In the design of a civil jet transport, such as the Boeing 777 shown in Fig. 6.27, the choice of engine size is usually based on having a 300 feet per minute rate-of- climb capability at the top-of-climb to cruising altitude. This is a safety margin. Assume the following cruise conditions at top-of-climb for the Boeing 777: L/D = 18, altitude = 31,000 ft, M∞ = 0.085, W = 550, 000 lb. (a) Obtain an equation for the required engine thrust, assuming that the climb angle is so small the L = W. (b) Calculate the required engine size (in terms of sea-level static thrust), and compare your result with the designers engine choice for the Boeing 777, which is two engines of the Rolls-Royce Tront type with a sea level static thrust of 34,000 lb each.
Figure 6.27 Three-view of the Boeing 777-200 twin-turbofan high-capacity commercial airliner.
Note: Intuition might tell you that, for a new airplane design, the engine should be sized to provide enough take-off thrust to get the airplane off the ground in a specified take-off distance. However, using the top-of-climb criteria discussed here, the resulting engine thrust is usually quite ample for take off.
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