Aircraft Design Projects - part 4 pot

“chap04” — 2003/3/10 — page 91 — #46

Project study: scheduled long-range business jet 91

21

550

500

450

400 12

10

8

14

20

19

18

17

16

Fig. 4.20 Trade-off study: cruise L/D ratio

46

550

500

450

400

12 8 10 14

44

42

40

38

32

34

36

1000 kg

Fig. 4.21 Trade-off study: stage fuel mass

4.8.5 Economic analysis

The results from the studies above can be used, together with operational data, to

assess the economic viability and sensitivity to the aircraft geometrical changes. The

aircraft price is related to the aircraft empty mass and engine size. The cost of fuel is

proportional to fuel mass. Other operational costs are related to aircraft take-off mass.

Hence, changes to the aircraft configuration will affect both aircraft selling price and

operating costs. For civil aircraft designs, these two cost parameters are often selected

as the principal design drivers (optimising criteria). Although the aircraft configuration

“chap04” — 2003/3/10 — page 92 — #47

92 Aircraft Design Projects

127

123

119

115

111

103

107

1000 kg

8

10

12

14

400

450

500

550

Fig. 4.22 Trade-off study: aircraft max. TO mass

320

300

280

260

240

180

200

220

sq. m

8

10

12

14

400

450

500

550

Fig. 4.23 Trade-off study: wing area

may not be selected at the optimum configuration for these parameters, the design team

will need to know what penalty they are incurring for designs of different configuration.

All of the cost calculations have been normalised to year 2005 dollars by applying

an inflation index based on consumer prices. Several separate cost studies have been

performed as described below.

“chap04” — 2003/3/10 — page 93 — #48

Project study: scheduled long-range business jet 93

64

66

68

70

72

74

76

78

80

82

84

86

88

8

10

12

400 14

450

500

550

$m (2005)

Fig. 4.24 Trade-off study: aircraft price

Aircraft price

Aircraft price is one component in the evaluation of total investment. This includes

the cost of airframe and engine spares. For this aircraft, the total investment is about

12 per cent higher than the aircraft price.

Figure 4.24 shows the variation of aircraft price for the geometrical changes con￾sidered previously. At the design point the price is estimated to be $70.5 m. The carpet

plot shows that this price would fall by about 5 per cent if the aspect ratio was reduced

to 8, and by about 9 per cent if the configuration was moved to point 550/8. The effect of

reducing wing loading progressively increases aircraft price (e.g. reducing wing loading

to 400 kg/sq. m increases the price by 9 per cent). Similarly, increasing wing aspect

ratio increases price (e.g. moving from 10 to 14 increases the price by over 10 per cent).

Without consideration of other operating costs, the main conclusion of this study is to

move the design point to lower values of both wing loading and aspect ratio.

Direct operating cost (DOC) per flight

There are two fundamentally different methods of estimating aircraft DOC. The tradi￾tional method includes the depreciation costs of owning the aircraft. On this aircraft,

this would be about 33 per cent of the total DOC. If the aircraft operator leases the

aircraft, the annual cost of the aircraft is regarded as a capital expenditure. This would

be considered as an indirect aircraft operating cost. In this case, the aircraft standing

charges (depreciation, interest and insurance) are not included in the calculation and

the resulting cost parameter is termed ‘Cash DOC’. It is important to calculate both

of the DOC methods. The results of the DOC calculations are shown in Figures 4.25

and 4.26.

The DOC per flight at the design point (500/10) is $72 740. This figure would be

reduced by 3 per cent if the design was moved to point 550/8 and still satisfy the technical

design requirements. Increasing wing area and/or aspect ratio from the design point

is not seen to be advantageous. At the design point the Cash DOC is estimated to be