10 Stability and Control

Manoeuvre Stability

Control and Stability 10

When the pilot pitches the aircraft, it rotates about the CG and the tailplane is subject to a pitching velocity, in this example, downwards. Due to the pitching velocity in manoeuvring flight, the longitudinal stability of the aeroplane is slightly greater than in steady flight conditions.

CHANGE IN TAIL LIFT

TAS

RELATIVE AIRFLOW

FROM ANGULAR ROTATION

PITCHING VELOCITY

INCREASE IN TAIL ANGLE OF

ATTACK DUE TO PITCHING

VELOCITY

Figure 10.37 Aerodynamic damping

Figure 10.37 shows that the tailplane experiences an upwards component of airflow due to its downwards pitching velocity. The vector addition of this vertical component to the TAS provides an increase in effective angle of attack of the tail, which creates an increase in tail lift, opposing the nose-up pitch displacement.

Since the negative pitching moment opposes the nose-up pitch displacement but is due to the nose-up pitching motion, the effect is a damping in pitch (aerodynamic damping).

It can be seen that an increase in TAS, for a given pitching velocity, decreases the angle of attack due to pitching velocity.

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Stability and Control 10

			MANOEUVRE MARGIN
		L
	L	x		y
				Lt
	AC			Lt
				AC
FWD CG
LIMIT			NEUTRAL POINT
HIGH				10
STICK
FORCE		AFT CG		Control
		LIMIT	MANOEUVRE POINT
		LOW		and
		STICK
				Stability
		FORCE
			Figure 10.38 Manoeuvre point

The pitching moment from aerodynamic damping will give greater stability in manoeuvres than is apparent in steady flight. The CG position when the tail moment would be the same as the wing moment during manoeuvring is known as the manoeuvre point, and this “neutral point” will be further aft than for 1g flight, as shown in Figure 10.38.

In most cases the manoeuvre point will not be a critical item; if the aeroplane demonstrates static stability in 1g flight, it will definitely have stability in manoeuvring flight.

Stick Force Per ‘g’

The most direct appreciation of the manoeuvring stability of an aeroplane is obtained from a plot of stick force versus load factor such as shown in Figure 10.39. The aeroplane with positive manoeuvring stability should demonstrate a steady increase in stick force with increase in load factor or “g”. The manoeuvring stick force gradient - or stick force per “g” - must be positive but should be of the proper magnitude. The stick force gradient must not be excessively high or the aeroplane will be difficult and tiring to manoeuvre. Also, the stick force gradient must not be too low or the aeroplane may be overstressed inadvertently when light control forces exist.

INCREASING ALTITUDE AT A CONSTANT IAS

DECREASES AERODYNAMIC DAMPING

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10 Stability and Control

	30
	20						MANOEUVRING STICK
							FORCE GRADIENT
	10
	1	2	3	4	5	6	7	8
				LOAD FACTOR, (n)
					or	(g)
10
Stability	CG POSITION
and	% MAC
								LOW
Control
		10					ALTITUDE
		20
		30						HIGH
								ALTITUDE
		40
	LOAD FACTOR							LOAD FACTOR
				Figure 10.39

When the aeroplane has high static stability, the manoeuvring stability will be high and a high stick force gradient will result, Figure 10.39. A possibility exists that the forward CG limit could be set to prevent an excessively high manoeuvring stick force gradient. As the CG moves aft, the stick force gradient decreases with decreasing manoeuvring stability and the lower limit of stick force gradient may be reached.

When asked to calculate ‘stick force per g’, remember that the aircraft is at 1g to start with. So 1g must be subtracted from the ‘g’ limit before dividing by the pull force.

The pitch damping of the aeroplane is related to air density. At high altitudes, the high TAS reduces the change in tail angle of attack for a given pitching velocity and reduces the pitch damping. Thus, a decrease in manoeuvring stick force stability can be expected with increased altitude.

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Stability and Control 10

Tailoring Control Forces

Control forces should reflect the stability of the aeroplane but, at the same time, should be of a tolerable magnitude. A manual flying control system may employ an infinite variety of techniques to provide satisfactory control forces throughout the speed, CG and altitude range of the aircraft.

EFFECT OF STICK CENTRING SPRING

STICK CENTRING

SPRING

Figure 10.40

Stick Centring Spring

If a spring is added to the control system as shown in Figure 10.40, it will tend to centre the stick and provide a force increment depending on stick displacement.

When the control system has a fixed gearing between stick position and surface deflection, the centring spring will provide a contribution to stick force stability according to stick position.

The contribution to stick force stability will be largest at low flight speeds where relatively large control deflections are required. The contribution will be smallest at high airspeed because of the smaller control deflections required. Thus, the stick centring spring will increase the airspeed and manoeuvring stick force stability, but the contribution decreases at high airspeeds.

A variation of this device would be a spring stiffness which would be controlled to vary with dynamic pressure (Q - Feel). In that case, the contribution of the spring to stick force stability would not diminish with speed.

Down Spring

A down spring added to a control system is a means of increasing airspeed stick force stability without a change in aeroplane static stability.

As shown in Figure 10.41, a down spring consists of a long pre-loaded spring attached to the control system which tends to rotate the elevators down (aircraft nose-down). The effect of the down spring is to contribute an increment of pull force independent of control deflection or airspeed.

Stability and Control 10

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10 Stability and Control

Control and Stability 10

EFFECT OF DOWN SPRING

PRELOADED SPRING

Figure 10.41

When the down spring is added to the control system of an aeroplane and the aeroplane is re-trimmed for the original speed, the airspeed stick force gradient is increased and there is a stronger feel for airspeed. The down spring would provide a “synthetic” improvement to an aeroplane deficient in airspeed stick force stability. Since the force increment from the down spring is unaffected by stick position or normal acceleration, the manoeuvring stick force stability would be unchanged.

Bobweight

The bobweight is an effective device for improving stick force stability. As shown in Figure 10.42, the bobweight consists of an eccentric mass attached to the control system which, in unaccelerated flight, contributes an increment of pull force identical to the down spring. In fact, a bobweight added to the control system of an aeroplane produces an effect identical to the down spring. The bobweight will increase the airspeed stick force gradient and increase the feel for airspeed.

The bobweight also has an effect on the manoeuvring stick force gradient since the bobweight mass is subjected to the same acceleration as the aeroplane. Thus, the bobweight will provide an increment of stick force in direct proportion to the manoeuvring acceleration of the aeroplane (load factor applied). This will prevent the pilot applying too much ‘g’ during manoeuvres; the more you pull back, the more resistance the bobweight adds to the control system.

EFFECT OF BOBWEIGHT

BOBWEIGHT

Figure 10.42

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