# Stability and Control Lesson

The stability of an aircraft is its tendency to return to the original trimmed position after having been displaced (i.e., by turbulence). Controllability of an aircraft is the ease with which a pilot can manoeuvre the aircraft.

Static Stability

This is a body’s initial reaction when disturbed. If an aircraft trimmed in steady level flight is disturbed in some way (i.e., by a vertical gust), it is said to be statically stable if it tends to return to the original equilibrium position, and statically unstable if it tends to depart further away from the equilibrium position. If it remains in the disturbed position, it is said to have neutral static stability. To illustrate this, we have used a ball.

Positive Static Stability: The ball is disturbed and returns to its original position.

Negative Static Stability: The ball is disturbed and continues away from its original position.

Neutral Static Stability: The ball is disturbed and remains stationary at another position.

Dynamic Stability

This is a body’s subsequent reaction after the disturbance has been removed. Dynamic stability occurs after the static stability reaction has taken place, thus an aircraft must have positive static stability before it can have dynamic stability.

There are three types of dynamic stability:

Positive Dynamic Stability: Following the initial disturbance, the aircraft returns to the original position.

Negative Dynamic Stability: Following the initial disturbance, the aircraft becomes more and more unstable.

Neutral Dynamic Stability: Following the initial disturbance, the aircraft remains in the disturbed motion.

Note: All aircraft in the picture above have positive static stability.

The stability of an aircraft refers to its stability about a given plane of motion. For example, the longitudinal stability of an aircraft refers to its stability around the lateral (pitching) axis. Lateral stability acts around the longitudinal axis, and directional stability acts around the normal, or vertical, axis.

Remember that, for an aircraft to be stable, it must have the natural tendency to return to its original flight attitude after having been disturbed—without pilot input. For example, if an aircraft is flying straight and level, and is disturbed in pitch, then is it deemed to be longitudinally stable if it tries to return to the straight and level attitude. For high-speed aircraft, we are particularly concerned with longitudinal and lateral stability.

Types of Stability Affecting Aircraft

Note: LATERAL AND DIRECTIONAL STABILITY ARE INTERRELATED. THEY AFFECT EACH OTHER, i.e., YAW and ROLL.

Longitudinal stability is in the pitching plane about the lateral axis. To be longitudinally stable, an aircraft must have a natural tendency to return to the same attitude in pitch. If an aircraft has been disturbed nose up by a gust, as the disturbance is taking place, the aircraft, due to inertia, will initially tend to continue on its original flight path. As a result, the angle of attack of both the wings and the tail plane will be increased. The resulting increase in lift on the tail plane may cause the nose of the aircraft to pitch down. The function of the tail plane is to provide a countering force to any residual pitching moment existing between the four main forces (lift and weight couple and the thrust and drag couple). If the angle of attack is increased by a disturbance, the increase in lift on the tail plane coupled with the distance from the centre of gravity can provide a resultant nose pitch down. Note: The horizontal stabiliser is the greatest longitudinally stabilising factor.

The factors are:

• Longitudinal dihedral
• Sweepback with washout
• Position of the centre of gravity
• Movement of the centre of pressure
• Downwash

In many aircraft, the tail plane is set at a lower angle of incidence than the angle of incidence of the wings. This design is called longitudinal dihedral. Longitudinal dihedral is the angle between the main and tail plane chord lines. If an aircraft has been disturbed nose-up by a gust, as the disturbance is taking place the aircraft, due to inertia, will initially tend to continue on its original flight path and the angle of attack of both the wings and the tail plane will be increased. If the restoring moment caused by the increase in lift on the tail plane overrides the unstable pitch of the main plane, then it is said to be a longitudinally stabilising factor. This is not to say, however, that the inclusion of longitudinal dihedral in aircraft design will provide any degree of longitudinal stability.

Before we look at this, we will explain what washout is. Washout is the decrease in angle of incidence from the wing root to wing tip. Washout is a design technique to ensure root-to-tip stall (i.e., the wing root reaches the stalling angle of attack before the wing tip does). The aircraft’s wings are attached to the fuselage at an angle of incidence of 4°, which decreases to 2° at the wing tip. This means that in any phase of flight, the angle of attack in the inboard sections is always greater. Although stalls cannot be eliminated, they can be made less severe by having the wing stall gradually. Since the ailerons are located near the wing tip, wings are designed so that the stalls progresses from the root to the wing tip, giving the pilot better aileron control throughout the stall, and preventing a ‘wing-drop stall’.

The opposite to washout is wash-in. Wash-in is an increase of angle of incidence from the wing root to wing tip, e.g., 2° at the wing root and 4° at the wing tip. There is a tendency for the aircraft to rotate in the opposite direction to the rotation of the propeller, known as torque reaction. One method of counteracting this tendency involves the use of washout on the wing that tends to rise, accompanied by wash-in on the other wing. The difference in lift causes a rolling moment opposing the torque reaction. Most high-performance aerobatic aircraft will have symmetrical wings with no washout or wash-in. This provides for an aerofoil that performs just as well inverted as it does upright!

Aircraft with the wings angled backwards along their span are said to have ‘sweepback’. It is a common feature of most jet aircraft and especially important for high-speed flight. There are also advantages with lower induced drag, etc. A disadvantage of a swept-back wing is its tip-stalling tendency; this can be somewhat overcome with washout. This, however, poses a problem when longitudinal stability is questioned. The root of the wing is producing lift further forward along the longitudinal axis compared to the wing tips. As airspeed and AoA are changed, the centre of pressure will move. This causes a change in aircraft pitch, which could be either stabilising or destabilising. This is vital for ‘flying-wing’ aircraft like hang-gliders.

The longitudinal stability of an aircraft is to a large extent determined by the position of the C of G and is, therefore, something over which the pilot has a lot of control. The forward limit of the centre of gravity is that at which the aircraft is most stable. If a stable aircraft in level flight is in trimmed and then disturbed position, it will quickly return to its trimmed position. The further forward the C of G, the longer the arm between the C of G and the horizontal stabiliser, the larger the leverage; therefore, longitudinal stability is increased.

The further aft the centre of gravity, the smaller the arm between the C of G and the horizontal stabiliser, the less the leverage; therefore, longitudinal stability is decreased.

The longer the arm, the greater the effect. The aircraft must be loaded so that the actual centre of gravity falls within limits stated in the aircraft flight manual. If the C of G is behind the legally allowable aft limit, then the handling characteristics of the aircraft may be adversely affected. Control authority may be so hindered that it becomes impossible to effect spin recovery. If the C of G is too far forward, the elevator may be so ineffective at low speed that flaring for landing may be impossible. Pilots will find that they have to apply more and more forward pressure to control the nose pitch as the airspeed falls.

The centre of pressure varies with the angle of attack. The C of P moves forward as the angle of attack increases, and aft as the angle of attack decreases. Movement of the centre of pressure should be small and stable. This can be achieved on some aircraft by the design of the reflex curve. Because the aircraft pitches about the centre of gravity as the C of P moves forward of the C of G, the aircraft nose will pitch up. This is said to be unstable.

As the C of P moves aft of the C of G, the aircraft nose will pitch down. This is said to be stable.

For most aircraft, especially high-wing aircraft with a low tail plane (i.e., Cessna 172) the downwash from the main plane changes the relative airflow over the tail plane. This will decrease the angle of attack of the tailplane and change the aircraft’s pitch as a result. This has a substantial effect on the longitudinal stability of an aircraft.

Lateral stability is the ability of an aircraft to return to an even keel when a slight roll has taken place about the longitudinal axis. After the aircraft has been displaced (by a roll disturbance in turbulence), the tilted lift vector from the wings now has a horizontal component. This causes a sideslip towards the lower wing.

Slip

When an aircraft is banked, the lift vector is inclined and produces a resultant sideslip into the turn. When the lift and weight force are not directly opposed, a small resultant sideslip force acts sideways and downwards. The aircraft is unbalanced and moves in the direction of the sideslip force, which causes a sideways flow of air in the opposite direction to that of the slip.

Factors that affect lateral stability are:

• Lateral dihedral
• High wing–low centre of gravity
• Sweepback
• High fin surface
• Blanketing

Lateral dihedral is the angle between each main plane and the horizontal. When the main plane is above the horizontal, the dihedral is positive and is known as positive dihedral or dihedral angle. When the main plane is inclined downwards below the horizontal, the dihedral is negative and is known as negative dihedral or anhedral angle.

Lateral dihedral takes advantage of the slip resulting from displacement in roll. When the wings are set a small positive dihedral angle, the new ‘sideways’ airflow caused by the sideslip meets the lower wing at a greater angle of attack than it meets the raised wing. Thus, the lower wing produces more lift than the raised wing does, which produces a restoring moment to correct the roll. Once the wings are levelled again, the sideslip is gone and the aircraft can continue to fly straight and level. Note that a negative dihedral or anhedral angle would have an unstable effect on the aircraft (i.e., it would want to roll further into the direction of the roll).

With a wing in a comparatively higher position in relation to the centre of gravity (i.e., a Cessna 172), a pendulous effect arises when the aircraft sideslips in the direction of the dropped wing following a disturbance in roll. As the aircraft sideslips, the centre of pressure moves towards the lower wing due to the direction of the relative airflow, thus producing more lift. This creates an arm between the lift and weight vectors, which produces a restoring moment that returns the wings laterally level. During the sideslip, the airflow striking the upper keel surfaces (those surfaces above the C of G) will create drag. The drag (of the higher wing) acting above the C of G will tend to help restore the aircraft to wings level. The raised wing will be partly protected (blanketed) from the resultant relative airflow caused by the sideslip and thus produces less lift than the lower wing does. This also helps to return the wings level.

Swept-back wings are very efficient in controlling lateral stability following a disturbance in roll. A correcting force is produced similar to the dihedral effect. When a sideslip develops from a dropped wing, because of sweepback, the lower wing presents more of its effective span relative to the airflow that is approaching it. This results in the down-going wing holding a greater aspect ratio compared to that of the raised wing for the period of the slip. As we are aware, the greater the AR, the greater the lifting efficiency of the wing in question. The relative airflow over the lower wing will also pass over a shorter chord, creating a greater effective camber than that of the raised wing. The combined effect of an increase in aspect ratio and increased camber of the dropped wing is to generate more lift than the raised wing. This creates a restoring moment to wings level.

A high fin surface with a with a low C of G will create a turning moment tending to raise the dropped wing, restoring the aircraft wings level. Large keel surfaces such as a high vertical stabiliser, a T-tail, high wings, undercarriage, and the fuselage, all present areas at right angles to any sideslip. The resulting sideways drag force will tend to roll the aircraft wings level. It is not uncommon to see tailplanes with some lateral dihedral as well. One reason for this is to assist the action of main plane dihedral in restoring wings level.

During the sideslip, the up-going wing becomes shielded by the fuselage; blanketing contributes to the dihedral effect, restoring the wings level.

Size and Distance from C of G: Directional stability of an aircraft is its natural, or built-in, ability to recover from a disturbance in the yawing plane, i.e., about the normal axis. It refers to the aircraft’s ability to WEATHERCOCK its nose into the relative airflow.

If the aircraft is disturbed from its original straight path by turbulence (or by the pilot), the aircraft’s inertia will keep it moving in the original direction. This will incline the longitudinal axis to the relative airflow. The vertical stabiliser is simply a symmetrical aerofoil. Presented to the airflow at an AoA, it will create an aerodynamic force. Due to its large area and the length of the moment arm between it and the centre of gravity, the vertical stabiliser acts to restore the nose to its original position. The greater the vertical stabiliser area and keel surface behind the C of G, the greater the directional stability of the aircraft. The further forward the C of G, the better, as it gives a longer moment arm for the vertical stabiliser. Too far forward, and pitching problems will occur. As well as being caused by turbulence, a yawing effect will also result from power changes. This is due to the effect slipstream has in yawing the aircraft.

The effects of lateral and directional stability are so closely interconnected that it is difficult to separate them. A disturbance that initially involves only lateral stability (roll) will, when the aircraft reacts, involve directional stability (yaw) at the same time.

Spiral Instability: If an aircraft has strong directional but weak lateral stability, e.g., a large vertical stabiliser but no dihedral, when it is disturbed in roll, it will continue to roll in the same direction. The increase in angle of bank leads to more slip, which leads to yaw and more roll, and so on, and the nose drops and the aircraft enters a spiral dive. Spiral instability is cured by more dihedral and smaller fin surfaces. Note that most aircraft are built with slight spiral instability. Spiral instability means directionally stable–laterally unstable.

Oscillatory Instability: Oscillatory instability is produced when the lateral stability of an aircraft is too strong in comparison to its directional stability (i.e., large dihedral angle and a small vertical stabiliser). Oscillatory instability is characterised by a combined rolling and yawing movement, or ‘wallowing’ motion. It is similar to back-pedalling on a bicycle.

When an aircraft is displaced in roll, slip is introduced in the direction of roll. Strong lateral stability starts to roll the aircraft back to level flight. At the same time, the weaker directional stability tries to correct for the sideslip by aligning itself with the ‘apparent’ relative airflow. Due to the weakness of the directional stability, it lags behind the restoring roll. The aircraft is now slipping in level flight and the process is reversed. When the aircraft is doing the rolling motion, this movement is called is called Dutch roll. When the aircraft is doing the yawing motion, this movement is called snaking. Oscillatory instability is cured by larger fin surfaces and less dihedral. Oscillatory instability means directionally unstable–laterally stable.

There are two types of undercarriage configurations:

• Tricycle undercarriage
• Taildragger undercarriage

TRICYCLE UNDERCARRIAGE is INHERENTLY STABLE—It tends to maintain direction when taxiing. The nose steering force counteracts the centrifugal force through the centre of gravity, making this configuration stable.

TAILDRAGGER AIRCRAFT are INHERENTLY UNSTABLE—When the aircraft is displaced from a straight line, the centrifugal reaction force increases the rate of turn and the pilot may lose control of the aircraft. Centrifugal force acts in the same direction as that of the tail wheel, making this configuration unstable. The result is the aircraft may spin about the main wheels. This is known as a ground loop.

When taxiing in moderate to strong winds, the position of the aircraft’s controls is very important. Correct positioning of the controls will reduce the risk of a wing being lifted.

The below image details the position of the controls when taxiing with moderate to strong winds.

Wind from ahead: With wind from ahead of the aircraft, the controls should be positioned into wind (so that the into-wind aileron is positioned up) with the elevators neutral.

Wind from behind: With wind from behind the aircraft, the controls should be positioned out of wind (so that the into-wind aileron is positioned down) with the elevators positioned down (meaning the controls forward).

All single-engine aircraft have an inherent tendency to swing on take-off. Note the tendency to swing one way on take-off is reversed when propeller rotation is reversed. American engines tend to spin clockwise when viewed from the pilot’s perspective. This tends to create a yaw to the left on take-off. English and Russian engines tend to rotate in the opposite sense. The following are the main reasons why the aircraft swings on take-off:

• Slipstream
• Torque
• Gyroscopic effect
• Asymmetric blade effect
• Crosswind

Slipstream: This is the air that is forced rearwards from the propeller and rotates around the aircraft’s fuselage. Depending on the direction of rotation, this will cause the aircraft to yaw left or right on application of power. For the aircraft at Ardmore Flying School, this causes a yaw to the left.

Torque: This effect is similar to an electric drill. While the drill bit rotates one way (generally clockwise), the actual body of the drill wants to rotate in the opposite direction (generally anti-clockwise). This is called torque. In relation to the aircraft, with the propeller rotating clockwise (from the pilots view), the airframe wants to rotate in the opposite direction, creating a downforce on the left wheel. This is like only braking on the left wheel, which will cause a yaw to the left.

Gyroscopic Effect: This refers to gyroscopic precession, where the action acts 90 degrees to the force in the direction of rotation. This is more prominent in taildragger aircraft on take-off when the tail lifts off the ground at the beginning of the take-off roll. As the tail lifts, a force is applied to the top of the propeller from behind, which acts 90 degrees in the direction of rotation, creating a yaw to the left.

Asymmetric Blade Effect: This is again more prominent on taildragger aircraft. This is because the propeller blades are inclined. The angle of attack on the down-going blade is larger than the angle of attack on the up-going blade, producing more thrust on the right side of the propeller rotation. This creates a yaw to the left. When the propeller axis is in the line of flight, the angles of attack and relative airflows on both the down-going blade and up-going blade are equal.

Crosswind: As we have already learned, the aircraft has a tendency to weathercock. In a crosswind situation, whether the wind is from the right or left will determine the direction of swing. Pilots should adopt the crosswind technique (to be discussed).

The main difficulty presented by a take-off in a strong crosswind is that of keeping the aircraft tracking straight down the runway. A crosswind will tend to weathercock the aircraft away from the runway direction and, depending on its direction, this will add or subtract to the already present tendency to swing for the reasons just described.

Crosswind Take-Off

• Prevent swing with rudder
• Aileron into wind, i.e., the down aileron produces some drag, thereby counteracting the weathercocking yaw
• The nose or tail wheel plays a major part in keeping the aircraft straight during the take-off roll—if the aircraft is rotated slowly, then it is likely to weathercock into wind; therefore, the aircraft should be rotated positively.

CAUTION: A rapid rotate may take the aircraft up beyond ground effect at a low airspeed; as a result, the aircraft may sink back onto the runway. A slow rotate or sink back on to the runway may exert a sideways force on the undercarriage.

On Approach

Keep the aircraft crabbed into wind to ‘bracket’ the final approach path. An alternative to this is to fly ‘wing down’ (out of balance) on approach.

Crosswind Landing

• Straighten aircraft with rudder (no longer ‘crabbing’—align with runway direction)
• Aileron into wind (the slip created by this roll will cancel out the tendency for the crosswind to push the aircraft across the runway)
• Rudder opposite to the aileron application in order to keep the aircraft tracking straight
• In the flare: touchdown on the windward wheel first, increasing aileron input as the aircraft slows. The other wheel will settle down than the nose wheel.
• You will then be left with crosswind taxiing technique as you finish the landing roll!

It represents the flight characteristics of an aircraft change when it is very close to the ground or any other surface. The aircraft can fly at a lower speed than when at altitude (VS reduced) or it can fly at the same speed using less thrust than when at altitude. An aircraft will fly more efficiently when it is just above the surface. There is a ‘cushioning’ effect caused by the air between the wing and the ground, known as ground effect. Pilots will experience this on landing. Within about one wingspan of the surface, wing tip vortices are unable to form freely due to surface interference. With reduction in the vertical velocities of air close to the wing, the reaction force is tilted further forward, reducing the induced drag otherwise experienced at altitude. This effect is more noticeable the closer the aircraft gets to the ground, hence the ‘floating’ sensation one experiences when entering the flare to land and, likewise, the ‘sag’ experienced when climbing out of ground effect. In free air, the upwash and downwash are not restricted, so induced drag is greater. Near the ground, upwash and downwash are restricted, so induced drag is reduced.

Discuss the following questions in class now.

1. Define Static Stability
2. Define Dynamic Stability
3. What two types of Stability are inter-related?
4. List the factors affecting Longitudinal Stability
5. List factors affecting Lateral Stability
6. List two factors affecting Directional Stability
7. Describe Spiral Instability
8. Describe Oscillatory Instability
9. Why is a Tail Dragger aircraft unstable when taxiing?
10. For a single engined aircraft list five factors that can produce swing on takeoff
11. In brief, describe crosswind takeoff and landing techniques

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