Stability and Control of an Aeroplane

From Wikipedia, the free encyclopedia

A fixed-wing aircraft's stability denotes its capacity to return to a particular condition of flight after experiencing some disturbance from the original condition without any efforts from the pilot.

While the degree of stability may vary, it is a very desirable phenomena in aircraft design and building. A plane may also be said to be stable under some conditions of flight while at the same time unstable in other conditions. For instance one that is stable while in straight and level flight would be unstable while turning or while inverted.

It is not uncommon for the balance of the aircraft to be confused with its stability. The balance of the aircraft is obtained when it is well trimmed. An aeroplane may fly with one wing higher than the other (a definite out of trim condition) but retain its stability.

When perturbed, an aeroplane that is stable tends to return to its original state, conversely if unstable, it will tend to move further away from the original position. There are times when some tend to do neither of the foregoing but instead retain and assume the new position. This is referred to as neutral stability.

A plane may have distinct stability characteristics abouts its three axes, the longitudinal, lateral or directional. The degree of stability may differ according to the parameters involved, for instance in pitching; while either in "stick-fixed" or "stick free" condition and whether it is being considered "power off" or "power on". Stick fixed implies that the elevators are held neutral relative to the tail plane, whereas stick free is when the pilot allows the elevators to take up their own positions.

Control is the capacity of the pilot to manoeuver the aeroplane into any desired position.

[edit] Longitudinal Stability

This is the stability/control that is concerned with pitching about the lateral axis. For stability longitudinally, the state of affairs must be such that if the angle of attack is temporarily increased, forces will act correctively as to depress the nose decreasing the angle of attack to its previous state. An ordinary upswept wing with a cambered aerofoil section cannot be balanced or 'trimmed' to give positive lift while simultaneously being stable because a positive increase in incidence produces a nose-down pitching moment about the centre of gravity (CG).

As regards the wing therefore, this can be improved by several features namely;

• Sweepback,
• Wash out (decreasing the angle of incidence toward the wing tips)
• Change in wing section towards the tips (currently common)
• Reflex curvature towards the trailing edge of the wing section

Other factors that affects the longitudinal stability as a whole can generally be summed up thus 1) The position of the CG. It must not be too far back, a chief consideration 2) The pitching moment on the main planes. 3) The pitching moment on the fuselage. 4) The tail plane; its area, angle at which it is set, aspect ratio and distance from the CG. This is always a stabilizing influence

If the restoring moment caused by the tail plane is greater than the upsetting moment caused by the main planes and possibly the fuselage, then the aircraft will be stable.

[edit] Lateral stability

To have stability laterally, the arrangement must be such that if a slight roll occurs, the forces acting on the aeroplane tend to restore it to an even keel. Overall, while flying at a small angle of attack, there exists resistance to roll owing to the angle of attack, and so the lift, will increase on the down-going wing, and decrease on the up-going wing. But this righting effect will only last while the aeroplane is actually rolling. This also happens when the angle of attack is small; at the stalling angle, the increased angle on the falling wing may cause a decrease in lift, and the decreased angle on the other an increase; thus the new forces and state of affairs will tend to roll the aeroplane still further. This eventuates to auto-rotation The common method of acquiring lateral stability is by using a dihedral angle on the main planes This angle being the angle between each plane and the horizontal. If the planes inclined upwards towards the wing tips, the dihedral is positive, if downwards, it is negative and termed as anhedral Another factor with considerable influence on lateral stability is the position of the various side surfaces i.e fuselage, fin, rudder and wheels. These presents areas at right angles to any sideslip, so there will be pressure upon them which, if they are high above the CG will tend to restore the aeroplane to an even keel. Conversely if the side surfaces are low the pressure on them will tend to roll the aircraft over still more causing lateral instability. Whatever the method of obtaining lateral stability, correction only takes place after a sideslip towards the low wing. It is the sideslip that effects the directional stability.

[edit] Directional stability

This is almost entirely a question of the side surface or 'fin area' but it is not a question of the relative height of this side surface, but whether it is in front or beind the CG. If the turning effect of the pressures behind the CG is greater than the truning effect in front of the CG, the aeroplane will tend to its original course. it is the turning effect or the moment that matters, and not the actual pressure, verily it is not a question of how much side surface but also of the distance from the CG [centre of Gravity] of each side surface.