Closure of the Fuselage
When the seating arrangement is determined in the midfuselage section, it must be closed at the front and aft ends for a streamlined shape, maintaining a fineness ratio from 7 to 14 (see Table 4.3). Typical front – and aft-fuselage closure ratios are in Table 4.4.
The fuselage upsweep angle of the aft-end closure depends on the type of aircraft. If it has a rear-loading ramp as in a cargo version, then the upsweep angle is higher, as shown in Figure 6.4. The fuselage clearance angle, в, depends on the main-wheel position of the undercarriage relative to the aircraft CG position (see
Chapter 7). The typical angle for в is between 12 and 16 deg to approach CLmax at aircraft rotation.
The next step is to construct a fuselage axis and set the zero reference plane normal to the fuselage axis, as explained in Section 3.23. In Figure 6.4, the fuselage axis is shown passing through the tip of the nose cone, where the zero reference plane starts. In this book, the zero reference plane is at the nose of the aircraft (it could be ahead of the nose cone tip). The zero reference plane and the fuselage axis are data for measuring relative distances of various aircraft components and for aerodynamic geometries for use in calculations.
The fuselage axis is an arbitrary line but it must be in the plane of aircraft symmetry. In general, for aircraft with a constant fuselage section, the fuselage axis is placed conveniently in the middle of the aircraft. The fuselage axis line could be the fuselage centerline. It is easier to assess if the reference lines are vertical and horizontal. If the aircraft’s normal position on the ground does not render the aircraft centerline horizontal, then the ground is tilted to show it with the associated angle. For simplification, this book keeps the centerline and ground horizontal, as shown in Figure 6.4. For military and smaller civil aircraft, there is no constant fuselage section, and the aircraft centerline must be conveniently chosen; it is the designer’s choice as long as the reference lines are clearly defined and adhered to for the entire life cycle of an aircraft that could encounter design modifications in its service life. The other possible choice is the fuselage axis as the principal inertia axis.
An interesting concept is to make variants of a modular fuselage – that is, with two types of aft ends easily interchangeable (see Figure 6.4). One type is for the conventional passenger version with a pointed aft-end closure, the other is for the cargo version with an increased upsweep to accommodate a rear-loading ramp. It can even be a “quick-change” version, swapping the type of fuselage needed for the mission; the changeover joint is located behind the main undercarriage.
Attaching the wing to the fuselage could have a local effect on the fuselage external shape. Following are the basic types of attachments:
1. Carry-through wing box. For larger aircraft, this is separately constructed and attached to the fuselage recess. Subsequently, wings are mated at each side in accurate assembly jigs. For smaller aircraft, it could be integral to the wing and then attached to the fuselage recess. In that case, the wing box is built into the wing, either in two halves or as a tip-to-tip assembly. A fairing at the junction reduces the interference drag. These wing boxes are primarily suited to civil aircraft designs. A central wing box is a part of the wing structure that integrates with the fuselage and is positioned high, low, or at a convenient mid-location (see Section 3.16).
2. Central beam and root attachments. These have a simpler construction and therefore are less costly, suited to smaller aircraft.
3. Wing roots (with multispar) joined to a series of fuselage frames. These are mostly suited to military aircraft designs. They are heavier and can be tailored to varying fuselage contours. The wing root is then secured to the fuselage structure, sometimes outside the shell, with attachments.
4. Strut/braced wing support. This is suited to smaller, low-speed, high-wing aircraft. Some low-wing agricultural aircraft have braced wings. Struts add to drag
but for a low-speed operation, the increment can be tolerated when it is less costly to build and lighter in construction.
5. Swing wing. Attachment of a swing wing is conveniently outside the fuselage such that the pivots have space around them to allow wing rotation.
For smaller aircraft, the wing must not pass through the fuselage interior, which would obstruct passenger movement. If the wing is placed outside the fuselage (i. e., top or bottom), then a large streamlined fairing on the fuselage would accommodate the wing box. The example of the Cessna Excel shows a low-wing design; the DO328 includes a fairing for the high-wing design. The Dornier 328 (see Figure 3.33) conceals the fairing that merges with the fuselage mould lines. The extra volume could be beneficial; however, to arrive at such a configuration, a proper DOC analysis must demonstrate its merits. High-wing aircraft must house the undercarriage in a fuselage fairing, although some turboprop aircraft have the undercarriage tucked inside the engine nacelles positioned below the wing (see Figure 10.19).