Flight Stability and Dynamics PDF - UniKL Malaysian Institute of Aviation Technology

12. FLIGHT STABILITY AND DYNAMICS 8 December 2021  LEARNING OUTCOMES On completion of this topic you should be able to: Describe about flight stability and dynamics. 1. Longitudinal, lateral and directional stability (active and passive). Prepared By: Wan...

12. FLIGHT STABILITY AND DYNAMICS 8 December 2021  LEARNING OUTCOMES On completion of this topic you should be able to: Describe about flight stability and dynamics. 1. Longitudinal, lateral and directional stability (active and passive). Prepared By: Wan Nur Shaqella Bte Wan 2 Abdul Razak  12.1 LONGITUDINAL, LATERAL, AND DIRECTIONAL STABILITY (ACTIVE AND PASSIVE) Prepared By: Wan Nur Shaqella Bte Wan 3 Abdul Razak  AXES ON AN AIRCRAFT ¢Aircraft is completely free to move in any direction. ¢Manoeuvre à dive, climb, turn and roll, or perform combinations of these. ¢Whenever an aircraft changes its attitude in flight, it must turn about one or all of these axes. ¢Axes – imaginary lines passing through the centre of the aircraft. Prepared By: Wan Nur Shaqella Bte Wan 4 Abdul Razak AXES ON AN AIRCRAFT Prepared By: Wan Nur Shaqella Bte Wan 5 Abdul Razak  Longitudinal Axis o Lengthwise from nose to tail through center of gravity. o Rotation about this axis is called roll. o Rolling is produced by movement of ailerons. Prepared By: Wan Nur Shaqella Bte Wan 6 Abdul Razak  Longitudinal Axis qControlling the aircraft about its longitudinal axis (rolling motion) qProvided by the ailerons qRolling motion – produce by increasing lift on one wing and reduce lift on the opposite wing qAilerons – Hinged to the trailing edge towards the wingtips and form part of a wing – Operated from the cockpit by mean of a control wheel or control stick or joystick Prepared By: Wan Nur Shaqella Bte Wan 7 Abdul Razak  Lateral Control q Sideways movement of the pilot’s control stick will cause the aileron on one wing to move upwards and, simultaneously, the aileron on the other wing to move downwards q The unequal wing lift on each side of the aircraft produces a roll Prepared By: Wan Nur Shaqella Bte Wan 8 Abdul Razak  Lateral Control q For aircraft to roll à one aileron deflected upward and one downward q Lowered aileron – lift increase + drag also increase (aileron drag or adverse yaw) q The increased drag tries to turn the aircraft in the direction opposite to that desired q Frise aileron or differential ailerons travel system used to overcome the problem of aileron drag Prepared By: Wan Nur Shaqella Bte Wan 9 Abdul Razak  Aileron Drag/Adverse Yaw Differential ailerons travel Frise aileron Prepared By: Wan Nur Shaqella Bte Wan 10 Abdul Razak  Lateral Axis o Spanwise from wingtip to wingtip through center of gravity. o Rotation about this axis is called pitch (nose up or nose down). o Pitching is produced by movement of the elevators. Prepared By: Wan Nur Shaqella Bte Wan 11 Abdul Razak  Longitudinal Control q Controlling the aircraft about the lateral axis (pitching motion) q Provided by elevators q Elevators are hinged to the trailing edge of the horizontal stabilizer q Pitching motion – Forward control column à elevators moves down giving the tailplane a positive camber thereby increasing its lift on the tail à nose pitch down (dive) – Backward control column à elevators moves up giving the tailplane a reverse camber, producing negative lift on the tail à nose pitch up (climb) Prepared By: Wan Nur Shaqella Bte Wan 12 Abdul Razak Longitudinal Control Prepared By: Wan Nur Shaqella Bte Wan 13 Abdul Razak  Normal or Vertical Axis o Passes from top to bottom of the aircraft through center of gravity. o Right angle to longitudinal and lateral axis. o Rotation about this axis is called yaw. o Yawing is produced by movement of the rudder. Prepared By: Wan Nur Shaqella Bte Wan 14 Abdul Razak  Directional Control q Involves rotation about the normal axis (yawing motion) q Controlled by rudder which is hinged to the trailing edge of the vertical stabilizer (Fin) q Movement of rudder is by a pair of rudder pedals located in the cockpit q Yawing motion – Yaw to the left move the left pedal forward, rudder is moved to the left and the nose will turn to the left about normal axis. – The opposite effect is obtained from the forward movement of the pilot’s right foot. Prepared By: Wan Nur Shaqella Bte Wan 15 Abdul Razak Directional Control Prepared By: Wan Nur Shaqella Bte Wan 16 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 17 Abdul Razak AIRCRAFT FLIGHT CONTROL Prepared By: Wan Nur Shaqella Bte Wan 18 Abdul Razak  STABILITY & CONTROL An aircraft is stable if it returns to its initial equilibrium flight conditions when it is perturbed. There are two main types of aircraft instability: An aircraft with static instability uniformly departs from an equilibrium condition An aircraft with dynamic instability oscillates about the equilibrium condition with increasing amplitude. There are two modes of aircraft control: one moves the aircraft between equilibrium states, the other takes the aircraft into a non-equilibrium (accelerating) state. Control is directly opposed to stability. Prepared By: Wan Nur Shaqella Bte Wan 19 Abdul Razak  STATIC STABILITY Stability can be defined as the ability of an object to return to it's original position after it has been disturbed. In general it is opposite to manoeuvrability. If an aircraft is very stable then it is not very manoeuvrable and vice versa. Static stability of an aircraft means that the aircraft after being disturbed from its attitude by a gust or other influences will return to its former attitude without the help of a pilot. Static stability is performed by construction. Prepared By: Wan Nur Shaqella Bte Wan 20 Abdul Razak  POSITIVE STATIC STABILITY The disturbs object will goes back to its original position. Prepared By: Wan Nur Shaqella Bte Wan 21 Abdul Razak  NEUTRAL STATIC STABILITY The disturbs object will move to new position and maintain there after the disturbance force is removed. Prepared By: Wan Nur Shaqella Bte Wan 22 Abdul Razak  NEGATIVE STATIC STABILITY The disturbs object will continue to move even though the disturbance forces is removed. Prepared By: Wan Nur Shaqella Bte Wan 23 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 24 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 25 Abdul Razak  DYNAMIC STABILITY Static stability is the production of a restorative force to bring an aircraft back to a condition of straight and level flight. Dynamic stability is the decrease of these forces with time. Dynamic Stability describes the time required for an airplane to responds to its static stability following a displacement from a condition of equilibrium. Dynamic stability refers to the oscillatory behaviour of an aircraft in response to a disturbing force. It is determine by its tendency to oscillate and damp out successive oscillations after initial displacement. Prepared By: Wan Nur Shaqella Bte Wan 26 Abdul Razak  DYNAMIC STABILITY 1. An aircraft is said to be dynamically stable when it returns to it’s original trimmed position with no overshoot. 2. It’s movement is said to be heavily damped. There are no oscillations and the aircraft returns steadily to it’s original flight path. 3. Dynamic Stability: a) Dynamic stability – heavy damping b) Dynamic stabiliy – some damping c) Neutral Dynamic stability d) Dynamically unstable Prepared By: Wan Nur Shaqella Bte Wan 27 Abdul Razak POSITIVE DYNAMIC STABILITY Prepared By: Wan Nur Shaqella Bte Wan 28 Abdul Razak  Aircraft with positive dynamic stability have oscillations that dampen out over time. The Cessna 172 is a great example. If your 172 is trimmed for level flight, and you pull back on the yoke and then let go, the nose will immediately start pitching down. Depending on how much you pitched up initially, the nose will pitch down slightly nose low, and then, over time, pitch nose up again, but less than your initial control input. Over time, the pitching will stop, and your 172 will be back to its original attitude. Prepared By: Wan Nur Shaqella Bte Wan 29 Abdul Razak NEGATIVE DYNAMIC STABILITY Prepared By: Wan Nur Shaqella Bte Wan 30 Abdul Razak  Aircraft with negative dynamic stability have oscillations that get worse over time. The diagram above pretty much sums it up. Over time, the pitch oscillations get more and more amplified. Prepared By: Wan Nur Shaqella Bte Wan 31 Abdul Razak NEUTRAL DYNAMIC STABILITY Prepared By: Wan Nur Shaqella Bte Wan 32 Abdul Razak  Aircraft with neutral dynamic stability have oscillations that never dampen out. As you can see in the diagram above, if you pitch up a trimmed, neutrally dynamic stable aircraft, it will pitch nose low, then nose high again, and the oscillations will continue, in theory, forever. Prepared By: Wan Nur Shaqella Bte Wan 33 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 34 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 35 Abdul Razak  AIRCRAFT STABILITY 1. Stability can be defined as the ability of an object to return to it’s original position after it has been disturbed. 2. Active stability – the aircraft is flown back to its trimmed flight path automatically by the controls. The flying controls are powered and controlled by the computers. 3. Passive stability – the aircraft flies itself back to its original path after being disturbed because of the aerodynamic design of the airframe. Prepared By: Wan Nur Shaqella Bte Wan 36 Abdul Razak  ACTIVE STABILITY The aircraft is flown back to its trimmed flight path automatically by the controls. The flying controls are powered and controlled by computers that note the aircraft's movement from gyros. The computers compare the aircraft movement with the pilot's input from the flight deck, and intervene if an un- commanded movement occurs. Used mostly on military aircraft, but also on some civil aircraft - for example the gust alleviation spoilers of the A320. Prepared By: Wan Nur Shaqella Bte Wan 37 Abdul Razak  PASSIVE STABILITY 1. Almost all aircraft are designed to be passively stable so the pilot need take little or no action to return the aircraft to its original path after it has been disturbed. 2. It is usual to consider stability in three separate forms: a) Lateral stability – about the longitudinal axis b) Directional stability – about the normal axis c) Longitudinal stability – about the lateral axis Prepared By: Wan Nur Shaqella Bte Wan 38 Abdul Razak  LATERAL STABILITY Passive Stability a) Lateral stability i. If the aircraft is disturbed about the longitudinal axis the movement of the centre of lift to one side of the centre of gravity will cause a correcting movement to help put the aircraft laterally level. ii. Design can could improve lateral stability: a. Wing Dihedral Angle b. Swept Wings c. Aerodynamic Shadow Prepared By: Wan Nur Shaqella Bte Wan 39 Abdul Razak LATERAL STABILITY Dihedral Sweepback Prepared By: Wan Nur Shaqella Bte Wan 40 Abdul Razak  DIHEDRAL The most common procedure for producing lateral stability is to build the wings with an angle of one to three degrees above perpendicular to the longitudinal axis. The wings on either side of the aircraft join the fuselage to form a slight V or angle called “dihedral.” The amount of dihedral is measured by the angle made by each wing above a line parallel to the lateral axis. Prepared By: Wan Nur Shaqella Bte Wan 41 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 42 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 43 Abdul Razak  When the aircraft is banked without turning, the tendency to sideslip or slide downward toward the lowered wing occurs. Prepared By: Wan Nur Shaqella Bte Wan 44 Abdul Razak  Since the wings have dihedral, the air strikes the lower wing at a much greater AOA than the higher wing. The increased AOA on the lower wing creates more lift than the higher wing. Increased lift causes the lower wing to begin to rise upward. As the wings approach the level position, the AOA on both wings once again are equal, causing the rolling tendency to subside. Prepared By: Wan Nur Shaqella Bte Wan 45 Abdul Razak  SWEEPBACK Sweepback is an addition to the dihedral that increases the lift created when a wing drops from the level position. 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. As a result, the low wing acquires more lift, rises, and the aircraft is restored to its original flight attitude. Prepared By: Wan Nur Shaqella Bte Wan 46 Abdul Razak  Adolf Busemann, proposed the use of swept-wings to reduce drag at high speed, at the Volta Conference in 1935. He noted that the airspeed over the wing was dominated by the normal component of the airflow, not the free stream velocity, so by setting the wing at an angle the forward velocity at which the shock waves would form would be higher. Prepared By: Wan Nur Shaqella Bte Wan 47 Abdul Razak The de Havilland DH 108, a prototype swept-wing aircraft, produced in 1944. Prepared By: Wan Nur Shaqella Bte Wan 48 Abdul Razak Prepared By: Wan Nur Shaqella Bte Wan 49 Abdul Razak  DIRECTIONAL STABILITY Stability about the vertical axis is called directional stability. Prepared By: Wan Nur Shaqella Bte Wan 50 Abdul Razak  DIRECTIONAL STABILITY Passive Stability b) Directional Stability i. This is assisted by the fin and rudder and the side area of the fuselage aft of the centre of gravity – taken all together called the Effective Keel Surface. ii. If the aircraft is caused to yaw, the airflow will blow it back to its original position. Prepared By: Wan Nur Shaqella Bte Wan 51 Abdul Razak DIRECTIONAL STABILITY Prepared By: Wan Nur Shaqella Bte Wan 52 Abdul Razak  When an airplane is flying straight into relative airflow , air strikes both sides of the vertical fin evenly and there is no side force. Prepared By: Wan Nur Shaqella Bte Wan 53 Abdul Razak  When a wind gust causes the nose to yaw to the right, air strikes the left side of the vertical fin to create an aerodynamic force that yaws the tail back to the right. Prepared By: Wan Nur Shaqella Bte Wan 54 Abdul Razak  DUTCH ROLL Dutch Roll involves movement about the longitudinal axis (roll) and movement about the normal axis (yaw). Prepared By: Wan Nur Shaqella Bte Wan 55 Abdul Razak  If the aircraft is disturbed about the normal axis (yaw) and the fin is moving to one side of it's normal position the wing on that side of the aircraft is going faster than the wing on the other side. Prepared By: Wan Nur Shaqella Bte Wan 56 Abdul Razak  So the initial yaw to one side causes the wing on that side to lift and cause a rolling moment. In this example as the fin moves to the right, the right wing goes faster, increasing the lifts. and the aircraft rolls to the left. Prepared By: Wan Nur Shaqella Bte Wan 57 Abdul Razak  During this time the airflow is acting on the fin and effective keel surface to move the fin to the left, as it does so, it moves the left wing faster so increasing it's lift. Prepared By: Wan Nur Shaqella Bte Wan 58 Abdul Razak Dutch Roll Prepared By: Wan Nur Shaqella Bte Wan 59 Abdul Razak  LONGITUDINAL STABILITY Passive Stability c) Longitudinal Stability i. This is normally associated with the tailplane or horizontal stabiliser. ii. If a gust of wind causes the nose of the aircraft to be deflected up or down then the tailplane will experience a change in AoA but the aircraft’s momentum will keep the aircraft going in the original direction for a short time. Prepared By: Wan Nur Shaqella Bte Wan 60 Abdul Razak  LONGITUDINAL STABILITY Longitudinal Stability is the stability of the aircraft pitching motion. A longitudinally unstable aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an aircraft with longitudinal instability becomes difficult and sometimes dangerous to fly. Prepared By: Wan Nur Shaqella Bte Wan 61 Abdul Razak  LONGITUDINAL STABILITY Static longitudinal stability or instability in an aircraft, is dependent upon three factors: 1. Location of the wing with respect to the C of G 2. Location of the horizontal tail surfaces with respect to the C of G 3. Area or size of the tail surfaces 62  To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced. Prepared By: Wan Nur Shaqella Bte Wan 63 Abdul Razak  When the aircraft is suddenly nose up, the wing moments and tail moments change so that the sum of their forces provides an unbalanced but restoring moment which, in turn, brings the nose down again. Prepared By: Wan Nur Shaqella Bte Wan 64 Abdul Razak  Similarly, if the aircraft is nose down, the resulting change in moments brings the nose back up. Prepared By: Wan Nur Shaqella Bte Wan 65 Abdul Razak  Longitudinal Static Stability cg Time = 0.0 Tail Force Aircraft encounters gust Nose pitches up Resulting Motion Time = 1.0 Tail Force Airflow Direction Time = 2.0 Tail Force Prepared By: Wan Nur Shaqella Bte Wan 66 Abdul Razak