Fuselage Shape

Planform Layout

Cabin Dimensions

The figure below shows a generic fuselage shape for a transport aircraft. The geometry is often divided into three parts: a tapered nose section in which the crew and various electronic components are housed, a constant section that contains the passenger cabin, and a mildly tapered tail cone.


Note that passengers or other payload may extend over more than just the constant section, especially when the fuselage diameter is large. Because of the long tail cone sections, the pressurized payload section often extends back into this region.


Additional area is required for lavatories, galleys, closets, and flight attendant seats. The number of lavatories depends on the number of passengers, with about 40 passengers per lavatory, a typical value. One must allow at least 34" x 38" for a standard lavatory. Closets take from a minimum 3/4" per passenger in economy class to 2" per first class passenger. Room for food service also depends on the airline operation, but even on 500 mi stage lengths, this can dictate as much as 1.5" of galley cabinet length per passenger. Attendant seats are required adjacent to door exits and may be stowed upright, but clear of exit paths. In addition, emergency exits must include clear aisles that may increase the overall length of the fuselage. The requirements are described in the FAR's.

On average the floor area per person ranges from 6.5 ft^2 for narrow body aircraft to 7.5 ft^2 for wide-bodies in an all-tourist configuration. A typical 3-class arrangement requires about 10 ft^2 per person. The figures below show two layouts for the 717. Note the fuselage nose and tailcone shapes.

Two-Class 717 configuration with 8 first-class seats with 36" pitch and 98 coach seats with 32" pitch.

Single-class 717 configuration with 117 seats at 32" and 31" pitch.

In addition to providing space for seats, galleys, lavatories, and emergency exits as set by regulations, the aircraft layout is important for maintainence and studies are done early in the program to determine that the layout is compatible with required ground services.

Aerodynamics

The fuselage shape must be such that separation and shock waves are avoided when possible. This requires that the nose and tail cone fineness ratios be sufficiently large so that excessive flow accelerations are avoided. Figure 2 shows the limit on nose fineness ratio set by the requirement for low wave drag on the nose.


Even when the Mach number is low, constraints on fuselage pressure gradients limit nose fineness ratios to values above about 1.5. The tail cone taper is chosen based on similar considerations and generally falls in the range of 1.8 to 2.0. The details of fuselage shaping may be determined by looking at the pressure distributions.


Several rules result from these analyses: The transition from nose to constant section, and constant section to tail cone should be smooth - free of discontinuities in slope (kinks). The tail cone slopes should resemble those shown in the examples. That is, the slope must change smoothly and the trailing edge should not be blunt. The closure angle near the aft end should not be too large (half angle less than 14°-20°).

Considerations Related to Fuselage Side-View

The shape of the fuselage in side view is determined based on visibility requirements for the cockpit and ground clearance of the tail cone. Usually aft-fuselage upsweep is required to provide the capability of rotating to high angles of attack on the ground (often about 14°). The upsweep cannot be set without estimating the length of the main gear, but this can be done early in the design process by comparison with similar aircraft.