Initial High-Speed Cruise (Bizjet)
An aircraft at the initial HSC is at Mach 0.74 (i. e., 716.4 ft/s) at a 41,000-ft altitude (p = 0.00055 slug/ft3). The fuel burned to climb is computed (but not shown) as 700 lb. The aircraft weight at initial cruise is 20,000 lb. At Mach 0.74, the aircraft lift coefficient Cl = MTOM/^SV = 20,000/(0.5 x 0.00055 x 716.42 x 323) = 20,000/45,627 = 0.438.
The clean aircraft drag coefficient from Figure 9.2 at CL = 0.438 gives CDciean = 0.0324. The clean aircraft drag, D = 0.0324 x (0.5 x 0.00055 x 716.42 x 323) = 0.0324 x 45,627 = 1,478 lb.
The available all-engines-installed thrust at maximum cruise rating at the speed and altitude from Figure 13.3 at Mach 0.74 is T = 2 x 790 = 1,580 lb (adequate). The capability satisfies the market requirement of Mach 0.74 at HSC.
13.5.1 Specific Range (Bizjet)
Specific range is a convenient way to present cruise performance. Using Equation 13.27, the Sp. Rn is computed. The details of the specific range are not a direct
substantiation requirement; it is needed to compute the cruise-segment performances (i. e., fuel burned, distance covered, and time taken). Figure 13.14 shows the specific range for the Bizjet (i. e., the worked-out example). When readers redo the specific-range computations, there may be minor differences in the results. From the Sp. Rn values, the fuel burned and distance covered is worked out, which in turn gives the time taken for the distance.