Aircraft Aerodynamics Structures & Systems PDF

THEORY OF FLIGHT PART-66 SYLLABUS LEVELS CERTIFICATION CATEGORY ¦ B2 Sub-Module 01 THEORY OF FLIGHT...

THEORY OF FLIGHT PART-66 SYLLABUS LEVELS CERTIFICATION CATEGORY ¦ B2 Sub-Module 01 THEORY OF FLIGHT Knowledge Requirements 13.1 - Theory of Flight (a) Airplane Aerodynamics and Flight Controls 1 Operation and effect of: - roll control: ailerons and spoilers, - pitch control: elevators, stabilators, variable incidence stabilizers and canards, - yaw control, rudder limiters; Control using elevons, ruddervators; High lift devices: slots, slats, flaps, drag inducing devices: spoilers, lift dumpers, speed brakes; Operation and effect of trim tabs, servo tabs, control surface bias; (b) High Speed Flight 1 Speed of sound, subsonic flight, transonic flight, supersonic flight; Mach number, critical Mach number; (c) Rotary Wing Aerodynamics 1 Terminology; Operation and effect of cyclic, collective and anti-torque controls. www.amequestionpaper.in > Visit our website to download all latest EASA / DGCA CAR 66 Module examination Question papers and study material Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.1 AEROPLANE AERODYNAMICS AND FLIGHT CONTROLS The directional control of a fixed-wing aircraft takes Rudder—Yaw Elevator—Pitch place around the lateral, longitudinal, and vertical axes Vertical Axis Lateral Axis by means of flight control surfaces designed to create (Longitudinal (Directional Aileron—Roll Stability) movement about these axes. These control devices are Stability) Longitudinal Axis (Lateral hinged or movable surfaces through which the attitude Stability) of an aircraft is controlled during takeoff, flight, and landing. They are usually divided into two major groups: 1) primary or main flight control surfaces and 2) secondary or auxiliary control surfaces. PRIMARY FLIGHT CONTROL SURFACES The primary f light control surfaces on a fixed-wing aircraft include: ailerons, elevators, and the rudder. The Primary ailerons are attached to the trailing edge of both wings and Control Airplane Axes of Type of Surface Movement Rotation Stability when moved, rotate the aircraft around the longitudinal axis. The elevator is attached to the trailing edge of the Aileron Roll Longitudinal Lateral horizontal stabilizer. When it is moved, it alters aircraft Elevator/ Pitch Lateral Longitudinal Stabilator pitch, which is the attitude about the horizontal or lateral Rudder Yaw Vertical Directional axis. The rudder is hinged to the trailing edge of the vertical stabilizer. When the rudder changes position, the Figure 1-1. Flight control surfaces move the aircraft rotates about the vertical axis (yaw). Figure 1-1 aircraft around the three axes of flight. shows the primary flight controls of a light aircraft and the movement they create relative to the three axes of flight. Aileron Hinge-pin Fitting Primary control surfaces are usually similar in construction Actuating Horn to one another and vary only in size, shape, and methods of attachment. On aluminum light aircraft, their structure is often similar to an all-metal wing. This is appropriate because the primary control surfaces are simply smaller aerodynamic devices. They are typically made from an Spar Lightning Hole aluminum alloy structure built around a single spar member or torque tube to which ribs are fitted and a Figure 1-2. Typical structure of an aluminum flight control surface. skin is attached. The lightweight ribs are, in many cases, stamped out from flat aluminum sheet stock. Holes in the materials and construction techniques are employed. ribs lighten the assembly. An aluminum skin is attached Figure 1-3 shows examples of aircraft that use composite with rivets. Figure 1-2 illustrates this type of structure, technology on primary flight control surfaces. Note that which can be found on the primary control surfaces of the control surfaces of fabric-covered aircraft often have light aircraft as well as on medium and heavy aircraft. fabric covered surfaces just as aluminum-skinned (light) aircraft typically have all-aluminum control surfaces. Primary control surfaces constructed from composite materials are also commonly used. These are found on OPERATION AND EFFECT OF ROLL many heavy and high-performance aircraft, as well as CONTROL DEVICES gliders, home-built, and light-sport aircraft. AILERONS The weight and strength advantages over traditional Ailerons are the primary f light control surfaces that construction can be signif icant. A wide variety of move the aircraft about the longitudinal axis. In other 1.2 Module 13 B2 - Aircraft Aerodynamic Structures and Systems words, movement of the ailerons in f light causes the trailing edge moves downward, camber is increased aircraft to roll. Ailerons are usually located on the and lift is increased. Conversely, on the other wing, the THEORY OF FLIGHT outboard trailing edge of each of the wings. They are raised aileron decreases lift. (Figure 1-5) built into the wing and are calculated as part of the wing's surface area. Figure 1-4 shows aileron locations The result is a sensitive response to the control input to on various wing tip designs. roll the aircraft. The pilot's request for aileron movement and roll are transmitted from the cockpit to the actual Ailerons are controlled by a side-to-side motion of the control surface in a variety of ways depending on the control stick in the cockpit or a rotation of the control aircraft. A system of control cables and pulleys, push- yoke. When the aileron on one wing def lects down, pull tubes, hydraulics, electric, or a combination of these the aileron on the opposite wing deflects upward. This can be employed. (Figure 1-6) amplif ies the movement of the aircraft around the longitudinal axis. On the wing on which the aileron Simple, light aircraft usually do not have hydraulic or electric fly-by-wire aileron control. These are found on heavy and high-performance aircraft. Large aircraft and some high performance aircraft may also have a second set of ailerons located inboard on the trailing edge of the wings. These are part of a complex system of primary and secondary control surfaces used to provide lateral control and stability in flight. At low speeds, the ailerons may be augmented by the use of flaps and spoilers. At high speeds, only inboard aileron deflection is required to roll the aircraft while the other control surfaces are locked out or remain stationary. Figure 1-4. Aileron location on various wings. Up Aileron Down Aileron Figure 1-3. Composite control surfaces and some Figure 1-5. Differential aileron control movement. When one aileron is of the many aircraft that utilize them. moved down, the aileron on the opposite wing is deflected upward. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.3  Stop are rigged to operate when the ailerons operate to assist with the lateral movement and stability of the aircraft. Elevator Cables On the wing where the aileron is moved up, the spoilers also raise thus amplifying the reduction of lift on that wing. (Figure 1-8) On the wing with downward aileron deflection, the spoilers remain stowed. As the speed of Tether Stop the aircraft increases, the ailerons become more effective and the spoiler interconnect disengages. Note that Stop spoilers are also used in as drag inducing devices. OPERATION AND EFFECT OF PITCH To Ailerons CONTROL DEVICES Note Pivots Not On Center Of Shaft ELEVATORS Figure 1-6. Transferring control surface inputs from the cockpit. The elevator is the primary flight control surface that Figure 1-7 illustrates the location of the typical flight moves the aircraft around the horizontal or lateral control surfaces found on a transport category aircraft. axis. This causes the nose of the aircraft to pitch up or down. The elevator is hinged to the trailing edge of the SPOILERS horizontal stabilizer and typically spans most or all of A spoiler is a device found on the upper surface of many its width. It is controlled in the cockpit by pushing or heavy and high-performance aircraft. It is stowed flush pulling the control yoke forward or aft. Light aircraft to the wing's upper surface. When deployed, it raises up use a system of control cables and pulleys or push pull into the airstream and disrupts the laminar airflow of the tubes to transfer cockpit inputs to the movement of the wing, thus reducing lift. Spoilers are made with similar elevator. High performance and large aircraft typically construction materials and techniques as the other flight employ more complex systems. Hydraulic power is control surfaces on the aircraft. At low speeds, spoilers commonly used to move the elevator on these aircraft. Speed Brakes Flight Spoilers Outboard Aileron Inboard Aileron Figure 1-7. Typical flight control surfaces on a transport category aircraft. 1.4 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  THEORY OF FLIGHT Figure 1-9. A stabilizer and index marks on a transport category aircraft. Variable Incidence Nose Down Figure 1-8. Spoilers deployed upon landing a transport category aircraft. Nose Up Jackscrew Pivot On aircraft equipped with f ly-by-wire controls, a combination of electrical and hydraulic power is used. Trim Motor or Trim Cable STABILATORS A movable horizontal tail section, called a stabilator, Figure 1-10. Some airplanes, including most jet transports, use is a control surface that combines the action of both an variable stabilizer to provide the required pitch trim forces. the horizontal stabilizer and the elevator. (Figure 1-9) Basically, a stabilator is a horizontal stabilizer that CANARDS can also be rotated about the horizontal axis to affect A canard utilizes the concept of two lifting surfaces. It the pitch of the aircraft. functions as a horizontal stabilizer located in front of the main wings. In effect, the canard is an airfoil similar to VARIABLE INCIDENCE STABILIZERS the horizontal surface on a conventional aft-tail design. A variable incidence stabilizer refers to any horizontal The difference is that the canard actually creates lift stabilizer in which the angle of incidence of the and holds the nose up, as opposed to the aft-tail design horizontal stabilizer is adjustable. Thus, a stabilator which exerts downward force on the tail to prevent the is a variable incidence horizontal stabilizer. Various nose from rotating downward. (Figure 1-11) mechanisms and operating rigging are available. Most large aircraft use a motorized jackscrew to alter The canard design dates back to the pioneer days the position of the stabilizer often energized by the of aviation, most notably used on the Wright Flyer. trim tab switch on the control yoke. The reason for a Recently, the canard conf iguration has regained stabilator or any horizontal stabilizer variable incidence popularity and is appearing on newer aircraft. Canard device is to minimize drag when trimming the aircraft designs include two types–one with a horizontal surface in flight. Deflection of the elevator via the use of a trim of about the same size as a normal aft-tail design, and tab causes drag and requires a relatively large elevator the other with a surface of the same approximate size on large aircraft to achieve all desired trim settings. and airfoil shape of the aft-mounted wing known as a By varying the angle of the horizontal stabilizer to tandem wing configuration. Theoretically, the canard is adjust pitch, less drag is created and elevator size and considered more efficient because using the horizontal deflection may be reduced. (Figure 1-10) surface to help lift the weight of the aircraft should result in less drag for a given amount of lift. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.5  RUDDER LIMITERS In flight, most large aircraft oscillate slightly from side to side. Yaw dampener units automatically detect this movement and send signals to the hydraulic power control unit (PCU) that moves the rudder so that it can correct for these yaw oscillations. Similarly, rudders are known to deflect without being commanded to do so by the flight crew. Again, the yaw dampener is designed to correct the fluctuations by signaling the PCU. However, too large of an involuntary deflection to a rudder can cause a loss of control of the aircraft. A rudder limiter is fitted to many aircraft to prevent any more than a few degrees of involuntary motion of the rudder. Essentially, it limits the Figure 1-11. The Piaggio P180 includes a variable-sweep canard movement unless it is commanded from the flight deck. design, which provides longitudinal stability about the lateral axis. SECONDARY OR AUXILIARY OPERATION AND EFFECT OF YAW CONTROL SURFACES CONTROL DEVICES There are several secondary or auxiliary flight control surfaces. Their names, locations, and functions of those RUDDERS for most large aircraft are listed in Figure 1-12. The rudder is the primary control surface that causes an aircraft to yaw or move about the vertical axis. This OPERATION AND EFFECT OF TABS provides directional control and thus points the nose of the aircraft in the direction desired. Most aircraft Trim Tabs have a single rudder hinged to the trailing edge of the The force of the air against a control surface during the vertical stabilizer. It is controlled by a pair of foot- high speed of flight can make it difficult to move and operated rudder pedals in the cockpit. When the right hold that control surface in the deflected position. A pedal is pushed forward, it deflects the rudder to the control surface might also be too sensitive for similar right which moves the nose of the aircraft to the right. reasons. Several different tabs are used to aid with The left pedal is rigged to simultaneously move aft. these types of problems. The table in Figure 1-13 When the left pedal is pushed forward, the nose of the summarizes the various tabs and their uses. While in aircraft moves to the left. flight, it is desirable for the pilot to be able to take his or her hands and feet off of the controls and have the As with the other primary flight controls, the transfer aircraft maintain its flight condition. of the movement of the cockpit controls to the rudder varies with the complexit y of the aircraft. Many Trims tabs are designed to allow this. Most trim tabs are aircraft incorporate the directional movement of the small movable surfaces located on the trailing edge of a nose or tail wheel into the rudder control system for primary flight control surface. A small movement of the ground operation. This allows the operator to steer the tab in the direction opposite of the direction the flight aircraft with the rudder pedals during taxi when the control surface is deflected, causing air to strike the tab, airspeed is not high enough for the control surfaces in turn producing a force that aids in maintaining the to be effective. Some large aircraft have a split rudder flight control surface in the desired position. Through arrangement. This is actually two rudders, one above linkage set from the cockpit, the tab can be positioned so the other. At low speeds, both rudders deflect in the that it is actually holding the control surface in position same direction when the pedals are pushed. At higher rather than the pilot. Therefore, elevator tabs are used speeds, one of the rudders becomes inoperative as to maintain the speed of the aircraft since they assist in the deflection of a single rudder is aerodynamically maintaining the selected pitch. Rudder tabs can be set to sufficient to maneuver the aircraft. hold yaw in check and maintain heading. Aileron tabs can help keep the wings level. 1.6 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  Secondary/Auxiliary Flight Control Surfaces THEORY OF FLIGHT Name Location Function Extends the camber of the wing for greater lift and slower flight. Flaps Inboard trailing edge of wings Allows control at low speeds for short field takeoffs and landings. Trailing edge of primary Trim Tabs flight control surfaces Reduces the force needed to move a primary control surface. Trailing edge of primary Balance Tabs Reduces the force needed to move a primary control surface. flight control surfaces Trailing edge of primary Anti-balance Tabs flight control surfaces Increases feel and effectiveness of primary control surface. Trailing edge of primary Servo Tabs Assists or provides the force for moving a primary flight control. flight control surfaces Spoilers Upper and/or trailing edge of wing Decreases (spoils) lift. Can augment aileron function. Extends the camber of the wing for greater lift and slower flight. Slats Mid to outboard leading edge of wing Allows control at low speeds for short field takeoffs and landings. Outer leading edge of wing Directs air over upper surface of wing during high angle of attack. Slots forward of ailerons Lowers stall speed and provides control during slow flight. Extends the camber of the wing for greater lift and slower flight. Leading Edge Flap Inboard leading edge of wing Allows control at low speeds for short field takeoffs and landings. NOTE: An aircraft may possess none, one, or a combination of the above control surfaces. Figure 1-12. Secondary or auxiliary control surfaces and respective locations for larger aircraft. Flight Control Tabs Direction of Motion Type Activation Effect (in relation to control surface) Statically balances the aircraft Trim Opposite Set by pilot from cockpit. in flight. Allows “hands off” Uses independent linkage. maintenance of flight condition. Moves when pilot moves control surface. Aids pilot in overcoming the force Balance Opposite Coupled to control surface linkage. needed to move the control surface. Directly linked to flight control Aerodynamically positions control Servo Opposite input device. Can be primary surfaces that require too much or back-up means of control. force to move manually. Increases force needed by pilot Anti-balance Same Directly linked to flight to change flight control position. or Anti-servo control input device. De-sensitizes flight controls. Located in line of direct linkage to servo Enables moving control surface Spring Opposite tab. Spring assists when control forces when forces are high. become too high in high-speed flight. Inactive during slow flight. Figure 1-13. Various tabs and their uses. Occasionally, a simple light aircraft may have a off condition when flying straight and level. The correct stationary metal plate attached to the trailing edge of a amount of bend can be determined only by flying the primary flight control, usually the rudder. This is also a aircraft after an adjustment. Note that a small amount trim tab as shown in Figure 1-14. It can be bent slightly of bending is usually sufficient. on the ground to trim the aircraft in flight to a hands Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.7 Balance Tabs control surface, not just reduce the force needed to do The aerodynamic phenomenon of moving a trim tab in so. It is usually used as a means to back up the primary one direction to cause the control surface to experience control of the flight control surfaces. (Figure 1-16) a force moving in the opposite direction is exactly what occurs with the use of balance tabs. (Figure 1-15) Often, On heavy aircraft, large control surfaces require too it is difficult to move a primary control surface due to its much force to be moved manually and are usually surface area and the speed of the air rushing over it. def lected out of the neutral position by hydraulic actuators. These power control units are signaled via a Deflecting a balance tab hinged at the trailing edge of system of hydraulic valves connected to the yoke and the control surface in the opposite direction of the desired rudder pedals. On f ly-by-wire aircraft, the hydraulic control surface movement causes a force to position the actuators that move the f light control surfaces are surface in the proper direction with reduced force to do signaled by electric input. In the case of hydraulic system so. Balance tabs are usually linked directly to the control failure(s), manual linkage to a servo tab can be used to surface linkage so that they move automatically when deflect it. This, in turn, provides an aerodynamic force there is an input for control surface movement. They also that moves the primary control surface. can double as trim tabs, if adjustable on the flight deck. Anti-Servo/Anti-Balance Tabs Servo Tabs Anti-servo tabs, as the name suggests, are like servo tabs A servo tab is similar to a balance tab in location and but move in the same direction as the primary control effect, but it is designed to operate the primary flight surface. On some aircraft, especially those with a movable horizontal stabilizer, the input to the control surface can be too sensitive. An Anti-servo tab tied through the control linkage creates an aerodynamic force that increases the effort needed to move the control surface. This makes flying the aircraft more stable for the pilot. Figure 1-17 shows an Anti-servo tab in the near neutral position. Deflected in the same direction as the desired stabilator movement, it increases the required control surface input. Anti-servo tabs are also known as anti- balance tabs. Control Surface Bias Ground Adjustable Rudder Trim When a control surface is in the neutral position, is Figure 1-14. Example of a trim tab. faired with the wing rudder or horizontal stabilizer and no effect on the aircrafts aerodynamic surfaces. Some aircraft are designed with control surface bias. Tab geared to deflect proportionally to the Lift control deflection, but in the opposite direction. Control Stick Free Link Fixed Surface Cont rol Tab Control Surface Hinge Line Figure 1-15. Balance tabs assist with forces Figure 1-16. Servo tabs can be used to position flight needed to position control surfaces. control surfaces in case of hydraulic failure. 1.8 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  inboard wing leading edge. The flaps are lowered to increase the camber of the wings and provide greater THEORY OF FLIGHT lift and control at slow speeds. They enable landing at slower speeds and shorten the amount of runway Antiservo Tab required for takeoff and landing. The amount that the flaps extend and the angle they form with the wing can be selected from the cockpit. Typically, flaps can Stabilator Pivot Point extend up to 45-50°. Figure 1-18 shows various aircraft with flaps in the extended position. Flaps are usually constructed of materials and with techniques used on the other airfoils and control surfaces of a particular aircraft. Aluminum skin and Figure 1-17. An Anti-servo tab moves in the same direction as the control structure flaps are the norm on light aircraft. Heavy and tab. Shown here on a stabilator, it desensitizes the pitch control. high performance aircraft flaps may also be aluminum, but the use of composite structures is also common. This means that a control surface is not naturally in the neutral position. It is designed to impart a force on the There are various kinds of f laps. Plain f laps form airfoil at all times. The force is generally used to counter the trailing edge of the wing when the f lap is in the balance a design imbalance and alter the aircraft's retracted position. (Figure 1-19A) The airflow over the aerodynamics for easy hands-off flight. This means that wing continues over the upper and lower surfaces of the when the aircraft is flying straight and level, the control flap, making the trailing edge of the flap essentially the surface bias has effect but all trim position gauges on the trailing edge of the wing. The plain flap is hinged so that flight deck indicate zero trim. the trailing edge can be lowered. This increases wing camber and provides greater lift. HIGH LIFT DEVICES Aircraft wings contain devices that are designed to A split flap is normally housed under the trailing edge increase the lift produced by the wing with the devices of the wing. (Figure 1-19B) It is usually just a braced deployed during certain phases of flight. flat metal plate hinged at several places along its leading edge. The upper surface of the wing extends to the FLAPS trailing edge of the flap. When deployed, the split flap Flaps are one such high lift device found on most trailing edge lowers away from the trailing edge of the aircraft. They are usually inboard on the wings' trailing wing. Airf low over the top of the wing remains the edges adjacent to the fuselage. Leading edge flaps are same. Airflow under the wing now follows the camber also common. They extend forward and down from the created by the lowered split flap, increasing lift. Figure 1-18. An aileron balance panel and linkage uses varying air pressure to assist in control surface positioning. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.9  A B Plain Flap Split Flap C Fowler Flap Figure 1-19. Various types of flaps. Fowler flaps not only lower the trailing edge of the wing Retracted when deployed but also slide aft, effectively increasing the area of the wing. (Figure 1-19C) This creates more lift via the increased surface area, as well as the wing camber. When stowed, the fowler flap typically retracts Fore Flap up under the wing trailing edge similar to a split flap. The sliding motion of a fowler flap can be accomplished Mid Flap with a worm drive and flap tracks. Aft Flap Figure 1-20. Triple slotted flap. An enhanced version of the fowler flap is a set of flaps that actually contains more than one aerodynamic surface. Figure 1-20 shows a triple-slotted flap. In this configuration, the flap consists of a fore flap, a mid Hinge Point flap, and an aft flap. Actuator When deployed, each f lap section slides aft on tracks as it lowers. The flap sections also separate leaving an open slot between the wing and the fore flap, as well as between each of the flap sections. Air from the underside of the wing flows through these slots. The result is that the laminar flow on the upper surfaces is enhanced. The Flap Extended Flap Retracted greater camber and effective wing area increase overall lift. Retractable Nose Heavy aircraft often have leading edge f laps that are Figure 1-21. Leading edge flaps. used in conjunction with the trailing edge flaps. (Figure 1-21) They can be made of machined magnesium or can FLAPERONS have an aluminum or composite structure. Some aircraft are equipped with f laperons. (Figure 1-23) Flaperons are ailerons which can also act as flaps. While they are not installed or operate independently, Flaperons combine both aspects of flaps and ailerons. In their use with trailing edge flaps can greatly increase addition to controlling the bank angle of an aircraft like wing camber and lift. When stowed, leading edge conventional ailerons, flaperons can be lowered together f laps retract into the leading edge of the wing. The to function much the same as a dedicated set of flaps. differing designs of leading edge flaps essentially provide The pilot retains separate controls for ailerons and flaps. the same effect. Activation of the trailing edge f laps A mixer is used to combine the separate pilot inputs into automatically deploys the leading edge flaps, which are this single set of control surfaces called flaperons. Many driven out of the leading edge and downward, extending designs that incorporate f laperons mount the control the camber of the wing. Figure 1-22 shows a Krueger surfaces away from the wing to provide undisturbed flap, recognizable by its flat mid-section. airflow at high angles of attack and/or low airspeeds. 1.10 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  THEORY OF FLIGHT Figure 1-22. Side view (left) and front view (right) of a Krueger flap on a Boeing 737. Flaperons Figure 1-23. Flaperons on a Skystar Kitfox MK 7. SLATS Another leading-edge device which extends wing camber is a slat. Slats can be operated independently of the flaps with their own switch in the cockpit. Slats not only extend out of the leading edge of the wing Figure 1-24. Air passing through the slot aft of the slat promotes increasing camber and lift, but most often, when fully boundary layer airflow on the upper surface at high angles of attack. deployed leave a slot between their trailing edges and the leading edge of the wing. (Figure 1-24) This increases the angle of attack at which the wing will maintain its laminar airflow, resulting in the ability to fly the aircraft slower and still maintain control. SLOTS A fixed device mounted to extend the leading edge of the wing forward and downward is known as a slot or cuff. (Figure 1-25) It essentially increases the camber of the wing and allows the aircraft to fly at slower speeds and higher angles of attack. Moreover, slots reduce the stall speed of the aircraft by mixing high speed air flow exiting the slot with boundary layer air. The result is a delay in boundary layer separation. However, slots Figure 1-25. A leading edge slot on a STOL aircraft. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.11 increase drag. The benefits of good low-speed handling This is possible on aircraft with V–tail empennages characteristics when weighed against the increased drag where the traditional horizontal and vertical stabilizers that a slot causes at higher speeds limits the use of slots. do not exist. Instead, two stabilizers angle upward and outward from the aft fuselage in a "V" configuration. Each Full-span slots span the full wing from root to tip. contains a movable ruddervator built into the trailing edge. They are commonly used on STOL (short takeoff and Movement of the ruddervators can alter the movement of landing) aircraft. Partial-span slots are positioned on the the aircraft around the horizontal and/or vertical axis. outboard section of the wing leading edge. This increases the angle of attack at which the outboard wing stalls and DRAG INDUCING DEVICES ensures that the wing root stalls first. When the wing root stalls first, stall characteristics are docile. Recovery SPOILERS is easier because the partial-span slots maintain air flow Spoilers are unique in that they may be fully deployed on over the ailerons during the stall. both wings to act as speed brakes. The reduced lift and increased drag can quickly reduce the speed of the aircraft ELEVONS AND RUDDERVATORS in flight. Spoilers are sometimes called lift dumpers. Elevons perform the combined functions of the ailerons and the elevator. (Figure 1-26) They are typically used SPEED BRAKES on aircraft that have no true separate empennage such as Dedicated speed brake panels similar to flight spoilers a delta wing or flying wing aircraft. in construction can be found on the upper surface of the wing trailing edge of heavy and high-performance They are installed on the trailing edge of the wing. aircraft. They are designed specifically to increase drag When moved in the same direction, the elevons cause a and reduce the speed of the aircraft when deployed. pitch adjustment. When moved in opposite directions, These speed brake panels do not operate differentially the aircraft rolls. Elevons may also move differentially in with the ailerons at low speed like the spoilers. the same direction causing adjustments to roll and pitch. The control yoke or stick activated elevon movement A speed brake control lever in the cockpit can deploy all through a mechanical or electronic mixing device. A spoiler and speed brake surfaces fully when operated. ruddervator combines the action of the rudder and Often, speed brakes surfaces are rigged to deploy on elevator. (Figure 1-27) the ground automatically when engine thrust reversers are activated. The location of speed brake panels is visible in Figure 1-7. HIGH SPEED FLIGHT SPEED OF SOUND MACH NUMBER, SUBSONIC, TRANSONIC Sound, in reference to aeroplanes and their movement AND SUPERSONIC FLIGHT through the a ir, is nothing more than pressure In high-speed f light and/or high-altitude f light, the disturbances in the air. It is like dropping a rock in the measurement of speed is expressed in terms of a "Mach water and watching the waves flow out from the center. number" - the ratio of the true airspeed of the aircraft to As an aeroplane flies through the air, every point on the the speed of sound in the same atmospheric conditions. aeroplane that causes a disturbance creates sound energy An aircraft traveling at the speed of sound is traveling in the form of pressure waves. These pressure waves flow at Mach 1.0. away from the aeroplane at the speed of sound, which at standard day temperature of 59 °F, is 761 mph. Aircraft speed regimes are defined approximately as follows: Subsonic - Mach numbers below 0.75 The speed of sound in air changes with temperature, Transonic - Mach numbers from 0.75 to 1.20 increasing as temperature increases. Figure 1-28 shows Supersonic - Mach numbers from 1.20 to 5.00 how the speed of sound changes with altitude. Hypersonic - Mach numbers above 5.00 1.12 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  Speed of Sound Altitude in Feet Temperature (°F) Elevons (mph) THEORY OF FLIGHT 0 59.00 761 1 000 55.43 758 2 000 51.87 756 3 000 48.30 753 4 000 44.74 750 5 000 41.17 748 6 000 37.60 745 7 000 34.04 742 8 000 30.47 740 9 000 26.90 737 Figure 1-26. Elevons. 10 000 23.34 734 15 000 5.51 721 20 000 –12.32 707 25 000 –30.15 692 30 000 –47.98 678 35 000 –65.82 663 * 36 089 –69.70 660 40 000 –69.70 660 Ruddervator 45 000 –69.70 660 50 000 –69.70 660 55 000 –69.70 660 60 000 –69.70 660 65 000 –69.70 660 70 000 –69.70 660 75 000 –69.70 660 80 000 –69.70 660 85 000 –64.80 664 Figure 1-27. Ruddervator. 90 000 –56.57 671 95 000 –48.34 678 When an aeroplane is flying at subsonic speed, all of the 100 000 –40.11 684 air flowing around the aeroplane is at a velocity of less *Altitude at which temperature stops decreasing than the speed of sound (known as Mach 1). Keep in Figure 1-28. Altitude and temperature versus speed of sound. mind that the air accelerates when it flows over certain parts of the aeroplane, like the top of the wing, so an separate from the wing. The shock wave also causes aeroplane f lying at 500 mph could have air over the the center of lift to shift aft, causing the nose to pitch top of the wing reach a speed of 600 mph. How fast down. The speed at which the shock wave forms is an aeroplane can fly and still be considered in subsonic known as the critical Mach number. flight varies with the design of the wing, but as a Mach number, it will typically be just over Mach 0.8. When When an aeroplane is f lying at supersonic speed, the an aeroplane is f lying at transonic speed, part of the entire aeroplane is experiencing supersonic airflow. At this aeroplane is experiencing subsonic airflow and part is speed, the shock wave which formed on top of the wing experiencing supersonic airflow. during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. Supersonic speed Over the top of the wing the velocity of the air will is from Mach 1.20 to 5.0. If an aeroplane flies faster than reach Mach 1 and a shock wave will form. The shock Mach 5, it is said to be in hypersonic flight. wave forms 90 degrees to the airf low approximately halfway between the leading and trailing edge of the SHOCK WAVE wing. It is known as a normal shock wave. Stability Sound coming from an aeroplane is the result of the air problems can be encountered during transonic flight, being disturbed as the aeroplane moves through it, and because the shock wave can cause the airf low to the resulting pressure waves that radiate out from the Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.13 source of the disturbance. For a slow moving aeroplane, jet aircraft normally operate in a cruise speed range of the pressure waves travel out ahead of the aeroplane, Mach 0.7 to Mach 0.90. traveling at the speed of sound. When the speed of the aeroplane reaches the speed of sound, however, the The speed of an aircraft in which airf low over any pressure waves (sound energy) cannot get away from part of the aircraft or structure under consideration the aeroplane. At this point the sound energy starts to first reaches (but does not exceed) Mach 1.0 is termed pile up, initially on the top of the wing, and eventually "critical Mach number" or "Mach Crit." Thus, critical attaching itself to the wing leading and trailing edges. Mach number is the boundar y bet ween subsonic This piling up of sound energy is called a shock wave. and transonic flight and is largely dependent on the If the shock waves reach the ground, and cross the path wing and airfoil design. Critical Mach number is an of a person, they will be heard as a sonic boom. Figure important point in transonic flight. When shock waves 1-29A shows a wing in slow speed f light, with many form on the aircraft, airf low separation followed by disturbances on the wing generating sound pressure buffet and aircraft control difficulties can occur. Shock waves that are radiating outward. Figure 1-29B is the waves, buffet (airflow becomes unsmooth), and airflow wing of an aeroplane in supersonic flight, with the sound separation take place above critical Mach number. A pressure waves piling up toward the wing leading edge. jet aircraft typically is most efficient when cruising at or near its critical Mach number. CRITICAL MACH NUMBER While flights in the transonic and supersonic ranges At speeds 5–10 percent above the critical Mach number, are common occurrences for military aircraft, civilian compressibility effects begin. Drag begins to rise sharply. Associated with the "drag rise" are buffet, trim and stability changes, and a decrease in control surface effectiveness. This is the point of "drag divergence." (Figure 1-30) (A) CD (Drag Coefficient) Force Divergence Mach Number Critical Mach Number CD= 0.3 0.5 1.0 M (Mach Number) (B) Figure 1-29. Sound energy in subsonic and supersonic flight. Figure 1-30. Critical Mach. ROTARY WING AERODYNAMICS TERMINOLOGY characteristics. One of the differences between a rotary wing and a fixed-wing aircraft is the main source of ROTARY WING AIRCRAFT CONFIGURATION lift. The fixed-wing aircraft derives its lift from a fixed T he st r uc t u res of t he rota r y w ing a i rc ra f t a re airfoil surface while the rotary wing aircraft derives lift designed to give rotary wing aircraft its unique flight from a rotating airfoil called the rotor. Changing the 1.14 Module 13 B2 - Aircraft Aerodynamic Structures and Systems angle of attack of the rotating airfoils (blades) increases or decreases lift, respectively raising or lowering the THEORY OF FLIGHT aircraft. Tilting the rotor plane of rotation causes the aircraft to move horizontally. The typical configurations of rotary wing aircraft are: Autogyro - an aircraft with a free-spinning Figure 1-31. An autogyro. horizontal rotor that turns due to passage of air upward through the rotor. This air motion is created from forward motion of the aircraft resulting from either a tractor or pusher configured engine/ propeller design. (Figure 1-31) Single rotor helicopter - An aircraft with a single horizontal main rotor that provides both lift and direction of travel is a single rotor helicopter. A secondary rotor mounted vertically on the tail counteracts the rotational force (torque) of the main rotor to correct yaw of the fuselage. (Figure 1-32) Dual rotor helicopter - An aircraft with two Figure 1-32. Single rotor helicopter. horizontal rotors that provide both the lift and directional control is a dual rotor helicopter. The rotors are counterrotating to balance the aerodynamic torque and eliminate the need for a separate antitorque system. (Figure 1-33) FLIGHT CONDITIONS Hovering Flight During hovering f light, a helicopter maintains a constant position over a selected point, usually a few feet above the ground. For a helicopter to hover, the lift Figure 1-33. Dual rotor helicopter. and thrust produced by the rotor system act straight up and must equal the weight and drag, which act straight down. (Figure 1-34) While hovering, the amount of main rotor thrust can be changed to maintain the Thrust desired hovering altitude. This is done by changing the Lift angle of incidence of the rotor blades and hence the angle of attack of the main rotor blades. Changing the angle of attack changes the drag on the rotor blades, and the power delivered by the engine must change as well to keep the rotor speed constant. The weight that must be supported is the total weight Weight of the helicopter and its occupants. If the amount of Drag lift is greater than the actual weight, the helicopter accelerates upwards until the lift force equals the weight gain altitude; if thrust is less than weight, the Figure 1-34. To maintain a hover at a constant altitude, enough helicopter accelerates downward. When operating near lift and thrust must be generated to equal the weight of the the ground, the effect of the closeness to the ground helicopter and the drag produced by the rotor blades. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.15 changes this response. An important consequence of Basically, these units do the same things, regardless producing thrust is torque. Newton's Third Law states of the type of helicopter on which they are installed; that for every action there is an equal and opposite however, the operation of the control system varies reaction. Therefore, as the engine turns the main rotor greatly by helicopter model. system in a counterclockwise direction, the helicopter fuselage tends to turn clockwise. This tendency for the Vertical Flight fuselage to rotate is called torque. The amount of torque Hovering is actually an element of vertical flight. Increasing is directly related to the amount of engine power being the angle of attack of the rotor blades (pitch) while keeping used to turn the main rotor system. The greater the their rotation speed constant generates additional lift and engine power, the greater the torque effect. The force the helicopter ascends. Decreasing the pitch causes the that compensates for torque and provides for directional helicopter to descend. In a no wind condition, when lift control can be produced by various means. The defining and thrust are less than weight and drag, the helicopter factor is dictated by the design of the helicopter, some descends vertically. If lift and thrust are greater than weight of which do not have a torque issue. Single main rotor and drag, the helicopter ascends vertically. (Figure 1-36) designs typically have an auxiliary rotor located on the end of the tail boom (Figure 1-32). This auxiliary rotor, generally referred to as a tail rotor, produces thrust in the direction opposite the torque Thrust reaction developed by the main rotor. A pilot can vary Vertical Ascent Lift the amount of thrust produced by the tail rotor in relation to the amount of torque produced by the engine. As the engine supplies more power to the main rotor, the tail rotor must produce more thrust to overcome the increased torque effect. Other methods of compensating for torque and providing directional control include the Fenestron® tail rotor system, an SUD Aviation design that employs a Weight ducted fan enclosed by a shroud. Another design, called Drag NOTAR®, a McDonald Douglas design with no tail rotor, employs air directed through a series of slots in the tail boom, with the balance exiting through a 90° duct Figure 1-36. To ascend vertically, more lift and thrust must be located at the rear of the tail boom. (Figure 1-35) generated to overcome the forces of weight and drag. Figure 1-35. Aerospatiale Fenestron tail rotor system (left) and the McDonnell Douglas NOTAR® System (right). 1.16 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  Resultant Forward Flight In steady forward flight with no change in airspeed or THEORY OF FLIGHT Lift vertical speed, the four forces of lift, thrust, drag, and weight must be in balance. Once the tip-path plane is tilted forward, the total lift-thrust force is also tilted forward. This resultant lift-thrust force can be resolved Thrust into two components - lift acting vertically upward and thrust acting horizontally in the direction of flight. In Drag addition to lift and thrust, there is weight (the downward Helicopter Movement acting force) and drag (the force opposing the motion of an airfoil through the air). (Figure 1-37) Weight In straight-and-level (constant heading and at a constant altitude), unaccelerated forward flight, lift equals weight Resultant and thrust equals drag. If lift exceeds weight, the Figure 1-37. The power required to maintain a straight- helicopter accelerates vertically until the forces are in and-level flight and a stabilized airspeed. balance; if thrust is less than drag, the helicopter slows until the forces are in balance. As the helicopter moves safely in the event of an engine failure; consequently, all forward, it begins to lose altitude because lift is lost as helicopters must demonstrate this capability in order to thrust is diverted forward. However, as the helicopter be certificated. (Figure 1-38) begins to accelerate, the rotor system becomes more efficient due to the increased airflow. OPERATION AND EFFECT OF ROTORCRAFT CONTROLS The result is excess power over that which is required to hover. Continued acceleration causes an even larger FLIGHTS CONTROLS CONFIGURATION increase in airf low through the rotor disk and more The flight controls of a helicopter differ slightly from excess power. In order to maintain unaccelerated flight, those found in an aircraft. The control units located the pilot must not make any changes in power or in in the flight deck of all helicopters are very nearly the cyclic movement. Any such changes would cause the same. There are three major controls in a helicopter that helicopter to climb or descend. Once straight-and-level the pilot must use during flight. They are the collective f light is obtained, the pilot should make note of the pitch control, cyclic pitch control, and antitorque pedals power (torque setting) required and not make major or tail rotor control. In addition to these major controls, adjustments to the flight controls. the pilot must also use the throttle control, which is mounted directly to the collective pitch control in order Autorotation to fly the helicopter. (Figure 1-39) Autorotation is the state of f light in which the main rotor system of a helicopter is being turned by the action Swash Plate Assembly of air moving up through the rotor rather than engine The purpose of the swash plate is to transmit control power driving the rotor. In normal, powered flight, air inputs from the collective and cyclic controls to the main is drawn into the main rotor system from above and rotor blades. It consists of two main parts: the stationary exhausted downward, but during autorotation, air moves swash plate and the rotating swash plate. (Figure 1-40) up into the rotor system from below as the helicopter descends. Autorotation is permitted mechanically by a The stationary swash plate is mounted around the main freewheeling unit, which is a special clutch mechanism rotor mast and connected to the cyclic and collective that allows the main rotor to continue turning even controls by a series of pushrods. It is restrained from if the engine is not running. If the engine fails, the rotating by an antidrive link but is able to tilt in all freewheeling unit automatically disengages the engine directions and move vertically. The rotating swash from the main rotor allowing the main rotor to rotate plate is mounted to the stationary swash plate by a freely. It is the means by which a helicopter can be landed uniball sleeve. It is connected to the mast by drive Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.17  Normal Powered Flight Autorotation ht Direction of Flight lig fF no tio ec Di r Figure 1-38. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. Ineffect, the blades are "gliding" in their rotational plane. Cyclic Control Stick Controls Attitude and Direction of Flight Throttle Controls rpm Collective Pitch Stick Controls Altitude Pedals Maintain Heading Figure 1-39. Controls of a helicopter and the principal function of each. links and is allowed to rotate with the main rotor mast. rotor blades simultaneously, or collectively, as the name Both swash plates tilt and slide up and down as one implies. As the collective pitch control is raised, there is a unit. The rotating swash plate is connected to the pitch simultaneous and equal increase in pitch angle of all main horns by the pitch links. rotor blades; as it is lowered, there is a simultaneous and equal decrease in pitch angle. This is done through a series Collective Pitch Control of mechanical linkages, and the amount of movement in The collective pitch control is located on the left side of the the collective lever determines the amount of blade pitch pilot's seat and is operated with the left hand. The collective change. (Figure 1-41) An adjustable friction control helps is used to make changes to the pitch angle of all the main prevent inadvertent collective pitch movement. 1.18 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  Pitch Link THEORY OF FLIGHT Stationary Swash Plate Twist Grip Throttle Rotating Swash Plate Control Rod Figure 1-40. Stationary and rotating swash plate. Figure 1-42. A twist grip throttle is usually mounted on the end of the collective lever. The throttles on some turbine helicopters are mounted on the overhead panel or on the floor in the cockpit. and when lowered, power is decreased. This system Throttle Control maintains rpm close to the desired value, but still requires adjustment of the throttle for fine tuning. Governors are common on all turbine helicopters (as it is a function of the fuel control system of the turbine engine), and used on some piston-powered helicopters. Some helicopters do not have correlators or governors and require coordination of all collective and throttle movements. Collective Figure 1-41. The collective changes the pitch of all Cyclic Pitch Control of the rotor blades simultaneously and by the same The cyclic pitch control is mounted vertically from the amount, thereby increasing or decreasing lift. cockpit floor, between the pilot's legs or, in some models, between the two pilot seats. (Figure 1-43) This primary Throttle Control f light control allows the pilot to f ly the helicopter in The function of the throttle is to regulate engine rpm. If any horizontal direction; fore, aft, and sideways (Figure the correlator or governor system does not maintain the 1-44). The total lift force is always perpendicular to the desired rpm when the collective is raised or lowered, or tip-path place of the main rotor. The purpose of the if those systems are not installed, the throttle must be cyclic pitch control is to tilt the tip-path plane in the moved manually with the twist grip to maintain rpm. direction of the desired horizontal direction. The cyclic The throttle control is much like a motorcycle throttle, control changes the direction of this force and controls and works almost the same way; twisting the throttle the attitude and airspeed of the helicopter. to the left increases rpm, twisting the throttle to the right decreases rpm. (Figure 1-42) The rotor disk tilts in the same direction the cyclic pitch control is moved. If the cyclic is moved forward, the rotor Governor/Correlator disk tilts forward; if the cyclic is moved aft, the disk tilts A governor is a sensing device that senses rotor and aft, and so on. Because the rotor disk acts like a gyro, the engine rpm and makes the necessary adjustments in order mechanical linkages for the cyclic control rods are rigged to keep rotor rpm constant. Once the rotor rpm is set in in such a way that they decrease the pitch angle of the rotor normal operations, the governor keeps the rpm constant, blade approximately 90° before it reaches the direction of and there is no need to make any throttle adjustments. cyclic displacement, and increase the pitch angle of the A correlator is a mechanical connection between the rotor blade approximately 90° after it passes the direction of collective lever and the engine throttle. When the displacement. An increase in pitch angle increases angle of collective lever is raised, power is automatically increased attack; a decrease in pitch angle decreases angle of attack. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.19  Helicopters that are designed with tandem rotors do not Cyclic Pitch Control have an antitorque rotor (Figure 1-33). These helicopters are designed with both rotor systems rotating in opposite directions to counteract the torque, rather than using a tail rotor. Directional antitorque pedals are used for directional control of the aircraft while in flight, as well as while taxiing with the forward gear off the ground. With the right pedal displaced forward, the forward rotor disk tilts to the right, while the aft rotor disk tilts to the left. The opposite occurs when the left pedal is pushed forward; the forward rotor disk inclines to the left, and Cyclic Pitch Control the aft rotor disk tilts to the right. Differing combinations of pedal and cyclic application can allow the tandem rotor helicopter to pivot about the aft or forward vertical axis, as well as pivoting about the center of mass. Swash Plate Figure 1-43. The cyclic pitch control may be mounted vertically between the pilot's knees or on a teetering bar from a single cyclic located in the center of the helicopter. The cyclic can pivot in all directions. For example, if the cyclic is moved forward, the angle of attack decreases as the rotor blade passes the right side of Sideware Flight Forward Flight the helicopter and increases on the left side. This results in maximum downward deflection of the rotor blade in front of the helicopter and maximum upward deflection behind CYCLIC CONTROL STICK MOVED SIDEWAYS CYCLIC CONTROL STICK MOVED FORWARD it, causing the rotor disk to tilt forward. Figure 1-44. The cyclic changes the angle of the swash plate which changes the plane of rotation of the rotor blades. This moves the aircraft Antitorque Pedals horizontally in any direction depending on the positioning of the cyclic. The antitorque pedals are located on the cabin floor by the pilot's feet. They control the pitch and, therefore, the thrust of the tail rotor blades. (Figure 1-45) Newton's Third Law applies to the helicopter fuselage and how it rotates in the opposite direction of the main rotor blades unless counteracted and controlled. To make flight possible and to compensate for this torque, most helicopter designs incorporate an antitorque rotor or tail rotor. The antitorque pedals allow the pilot to control the pitch angle of the tail rotor blades which in forward flight puts the helicopter in longitudinal trim and while at a hover, enables the pilot to turn the helicopter 360°. The antitorque pedals are connected to the pitch change mechanism on the tail rotor gearbox and allow the pitch Figure 1-45. Antitorque pedals compensate for changes angle on the tail rotor blades to be increased or decreased. in torque and control heading in a hover. 1.20 Module 13 B2 - Aircraft Aerodynamic Structures and Systems  QUESTIONS Question: 1-1 Question: 1-5 Around what three axis do the primary flight controls __________________ and __________________ are move an aeroplane? lowered to increase the camber of the wings and provide greater lift and control at slow speeds. Question: 1-2 Question: 1-6 Movement of the __________________ in flight causes Elevons perform the combined functions of the the aircraft to roll. __________________ and the __________________. Question: 1-3 Question: 1-7 The __________________ is the primary flight control At which speed a shock wave is generated during that moves the aircraft around the horizontal or transonic flight? lateral axis. Question: 1-4 Question: 1-8 An __________________ tab is used to maintain the Which are the three main flight controls of a speed of an aircraft since it assists in maintaining the helicopter? selected pitch. Module 13 B2 - Aircraft Aerodynamic Structures and Systems 1.21  ANSWERS Answer: 1-1 Answer: 1-5 Lateral or Horizontal. Flaps, slats. Longitudal. Vertical. Answer: 1-2 Answer: 1-6 ailerons. Ailerons, Elevator. Answer: 1-3 Answer: 1-7 elevator. Critical Mach Number Answer: 1-4 Answer: 1-8 Elevator. Collective pitch control, cyclic pitch control, and antitorque pedals or tail rotor control 1.22 Module 13 B2 - Aircraft Aerodynamic Structures and Systems