design features

If an aircraft is to be stable, it is obvious from the previous paragraphs that if the aircraft has been momentarily displaced relative to its flight path, there must be a restoring force or moment to return it to its original altitude. Recalling that a moment is the product of force and distance, we then deduce that an

aerodynamic force must be generated at some distance from the aircraft's centre of gravity (about which the aircraft has been displaced / rotated).

Displacements about all three axes must be considered.

The easiest one to consider is displacement (yaw) about the normal axis. The

will again 'heads' towards the relative airflow - just like a weathercock heads into wind).

The fin gives an aircraft directional stability (about the normal axis).

The manner in which the tailplane (horizontal stabiliser) acts is similar in principle but somewhat more complicated in detail. The diagram below shows the aircraft displaced in the pitching plane. Now two aerofoils are involved, the mainplane and tailplane.

The mainplane angle of attack increases, and as drawn, this creates more lift and a forward movement of the centre of pressure. This creates an upsetting moment tending to destabilise the aircraft. (A tail-less aircraft is therefore inherently unstable).

The tailplane also generates lift so as to create a restoring moment. For the aircraft to be statically stable, clearly the restoring moment must be greater than the upsetting moment. By comparing these moments, it becomes clear how important the position of the centre of gravity becomes.

As the centre of gravity moves aft, the aircraft becomes less stable, due to the changing distances and the effect on the moments.

As the centre of gravity moves forward, the aircraft becomes more stable.

The tailplane gives an aircraft longitudinal stability (about the lateral axis).

If an aircraft has 'dropped' a wing, it should be obvious from the preceding paragraphs that a moment to raise that wing is required. But how is this to be achieved? Consider the first diagram. An aircraft that has 'dropped' a wing will side-slip towards that wing because of the imbalance of the two forces which has resulted. It is the change in aerodynamic forces resulting from this side-slipping motion which will create a restoring moment.

The most common design feature employed to promote lateral stability is the introduction of dihedral. The diagram indicates the angle concerned. Dihedral results in the 'dropped' wing meeting the revised relative airflow (due to side-slip) at a greater angle of attack than the upper wing. The net effect is therefore to create a restoring moment which is tending to roll the aircraft back towards straight and level (at which point the side-slip stops and the restoring moment becomes zero).

The next diagram shows the effect of the 'keel' area above the centre of gravity.

This will also 'right' the aircraft (similar to a yacht-keel). Note that if the keel-area is mostly aft of the centre of gravity, then an additional effect is to yaw the aircraft towards the dropped-wing.

In later studies, it will be appreciated that designers employ swept-wings to allow flight at high speeds. But an added bonus is that swept-wings encourage lateral stability. Consider the diagrams. In the first, the aircraft is flying straight and level.

The relative airflow meets both left and right leading edges at the same angle.

(The RAF is then shown as two components - one normal and one parallel to the leading edges).

In the second diagram, the aircraft has dropped the left wing and is side-slipping.

Due to the angle of sweep-back, the RAF now meets the leading-edges at different angles, and now has different components in respect of each wing. It will be recalled that it is the chordwise (or normal) component that creates lift and reference to the diagram shows that greater chordwise component occurring over the dropped-wing will therefore generate more lift, so as to create a rolling moment that restores the aircraft to (straight) and level flight.

Another feature which results in enhanced lateral stability is that of a high-(mounted) wing. The designer has probably employed a high-wing because of the intended role for the aircraft but with the centre of pressure above the centre of gravity, there is an inherent 'righting' effect, in the manner of a pendulum.

Several design features have been considered which result in lateral stability.

But an aircraft that is very stable will be unresponsive to control movements.

Stability requirements have to complement control requirements. An aircraft that has excessive stability is as undesirable as one that lacks stability. The right 'balance' between stability and control is often dictated by the intended role of the aircraft. An aircraft that possessed all the features described would probably be too stable. So a swept-wing, high-wing aircraft might incorporate anhedral (the opposite to dihedral) in order to reduce the degree of stability.

The above paragraphs have analysed features which create a moment so as to restore the aircraft towards its undisturbed or original position. They contribute static stability. Dynamic stability in the manner in which the aircraft moves or oscillates towards / about that position. This will depend on the variation of the forces in respect of displacement / time and is too complex for this module.