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		L	INCREASED
			DECREASED
	L
		x	y
			Lt
		AC	Lt
			AC
			NEUTRAL POINT
			POSITION OF CG WHEN
			TAIL MOMENT AND
			WING MOMENT
			ARE EQUAL

Figure 10.9 Neutral point

Neutral Point

If you consider the CG moving rearwards from a position of static longitudinal stability:

•the tail arm ‘y’ will decrease and the wing arm ‘x’ will increase; consequently,

•the (negative) tail moment will decrease and the (positive) wing moment will increase,

Figure 10.9.

Eventually the CG will reach a position at which the tail moment is the same as the wing moment. If a vertical gust displaces the aircraft nose-up, the sum of the moments will be zero and there will be no angular acceleration about the CG to return the aircraft towards its original position of equilibrium.

Because there is no resultant moment, either nose-up or nose-down, the aircraft will remain in its new position of equilibrium; the aircraft will have neutral static longitudinal stability. See page 245.

The position of the CG when the sum of the changes in the tail moment and wing moment caused by the gust is zero is known as the neutral point, Figure 10.9.

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Static Margin

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We have established that with the CG on the neutral point the aircraft will have neutral static longitudinal stability, i.e. the sum changes in the wing moment and the tail moment caused by a disturbance is zero.

If the CG is positioned just forward of the neutral point, the tail moment will be slightly greater than the wing moment (arm ‘y’ increased and arm ‘x’ decreased). A vertical gust which increases the angle of attack will generate a small nose-down angular acceleration about the CG, which will gently return the aircraft towards its original position of trim (equilibrium).

The further forward the CG, the greater the nose-down angular acceleration about the CG - the greater the degree of static longitudinal stability.

		STATIC MARGIN
	L
L	x	y
		Lt

AC	L t
	AC
AFT CG
LIMIT	NEUTRAL POINT

Figure 10.10 Static margin & aft CG limit

The neutral point is an important point of reference in the study of static longitudinal stability. In practice, the CG will never be allowed to move so far aft that it reached the neutral point. The aircraft would be much too sensitive to the controls.

It has been stated that the further forward the CG is from the neutral point, the greater the static longitudinal stability. The distance the CG is forward of the neutral point will give a measure of the static longitudinal stability; this distance is called the static margin, Figure 10.10. The greater the static margin, the greater the static longitudinal stability.

A certain amount of static longitudinal stability is always required, so the aft CG limit will be positioned some distance forward of the neutral point. The distance between the neutral point and the aft CG limit gives the required minimum static stability margin.

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Stability and Control

Trim and Controllability

An aircraft is said to be trimmed (in trim) if all moments in pitch, roll, and yaw are equal to zero. The establishment of trim (equilibrium) at various conditions of flight may be accomplished by:

•pilot effort

•trim tabs

•variable incidence trimming tailplane

•moving fuel between the wing tanks and an aft located trim tank, or

•bias of a surface actuator (powered flying controls)

The term controllability refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. Adequate controllability must be available to perform take-off and landing and accomplish the various manoeuvres in flight.

A contradiction exists between stability and controllability. A high degree of stability gives reduced controllability. The relationship between static stability and controllability is demonstrated by the following four illustrations.

POSITIVE STATIC STABILITY

Figure 10.11

Degrees of static stability are illustrated by a ball placed on various surfaces. Positive static stability is shown by the ball in a trough, Figure 10.11; if the ball is displaced from equilibrium at the bottom of the trough, there is an initial tendency to return to equilibrium. If it is desired to “control” the ball and maintain it in the displaced position, a force must be supplied in the direction of displacement to balance the inherent tendency to return to equilibrium.

This same stable tendency in an aircraft resists displacement from trim equally, whether by pilot effort on the controls (stick force) or atmospheric disturbance.

10

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10 Stability and Control

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INCREASED POSITIVE

STATIC STABILITY

Figure 10.12

The effect of increased static stability (forward CG movement) on controllability is illustrated by the ball in a steeper trough, Figure 10.12. A greater force is required to “control” the ball to the same position of displacement when the static stability is increased. In this manner, a large degree of static stability tends to make the aircraft less controllable. It is necessary to achieve the proper proportion between static stability and controllability during the design of an aircraft because too much static stability (forward CG position) reduces controllability. The forward CG limit is set to ensure minimum controllability, Figure 10.13.

	STATIC MARGIN
	L
L	x	y
		Lt
AC		L t
		AC
FWD CG
LIMIT	NEUTRAL POINT
HIGH
STICK
FORCE	AFT CG
	LIMIT
	LOW
	STICK
	FORCE

Figure 10.13

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NEUTRAL STATIC STABILITY

Figure 10.14

The effect of reduced static stability on controllability is shown by the ball on a flat surface, Figure 10.14. If neutral static stability exists (CG on the neutral point), the ball may be displaced from equilibrium and there is no tendency to return. A new point of equilibrium is obtained and no force is required to maintain the displacement. As static stability approaches zero, controllability increases to infinity and the only resistance to displacement is a resistance to the motion of displacement, aerodynamic damping. For this reason, decreased static stability (aft CG movement) increases controllability. If the stability of the aircraft is too low, control deflections may create exaggerated displacements of the aircraft.

NEGATIVE STATIC STABILITY

Figure 10.15

The effect of static instability on controllability (CG aft of the neutral point) is shown in Figure 10.15 by the ball on a hill. If the ball is displaced from equilibrium at the top of the hill, the initial tendency is for the ball to continue in the displaced direction. In order to “control” the ball at this position of displacement, a force must be applied opposite to the direction of displacement.

This effect would be apparent during flight by an unstable “feel” to the aircraft. If the controls were deflected to increase the angle of attack, the aircraft would need to be ‘held’ at the higher angle of attack by a push force to keep the aircraft from continuing in the nose-up direction. The pilot would be supplying the stability by his attempt to maintain the equilibrium; this is totally unacceptable!

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