SAWE Report - Cal Poly San Luis Obispo

consumption. The main aspects of the mission that were optimized were the initial climb
sequence, the cruise and dash altitudes (dash altitude must be greater than 50,000 ft (15,240 m)),
and the loiter speed. The optimum climb sequence was found by creating a flight envelope with
lines of constant climb rate to fuel flow ratio (dh/dW F ) at the average climb weight of the
aircraft. The climb profile that minimizes the fuel required to climb the aircraft to a given cruise
condition is then found by drawing a flight path between the initial and cruise conditions that
follow the maximum climb rate to fuel flow ratio. The resulting flight path and fuel
consumption envelope are shown in Figure 11.1.
50,000
dh /dW F = 0 ft/lb
100 ft/lb
40,000
Stall Limit
50 ft/lb
Altitude (ft)
30,000
20,000
100 ft/lb
150 ft/lb
200 ft/lb
250 ft/lb
10,000
300 ft/lb
q Limit
0
0 0.5 1 1.5 2
Mach
Figure 11.1 - Fuel Consumption Envelope at Average Climb Weight
The optimum cruise and dash altitudes were found by running a series of missions at different
altitudes and finding the mission with the lowest fuel consumption. Because the aircraft’s
weight decreases as fuel is burned, the optimum cruise altitude increases over the mission
profile. It was found that the optimum sequence of cruise altitudes began at 52,000 ft (15,850 m)
for the initial cruise and increased by 2,000 ft (610 m) for each successive cruise or dash segment
resulting in a final cruise altitude of 58,000 ft (17,678 m). The optimum loiter speed was found
as the speed at which the minimum drag occurred under loiter conditions (sea level and 61,000 lb
(27,669 kg) weight). The drag on the aircraft under these conditions is plotted as a function of
Mach number in Figure 11.2 showing that the minimum drag occurs at Mach 0.35 or 391 ft/s
(119.2 m/s). The resulting detailed mission is listed in Table 11.III.
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