e. the oscillations do not damp
dynamically stable aircraft will continue oscillating after
disturbance but the magnitude of those oscillations will neither
diminish nor increase. If these were oscillations in pitch the aircraft
will just continue 'porpoising' – if there were no other disturbances
and the pilot did not intervene.
The negatively stable
or fully unstable aircraft may be statically unstable and never
attempt to return towards the trimmed state. Or it can be statically
stable but dynamically unstable, where it will continue oscillating
after disturbance with the magnitude of those oscillations getting
larger and larger. Significant instability is an undesirable
characteristic, except where an extremely manoeuvrable aircraft is
needed and the instability can be continually corrected by on-board
'fly-by-wire' computers rather than the pilot – for example, a
supersonic air superiority fighter. The best piston-engined WW2 day
fighters were generally designed to be just stable longitudinally,
neutrally stable laterally and positively stable directionally.
Longitudinal stability is
associated with the restoration of aoa to the trimmed aoa after a
disturbance changes it.
Angle of incidence
Angle of incidence is a
term which is sometimes confusingly used as being synonymous with wing
angle of attack, however the former cannot be altered in flight. Angle of
incidence, usually just expressed as incidence, is within the
province of the aircraft designer who calculates the wing aoa to be
employed in the main role for which the aircraft is being designed,
probably the aoa in performance cruise mode. The designer might then plan
the fuselage wing mounting so that the fuselage is aligned to produce the
least drag when the wing is flying at the cruise aoa. Wings which
incorporate washout will have differing angles of incidence at the wing
root and at the outer section.
A notional horizontal datum line is drawn longitudinally through the
fuselage and the angle between that fuselage reference line (FRL)
and the wing chord line is the angle of incidence. Incidence should be
viewed as the mounting angle of the fuselage rather than the mounting
angle of the wings.
Incidence may also be called the 'rigger's incidence' or some similar
expression carried over from the earlier days of aviation.
An angle of incidence is
also calculated for the horizontal stabiliser with reference to the FRL
and the angular difference between wing and stabiliser angles is called
the longitudinal dihedral, although it is probably more correct to
say that the longitudinal dihedral is the angular difference between the
two surfaces at their zero lift aoa. Incidentally the angle of the line
of thrust is also expressed relative to the FRL.
It is the longitudinal dihedral, combined with the horizontal stabiliser
area and moment arm, which provides the restoring moment to return aoa to
the trimmed state. However bear in mind that the moment arm, which
supplies the restoring leverage and thus the stability, is affected by the
cg position and if the cg lies outside its limits the aircraft will be
When flying with level wings, at a particular weight, each aoa is
associated with a particular IAS. We might as well take advantage of that
by arranging the longitudinal dihedral so that the built-in state of trim
produces a particular indicated airspeed.
Directional stability is
associated with the realigning of the longitudinal axis with the flight
path (the angle of zero slip) after a disturbance causes the aircraft to
yaw out of alignment and produce slip; remember yaw is a rotation about
the normal (vertical) axis. The restoring moment – the static stability –
provided by the fin is the product of the fin area and the moment arm and
the moment arm leverage will vary according to the cg position – the
The area required for the fin has some dependency on the net sum of all
the restoring moments associated with the aircraft fuselage and
undercarriage side surfaces fore (negative moments) and aft (positive
moments) of the cg. Some aircraft have ventral or dorsal fins added to
increase their directional stability.
The areas of side surface above and below the cg also affect other aspects
The similar term 'weathercocking' refers to
the action of an aircraft, moving on the ground, attempting to swing into
wind. It is brought about by the pressure of the wind on the rear keel
surfaces, fin and rudder causing the aeroplane to pivot about one or both
of its main wheels. It is usually more apparent in tailwheel aircraft
because of the longer moment arm between the fin and the main wheels:
although if a nosewheel aircraft is 'wheelbarrowing' with much of the
weight on the nose wheel, then there will be a very long moment arm
between the nose wheel pivot point and the fin.
Lateral stability refers
to roll stability about the longitudinal axis and ailerons provide the
means whereby the aircraft is rolled in the lateral plane. However, unlike
the longitudinal and normal planes where the horizontal and vertical
stabilisers provide the restoring moments necessary for pitch and yaw
stability, no similar restoring moment device exists in the lateral plane.
But let's imagine that some atmospheric disturbance has prompted the
aircraft to roll to the left, thus the left wingtip will be moving forward
and down, the right wingtip will be moving forward and up. Now think about
the aoa for each wing – the wing that is moving down will be meeting a
relative airflow coming from forward and below and consequently has a
greater aoa than the rising wing. A greater aoa, with the same airspeed,
means more lift generated on the downgoing side and thus the left wing
will stop going further down or perhaps even rise and return to a wings
level state. This damping of the roll is known as lateral damping.
So roll stability, except at or very close to the stall, is intrinsic to
practically all single-engined light aircraft. (When the aircraft is
flying close to the stall the aoa of the downgoing wing could exceed the
critical aoa and thus stall, which will exacerbate the wing drop and might
lead to an incipient spin condition.
But, and there always seems to be a 'but', when the aircraft is banked
other forces come into play and affect the process. If you re-examine the
turn forces diagram in the manoeuvring forces module you will see that
when an aircraft is banked the lift vector has a substantial sideways
component, in fact for bank angles above 45° that sideways force is
greater than weight. So we can say that any time the aircraft is banked,
with the rudder and elevators in the neutral position, an additional force
will initiate a movement in the direction of bank i.e. creating a slip.
The aircraft's directional stability will then yaw the nose to negate the
slip and the yaw initiates a turn, which will continue as long as the same
bank angle is maintained.
There are several design features that stop that slip and level the wings
thus promoting lateral stability, for instance placing the wing as high as
possible above the cg promotes 'pendulum' stability; the feature usually
employed with low wing monoplanes is wing dihedral, where the wings
are tilted up from the wing root a few degrees. Another design method is
anhedral where the wings are angled down from the wing root, but it
is unlikely to be used in light aircraft.
An aircraft with positive
spiral stability tends to roll out of a turn by itself if the
controls are centred. Some light aircraft with little or no wing dihedral
and a large fin tend to have strong static directional stability but are
not so stable laterally. If a sideslip is introduced by turbulence – and
left to their own devices – such aircraft will gradually start to bank and
turn, with increasing slip and hence increasing turn rate and rapid
increase in height loss. The condition is spiral instability and
the process is spiral divergence which, if allowed to continue and
given sufficient height, will turn into a high speed spiral dive. Neutral
spiral stability is the usual aim of the designer.
It is evident that directional stability and lateral stability are coupled
and to produce a balanced turn, i.e. with no slip or skid, the
aileron, rudder and elevator control movements and pressures must be
balanced and co-ordinated.