6.4 Civil Aircraft Fuselage: Typical Shaping and Layout |
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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.
6.4.3 Worked-Out Example: Civil Aircraft Fuselage Layout
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 |
× 100 + 300 = 1,100 kg |
|
Low Comfort Level: |
10 |
× 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 |
× 100 + 300 = 1,500 kg |
|
Low Comfort Level: |
14 |
× 90 (averaged) + 240 = 1,500 kg |
|
Range: |
2,000 miles + reserve |
||
Shorter Variant (4 to 6 passengers) |
|||
Payload: |
600 kg |
||
High Comfort Level: |
4 |
× 100 + 200 = 600 kg |
|
Low Comfort Level: |
6 |
× 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
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Configuring Aircraft |
Figure 6.8. Fuselage lengths of the three variants
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
6.4 Civil Aircraft Fuselage: Typical Shaping and Layout |
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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 twoabreast 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 × 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 × 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 × 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
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Configuring Aircraft |
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.
6.5 Configuring a Civil Aircraft Wing: Positioning and Layout
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 CLmax as well as a high lift-curve slope (dCL/dα), 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.