The aircraft's response to momentary disturbance is associated with its
inherent degree of stability built in by the designer, in each of the three axes,
and occurring without any reaction from the pilot.
There is another condition affecting flight, which 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.
Free flight models generally have to rely on the state of trim built in by the
designer and adjusted by the rigger, while the remote controlled models have
some form of trim devices which are adjustable during the flight.
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).
Lateral and directional stabilities are inter-dependent.
Stability may be defined as follows:
- Positive stability: tends to return to original condition after a disturbance.
- Negative stability: tends to increase the disturbance.
- Neutral stability: remains at the new condition.
- Static stability: refers to the aircraft's initial response to a disturbance.
A statically unstable aircraft will uniformly depart from a condition of equilibrium.
- Dynamic stability: refers to the aircraft's ability to damp out oscillations, which
depends on how fast or how slow it responds to a disturbance.
A dynamically unstable aircraft will (after a disturbance) start oscillating with
increasing amplitude.
A dynamically neutrally stable aircraft will continue oscillating after a disturbance
but the amplitude of the oscillations will not change.
So, a statically stable aircraft may be dynamically unstable.
Dynamic instability may be prevented by an even distribution of weight inside the
fuselage, avoiding too much weight concentration at the extremities or at the CG.
Also, control surfaces' max throws may affect the flight stability, since a too much
control throw may cause instability, e.g. Pilot Induced Oscillations (PIO).
Static stability is proportional to the stabilizer area and the tail moment.
You get double static stability if you double the tail area or double the tail moment.
Dynamic stability is also proportional to the stabilizer area but increases with the
square of the tail moment, which means that you get four times the dynamic stability
if you double the tail arm length.
However, making the tail arm longer or encreasing the stabilizer area will move
the mass of the aircraft towards the rear, which may also mean the need to make
the nose longer in order to minimize the weight required to balance the aircraft...
A totally stable aircraft will return, more or less immediately, to its trimmed state
without pilot intervention.
However, such an aircraft is rare and not much desirable. We usually want an
aircraft just to be reasonably stable so it is easy to fly.
If it is too stable, it tends to be sluggish in manoeuvring, exhibiting too slow
response on the controls.
Too much instability is also 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, such as a
supersonic air superiority fighter.
Lateral stability is achieved through dihedral, sweepback, keel effect and
proper distribution of weight.
The dihedral angle is the angle that each wing makes with the horizontal (see
Wing Geometry).
If a disturbance causes one wing to drop, the lower wing will receive more lift
and the aircraft will roll back into the horizontal level.
A sweptback wing is one in which the leading edge slopes backward.
When a disturbance causes an aircraft with sweepback to slip or drop a wing,
the low wing presents its leading edge at an angle more perpendicular to the
relative airflow. As a result, the low wing acquires more lift and rises, restoring
the aircraft to its original flight attitude.
The keel effect occurs with high wing aircraft. These are laterally stable simply
because the wings are attached in a high position on the fuselage, making the
fuselage behave like a keel.
When the aircraft is disturbed and one wing dips, the fuselage weight acts like
a pendulum returning the aircraft to the horizontal level.
The tail fin determines the directional stability.
If a gust of wind strikes the aircraft from the right it will be in a slip and the fin
will get an angle of attack causing the aircraft to yaw until the slip is eliminated.
Longitudinal stability depends on the location of the center of gravity, the
stabilizer area and how far the stabilizer is placed from the main wing.
Most aircraft would be completely unstable without the horizontal stabilizer.
Non-symmetrical cambered airfoils have a higher lift coefficient, but they also
have a negative pitching moment (Cm) tending to pitch nose-down, and thus
being statically unstable, which requires the counter moment produced by the
horizontal stabilizer to get adequate longitudinal stability.
The stabilizer provides the same function in longitudinal stability as the fin does
in directional stability.
