British Airways Global Learning Academy - Basic Aerodynamics PDF

British Airways Global Learning Academy – Basic Aerodynamics 8.4 – Flight Stability and Dynamics Axes of an Aircraft and Pitch, Roll and Yaw An aircraft in flight is free to rotate about three axes, and its’ flight controls are designed to allow the pilot to control its rotation about each axis. Th...

British Airways Global Learning Academy – Basic Aerodynamics 8.4 – Flight Stability and Dynamics Axes of an Aircraft and Pitch, Roll and Yaw An aircraft in flight is free to rotate about three axes, and its’ flight controls are designed to allow the pilot to control its rotation about each axis. They are:  The Longitudinal, or Roll axis.  The Lateral, or Pitch axis.  The Vertical, Yaw or Normal axis. The longitudinal axis is a straight line through the fuselage of an aircraft, passing through the centre of gravity, is called the longitudinal, or roll, axis of the aircraft. The lateral, or pitch, axis passes through the centre of gravity and extends parallel to the wing span. The vertical axis that passes through the centre of gravity and is perpendicular to the other two is the vertical axis, which is also called the yaw axis of the aircraft. Figure 1 shows all of the axes of an aircraft and their geometric relationship too each other. Module 08B ETBN 0492 October 2023 Edition Fig 1 – The Three Axes Of An Aircraft 4 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Roll control is through the aircraft’s control column or yoke, connecting to the ailerons as shown in the simplified diagram in Figure 2a Longitudinal Axis The axis that extends lengthwise through fuselage from the nose to the tail or empennage is the longitudinal (roll) axis (Figure 2). Control of movement (Roll) about this axis is through the use of ailerons. Fig 2 – Longitudinal Axis Figure 2a – Roll Control Yoke (Control Column) and Ailerons A secondary effect of roll is unwanted yaw, called “adverse yaw” in the opposite direction of the roll, caused by the drag of the downward moving aileron on the higher wing in the roll and the additional lift of the wing. This can be counteracted by applying a small opposite rudder pedal input in the same direction as the roll. Other methods are discussed under “Adverse Yaw” part of these notes. Module 08B ETBN 0492 October 2023 Edition 5 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Figure 3a shows the basic lateral (pitch) axes control mechanism in an aircraft. Lateral Axis The axis that extends through the aircraft's fuselage from wingtip to wingtip is the Lateral (Pitch) axis (Figure 3). Control about this axes is via the control column (yoke) connected to the elevators. Figure 3a - Pitch Control from Control Column to Elevator Figure 3 - Lateral Axis Control Module 08B ETBN 0492 October 2023 Edition 6 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Figure 4a shows the basic vertical (yaw) axis control mechanism in an aircraft from the Rudder pedals in the cockpit to the rudder. Vertical Axis The vertical (or yaw) axis passes vertically through the fuselage at the centre of gravity (C of G) as shown in Figure 4 and is controlled by the aircraft rudder pedals and rudder. Figure 4a – Directional (Yaw) Control From Pedals to Rudder A secondary effect of aircraft yaw is unwanted roll in the same direction as the yaw, due to the faster outer wing in the turn generating more lift. Correction can be made by applying a small aileron input in the opposite direction of the rudder direction. Figure 4 – Vertical (Yaw) Axis Module 08B ETBN 0492 October 2023 Edition 7 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Adverse Yaw To avoid this undesirable effect, various methods have been adapted. The main ones are use are:- The ailerons and rudder are all used in turning the aircraft. If you want to turn to the left move the control wheel to the left. The right aileron moves down and the left aileron moves up. The deflected ailerons which cause the bank also cause the aircraft to yaw. Differential Ailerons The aileron linkage causes the up-going aileron move through a large angle than the down going aileron as in Figure 5a. The problem is that the direction of yaw is opposite that which we want. The right aileron moving down increases not only the lift of the right wing, but it also creates a good deal of induced drag cantered out near the wing tip, and this drag, since it is not countered with a similar drag on the left wing, will cause the nose of the aircraft to start to move to the right as in Figure 5 Figure 5a - Differential Ailerons Figure 5 – Adverse Yaw Module 08B ETBN 0492 October 2023 Edition 8 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Frise Ailerons Aileron-Rudder Coupling Have an asymmetric leading edge. The leading edge of up-going aileron protrudes below the lower surface of the wing, causing high drag. The leading edge of the down-going aileron shrouded and cause less drag as seen in Figure 5b. In this system the aileron and rudder controls are interconnected, so that when the ailerons are deflected the rudder automatically moves to counter the adverse yaw. Figure 5b - Frise Ailerons Figure 5c - Aileron- Rudder Coupling Module 08B ETBN 0492 October 2023 Edition 9 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Aircraft Stability Aerodynamic stability is somewhat different from other types of mechanical stability in that a stable aircraft does not necessarily try to keep its wings level with respect to the earth, nor does it even try to keep its nose level with the horizon. It is not stable in its attitude, with respect to the earth, but it is stable with regard to its relative wind. A stable aircraft will return to the angle of attack for which it is trimmed any time it is disturbed from this angle. Static Stability An aircraft is in equilibrium when there are no forces trying to disturb its condition of steady flight; that is, there are no forces trying to change its angle of attack. But, if the plane is disturbed from steady flight, its static stability will try to return it to its original angle of attack, and this may result in a nose-up or nose-down attitude. Dynamic Stability Static stability, as was stated, creates a force that tends to return the aircraft to its original angle of attack, but it is the dynamic stability of the aircraft that determines the way it will return. It is concerned with the way the restorative forces act with regard to time. Module 08B ETBN 0492 October 2023 Edition 10 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Conditions of Stability Positive Static Stability If we roll a ball up the side of a valley and release it, it will tend to roll back down to its original position (Figure 6). This is caused by positive static stability possessed by the ball. Positive Dynamic Stability When we release the ball so it will return to its original position at the bottom of the valley, the ball will probably overshoot its position of equilibrium and will start up the opposite side. But as soon as it starts up the slope, its positive static stability will tend to return it again to the bottom. The ball will thus rock back and forth, each time moving a shorter distance up the slope, until it finally stops at the bottom. This is an example of positive dynamic stability—the restorative forces of static stability have lessened with time. Figure 6 shows a practical example with Displacement/Time graphs of Positive Static and Dynamic Stability. Figure 6 – Positive Dynamic Stability Module 08B ETBN 0492 October 2023 Edition 11 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Negative Static Stability Figure 6a shows a practical example with Displacement/Time graphs of Negative Static and Dynamic Stability If we release a ball from the top of a slope, it will have no tendency to return to its original position and, in this condition, the ball is said to have negative static stability, or to be statically unstable. Negative Dynamic Stability If the corrective forces increase with time, the body has negative dynamic stability. In Figure 6a, each oscillation is larger than the one previous and a body having these characteristics is said to have positive static stability but negative dynamic stability. Figure 6a – Negative Dynamic Stability Module 08B ETBN 0492 October 2023 Edition 12 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Neutral Static Stability If we roll a ball across a perfectly level surface, it will have no tendency to return to its original position, nor will it have a tendency to continue to move away from this position. As soon as the energy from the original movement is dissipated in friction, the ball will stop rolling. Neutral Dynamic Stability If the restorative forces that tend to bring a disturbed object back to its position of equilibrium neither increase nor decrease in amplitude with time, the body is said to have neutral dynamic stability. No energy is added to the body, nor is any taken away from it in the form of damping. Figure 6b – Neutral Static And Dynamic Ability Module 08B ETBN 0492 October 2023 Edition 13 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Stability About The Axes Longitudinal Stability About The Lateral Axis (Pitch) The longitudinal, or pitch, stability of an aircraft determines its ability to be flown hands - off at any airspeed. The centre of gravity is located ahead of the aerodynamic centre of the wing, and in straight and level flight, the wing produces a nosedown moment. In order to balance this rotational force, the horizontal tail surface is installed in such a way that it produces a down load that causes a nose-up moment, and aircraft are equipped with methods of varying the tail load in flight. If the aircraft is flying at 120 knots, the downward tail load can be trimmed to exactly balance the tendency of the wing to rotate the nose down. When the aircraft is slowed to 80 knots for an approach, the lower airspeed over the tail will decrease the tail load enough that the pilot will have to hold back pressure on the control wheel. Figure 7 – Stabilising Tail Load (Provides Longitudinal Stability) Module 08B ETBN 0492 October 2023 Edition 14 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics If the aircraft is trimmed for any given airspeed, let us say for a speed of 100 knots, and it encounters a downdraft, the nose will drop and the aircraft will start down. The airspeed will build up and the tail load will increase and bring the nose up. The opposite will happen if the nose is pitched up: the airspeed will drop off and the tail load will decrease, so the nose-down moment of the wing will lower the nose and restore level flight. We can also say that stability about the lateral axis is the longitudinal stability and is controlled by the elevators that are fitted in the trailing edges of the tail plane or horizontal stabilizer. If the aircraft is put into a dive or climb attitude and the control column (yoke) released, the aircraft should return to its original level flight path automatically. If the aircraft does NOT have longitudinal stability, it may increase the angle of dive after being placed in a dive attitude or it may "porpoise" (oscillate through a series of dives and climbs [pitch up and down]), unless controlled by the pilot. Module 08B ETBN 0492 October 2023 Edition 15 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Effect Of C of G On Stability Longitudinal stability, for the most part, is determined by the centre of gravity (C of G) with respect to the centre of lift. Neutral longitudinal stability (Fig 7a) is when the centre of lift is directly over the centre of gravity or weight. An aircraft with neutral stability will produce no permanent pitch moments about the centre of gravity. With the centre of lift forward of the C of G (Figure 7b) negative stability and an undesirable pitch-up moment during flight would be experienced. If disturbed during flight, the up and down pitching moment will tend to increase in magnitude. This condition can be made worse if the aircraft payload is distributed in such a way that the centre of gravity is rearwards of the designed aft loading limits. Figure 7b – Negative Longitudinal Stability Figure 7a – Neutral Longitudinal Stability Module 08B ETBN 0492 October 2023 Edition With the centre of lift rearwards of the C of G as shown in Figure 7c, negative stability is produced. In the design of the aircraft, some force must balance out the down force of the forward weight condition. 16 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics This can be accomplished by designing the aircraft so that the air flowing off the wing trailing edges will strike the upper surfaces of the tailplane. This condition will create a downward tail force to counteract the tendency to pitch aircraft nose-down. Successful design such as this will create a positive longitudinal stability. Figure 7c – Downward Tail force for Positive Longitudinal Stability Module 08B ETBN 0492 October 2023 Edition 17 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics When an aircraft is flying straight and level, the angle of attack is the same for both wings; but if a wing drops, the aircraft will start to slip to the side, and the upturned wing on the side to which the aircraft is slipping will immediately have a greater angle of attack than the wing opposite the direction of the slip (Figures 8a and 8b). Lateral Stability About The Longitudinal Axis (Roll) Lateral stability- stability about the longitudinal axis, or roll stability - Is provided primarily by dihedral in the wings. Dihedral (Figure 8) is the positive acute angle between the wing and the lateral axis of the aircraft, and this angle is considerably larger on a low-wing aircraft than it is on a high-wing model. The increased angle of attack will increase the lift on the downwardmoving wing and restore the aircraft to straight and level flight. The pendulum effect caused by the fuselage being below the centre of lift on a high-wing aircraft provides a righting action when the aircraft rolls, so the need for a steep dihedral angle is not as great as it is on a low-wing aircraft, whose centre of gravity is above its centre of lift. Figure 8a – Aircraft In A Sideslip And Self-Righting Action The aircraft's lateral stability is controlled with the ailerons that are fitted in the trailing edges of the wing surfaces. An aircraft that tends to return to a wings-level attitude after being displaced from a level attitude by some outside force (turbulent air) is considered laterally stable. The factors that primarily affect lateral stability are dihedral and sweepback. Figure 8 Dihedral On A Low Wing Aircraft Module 08B ETBN 0492 October 2023 Edition 18 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Dihedral Angle Increased AoA Reduced AoA Resultant Airflow Resultant Airflow Resultant Airflow Caused by Sideslip Fig 8b – Sideslip And Wing AoA Module 08B ETBN 0492 October 2023 Edition 19 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Wing Dihedral A dihedral angle is the angle formed by the intersection of two planes (see Figures 8a and 8b). The term dihedral means the lateral angle of the wing with respect to a horizontal plane. The purpose of dihedral is to provide lateral stability for the aircraft. A dihedral angle wing gives a greater lift force to the down-going wing in a roll that helps reduce the action known as sideslip. The side force on a vertical fin also assists in controlling sideslip. So one can say the vertical fin will also provide some lateral stability. Wing Sweepback Sweepback is the angle at which the wings are slanted rearward from the wing root to the wingtip (See Figure 9). Sweepback contributes to the lateral stability of an aircraft in flight. Sweepback wing design also places the centre of lift further rearward which affects longitudinal stability more than it does the lateral stability. Figure 9 – Wing Sweepback Module 08B ETBN 0492 October 2023 Edition 20 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Sweepback of an aircraft wings affects the lateral stability of an aircraft, as when an aircraft sideslips, the lower wing because of its angle of sweep will now present more of its span to the relative airflow than the upper wing illustrated in Figure 9a. This produces a change in aspect ratio (AR) of the upper and lower wings, as the effective chord of the lower wing is decreased whilst that of the upper wing is increased. As this will increase the aspect ratio (AR) of the lower wing and decreases the aspect ratio of the upper wing, the lower wing will produce a greater amount of lift than the upper wing and restore the aircraft back to a wings level position. Sweepback, therefore, like dihedral, when combined with the damping effect produces enough lift to roll the aircraft back to its former position of equilibrium. In fact approximately 100 of sweepback, has about the same effect as 1° of dihedral. As sweepback has the same effect as dihedral, aircraft incorporating both, may become to stable especially at low airspeeds, as sweepback and changes in aspect ratio result in variations of the lift forces of the leading and trailing wings at a high angle of attack. To prevent the aircraft becoming too stable and increase the handling characteristics at high angles of attack, it may be necessary to incorporate some negative dihedral (anhedral) on aircraft that are fitted with large swept backed wings. Figure 9a – Effect Of Wing Sweep During A Sideslip Module 08B ETBN 0492 October 2023 Edition 21 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics If the aircraft is yawed out of its flight path the relative wind will exert a force on one side of the vertical fin and return the aircraft to its original direction of flight, as shown in the picture below. Vertical (Directional) Stability About The Yaw Axis The vertical fin—that is, the fixed portion of the vertical tail of an aircraft—gives the plane directional stability, or the tendency to align with the relative wind. Any streamlined body will turn crosswise to the relative wind unless it has some form of stabilizing surface on its tail, and the tendency for an aircraft to yaw (rotate about its vertical axis) is immediately countered by the weather-vaning action of the vertical fin, and the nose of the aircraft will point into the relative wind (Figure 9b). The relationship between the vertical area ahead of and that behind the centre of gravity has a great deal to do with the directional stability of an aircraft. We can also say that stability of the aircraft about the vertical axis is called directional stability. This means the aircraft will return to a straight and level flight path after having yawed one way or the other. Directional stability is obtained by constructing the aircraft with a vertical fin or stabilizer rearwards of the C of G and on the upper portion of the tail plane. Control about the vertical axis is accomplished with the use of the rudder installed in the trailing edge of the vertical fin of the empennage section of the aircraft explained earlier in the Axes Of An Aircraft section of these notes. Module 08B ETBN 0492 October 2023 Edition Figure 9b – Aircraft “Weather Vanning” Into Wind The amount of directional stability is directly proportional to both the size of the vertical fin and the distance the fin is located aft of the C of G. The larger the surface area of the fin and the further aft it is located will determine the degree of stability (See Figure 9c). 22 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Because the vertical fin is the primary directional stabilizing force, it must be located aft of the C of G for a stable aircraft configuration. Fig 9c – A380 With Large Tail Fin Module 08B ETBN 0492 October 2023 Edition 23 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Gust of air disturbs aircraft and makes it yaw As aircraft continues to move forward AoA on tail plane increases producing extra lift Longitudinal AND Lateral axis rotating ABOUT Normal axis Moment arm created by distance from CoG to CoP of tailplane Restoring moment created by tailplane moment arm and lift created by tailplane Fig 9d – Lateral Stability Module 08B ETBN 0492 October 2023 Edition 24 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Corkscrewing Slipstream With single engine propeller aeroplanes a corkscrewing slipstream pushed back by the propeller strikes the vertical tail surfaces on the left side and tries to push the tail to the right. On most aircrafts this effect is cancelled by offsetting the leading edge of the vertical fin slightly to the left. The slipstream passes evenly around the fin at cruise power and cruise airspeed, but during take-off when the engine power is high and the airspeed is low, a strong force will push the tail to the right, and the pilot must counteract this with some right rudder. In Figure 10, the vertical fin is off-set from the aircraft centreline to counteract the effect of the corkscrewing slipstream. During a power-off glide, when there is no corkscrewing slipstream, the air passes straight down the fuselage, and the offset fin causes the tail to move to the left. To maintain straight flight, the pilot must use some left rudder. Figure 10 - Corkscrew Slipstream Module 08B ETBN 0492 October 2023 Edition 25 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Asymmetric Thrust When an engine fails on a multi-engine aircraft there will be a decrease in thrust and an increase in drag on the side with the failed engine:  Airspeed will decay.  The nose will drop and  Most significantly, there will be an immediate yawing moment towards the failed engine. Figure 11 shows the forces and moments acting on an aircraft following failure of the left engine. The aircraft has a yawing moment towards the failed engine. Rudder is used to stop the yaw. The yawing moment is the product of thrust and the operating engine, multiplied by the distance between the thrust line and the CG (thrust arm) plus the drag from the failed engine, multiplied by the distance between the engine centre line and the CG. The strength of the yawing moment will depend on:  How much thrust the operating engine is developing (throttle setting and density altitude.  The distance between the thrust line and the CG (thrust arm)  How much drag is being produced by the failed engine. Figure 11 – Asymmetric Thrust Module 08B ETBN 0492 October 2023 Edition 26 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics The rudder moment, which balances the yawing moment, is the product of rudder force multiplied by the distance between the fin CP and the CG (rudder arm). Thus, at this stage, the ability of the pilot to counteract the yawing moment due to asymmetric thrust will depend on:  Rudder displacement  Cg position (affecting rudder arm)  The IAS (affecting rudder force) Assume the rudder is at full deflection, CG is at the rear limit (shortest rudder arm) and the IAS (dynamic pressure) is just sufficient for the rudder force to give a rudder moment equal to the yawing momentthere will be no yaw. But, any decrease in IAS will cause the aircraft to yaw uncontrollably towards the failed engine. The uncontrollable yaw to the left, in this example will cause the aircraft to roll uncontrollably to the left due to greater lift on the right wing. The aircraft will enter a spiral dive to the left. Figure 12 – Effect Of Torque Thus, there is a minimum IAS at which directional control can be maintained following engine failure on a multi-engine aircraft. The minimum IAS is called VMC (minimum control speed). Torque is the tendency of the aircraft to want to roll in the opposite direction to that in which the propeller is rotating. If the propeller rotates clockwise as seen from the cockpit, the aircraft will try to roll anti-clockwise. Module 08B ETBN 0492 October 2023 Edition 27 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Dutch Roll And Spiral Instability Oscillating instability is not easily corrected by the pilot and if the aircraft is prone to this form of dynamic instability, an automatic yaw damper will be installed in the aircraft. The yaw damper will sense a roll or yaw motion and activate the correct flight controls to overcome them. The effect of dihedral is to roll the aircraft in the direction opposite that in which it is slipping, and the vertical fin will try to yaw the aircraft in the direction of the slip. These two forces both affect the lateral and directional stability of an aircraft. If the dihedral effect is greater than that of the fin, the aircraft will have a tendency to Dutch roll in flight (Figure 13). A Dutch roll is an oscillation of very low magnitude about both the longitudinal and vertical axes. It is objectionable as far as flight comfort, but it is generally not a serious flight condition. If the aircraft is yawed to the right, the left wing advances (sideslip) and generates more lift, whilst the right wing slows down and produces less lift. The result of the imbalance in lift is to roll the aircraft in the direction of the initial yaw. The left wing also produces greater drag due to the larger areas exposed to the airflow, which causes the aircraft to yaw in the opposite direction. Figure 13 – Dutch Roll This results in the right wing producing more lift than the left wing, reversing the direction of the roll. The final result is a snaking motion, where the rolling and yawing oscillations have the same frequency, but are out of phase with each other. Dutch Roll is characterised by a coupling of directional and lateral oscillation that produces the tendency of the aircraft to "wander" about the roll and yaw axes. Dutch Roll generally occurs when an aircraft's dihedral effect is larger than its static directional stability. Module 08B ETBN 0492 October 2023 Edition 28 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Manoeuvrability Manoeuvrability is also an important characteristic of an aircraft design process. This is the ability of an aircraft to be directed along a selected flight path. The smooth and easy response of the aircraft to its controls is very important. Aircraft are designed to have varying degrees of stability depending on their role. If the aircraft lacks manoeuvrability, it will be easy to fly straight and level but difficult and tiring through manoeuvres. It is important to achieve a good balance between stability and manoeuvrability in any aircraft design process. Commercial transport aircraft need to be positively stable because they spend many hours in straight and level cruising flight but machines required to be highly manoeuvrable such as fighters or aerobatic types are only marginally stable and require a lot of attention from the pilot. Module 08B ETBN 0492 October 2023 Edition 29 Basic Aerodynamics – Flight Stability