A continuous gusset stiffens cap strips in the plane of the rib. This aids in preventing buckling and helps to obtain better rib/skin glue joints where nail-gluing is used because such a rib can resist the driving force of nails better than the other types. Continuous gussets are more easily handled than the many small separate gussets otherwise required.
Figure 1-18 shows the basic rib and spar structure of a wooden wing frame, together with some of the other wing structural members. In addition to the front and rear spars, an aileron spar, or false spar, is shown in figure 1-18. This type of spar extends only part of the spanwise length of the wing and provides a hinge attachment point for the aileron.
Various types of ribs are also illustrated in figure
1-18. In addition to the wing rib, sometimes called "plain rib" or even "main rib," nose ribs and the butt rib are shown. A nose rib is also called a false rib, since it usually extends from the wing leading edge to the front spar or slightly beyond. The nose ribs give the wing leading edge area the necessary curvature and support. The wing rib, or plain rib, extends from the leading edge of the wing to the rear spar and in some cases to the trailing edge of the wing. The wing butt rib is normally the heavily stressed rib section at the inboard end of the wing near the attachment point to the fuselage. Depending on its location and method of attachment, a butt rib may be called a bulkhead rib or a compression rib, if it is designed to receive compression loads that tend to force the wing spars together.
Since the ribs are laterally weak, they are strengthened in some wings by tapes that are woven above and below rib sections to prevent sidewise bending of the ribs.
Drag and antidrag wires (figure 1-18) are crisscrossed between the spars to form a truss to resist forces acting on the wing in the direction of the wing chord. These tension wires are also referred to as tie rods. The wire designed to resist the backward forces is called a drag wire; the antidrag wire resists the forward forces in the chord direction.
The wing attachment fittings, shown in figure 1-18, provide a means of attaching the wing to the aircraft fuselage.
The wing tip is often a removable unit, bolted to the outboard end of the wing panel. One reason for this is the vulnerability of the wing tips to damage, especially during ground handling and taxiing.
Figure 1-19 shows a removable wing tip for a large aircraft wing. The wing-tip assembly is of aluminum alloy construction. The wing-tip cap is secured to the tip with countersunk screws and is Wing tip Leading edge strip Nose rib or false rib Antidrag wire Front spar or tie rod Drag wire or tie rod False spar or aileron spar Aileron Aileron Hear spar Wing rib or plain rib or compression rib (or bulkhead rib Wing butt rib hinge Wing attach fittings FIGURE 1-18. Basic rib and spar structure, 11
secured to the interspar structure at four points with ¼-in. bolts. The tip leading edge contains the heat anti-icing duct. Wing-heated air is exhausted through a louver on the top surface of the tip. Wing position lights are located at the center of the tip Points of attachment to front and rear spar fittings (2 upper, 2 lower) Louver and are not directly visible from the cockpit. As an indication that the wing tip light is operating, some wing tips are equipped with a lucite rod to transmit the light to the leading edge.
Figure 1-20 shows a cross sectional view of an Access door Upper skin Wing tip navigation light Leading edge • Anti-icing outer skin Heat duct - Wing cap exhaust air outlet Reflector rod Corrugated inner skin FICURE 1-19. Removable wing tip.
FIGURE 1-20. All-metal wing with chemically milled channels.
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