British Airways Global Learning Academy - Basic Aerodynamics PDF

British Airways Global Learning Academy – Basic Aerodynamics UK Part 66 Module 8B Basic Aerodynamics 8.3 Theory Of Flight Module 08B ETBN 0492 October 2023 Edition Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Book 3 of 4 Contents Syllabus .....

British Airways Global Learning Academy – Basic Aerodynamics UK Part 66 Module 8B Basic Aerodynamics 8.3 Theory Of Flight Module 08B ETBN 0492 October 2023 Edition Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Book 3 of 4 Contents Syllabus ................................................................................................................................................................................................................... 4 8.3 – Theory Of Flight .............................................................................................................................................................................................. 5 Relationship Between Lift Weight, Thrust And Drag .................................................................................................................................................. 5 Four Forces Of Flight ............................................................................................................................................................................................ 5 Forces In Steady State Level Flight....................................................................................................................................................................... 5 Forces In The Climb ........................................................................................................................................................................................... 10 Forces In A Glide And Glide Ratio ...................................................................................................................................................................... 12 Theory Of The Turn ............................................................................................................................................................................................... 14 Forces In The Turn ............................................................................................................................................................................................. 15 Load Factor ........................................................................................................................................................................................................ 17 Co-ordinating Turns ............................................................................................................................................................................................ 19 Lift, Load And Sideslip ........................................................................................................................................................................................ 20 Influence Of Load Factor ........................................................................................................................................................................................ 22 Flight Envelope And Structural Limitations .......................................................................................................................................................... 22 Aerodynamic Stall............................................................................................................................................................................................... 24 Lift Augmentation ................................................................................................................................................................................................... 26 High Lift Devices ................................................................................................................................................................................................ 26 Leading Edge Devices ........................................................................................................................................................................................ 28 Module 08B ETBN 0492 October 2023 Edition Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Slats ................................................................................................................................................................................................................... 28 Trailing Edge Devices ......................................................................................................................................................................................... 30 Drag Inducing Devices ........................................................................................................................................................................................ 33 Vortex Generators .............................................................................................................................................................................................. 35 Module 08B ETBN 0492 October 2023 Edition Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Syllabus UK Part 66 Syllabus Level Basic Aerodynamics 8.3 Theory Of Flight A Relationship between lift, weight, thrust and drag; 1 B B2 / B2L B3 2 2 1 Glide ratio; Steady state flights, performance; Theory of the turn; Influence of load factor: stall, flight envelope and structural limitations; Lift augmentation Module 08B ETBN 0492 October 2023 Edition 4 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics 8.3 – Theory Of Flight Relationship Between Lift Weight, Thrust And Drag Four Forces Of Flight Lift This force is produced mainly by the aircraft’s wings. It acts at right angles to the line of flight and through the centre of pressure (CoP) of the wings. Weight The weight acts vertically downwards through the aircraft’s centre of gravity (CoG) no matter what attitude the aircraft assumes. Thrust Thrust is the propelling force delivered by the aircraft’s powerplant which are normally arranged symmetrically about the aircraft’s centre line. The thrust force may be taken to act parallel to the line of flight. Drag Figure 1 – The Four Forces In Flight Forces In Steady State Level Flight For simplicity, thrust and drag forces are considered as acting parallel to the longitudinal axis, and their displacement from this axis depends on the design of the aircraft, high wing or low wing, the position of the engine(s), and so on. Drag opposes the forward motion of the aircraft and can be regarded as a rearward acting force through the centre of pressure. Even when the conditions above have been satisfied the aircraft would still have a tendency to rotate nose up or down if the lines of action of the four forces were not arranged correctly. If an aircraft is flying at a constant height and speed, lift equals weight and thrust equals drag as shown in Figure 1 The arrangement depends on the design of the aircraft, but usually the CoP is designed to be aft of the CoG so that the lift and weight forces, acting together, tend to force the aircraft’s nose down. This tendency is counteracted by arranging the thrust line to be below the line of drag, thus providing a nose up couple as shown in Figure 1a. Under these conditions the lift has been adjusted by altering the angle of attack until it exactly equals the weight which it must support. Similarly, the thrust of the engine(s) has been set to exactly equal the aircraft drag generated at that particular speed. Module 08B ETBN 0492 October 2023 Edition 5 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics LIFT LIFT DRAG DRAG THRUST THRUST WEIGHT WEIGHT LIFT LIFT / WEIGHT COUPLE THRUST / DRAG COUPLE CoG Nose DOWN moment THRUST WEIGHT DRAG Nose UP moment Fig 1a – Lift/Weight And Thrust/Drag Turning Moments Module 08B ETBN 0492 October 2023 Edition 6 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics The particular arrangement shown in Figure 1a has the advantage that if the thrust force disappears, through engine failure, the lift weight couple will force the nose down and the aircraft will assume a gliding attitude. It will, therefore, maintain enough speed to prevent it stalling. The couple between the thrust and drag forces may compliment the lift/weight pitching moment (Figure 1b - top), or oppose it (Figure 1b bottom). With the couples arranged as shown at figure 1a – top, it would be necessary for the tailplane to produce negative lift, in order to maintain level flight. In order to maintain steady flight the forces acting on an aeroplane must be in balance, with no turning moment about any axis. In this condition the aircraft is said to be trimmed. The consequence of this downward force produced by the tailplane is effectively an increase in the aircraft weight. Consequently, for a given speed, a greater angle of attack must be used to produce the required lift, and this results in an increase in the amount of drag produced. This is one example of trim drag, we'll come across others later. The condition is achieved by balancing the lift, weight, thrust and drag forces acting at the aircraft's CoG and CoP so that:  Lift equals weight, otherwise the aircraft would climb or descend.  Thrust equals drag, otherwise the aircraft would accelerate or decelerate. A change in any one of the forces will disturb the trim of the aircraft, causing it to pitch either nose up or nose down. One important consideration in respect of the lift/weight force couple is the way in which the aircraft behaves in the event that the thrust/drag couple is destroyed, in other words during engine failure with a single engine aircraft. Providing that the centre of gravity and the centre of pressure are not coincident a force couple will be set up by the lift and the weight forces, and this will result in a pitching moment, as shown at figure 1. With the centre of gravity forward of the centre of pressure, the natural tendency will be for the nose to drop when the thrust/drag couple is removed as the engine fails, and this is desirable. The magnitude of the pitching moment will depend on the magnitude of lift and weight forces, but also on the distance between the centre of gravity and the centre of pressure. The position of the CoG will depend on the way in which the aircraft is loaded, and on the way in which fuel is transferred/consumed in flight. The position of the centre of pressure depends on the angle of attack, with the CoP moving slowly forward as the angle increases, and then rapidly backwards at the stalling angle. Module 08B ETBN 0492 October 2023 Edition 7 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics LIFT / WEIGHT COUPLE THRUST / DRAG COUPLE LIFT / WEIGHT COUPLE THRUST / DRAG COUPLE Fig 1b – Different Thrust/Drag Couples Module 08B ETBN 0492 October 2023 Edition 8 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Tailplane One reason for fitting a tailplane is to counter any residual out-ofbalance pitching moments arising from inequalities of the two main couples as already stated. The tailplane, although small in area and in lifting capabilities when compared to the main aerofoils, is able to balance large residual pitching moments due to the fact that it is placed some distance behind the Centre Of Gravity (CoG), and can exert considerable leverage because of the moment produced. At high speed the angle of attack of the mainplane will be small, which, in turn, will cause the Centre Of Pressure (CoP) to move rearwards, tending to make the aircraft’s nose drop as seen in Figure 1c. At low speed we see the opposite effect of the nose rising due to the CoP moving forward of the CoG as seen in Figure 1d. Fig 1c – High Speed – Nose Drops To counteract this tendency, the tailplane must carry a download force to re-balance the aircraft at high speed and upload force for low speed. Where a tailplane is likely, as in the above case, to carry loads in either direction, it may well be of symmetrical camber. This will mean that when the angle of attack of the tailplane is 0°, the chord line of the section will also be the neutral lift line. Whilst a tailplane is designed to be set at a definite angle of attack for normal flight, the variables of speed, angle of attack and changing loads, will, at times, require a different tailplane angle of incidence and to provide for this, some tailplanes are adjustable in flight. Module 08B ETBN 0492 October 2023 Edition Fog 1d – Low Speed – Nose Raises 9 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Forces In The Climb Strangely enough the lift required is now less than was needed for level flight, since the lift now supports only that component of the weight given by: During a climb the aircraft gains potential energy by virtue of increased altitude, which is achieved by either or both of two means: 𝑊 𝐶𝑜𝑠 𝛳 Where ϴ is the angle of climb.  By increasing the thrust above that required for level flight at a given speed.  By using the aircraft's kinetic energy (without increasing power, and accepting a gradual loss of airspeed). We obviously haven't been given something for nothing, since the thrust available now has to match not only the drag, but also the component of the weight given by It is normal to combine these two means so aircraft tend to climb at a reduced airspeed and with an increased power setting. 𝑊 𝑆𝑖𝑛 𝛳 During the climb the lift continues to act at right angles to the flight path and the weight vertically downwards, however the two are now no longer directly opposed as in figure 2. The weight must now be resolved into 2 components:  One acting in at 90º to the flight path which is supported by lift.  One acting parallel to the flight path but in the opposite direction to the direction of travel, and is supplemented by drag. Module 08B ETBN 0492 October 2023 Edition 10 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics LIFT THRUST DRAG Angle of Climb 𝜽 Vertical component of weight 𝑾𝑪𝒐𝒔𝜽 WEIGHT Horizontal component of weight 𝑾𝑺𝒊𝒏𝜽 VECTOR DIAGRAM True Airspeed Vector (V) Rate of Climb (ROC) Ground Speed (VG) Angle of Climb (𝜽) Figure 2 – Forces Components Acting In A Climb Module 08B ETBN 0492 October 2023 Edition 11 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Glide ratio is the ratio of the forward distance travelled to the vertical distance an aircraft descends when operated without power. Forces In A Glide And Glide Ratio The forces acting on an aircraft in a glide descent – that is, when no thrust is present; are as shown at figure 3. In looking at gliding angle and range, we have not yet considered one of the primary factors that will affect the gliding range – the prevailing wind. Gliding with a tail wind increases glide range, whereas gliding into a headwind reduces glide range. From this diagram it can be seen that a part of the aircraft’s weight vector is used to overcome the drag of the aircraft, while the other half of the vector must be overcome by lift in order to keep the aircraft in the air. The rate of descent is not affected by the prevailing wind. In other words, whether gliding into a headwind or with a tailwind, the aircraft will still reach the ground in the same time. What will alter is the distance it covers in the glide as shown in Figure 3a. In other words the aircraft’s potential energy (altitude) is used in the glide descent. The best glide angle depends upon the lift/drag ratio of the aircraft. We know that in order for an aircraft to stay in the air with the least amount of effort the wings must be operating at the optimum Lift/Drag ratio. We already know that to keep the aircraft in the air the lift produced by the wings must be equal to the weight of the aircraft, thus: 𝐿𝑖𝑓𝑡 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑎𝑐𝑡𝑖𝑛𝑔 𝑎𝑡 90° 𝑡𝑜 𝑡ℎ𝑒 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡𝑟𝑎𝑣𝑒𝑙 𝐿𝑖𝑓𝑡 = 𝑊 Cos 𝛳 Similarly we know that the drag force produced by the aircraft is fighting against the component of weight that is acting parallel to the direction of travel, thus: Figure 3a – Effects of Wind On Gliding Range 𝐷𝑟𝑎𝑔 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑎𝑐𝑡𝑖𝑛𝑔 𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑑 𝑡𝑜 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡𝑟𝑎𝑣𝑒𝑙 𝐷𝑟𝑎𝑔 = 𝑊 Sin 𝛳 Module 08B ETBN 0492 October 2023 Edition 12 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics LIFT DRAG Vertical component of weight 𝑾 𝑪𝒐𝒔 𝜽 WEIGHT Angle of Glide (𝜽) Horizontal component of weight 𝑾 𝑺𝒊𝒏 𝜽 VECTOR DIAGRAM True Airspeed Vector (V) Rate of Descent (ROD) Angle of Glide (𝜽) Ground Speed (VG) Figure 3 - Forces In A Glide Module 08B ETBN 0492 October 2023 Edition 13 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Theory Of The Turn A smooth turn entry can be made only by the coordinated use of ailerons and rudder. When you move the wheel to the right, apply a bit of right rudder pressure, just enough to prevent the nose moving to the left. Once the nose has started in the correct direction, the rudder is no longer needed and it can be centred. An aircraft is turned by banking it with the ailerons. When it is banked over, the lift force is tilted, since lift always acts perpendicular to the wings and the horizontal component of the lift vector, Figure 4, will move the aircraft in the direction it is tilted. The wind pushing on the tail surface will align the nose of the aircraft with its relative wind and the aircraft will turn in a smooth circular flight path. Figure 4a – Load Factor The vertical component of the lift vector must be equal to the weight of the aircraft and while the aircraft is turning, the centrifugal force adds an apparent weight, and the total lift must be equal to both the weight of the aircraft and the centrifugal force. In order to prevent the nose of the aircraft dropping in a turn, the pilot must pull the control wheel back to increase the angle of attack enough to produce the additional lift needed. To appreciate the amount of apparent weight the centrifugal force adds to the load on the wings in a turn, look at figure 4a, where we see the load factors that are developed in a coordinated turn. If a 6.000-pound aircraft is in a coordinated 60-degree banked turn, its load factor is 2.0, which means that the wings will have to support 12,000 pounds. Figure 4 – Forces In A Turn A turn is caused by tilting the lift produced by the wing. The vertical tail surfaces keep the aircraft turned into its relative wind. Module 08B ETBN 0492 October 2023 Edition 14 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Forces In The Turn During a turn the weight still acts vertically downward, and the lift continues to act parallel to the aircraft’s normal axis. The amount of lift which was sufficient to maintain straight and level flight is therefore insufficient to maintain straight flight with the aircraft tuning, see Figure 5. The horizontal component of the total lift vector is used to provide the centripetal force which causes the aircraft to follow a curved path and stops it side slipping in the direction of the turn. The additional lift which is required is necessarily generated by increasing the angle of attack, and this will result in a decrease in airspeed (and subsequent decrease in lift) unless the power is increased during turns using significant bank angles. The increase in lift which is required in a turn may be considered to compensate for the apparent increase in aircraft weight during the turn since we know that in order to keep an aircraft from descending lift must equal weight. In a turn, it is the lift force acting at 90º to the wings that must increase in order to increase its vertical component that now has to overcome the weight. Module 08B ETBN 0492 October 2023 Edition 15 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics LIFT LIFT Additional lift required for turn Additional lift required for turn Lift required for level flight Bank Angle (𝜃) Lift required for level flight 30º Bank Angle WEIGHT Horizontal component of Lift 𝑾 𝑺𝒊𝒏 𝜽 WEIGHT 60º Bank Angle Vertical component of Lift (lift required for level flight) 𝑾 𝑪𝒐𝒔 𝜽 Figure 5 – Forces In A Turn Module 08B ETBN 0492 October 2023 Edition 16 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Negative Load Factor Load Factor Previously it has been demonstrated that to bank an aircraft and maintain altitude, lift has to be greater than weight. And that, additional lift in a turn is obtained by increasing the angle of attack. To consider the relationship between lift and weight we use Load Factor. Under certain conditions, an abrupt deviation from the aeroplane’s equilibrium can cause an inertia acceleration that in turn will cause the weight to become greater than the lift. For example during a stall, the load factor may be reduced to zero. This will cause the pilot to feel “weightless”. Load Limits Both excessive deviation from positive and negative load factor limits must be avoided because of the possibility of exceeding the structural load limits of the aeroplane. Load Factor is the ratio of the total load supported by the wing to the total weight of the aeroplane. In straight and level flight the load on the wings is equal to lift and to the weight. Consequently, the load factor equals 1. A sudden and forceful elevator control movement forward can also cause the load factor to move into a negative region. The Load Factor is affected by: a) b) c) d) e) Both excessive deviation from positive and negative load factor limits must be avoided because of the possibility of exceeding the structural load limits of the aeroplane. Increasing lift in a turn Increasing the bank angle Increasing speed Flight manoeuvres Turbulence The load factor may either be positive or negative. Positive Load Factor: During normal flight, the load factor is 1 or greater than 1. Whenever the load factor is 1 or greater the load factor is defined as positive. Module 08B ETBN 0492 October 2023 Edition 17 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Figure 6 – Effect Of Wing Loading In A turn The load supported by the wings increases as the angle of bank increases. The increase in load is shown by the relative lengths of the arrows. During a curved flight path, the load the wings must support will be equal to the weight of the aircraft plus the load imposed by the centrifugal force. Module 08B ETBN 0492 October 2023 Edition 18 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Co-ordinating Turns When an aircraft changes its flight path from straight and level flight to a curved path, a force is required to overcome the inertia which wants to maintain the original flightpath. The force required to overcome this inertia is a centripetal force. An equal and opposite force is produced which is a centrifugal force. This acts towards the outside of the turn. (Figure 6a). As we have already seen, when an aircraft banks by use of the ailerons, lift is no longer acting vertically and has an unopposed horizontal component as in Figure 6b. Figure 6a - Turn Coordination Module 08B ETBN 0492 October 2023 Edition 19 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Lift, Load And Sideslip Curved flight producing a positive load is the result of increasing the angle of attack and lift. Increasing the lift of an aircraft always increases the positive load imposed on the wings at the time the angle of attack is being increased. Once the angle of attack is established, the load remains constant. Figure 6c - Positive and Negative AOA Figure 6b - Sideslip Figure 6b and 6c shows the action of an aircraft following a disturbance with airflow meeting the lower wing at a positive angle of attack and the higher wing at a negative angle of attack. Module 08B ETBN 0492 October 2023 Edition 20 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Figure 6e – Sideslip Recovery With Dihedral And Fin During a right or left turn manoeuvre in an aircraft, a partial sideways movement is experienced which is known as slip or skid. Slip is downward and inward toward the turn. Figure 6d – Sideslip A skidding movement is sideways and outward from the turn. Module 08B ETBN 0492 October 2023 Edition 21 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics The manoeuvre envelope (or VN Diagram) is a graphic representation of the operating limits of an aircraft. The envelope is used to: Influence Of Load Factor Flight Envelope And Structural Limitations a) b) c) Lay down design requirement for a new aircraft Illustration of an aircraft’s capabilities A means to compare different types The vertical axis is load factor or “g”, both positive and negative and represented in figure 7 as “n”, the horizontal axis is indicated airspeed (IAS). A typical manoeuvring load diagram is illustrated in Figure 7. Identifying the varying features on the diagram; Load Factor ‘n’ The load factor is the vertical scale and as the aircraft manoeuvres more load is felt, there is a normal limit not to be exceeded given as the limit load factor. There is normally a safety margin such that if an excursion is experienced structural damage may result, the ultimate load factor is the point where it is known that structural failure will occur. CLmax Boundary The CLmax boundary is the point at which the stall occurs at various speeds and loading. On the diagram the positive and negative CLmax boundary lines are marked. VA Design Manoeuvring Speed The VA design manoeuvring speed is the maximum speed at which the aircraft can be manoeuvred to the g limits without damaging the structure, the speeds are different for negative and positive g and the single VA is taken as the higher. These speed are illustrated with a dotted line. Figure 7 – Manoeuvring (Flight) Envelope Module 08B ETBN 0492 October 2023 Edition 22 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics VC Design Cruising Speed light aircraft. The limiting load factors are based on the maximum weight of the aircraft. The design cruising speed is selected by the manufacturer and requires a margin so that an upset will not cause the aircraft to exceed the maximum speed, another term is V normal operating or VNO. Load Factor Limits The Load factor limits for Part 23 (Commuter) aircraft in a typical cruise condition and with flaps extended are given in CAA/EASA Part 23 for limit manoeuvring load factors. The following limits are for information only. The positive limit manoeuvring load factor n may not be less than:  2.1+ 24,000 / W + 10,000 for normal and commuter category aeroplanes (where W = design maximum take-off weight in pounds) n need not be more than 3.8g.  4.4g for utility category aeroplanes; or  6.0 for aerobatic aeroplanes. The negative limit manoeuvring load factor may not be less than:  0.4 times the positive load factor for the normal, utility and commuter categories (-1 for normal category and -1.76 for utility category aircraft).  0.5 times the positive load factor for the aerobatic category (-3 for aerobatic category aircraft). The limiting load is given in terms of load factor to make the requirement general to all aircraft. However, it should be appreciated that failure of the structure will occur at some particular applied load. For example, if the structure fails at 10000Lbs load, an aircraft weighing 4000Lbs will reach this load at a load factor of 2.5. However, if the aircraft weighs 5000Lbs the failing load is reached at a load factor of 2, i.e. it takes less ‘g’ to over stress a heavy aircraft than a Module 08B ETBN 0492 October 2023 Edition 23 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics As the aircraft wing loading increases (perceived aircraft weight increasing) the α Angle Of Attack (AoA) needs to increase to generate more lift to keep the aircraft flying safely. Note that if you look at the curve in Figure 7a, as positive or negative Load Factor increases, so does stall speed, meaning the aircraft has to fly faster to compensate for the load factor to avoid the aerodynamic stall. If an aeroplane is pulled up too sharply, until its forward speed reduces to a point where lift is less than gravity, the aeroplane will begin to lose altitude. High-speed stalls occur when an aeroplane pulls up so quickly that the angle of attack exceeds the stall angle. Stalls are more likely to occur during turns than in straight and level flight because greater lift is required to maintain level flight in a turn due to Load Factor. An experienced pilot can usually sense or “feel” an upcoming stall condition because of the way the controls feel to them and by the manner in which the aircraft is reacting. Aerodynamic Stall Many times the aircraft will start to "buffet", or shake, because of the flow separation. Flow separation occurs on the wing and turbulent air buffeting can occur on the tail surfaces. The flight controls in the cockpit become "sloppy" and do not have the solid feel of normal flight. A stall occurs when the angle of attack becomes so great that the laminar airflow separates from the surface of the aerofoil destroying the low-pressure area normally existing on the upper surface of the wing in flight. Aircraft are equipped with stall warning devices such as a small vane mounted near the wing leading edges arranged so it will actuate a switch when it rises because of excessive angles of attack. From Figure 7a, it can be seen that Positive and Negative Load Factors affect the Indicated Airspeed (IAS) at which a stall occurs and also “G” limits for the airframe. This causes a warning horn to sound when the angle of attack approaches the stalling speed. Other stall warning devices can be a stick shaker and/or a stick pusher. Figure 7a – Flight Envelope Diagram Example Module 08B ETBN 0492 October 2023 Edition 24 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Figure 8a – The Lift Curve Figure 8a shows that as the angle of attack increases from the zero lift value, the curve is linear over a considerable range. As the effects of separation being to be felt, the slope of the curve begins to fall off. Figure 8 – Stall And Airflow Separation Stages Eventually, lift reaches a maximum and begins to decrease. The angle at which it does so is called the stalling angle or critical angle of attack, and the corresponding value of lift coefficient is CL MAX. An aeroplane can be stalled at any true airspeed or attitude. Module 08B ETBN 0492 October 2023 Edition 25 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Lift Augmentation High Lift Devices An aeroplane is a series of engineering compromises. We must choose between stability and manoeuvrability and between high cruising speed and low landing speed, as well as between high utility and low cost. High lift devices allow the designer to not only make a wing that is efficient at cruise, but also efficient at landing and take-off – thus allowing for lower take-off and landing speeds. There are a variety of high lift devices (shown in Figure 9) that work by changing the camber of the wing once deployed. These devices have a range of deployment states, ranging from fully retracted – in cruise; to fully deployed – at landing. The degree of deployment is different for various aircraft. High lift devices can be separated into 3 groups depending on their location on the wing:  Leading edge  Trailing edge  Boundary Layer Control Module 08B ETBN 0492 October 2023 Edition 26 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Highlift liftdevices devices – leading High – leading edge edge High – leading edge edge Highlift liftdevices devices – trailing Figure 9 – High Lift Devices Locations On Aircraft Module 08B ETBN 0492 October 2023 Edition 27 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Leading Edge Devices Slats Leading edge devices not only increase the camber, and hence the lift produced by the wing at various phases of flight, but they also increase the angle of attack of the wing – thus increasing lift even further. A slat is a moveable leading edge. When not in use, during cruise, it forms the leading edge of the wing. When required for additional lift during take-off, the slat can be moved on programming tracks to an intermediate position. The trailing edge of the slat is still in contact with the wing upper surface, forming an increased camber and wing surface area. This increases lift without undue increase in drag. Slots Slots are nozzle-shaped passage through a wing, designed to improve the airflow conditions at high angles of attack and slow speeds. It is normally placed very near the leading edge and is built into the wing. As the angle of attack of the wing increases, more of the air is deflected through the slot, thus maintaining a stream line flow around the wing. For landing, the slat is deployed further into the airflow. This results in the trailing edge of the slat moving away from the wing structure, forming a slot, which acts as previously described to allow energised air to flow to the wing upper surface. The slat, in this position, further increases camber and wing area, increasing lift, whilst also generating increased drag, which is useful in reducing aircraft speed, Since the slot is of use only at high angles of attack, at the normal angles its presence serves only to increase drag. This disadvantage can be overcome by making the slot movable so that when not in use it lies flush against the leading edge of the wing. In this case the slat is hinged on its supporting arms so that it can move to the operating position at which it gives least drag. This type of slot is fully automatic in that its action needs no separate control Module 08B ETBN 0492 October 2023 Edition 28 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Kruger Flap Another method for providing the leading edge flap is to design an extendible surface that ordinarily fits smoothly into the lower part of the leading edge. When the flap is required, the surface extends forward and downward. This flap is mainly used where there is no room to use a slotted flap, such as around the pylon to wing area. Figure 10 illustrates the location, profile and basic operation of the Kruger Flaps and Slats Module 08B ETBN 0492 October 2023 Edition 29 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Slotted Flaps Trailing Edge Devices Wing Flaps Have been developed to provide even more lift than the flaps described previously. When such flaps are extended, either partially or completely, one or more slots are formed near the trailing edge of the wing (Figure 11). A wing flap, illustrated in Figure 11 is defined as a hinged, pivoted, or sliding aerofoil, usually near the trailing edge of the wing. It is designed to increase the lift and drag, when deflected. Wing flaps are used for both take-off and landing phases of flight. The slots allow air from the bottom of the wing (high-energy air) to flow to the upper portion of the flaps and downward at the trailing edge of the wing. For take-off, an intermediate setting is used. This gives an increase of lift with little increase in parasite drag, allowing a shorter take-off run and lower take-off speed. This aids in preventing the airflow from breaking away into turbulence. When lowered there is increased lift for similar angles of attack of the basic aerofoil and the maximum lift coefficient is greatly increased. For landing, the flaps are lowered fully. The increase in camber and in some cases surface area gives an increase of lift for any given speed. This allows a lower approach speed. At the same time parasite drag is increased significantly. This allows for a steep approach without an increase in speed. Slotted Fowler Flap This flap (Figure 11) is constructed so that the lower part of the trailing edge of the wing rolls back on a track, thus increasing the effective area of the wing and at the same time lowering the trailing edge. The advantages are that obstacles on the approach can be cleared easily and the landing run will be shorter with less wear on the landing gear. The Fowler flap not only increases the surface area of the wing, as well as camber, but also uses a similar principle to the slotted flap for renewing the boundary layer over the top of the flap – thus preventing early separation of the boundary layer and hence stalling of the airfoil. On large transport category aircraft, it is standard practice to have higher lift producing flaps located on the inboard trailing edge, and lower lift creating flaps on the outboard trailing edge. Flower flaps can be single – only one rearwards moving surface (Boeing 777 outboard flap), double – two rearwards moving flaps (Airbus A320), and even triple – three rearwards moving flaps (Boeing 747) The reason for this is to prevent the over stressing of the wing due to the bending moments produced by the additional lift. Module 08B ETBN 0492 October 2023 Edition 30 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Area of energized airflow Area of energized airflow Single Slotted Fowler Flap Slotted Flap Double Slotted Fowler Flap Areaofofenergized energized airflow Area airflow Area of energized airflow Triple Slotted Fowler Flap Area of energized airflow Figure 11 – Trailing Edge Devices Module 08B ETBN 0492 October 2023 Edition 31 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Flaperons Flaperons are flight controls that, much like elevons and ruddervators; have a dual purpose – to act as a flap and also as an aileron. They are usually in the position that you would expect to find the inboard ailerons. An example of this can be found on the Boeing 777. On the A320 and A330, this function is referred to as ‘aileron droop’, but in essence it achieves the same goal. The flaperon is used to aid the rolling of the aircraft, just as a normal aileron – that is, it is lowered on the up going wing, and raised on the down going wing. Droop (Degrees) When operating as a flap, the deployment of the flaperon may not exactly mimic that of the flaps, and may even be assisted by the use of the aileron under certain conditions. The difference between an aileron lowering during the extension of the flaps and the operation of a flaperon, is put simply – a flaperon droops more, and an aileron is only used to supplement the other high lift devices. The operating profile of a Flaperon and standard Aileron is shown in Figure 11a. Flap Position (Units) Figure 11a – Flaperon And Aileron Operating Profiles Module 08B ETBN 0492 October 2023 Edition 32 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Drag Inducing Devices Aerodynamic brakes are devices, which when deployed disturb the patterns of smooth airflow. This produces an increment of drag and also decrement of lift, depending on the kind of device. They are two kind of devices mainly in use: 1. 2. Wing installed (drag increment and lift decrement) Fuselage installed (drag increment) Drag inducing devices are used in the following flight manoeuvres:     Approach (reduction of glide ratio) Rapid descent Landing (shortening of roll-out distance) Turning flight (spoilers only) Figure 12 – Spoiler (Speed Brake Use And Effectiveness) Module 08B ETBN 0492 October 2023 Edition 33 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Spoilers If roll spoilers (see in Figure 12b) are used to augment the roll rate obtained from the ailerons, they will reduce the adverse yaw, as the down-going wing will have an increase in drag due to the raise spoiler. A spoiler is a control device that destroys lift over a part of the wing. They are deployed to allow a rapid rate of descent, while still retaining full control. This function of spoilers is normally named “speed-brake” or “flight spoilers”. They can be retracted to regain full lift when the desired altitude is reached. Spoilers can also be given the name “lift dumpers” or “ground spoilers”. In this configuration the spoilers are extended fully on both wings to completely destroy all the lift on both wings when the aircraft has landed. When this happens the complete weight of the aircraft is transferred to the undercarriage – this increasing the friction between the tyres and the runway – increasing the effectiveness of the braking system. Figure 12b - Roll Control Spoilers Fig 12a – Spoilers Deployed During Landing Depending which configuration the spoiler is being used in will dictate the extent to which the spoiler is extended. Module 08B ETBN 0492 October 2023 Edition 34 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Some older aircraft used stall wedges located on the inboard leading edge structure of the wing so as to ensure that the inboard part of the wing will stall before the outboard section – containing the flight controls – thus giving the pilot opportunity to take corrective action. Vortex Generators Vortex generators (Figure 13) are low-aspect-ratio airflows arranged in pairs. The tip vortices produced by the aero foils pull high energy air down into the boundary layer and prevent the separation. These can be found on medium commercial air transport aircraft where air disturbances from the engines can adversely affect the airflow over the horizontal stabiliser and control surfaces – such as Boeing 737 (Figure 13b) Fences and other devices may also be used to prevent air from flowing toward wing tip. Turbulent Airflow Straightened Airflow Figure 13a – Leading Edge Vortex Generators Figure 13 – Vortex Generators Positioning And Function Module 08B ETBN 0492 October 2023 Edition 35 Basic Aerodynamics – Theory Of Flight British Airways Global Learning Academy – Basic Aerodynamics Boundary layer airflow re-energised by vortex generators, producing laminar flow over empennage and control surfaces Boundary layer airflow re-energised by vortex generators, producing laminar flow over empennage and control surfaces Jet Exhaust Figure 13b – Vortex Generators On An Empennage Module 08B ETBN 0492 October 2023 Edition 36 Basic Aerodynamics – Theory Of Flight