Table 11.3. Bizjet cruise sizing

Computing and listing in tabular form (use Figure 9.1 for the drag polar):

In SI:

TSLS/ W = 4.5 × 0.5 × 0.289 × 42,662.5 × CD/(W/SW) = 27,741.3 × CD/(W/SW)

11.4.4 Landing

From the market requirements, Vapp = 120 knots = 120 × 1.68781 = 202.5 ft/s (61.72 m/s). Landing CLmax = 2.1 at a 40-deg flap setting (from testing and CFD analysis). For sizing purposes, the engine is set to the idle rating, producing zero thrust using Equation 11.22.

In the FPS system, W/SW = 0.311 × 0.002378 × 2.1× (202.5)2 = 63.8 lb/ft2. In the SI system, W/SW = 0.311 × 1.225 × 2.1 × (61.72)2 = 3,052 N/m2. Because the thrust is zero (i.e., idle rating) at landing, the W/SW remains constant.

Performance. Chapter 13 verifies whether the design meets the aircraft performance specifications.

11.5 Coursework Exercises: Military Aircraft Design (AJT)

This extended section of the book on coursework exercises – military aircraft design (AJT) is found on the Web at www.cambridge.org/Kundu and includes the following subsections.

11.5.1 Takeoff – Military Aircraft

Table 11.4. AJT takeoff sizing

11.5.2 Initial Climb – Military Aircraft

Table 11.5. AJT climb sizing

11.5.3 Cruise – Military Aircraft

Table 11.6. AJT cruise sizing

11.5.4 Landing – Military Aircraft

11.6 Sizing Analysis: Civil Aircraft (Bizjet)

The four sizing relationships (Sections 11.3.1 through 11.3.4) for wing-loading, W/SW, and thrust-loading, TSLS INSTALLED/W, meet (1) takeoff, (2) approach speed

Figure 11.3. Aircraft sizing: civil aircraft

for landing, (3) initial cruise speed, and (4) initial climb rate. These are plotted in Figure 11.3.

The circled point in Figure 11.3 is the most suitable for satisfying all four requirements simultaneously. To ensure performance, there is a tendency to use a slightly higher thrust-loading TSLS INSTALLED/W; in this case, the choice becomes TSLS INSTALLED/W = 0.32 at a wing-loading of W/SW = 63.75 lb/ft2 (2,885 N/m2).

Now is the time for the iterations for the preliminary configuration generated in Chapter 6 from statistics, in which only the fuselage was deterministic. At 20,720 lb (9,400 kg) MTOM, the wing planform area is 325 ft2, close to the original area of 323 ft2; hence, no iteration is required. Otherwise, it is necessary to revisit the empennage sizing and revise the weight estimates. The TSLS INSTALLED per engine then becomes 0.32 × 20,720/2 = 3,315 lbs. At a 7% installation loss at takeoff, this gives uninstalled TSLS = 3,315/0.93 = 3,560 lb/engine (TSLS/W = 3,560 × 2/20,720 = 0.344). This is very close to the TFE731–20 class of engine; therefore, the engine size and weight remain the same. For this reason, iteration is avoided; otherwise, it must be carried out to fine-tune the mass estimation.

The entire sizing exercise could have been conducted well in advance, even before a configuration was settled – if the chief designer’s past experience could “guesstimate” a close drag polar and mass. Statistical data of past designs are useful in guesstimating aircraft close to an existing design. Mass fractions as provided in Section 8.8 offer a rapid mass estimation method. Generating a drag polar requires some experience with extraction from statistical data.

In the industry, more considerations are addressed at this stage – for example, what type of variant design in the basic size can satisfy at least one larger and one smaller capacity (i.e., payload) size. Each design may have to be further varied for more refined variant designs.

11.6.1 Variants in the Family of Aircraft Design

The family concept of aircraft design is discussed in previous chapters and highlighted again at the beginning of this chapter. Maintaining large component commonality (genes) in a family is a definite way to reduce design and manufacturing costs – in other words, “design one and get two or more almost free.” This

Figure 11.4 Variant designs in the family of civil aircraft

encompasses a much larger market area and, hence, increased sales to generate resources for the manufacturer and nation. The amortization is distributed over larger numbers, thereby reducing aircraft costs.

Today, all manufacturers produce a family of derivative variants. The Airbus 320 series has 4 variants and more than 3,000 have been sold. The Boeing 737 family has 6 variants, offered for nearly 4 decades, and nearly 6,000 have been sold. It is obvious that in three decades, aircraft manufacturers have continuously updated later designs with newer technologies. The latest version of the Boeing 737–900 has vastly improved technology compared to the late 1960s 737–100 model. The latest design has a different wing; the resources generated by large sales volumes encourage investing in upgrades – in this case, a significant investment was made in a new wing, advanced cockpit/systems, and better avionics, which has resulted in continuing strong sales in a fiercely competitive market.

The variant concept is market and role driven, keeping pace with technology advancements. Of course, derivatives in the family are not the optimum size (more so in civil aircraft design), but they are a satisfactory size that meets the demands. The unit-cost reduction, as a result of component commonalities, must compromise with the nonoptimum situation of a slight increase in fuel burn. Readers are referred to Figure 16.6, which highlights the aircraft unit-cost contribution to DOC as more than three to four times the cost of fuel, depending on payload-range capability.

The worked-out examples in the next section offer an idea of three variants in the family of aircraft.

11.6.2 Example: Civil Aircraft

Figure 11.4 shows the final configuration of the family of variants; the baseline aircraft is in the middle (see Figure 6.1 for the plug sizes).

Section 6.10 proposes one smaller (i.e., four to six passengers) and one larger (i.e., fourteen to sixteen passengers) variant from the baseline design that carries ten to twelve passengers by subtracting and adding fuselage plugs from the front and aft