Study of Chattering Cruise

TLDR: In this paper, a reduced-order energy model was used for analysis of chattering cruise, an idealization of time-shared operation between two Mach-number/altitude points, proceeding along the general lines of a study by Gilbert and Parsons.

Abstract: That steady-state cruise is not, in general, fuel-range optimal has been shown in studies by Speyer and others. The presently reported investigation makes use of a reduced-order ("energy") model for analysis of "chattering" cruise, an idealization of time-shared operation between two Mach-number/altitude points, proceeding along the general lines of a study by Gilbert and Parsons. The characteristic nonconvexity of the hodograph figure, which leads to chattering, is examined and attempts are made to relate the extent of the effect to aerodynamic and propulsion-system parameters. One such is an analytical attack on a highly simplified vehicle model featuring Mach-number and altitude independence. Another is a computational study of several aircraft modeled with the usual altitude dependence, and with and without Mach dependence. The trends noted are examined further in a look at specific-energy dependence of the hodograph figure's shape. Although substantial improvements in fuel economy appear in low-energy situations, the best improvement found in the cruise-energy range is about 5%.