Figure 6.5. Narrow-body, single-aisle fuselage layout (not to scale)

configuration. Three-engine designs (e.g., B727, DC10, and Lockheed Tristar) are no longer pursued except for a few designs. An underwing-mounted nacelle should remain clear of the ground in the event of a nose-wheel collapse. A minimum of 30 deg of separation (see Chapter 9) is necessary to avoid wheel-spray ingestion.

Nacelles should have their thrust lines positioned close to the aircraft CG to minimize associated pitching moments. In general, the nacelle aft end is slightly inclined (i.e., 1 to 1.5 deg) downward, which also assists in takeoff. Because of the lack of ground clearance for smaller aircraft, engines are mounted on the fuselage aft end, forcing the H-tail to be placed higher. Aft-mounted engines are less desirable than wing-mounted engines. Therefore, when aircraft size and wing position allows, engines take the natural position mounted on the wing, generally slung underneath. It is for this reason that the designers of smaller aircraft are currently considering mounting the engine over the wing, as in the Honda small-jet-aircraft design.

6.4 Civil Aircraft Fuselage: Typical Shaping and Layout

Passenger-capacity and seating-arrangement requirements dictate the layout, which is generally limited to the constant cross-section midpart of the fuselage. Options for various types of fuselage cross-sections are described in Section 4.7.1. Typical geometric and interior details for aircraft with 2- to 10-abreast seating accommodating from 4 to 600 passengers with possible cabin width, fuselage length, and seating arrangement are described in this subsection and shown in Figures 6.5 and 6.6. The figures are from the stabilized statistics of market demand, which varies slightly among cases. The public domain has many statistics for seating and aisle dimensions relative to passenger number, cabin volume, and so forth. The diagrams in this section reflect current trends. Figures 6.5 and 6.6 show the spaces for toilets, galleys, wardrobes, attendant seating, and so forth but are not indicated as such. There are considerable internal dimensional adjustments required for the

Figure 6.6. Wide-body, double-aisle fuselage layout (not to scale)

compromise between comfort and cost. The fuselage fineness ratio is kept from 7 to 14 (in the family of variants; the baseline design can start at around 10). Table 4.2 lists the typical relationship between the number of passengers and the number of abreast seating.

The first task is to determine the abreast seating for passenger capacity. The standard practice for seat dimensions is to cater to the 95th percentile of European men. Section 4.7.6 describes typical seat and aisle dimensions. Elbowroom is needed on both sides of a seat; in the middle seats, it is shared. Typical elbowroom is from 1.5 to 2 inches for economy class and double that for first class. In addition, there is a small space between the window elbowrest and the fuselage wall, larger for more curved, smaller aircraft – typically, about an inch (see Figure 3.50). A wider cabin provides more space for passenger comfort at an additional cost and drag. A longer seat pitch and wider seats offer better comfort, especially for oversized people. Aircraft with a seating capacity of 150 to 200 passengers and as many as 6 abreast with a single aisle is known as a narrow body. With more than six abreast, a two-aisle arrangement is the general practice. Fuselage width is the result of adding the thickness of the fuselage structural shell and soft wall furnishings to the cabin width (see Figure 3.50). During Phase 2 (i.e., the project-definition stage), when sufficient structural details emerge, the interior-cabin geometric dimensions are defined with better resolution; the external geometry remains unaffected. The number of abreast seating and total passenger capacity determine the number of rows. Table 4.5 lists typical dimensions of seat pitch and width.

When the interior arrangement is determined, the constant cross-section midfuselage needs to be closed at the front and aft ends. The midsection fuselage could exhibit closure trends at both the front and aft ends, with diminishing interior arrangements at the extremities. The front-end fuselage mould lines have a favorable pressure gradient and therefore are blunter with large curvatures for rapid

Table 6.1. Fuselage seating dimensions – narrow body (in inches)

Recessed floor.

front-end closure. Basically, a designer must consider the space for the flight crew at the front end and ensure that the pilot’s view polar is adequate. Conversely, the aft end is immersed in an adverse pressure gradient with low energy and a thick boundary layer – therefore, a gradual closure is required to minimize airflow separation (i.e., minimize pressure drag). The aft end also contains the rear pressure-bulkhead structure (see Section 4.7.3 and Figure 4.16 for closure shapes). The longer aft-end space could be used for payload (i.e., cargo) and has the scope to introduce artistic aesthetics without incurring cost and performance penalties.

An important current trend is a higher level of passenger comfort (with the exception of low-cost airlines). Specifications vary among customers. Designers should conduct trade-off studies on cost versus performance in consultation with customers (i.e., operators) to satisfy as many potential buyers as possible and to maximize sales. This is implied at every stage of aircraft component sizing, especially for the fuselage.

Dimensions listed in Tables 6.1 and 6.2 are estimates. The figures of seat pitch, seat width, and aisle width are provided as examples of what exists in the market.

Table 6.2. Fuselage seating dimensions: wide body (in inches)

The dimensions in the tables can vary to a small extent, depending on customer requirements. The seat arrangement is shown by numbers in clusters of seats, as a total for the full row with a dash for the aisle. For example, “3–4–3” indicates that the row has a total of 10 seats, in a cluster of 3 at the 2 window sides of the fuselage and a cluster of 4 in the middle flanked by 2 aisles.

Variants in the family of aircraft are configured by using a constant cross-section fuselage plug in units of one row of pitch. The changes in passenger numbers are discreet increases in the total number of passengers in a row (an example of sixabreast seating is shown in Figure 6.5). An increase in capacity results from adding plugs as required. If more than one, they are distributed in front and aft of the wing. When in odd numbers, their distribution is dictated by the aircraft CG position. In most cases, the front of the wing has the extra row. Conversely, a decrease in passenger numbers is accomplished by removing the fuselage plug using the same logic. For example, a 50-passenger increase of 10-abreast seating has 2 plugs distributed as 3 rows in a subassembly in front of the wing and a subassembly of 2 rows aft of the wing. Conversely, a 50-passenger decrease is accomplished by removing 3 rows from the rear and 2 from the front. For smaller aircraft with smaller reductions, unplugging may have to be entirely from the front of the wing.

Readers are required to work out dimensions using the information provided in the following subsections – intensive coursework begins now. However, readers should be aware that the worked-out examples demonstrate only the proposed methodology. Designers are free to configure aircraft with their own choices, which are likely to be within the ranges defined herein.

6.4.1 Narrow-Body, Single-Aisle Aircraft

Figure 6.5 shows a typical seating arrangement for single-aisle, narrow-body aircraft carrying up to about 220 passengers (all economy class). Section 6.3.1 lists the general considerations regarding doors, fineness ratio, closure angles, seat and aisle dimensions, internal facilities, and so forth for each type.

Table 6.1 provides typical dimensions for establishing narrow-body fuselage widths. All dimensions are in inches. Figure 3.50 defines the symbols used. Additional fuselage interior details follow. Figure 6.5 shows examples of seating arrangements from two to six passengers abreast.

Two abreast (4 to 24 passengers). Two-abreast seating is the lowest arrangement. The passenger comfort level demands relatively large variations in fuselage width. The typical passenger capacity extends from 4 to 19 (e.g., Beech 1900D) and could expand to 24 passengers in an extreme derivative version.

A circular cross-section is ideal to obtain the minimum weight for a pressurized cabin; however, a circular cross-section may not always prove to be best. The aircraft fuselage diameter for two-abreast seating does not provide enough space for passengers to straighten their legs when seated; therefore, a widening of the bottom half could provide more comfort, as shown in Figure 6.7. The fuselage top is semicircular, making headroom clearance a fallout of the design. Cabin height is on the order of 60 inches and most passengers would have to bend down during boarding. A toilet facility is preferred.

Figure 6.7. Example of configuring the fuselage for the medium comfort level (in inches)

Current regulations do not require a cabin crew for up to 19 passengers, but some operators prefer to have one crew member, who uses a folding seat secured in a suitable location. An expanded variant of 2-abreast seating can exceed 19 passengers, but a new high-capacity design should move into 3-abreast seating, described next. The baggage area is at the rear, which is the preferred location in smaller aircraft.

Summary. A typical two-abreast fuselage would have the following features:

Cabin Width: This consists of one seat on each side of the center aisle. To avoid tightness of space in a smaller aircraft, seats could be slightly wider, sacrificing aisle width where there is little traffic. Typically, cabin width is between 64 and 70 inches.

Cross-Section: The fuselage cross-section is typically circular or near circular (i.e., the overall width is greater than the height). Designers must compromise their choices to maximize the sales. The bottom half could be opened up for better legroom. There is no payload space below the floorboards but it could be used for aircraft equipment and fuel storage. Luggage space is located in the aft fuselage.

Front/Aft Closure: See Table 4.4 for the range of dimensions.

Fuselage Length: This depends on the number of passengers and facilities provided (see Figure 6.5). Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of four rows, conveniently distributed on each side of the wing, is possible. The worked-out example baseline version starts with ten passengers (see Figure 6.7).

Three abreast (24 to 50 passengers). A typical 3-abreast seating arrangement accommodates 24 to 45 passengers, but variant designs change that from 20 to 50 passengers (e.g., ERJ145). Full standing headroom is possible; for smaller designs, a floorboard recess may be required (see Figure 4.12). A floorboard recess could trip passengers when they are getting to their seat. Space below the floorboards is still not adequate for accommodating any type of payload. Generally, space for luggage in the fuselage is located in a separate compartment at the rear but in front of the aft pressure bulkhead (the luggage-compartment door is sealed).

At least 1 cabin crew member is required for up to 30 passengers. With more passengers, 2 crew members are required for up to 50 passengers. A new design with potential for growth to more than 50 passengers should start with 4-abreast seating, described next.

Summary. A typical three-abreast fuselage would have the following features:

Front/Aft Closure: See Table 4.4 for the range of dimensions.

Fuselage Length: This depends on the number of passengers and facilities provided (see Figure 6.5). Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of five rows, conveniently distributed on each side of wing, is possible. The baseline version could start with 36 passengers and range from 24 to 50 passengers (Figure 6.5 shows the largest in the family).

Four abreast (44 to 80 passengers). A typical 4-abreast seating arrangement accommodates 44 to 80 passengers, but variant designs could change that number from 40 to 96 passengers (e.g., the Bombardier CRJ1000; the Canadair CL-600 is an executive version that accommodates 19 passengers – another example of a derivative). The cabin crew increases to at least three for higher passenger loads. The increase in the fuselage diameter can provide space below the floorboards for payload, but it is still somewhat limited. To maximize the below-floorboard space, the fuselage height could be slightly oval, with the upper-half semicircular and the bottom half elongated to suit smaller container sizes. Figure 4.12 shows a fourabreast seating arrangement; note the facilities and luggage-compartment arrangement. As the fuselage radius increases, the gap between the elbowrest and the fuselage wall can be reduced to 1 inch (2.54 cm) on each side, increasing the seat width.

Summary. A typical four-abreast fuselage would have the following features:

Cabin Width: A four-abreast arrangement is two seats in a cluster on both sides of a center aisle. Cabin width is from 100 to 106 inches depending on the customer’s demand for the comfort level. The aisle width could be increased to ease cabin-crew access and passenger traffic.

Cross-Section: The fuselage cross-section is typically circular but can be elongated. It follows the cabin-section contour with added wall thickness (see Table 6.1). Full standing headroom is easily achievable. There is aft-fuselage luggage space.

Front/Aft Closure: See Table 4.4 for the range of dimensions.

Fuselage Length: This depends on the number of passengers and facilities provided. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of seven rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 60 passengers and range from 40 to 96 passengers.

Five abreast (80 to 150 passengers). A typical 5-abreast seating arrangement can accommodate 85 to 130 passengers, but variant designs could extend that number somewhat on both sides. The number of cabin crew increases with passenger capacity. There are not many aircraft with five-abreast seating because the increase from four abreast to six abreast better suited market demand. A prominent five-abreast design is the MD-9 series (now the Boeing 717).

The fuselage diameter widens to provide more generous space. Space below the floorboards is conspicuous to accommodate standard containers (see Section 4.7.8).

The fuselage aft closure could affect seating – that is, the last row could be reduced to four abreast. To ease cabin access, the aisle width widens to at least 20 inches plus the armrest at each side. To maximize the below-floor space, the fuselage could be slightly elongated, with the bottom half stretched to accommodate container sizes. A separate cargo space exists at the rear fuselage in the closure area.

Summary. A typical five-abreast fuselage would have the following features:

Front/Aft Closure: See Table 4.4 for the range of dimensions.

Fuselage Length: This depends on the number of passengers and facilities. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of eight rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 100 passengers and range from 85 to 150 passengers.