Theory and Practice of Aircraft Performance

CHAPTER 13Cruise Performance and Endurance

13.1 Overview

After estimating takeoff, climb, descent and landing, the last of the segments to complete the mission profile is the cruise segment in the middle. This is the longest segment of the mission. Operationally, it may appear to be the least stressful to pilot; the cruise segment continues in a quasi‐steady state. There is no airworthiness requirement for the cruise segment (except at engine failure), but designers need to substantiate the customer requirement of meeting the maximum initial cruise speed.

Chapter 10 compared parabolic drag polar with computed actual drag polar to demonstrate that there is a difference between the two. Being more accurate, industries use computed actual drag polar verified in wind‐tunnel tests for high subsonic aircraft application. The difference reduces for low‐speed aircraft such as the TPT (turboprop trainer) flying below 0.5 Mach (no wave drag); use of parabolic drag polar is common in industrial practice. In civil applications, where economics and/or time constraints dictate, it is important to find an operational schedule at better accuracy to suit the requirements. For long ranges, substantial weight change takes place on account of fuel burnt, requiring fine‐tuning of speed–altitude schedules as discussed in this chapter. For this reason, commercial transport aircraft have aircraft performance monitoring (APM) systems to fly at the most economic schedule.

Low drag at higher altitudes ...