Stability and Control

Aim To learn what stability is and how it affects flight characteristics

Objectives Define Stability State different types of stability State the design features which aid stability Explain the relationship between stability and controllability Explain how the aircraft is controlled about all axis

1. Define Stability Stability Stability is the inherent quality of an aircraft to correct for conditions that may disturb equilibrium, and to return to or to continue on the original flight path By inherent we mean the natural tendencies of the aircraft without any input by the pilot

2. Stability Static Stability Static stability refers to the initial tendency, or direction of movement back to equilibrium Static stability can occur in each of the three axis of movement It refers to the aircraft’s initial response when disturbed from a given AoA There are 3 types of static stability: Positive static stability Neutral static stability Negative static stability

2. Stability Static Stability Positive static stability - is the initial tendency to return to original state of equilibrium after being disturbed

2. Stability Static Stability Neutral static stability - is the tendency to remain in a new condition after equilibrium has been disturbed

2. Stability Static Stability Negative static stability - is the tendency to continue away from the original state of equilibrium after being disturbed.

2. Stability Dynamic Stability We have seen that static stability is the initial tendency to return to equilibrium after the aircraft has been disturbed from its trimmed condition Dynamic stability refers to the aircrafts subsequent response over time when disturbed Dynamic stability also has 3 subtypes: Positive dynamic stability Neutral dynamic stability Negative dynamic stability

Returns to original attitude 2. Stability Dynamic Stability Positive dynamic stability – over time, the motion of the displaced object decreases in amplitude and returns back towards its original equilibrium state Positive static stability Positive Dynamic Stability Returns to original attitude Aircraft is disturbed Original attitude

2. Stability Dynamic Stability Neutral dynamic stability – once displaced, the displaced object neither decreases or increases in amplitude. Positive static stability Neutral Dynamic Stability Aircraft is disturbed Original attitude Will continue to oscillate about original attitude

2. Stability Dynamic Stability Negative dynamic stability – over time, the motion of the displaced object increases and becomes more divergent. Positive static stability Diverges from original attitude Aircraft is disturbed Original attitude Negative Dynamic Stability

2. Stability Longitudinal, Lateral and Directional Stability When designing an aircraft, a great deal of effort is spend in developing stability around all three axis. Stability around the longitudinal axis is known as lateral stability Stability around the lateral axis is known as longitudinal stability Stability around the normal axis is known as directional stability

3. Stability – Design Features Longitudinal stability Longitudinal stability is the quality that makes an aircraft stable about the lateral axis It involves the pitching motion of the aircrafts nose up and down in flight A longitudinally unstable aircraft has the tendency to dive or climb into a progressively steep dive or climb, or even a stall An aircraft with longitudinal instability becomes difficult and sometimes dangerous to fly Static longitudinal stability or instability is dependant upon these factors: Location of the wing with respect to the CoG Location of the horizontal tail surfaces with respect to the CoG Area or size of the tail surfaces. Position of CoG

3. Stability – Design Features Longitudinal stability – design features Longitudinal Dihedral The difference in angles of incidence between the wings an tail plane The tail plane is typically set at a lower angle of incidence than the main plane If the aircraft is subject to a disturbance the tail plane will have a greater % increase/decrease in lift, causing a restoring moment 4˚ 2˚

3. Stability – Design Features Longitudinal stability – design features If the aircraft is displaced in the lateral axis by a gust into a higher nose attitude, the tailplane is exposed to a greater AoA This will increase the lift on the tailplane therefore causing the aircraft to rotate about the lateral axis and pitch the nose back down Because of the length of the moment arm between the CG and the tailplane, the aerodynamic force need not be great for it to have an effect The further forward the CoG, the greater the moment arm of the tailplane, therefore the greater the force of the tailplane. This has a strong stabilising effect longitudinally An aft CoG reduces the moment arm and therefore reduces longitudinal stability. CoG CoP Tailplane CoG CoP Tailplane Long arm - stable Short arm – less stable

3. Stability – Design Features Longitudinal stability – design features Wing pitching Moment We know in a correctly loaded aircraft the centre of pressure should be behind the centre of gravity As AoA increase the centre of pressure moves forward LIFT WEIGHT

3. Stability – Design Features Longitudinal stability – design features Wing pitching Moment We know in a correctly loaded aircraft the centre of pressure should be behind the centre of gravity As AoA increase the centre of pressure moves forward If the CoG is placed too far reward the CoP will then go in front of the CoG This will create a Nose pitch up moments making the aircraft unstable LIFT WEIGHT

3. Stability – Design Features Directional Stability Stability about the aircrafts normal axis is called directional stability The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors which make the aircraft act like the well known weather vane of arrow, pointing its nose into the relative wind In examining a wind vane, if the same amount of surface were exposed to the wind in front of the pivot point as behind it, the forces fore and aft would be in balance and little or not directional movement would result. Consequently, it is necessary to have a greater surface aft of the pivot point than forward of it.

3. Stability – Design Features Directional Stability Similarly, the aircraft designer must ensure positive directional stability by making the side surface greater aft than ahead of the CG To provide additional positive stability to that provided by the fuselage, a vertical fin is added. The fin acts much like the feather of an arrow

3. Stability – Design Features Lateral Stability The stability about the aircraft’s longitudinal axis is called lateral stability. This helps to stabilise the lateral or “rolling effect” when one wings gets lower than the wing on the opposite side of the aircraft There are a number of design factors which make an aircraft laterally stable: Dihedral Sweepback Keel effect Wing position Weight distribution

3. Stability – Design Features Lateral Stability When considering lateral stability in order to get any restoring moment the aircraft must begin to slip We know that if the aircraft is banked the lift vector is inclined, this creates a lateral component of relative airflow

3. Stability – Design Features Lateral Stability – Design Features Dihedral is the most common way for producing lateral stability, here the wingtips are inclined upwards with respect to the wing roots If the aircraft is displaced, for example by a gust of wind, it will roll then begin to slip The Greater lateral component of relative airflow on the down going wing creates more lift on the down going wing resulting in a rolling moment opposing the initial displacement Dihedral Dihedral Restoring Moment Lift Lift RaF

3. Stability – Design Features Lateral Stability – Design Features A high wing aircraft is more laterally stable than a low wing for two reasons: 1. Here we can see the aircraft has been displaced in roll, as the aircraft begins to slip the fuselage will block the lateral component of the relative airflow on the up going wing This will result in the up going wing creating less lift resulting in the restoring moment, this is known as shielding Restoring Moment Lift Lift Shielding RaF

3. Stability – Design Features Lateral Stability – Design Features 2. The second reason is due to the distance between the CoP and CoG Here we can see the aircraft has been displaced in roll creating a lateral couple between the CoP and CoG resulting in the restoring moment The combined effect of these two restoring moments may make the aircraft so stable that it becomes uncontrollable, in this case the aircraft may be built with anhedral wings which can be seen as the opposite of dihedral Restoring Moment Lift Weight

3. Stability – Design Features Lateral Stability – Design Features Where the CoP of the keel surface is above the CoG the drag created when the aircraft begins to slip will create a rolling moment away from the direction of slip This effect increases with an increase of tail size, ‘T’ tail design and low Cog Restoring Moment Lift Weight

3. Stability – Design Features Lateral Stability When a disturbance causes an aircraft with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow thus increasing its effective span As a result, the low wing acquires more lift, rises, and the aircraft is restored to original flight attitude Sweepback also contributes to directional stability. When turbulence or rudder application causes the aircraft to yaw, one wing presents itself perpendicular to the relative airflow. The airspeed of the in to wind wing increases and therefore the induced drag also increases. The additional drag pulls it back, turning the aircraft to its original path

4. Stability vs Control Spiral Instability Found in aircraft with strong directional but weak lateral stability If it is disturbed in roll it will continue to roll in the same direction, the increased angle of bank leads to more yaw and more roll so the nose begins to drop Without pilot input the aircraft will enter a steep spiral dive

4. Stability vs Control Dutch Roll Found on aircraft with strong lateral stability and weak directional stability Characterised by a combined yawing and rolling motion A simplified explanation of the process can be seen below: Aircraft is disturbed in yaw, because the restoring moment is weak the aircraft begins to oscillate in yaw The yawing moment generates a rolling moment due to the strong lateral stability Because the lateral recovery is faster the moments soon get out of phase and ‘feed’ each other

4. Stability vs Control Control The movement of an aircraft is done by moving controls which are there to overcome static stability There are 3 sets of primary controls: Elevators Aileron Rudder The deflection of the controls changes airflow characteristics and therefore lift over certain parts of the airframe causing a change in attitude

5. Control Control in pitch The primary control to change pitch is called the elevator The pilot changes the elevator by the forward and aft movement of the control column Pulling back causes elevator deflection up and nose up pitch about the CG Pushing forward causes elevator deflection down and a nose down pitch about the CG

5. Control Control in pitch The strength of the tail moment depends on the force it produces and the length of the arm between it and the CG The force generated by the tailplane-elevator combination depends on the relative sizes and shape The larger the relative size, the greater the control

5. Control Control in pitch To retain handling throughout a speed range, the position of the CG must be within a prescribed range (flight envelope) Outside of the envelope the aircraft can be difficult to control If the CG is too far forward, the aircraft will be too stable longitudinally because of the long arm A forward CG makes the aircraft nose heavy and resistant to changes in pitch, this can cause problems in the round out and landing.

5. Control Control in roll The primary control to control the aircraft in roll is the Aileron The Ailerons are usually positioned on the outboard trailing edges of the Wings

5. Control Control in roll The ailerons move contrary to each other, ie when one goes up, the other goes down This causes the lift to increase on the down-going aileron and to reduce on the up-going aileron The pilot moves the ailerons with the rotation of the control wheel sideways The magnitude of the rolling moment depends on the arm from the CG and the magnitude of the differing lift forces

5. Control Control in yaw The primary control used to control the aircraft in Yaw is the Rudder The rudder is hinged to the rear of the fin

5. Control Control in yaw It is controlled in the cockpit by the rudder pedals Pushing the left pedal will move the rudder left, and cause a left yaw Pushing the right pedal will move the rudder right, and cause right yaw Like all controls, rudder effectiveness increases with speed as we are altering the lift Rudder is important as we use it for balance and crosswind take-offs and landings

Questions?