MOD 8.3 PART 2.pdf

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 –...

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