CIVIL JET AIRCRAFT DESIGN

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Case Studies
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A good example of the use of the data (A, B and C) in aircraft design is given in the case study described in Chapter 16 of the book. This study is concerned with the design of a small regional jet to replace ageing aircraft currently used by airlines. Although these old aircraft are relatively cheap to buy (or lease) they are expensive to operate due to the old technologies used in their original manufacture. The study was undertaken to investigate the feasibility of designing a 70 seat aircraft incorporating advanced technology in airframe and engine designs.

The description below shows how the study used data from this Website to progress the design. The table and figure numbers refer to Chapter 16 of the book.

Searching Data A gives the current/existing aircraft type in the 70 to 100 seat size (Table 16.1) that are currently used for regional services. Published airline schedules were studied to provide a list of city airport pairs typical of such services. Data C was used to determine the stage length (great circle distance) between each city pair (Table 16.2). These distances were compared to the aircraft design ranges for the specimen aircraft types (Table 16.1). Data A also provided data on cruise speed and cruise altitude (Table 16.3) and field performance (Table 16.4) for the ten specimen aircraft. This data was used to fix the initial operational specification for the aircraft. To check the validity of the choice of field length, all UK, French and German airfields in Data C were analysed to provide a cumulative frequency curve (Figure 16.1) for the available runway lengths. The chosen aircraft field length shows that over 70% of these airfields will be suitable. To assist with technical (mass, aerodynamics and performance) estimates for the case study, Data A was used to produce graphs for empty mass ratio against aircraft maximum take-off mass (Figure 16.2) and lift to drag ratio against wing loading (Figure 16.3) for the specimen aircraft. Aircraft layout and geometrical decisions were supported by comparing wing layout, fuselage geometry (seat layout and overall diameter) (Table 16.6) and tail sizes (area ratios and tail volume coefficients) (Table 16.7 for horizontal tail and Table 16.8 for vertical tail) from Data A. Engine details for typical engines used on the specimen aircraft was found from Data B and typical engine performance charts and scaling rules found from Chapter 9 of the book.

The case study shows how the initial decisions on aircraft configuration were modified by parametric analysis to produce an ‘optimum’ layout. The final design is shown below.

Fig. 16.9

The ‘stubby’ appearance of the fuselage on this layout results from the provision of a more comfortable fuselage/cabin layout than is usual for this class of aircraft. This configuration will allow a fuselage stretch to 100 seats for future development of the aircraft type. The relatively small wing planform is a consequence of the efficient aerodynamic design and the high strength materials used. The concluding part of Chapter 16 describes the final design in detail.

 

Suggested Applications

These are some suggested applications for the book.

Example 1 (Flaps)

In selecting the type of flap and its geometry for a projected aircraft it is useful to understand what previous/existing aircraft have used and achieved. Data A can be interrogated to show the type of flap used on specimen aircraft. A graph showing values of aircraft maximum lift coefficient against wing sweepback angle is shown in Chapter 6 (Figure 6.11, page 118), and further details are given in Chapter 8 (pages 167-9).

Example 2 (Mass estimation)

To determine the mass components for the initial estimation of aircraft maximum take-off mass (MTOM) it is necessary to assume a value for the aircraft empty mass fraction. To assist in this process it is helpful to plot this ratio against MTOM using data of specimen aircraft taken from Data A. Such a plot is shown in Chapter 7 (Figure 7.3, page 130).

Example 3 (Fuel mass)

The equation used to predict the fuel mass ratio for a specified range of duration requires a knowledge of the engine specific fuel consumption (sfc) and aircraft lift to drop ratio (L/D) as described on page 131. Data B shows published volumes of sfc for typical engines and this can be related to the cruise conditions (speed and height) using the generalised engine performance charts in Chapter 9 (pages 203-213). The aircraft L/D ratio can be determined for specimen aircraft from Data A.

Example 4 (Engine Size)

To size the engine for a particular project aircraft it is useful to use the engine data (from Data B) for specimen engines of the same operating/design conditions (e.g. by-pass ratio). This can then be scaled to determine geometry and performance for your aircraft using the charts and rules given in Chapter 9 (page 201 onwards).

Example 5 (Unit Conversions)

Although most technical analysis on a project will be done using SI units it is necessary to use Data D and E to convert values to units that are commonly used and understood in industry, or to transfer published data into SI units for use in your design work. An example of such complications is given on page 318 of the book. Here it is necessary to convert aviation fuel density and volume from SI units to US gallons to predict the cost of fuel used in the flight (e.g. US$ per hour).