FUEL TANKS
Aircraft typically use three types of fuel tanks: integral, rigid removable, and bladder.
• Integral tanks are areas inside the aircraft structure that have been sealed to allow fuel storage. Since these tanks are part of the aircraft structure, they cannot be removed for service or inspection. Inspection panels must be provided to allow internal inspection, repair, and overall servicing of the tank. Most large transport aircraft use this system, storing fuel in the wings and/or tail of the airplane.
• Rigid removable tanks are installed in a compartment designed to accommodate the tank. They are typically of metal construction, and may
be removed for inspection, replacement, or repair. The aircraft does not rely on the tank for structural integrity.
• Bladder tanks are reinforced rubberized bags installed in a section of aircraft structure designed to accommodate the weight of the fuel. The bladder is rolled up and installed into the compartment through the fuel filler neck or access panel, and is secured by means of metal buttons or snaps inside the compartment. Many high-performance light aircraft and some smaller turboprops use bladder tanks.
Pertaining to the initial design carried out to the aircraft, all commercial aircrafts follow the integral type tank for safety and easier access of fuel to the engine.
RIB LOCATION AND DIRECTION
The span-wise location of ribs is of some consequence.
Ideally, the rib spacing should be determined to ensure adequate overall buckling support to the distributed flanges. This requirement may be considered to give a maximum pitch of the ribs. In practice other considerations are likely to determine the actual rib locations such as:
a) Hinge positions for control surfaces and attachment/operating points for flaps, slats, and spoilers.
b) Attachment locations of power plants, stores and landing gear structure.
c) A need to prevent or postpone skin local shear or compression buckling, as opposed to overall buckling.
d) Ends of integral fuel tanks where a closing rib is required. When the wing is upswept, it is usual for the ribs to be arranged in the flight direction and thereby define the aerofoil section.
Ribs placed at right angles to the rear spar are usually he most satisfactory in facilitating hinge pick-ups, but they do cause layout problems in the root regions. There is always the possibility of special exceptions, such as power plant or store mounting ribs, where it may be preferable to locate them in the flight direction.
FIXED SECONDARY STRUCTURE
A fixed leading edge is often stiffened by a large number of closely pitched ribs, span-wise members. Considering design of the skin attachment it is possible to arrange for little span-wise end load to be diffused into the leading edge and buckling of the relatively light structure is avoided.
This may imply short spam-wise sections. The presence of thermal de-icing, high-lift devices or other installations in the leading edge also has a considerable influence upon the detail design. Bird strike considerations are likely to be important.
Installations also affect the trailing edge structure where much depends upon the type of flaps, flap gear, controls and systems. It is always aerodynamically advantageous to keep the upper surfaces as complete and smooth as is possible. Often spoilers can be incorporated in the region above flaps or hinged doors provided for ease of access.
HORIZONTAL STABILISER
When the horizontal stabilizer is constructed as a single component across the centreline of the aircraft, the basic structural requirements are very similar to those of a wing. Here for our aircraft we have, the basic structural requirements are very similar to those of a wing.
VERTICAL STABILISER Conventional tail
In conventional tail the vertical stabilizer is exactly vertical.
The vertical stabilizer is mounted exactly vertically, and the horizontal
stabilizer is directly mounted to the empennage (the rear fuselage). This is the most common vertical stabilizer configuration.
T-tail
A T-tail has the horizontal stabilizer mounted at the top of the vertical stabilizer.
It is commonly seen on rear-engine aircraft
The vertical stabilizer presents a set of issues which are different from those of the main plane or horizontal stabilizer. Relevant matters are:
It is not unusual to build the vertical stabilizer integrally with the rear fuselage.
The spars are extended to form fuselage frames or bulkheads. A ‘root’ rib is made to coincide with the upper surface of the fuselage and is used to transmit the fin root skin shears directly into the fuselage skin. Fin span-wise bending results in fuselage torsion. Sometimes on smaller aircraft the fin is designed as a separate component which may readily be detached. The fin attachment lugs are arranged in both lateral and fore and aft directions so that in addition to vertical loads they react side and drag loads.
There is a special situation when the horizontal stabilizer is attached at some location across the height of the fin. The horizontal stabilizer transmits substantial loads to the fin, usually of the same order of magnitude as the loads on the fin itself. A particular hg loading results from the reaction of horizontal stabilizer asymmetrical lift case, which always adds to fin lateral air-loads
AUXILIARY SURFACES
The structural layout of the auxiliary lifting surfaces is generally similar to that of the wing but there are differences, in part due to the smaller
size and in part due to the need to provide hinges or supports. The latter implies that each auxiliary surface is a well-defined.
HINGED CONTROL SURFACES
Conventional training edge control surfaces are almost invariably supported by a number of discrete hinges, although continuous, piano type, hinges may be used for secondary tabs. To some degree the number and location of the discrete hinges depends upon the length of the control. The major points to be considered are:
a) The bending distortion of the control relative to the fixed surface must be limited so that the nose of the control does mot fouls the fixed shroud.
b) The control hinge loads and the resulting shear forces and bending moments should be equalized as far as is possible.
c) Structural failure of a single hinge should be tolerated unless each hinge is of fail-safe design and can tolerate cracking one load path.
PIVOTED CONTROL SURFACES
In certain high-performance aircraft, the whole of a stabilizing or control surface on one side of the aircraft may be pivot about a point on its root chord. Clearly in this case, the structural considerations are dominated by the need to react all the forces and moments at the pivot and operating points.
Some designs incorporate the pivot into the moving surface with the support bearings on the fuselage, while on others the pivot is attached to the fuselage and the bearings are in the surface. The bearings should be as far apart
as the local geometry allows to minimize loads resulting from the reaction of the surface bending moment.
HIGH LIFT SYSTEMS
There is a wide variety of leading and trailing edge high-lift systems.
Some types are simply hinged to the wing, but many require some degree of chord-wise extension. This can be achieved by utilizing a linkage, a mechanism, a pivot located outside the aerofoil contour or, perhaps most commonly, by some form of track. Trailing edge flaps may consist of two or more separate chord-wise segments, or slats, to give a slotted surface and these often move on tracts attached to the main wing structure.
The structural design of flaps is similar to that of control surfaces but it s simpler as there is no requirement for mass balance, the operating mechanisms normally being irreversible. On large trailing edge flap components, there is often more than one spar member. Especially when this assists in reacting the support or operating loading. There may be a bending stiffness problem in the case of relatively small chord slat segments and full depth honey combs can be used to deal with this.
ATTACHMENT OF LIFTING SURFACES
The joint of the fuselage with the wing is subjected to heavy load inputs and there is a potential for considerable relative distortion. This distortion is usually accepted and the wing centre box is built completely into the fuselage.
It is sometimes possible to arrange the wing pick-ups as pivots on the neutral axis or set them on swinging links. In this case, the relative motion is
allowed to take place and there are no induced stresses. Structural assembly of the wing to the fuselage is relatively simple.
Fins are usually built integrally with the rear fuselage. This is mainly due to the different form of loading associated with the geometric asymmetry.