CASAMod11aAeroplaneAerodynAndFltContSysB111aSR20221004sml.pdf
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MODULE 11A Category B1 Licence CASA B1-11a Aeroplane Aerodynamics and Flight Control Systems Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduce...
MODULE 11A Category B1 Licence CASA B1-11a Aeroplane Aerodynamics and Flight Control Systems Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the written permission of Aviation Australia. CONTROLLED DOCUMENT 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 2 of 209 CASA Part Part 66 - Training Materials Only Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 3 of 209 CASA Part Part 66 - Training Materials Only Table of Contents Aeroplane Aerodynamics and Flight Controls (11.1.1) 8 Learning Objectives 8 Aeroplane Axes and Primary Flight Controls 9 Aeroplane Axes 9 Flight Control Systems 9 Primary Flight Controls 11 Roll Control 13 Ailerons 13 Roll Spoilers 14 Pitch Control 16 Elevators 16 Stabilator 16 Variable Incidence Stabiliser 17 Drag Reduction 19 T-Tail 20 Canards 21 Yaw Control and Rudder Limiters 23 Yaw Control and Rudder Limiters 23 Rudder Limiters 24 Elevons and Ruddervators 27 Elevons 27 Ruddervators 27 High-Lift Devices 30 Introduction to High-Lift Devices 30 Trailing Edge Flaps 30 Plain Flaps 31 Split Flaps 32 Slotted Flaps 33 Fowler Flaps 33 Slotted Fowler Flaps 34 Flaperons 36 Leading Edge High-Lift Devices 37 Krueger Flaps 37 Drooped Leading Edge Flap 38 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 4 of 209 CASA Part Part 66 - Training Materials Only Slots 39 Slats 40 Effect of Flaps and Slats on Stall Angle 41 Drag-Inducing Devices 42 Drag-Inducing Devices 42 Spoilers 42 Speed Brakes 44 Boundary Layer Control 48 The Boundary Layer 48 Wing Fences 48 Saw-Cut and Dog-Tooth Leading Edges 49 Vortex Generators 51 Stall Strips 53 Flight Control Tabs 56 Introduction to Flight Control Tabs 56 Trim Tab 56 Balance Tab 58 Anti-Balance Tab 58 Servo Tab 59 Spring Tab 60 Control Surface Bias Ground-Adjustable Tab (Fixed Tab) 60 Control Surface Balancing 62 Types of Control Surface Balancing 62 Control Surface Mass Balance 62 Aerodynamic Balance 63 Aerodynamic Balance Panels 64 Supplementary Media 66 Video Glossary 66 High-Speed Flight (11.1.2) 67 Learning Objectives 67 Subsonic, Transonic and Supersonic Flight 68 Introduction to High-Speed Flight 68 The Speed of Sound 69 The Doppler Effect 69 Standard Atmosphere 71 Mach Number 72 Subsonic Flight 73 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 5 of 209 CASA Part Part 66 - Training Materials Only Transonic Flight 74 Supersonic Airflow 75 Properties of High-Speed Flight 79 Critical Mach Number 79 Shock Waves 80 Supersonic Flight 85 Compression Buffet (High-Speed Stall Buffet) 87 Area Rule 88 Aerodynamic Heating 92 Engine Intakes of High-Speed Aircraft 94 Supersonic Engine Inlet Ducts 94 Flight Controls I (11.9) 96 Learning Objectives 96 Flight Controls Systems and Devices 97 Aircraft Flight Controls 97 Primary Control Surfaces 99 Ailerons 99 Elevator 100 Rudder 101 Instinctive Controls 102 Trim Control 104 Roll, Pitch and Yaw Trimming 104 Aileron Trim (Electrical) 107 Pitch Trimming (Manually, Electrically and Hydraulically) 109 Automatic Pitch Trim 110 Active Load Control 114 Load Alleviation 114 High-Lift Devices 115 Trailing Edge Flaps 115 Hydraulic Flap Drive Systems 115 Mechanically Controlled, Hydraulically Operated Flap System 116 Electrically Controlled, Hydraulically Operated Flap System 116 Electrically Controlled Flap System 119 Pneumatic Flap Systems 121 Leading Edge Flaps 122 Slats 124 Leading and Trailing Edge Devices Position Indication 127 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 6 of 209 CASA Part Part 66 - Training Materials Only Lift Dump, Speed Brakes and Spoilers 133 Spoilers 133 Speed Brake System Operation 134 Ground Spoilers 134 Flight Controls II (11.9) 141 Learning Objectives 141 Flight Control System Operation 142 Operating Methods 142 Aerodynamically Controlled Systems 147 Hydraulically Assisted Control Systems 148 Hydraulic Power-Operated Systems 150 Follow-Up Control 152 Faults and Testing 154 Electrical Flight Control 155 Fly-by-Wire Philosophy 156 Cockpit Controls 157 Control Configurations 158 Mechanical Backup 161 Fly-by-Wire Operation 162 Flight Control Supporting Systems 168 Artificial Feel and Centring 168 Computer-Controlled Artificial Feel and Centring System 170 Yaw Dampers 172 Mach Trim Systems 175 Rudder Limiters 176 Gust Locks 180 Balancing and Rigging 185 Mass Balancing 185 Control System Rigging 188 Independent Inspections 198 Stall Protection Systems 199 Stall Systems 199 Stall Warning Devices 200 Large Aeroplane Stall Warning 203 High Angle of Attack Protection in Fly-by-Wire Systems 206 Stall Strips 208 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 7 of 209 CASA Part Part 66 - Training Materials Only Aeroplane Aerodynamics and Flight Controls (11.1.1) Learning Objectives 11.1 Describe aeroplane axes of rotation (S). 11.1.1.1.1 Describe the operation and effects of roll control using ailerons and spoilers (Level 2). 11.1.1.1.2 Describe the operation and effects of pitch control using elevators, stabilators, variable incidence stabilisers and canards (Level 2). 11.1.1.1.3 Describe the operation and effects of yaw control using rudder and rudder limiters (Level 2). 11.1.1.2 Describe the operation and effect of flight control using elevons and ruddervators (Level 2). 11.1.1.3 Describe the operation and effect of high-lift devices including slots, slats, flaps and flaperons (Level 2). 11.1.1.4 Describe the operation and effect of drag-inducing devices including spoilers, lift dumpers and speed brakes (Level 2). 11.1.1.5 Describe the aerodynamic effects of wing fences and sawtooth leading edges (Level 2). 11.1.1.6 Describe the aerodynamic effects of boundary layer control including vortex generators, stall wedges or leading-edge devices (Level 2). 11.1.1.7 Describe the operation and effect of trim tabs, balance and anti-balance (leading) tabs, servo tabs, spring tabs, mass balance, control surface bias and aerodynamic balance panels (Level 2). 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 8 of 209 CASA Part Part 66 - Training Materials Only Aeroplane Axes and Primary Flight Controls Aeroplane Axes 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. The three axes pass through a common reference point called the centre of gravity (CG), which is the theoretical point where the entire weight of the aircraft is concentrated. Since all three axes pass through the CG, the aeroplane always moves about its CG regardless of which axis is involved. The Longitudinal Axis The longitudinal axis is a straight line passing through the fuselage from nose to tail. Motion about the longitudinal axis is called roll, and this axis is often referred to as the roll axis. Roll control is provided by ailerons and/or roll spoilers. Control about the longitudinal axis is called lateral control. The Lateral Axis The lateral axis is a straight line passing through the CG at right angles to the longitudinal and vertical (normal) axes. It is parallel to a line joining the wing tips. Rotary motion about the lateral axis is called pitching. Pitch control is provided by the elevators. Control about the lateral axis is called longitudinal control. Vertical or Normal Axis The vertical axis is a straight line passing through the CG at right angles to the longitudinal and lateral axes. Rotary motion about the vertical axis is called yaw. Yaw control is provided by the rudder. Control about the vertical axis is called directional control. The three axes 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 9 of 209 CASA Part Part 66 - Training Materials Only Flight Control Systems Flight control systems are usually divided into two major groups: Primary or main flight control surfaces Secondary or auxiliary control surfaces. Flight control groups 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 10 of 209 CASA Part Part 66 - Training Materials Only Flight Controls The flight controls of an aircraft do no more than modify the camber, or aerodynamic shape, of the surface to which they are attached. This change in camber creates a change in the lift and drag produced by the surface, with the immediate result of rotating the aeroplane about one of its axes. This rotation produces the desired changes in the flight path of the aircraft. Effect of flight control movement on camber Relevant Youtube link: The aerodynamics of flight 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 11 of 209 CASA Part Part 66 - Training Materials Only Primary Flight Controls An aeroplane is controlled by rotating it about one or more of its three axes. The ailerons rotate it about its longitudinal axis to produce roll, elevators or their equivalent rotate it about its lateral axis to produce pitch, and the rudder rotates it about its vertical axis to produce yaw. Aviation Australia Ailerons Relevant Youtube link: How Flight Controls Work 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 12 of 209 CASA Part Part 66 - Training Materials Only Roll Control Ailerons The wings have two types of control surfaces attached to the rear, or trailing, edges. These are referred to as ailerons and flaps. Ailerons extend from about the midpoint of each wing outwards to the tip. They move in opposite directions: when one aileron goes up, the other goes down. The ailerons are moved by turning the control wheel in the cockpit. When the wheel is turned to the left, the left aileron moves up and the right one moves down. Turning the wheel to the right has the opposite effect. The flaps operate using a switch or handle located in the cockpit. Aileron Drag (Adverse Yaw) Displacement of the ailerons causes an undesirable effect called aileron drag. The aileron that moves downwards has the most influence on aileron drag. It creates more of both lift and drag. This drag out near the wing tip pulls the nose of the aircraft around in the direction opposite the turn. This effect is called adverse yaw. Aileron drag is a big problem caused by the displacement of the ailerons. When an aeroplane is rolled to the right, the lift produced by the wing, which acts perpendicular to the lateral axis, now has a horizontal component that pulls the nose to the right. But when the left aileron moves down to increase lift on the left wing and start the bank, it also increases the induced drag that pulls the nose to the left. Differential Ailerons The movement of the nose in the wrong direction at the beginning of the turn is called adverse yaw. This condition can be minimised using differential ailerons. The aileron moving upwards travels a greater distance than the one moving downwards. The extra upward travel creates just enough parasite drag to counteract the induced drag caused by the lowered aileron. Differential ailerons 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 13 of 209 CASA Part Part 66 - Training Materials Only Frise Ailerons Another way to minimise adverse yaw is to use Frise ailerons. Extending the leading edge of the up aileron below the lower surface of the wing produces additional parasite drag. This counteracts the increased induced drag caused by the down-going aileron on the opposite wing. Frise ailerons Roll Spoilers Roll spoilers are hinged surfaces located ahead of the flaps on the upper surface of the wing. They are used in conjunction with the ailerons to assist in roll control. Ordinarily they lie flush with the wing surface and have no effect on the performance of the aerofoil, but they can be connected to the aileron controls so that that when an aileron is moved up beyond a certain angle, the spoiler is raised. Aviation Australia Roll spoilers The roll spoilers on the wing with the up-moving aileron automatically deploy to decrease the lift on the wing that is moving down. They also produce additional parasite drag to overcome adverse yaw. Thus, we have a large rolling and yawing effect in the right direction. When a large amount of aileron is used, the spoilers account for about 80% of the roll rate. In the following example, roll spoilers 2, 3, 4 and 5 are used in conjunction with the aileron. Transport category aircraft use spoilers as a part of the secondary flight control system. They can be used as an aid for the ailerons, to relieve control pressures, and to increase and decrease lift. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 14 of 209 CASA Part Part 66 - Training Materials Only Roll spoilers 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 15 of 209 CASA Part Part 66 - Training Materials Only Pitch Control Elevators The elevators, which are the horizontal movable control surfaces mounted on the tail of the aircraft, cause the aircraft to rotate about its lateral axis. Elevators are attached to the back of the horizontal stabiliser. During flight, the elevator is used to move the nose up and down to direct the aeroplane to the desired altitude. When the control wheel is pulled back, the trailing edge of the elevator moves upwards and deflects the airstream upwards. This increases the download on the tail and rotates the nose of the aircraft upwards. When the control wheel is pushed forward, the trailing edge of the elevators moves downwards and deflects the air downwards. This pushes the tail up, lowering the nose. Elevator movement is intuitive: pull the control column back, and the elevator moves up, the aeroplane rotates about its lateral axis and the nose moves up. If you push the control column forward, the elevator moves down and the nose of the plane moves down. Aviation Australia Elevators 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 16 of 209 CASA Part Part 66 - Training Materials Only Stabilator The stabilator, or all-movable tail, is one form of horizontal tail surface that is finding a good deal of popularity. This type of horizontal tail surface has no fixed stabiliser, but has an almost full-length anti-balance tab on its trailing edge. The area ahead of the hinge line makes stabilators overly sensitive, and the anti-balance tab helps decrease this sensitivity. These tabs move in the same direction as the surface. The horizontal stabilator pivots up and down from a central hinge point and requires no elevator. The stabilator is moved using the control wheel, just as you would the elevator. When the control wheel is pulled back, the leading edge of the stabilator moves down and increases the downward force produced by the tail. This rotates the nose up. When the wheel is pushed forward, the nose of the stabilator moves up, decreasing tail load, and the aeroplane rotates nose down. Stabilator and anti-balance tab Stabilator 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 17 of 209 CASA Part Part 66 - Training Materials Only Variable Incidence Stabiliser Variable Incidence Stabiliser Pitch Trimming The purpose of the adjustable stabiliser is to maintain a balance of forces to prevent undesired rotation along the pitch axis. The forces that are exerted on the aircraft are: A downward force caused by the pull of gravity on the aircraft An equal upward force during horizontal flight (lift). These two forces do not have the same point of application. Gravity is applied to the aircraft's CG. The position of the CG is not fixed and depends on the: Amount of fuel Number of passengers Amount and distribution of the cargo. The CG position may vary slightly, but not too much. During flight, it will move slowly in the direction of the nose of the aircraft as a result of fuel consumption. Lift is applied to the wing (pressure point). Just as with the CG, the pressure point is not a fixed point. Its position depends on the airspeed and moves backwards when the airspeed increases. Depending on the position of the CG and the pressure point, a combination is created which causes an upward or downward movement of the aircraft's nose (rotation along the pitch axis). To compensate, the stabiliser must create an opposite force. This force is created by giving the stabiliser another position in relation to the airflow. Adjusting the stabiliser is done by means of screw spindles, which are driven hydraulically, electrically or by a combination of both. Aviation Australia Variable incidence stabiliser 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 18 of 209 CASA Part Part 66 - Training Materials Only Aviation Australia Trimmable horizontal stabiliser Trimmable horizontal stabiliser The pilot operates the trim actuator using the pitch trim wheel or trim switches in the cockpit. Variable incidence horizontal stabilisers are used in high-speed Transport category aeroplanes. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 19 of 209 CASA Part Part 66 - Training Materials Only Drag Reduction Forces of air on the elevator and an adjustable stabiliser reduce drag. Using the trimmable stabiliser to correct pitch trim instead of the elevator trim reduces the amount of parasite drag generated. Forces on a trimmable stabiliser 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 20 of 209 CASA Part Part 66 - Training Materials Only T-Tail Several modern aeroplanes use the T-tail configuration. The horizontal tail surfaces are mounted on top of the vertical surfaces. This arrangement is mainly used with fuselage- or tail-mounted engines. The T-tail configuration places the horizontal tail surface at the top of the vertical surface. T-Tail 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 21 of 209 CASA Part Part 66 - Training Materials Only Canards Any aircraft that has the equivalent of two lifting surfaces, instead of the conventional horizontal stabiliser that provides a download, can be classified as a canard. The canard is the forward surface and is often also a control surface. In a conventional aeroplane, the wing stalls, aileron control is lost, the CG shifts forward, and then speed builds up and control is regained. During this sequence of events, there is always the chance that lateral control will also be lost, particularly when the aeroplane is in a turn, causing an accidental spin. With a canard configuration, the sequence changes somewhat: the canard stalls first, causing the nose to drop and the aircraft to build up speed. The canard regains full lift and the nose comes back up. The CG does not change, and full aileron control is always available. This virtually eliminates the chance of an inadvertent stall/spin accident. Canard 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 22 of 209 CASA Part Part 66 - Training Materials Only Yaw Control and Rudder Limiters Yaw Control and Rudder Limiters Yaw Control (Rotation About the Vertical Axis) The rudder is attached to the back of the vertical stabiliser to move the aeroplane’s nose left and right. The rudder is used in combination with the ailerons during flight to initiate a turn. When pressure is applied to the rudder pedals, the rudder deflects into the airstream. This produces an aerodynamic force that rotates the aeroplane about its vertical axis, referred to as yawing the aeroplane. The rudder may be displaced to either the left or right of centre, depending on which rudder pedal is depressed. The rudder is used only at the beginning of the turn to overcome adverse yaw and start the nose moving in the correct direction. It is also used for such flight conditions as crosswind and one-engine operations. Since the vertical stabiliser is also an aerofoil, deflection of the rudder alters the stabiliser’s effective camber and chord line. In this case, left rudder pressure causes the rudder to move to the left. With a change in the chord line, the angle of attack is altered, generating an aerodynamic force towards the right side of the vertical fin. This causes the tail section to move to the right and the nose of the aeroplane to yaw to the left. Some aeroplanes have connected the rudder to the aileron controls so that when a turn is started, the rudder automatically moves in the correct direction. This allows flight operation without actuation of the rudder pedals. Many aircraft incorporate the directional movement of the nose or tail wheel into the rudder control system for ground operation. This allows the operator to steer the aircraft with the rudder pedals during taxi, when the airspeed is not high enough for the control surfaces to be effective. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 23 of 209 CASA Part Part 66 - Training Materials Only Rudder, yaw control 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 24 of 209 CASA Part Part 66 - Training Materials Only Rudder Limiters The maximum deflection of the rudder, directly controlled by the pedals, is limited as a function of the aircraft speed. The amount of limitation applied keeps the loads exerted on the structure below the maximum allowable limit, yet maintains the authority necessary to enable aircraft trimming throughout the flight envelope. This includes the ability to correct for asymmetric flight caused by an engine failure on multi-engine aircraft. This function is achieved by two identical units installed at two different locations: the pedal travel limitation unit and the rudder travel limitation unit. Pedal Travel Limiter Operation As airspeed increases, airspeed data is fed to digital primary and secondary controllers, which energise the two electric motors. The motors rotate jack screws through gear trains. The jack nuts move along the jack screws as they rotate and position the movable stops. The movable stops limit the travel of the rudder input and output links (the output is linked to the rudder hydraulic actuator’s control rods), thus limiting both the pilot’s input to the rudder and the amount of rudder deflection. Rudder limiter position is fed back to the controllers by the four position and feedback transmitters. The controlling and feedback systems are duplicated for redundancy. Rudder pedal travel limiter 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 25 of 209 CASA Part Part 66 - Training Materials Only Aviation Australia Rudder limiting versus speed 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 26 of 209 CASA Part Part 66 - Training Materials Only Elevons and Ruddervators Elevons Control Using Elevons The ailerons, elevators and rudder are considered conventional primary control surfaces. However, some aircraft are designed with a control surface that may serve a dual purpose. Elevons are control surfaces used on tailless aeroplanes like the Concorde. It combines the function of both aileron and elevator. Movement of the control column backwards and forward causes the elevons to act as elevators, while rotating the control wheel causes the elevons to act as ailerons. A mixing unit makes it possible to move the surfaces so that you perform both functions at the same time. Elevons 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 27 of 209 CASA Part Part 66 - Training Materials Only Ruddervators Control Using Ruddervators On aircraft with tail empennages where the traditional horizontal and vertical stabilisers do not exist, two stabilisers angle upwards and outwards from the aft fuselage in a V configuration. For example, some Beechcraft Bonanza models have ruddervators that combine rudders and elevators into one control surface. The two fixed surfaces act as both horizontal and vertical stabilisers, while the movable surfaces are connected through a mixing type linkage that allows in-and-out movement of the control wheel to move both surfaces together for pitch and rudder control. Longitudinal and directional control are achieved with two fixed and two movable surfaces. Depending on control inputs, the ruddervators move in the same direction for pitch control, and in opposite directions for yaw control. Aviation Australia Ruddervators: (A) pitch control and (B) yaw control 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 28 of 209 CASA Part Part 66 - Training Materials Only Aircraft with ruddervator 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 29 of 209 CASA Part Part 66 - Training Materials Only High-Lift Devices Introduction to High-Lift Devices An aeroplane is a series of engineering compromises. Designers must choose between stability and manoeuvrability, and between high cruising speed and low landing speed, as well as between high utility and low cost. Primary flight controls rotate the aeroplane about its three axes, but secondary controls are used to assist or to modify the effect of the primary controls. Secondary flight controls may be extended only when needed, then tucked away into the structure at all other times. Aerodynamic lift is determined by the shape and size of the wing aerofoil section. Lift can be changed by modifying the shape and/or size of an aerofoil using devices known as slots and flaps. Increasing the camber of the wing is the easiest way to provide a greater pressure difference between the upper and lower wing surfaces when increasing speed and/or angle of attack is not a viable option (such as during landing and take-off). Slots and flaps are used, as and where necessary, to increase lift, change the stall angle or change the aircraft’s trim. This creates four basic types of secondary flight controls: Those that modify the amount of lift a surface produces: Trailing edge high-lift devices Leading edge high-lift devices. Those that change the amount of force needed to operate the primary controls: Those that control the aircraft’s trim Those that induce drag. Aviation Australia Flaps and camber 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 30 of 209 CASA Part Part 66 - Training Materials Only Trailing Edge Flaps Perhaps the most universal lift-modifying devices used on modern aircraft are flaps on the trailing edge of the wing. These surfaces change the cambers of the wing, increasing both lift and drag for any given angle of attack. They enable landing at slower speeds and shorten the amount of runway required for take-off and landing. The amount that the flaps extend and the angle they form with the wing can be selected from the cockpit. There are several configurations of trailing edge flaps. When most types of flaps are lowered to their full deflection, the airflow breaks away from the upper surface. When this happens, the increased lift produced by the flap is lost and only a great deal of drag results. To deflect the flaps the maximum amount without this airflow breakaway, leading edge slots or slats are often used. Plain Flaps Plain flaps are simple devices that consist merely of sections of the wing's trailing edge, inboard of the ailerons. They are about the same size as the aileron and are hinged from the top so they can be deflected. The effect of these flaps is minimal, and they are seldom found on modern aeroplanes. When deployed, this flap increases the angle of attack of the aerofoil, increasing wing camber and total drag. It is used only on low-speed aeroplanes. Aviation Australia Plain flap 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 31 of 209 CASA Part Part 66 - Training Materials Only Aviation Australia Plain flap extended Split Flaps The split flap is another design used with a great deal of success in the past, but seldom used today. On the extremely popular Douglas DC-3, a portion of the lower surface of the wing's trailing edge from one aileron to the other, across the bottom of the fuselage, could be hinged down into the airstream. The lift change was similar to that produced by a plain flap, but it produced much more drag at low lift coefficients. This drag coefficient changed very little with the angle of attack. Split flap 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 32 of 209 CASA Part Part 66 - Training Materials Only Split flap extended Slotted Flaps The most popular flap on aeroplanes today is the slotted flap. Variations of this design are used on small aeroplanes as well as on large ones. Slotted flaps increase the lift coefficient a good deal more than the simple flap. On small aeroplanes, the hinge is located below the lower surface of the flap, and when the flap is lowered, it forms a duct between the flap well in the wing and the leading edge of the flap. When the flap is lowered all the way and the airflow tends to break away from its surface, air from the high-pressure area below the wing flows up through the slot and blows back over the top of the flap. This high-energy flow on the surface pulls air down and prevents the flap stalling. It is not uncommon on large aeroplanes to have double and even triple-slotted flaps to allow the maximum increase in drag without the airflow over the flaps separating and destroying the lift they produce. Aviation Australia Slotted flap 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 33 of 209 CASA Part Part 66 - Training Materials Only Fowler Flaps This flap is constructed so that the lower part of the wing's trailing edge rolls back on a track, increasing the effective area of the wing and lowering the trailing edge. The flap itself is a small aerofoil that fits neatly into the trailing edge of the main wing when retracted. When deployed, initially the flap greatly increases wing area, giving more lift without a substantial increase in parasite drag. As the flap continues to move rearwards and then downwards, the wing angle of attack, camber and drag are increased. A disadvantage of this type of flap is that as the wing area is increasing, the boundary layer thickens, becoming turbulent towards the trailing edge and over the flap. As a result, the flap loses some of its effectiveness. Fowler flap 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 34 of 209 CASA Part Part 66 - Training Materials Only Slotted Fowler Flaps Some aeroplanes have single-, double- or triple-slotted fowler flaps to balance the lift and drag necessary for reasonable take-off and landing speeds with the requirements for high-speed cruising flight. The disadvantage of the Fowler flap is negated by the incorporation of slots. Triple-slotted Fowler flap Triple-slotted Fowler flap extended 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 35 of 209 CASA Part Part 66 - Training Materials Only Relevant Youtube link: Triple-slotted flap animation Relevant Youtube link: Boeing 747 – triple-slot flaps Flaperons Flaperons are ailerons which can also act as flaps. They combine the designs of ailerons and flaps. Differential movement imparts the same effect as ailerons, while collective movement produces the same effect as flaps. The Boeing 767, 777 and 787 use the inboard aileron as a flaperon. The 787 uses the inboard aileron differently depending on the flight phase. During cruise, it is an aileron. During the landing phase, it is a flaperon. During the landing roll-out, it is a combined spoiler and aileron. In some aircraft, the entire trailing edge is lowered to increase lift by increasing the camber, as is normal. Aileron control is provided by outer sections of the flaps or by spoilers on the upper surface. Aviation Australia Flaperons 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 36 of 209 CASA Part Part 66 - Training Materials Only Flaperons Twin Otter Leading Edge High-Lift Devices Some high-performance aeroplanes have flaps on the leading edges as well as on the trailing edges. Leading edge flaps are extended at the same time as trailing edge flaps to increase the camber of the wing and allow it to attain a higher angle of attack before the airflow breaks away over the upper surface. They extend forward and downwards. A leading edge device is a high-lift device which reduces the severity of the pressure peak above the wing at high angles of attack. This enables the wing to operate at higher angles of attack than would be possible without the flap. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 37 of 209 CASA Part Part 66 - Training Materials Only Krueger Flaps The swept wings of large turbine-engine transports develop little lift at low speeds. To remedy this problem, leading edge devices called Krueger flaps are used to effectively increase a wing’s camber and hence its lift. A Krueger flap is hinged to a wing’s leading edge and lies flush with its lower surface when stowed. When the flap is deployed, it extends down and forward to alter the wing profile. The Krueger flaps shown here are on the wing of the Boeing 737. These flaps are electrically or hydraulically actuated and are used in conjunction with trailing edge flaps. They are controlled by movement of the trailing edge flaps. Krueger flap Krueger flap on an aircraft 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 38 of 209 CASA Part Part 66 - Training Materials Only Drooped Leading Edge Flap The drooped leading edge flap uses a jack-screw arrangement to push the leading edge of the wing against a hinge on its lower surface. This causes the leading edge to droop and increase the camber of the wing so it will deflect more air. Aviation Australia Drooped leading edge Slots Slots are nozzle-shaped passages through a wing, designed to improve airflow conditions at high angles of attack and slow speeds. They are normally placed near the leading edge and built into the wing to increase the stall angle. As the wing's angle of attack increases, more of the air is deflected through the slot, thus maintaining a streamlined flow around the wing. In normal flight, the slot causes a slight increase in drag due to turbulence around the openings. This is acceptable for slow-speed aircraft, but as speeds increase, the drag penalty becomes unacceptable. Slots 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 39 of 209 CASA Part Part 66 - Training Materials Only Slots on aircraft Slats The effect of the slat at the highest angles of attack is to boost the extent of the low-pressure area over the wing. Since the slat is useful only at high angles of attack, at the normal angles, its presence only increases drag. This disadvantage can be overcome by making the slat movable so that when it is 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 the least drag. Some aircraft have slats operated by hydraulic or electric actuators. Aviation Australia Slats 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 40 of 209 CASA Part Part 66 - Training Materials Only Relevant Youtube link: Flap and slat operation - growing wings Effect of Flaps and Slats on Stall Angle CL refers to the coefficient of lift. In the following graph, notice that the stall angle reduces with flaps only (however, the associated increase in CL permits a lower stall speed). The reason for the decrease in stall angle is the local increase in angle of attack at the location of the flap, and the increased camber at the flap location moves the separation point forward. Deploying the slats along with the flaps restores the stall angle and actually increases it beyond the no-flap position. This is because the associated slot increases the energy and approach of the airflow over the wing at that critical angle of attack. It is also because the associated drooping of the leading edge on some slat designs decreases the local angle of attack slightly. This is why (on larger airliners) slats are always deployed with flaps, often automatically, and usually only after the first stage of flap deployment. Slat deployment also increases the camber significantly, resulting in a large increase in lift coefficient. Thus, the deployment of slats with flaps is by far the preferred design configuration on large Transport category aircraft. Effect of Flap Deployment on Pitch When a flap is deployed, the bulk of the lift moves to the trailing edge of the wing, at the location of the flap. This produces a nose-down moment on the wing. Aviation Australia Effect of slat and flap deployment on stall angle 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 41 of 209 CASA Part Part 66 - Training Materials Only Drag-Inducing Devices Drag-Inducing Devices Aerodynamic brakes are devices which, when deployed, disturb the pattern of smooth airflow. This produces an increment of drag. Two kinds of devices are mainly in use: Wing-installed, which increase drag and reduce lift Fuselage-installed, which increase drag. Drag-inducing devices are used in the following flight manoeuvres: Approach and rapid descent – Speed brake function is commanded in the flight phase and used during approach to help reduce the glide ratio. Surfaces on both the left and right wings extend symmetrically, depending on the amount of pilot action on the speed brake lever. Roll order has priority over speed brake function. Landing – Ground spoiler function is armed during the flight phase by pilot action on the speed brake lever. This function is automatic; as soon as both main landing gear legs are compressed, it is activated. All spoilers of the left and right wings extend fully. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 42 of 209 CASA Part Part 66 - Training Materials Only Spoilers A spoiler is a control device that destroys lift by disrupting the airflow over a part of the wing. Spoilers are 'popped up' to kill lift on a portion of the wing, allowing a rapid rate of descent while still retaining full control. This function of spoilers is normally called the speed brake. Spoilers can be retracted to regain full lift when the desired altitude is reached. On the ground, both spoilers can be raised to help increase braking efficiency by increasing tyre contact pressure on the ground and providing additional drag, thus becoming ground spoilers. Spoiler operation 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 43 of 209 CASA Part Part 66 - Training Materials Only Effect of Spoilers Deploying wing spoilers affects airflow. The turbulent flow reduces the resultant lift, produces drag and increases stall speed. Therefore, flight spoiler use is usually inhibited when flaps are fully extended (very slow flight). Effect of Spoilers Speed Brakes High-performance aircraft have speed brakes, or spoilers, installed on their upper wing surfaces. When raised in flight, speed brakes reduce airspeed and/or allow the aeroplane to make a steep descent without gaining excessive speed. Speed brake function 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 44 of 209 CASA Part Part 66 - Training Materials Only Fuselage-Mounted Speed Brakes Some aircraft have large panels attached to the rear of their fuselage that, when opened in flight, create drag to reduce airspeed. They are also used to slow an aircraft after touchdown. Fuselage- mounted speed brakes have no effect on lift or stall speed. Fuselage mounted speed brake 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 45 of 209 CASA Part Part 66 - Training Materials Only Ground Spoilers Ground spoilers destroy lift and increase drag to help slow the aircraft and reduce landing roll. They deploy to their full opening when the aeroplane weight is on the landing gear and the cockpit speed brake lever is moved to the DEPLOY position. Another name for ground spoilers is lift dumpers. Ground spoilers Ground spoilers function 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 46 of 209 CASA Part Part 66 - Training Materials Only Aviation Australia Ground spoilers function 2 Relevant Youtube link: How flaps and spoilers work during landing 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 47 of 209 CASA Part Part 66 - Training Materials Only Boundary Layer Control The Boundary Layer The boundary layer is the region of air that flows immediately adjacent to the surface of the aerofoil. Boundary layer control devices are designed to delay airflow separation over the wing. Separation of the boundary layer causes wing stalls. Not only do high angles of attack cause turbulent boundary layers, but so do shock waves along the upper wing when flying near the speed of sound. Separation of the boundary layer also increases drag. Using boundary layer control devices, a drag reduction of up to 50% can be achieved. Devices used to control the boundary layer include: Wing fences Saw-cut and dog-tooth leading edges Vortex generators Stall wedges. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 48 of 209 CASA Part Part 66 - Training Materials Only Wing Fences The movement of air flow parallel to the leading edge of a swept-wing aircraft affects the boundary layer. A wing fence is a fixed vane that extends chord-wise across the wing of a swept-wing aircraft. Its purpose is to prevent air from flowing outwards along the span of the wing from inboard towards the wing tip, which is likely to cause airflow separation near the wing tips and lead to tip stalling and pitch-up. Wing fence Wing fence 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 49 of 209 CASA Part Part 66 - Training Materials Only Saw-Cut and Dog-Tooth Leading Edges On swept-wing aircraft, boundary layer separation can occur towards the tip first. Separation in this region can cause large changes in pitching moments. This effect is reduced by a saw cut or a dog tooth in the leading edge. Each cut or tooth generates a strong vortex which controls the boundary layer in the tip region. Saw-cut leading edge Dog tooth leading edge 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 50 of 209 CASA Part Part 66 - Training Materials Only Military aircraft 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 51 of 209 CASA Part Part 66 - Training Materials Only Vortex Generators Vortex generators prevent or delay separation or breakaway of the airflow from the surface of the aerofoil. The reason for the breakaway is that the boundary layer over the rear portion of the aerofoil becomes sluggish or slows down. To reinvigorate the airflow towards the surface of the aerofoil, vortex generators may be installed on the aerofoil at the point where this separation is most likely to occur. Vortex generators are short, low-aspect-ratio aerofoils arranged in pairs. The tip vortices of these aerofoils pull high-energy air down into the boundary layer, helping prevent the separation. Aviation Australia Vortex generators Vortex generator 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 52 of 209 CASA Part Part 66 - Training Materials Only Vortex generators are usually mounted in pairs Relevant Youtube link: How do vortex generator work on the Kitfox Wing? Where to place them? 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 53 of 209 CASA Part Part 66 - Training Materials Only Stall Strips It is important that a wing stalls progressively from root to tip so the ailerons can still provide lateral control throughout the stall. If the wing does not have this characteristic naturally, small, triangular stall strips are installed on the leading edge of the wing in the root area. When the angle of attack is increased enough for a stall to occur, the strips provide enough air disturbance to hasten the stall on the section of wing behind them. This loss of lift usually causes the nose of the aeroplane to drop while the outer portion of the wing is still flying, and the ailerons are still effective. It also causes the disturbed air to buffet the horizontal tail surfaces, providing the pilot with a feeling in the controls of the impending stall. Stall strip Stall strips are also used on some smaller aircraft. Where no slats are installed, aircraft can be equipped with stall strips at the wings’ leading edges. The purpose of the strips is to produce a small vortex which reaches the horizontal stabiliser. If the angle of attack becomes extreme, the pilot can feel the vibration caused by the vortex via the elevator surface. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 54 of 209 CASA Part Part 66 - Training Materials Only Stall strip Stall strip producing vortex 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 55 of 209 CASA Part Part 66 - Training Materials Only Flight Control Tabs Introduction to Flight Control Tabs The force of air against a control surface during high-speed flight can make it difficult to move and hold that control surface in the deflected position. A control surface might also be too sensitive for similar reasons. Several different tabs are used to overcome these types of problems. Flight control tabs 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 56 of 209 CASA Part Part 66 - Training Materials Only Trim Tab The longitudinal balance of an aeroplane is the one that is most affected by the airspeed variations in normal flight, so almost all aeroplanes have some method of adjusting the downward tail load as the airspeed is changed in flight to allow for hands-off flight at any airspeed. Small auxiliary devices on the trailing edges of the various primary control surfaces are used to produce aerodynamic forces. Trim tabs are used by the pilot to fly the aircraft at the desired attitude with little or no control input. Trim tabs are operated by separate controls in the cockpit. The simple adjustable trim tab is hinged to the trailing edge of the elevator. A jack screw mechanism inside the elevator varies the length of the actuating rod to the tab horn in flight. When the length of this rod is adjusted, the relationship between the tab and the elevator remains fixed for all positions of the elevator. Aviation Australia Trim tab Elevator trim tab on aircraft 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 57 of 209 CASA Part Part 66 - Training Materials Only Relevant Youtube link: Trim tab operating Balance Tab A balance tab works automatically to produce an air load on the control surface that helps the pilot move the surface. When the cockpit control is moved to raise the trailing edge of the control surface, the linkage pulls the balance tab so that it moves in the opposite direction. This opposite deflection produces an aerodynamic force that helps the pilot move the surface. The linkage for many balance tabs is adjustable to allow the position of the tab to be changed in flight so the tab can serve as both a trim tab and a balance tab. Balance tab 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 58 of 209 CASA Part Part 66 - Training Materials Only Anti-Balance Tab Anti-balance tabs are used where the control forces are excessively light. Rather than requiring help to move a stabilator or elevator against air loads, the area ahead of the hinge line makes them overly sensitive. To decrease this sensitivity, most stabilators or stabiliser-elevator configurations have anti- balance tabs extending across their entire trailing edge. These tabs move in the same direction as the surface. Anti-balance tabs are also constructed to serve as trim tabs by making the length of the actuating rod adjustable in flight. If the pilot needs to trim the aeroplane slightly nose-up, they lengthen the actuating rod, which brings the trailing edge of the anti-servo tab down relative to the stabilator. This deflects the air downwards over the stabilator, raising its trailing edge for this flight condition. Raising the trailing edge aerodynamically has the same effect on flight as the pilot holding a slight back- pressure on the control wheel. Anti-balance tab 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 59 of 209 CASA Part Part 66 - Training Materials Only Servo Tab Many larger high-speed aeroplanes do not have control systems connected directly to their large control surfaces, but rather use a servo tab to generate aerodynamic forces that move the large surfaces. A servo tab creates an aerodynamic force that moves the control surface to which it is attached. A servo tab is installed on the control surfaces of aeroplanes requiring such high control forces that it is impractical to move the primary surface itself. The cockpit control is attached to the servo tab so that it moves in the direction opposite that desired for the primary surface. Deflection of the servo tab produces an aerodynamic force that deflects the primary surface, which in turn rotates the aeroplane about the desired axis. Servo tab Relevant Youtube link: Servo tab Spring Tab Another device that helps pilots of high-speed aircraft is the spring tab. The control horn is free to pivot on the hinge axis of the surface, but it is restrained by a spring. For normal operation, when control forces are light, the spring is not compressed. The horn acts as though it is rigidly attached to the surface. At high airspeeds, when the control forces are too high for the pilot to operate properly, the spring collapses and the control horn deflects the tab in the direction required to produce an aerodynamic force that aids the pilot in moving the surface. A spring tab deflects only when the control forces are great enough to collapse the spring. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 60 of 209 CASA Part Part 66 - Training Materials Only Control Surface Bias Ground-Adjustable Tab (Fixed Tab) Some tabs are fixed to the surface and are adjustable only when the plane is on the ground. These tabs are used to produce a fixed air load on the control surface to trim the aeroplane against a permanent out-of-balance condition; the correction is accurate only at a given airspeed. The fixed tab deflects the primary control surface in one direction only. The control surface is said to be biased in the opposite direction to the tab. Fixed Tabs 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 61 of 209 CASA Part Part 66 - Training Materials Only Control Surface Balancing Types of Control Surface Balancing There are two types of control surface balance: Mass balance – prevents 'flutter' of the flight controls during flight Aerodynamic balance – assists the pilot to move the controls. Control Surface Mass Balance A phenomenon known as control surface flutter occurs at specific speeds (usually high speeds) and can cause vibration of the airframe. In severe conditions, it can lead to structural divergence and catastrophic structural failure. Flutter is an oscillation of the control surface which occurs due to bending and twisting of the structure under load. If the CG of the control surface is behind the hinge, its inertia causes it to oscillate about its hinge when the structure distorts. Flutter is eliminated by statically balancing the control surface using balance weights. The remedy for this problem is to set a specific mass some distance ahead of the control surface hinge-line. This makes the moment about the control surface hinge equal and opposite to the mass- moment of the control surface itself. Mass balance weights can be external or internal. Flight control surface mass balance (internal) The control surfaces for new aeroplanes are properly balanced at the factory. After the aeroplane undergoes overhaul, painting or repair of the control surfaces, the balance may be altered to the extent that flutter will occur in flight. The surface therefore needs to be mass balanced again. The horn may also house a lead weight which is used for mass balancing the control surface. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 62 of 209 CASA Part Part 66 - Training Materials Only Aviation Australia Flight control surface mass balance Aviation Australia Flight control surface mass balance 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 63 of 209 CASA Part Part 66 - Training Materials Only Aerodynamic Balance The control force that the pilot must exert to move the control surface depends on the airspeed and the area of the control surface. The larger the surface area and the faster the aircraft flies, the greater the force required to manoeuvre. For this reason, controls are often aerodynamically balanced to assist the pilot’s input force during manoeuvres. Horn Balance In the case of the rudder or elevator, aerodynamic balance is achieved when a portion of the flight control surface is extended out ahead of the hinge line. The portion ahead of the hinge line is known as a horn balance or overhang. The horn moves into the opposite airflow from the rest of the control surface and uses the airflow to aid in moving the surface. Aerodynamic horn balance 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 64 of 209 CASA Part Part 66 - Training Materials Only Aerodynamic Balance Panels For some types of aircraft, aerodynamic loads acting on the ailerons and elevators are reduced by balance panels that operate in conjunction with balance tabs. The panel is connected between the leading edge of the primary control surface and the rear spar of the wing or horizontal stabiliser using hinged fittings. The hinged panel creates two vented compartments in the trailing edge of the wing. With the primary control surface in its neutral position, the air pressure vented into each compartment is balanced. When the control surface is moved, for example, upwards, a higher pressure is developed in the upper compartment. The resulting force acts on the hinged balance panel, moving it downwards and assisting the tab to move the primary control surface upwards. An aerodynamic balance panel helps the pilot move the control. The amount of aid increases as the deflection of the surface increases. Aviation Australia Aerodynamic balance panel 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 65 of 209 CASA Part Part 66 - Training Materials Only Supplementary Media Video Glossary This is a quick access area compiling all of the videos linked in the text. The videos shown here and throughout this topic are supplementary and aimed to help better understand the topics described in this resource. If it is discussed in supplementary media (such as a video) but not mentioned explicitly in the student resource it is not examinable. Relevant Youtube link: The Aerodynamics of Flight Relevant Youtube link: How Flight Controls Work Relevant Youtube link: Triple Slotted Flap Animation Relevant Youtube link: Boeing 747 – Triple Slot Flaps Relevant Youtube link: Flap and Slat Operation - Growing Wings Relevant Youtube link: How Flaps and Spoilers Work During Landing? Relevant Youtube link: How Do Vortex Generator Work on the Kitfox Wing? Where to Place Them? Relevant Youtube link: Trim Tab Operating Relevant Youtube link: Servo Tab 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 66 of 209 CASA Part Part 66 - Training Materials Only High-Speed Flight (11.1.2) Learning Objectives 11.1.2.1.1 Describe the speed of sound and the effect of local speed of sound on aircraft operation (Level 2). 11.1.2.1.2 Describe the characteristics of subsonic, transonic and supersonic flight (Level 2). 11.1.2.2.1 Describe the Mach number and the effect of the Mach number on flight (Level 2). 11.1.2.2.2 Describe the critical Mach number, compressibility buffeting, shockwaves and shockwave formation and their associated effects on flight (Level 2). 11.1.2.2.3 Describe aerodynamic heating and the area rule and how these factors affect aircraft design and flight (Level 2). 11.1.2.3 Describe the factors affecting airflow in engine intakes of high-speed aircraft (Level 2). 11.1.2.4 Describe the effects of sweepback on the critical Mach number (Level 2). 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 67 of 209 CASA Part Part 66 - Training Materials Only Subsonic, Transonic and Supersonic Flight Introduction to High-Speed Flight Low-speed aerodynamics assumes air is incompressible; the attendant errors are negligible since at low speeds the amount of compression is negligible. At speeds approaching that of sound, however, compression and expansion near the aircraft are sufficiently marked to affect the streamline pattern about the aircraft. At low subsonic speeds, a flow pattern is established about the aircraft, but at high subsonic and supersonic speeds, the flow around a given wing can be controlled and its behaviour predicted. In the transonic range, where a mixture of subsonic and supersonic flows exist, marked problems of control and stability arise, necessitating special design features to minimise the effects of compressibility. Subsonic Compressibility Effects At low speeds, the study of aerodynamics is simplified by the fact that as air passes over a wing, it experiences a relatively small change in pressure and density. This airflow is termed incompressible since the air undergoes changes in pressure without apparent changes in density. This is seen when a fluid passes through a venturi. As the fluid enters a restriction, its velocity increases and its pressure decreases. Subsonic airflow through a Venturi 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 68 of 209 CASA Part Part 66 - Training Materials Only Sonic Compressibility Effects At high speeds, changes in air pressure and density are significant. For example, as air enters a venturi at supersonic speeds, the airflow slows down and therefore must compress to pass through the restriction. Once a fluid compresses, its pressure and density increase. The study of high-speed airflow must account for these changes in air density and consider that the air is compressible. Sonic airflow through a Venturi The Speed of Sound Advancements in technology have produced high-performance aeroplanes that are capable of high- speed flight. The study of aerodynamics at these high flight speeds is significantly different from the study of low-speed aerodynamics. As an aircraft flies, it disturbs the air it passes through, and pressure waves radiate from every part of its surface. At any particular time, the sound waves are moving faster than the speed of the aircraft, so as the aircraft moves forward, the sound waves are well clear of the aircraft. As long as the aircraft is moving slower than the pressure waves, the waves move out in all directions. Pressure waves radiate from every part of a surface 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 69 of 209 CASA Part Part 66 - Training Materials Only The Doppler Effect A stationary object, which vibrates at a certain frequency, is the source of a continuous series of compressed air pulses. These small disturbances, corresponding to the ripples produced when a stone is dropped into water, move out as expanding spheres travelling at the speed of sound. If the source of disturbance starts moving, it closes on the pressure waves ahead of it. Thus, an observer standing ahead of the object receives more sound waves per second (a higher frequency) than one standing behind it. The ears interpret this higher frequency as a higher-pitched note that drops to a lower note after the object passes. This is known as the Doppler Effect and is evident whenever a low-flying aircraft approaches rapidly and passes overhead. However, we are less concerned about hearing these waves than we are about the way they travel. Pressure Waves at Subsonic Speed As discussed earlier, when air is disturbed, longitudinal waves are created. This causes the air pressure to increase and decrease. These changes radiate concentrically from the point of disturbance as pressure waves. Pressure waves at subsonic speed When the source of the wave moves at the same speed as the pressure waves, the pressure waves cannot move out ahead of the source. Instead, they form a compression wave where the pressure energy, in effect, piles up at the front edge of the aircraft. This compression wave causes a large difference in the static pressure and density of the air. The pressure changes in the sound wave are caused by the movement of the molecules in the air, and as the temperature changes, so does the speed of the molecular movement. The way the speed of sound varies in standard air is indicated in the chart. 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 70 of 209 CASA Part Part 66 - Training Materials Only Pressure waves at sonic speed Relevant Youtube link: Doppler effect animated examples Relevant Youtube link: The Doppler effect: what does motion do to waves? (Video) 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 71 of 209 CASA Part Part 66 - Training Materials Only Standard Atmosphere At standard sea level, sound waves travel at 661.7 kt. However, when the air temperature changes, so does the speed of sound. One of the most important measurements in high-speed aerodynamics is based on the speed of sound. The Mach number is the ratio of the true airspeed of an aircraft to the speed of sound. When an aircraft flies at the speed of sound, it is said to be travelling at Mach 1. Flight below the speed of sound is expressed as a fractional Mach number. Relationship of altitude and temperature to the speed of sound 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 72 of 209 CASA Part Part 66 - Training Materials Only Mach Number The Mach number (M) refers to the speed at which an aircraft is travelling in relation to the speed of sound. It has no units, and its expression is as follows: u T rue airspeed (velocity) M = = c Local speed of sound Thus, a Mach number of 0.5 means the aircraft is travelling at half the speed of sound. Both the speed of the aircraft and the speed of sound are true speeds, i.e. the only real speeds. Worked Example An aircraft’s true airspeed is 480 km/h and the local speed of sound is 800 km/h. What is the Mach number? 480 = 0.6 800 480 / 800 = 0.6 Subsonic Flight In low-speed flight, air is considered incompressible and acts in much the same way as a liquid. It can undergo changes in pressure without any appreciable change in its density. But in high-speed flight, air acts as a compressible fluid, and its density changes with changes in its pressure and velocity. An aeroplane passing through the air creates pressure disturbances that surround it. When the aeroplane is flying at a speed below the speed of sound, these disturbances move out in all directions, the air immediately ahead of the aeroplane is affected and its direction changes before the air reaches the surface. Flight below Mach 0.75 is called subsonic flight. In this speed range, all airflow is subsonic, and aircraft behave in accordance with the concepts discussed in basic aerodynamics. Subsonic airflows can be as high as Mach 0.9 before transonic range effects are felt. Aviation Australia Subsonic flight 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 73 of 209 CASA Part Part 66 - Training Materials Only Mach Cone and Mach Angle at Subsonic Flight Sonic fronts leave any particular point at the speed of sound. The aeroplane is travelling below the speed of sound. There is no shock front. Mach cone angle at subsonic flight Mach cone angle at subsonic flight 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 74 of 209 CASA Part Part 66 - Training Materials Only Transonic Flight The most difficult realm of flight is at transonic speed, which is between Mach 0.75 and 1.20. At this speed, some of the airflow passing over the wings is subsonic and some is supersonic. Normal shock waves form and an aerofoil’s aerodynamic centre of lift shifts from 25% of the chord at subsonic speeds to 50% at transonic speeds. This shift causes large changes in aerodynamic trim and stability. Air passing through this normal shock wave slows to a subsonic speed without changing its direction. The shock wave can cause the air that passes through it to be turbulent and to separate from the wing surface. Shock-induced flow separation can create serious drag and control problems. Aviation Australia Transonic flight Relevant Youtube link: Shock wave formation in transonic flight 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 75 of 209 CASA Part Part 66 - Training Materials Only Supersonic Airflow Supersonic speed is above Mach 1.20. At this speed, all the airflow passing over the aeroplane is considered supersonic. Mach 1 At Mach 1, sonic fronts leave any particular point at the speed of sound. Aeroplanes also move at the speed of sound. The so-called sonic barrier is created because all the pressure waves created previously move at the same speed as the aeroplane and therefore arrive at the same point as the aeroplane. The shock front is at 90° to the axis of motion. Mach 1 Mach 1 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 76 of 209 CASA Part Part 66 - Training Materials Only Mach 2 At Mach 2, sonic fronts leave any particular point at the speed of sound. An aeroplane moves at twice the speed of sound. Because the aeroplane is moving twice as fast as the sonic fronts, the boundaries of all the sonic fronts are tangent to a conical surface. An infinite number of pressure waves would produce a continuous line, inclined backwards in the form of a cone. The angle of the cone, called the Mach angle, would become smaller as the speed rose. This cone, which marks the boundary of the sphere of influence of the body, is called a Mach cone. All objects within the Mach cone would experience the effects of the passage of the body. All those outside it would be unaffected. Mach 2 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 77 of 209 CASA Part Part 66 - Training Materials Only Mach 2 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 78 of 209 CASA Part Part 66 - Training Materials Only Properties of High-Speed Flight Critical Mach Number The airflow over the upper surface of the wing is deliberately accelerated to produce lift, and even though the aircraft itself may be flying below the speed of sound, some of the air flowing over the wings may be accelerated to Mach 1.0. When this happens, it is termed the critical Mach number or Mcrit. When this point is reached, a shock wave forms over the upper surface of the wing because the pressure waves from the rear of the wing that are trying to move forward are meeting air travelling at exactly the same speed flowing backwards. This is similar to trying to move along a moving walkway in the wrong direction at the same speed as the walkway is travelling. The point at which this shock wave usually forms is just aft of the point of maximum camber of the wing, where the acceleration of the air is greatest. In front of the shock wave, the flow is at or higher than Mach 1, while behind the flow, it returns to subsonic. These effects may be delayed until a higher airspeed is reached by increasing Mcrit using a thin aerofoil. This aerofoil's point of maximum thickness is well back from the leading edge and sweeps the wings back. The critical Mach gives the first indication of local sonic airflow 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 79 of 209 CASA Part Part 66 - Training Materials Only Effects of Sweepback on Critical Mach Number One of the most common ways to prevent drag rise and control problems with an aeroplane flying in the transonic range is to sweep the wing back. Sweeping the wing back also increases the critical Mach number by effectively decreasing the thickness ratio of the wing. The air flowing across the wing in the line of flight travels farther than the distance perpendicular to the leading edge. This longer travel for the same thickness has the same effect on the critical Mach number as making the wing thinner, yet it allows a thicker wing for structural strength, i.e. it reduces the apparent thickness/chord ratio. Critical Mach number can be increased by wing sweepback 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 80 of 209 CASA Part Part 66 - Training Materials Only Shock Waves There are three types of pressure waves: normal and oblique shock waves and expansion waves. Normal Shock Waves When the airstream passes through a normal shock wave, its direction does not change. However, the airstream does slow to a subsonic speed with a large increase in its static pressure and density. A normal shock wave forms the boundary between supersonic and subsonic airflow when there is no change in the direction of air as it passes through the wave. Air flowing over an aerofoil acts in the same way it does as it flows through a converging and diverging duct. When the supersonic airstream passes through a normal shock wave: The airstream slows to subsonic. The airflow direction immediately behind the wave is unchanged. The static pressure of the airstream behind the wave increases greatly. The density of the airstream behind the wave increases greatly. The energy of the airstream is greatly reduced. Initial formation of the shock wave occurs on the wing’s upper surface. Normal shock wave on wing Relevant Youtube link: Shock wave formation in transonic flight 2022-10-04 B1-11a Turbine Aeroplane Aerodynamics, Structures and Systems Page 81 of 209 CASA Part Part 66 - Training Materials Only Subsonic Blunt-Nose Aerofoil As the air approaches a relatively blunt-nose subsonic aerofoil at a supersonic sp
