Ch 1 19-37 PDF Aircraft Flight Control Surfaces

Flaperons Figure 1-58. A stabilizer and index marks on a transport category aircraft. Figure 1-60. Flaperons. increases wing camber and provides greater lift. Ruddervator Figure 1-59. Ruddervator. on the wings’ trailing edges adjacent to the fuselage. Leading edge flaps are also common. They extend...

Flaperons Figure 1-58. A stabilizer and index marks on a transport category aircraft. Figure 1-60. Flaperons. increases wing camber and provides greater lift. Ruddervator Figure 1-59. Ruddervator. on the wings’ trailing edges adjacent to the fuselage. Leading edge flaps are also common. They extend forward and down from the inboard wing leading edge. The flaps are lowered to increase the camber of the wings and provide greater lift and control at slow speeds. They enable landing at slower speeds and shorten the amount of runway required for takeoff and landing. The amount that the flaps extend and the angle they form with the wing can be selected from the flight deck. Typically, flaps can extend up to 45–50°. Figure 1-62 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 structure flaps are the norm on light aircraft. Heavy and high-performance aircraft flaps may also be aluminum, but the use of composite structures is also common. There are various kinds of flaps. Plain flaps form the trailing edge of the wing when the flap is in the retracted position. [Figure 1-63A] The airflow over the wing continues over the upper and lower surfaces of the flap, making the trailing edge of the flap essentially the trailing edge of the wing. The plain flap is hinged so that the trailing edge can be lowered. This A split flap is normally housed under the trailing edge of the wing. [Figure 1-63B] It is usually just a braced flat metal plate hinged at several places along its leading edge. The upper surface of the wing extends to the trailing edge of the flap. When deployed, the split flap trailing edge lowers away from the trailing edge of the wing. Airflow over the top of the wing remains the same. Airflow under the wing now follows the camber created by the lowered split flap, increasing lift. Fowler flaps not only lower the trailing edge of the wing when deployed but also slide aft, effectively increasing the area of the wing. [Figure 1-63C] This creates more lift via the increased surface area, as well as the wing camber. When stowed, the Fowler flap typically retracts up under the wing trailing edge similar to a split flap. The sliding motion of a Fowler flap can be accomplished with a worm drive and flap tracks. An enhanced version of the Fowler flap is a set of flaps that actually contains more than one aerodynamic surface. Figure 1-64 shows a triple-slotted flap. In this configuration, the flap consists of a fore flap, a mid flap, and an aft flap. When deployed, each flap 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 greater camber and effective wing area increase overall lift. Heavy aircraft often have leading edge flaps that are used in conjunction with the trailing edge flaps. [Figure 1-65] They can be made of machined magnesium or can have an aluminum or composite structure. While they are not installed or operate independently, their use with trailing edge flaps can greatly increase wing camber and lift. When stowed, leading edge flaps retract into the leading edge of the wing. 1-27 Secondary/Auxiliary Flight Control Surfaces Name Location Function Flaps Inboard trailing edge of wings Extends the camber of the wing for greater lift and slower flight. Allows control at low speeds for short field takeoffs and landings. Trim tabs Trailing edge of primary flight control surfaces Reduces the force needed to move a primary control surface. Balance tabs Trailing edge of primary flight control surfaces Reduces the force needed to move a primary control surface. Anti-balance tabs Trailing edge of primary flight control surfaces Increases feel and effectiveness of primary control surface. Servo tabs Trailing edge of primary flight control surfaces Assists or provides the force for moving a primary flight control. Spoilers Upper and/or trailing edge of wing Decreases (spoils) lift. Can augment aileron function. Slats Mid to outboard leading edge of wing Extends the camber of the wing for greater lift and slower flight. Allows control at low speeds for short field takeoffs and landings. Slots Outer leading edge of wing forward of ailerons Directs air over upper surface of wing during high angle of attack. Lowers stall speed and provides control during slow flight. Leading edge flap Inboard leading edge of wing Extends the camber of the wing for greater lift and slower flight. 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 1-61. Secondary or auxiliary control surfaces and respective locations for larger aircraft. Figure 1-62. Various aircraft with flaps in the extended position. A B Plain flap Split p flap C Fowler flap Figure 1-63. Various types of flaps. 1-28 Retracted the camber of the wing. Figure 1-66 shows a Krueger flap, recognizable by its flat mid-section. 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 flight deck. Slats not only extend out of the leading edge of the wing increasing camber and lift, but most often, when fully deployed leave a slot between their trailing edges and the leading edge of the wing. [Figure 1-67] This increases the angle of attack at which the wing will maintain its laminar airflow, resulting in the ability to fly the aircraft slower with a reduced stall speed, and still maintain control. Fore flap Mid flap Aft flap Figure 1-64. Triple-slotted flap. Spoilers & Speed Brakes Hinge point Actuator Flap extended Flap retracted Retractable nose Figure 1-65. Leading edge flaps. The differing designs of leading edge flaps essentially provide the same effect. Activation of the trailing edge flaps automatically deploys the leading edge flaps, which are driven out of the leading edge and downward, extending A spoiler is a device found on the upper surface of many heavy and high-performance aircraft. It is stowed flush to the wing’s upper surface. When deployed, it raises up into the airstream and disrupts the laminar airflow of the wing, thus reducing lift. Spoilers are made with similar construction materials and techniques as the other flight control surfaces on the aircraft. Often, they are honeycomb-core flat panels. At low speeds, spoilers are rigged to operate when the ailerons operate to assist with the lateral movement and stability of the aircraft. On the wing where the aileron is moved up, the spoilers also raise thus amplifying the reduction of lift on that wing. [Figure 1-68] On the wing with downward aileron deflection, the spoilers remain stowed. As the speed of the aircraft increases, the ailerons become more effective and the spoiler interconnect disengages. Spoilers are unique in that they may also be fully deployed on both wings to act as speed brakes. The reduced lift and increased drag can quickly reduce the speed of the aircraft in Figure 1-66. Side view (left) and front view (right) of a Krueger flap on a Boeing 737. 1-29 surfaces are also rigged to deploy on the ground automatically when engine thrust reversers are activated. Tabs The force of the air against a control surface during the high speed of 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 aid with these types of problems. The table in Figure 1-69 summarizes the various tabs and their uses. Figure 1-67. Air passing through the slot aft of the slat promotes boundary layer airflow on the upper surface at high angles of attack. flight. Dedicated speed brake panels similar to flight spoilers in construction can also be found on the upper surface of the wings of heavy and high-performance aircraft. They are designed specifically to increase drag and reduce the speed of the aircraft when deployed. These speed brake panels do not operate differentially with the ailerons at low speed. The speed brake control in the flight deck can deploy all spoiler and speed brake surfaces fully when operated. Often, these While in flight, it is desirable for the pilot to be able to take their hands and feet off of the controls and have the aircraft maintain its flight condition. Trims tabs are designed to allow this. Most trim tabs are small movable surfaces located on the trailing edge of a primary flight control surface. A small movement of the tab in the direction opposite of the direction the flight control surface is deflected, causing air to strike the tab, in turn producing a force that aids in maintaining the flight control surface in the desired position. Through linkage set from the flight deck, the tab can be positioned so that it is actually holding the control surface in position rather than the pilot. Therefore, elevator tabs are used to maintain the speed of the aircraft since they assist in maintaining the selected pitch. Rudder tabs can be set to hold yaw in check and maintain heading. Aileron tabs can help keep the wings level. Occasionally, a simple light aircraft may have a stationary metal plate attached to the trailing edge of a primary flight control, usually the rudder. This is also a trim tab as shown in Figure 1-70. It can be bent slightly on the ground to trim the aircraft in flight to a hands-off condition when flying straight and level. The correct amount of bend can be determined only by flying the aircraft after an adjustment. Note that a small amount of bending is usually sufficient. The aerodynamic phenomenon of moving a trim tab in one direction to cause the control surface to experience a force moving in the opposite direction is exactly what occurs with the use of balance tabs. [Figure 1-71] Often, it is difficult to move a primary control surface due to its surface area and the speed of the air rushing over it. Deflecting a balance tab hinged at the trailing edge of the control surface in the opposite direction of the desired control surface movement causes a force to position the surface in the proper direction with reduced force to do so. Balance tabs are usually linked directly to the control surface linkage so that they move automatically when there is an input for control surface movement. They also can double as trim tabs, if adjustable in the flight deck. Figure 1-68. Spoilers deployed upon landing on a transport category aircraft. A servo tab is similar to a balance tab in location and effect, but it is designed to operate the primary flight control surface, not just reduce the force needed to do so. It is usually used as 1-30 Flight Control Tabs Type Direction of Motion (in relation to control surface) Trim Opposite Set by pilot from cockpit. Uses independent linkage. Statically balances the aircraft in flight. Allows “hands off” maintenance of flight condition. Balance Opposite Moves when pilot moves control surface. Coupled to control surface linkage. Aids pilot in overcoming the force needed to move the control surface. Servo Opposite Directly linked to flight control input device. Can be primary or back-up means of control. Aerodynamically positions control surfaces that require too much force to move manually. Anti-balance or Anti-servo Same Directly linked to flight control input device. Increases force needed by pilot to change flight control position. De-sensitizes flight controls. Spring Opposite Located in line of direct linkage to servo tab. Spring assists when control forces become too high in high-speed flight. Enables moving control surface when forces are high. Inactive during slow flight. Activation Effect Figure 1-69. Various tabs and their uses. Lift Tab geared to deflect proportionally to the control deflection, but in the opposite direction Fixed surface Con trol Tab Ground adjustable rudder trim Figure 1-70. Example of a trim tab. a means to back up the primary control of the flight control surfaces. [Figure 1-72] On heavy aircraft, large control surfaces require too much force to be moved manually and are usually deflected out of the neutral position by hydraulic actuators. These power control units are signaled via a system of hydraulic valves connected to the yoke and rudder pedals. On fly-by-wire aircraft, the hydraulic actuators that move the flight control surfaces are signaled by electric input. In the case of hydraulic system failure(s), manual linkage to a servo tab can be used to deflect it. This, in turn, provides an aerodynamic force that moves the primary control surface. Figure 1-71. Balance tabs assist with forces needed to position control surfaces. A control surface may require excessive force to move only in the final stages of travel. When this is the case, a spring tab can be used. This is essentially a servo tab that does not activate until an effort is made to move the control surface beyond a certain point. When reached, a spring in line of the control linkage aids in moving the control surface through the remainder of its travel. [Figure 1-73] Figure 1-74 shows another way of assisting the movement of an aileron on a large aircraft. It is called an aileron balance panel. Not visible when approaching the aircraft, it is positioned in the linkage that hinges the aileron to the wing. Balance panels have been constructed typically of aluminum skin-covered frame assemblies or aluminum honeycomb structures. The trailing edge of the wing just forward of the leading edge of the aileron is sealed to allow controlled airflow 1-31 Control stick Control stick Spring Free link Free link Control surface hinge line Figure 1-73. Many tab linkages have a spring tab that kicks in as the forces needed to deflect a control increase with speed and the angle of desired deflection. Figure 1-72. Servo tabs can be used to position flight control surfaces in case of hydraulic failure. stable for the pilot. Figure 1-76 shows an antiservo tab in the near neutral position. Deflected in the same direction as the desired stabilator movement, it increases the required control surface input. in and out of the hinge area where the balance panel is located. [Figure 1-75] When the aileron is moved from the neutral position, differential pressure builds up on one side of the balance panel. This differential pressure acts on the balance panel in a direction that assists the aileron movement. For slight movements, deflecting the control tab at the trailing edge of the aileron is easy enough to not require significant assistance from the balance tab. (Moving the control tab moves the ailerons as desired.) But, as greater deflection is requested, the force resisting control tab and aileron movement becomes greater and augmentation from the balance tab is needed. The seals and mounting geometry allow the differential pressure of airflow on the balance panel to increase as deflection of the ailerons is increased. This makes the resistance felt when moving the aileron controls relatively constant. Other Wing Features There may be other structures visible on the wings of an aircraft that contribute to performance. Winglets, vortex generators, stall fences, and gap seals are all common wing features. Introductory descriptions of each are given in the following paragraphs. A winglet is an obvious vertical upturn of the wing’s tip resembling a vertical stabilizer. It is an aerodynamic device designed to reduce the drag created by wing tip vortices in flight. Usually made from aluminum or composite materials, winglets can be designed to optimize performance at a desired speed. [Figure 1-77] Antiservo tabs, as the name suggests, are like servo tabs but move in the same direction as the primary control surface. On some aircraft, especially those with a movable horizontal stabilizer, the input to the control surface can be too sensitive. An antiservo 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 Vortex generators are small airfoil sections usually attached to the upper surface of a wing. [Figure 1-78] They are designed to promote smooth, or non-turbulent, airflow over the wing and control surfaces. Usually made of aluminum Hinge Balance panel Vent gap Control tab AILERON WING Vent gap Lower pressure Figure 1-74. An aileron balance panel and linkage uses varying air pressure to assist in control surface positioning. 1-32 Balance panel Figure 1-75. The trailing edge of the wing just forward of the leading edge of the aileron is sealed to allow controlled airflow in and out of the hinge area where the balance panel is located. Figure 1-77. A winglet reduces aerodynamic drag caused by air spilling off of the wing tip. Antiservo tab Stabilator pivot point Figure 1-78. Vortex generators. Figure 1-76. An antiservo tab moves in the same direction as the control tab. Shown here on a stabilator, it desensitizes the pitch control. and installed in a spanwise line or lines, the vortices created by these devices swirl downward assisting maintenance of the boundary layer of air flowing over the wing. They can also be found on the fuselage and empennage. Figure 1-79 shows the unique vortex generators on a Symphony SA-160 wing. A chordwise barrier on the upper surface of the wing, called a stall fence, is used to halt the spanwise flow of air. During low speed flight, this can maintain proper chordwise airflow reducing the tendency for the wing to stall. Usually made of aluminum, the fence is a fixed structure most common on swept wings, which have a natural spanwise tending boundary air flow. [Figure 1-80] Often, a gap can exist between the stationary trailing edge of a wing or stabilizer and the movable control surface(s). At high angles of attack, high pressure air from the lower wing surface can be disrupted at this gap. The result can be turbulent airflow, which increases drag. There is also a tendency for some lower wing boundary air to enter the gap and disrupt the upper wing surface airflow, which in turn reduces lift and control surface responsiveness. The use of gap seals is common to promote smooth airflow in these gap areas. Gap seals can be made of a wide variety of materials Figure 1-79. The Symphony SA-160 has two unique vortex generators on its wing to ensure aileron effectiveness through the stall. ranging from aluminum and impregnated fabric to foam and plastic. Figure 1-81 shows some gap seals installed on various aircraft. Landing Gear The landing gear supports the aircraft during landing and while it is on the ground. Simple aircraft that fly at low speeds generally have fixed gear. This means the gear is stationary and does not retract for flight. Faster, more complex aircraft 1-33 for aviation use and have unique operating characteristics. Main wheel assemblies usually have a braking system. To aid with the potentially high impact of landing, most landing gear have a means of either absorbing shock or accepting shock and distributing it so that the structure is not damaged. Stall fence Figure 1-80. A stall fence aids in maintaining chordwise airflow over the wing. have retractable landing gear. After takeoff, the landing gear is retracted into the fuselage or wings and out of the airstream. This is important because extended gear create significant parasite drag which reduces performance. Parasite drag is caused by the friction of the air flowing over the gear. It increases with speed. On very light, slow aircraft, the extra weight that accompanies a retractable landing gear is more of a detriment than the drag caused by the fixed gear. Lightweight fairings and wheel pants can be used to keep drag to a minimum. Figure 1-82 shows examples of fixed and retractable gear. Landing gear must be strong enough to withstand the forces of landing when the aircraft is fully loaded. In addition to strength, a major design goal is to have the gear assembly be as light as possible. To accomplish this, landing gear are made from a wide range of materials including steel, aluminum, and magnesium. Wheels and tires are designed specifically Not all aircraft landing gear are configured with wheels. Helicopters, for example, have such high maneuverability and low landing speeds that a set of fixed skids is common and quite functional with lower maintenance. The same is true for free balloons which fly slowly and land on wood skids affixed to the floor of the gondola. Other aircraft landing gear are equipped with pontoons or floats for operation on water. A large amount of drag accompanies this type of gear, but an aircraft that can land and take off on water can be very useful in certain environments. Even skis can be found under some aircraft for operation on snow and ice. Figure 1-83 shows some of these alternative landing gear, the majority of which are the fixed gear type. Amphibious aircraft are aircraft than can land either on land or on water. On some aircraft designed for such dual usage, the bottom half of the fuselage acts as a hull. Usually, it is accompanied by outriggers on the underside of the wings near the tips to aid in water landing and taxi. Main gear that retract into the fuselage are only extended when landing on the ground or a runway. This type of amphibious aircraft is sometimes called a flying boat. [Figure 1-84] Many aircraft originally designed for land use can be fitted with floats with retractable wheels for amphibious use. [Figure 1-85] Typically, the gear retracts into the float when not needed. Sometimes a dorsal fin is added to the aft underside of the fuselage for longitudinal stability during water operations. It is even possible on some aircraft to direct this type of fin by tying its control into the aircraft’s rudder pedals. Skis can also be fitted with wheels that retract to allow landing on solid ground or on snow and ice. Aileron gap seal Tab gap seal Figure 1-81. Gap seals promote the smooth flow of air over gaps between fixed and movable surfaces. 1-34 Figure 1-84. An amphibious aircraft is sometimes called a flying boat because the fuselage doubles as a hull. Figure 1-82. Landing gear can be fixed (top) or retractable (bottom). Tail Wheel Gear Configuration There are two basic configurations of airplane landing gear: conventional gear or tail wheel gear and the tricycle gear. Tail wheel gear dominated early aviation and therefore has become known as conventional gear. In addition to its two main wheels which are positioned under most of the weight of the aircraft, the conventional gear aircraft also has a smaller wheel located at the aft end of the fuselage. [Figure 1-86] Often this tail wheel is able to be steered by rigging cables attached to the rudder pedals. Other conventional gear have no tail wheel at all using just a steel skid plate under the aft fuselage instead. The small tail wheel or skid plate allows the fuselage to incline, thus giving clearance for the long propellers that prevailed in aviation through WWII. It also gives greater clearance between the propeller and loose debris when operating on an unpaved runway. But the inclined fuselage blocks the straight-ahead vision of the pilot during ground operations. Until up to speed where the elevator becomes effective to lift the tail wheel off the ground, the pilot must lean his head out the side of the flight deck to see directly ahead of the aircraft. The use of tail wheel gear can pose another difficulty. When Figure 1-83. Aircraft landing gear without wheels. 1-35 the air flowing over the elevator is sufficient for it to raise the tail off the ground. As speed increases further, the two main wheels under the center of gravity are very stable. Tricycle Gear Configuration Tricycle gear is the most prevalent landing gear configuration in aviation. In addition to the main wheels, a shock absorbing nose wheel is at the forward end of the fuselage. Thus, the center of gravity is then forward of the main wheels. The tail of the aircraft is suspended off the ground and clear view straight ahead from the flight deck is given. Ground looping is nearly eliminated since the center of gravity follows the directional nose wheel and remains between the mains. Figure 1-85. Retractable wheels make this aircraft amphibious. landing, tail wheel aircraft can easily ground loop. A ground loop is when the tail of the aircraft swings around and comes forward of the nose of the aircraft. The reason this happens is due to the two main wheels being forward of the aircraft’s center of gravity. The tail wheel is aft of the center of gravity. If the aircraft swerves upon landing, the tail wheel can swing out to the side of the intended path of travel. If far enough to the side, the tail can pull the center of gravity out from its desired location slightly aft of but between the main gear. Once the center of gravity is no longer trailing the mains, the tail of the aircraft freely pivots around the main wheels causing the ground loop. Conventional gear is useful and is still found on certain models of aircraft manufactured today, particularly aerobatic aircraft, crop dusters, and aircraft designed for unpaved runway use. It is typically lighter than tricycle gear which requires a stout, fully shock absorbing nose wheel assembly. The tail wheel configuration excels when operating out of unpaved runways. With the two strong main gear forward providing stability and directional control during takeoff roll, the lightweight tail wheel does little more than keep the aft end of the fuselage from striking the ground. As mentioned, at a certain speed, Figure 1-86. An aircraft with tail wheel gear. Light aircraft use tricycle gear, as well as heavy aircraft. Twin nose wheels on the single forward strut and massive multistrut/ multiwheel main gear may be found supporting the world’s largest aircraft, but the basic configuration is still tricycle. The nose wheel may be steered with the rudder pedals on small aircraft. Larger aircraft often have a nose wheel steering wheel located off to the side of the flight deck. Figure 1-87 shows aircraft with tricycle gear. Chapter 13, Aircraft Landing Gear Systems, discusses landing gear in detail. Maintaining the Aircraft Maintenance of an aircraft is of the utmost importance for safe flight. Certificated technicians are committed to perform timely maintenance functions in accordance with the manufacturer’s instructions and under Title 14 of the Code of Federal Regulations (14 CFR). At no time is an act of aircraft maintenance taken lightly or improvised. The consequences of such action could be fatal, and the technician could lose their certificate and face criminal charges. Airframe, engine, and aircraft component manufacturers are responsible for documenting the maintenance procedures that guide managers and technicians on when and how to perform maintenance on their products. A small aircraft may only require a few manuals, including the aircraft maintenance manual. This volume usually contains the most frequently used information required to maintain the aircraft properly. The Type Certificate Data Sheet (TCDS) for an aircraft also contains critical information. Complex and large aircraft require several manuals to convey correct maintenance procedures adequately. In addition to the maintenance manual, manufacturers may produce such volumes as structural repair manuals, overhaul manuals, wiring diagram manuals, component manuals, and more. Note that the use of the word “manual” is meant to include electronic as well as printed information. Also, proper maintenance extends to the use of designated tools and fixtures called out in the manufacturer’s maintenance documents. In 1-36 maintenance performed on large aircraft such as airliners is common. The importance of correct maintenance recordkeeping should not be overlooked. Location Numbering Systems Even on small, light aircraft, a method of precisely locating each structural component is required. Various numbering systems are used to facilitate the location of specific wing frames, fuselage bulkheads, or any other structural members on an aircraft. Most manufacturers use some system of station marking. For example, the nose of the aircraft may be designated “zero station,” and all other stations are located at measured distances in inches behind the zero station. Thus, when a blueprint reads “fuselage frame station 137,” that particular frame station can be located 137 inches behind the nose of the aircraft. Figure 1-87. Tricycle landing gear is the most predominant landing gear configuration in aviation. the past, not using the proper tooling has caused damage to critical components, which subsequently failed and led to aircraft crashes and the loss of human life. The technician is responsible for using the correct information, procedures, and tools needed to perform appropriate maintenance or repairs. Standard aircraft maintenance procedures do exist and can be used by the technician when performing maintenance or a repair. These are found in the Federal Aviation Administration (FAA) approved advisory circulars (AC) 43.13-2, Acceptable Methods, Techniques, and Practices - Aircraft Alterations and AC 43.13-1, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair. If not addressed by the manufacturer’s literature, the technician may use the procedures outlined in these manuals to complete the work in an acceptable manner. These procedures are not specific to any aircraft or component and typically cover methods used during maintenance of all aircraft. Note that the manufacturer’s instructions supersede the general procedures found in AC 43.13-2 and AC 43.13-1. All maintenance related actions on an aircraft or component are required to be documented by the performing technician in the aircraft or component logbook. Light aircraft may have only one logbook for all work performed. Some aircraft may have a separate engine logbook for any work performed on the engine(s). Other aircraft have separate propeller logbooks. Large aircraft require volumes of maintenance documentation comprised of thousands of procedures performed by hundreds of technicians. Electronic dispatch and recordkeeping of To locate structures to the right or left of the center line of an aircraft, a similar method is employed. Many manufacturers consider the center line of the aircraft to be a zero station from which measurements can be taken to the right or left to locate an airframe member. This is often used on the horizontal stabilizer and wings. The applicable manufacturer’s numbering system and abbreviated designations or symbols should always be reviewed before attempting to locate a structural member. They are not always the same. The following list includes location designations typical of those used by many manufacturers. Fuselage stations (Fus. Sta. or FS) are numbered in inches from a reference or zero point known as the reference datum. [Figure 1-88] The reference datum is an imaginary vertical plane at or near the nose of the aircraft from which all fore and aft distances are measured. The distance to a given point is measured in inches parallel to a center line extending through the aircraft from the nose through the center of the tail cone. Some manufacturers may call the fuselage station a body station, abbreviated BS. Buttock line or butt line (BL) is a vertical reference plane down the center of the aircraft from which measurements left or right can be made. [Figure 1-89] Water line (WL) is the measurement of height in inches perpendicular from a horizontal plane usually located at the ground, cabin floor, or some other easily referenced location. [Figure 1-90] Aileron station (AS) is measured outboard from, and parallel to, the inboard edge of the aileron, perpendicular to the rear beam of the wing. Flap station (KS) is measured perpendicular to the rear 1-37 9. (5Ŧ (5Ŧ (5Ŧ (5Ŧ (5Ŧ (5Ŧ (5Ŧ (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 (5 Figure 1-88. The various fuselage stations relative to a single point of origin illustrated in inches or some other measurement (if of foreign development). In addition to the location stations listed above, other measurements are used, especially on large aircraft. Thus, there may be horizontal stabilizer stations (HSS), vertical stabilizer stations (VSS) or powerplant stations (PPS). [Figure 1-91] In every case, the manufacturer’s terminology and station location system should be consulted before locating a point on a particular aircraft. Another method is used to facilitate the location of aircraft components on air transport aircraft. This involves dividing the aircraft into zones. These large areas or major zones are further divided into sequentially numbered zones and subzones. The digits of the zone number are reserved and indexed to indicate the location and type of system of which the component is a part. Figure 1-92 illustrates these zones and subzones on a transport category aircraft. BL 76.50 BL 61.50 BL 47.27 BL 34.5 BL 21.50 BL 47.50 BL 96.50 BL 96.62 Nacelle station (NC or Nac. Sta.) is measured either forward of or behind the front spar of the wing and perpendicular to a designated water line. BL 86.56 BL 96.50 BL 96.62 BL 47.50 BL 86.56 BL 21.50 beam of the wing and parallel to, and outboard from, the inboard edge of the flap. BL 76.50 BL 61.50 BL 47.27 BL 34.5 BL 23.25 BL 16.00 Figure 1-89. Butt line diagram of a horizontal stabilizer. Access & Inspection Panels Quick access to the accessories and other equipment carried in the fuselage is provided for by numerous access doors, inspection plates, landing wheel wells, and other openings. WL 123.483 WL 97.5 WL 79.5 WL 7.55 WL 73.5 Ground line WL 9.55 Figure 1-90. Water line diagram. 1-38 943 903 886 863 843.8 652.264 568.5 585 536 511.21 437 411 379 CL FUS-WING STA 0 16.5 25.7 41.3 56.9 72.5 88.1 15.2 NAC C L 65.7 76.5 85.5 2° 106.4 BL 86.179 199 242 258 264 274 282 294.5 NAC C L BL 86.179 200 218.17 230.131 4° 177 185 2° 315.5 329.5 343.5 353 371 135.845 151.14 155.315 353 371 WGLTS 0.00 FUS CL 220 100.72 WGLTS 49.89 104.1 122 FS 674.737 FS 625.30 177.0 148 163 178 15° FS 652.264 111 127.2 Figure 1-91. Wing stations are often referenced off the butt line, which bisects the center of the fuselage longitudinally. Horizontal stabilizer stations referenced to the butt line and engine nacelle stations are also shown. Servicing diagrams showing the arrangement of equipment and location of access doors are supplied by the manufacturer in the aircraft maintenance manual. Knowing where a particular structure or component is located on an aircraft needs to be combined with gaining access to that area to perform the required inspections or maintenance. To facilitate this, access and inspection panels are located on most surfaces of the aircraft. Small panels that are hinged or removable allow inspection and servicing. Large panels and doors allow components to be removed and installed, as well as human entry for maintenance purposes. The underside of a wing, for example, sometimes contains dozens of small panels through which control cable components can be monitored and fittings greased. Various drains and jack points may also be on the underside of the wing. The upper surface of the wings typically have fewer access panels because a smooth surface promotes better laminar airflow, which causes lift. On large aircraft, walkways are sometimes designated on the wing upper surface to permit safe navigation by mechanics and inspectors to critical structures and components located along the wing’s leading and trailing edges. Wheel wells and special component bays are places where numerous components and accessories are grouped together for easy maintenance access. Panels and doors on aircraft are numbered for positive identification. On large aircraft, panels are usually numbered sequentially containing zone and subzone information in the panel number. Designation for a left or right side location on the aircraft is often indicated in the panel number. This could be with an “L” or “R,” or panels on one side of the aircraft could be odd numbered and the other side even numbered. The manufacturer’s maintenance manual explains the panel numbering system and often has numerous diagrams and tables showing the location of various components and under which panel they may be found. Each manufacturer is entitled to develop its own panel numbering system. Helicopter Structures The structures of the helicopter are designed to give the helicopter its unique flight characteristics. A simplified explanation of how a helicopter flies is that the rotors are rotating airfoils that provide lift similar to the way wings provide lift on a fixed-wing aircraft. Air flows faster over the curved upper surface of the rotors, causing a negative pressure and thus, lifting the aircraft. Changing the angle of attack of the rotating blades increases or decreases lift, respectively raising or lowering the helicopter. Tilting the rotor plane of rotation causes the aircraft to move horizontally. Figure 1-93 shows the major components of a typical helicopter. Airframe The airframe, or fundamental structure, of a helicopter can be made of either metal or wood composite materials, or some combination of the two. Typically, a composite component consists of many layers of fiber-impregnated resins, bonded to form a smooth panel. Tubular and sheet metal substructures are usually made of aluminum, though stainless steel or 1-39 ZONE 300—Empennage 326 ZONE 300—Empennage 324 344 343 345 342 341 335 334 351 322 321 323 325 333 312 332 331 311 Zone 600—Right wing ZONE 800—Doors Zone 400—Engine nacelles 824 Zone 200—Upper half of fuselage 825 822 823 821 811 Zone 700—Landing gear and landing gear doors ZONE 100—Lower half of fuselage 133 123 144 132 143 145 131 134 112 Zone 500—Left wing 141 122 111 146 142 135 Zones Subzones 121 Figure 1-92. Large aircraft are divided into zones and subzones for identifying the location of various components. titanium are sometimes used in areas subject to higher stress or heat. Airframe design encompasses engineering, aerodynamics, materials technology, and manufacturing methods to achieve favorable balances of performance, reliability, and cost. Fuselage As with fixed-wing aircraft, helicopter fuselages and tail booms are often truss-type or semimonocoque structures of stress-skin design. Steel and aluminum tubing, formed aluminum, and aluminum skin are commonly used. Modern helicopter fuselage design includes an increasing utilization of advanced composites as well. Firewalls and engine decks are usually stainless steel. Helicopter fuselages vary widely from those with a truss frame, two seats, no doors, and a monocoque shell flight compartment to those with fully enclosed airplane-style cabins as found on larger twinengine helicopters. The multidirectional nature of helicopter flight makes wide-range visibility from the flight deck essential. Large, formed polycarbonate, glass, or plexiglass windscreens are common. Landing Gear or Skids As mentioned, a helicopter’s landing gear can be simply a set of tubular metal skids. Many helicopters do have landing gear with wheels, some retractable. Powerplant & Transmission The two most common types of engine used in helicopters are the reciprocating engine and the turbine engine. Reciprocating engines, also called piston engines, are generally used in smaller helicopters. Most training helicopters use reciprocating engines because they are relatively simple and inexpensive to operate. Turbine Engines Turbine engines are more powerful and are used in a wide variety of helicopters. They produce a tremendous amount of power for their size but are generally more expensive to operate. The turbine engine used in helicopters operates differently than those used in airplane applications. In most applications, the exhaust outlets simply release expended 1-40 Tail rotor Tail boom Main rotor hub assembly Stabilizer Main rotor blades Pylon Tail skid Powerplant Airframe Fuselage Transmission Landing gear or skid Figure 1-93. The major components of a helicopter are the airframe, fuselage, landing gear, powerplant/transmission, main rotor system, and antitorque system. gases and do not contribute to the forward motion of the helicopter. Because the airflow is not a straight line pass through as in jet engines and is not used for propulsion, the cooling effect of the air is limited. Approximately 75 percent of the incoming airflow is used to cool the engine. The gas turbine engine mounted on most helicopters is made up of a compressor, combustion chamber, turbine, and accessory gearbox assembly. The compressor draws filtered air into the plenum chamber and compresses it. Common type filters are centrifugal swirl tubes where debris is ejected outward and blown overboard prior to entering the compressor, or engine barrier filters (EBF), a paper element type filter, encased in a frame with a screen/grill over the inlet, and usually coated with an oil. This design significantly reduces the ingestion of foreign object debris (FOD). The compressed air is directed to the combustion section through discharge tubes where atomized fuel is injected into it. The air-fuel mixture is ignited and allowed to expand. This combustion gas is then forced through a series of turbine wheels causing them to turn. These turbine wheels provide power to both the engine compressor and the accessory gearbox. Depending on model and manufacturer, the rpm range can vary from a range low of 20,000 to a range high of 51,600. Power is provided to the main rotor and tail rotor systems through the freewheeling unit which is attached to the accessory gearbox power output gear shaft. The combustion gas is finally expelled through an exhaust outlet. The temperature of gas is measured at different locations and is referenced differently by each manufacturer. Some common terms are: inter-turbine temperature (ITT), exhaust gas temperature (EGT), or turbine outlet temperature (TOT). TOT is used throughout this discussion for simplicity purposes. [Figure 1-94] Transmission The transmission system transfers power from the engine to the main rotor, tail rotor, and other accessories during normal flight conditions. The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. The freewheeling unit, or autorotative clutch, allows the main rotor transmission to drive the tail rotor drive shaft during autorotation. Helicopter transmissions are normally lubricated and cooled with their own oil supply. A sight gauge is provided to check the oil level. Some transmissions have chip detectors located in the sump. These detectors are wired to warning lights located 1-41 Compression Section Gearbox Section Exhaust air outlet Turbine Section N2 Rotor Stator Combustion Section N1 Rotor Compressor rotor Igniter plug Air inlet Fuel nozzle Inlet air Compressor discharge air Combustion gases Exhaust gases Gear Output Shaft Combustion liner Figure 1-94. Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main difference between a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine rather than producing thrust through the expulsion of exhaust gases. on the pilot’s instrument panel that illuminate in the event of an internal problem. Some chip detectors on modern helicopters have a “burn off” capability and attempt to correct the situation without pilot action. If the problem cannot be corrected on its own, the pilot must refer to the emergency procedures for that particular helicopter. Main Rotor System The rotor system is the rotating part of a helicopter which generates lift. The rotor consists of a mast, hub, and rotor blades. The mast is a cylindrical metal shaft that extends upwards from and is driven, and sometimes supported, by the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. The rotor blades are then attached to the hub by any number of different methods. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. There are three basic classifications: rigid, semirigid, or fully articulated. Rigid Rotor System The simplest is the rigid rotor system. In this system, the rotor blades are rigidly attached to the main rotor hub and are not free to slide back and forth (drag) or move up and down (flap). [Figure 1-95] The forces tending to make the rotor blades do so are absorbed by the flexible properties of the blade. The pitch of the blades, however, can be adjusted by rotation about the spanwise axis via the feathering hinges. Semirigid Rotor System The semirigid rotor system in Figure 1-96 makes use of a teetering hinge at the blade attach point. While held in check from sliding back and forth, the teetering hinge does allow the blades to flap up and down. With this hinge, when one blade flaps up, the other flaps down. Flapping is caused by a phenomenon known as dissymmetry of lift. As the plane of rotation of the rotor blades is tilted and the helicopter begins to move forward, an advancing blade and a retreating blade become established (on two-bladed systems). The relative windspeed is greater on an advancing blade than it is on a retreating blade. This causes greater lift to be developed on the advancing blade, causing it to rise up or flap. When blade rotation reaches the point where the blade becomes the retreating blade, the extra lift is lost and the blade flaps downward. [Figure 1-97] Fully Articulated Rotor System Fully articulated rotor blade systems provide hinges that allow the rotors to move fore and aft, as well as up and down. This lead-lag, drag, or hunting movement as it is called is in response to the Coriolis effect during rotational speed changes. When first starting to spin, the blades lag until centrifugal force is fully developed. Once rotating, a reduction in speed causes the blades to lead the main rotor hub until forces come into balance. Constant fluctuations in rotor blade speeds cause the blades to “hunt.” They are free to do so in a fully articulating system due to being mounted on the vertical drag hinge. One or more horizontal hinges provide for flapping on a fully articulated rotor system. Also, the feathering hinge allows blade pitch changes by permitting rotation about the spanwise axis. Various dampers and stops can be found on 1-42 Blade pitch horns Main rotor hub Main rotor blades Pitch change links Main rotor blades Main rotor mast Figure 1-95. Four-blade hingeless (rigid) main rotor. The hub is a single piece of forged rigid titanium. Blade grip Retreating Side Blade pitch change horn Coning hinge a rot de Advancing Side tion Blade tip speed minus helicopter speed (200 knots) Blade tip speed plus helicopter speed (400 knots) io at rot n Pitch link Relative wind Bl a Blade grip Direction of Flight Teetering hinge Relative wind Coning hinge Swash plate Blade Forward Flight 100 knots Figure 1-96. The semirigid rotor system of the Robinson R22. Figure 1-97. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100 knots, the relative windspeed on the advancing side is 400 knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymmetry of lift. 1-43 Pitch change axis (feathering) Pitch horn Flap hinge Drag hinge Damper Figure 1-99. Five-blade articulated main rotor with elastomeric bearings. different designs to reduce shock and limit travel in certain directions. Figure 1-98 shows a fully articulated main rotor system with the features discussed. Numerous designs and variations on the three types of main rotor systems exist. Engineers continually search for ways to reduce vibration and noise caused by the rotating parts of the helicopter. Toward that end, the use of elastomeric bearings in main rotor systems is increasing. These polymer bearings have the ability to deform and return to their original shape. As such, they can absorb vibration that would normally be transferred by steel bearings. They also do not require regular lubrication, which reduces maintenance. Some modern helicopter main rotors have been designed with flextures. These are hubs and hub components that are made out of advanced composite materials. They are designed to take up the forces of blade hunting and dissymmetry of lift by flexing. As such, many hinges and bearings can be eliminated from the traditional main rotor system. The result is a simpler rotor mast with lower maintenance due to fewer moving parts. Often, designs using flextures incorporate elastomeric bearings. [Figure 1-99] Antitorque System Ordinarily, helicopters have between two and seven main rotor blades. These rotors are usually made of a composite structure. The large rotating mass of the main rotor blades of a helicopter produce torque. This torque increases with engine power and tries to spin the fuselage in the opposite direction. The tail boom and tail rotor, or antitorque rotor, counteract this torque effect. [Figure 1-100] Controlled with foot pedals, the countertorque of the tail rotor must be modulated as engine power levels are changed. This is done by changing the pitch of the tail rotor blades. This, in turn, changes the amount of countertorque, and the aircraft can be rotated about its vertical axis, allowing the pilot to control the direction the helicopter is facing. Similar to a vertical stabilizer on the empennage of an airplane, a fin or pylon is also a common feature on rotorcraft. Normally, it supports the tail rotor assembly, although some tail rotors are mounted on the tail cone of the boom. Additionally, a horizontal member called a stabilizer is often constructed at the tail cone or on the pylon. A Fenestron® is a unique tail rotor design which is actually a multiblade ducted fan mounted in the vertical pylon. It works the same way as an ordinary tail rotor, providing sideways thrust to counter the torque produced by the main rotors. [Figure 1-101] A NOTAR® antitorque system has no visible rotor mounted otation de r Bla Tor qu e Figure 1-98. Fully articulated rotor system. Resultant torque from main rotor blades Tor qu e tion Blade rota Tail rotor thrust Figure 1-100. A tail rotor is designed to produce thrust in a direction opposite to that of the torque produced by the rotation of the main rotor blades. It is sometimes called an antitorque rotor. 1-44 Figure 1-101. A Fenestron or “fan-in-tail” antitorque system. This design provides an improved margin of safety during ground operations. on the tail boom. Instead, an engine-driven adjustable fan is located inside the tail boom. NOTAR® is an acronym that stands for “no tail rotor.” As the speed of the main rotor changes, the speed of the NOTAR® fan changes. Air is vented out of two long slots on the right side of the tail boom, entraining main rotor wash to hug the right side of the tail boom, in turn causing laminar flow and a low pressure (Coanda Effect). This low pressure causes a force counter to the torque produced by the main rotor. Additionally, the remainder of the air from the fan is sent through the tail boom to a vent on the aft left side of the boom where it is expelled. This action to the left causes an opposite reaction to the right, which is the direction needed to counter the main rotor torque. [Figure 1-102] Controls The controls of a helicopter differ slightly from those found in an aircraft. The collective, operated by the pilot with the left hand, is pulled up or pushed down to increase or decrease the angle of attack on all of the rotor blades simultaneously. This increases or decreases lift and moves the aircraft up or down. The engine throttle control is located on the hand grip at the end of the collective. The cyclic is the control “stick” located between the pilot’s legs. It can be moved in any direction to tilt the plane of rotation of the rotor blades. This causes the helicopter to move in the direction that the cyclic is moved. As stated, the foot pedals control the pitch of the tail rotor blades thereby balancing main rotor torque. Figures 1-103 and 1-104 illustrate the controls found in a typical helicopter. Throttle control Collective Figure 1-103. The collective changes the pitch of all of the rotor blades simultaneously and by the same amount, thereby increasing or decreasing lift. Downwash Air jet Main rotor wake Swash plate Lift Sideware flight Air intake Rotating nozzle Figure 1-102. While in a hover, Coanda Effect supplies approximately two-thirds of the lift necessary to maintain directional control. The rest is created by directing the thrust from the controllable rotating nozzle. Cyclic control stick moved sideways Forward flight Cyclic control stick moved forward Figure 1-104. The cyclic changes the angle of the swash plate which changes the plane of rotation of the rotor blades. This moves the aircraft horizontally in any direction depending on the positioning of the cyclic. 1-45