What is directional stability?

Directional stability is the tendency for an aircraft to regain its direction (heading) after the aircraft has been directionally disturbed (e.g., an induced yaw) from its straight path. This is achieved naturally because the fin (vertical tailplane) becomes presented to the airflow at a greater angle of incidence, which generates a restoring aerodynamic force.

What is spiral stability and instability?

Spiral stability (or a spirally stable aircraft) is defined as the tendency of an aircraft in a properly coordinated banked turn to return to a laterally level flight attitude on release of the ailerons. Spirally stable aircraft have dominant lateral surfaces (e.g., wings). Spiral instability or a spirally unstable aircraft will see a banked turn increase fairly quickly, followed by the nose falling into the turn, leading to the aircraft entering into a spiral dive when the ailerons are released in a coordinated turn. Spirally unstable aircraft have dominant (too large) vertical surfaces (e.g., tailplane). What happens is that as the aircraft starts to slip into the turn on release of the ailerons and before the rolling moment due to the sideslip can take effect, the rather dominant fin jumps into play. This is so because the fin/tailplane area (outside) becomes exposed to the relative airflow, which exerts two forces on the aircraft:

1. Around the vertical axis, which straightens the aircraft directionally

2. Around the longitudinal axis, which increases the bank

This accelerates the outer (upper) wing and causes the bank to be increased further. The increased bank causes another slip, which the fin again straightens. This sequence repeats, and the turn is thus made steeper. Once the bank angle exceeds a given type-specific amount (say, 30°), the nose falls into the turn, the speed increases as the roll increases, and the aircraft enters into a spiral dive.

What is lateral stability?

Lateral stability is the tendency for an aircraft to return to a laterally level position around the longitudinal axis on release of the ailerons in a sideslip. There are two principal features that make an aircraft naturally laterally stable, namely,

1. Wing dihedral. The airflow due to a sideslip causes an increase in the angle of attack (lift) on the lower (leading) wing and a decrease in angle of attack on the raised wing because of the dihedral angle. The lower wing thus produces an increase in lift because of the increased angle of attack, and the raised wing produces less lift. The difference in lift causes a rolling moment that tends to restore the wing to its laterally level position.

2. Side loads produced on the keel surface. When the aircraft is sideslipping, a side load will be produced on the keel surface, particularly the fin. This side load will produce a moment to roll the aircraft laterally level, which in general terms is stabilizing. The magnitude of this effect depends on the size of the fin, but regardless, its effect is small compared with other laterally stabilizing effects.

What is longitudinal stability?

Longitudinal stability is an aircraft’s natural ability to return to a stable pitch position around its lateral axis after a disturbance. When an aircraft is in equilibrium, the tailplane in general will be producing an up or down load to balance the moments about the center of gravity. (It is assumed that throughout the elevator remains in its original position during any disturbance in pitch.) If the aircraft is disturbed in pitch (say, nose-up), there will be a temporary increase in the angle of attack. The increase in tailplane angle of attack produces an increase in tailplane lift, which will cause a nose-down pitching moment. (The tailplane is thus able to produce a stabilizing moment due to a displacement in pitch as long as the center of gravity remains within its limits.) The wings also experience this increase in angle of attack, resulting in the wings producing an increase in lift. The moment and the direction of the moment produced by this lift will depend on the relative positions of the center of pressure and the center of gravity.

Stability at high altitudes

Longitudinal, lateral, directional, and oscillatory stability in general are reduced at high altitudes, in terms of dynamic stability, mainly because aerodynamic damping decreases with altitude. The aircraft will feel and is less stable except for spiral stability, which improves with altitude, whereas oscillatory stability deteriorates very rapidly with altitude. This is so because for a constant indicated airspeed (IAS), the fin suffers a smaller angle of incidence and therefore has a smaller restoring force the higher the altitude. Therefore, the fin is less dominant, which is detrimental to oscillatory stability but as a consequence means that the aircraft’s lateral surfaces (wings) become more dominant. This improves the aircraft’s spiral stability qualities (spiral stability always opposes oscillatory stability, and vice versa).

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