all-metal full cantilever (no external bracing) wing Skin section. The wing is made up of spars, ribs, and lower and upper wing skin covering. With few exceptions, wings of this type are of the stressed-skin design (the skin is part of the wing structure and carries part of the wing stresses).
The top and bottom wing skin covers are made up of several integrally stiffened sections. This type Skin of wing construction permits the installation of A. Constant thickness bladder-type fuel cells in the wings or is sealed to hold fuel without the usual fuel cells or tanks. A wing which is constructed to allow it to be used as a fuel cell or tank is referred to as a "wet-wing." Skin A wing that uses a box-beam design is shown in figure 1-21. This type of construction not only increases strength and reduces weight, but it also enables the wing to serve as a fuel tank when prop- Skün erly sealed.
B. Tapered core FIcURE 1-22. Constant-thickness and tapered-core honeycomb sections.
FIGURE 1-21. Box-beam milled wing- Both aluminum honeycomb and fiber glass honeycomb sandwich material are commonly used in the construction of wing and stabilizer surfaces, bulkheads, floors, control surfaces, and trim tabs. Aluinum honeycomb material is made of aluminum foil honeycomb core, bonded between sheets of aluminum. Fiber glass honeycomb material consists of fiber glass honeycomb core bonded between layers of fiber glass cloth.
In the construction of large aircraft structures, and in some small aircraft as well, the honeycomb sandwich structure employs either aluminum or reinforced plastic materials. Honeycomb panels are usually a lightweight cellular core sandwiched between two thin skins or facing materials such as aluminum, wood, or plastic.
Aircraft honeycomb material is manufactured in various shapes, but is usually of the constant thickness or tapered core types. An example of each is shown in figure 1-22.
Figure 1-23 shows a view of the upper surface of a large jet transport wing. The various panels manufactured from honeycomb material are outlined by diagonal lines and labeled.
Still another type of construction is illustrated in figure 1-24. In this case the sandwich structure of the wing leading edge is bonded to the metal spar.
Also shown is the integrally bonded deicer panel.
NACELLES OR PODS Nacelles or pods are streamlined enclosures used 10% on multi-engine aircraft primarily to house the engines. They are round or spherical in shape and are usually located above, below, or at the leading edge of the wing on multi-engine aircraft. If an aircraft has only one engine, it is usually mounted at the forward end of the fuselage, and the nacelle is the streamlined extension of the fuselage.
An engine nacelle or pod consists of skin, cowling, structural members, a firewall, and engine mounts. Skin and cowling cover the outside of the nacelle. Both are usually made of sheet aluminum alloy, stainless steel, magnesium, or titanium. Regardless of the material used, the skin is usually attached to the framework by rivets.
The framework usually consists of structural members similar to those of the fuselage. The framework includes lengthwise members, such as 13
Trailing edge sandwich panels (constant-thickness core) Trailing edge sandwich panel (constant-thickness core) Spoiler sandwich panel (tapered core, solid wedge) Wing leading edge Trailing edge sandwich panels (constant-thickness core) Outboard flap
- Inboard flap Spoiler sandwich panel (tapered core, solid wedge) Aileron tab sandwich panels (tapered core, phenolic wedge) Aileron sandwich panel (constant-thickness core) Trailing edge wedge sandwich panel (tapered core, cord wedge) FIGURE 1-23. Honeycomb wing construction on a large jet transport aircraft.
longerons and stringers, and widthwise/vertical members, such as bulkheads, rings, and formers.
A nacelle or pod also contains a firewall which separates the engine compartment from the rest of the aircraft. This bulkhead is usually made of stainless steel sheet metal, or as in some aircraft, of titanium.
Another nacelle or pod member is the engine mount. The mount is usually attached to the firewall, and the engine is attached to the mount by nuts, bolts, and vibration-absorbing rubber cushions or pads. Figure 1-25 shows examples of a semimonocoque and a welded tubular steel engine mount used with reciprocating engines.
Engine mounts are designed to meet particular conditions of installation, such as the location and the method of attachment of the engine mount and the size, type, and characteristics of the engine it is intended to support. An engine mount is usually constructed as a single unit which can be detached quickly and easily from the remaining structure.
Engine mounts are commonly made of welded chrome/molybdenum steel tubing, and forgings of chrome/nickel/molybdenum are used for the highly stressed fittings.
To reduce wind resistance during flight, the landing gear of most high-speed or large aircraft is retracted (drawn up into streamlined enclosures).
The part of the aircraft which receives or encloses the landing gear as it retracts is called a wheel well.
In many instances, the wheel well is part of the nacelle; however, on some aircraft the landing gear retracts into the fuselage or wing.
Cowling Cowling usually refers to the detachable covering of those areas into which access must be gained regularly, such as engines, accessory sections, and engine mount or firewall areas. Figure 1-26 shows an exploded view of the pieces of cowling for a horizontally opposed engine on a light aircraft.
Laminated metal structure Deicer panel with chordwise ribs sandwich core Honeycomb Glass reinforced plastica sandwich faces spanwise and chordwise Wooden members bonded to sandwich Metal member Ficout 1-24. Leading edge sandwich material bonded to metal wing member.
Samimonocoque Tubular brackets by pins which automatically lock in place, hinges when the cowl is opened (figure 1-27). tached to the firewall by mounts which also serve as "orange-peel" cowl panels, The cowl panels are at- The lower cowl mounts are secured to the hinge Some large reciprocating engines are enclosed by FIcURE 1-25. Semimonocoque and welded tubular steel engine mounts. a spring and cable. lower panel is restrained in the "open" position by top panel is held open by a longer rod, and the The side panels are held open by short rods; the but can be removed by simply pulling on a ring. FiGURe 1-26. Cowling for horizontally oppoeed engine, 15 tion by over-center steel latchen, which are secured All four panels are locked in the "closed" posi
FIGURE 1-27. "Orange-peel" cowling opened.
in the closed position by spring loaded safety catches. Cowl panels are generally of aluminum alloy construction; however, stainless steel is generally used as the inner skin aft of the power section, for cowl flaps and near the cowl flap openings, and for oil cooler ducts.
On turbojet engine installations, cowl panels are designed to provide a smooth airfiow over the engines and to protect the engine from damage. The entire engine cowling system includes a nose cowl, upper and lower hinged removable cowl panels, and fixed cowl panel. Typical upper and lower hinged removable panels are shown in figure 1-28.
EMPENNAGE The empennage is also called the tail section and most aircraft designs consist of a tail cone, fixed surfaces, and movable surfaces.
The tail cone serves to close and streamline the aft end of most fuselages. The cone is made up of structural members (figure 1-29) like those of the fuselage; however, cones are usually of lighter construction since they receive less stress than the fuselage.
Other components of the typical empennage are of heavier construction than the tail cone. These members include fixed surfaces that help steady the aircraft and movable surfaces that help to direct an aircraft's flight. The fixed surfaces are the horizontal and vertical stabilizers. The movable surfaces are usually a rudder and elevators.
Figure 1-30 shows how the vertical surfaces are braced, using spars, ribs, stringers, and skin in a similar manner to the systems used in a wing.
Stress in an empennage is also carried like stress 16
Hold open rod Hold open rod Forward snubber
- Aft snubber FreURE 1-28. Side-mounted turbojet engine cowling.
in a wing. Bending, torsion, and shear, created by airioads, pass from one structural member to another. Each member absorbs some of the stress and Vertical stabilizer passes the remainder to other members. The overload of stress eventually reaches the spars, which transmit it to the fuselage structure.
‚Frame Longeron • Skin Stringer - Skin Rib Bulkhead Spar Rudder Stringer FICURE 1-30. Construction features of rudder and FIcURE 1-29. The fuselage terminates in a tail cone. vertical stabilizer.
FLIGHT CONTROL SURFACES The directional control of a fixed-wing aircraft takes place around the lateral, longitudinal, and vertical axes by means of flight control surfaces.
These control devices are hinged or movable surfaces through which the attitude of an aircraft is controlled during takeoff, flight, and landing. They are usually divided into two major groups, the primary or main, and the auxiliary control surfaces.
The primary group of flight control surfaces consists of ailerons, elevators, and rudders. Ailerons are attached to the trailing edge of both wings of an aircraft. Elevators are attached to the trailing edge of the horizontal stabilizer. The rudder is hinged to the trailing edge of the vertical stabilizer.
Primary control surfaces are similar in construction and vary only in size, shape, and methods of attachment, In construction, control surfaces are similar to the all-metal wing. They are usually made of an aluminum alloy structure built around a single spar member or torque tube. Ribs are fitted to the spar at the leading and trailing edges and are joined together with a metal strip. The ribe, in many cases, are formed from flat sheet stock. They are seldom solid; more often, the formed, stampedout ribs are reduced in weight by holes which are punched in the metal.
The control surfaces of some aircraft are fabric covered. However, all turbojet powered aircraft have metal-covered surfaces for additional strength.
The control surfaces previously described can be considered conventional, but on some aircraft, a control surface may serve a dual purpose. For example, one set of control surfaces, the elevons, combines the functions of both ailerons and elevators.
Flaperons are ailerons which can also act as flaps.
A movable horizontal tail section is a control surface which supplies the action of both the horizontal stabilizer and the elevators.
The secondary or auxiliary group of control surfaces consists of such members as trim tabs, balance tabs, servo tabs, flaps, spoilers, and leading edge devices. Their purpose is to reduce the force required to actuate the primary controls, to trim and balance the aircraft in flight, to reduce landing speed or shorten the length of the landing roll, and to change the speed of the aircraft in flight. They are usually attached to, or recessed in, the main control surfaces.
Ailerons Ailerons are primary control surfaces which make up part of the total wing area. They are movable through a pre-designed are and are usually hinged to the aileron spar or rear wing spar. The ailerons are operated by a lateral (side-to-side) movement of the aircraft control stick, or a turning motion of the wheel on the yoke.
In a conventional configuration, one aileron is hinged to the outboard trailing edge of each wing.
Figure 1-31 shows the shape and location of typical small-aircraft ailerons on various wing-tip designs.
FICURE 1-31. Aileron location on various wing-tip designs.
The ailerons are interconnected in the control system to operate simultaneously in opposite directions. As one aileron moves downward to increase lift on its side of the fuselage, the aileron on the opposite side of the fusclage moves upward to decrease lift on its side. This opposing action results in more lift being produced by the wing on one side of the fuselage than on the other, resulting in a controlled movement or roll due to unequal aerodynamic forces on the winge.
An end view of a typical metal rib in an aileron is shown in figure 1-32. The hinge point of this type of aileron is behind the leading edge of the aileron to provide a more sensitive response to control movements. The horns attached to the aileron spar are levers to which the aileron control cables are secured.
Aileron hinge-pin fitting
- Actuating hom Spar - Lightening hole FIGURE 1-32. End view of aileron rib.
Large aircraft may use all-metal ailerons, except wing spar in at least four places. Figure 1-33 shows for fiber glass trailing edges, hinged to the rear several examples of aileron installation.
FICURE 1-33. Aileron hinge locations.
All the control surfaces of a large turbojet aircraft are shown in figure 1-34. As illustrated, each wing has two ailerons, one in the conventional position at the outboard trailing edge of the wing and another hinged to the trailing edge of the wing center section.
The complex lateral control system in large turbojet aircraft is far more sophisticated than the type employed in a light airplane. During low-speed flight all lateral control surfaces operate to provide maximum stability. This includes all four ailerons, flaps, and spoilers. At high speeds, flaps are retracted and the outboard ailerons are locked out of the aileron control system.
The major part of the skin area of the inboard ailerons is aluminum honeycomb panels. Exposed honeycomb edges are covered with sealant and protective finish. The aileron nose tapers and extends forward of the aileron hinge line. Each inboard aileron is positioned between the inboard and out- 19
Rudder Rudder control tab Outboard aileron Stabilizer actuated elevator tab Elevator control tab
- Balance tab Elevator
• Outboard spoilers
- Outboard flap Inboard aileron Stabilizer 20
- Fillet flap
- Inboard flap
- Inboard spoilers Control tab Leading edge flaps Vortex generators FIGURE 1-34. Control surfaces on a large turbojet aircraft.
board flaps at the trailing edge of the wing. The aileron hinge supports extend aft and are attached to aileron hinge bearings to support the aileron.
The outboard ailerons are made up of a nose spar and ribs covered with aluminum honeycomb panels. A continuous hinge attached to the forward edge of the nose is grooved to mate with the hem of a fabric seal.
The outboard ailerons are located in the trailing edge of each outboard wing section. Hinge supports extend aft from the wing and are attached to the aileron hinge bearing to support the aileron. The nose of the aileron extends into a balance chamber in the wing and is attached to balance panels.
Aileron balance panels (figure 1-35) reduce the force necessary to position and hold the ailerons.
The balance panels may be made of aluminum honeycomb skin bonded to an aluminum frame, or of Hinge Balance panel aluminum skin-covered assemblies with hat-section stiffeners. Clearance between the aileron nose and wing structure provides a controlled airflow area necessary for balance panel action. Seals attached to the panels control air leakage.
Air loads on the balance panels (figure 1-35) depend on aileron position. When the ailerons are moved during flight to either side of the streamline position, differential pressure is created across the balance panels. This differential pressure acts on the balance panels in a direction that assists aileron movement. Full balance panel force is not required for small angles of aileron displacement because the manual force necessary to rotate the control tab through small angles is slight. A controlled air bleed is progressively decreased as the aileron displacement angle is increased. This action increases the differential air pressure on the balance panels as Control tab Vent gap
AILERON WING -Aileron hinge point FIGURE 1-35. Aileron balance panel.
the ailerons rotate from the streamline position. The increasing load on the balance panel counteracts the increasing load on the ailerons.
Auxiliary Wing Flight Surfaces The ailerons are the primary wing flight surfaces.
Auxiliary wing flight surfaces include trailing edge flaps, leading edge flaps, speed brakes, spoilers, and leading edge slats. The number and type of auxiliary wing flap surfaces on an aircraft vary widely, depending on the type and size of aircraft.
Wing flaps are used to give the aircraft extra lift.
They reduce the landing speed, thereby shortening the length of the landing rollout to facilitate landing in small or obstructed areas by permitting the gliding angle to be increased without greatly increasing the approach speed. In addition, the use of flaps during takeoff reduces the length of the takeoff run.
Most flaps are hinged to the lower trailing edges of the wings, inboard of the ailerons. Leading edge flaps are also used, principally on large high-speed aircraft. When they are in the "up" (or retracted) position, they fair in with the wings and serve as part of the wing trailing edge. When in the "down" (or extended) position, the flaps pivot on the hinge points and drop to about a 45° or 50° angle with the wing chord line. This increases the wing camber and changes the airflow, providing greater lift.
Some common types of flaps are shown in figure
1-36. The plain flap (figure 1-36A) forms the trailing edge of the wing when the flap is in the up (or 21
- Retracted Fore flap A. Plain flap (down) Mid flap Aft flap FIcURE 1-37. Triple-slotted trailing edge flaps.
B. Split flap (down) flap area. The resulting slots between flaps prevents separation of the airflow over the flap area.
The leading edge flap (figure 1-38) is similar in operation to the plain flap; that is, it is hinged on the bottom side, and, when actuated, the leading edge of the wing extends in a downward direction C. Fowler flap (down) to increase the camber of the wing. Leading edge flaps are used in conjunction with other types of FICURE 1-36. Wing flaps. flaps.
retracted) position. It contains both the upper and lower surface of the wing trailing edge.
The plain split flap (figure 1-36B) is normally housed flush with the undersurface of the wing. It is similar to a plain flap except that the upper surface of the wing extends to the flap trailing edge and does not droop with the flap. This flap is also called the split-edge flap. It is usually just a braced, flat metal plate hinged at several points along its leading edge.
Aircraft requiring extra wing area to aid lift often use Fowler faps (figure 1-36C). This system houses the flaps flush under the wings much as does the plain split flap system. But, instead of the flaps hinging straight down from a stationary hinge line, worm-gear drives move the flaps leading edge rearward as the flaps droop. This action provides normal flap effect, and, at the same time, wing area is increased when the flaps are extended.
An example of a triple-slotted segmented flap used on some large turbine aircraft is shown in figure 1-37. This type of trailing edge flap system provides high lift for both takeoff and landing.
Each flap consists of a foreflap, a mid-flap, and an aft-flap. The chord length of each flap expands as the flap is extended, providing greatly increased FIGURE 1-38. Cross section of a leading edge flap.
Figure 1-34 shows the location of the leading edge flaps on a large multi-engine turbine aircraft.
Three Kruger-type flaps are installed on each wing.
The flaps are machined magnesium castings with integral ribs and stiffeners. The magnesium casting of each flap is the principal structural component and consists of a straight section with a hollow core called the torque tube extending from the straight section at the forward end.
Each leading edge flap has three gooseneck hinges attached to fittings in the fixed wing leading edge, and a hinged fairing is installed on the trailing edge of each flap. Figure 1-39 shows a typical leading edge flap in retracted position, with an outline of the extended position.
Speed brakes, sometimes called dive flaps or dive brakes, serve to slow an aircraft in flight. These brakes are used when descending at a steep angle or when approaching the runway for a landing. The brakes themselves are manufactured in many 22
Hinge point Actuator
• Flap extended Flap retracted
- Retractable nose FICURE 1-39. Leading edge flap.
shapes, and their location depends on the design of the aircraft and the purpose of the brakes.
The brake panels may be located on certain parts of the fuselage or on the wing surfaces. Brakes on the fuselage are small panels that can be extended into the smooth airflow to create turbulence and drag. Wing-type brakes may be multiple-finger channels extending above and below the wing surfaces to break up smooth airflow. Usually speed brakes are controlled by electrical switches and actuated by hydraulic pressure.
Another type of air brake is a combination of spoilers and speed brakes. A typical combination consists of spoiler flaps located in the upper wing surfaces ahead of the ailerons. When the operator wishes to use both air brakes and spoilers, he can slow the flight speed and maintain lateral control as wel.
Spoilers are auxiliary wing flight control surfaces, mounted on the upper surface of each wing, which operate in conjunction with the ailerons to provide lateral control.
Most spoiler systems can also be extended symmetrically to serve a secondary function as speed brakes. Other systems are equipped with separate ground and fight spoilers. Most spoiler panels are bonded honeycomb structures with aluminum skin.
They are attached to the wing structure by machined hinge fittings which are bonded into the spoiler panel.
Tabs One of the simplest yet most important devices to aid the pilot of an aircraft is the tab attached to a control surface. Although a tab does not take the place of a control surface, it is mounted on or attached to a movable control surface and causes easier movement or better balance of the control surface.
All aircraft, except a few of the very lightest types, are equipped with tabs that can be controlled from the cockpit. Tabs on some of these aircraft are usually adjustable only when the aircraft is on the ground. Figure 1-40 shows the location of a typical rudder tab.
Hinge line
• Rudder trailing Vertical edge stabilizer
• Tab Fuselage Aircraft movement FIGURE 1-40. Typical location of rudder control tab.
LANDING GEAR The landing gear is the assembly that supports the aircraft during landing or while it is resting or moving about on the ground. The landing gear has shock struts to absorb the shock of landing and taxiing. By means of a gear-retraction mechanism, the landing gear attached to the aircraft structure and enables the gear to extend and retract. The landing gear arrangement either has a tailwheel or a nosewheel. Landing gear arrangements having a nosewheel are usually equipped for nosewheel steering. Nosewheel aircraft are protected at the fuselage tail section with a tail skid or bumper. By means of wheels and tires (or skia), the landing gear forms a stabilizing contact with the ground during landing and taxiing. Brakes installed in the wheels enable the aircraft to be slowed or stopped during movement on the ground.
SKIN AND FAIRING The smooth outer cover of the aircraft is referred to as skin. The skin covers the fuselage, wings, empennage, nacelles, and pods. The material used for the skin covering is usually sheet aluminum alloy, treated so that it will not corrode. Magnesium and stainless steel may also be used to a limited extent. The thickness of the skin materials covering a structural unit may differ, depending on the load and streases imposed within and throughout the structure. To smooth out the airflow over the angles formed by the wings and other structural units with the fuselage, shaped and rounded panels or metal skin are attached. This paneling or skin is called fairing. Fairing is sometimes referred to as a fillet.
Some fairing is removable to provide access to aircraft components, whereas other fairing is riveted to the aircraft structure.
ACCESS AND INSPECTION DOORS Access doors permit normal or emergency entrance into or exit from the aircraft. Also, they provide access to servicing points and manually operated drains. Inspection doors provide access to a particular part of the aircraft being inspected or maintained. Access or inspection doors are either hinged or removable. They are fastened in the closed position with catch and locking mechanisms, screws, quick-release devices, or cowling type fasteners. Access and inspection doors that are removable often have a stenciled identification number that is identical to a number stenciled near the opening that they cover. Other access and inspection doors have a stenciled nomenclature to identify the opening that they cover.
HELICOPTER STRUCTURES Like the fuselages in fixed-wing aircraft, helicopter fuselages may be welded truss or some form of monocoque construction. Although their fuselage configurations may vary a great deal, most helicopter fuselages employ structural members similar to those used in fixed-wing aircraft. For example, most helicopters have such vertical/widthwise braces as bulkheads, formers, rings, and frames. They are also provided with such lengthwise braces as stringers and longerons. In addition, the gussets, joiners, and skin hold the other structural members together.
The basic body and tail boom sections of a typical helicopter are of conventional, all-metal, riveted structures incorporating formed aluminum alloy bulkheads, beams, channels, and stiffeners. Stressed skin panels may be either smooth or beaded. The firewall and engine deck are usually stainless steel.
The tail boom is normally of semimonocoque construction, made up of formed aluminum bulkheads, extruded longerons, and skin panels or of welded tubular steel.
The major structural components of one type of helicopter are shown in figure 1-41. The members of a helicopter's tail group vary widely, depending on the individual type and design. In this case, a stabilizer is mounted on a pylon to make up the group. In other cases, the stabilizer may be mounted on the helicopter tail cone or fuselage. In either case, both the pylon and stabilizer usually contain aluminum alloy structural members covered with magnesium alloy skin. The types of structural members used, however, usually vary. A pylon usually has bulkheads, formers, frames, stringers, and beams, making it somewhat of a blend of aircraft wing and fuselage structural members. The stabilizer is usually built more like an aircraft wing, with ribs and spars.
In a typical helicopter, the tail, body, and tail boom are constructed of all-metal stressed skin and metal reinforcing members. The helicopter cabin is normally a plexiglass enclosure which is supported by aluminum tubing in some models.
A large single-rotor helicopter is shown in figure
1-42. It is all-metal and is basically composed of two major sections, the cabin and the tail cone. The cabin section is further divided into passenger or cargo compartments, which provide space for the crew, passengers, cargo, fuel and oil tanks, controls, and powerplant. In multi-engine helicopters, the powerplants are usually mounted in separate engine nacelles.
As shown in figure 1-42, the aft section of a typical single-rotor helicopter consists of the tail cone, the fin, the tail-cone housing, the tail-rotor pylon, and the tail-end fairing. The tail cone is bolted to the rear of the forward section and supports the tail rotor, tail-rotor drive shafts, stabilizers, tail-cone housing, and tail-rotor pylon. The tail cone is of magnesium alloy and aluminum alloy construction. The tail-cone housing is bolted to the aft end of the tail cone. Trim stabilizers extend out 24