Category AIRCRAF DESIGN

Chapter 3 introduces a host of wing-mounted control surfaces (e. g., aileron, flap, slat, spoilers, and trim tabs), none of which are sized in this book; however, geometry from current designs is extracted and their placement should be earmarked. Control surface sizing is accomplished after the wing is sized and is addressed in subsequent design phases.

Flaps and slats are wing components that are selected for field-performance demands to generate high lift. In general, the more demanding aircraft-performance requirements, the more sophisticated are the high-lift devices, which are progres­sively more complex and therefore more expensive (see Section 3.12). Associated incremental lift gains by each type are shown in Figure 3.21. In general, a single – or double-slotted Fowler action flap suffices for the majority of civil transport aircraft. The simpler types are less costly to manufacture and are used in low-speed, low-cost smaller aircraft, usually compensated by the relatively larger wing area.

The aileron span is about a third of the wing span at the extremities. Ailerons and flaps are hinged aft of the rear spar for up and down movements; provision for them should be made during the conceptual design phase. On some designs, flap tracks are used to support the flaps traveling outward to increase lift. A flaperon serves as both a flap and an aileron.

Flaps are positioned behind the wing rear spar (about 60 to 66% of the chord) and typically run straight or piecewise. Flaps take up about two thirds of the inner wing span. It is apparent that designers must have a good knowledge of the inter­nal structural layout to configure an aircraft. Chapter 15 provides information on aircraft structure pertaining to the aircraft-configuration study.

Not all aircraft have wing spoilers; however, aircraft with speed over Mach 0.6 generally have spoilers. These are installed close to the aircraft CG line to mini­mize pitch change. Spoilers also act as air brakes. The differential use of spoilers is for lateral control and they are referred to as spoilerons. This book does not size spoilerons or air brakes but schematically earmarks their position on the wing.

Wing sweep, Л, is a function of aircraft speed to delay transonic effects. For aircraft flying at less than Mach 0.6, a wing sweep is not required. A tapered wing with a zero quarter-chord sweep has some LE sweep; the trailing-edge sweep depends on the taper ratio.

Wing Twist

It is an essential geometrical adjustment to ensure that wing-tip effects do not create adverse conditions. A major requirement is to make the wing root stall earlier to retain aileron effectiveness at a high angle of attack (low speed) – especially during landings. A wing twist with washout would favor such behavior (and is the prevailing practice).

Wing Dihedral/Anhedral

To ensure roll stability (see Section 12.3.3), wing dihedral and anehedral angles are used. Generally, the dihedral is associated with low-wing design and the anhedral with high-wing design; however, there are designs that are the reverse: a high wing can accommodate a dihedral. The type and extent are settled through stability analysis, which is not discussed in this book. All civil aircraft have some dihedral or anhedral angle between 1 and 5 deg. If a high wing and/or a high-wing sweep increases lateral stability more than what is required, the anhedral angle is required to reduce it to the desired level. Some low-wing Russian bombers have a high-wing sweep that necessitates an anhedral angle, when the undercarriage struts must be longer to provide the desired ground clearance.

A civil aircraft designer would seek the maximum possible aspect ratio that a struc­ture would allow. This minimizes induced drag (see Equation 3.13). The V-n dia­gram (see Section 5.7) determines the strength requirement in pitching maneuvers creating maximum stress from the bending moment at the wing root. Civil aircraft do not have high roll rates (unless it is a small aerobatic aircraft). Choice of material and aerofoil t/c ratio contributes to structural integrity. For civil aircraft, a trape­zoidal wing planform (with or without extensions; see Section 3.14) would be the dominant choice. The least expensive to manufacture is a rectangular planform, but there is no cost benefit for highly utilized commercial aircraft to offset drag reduc­tion (i. e., fuel-saving). Rectangular planforms are used in smaller club and sports aircraft with a low level of utilization.

Wing Reference Area

The wing reference area is obtained from the sizing of wing-loading W/S. At this stage, without knowing the aircraft weight, an initial estimate is derived from statis­tical values reflecting successful past designs. Subsequently, the wing will be sized to the requirement (see Chapter 1). Some iteration is required because component weights are revised at the stages of the study. In coursework activity, one iteration is sufficient.

When the aerofoil section has been selected, the next task is to obtain the following information, which would be iterated to the final size through various design phases, as shown in Chart 2.1. Initially, all geometric details are taken from past experience (i. e., the statistical data of the aircraft class), followed by formal sizing, fine-tuned through CFD analyses and wind-tunnel testing, and finally substantiated through flight-testing (modifications are made, if required).

1. Determine the wing planform shape and its reference area. It should max­imize the aspect ratio and optimize the taper ratio. In addition, the wing ensures adequate fuel volume. At this stage, it is considered that the wing struc­tural layout can accommodate fuel capacity and movable control and lifting surfaces.

2. Determine the wing sweep, which is dependent on maximum cruise speed (see Section 3.16).

3. Determine the wing twist; a typical statistical value is 1 to 2 deg, mostly as washout (see Section 3.14).

4. Determine the wing dihedral/anhedral angle; initially, this is from the statistical data (see Section 3.14).

5. Determine high-lift devices and control areas. At first, the type is selected to satisfy the requirements at low cost. The values of its aerodynamic properties initially are taken from statistical data (see Section 3.10).

Section 6.3.2 discusses general considerations for wing design. Given here are suggestions to establish these parameters (see also Section 3.16).

The first task for designing the wing is to select a suitable aerofoil. Aerofoil design is a protracted and complex process that is beyond the scope of this book. After an aerofoil is selected (it could vary spanwise), the next task is to configure a wing planform with reference area. It is not like the fuselage sizing determined by the passenger number; initially, it is from statistics for the aircraft class. At the concep­tual stage of the project study, typical values of wing twist and other refinements are taken from the past experience of a designer. The values must be substantiated and, if required, modified through CFD analysis and wind-tunnel testing to a point when the flight test may require final local refinements (e. g., flap and aileron rigging). Ini­tially, an isolated wing is analyzed to quickly arrive at a suitable geometry and then studied with the fuselage integrated. Subsequently, the wing is sized formally (see Chapter 11).

6.5.1 Aerofoil Selection

Section 3.7 outlines the strategy to search for an aerofoil that would provide a high Cbmax as well as a high lift-curve slope (dCL/da), a high L/D ratio for the prescribed cruise speed, a low pitching moment, and gentle stalling characteristics. While retaining these characteristics, consultation with structural designers should decide an aerofoil t/c ratio that would permit good structural integrity to increase the aspect ratio. This is an area in which designers should gain over the competi­tion with a better aerofoil and material. Finally, for high-subsonic cruise speed, the aerofoil shape should minimize compressibility effects (i. e., wave drag). Typically, a supercritical aerofoil with a relatively flat upper-surface profile (i. e., Whitcomb) reduces the transonic effects. Figure 6.9 shows a typical flat upper-surface pressure distribution at cruise (i. e., supercritical aerofoil). Good aerofoil sections are propri­etary information and mostly are not available in the public domain.

To minimize repeating work that is similar in nature, the chosen aerofoil sec­tion for worked-out examples is kept the same for both civil and military aircraft designs. For a relatively low cruise Mach number of 0.65 at the LRC and 0.74 at the HSC, the NACA 65-410 is chosen for both designs. It is not exactly a super­critical aerofoil but serves the learning process because it is a known aerofoil suc­cessfully applied to many aircraft (Appendix C gives the details of NACA 65-410 aerofoil).

The purpose of the worked-out example is only to substantiate the methodology outlined. Readers can decide their own dimensions of the class of aircraft on which they are working. The available range of dimensions offers several choices; it is unlikely that fuselage sizing would fall outside of the given ranges – if at all, a marginal deviation is possible in an extreme design. Readers need not be confined to this classwork example and may explore freely; simplicity can be an asset.

Example 2 in Section 2.6 is used here to provide an example of configuring a civil aircraft: a Learjet45 class Bizjet that offers variants in a family of designs. Following are the important specifications for the aircraft:

Baseline Version (8 to 10 passengers)

Payload: 1,100 kg

High Comfort Level: 8 x 100 + 300 = 1,100 kg

Low Comfort Level: 10 x 90 (averaged) + 200 = 1,100 kg

Range: 2,000 miles + reserve

Longer Variant (12 to 14 Passengers)

Payload: 1,500 kg

High Comfort Level: 12 x 100 + 300 = 1,500 kg

Low Comfort Level: 14 x 90 (averaged) + 240 =

Range: 2,000 miles + reserve

Shorter Variant (4 to 6 passengers)

Payload: 600 kg

High Comfort Level: 4 x 100 + 200 = 600 kg

Low Comfort Level: 6 x 90 (averaged) + 60 = 600 kg

Range: 2,000 miles + reserve

The fuselage size is determined from the required passenger load. Following the considerations listed in Section 6.3.1, a stepwise approach is suggested.

Step 1: Configure the mid-fuselage width, which mostly consists of the con­stant cross-section.

Decide the number of abreast seating using Table 4.2 and the comfort level (aisle and seat width are made more comfortable at the

Short variant (4 to 6 passengers)

41 71 ft (12 71m)

Baseline arcraftt (8 to 10 passengers)

50 ft (15 24m)

Long vanant(12to 14 passengers)

58 2 ft (17 74m) expense of cost). In this case, it is two-abreast seating. This gives the cabin width and, adding the fuselage thickness, the result is the fuse­lage width. For a pressurized cabin, keeping the cross-section as close as possible to a circular shape is preferred; for an unpressurized cabin, it can approach a rectangular shape.

Step 2: Configure the mid-fuselage length, which consists mostly of the con­stant cross-section.

Determine the number of seat rows by dividing the total passenger capacity by the number of abreast seating. If it is not divisible, then the extreme rows will have seating with fewer abreast. Decide the passen­ger facilities (e. g, toilets, galleys, closets, and cabin-crew seating) and add those dimensions. The extremities of the fuselage midsection can be tapered to begin the fuselage closure. With Step 1, this provides the fuselage midsection size.

Step 3: Configure the front and aft closures.

Section 4.7.3 suggests various fuselage closures; there are many to choose from as observed from past designs in the aircraft class. Although there are benefits from past experience, designers should develop their own configuration based on pilot vision, drag considera­tions, space for storage, rotation for takeoff, and so forth. Following is a worked-out example to configure a baseline aircraft with a midsec­tion fuselage.

The baseline aircraft cabin with medium comfort and a 10 seat layout is shown in Figure 6.8.

As discussed previously, two-abreast seating in the cross-section results in a widening of the bottom half for legroom, shown here in the inclined position for a man in the 95-percentile size. The fuselage width is 173 cm (68.11 inches) and the fuselage height is 178 cm (70 inches). To simplify the computation, an equivalent approximation uses an average circular diameter of the cross-section of 175.5 cm (69.1 inches) (e. g., for estimation of the fuselage wetted area). Standing height
inside the cabin is 152 cm (59.85 inches). The total fuselage shell thickness is 14 cm (5.5 inches), which makes the cabin width 159 cm (62.6 inches). The two – abreast seating arrangement is in accordance with Section 6.4.1, with detailed dimensions adding up to 4.5 + 53S + 44A + 53S + 4.5 = 159 cm (or 1.78 + 21S + 17A + 21S + 1.78 = 62.6 inches), fitting the cabin width exactly.

The medium-comfort seat pitch is 32 inches (81.28 cm). With the toilet facility 1 m (39.65 inches) long, the entry door of 76.2 cm (30 inches), and the interior space of 12.4 cm, the fuselage midsection (i. e., cabin) length totals (5 x 81.28) + 100 +

76.2 + 12.4 = 595 cm (234.3 inches). With the flight-crew cockpit space (1.85 m in length), the total is 7.8 m (303.12 inches), as shown in Figure 6.7. It is suggested that readers compare this with competition aircraft – this design has more room than is typical of the class.

For the longer variant with a higher density, a 14-passenger seat pitch is 30 inches (76.2 cm). The variant cabin dimension is now (7 x 76.2) + 100 + 76.2 +

12.4 = 722 cm (284.25 inches). Adding the cockpit length, the length becomes (7.22 + 1.85) = 9.07 m.

The shorter 6-passenger variant has the scope to retain a seat pitch of 32 inches (81.28 cm). The cabin midsection length becomes (3 x 81.28) + 100 +

76.2 + 12.4 = 432.44 cm (170.25 inches). With the length of the flight-crew cock­pit space added in (1.85 m), it totals 6.37 m (250.8 inches).

The overall fuselage length is reached after adding front and aft closures, as given in Table 4.4. The windscreen shape and size must comply with FAR reg­ulations, as shown in Figure 4.17. This is an opportunity to streamline the fuse­lage, incorporating aesthetics without incurring additional cost and performance degradation. After streamlining, the various ratios are checked out to be within the acceptable range. Choosing a suitable ratio, the following dimensions are estimated:

• The front-fuselage closure length is 11.48 ft (3.5 m), of which 1.85 m is the cock­pit length.

• The front-fuselage closure ratio becomes Lf = 350/175.5 = 1.994 (see Sec­tion 4.7.3).

• The aft-fuselage closure length works out to be 18.54 ft (5.65 m), with the upsweep angle to be checked out later.

• The aft-fuselage closure ratio becomes La = 565/175.5 = 3.22, within the range. Therefore, the baseline version fuselage length, L = Lf + Lm + La = 3.5 + 5.95 + 5.65 = 15.1 m (49.54 ft).

• Fineness ratio = 1,510/175.5 = 8.6.

• Use the same closure lengths for the variants. The longer variant has a fineness ratio = (722 + 350 + 565)/175.5 = 9.33, well within the prescribed range. Here, one fuselage plug of 64 inches in the front and 40 inches in the aft of the wing are added (see Figure 6.2).

• The shorter variant has a fineness ratio = (432.44 + 350 + 565)/175.5 = 7.68, within the prescribed range. Here, one fuselage plug of 64 inches in the front and 30 inches in the aft of the wing are substracted (see Figure 6.2).

The three variants of the family are shown in Figures 6.2 and 6.8 along with the wing positioned nearly at the middle of the fuselage. The rotation clearance is to be checked out after the undercarriage is positioned. This is not a problem because the

Figure 6.9. A CAST 7 (Germany) aero­foil and its characteristics

main undercarriage length can be tailored in conjunction with the longest fuselage; this is the iterative process.

Seven-abreast seating and more would require more than one aisle to facilitate passenger and crew traffic in the cabin. These aircraft are also known as wide­bodied aircraft. Figure 6.6 shows a typical seating arrangement for a double-aisle, wide-body aircraft carrying up to 555 passengers; however, high-density seating of all economy-class passengers can exceed 800 (e. g., A380). These large passenger numbers require special attention to manage comfort, amenities, and movement.

Section 6.3.1 discusses general considerations for each type of aircraft seating (e. g., doors, fineness ratio, closure angles, seat and aisle dimensions, and internal facil­ities). A typical cross-section is circular but can be elongated, as shown in Fig­ure 4.12. A double-deck aircraft has an elongated cross-section.

Table 6.2 provides typical dimensions to establish a wide-body fuselage width. All dimensions are in inches, and decimals are rounded up. Refer to Figure 3.50 for the symbols used. More fuselage-interior details are given in Table 6.2. Designers are free to adjust the dimensions – the values in the table are typical.

Seven abreast (160 to 260passengers). The Boeing 767 appears to be the only air­craft with seven-abreast seating and it can reconfigure to eight-abreast seating. Typi­cal 7-abreast seating accommodates 170 to 250 passengers, but variant designs could change that number on either side. The number of cabin crew increases accord­ingly. The fuselage diameter is wider to provide generous space. Space below the floorboards can accommodate cargo containers (see Section 4.7.8). To maximize the below-floorboard space, the fuselage height could be slightly elongated, with the bottom half suitable for container sizes. A separate cargo space is located at the rear fuselage.

Summary. A typical seven-abreast fuselage (with better comfort) would have the following features:

Cabin Width: Seven-abreast seating is arranged as 2-3-2 in a cluster of two at

the window sides and a cluster of three at the center between the two aisles. Very little gap is required between the armrest and the cabin wall because the fuselage radius is adequate. The cabin width is from 190 to 196 inches, depending on the cus­tomer’s demand for the comfort level. The aisle width could be increased to facilitate cabin-crew access and passenger move­ment.

Cross-Section: The fuselage cross-section is typically circular but can be oval.

It follows the cabin-section contour with added wall thickness (see Table 6.2). Full standing headroom is no longer an issue. There is potential for 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. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of

ten rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 200 passengers and range from 160 to 260 passengers.

Eight abreast (250 to 380 passengers, wide-body aircraft). The Airbus 300/310/ 330/340 series has been configured for eight-abreast seating. Figure 6.6 shows an example of an 8-abreast seating arrangement for a total of 254 passengers (in mixed classes; for all economy-class, 380 passengers in a variant design is possible). Space below the floorboards can accommodate larger containers. Seat width, pitch, and layout with two aisles results in considerable flexibility to cater to a wide range of

customer demands. The cross-section is typically circular, but to maximize below – floor board space, it could be slightly elongated, with the bottom half suitable for cargo container sizes. There is potential for a separate cargo space at the rear fuselage.

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

Cabin Width: Eight-abreast seating is arranged as 2-4-2 in a cluster of two

at the window sides and a cluster of four in the center between the two aisles. Very little gap is required between the armrest and the cabin wall because the fuselage radius is adequate. The cabin width is from 210 to 216 inches, depending on the cus­tomer’s demand for the comfort level. The aisle width is nearly the same as for a wide-bodied layout to facilitate cabin-crew and passenger movement.

Cross-Section: The fuselage cross-section is typically circular but can be oval.

It follows the cabin-section contour with added wall thickness (see Table 6.2). Full standing headroom is adequate. There is potential for 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. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of

eleven rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 300 passen­gers and range from 250 to 380 passengers.

Nine to Ten Abreast (350 to 480passengers, wide-body aircraft). The current ICAO restriction for fuselage length is 80 m. The associated passenger capacity for a single-deck aircraft is possibly the longest currently in production. It appears that only the Boeing 777 has been configured to nine – or ten-abreast seating in a single deck.

Figure 6.6 is an example of a 9-abreast seating layout for a total of 450 pas­sengers. Seat width, pitch, and a layout with two aisles has a similar approach to the earlier seven-abreast seating designs, which embeds considerable flexibility for catering to a wide range of customer demands. Cabin-crew numbers can be as many as twelve. Space below the floorboards can carry larger containers (i. e., LD3). The cross-section is typically circular, but to maximize below-floorboard space, it could be slightly elongated, with the bottom half suitable for container sizes. There is potential for a separate cargo space at the rear fuselage.

Summary. A typical nine – or ten-abreast fuselage seating arrangement would have the following features:

Cabin Width: Nine-abreast seating is arranged as 2-5-2 in a cluster of two at

the window sides and a cluster of five in the center between the two aisles. A 3-3-3 arrangement is also possible but not shown. Very little gap is required between the armrest and the cabin

wall because the fuselage radius is adequate. The cabin width is from 230 to 236 inches, depending on the customer’s demand for the comfort level. The aisle width is nearly the same as for the wide-bodied layout to facilitate cabin-crew access and pas­senger movement.

Cross-Section: The fuselage cross-section is typically circular but can be oval.

It follows the cabin-section contour with added wall thickness (see Table 6.2). Full standing headroom is no longer an issue. There is potential for an 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. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of

eleven rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 400 passen­gers and range from 300 to 480 passengers.

Ten abreast and more (more than 400 to almost 800 passenger capacity, wide-body and double-decked). A more than 450-passenger capacity provides the largest class of aircraft with variants exceeding an 800-passenger capacity. This would invariably become a double-decked configuration to keep fuselage length below the current ICAO restriction of 80 m. Double-decking could be partial (e. g., Boeing 747) or full (e. g., Airbus 380), depending on the passenger capacity; currently, there are only two double-decked aircraft in production.

With a double-decked arrangement, there is significant departure from the rou­tine adopted for a single-decked arrangement. Passenger numbers of such large capacity would raise many issues (e. g., emergency escape compliances servicing and terminal handling), which could prove inadequate compared to current practice. Reference [4] may be consulted for double-decked aircraft design. The double­decked arrangement produces a vertically elongated cross-section. Possible and futuristic double-decked arrangements are shown in Figure 4.12. The number of cabin crew increases accordingly. The space below the floorboards is sufficient to accommodate larger containers (i. e., LD3).

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

Cabin Width: The lower deck of a double-decked aircraft has at most 10

abreast, arranged as 3-4-3 in a cluster of 3 at the window sides and a cluster of 4 in the center between the 2 aisles. Very lit­tle gap is required between the armrest and the cabin wall because the fuselage radius is adequate. The cabin width is from 250 to 260 inches, depending on the customer’s demand for the comfort level. The aisle width is nearly the same as for a wide-bodied layout to facilitate cabin-crew and passen­ger movement.

Cross-Section: A double-decked fuselage cross-section is elongated at this design stage. It follows the cabin-section contour with added wall thickness (see Table 6.2). Full standing headroom is no longer an issue. There is potential for 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.

Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of

10 rows, conveniently distributed on each side of the wing, is possible. Fuselage length is less than 80 m.

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 24passengers). 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.

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:

Cabin Width: This consists of two seats in a cluster and one seat on each side

of the aisle. The aisle width could be increased to ease cabin – crew access. Cabin width is from 82 to 88 inches, depending on the customer’s demand for the comfort level.

Cross-Section: The fuselage cross-section is typically circular but follows the cabin-section contour with added wall thickness. There is no payload space below the floorboards, but it can be used for aircraft equipment and fuel storage.

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 80passengers). 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 four – abreast 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.

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.

See Table 4.4 for the range of dimensions.

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

Cabin Width: Five-abreast is seating arranged as three in a cluster on one side

of the single aisle and two in a cluster on the other side. Very little gap is required between the armrest and the cabin wall because the fuselage radius is adequate. Cabin width is from 122 to 130 inches depending on the customer’s demand for the comfort level. The aisle width could be increased to facilitate passenger and crew 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 potential for 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. 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.

Six abreast (120 to 230 passengers). This class of passenger capacity has the most commercial transport aircraft in operation (more than 8,000), including the Airbus 320 family and the Boeing 737 and 757 families. The Boeing 757-300 has the largest passenger capacity of 230 and the highest fineness ratio of 14.7. There is considerable flexibility in the seating arrangement to accommodate a wide range of customer demands.

Figure 6.5 shows an aircraft family of variant designs to accommodate three different passenger-loading capacities in mixed classes. A typical 6-abreast seat­ing arrangement accommodates 120 to 200 passengers, but variant designs could change that number from 100 to 230 passengers. The number of cabin crew increases accordingly. The fuselage diameter is wider to provide generous space. Space below the floorboards can accommodate standard containers (see Section 4.7.8). To maxi­mize the below-floor space, the fuselage height could be slightly elongated, with the bottom half suitable for container sizes. A separate cargo space is located at the rear fuselage.

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

Cabin Width: Six-abreast seating is arranged as three in a cluster on both

sides of the single center aisle. Very little gap is required between the armrest and the cabin wall because the fuselage radius is adequate. Cabin width is from 138 to 145 inches, depending on the customer’s demand for the comfort level. The aisle width is increased to facilitate passenger and crew 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 adequate. There is potential for 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. Add front and aft closures to the fuselage midsection.

Family Variants: Addition or subtraction of fuselage plugs, to a maximum of

ten rows, conveniently distributed on each side of the wing, is possible. The baseline version could start with 150 passengers and range from 85 to 210 passengers. The Boeing 757 base­line starts with a higher passenger load, enabling the variant to reach 230 passengers.

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

Toilet, galley, wardrobe, attendant seat provided Family variants not shown

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

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.

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 six – abreast 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.

Following are general considerations important for configuring the nacelle (see also Section 6.7):

Civil aircraft designs are invariably externally pod-mounted on either the wing or the aft fuselage (smaller low-wing turbofan engines). The demonstration of high engine reliability enables an ETOPS clearance by the FAA for a two-engine

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.