Stability and Control 10

Summary

Self Study

Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and to return to, or continue on its original flight path. An aircraft can have two basic types of stability: static and dynamic, and three condition of each type: positive, neutral, and negative.

Static stability describes the initial reaction of an aircraft after it has been disturbed from equilibrium about one or more of its three axes.

Positive static stability is the condition of stability in which restorative forces are set-up that will tend to return an aircraft to its original condition anytime it’s disturbed from a condition of equilibrium. If an aircraft has an initial tendency to return to its original attitude of equilibrium, it has positive static stability. (statically stable).

An aircraft with neutral static stability produces forces that neither tend to return it to its original condition, nor cause it to depart further from this condition. If an aircraft tends to remain in its new, disturbed state, it has neutral static stability. (statically neutral).

If an aircraft has negative static stability, anytime it is disturbed from a condition of equilibrium, forces are set up that will tend to cause it to depart further from its original condition. Negative static stability is a highly undesirable characteristic as it can cause loss of control. When an aircraft continues to diverge, it exhibits negative static stability. (statically unstable).

Most aeroplanes have positive static stability in pitch and yaw, and are close to being neutrally statically stable in roll.

When an aircraft exhibits positive static stability about any of its three axes, the term “dynamic stability” describes­ the long term tendency of the aircraft.

When an aircraft is disturbed from equilibrium and then tries to return, it will invariably overshoot the original ATTITUDE (due to its momentum) and then start to return again. This results in a series of oscillations.

Positive dynamic stability is a condition in which the forces of static stability decrease with time. Positive dynamic stability is desirable. If oscillations become smaller with time, an aircraft has positive dynamic stability. (dynamically stable).

Neutral dynamic stability causes an aircraft to hunt back and forth around a condition of equilibrium, with the corrections getting neither larger or smaller. (dynamically neutral). Neutral dynamic stability is undesirable.

If an aircraft diverges further away from its original attitude with each oscillation, it has negative dynamic stability. Negative dynamic stability causes the forces of static stability to increase with time. (dynamically unstable). Negative dynamic stability is extremely undesirable.

The overall design of an aircraft contributes to its stability (or lack of it) about each of its three axes of motion.

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10 Stability and Control

Control and Stability 10

The vertical stabilizer (fin) is the primary source of directional stability (yaw).

The horizontal stabilizer (tailplane) is the primary source of longitudinal stability (pitch).

The wing is the primary source of lateral stability (roll).

CG location also affects stability.

If the CG is close to its aft limit, an aircraft will be less stable in both pitch and yaw. As the CG is moves forward, stability increases.

Even though an aeroplane will be less stable with an aft CG, it will have some desirable aerodynamic characteristics due to reduced aerodynamic loading of the horizontal tail surface. This type of an aeroplane will have a slightly lower stall speed and will cruise faster for a given power setting.

Manoeuvrability is the quality of an aircraft that permits it to be manoeuvred easily and to withstand the stresses imposed by those manoeuvres.

Controllability is the capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude.

An aircraft is longitudinally stable if it returns to a condition of level flight after a disturbance in pitch, caused by either a gust or displacement of the elevator by the pilot. The location of the CG and the effectiveness of the tailplane determines the longitudinal stability, and thus the controllability of an aircraft.

Increasing stability about any axis:

•decreases manoeuvrability and controllability, and

•increases stick (or pedal) forces.

Phugoid oscillation is a long-period oscillation in which the pitch attitude, airspeed and altitude vary, but the angle of attack remains relatively constant. It is a gradual interchange of potential and kinetic energy about some equilibrium airspeed and altitude. An aircraft experiencing longitudinal phugoid oscillation is demonstrating positive static stability, and it is easily controlled by the pilot.

An aircraft will return towards wing level after a wing drop if it has static lateral stability.

The wing of most aircraft has a positive geometric dihedral angle (dihedral). This is the angle produced by the wing tips being higher than the wing root. If the left wing drops in flight, an aircraft will momentarily begin to slip to the left, and the effective angle of attack of the left wing will increase and the effective angle of attack of the right wing will decrease. The change in angle of attack of both wings will cause the wing to return back towards a level attitude.

Sweepback also has a “dihedral effect”. This is a by-product. A wing is swept-back to give an

aircraft a higher MCRIT. An aircraft with a swept-back wing will not require as much geometrical dihedral as a straight wing.

Some aircraft have the wing mounted on top of the fuselage for various reasons. Also as a by-product, a high mounted wing will give a “dihedral effect” due to the direction of airflow around the fuselage and wing during a sideslip. An aircraft with a high mounted wing does not require as much geometric dihedral.

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An aircraft which has a high mounted, swept-back wing will have so much lateral stability that the wing is usually given anhedral (negative dihedral).

Too much static lateral stability could result in dynamic instability - Dutch roll.

Static directional stability is the tendency of the nose of an aircraft to yaw towards the relative airflow. It is achieved by the keel surface behind the CG being larger than that in front of the CG.

A swept-back wing also provides a measure of static directional stability.

Too much static directional stability could result in dynamic instability - Spiral Instability.

Interaction between static lateral stability and static directional stability. If a wing drops and the aircraft begins to slip to the side, directional stability will cause the nose to yaw into the relative airflow.

“Dihedral effect” tends to roll an aircraft when a wing drops, and directional stability causes the nose to yaw into the direction of the low wing.

These two forces interact (coupled motion):

•An aircraft with strong static directional stability and weak “dihedral effect” will have a tendency towards spiral instability.

•When a wing drops, the nose will yaw toward the low wing and the aeroplane will begin to turn. The increased speed of the wing on the outside of the turn will increase the angle of bank, and the reduction in the vertical component of lift will force the nose to a low pitch angle. This will cause the aircraft to enter a descending spiral.

•An aircraft with strong “dihedral effect” and weak directional stability will have a tendency towards dutch roll instability.

A Mach trim system maintains the required stick force gradient at high Mach numbers by adjusting the longitudinal trim. The Mach trim system only operates at high Mach numbers.

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10 Questions

Questions

		1.	An aeroplane which is inherently stable will:
			a.	require less effort to control.
			b.	be difficult to stall.
			c.	not spin.
			d.	have a built-in tendency to return to its original state following the removal of
				any disturbing force.
		2.	After a disturbance in pitch an aircraft oscillates in pitch with increasing amplitude.
			It is:
			a.	statically and dynamically unstable.
			b.	statically stable but dynamically unstable.
			c.	statically unstable but dynamically stable.
			d.	statically and dynamically stable.
	10	3.	Longitudinal stability is given by:
	Questions
			b.	the wing dihedral.
			a.	the fin.
			c.	the horizontal tailplane.
			d.	the ailerons.
		4.	An aircraft is constructed with dihedral to provide:
			a.	lateral stability about the longitudinal axis.
			b.	longitudinal stability about the lateral axis.
			c.	lateral stability about the normal axis.
			d.	directional stability about the normal axis.
		5.	Lateral stability is reduced by increasing:
			a.	anhedral.
			b.	dihedral.
			c.	sweepback.
			d.	fuselage and fin area.
		6.	If the wing AC is forward of the CG:
			a.	changes in lift produce a wing pitching moment which acts to reduce the
				change of lift.
			b.	changes in lift produce a wing pitching moment which acts to increase the
				change of lift.
			c.	changes in lift give no change in wing pitching moment.
			d.	when the aircraft sideslips the CG causes the nose to turn into the sideslip thus
				applying a restoring moment.
		7.	The longitudinal static stability of an aircraft:
			a.	is reduced by the effects of wing downwash.
			b.	is increased by the effects of wing downwash.
			c.	is not affected by wing downwash.
			d.	is reduced for nose-up displacements, but increased for nose-down
				displacements by the effects of wing downwash.

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			Questions		10
	8.	To ensure some degree of longitudinal stability in flight, the position of the CG:
		a.	must always coincide with the AC.
		b.	must be forward of the Neutral Point.
		c.	must be aft of the Neutral Point.
		d.	must not be forward of the aft CG limit.
	9.	When the CG is close to the forward limit:
		a.	very small forces are required on the control column to produce pitch.
		b.	longitudinal stability is reduced.
		c.	very high stick forces are required to pitch because the aircraft is very stable.
		d.	stick forces are the same as for an aft CG.
	10.	The static margin is equal to the distance between:
		a.	the CG and the AC.
		b.	the AC and the neutral point.	10
		c.	the CG and the neutral point.
		d.	the CG and the CG datum point.		Questions
	11.	If a disturbing force causes the aircraft to roll:
		a.	wing dihedral will cause a rolling moment which reduces the sideslip.
		b.	the fin will cause a rolling moment which reduces the sideslip.
		c.	dihedral will cause a yawing moment which reduces the sideslip.
		d.	dihedral will cause a nose-up pitching moment.
	12.	With flaps lowered, lateral stability:
		a.	will be increased because of the effective increase of dihedral.
		b.	will be increased because of increased lift.
		c.	will be reduced because the centre of lift of each semi-span is closer to the
			wing root.
		d.	will not be affected.
	13.	Dihedral gives a stabilizing rolling moment by causing an increase in lift:
		a.	on the up-going wing when the aircraft rolls.
		b.	on the down-going wing when the aircraft rolls.
		c.	on the lower wing if the aircraft is sideslipping.
		d.	on the lower wing whenever the aircraft is in a banked attitude.
	14.	A high wing configuration with no dihedral, compared to a low wing configuration
		with no dihedral, will provide:
		a.	greater longitudinal stability.
		b.	the same degree of longitudinal stability as any other configuration because
			dihedral gives longitudinal stability.
		c.	less lateral stability than a low wing configuration.
		d.	greater lateral stability due to the airflow pattern around the fuselage when
			the aircraft is sideslipping increasing the effective angle of attack of the lower
			wing.

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10 Questions

	15.	At a constant IAS, what effect will increasing altitude have on damping in roll?
		a.	It remains the same.
		b.	It increases because the TAS increases.
		c.	It decreases because the ailerons are less effective.
		d.	It decreases because the density decreases.
	16.	Sweepback of the wings will:
		a.	not affect lateral stability.
		b.	decrease lateral stability.
		c.	increases lateral stability at high speeds only.
		d.	increases lateral stability at all speeds.
	17.	At low forward speed:
		a.	increased downwash from the wing will cause the elevators to be more
	10		responsive.
		b.	due to the increased angle of attack of the wing the air will flow faster over
	Questions		the wing giving improved aileron control.
		c.	a large sideslip angle could cause the fin to stall.
		d.	a swept-back wing will give an increased degree of longitudinal stability.
	18.	Following a lateral disturbance, an aircraft with Dutch roll instability will:
		a.	go into a spiral dive.
		b.	develop simultaneous oscillations in roll and yaw.
		c.	develop oscillations in pitch.
		d.	develop an unchecked roll.
	19.	To correct Dutch roll on an aircraft with no automatic protection system:
		a.	use roll inputs.
		b.	use yaw inputs.
		c.	move the CG.
		d.	reduce speed below MMO.
	20.	A yaw damper:
		a.	increases rudder effectiveness.
		b.	must be disengaged before making a turn.
		c.	augments stability.
		d.	increases the rate of yaw.
	21.	A wing which is inclined downwards from root to tip is said to have:
		a.	wash out.
		b.	taper.
		c.	sweep.
		d.	anhedral.

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Questions 10

22.The lateral axis of an aircraft is a line which:

a.passes through the wing tips.

b.passes through the centre of pressure, at right angles to the direction of the airflow.

c.passes through the quarter chord point of the wing root, at right angles to the longitudinal axis.

d.passes through the centre of gravity, parallel to a line through the wing tips.

23.Loading an aircraft so that the CG exceeds the aft limits could result in:

a.loss of longitudinal stability, and the nose to pitch up at slow speeds.

b.excessive upward force on the tail, and the nose to pitch down.

c.excessive load factor in turns.

d.high stick forces.

24	The tendency of an aircraft to suffer from Dutch roll instability can be reduced:
	a.	by sweeping the wings.	10
	b.	by giving the wings anhedral.	Questions
	c.	by reducing the size of the fin.
	d.	by longitudinal dihedral.

25.What determines the longitudinal static stability of an aeroplane?

a.The relationship of thrust and lift to weight and drag.

b.The effectiveness of the horizontal stabilizer, rudder, and rudder trim tab.

c.The location of the CG with respect to the AC.

d.the size of the pitching moment which can be generated by the elevator.

26.Dihedral angle is:

a.the angle between the main plane and the longitudinal axis.

b.the angle measured between the main plane and the normal axis.

c.the angle between the quarter chord line and the horizontal datum.

d.the upward and outward inclination of the main planes to the horizontal datum.

27.Stability around the normal axis:

a.is increased if the keel surface behind the CG is increased.

b.is given by the lateral dihedral.

c.depends on the longitudinal dihedral.

d.is greater if the wing has no sweepback.

28.If the Centre of Gravity of an aircraft is found to be within limits for take-off:

a.the C of G will be within limits for landing.

b.the C of G for landing must be checked, allowing for fuel consumed.

c.the C of G will not change during the flight.

d.the flight crew can adjust the CG during flight to keep it within acceptable limits for landing.

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10 Questions

	29.	The ailerons are deployed and returned to neutral when the aircraft has attained
		a small angle of bank. If the aircraft then returns to a wings-level attitude without
		further control movement it is:
		a.	neutrally stable.
		b.	statically and dynamically stable.
		c.	statically stable, dynamically neutral.
		d.	statically stable.
	30.	The property which tends to decreases rate of displacement about any axis, but
		only while displacement is taking place, is known as:
		a.	stability.
		b.	controllability.
		c.	aerodynamic damping.
		d.	manoeuvrability.
10	31.	If an aircraft is loaded such that the stick force required to change the speed is zero:
Questions		a.	the CG is on the neutral point.
		b.	the CG is behind the neutral point.
		c.	the CG is on the manoeuvre point.
		d.	the CG is on the forward CG limit.

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