# Cross-Section Design

It is often reasonable to start the fuselage layout with a specification of the cross-section: its shape and dimensions.

### Cross-Section Shape

Most fuselage cross-sections are relatively circular in shape. This is done for two reasons
1. By eliminating corners, the flow will not separate at moderate angles of attack or sideslip
2. When the fuselage is pressurized, a circular fuselage can resist the loads with tension stresses, rather than the more severe bending loads that arise on non-circular shapes.

Many fuselages are not circular, however. Aircraft with unpressurized cabins often incorporate non-circular, even rectangular cabins in some cases, as dictated by cost constraints or volumetric efficiency.

Sometimes substantial amounts of space would be wasted with a circular fuselage when specific arrangements of passenger seats and cargo containers must be accommodated. In such cases, elliptical or double-bubble arrangements can used. The double-bubble geometry uses intersecting circles, tied together by the fuselage floor, to achieve an efficient structure with less wasted space.

### Fuselage Diameter

The dimensions are set so that passengers and standard cargo containers may be accommodated.
Typical dimensions for passenger aircraft seats are shown by way of the several examples below.

In addition, space must be available for cargo: either revenue cargo or lugggage. Typical cargo weighs 10 lb/ft^3 while luggage averages 12.5 lb/ft^3 (Torenbeek). Passengers are generally allotted 35 to 40 lbs for bags. This means about 4 ft^3 per passenger for baggage. Most large airplanes have much more room than this, thus allowing space for revenue cargo. 767/ MD-11 / 747 values are more like 12 ft^3 per person, although this is not a requirement. A 757 provides about 10 ft^3 per passenger of bulk cargo volume. Since substantial income is generated by revenue cargo, it is often desirable to allow room for extra cargo. The preferred approach is to accommodate standard size containers, some of which are shown below.

One must provide for a sidewall clearance of about 3/4" to account for shell deflection, seat width tolerances, and seat track location tolerances. Finally, the fuselage frame, stringers, and insulation thickness must be added to determine the fuselage outer diameter. Typically, the outer diameter is about 8% larger than the cabin diameter.

### Busness Jets

The diameter of smaller aircraft such as commuters and business aircraft is dictated by similar considerations, although cargo is not carried below the floor and the cabin height is much more a market-driven decision.

The interiors of business aircraft are laid out more flexibly than are commercial transports. Interior appointments often cost millions of dollars and can be very luxurious, especially for the larger long range aircraft such as the Gulfstream V or Global Express. Business aircraft based on commercial transports such as the Boeing Business Jet provide even greater possibilities.

### Very Large Aircraft

Recent interest in very large aircraft suggests that additional creative possibilities exist for the aircraft interior. The figure below illustrates some concepts for large aircraft fuselage cross sections as described by Douglas Aircraft in 1966.

More recently, aircraft such as the A380 have been designed with interesting interior possibilities. The figures below show some of the options that were considered in the early design process.

The cross section of the A380 departs from the double-bubble concept with a rather eccentric ellipse as shown in the cross sections below.

The table available here gives the external cross-section dimensions and seating layouts for a number of aircraft. Use the interactive layout computation in exercise 2 to check your hand layout.