Climb Performance Requirements (Bizjet)

The three requirements for substantiation of the climb performance are given as fol­lows for a two-engine aircraft (the first two are FAR requirements; Table 13.5 pro­vides an aircraft configuration and FAR requirements for the first – and second – segment climb):

1. Verify that the FAR first-segment climb requirement of a positive gradient is maintained.

2. Verify that the FAR second-segment climb gradient requirement exceeds 2.4%.

3. Verify that the market requirement of the initial enroute rate of climb equals or exceeds 2,600 ft/min. The cabin-pressurization system should handle the rate of climb. (This is a customer requirement, not a FAR requirement.)

The second segment starts at a 400-ft altitude with flaps extended and the undercar­riage retracted (i. e., one engine inoperative). From a 400- to 1,000-ft altitude, the undercarriage is retracted in the second-segment climb. An aircraft is maintained at the V2 speed for the best gradient – a 50% loss of thrust does not favor an acceler­ated climb, which will be low in this case. The engine is at the takeoff rating. The available one-engine-installed thrust is from Figure 13.1. The thrust is kept invariant at the takeoff rating through the first – and second-segment climb. Table 13.17 sum­marizes the first – and second-segment climbs for both 8-deg and 20-deg flap settings. At one engine failed, the aircraft must return to the base immediately.

When verifying initial enroute rate of climb, the specification requirement is 2,600 ft/min. When the initial enroute climb starts at a 1,000-ft altitude (p = 0.0023 slug/ft3, a = 0.9672) and all engines are throttled back to maximum climb rating, an aircraft has a clean configuration. An aircraft makes an accelerated climb from V2 to reach 250 KEAS, which is kept constant in a quasi-steady-state climb until it reaches Mach 0.7 at about a 32,000-ft altitude. From there, the Mach number is held constant in the continued quasi-steady-state climb until it reaches the cruise altitude. Fuel consumed during the second-segment climb is small and assumed empirically (from statistics) to be 120 lb (see Table 13.17). Therefore, the aircraft weight at the beginning of the enroute climb is M = 20,600 lb. At 250 kts (422 ft/s, Mach

0. 35), the aircraft lift coefficient CL = M/qSW = 20,600/(0.5 x 0.0023 x 4222 x 33) = 20,600/66,150 = 0.311.

The clean aircraft drag coefficient from Figure 9.2 at CL = 0.311 gives CDclean = 0.0242. The clean aircraft drag, D = 0.0242 x (0.5 x 0.0023 x 4222 x 323) = 0.0242 x 66,150 = 1,600 lb. The available all-engine-installed thrust at the maximum climb rating from Figure 13.2 at Mach 0.378 is T = 2 x 2,260 = 4,520 lb. From Equa­tion 13.10, the quasi-steady-state rate of climb is given by:

R/C Vqq[(T – D)/W]

/ accl 1 + (V/g)(dV/dh)

At the quasi-steady-state climb, Table 13.5 gives V (dV) = 0.56 m2 = 0.56 x 0.352 = 0.0686. Hence:

R/Caccl = {[422 x (4,520 – 1,600)]/20,680}/[1 + 0.0686] = 55.8ft/s = 3,345 ft/min(17 m/s)

This capability satisfies the market requirement of 2,600 ft/min (13.2 m/s). (The civil aircraft rate of climb is limited by the cabin-pressurization schedule. An aircraft is limited to 2,600 ft/min at an altitude where the cabin pressurization rate reaches its maximum capability. Naturally, at the low altitude of 1,000 ft, this limit is not applicable.)

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