Technical Comments. Comment on "Effect of Thrust/Speed Dependence on Long-Period Dynamics in Supersonic Flight"

Introduction R EFERENCE 1 discusses effects of speed-dependent variations in thrust on the long-period motions of aircraft in supersonic flight. Its principal contribution is the demonstration of effects that a complex pair of zeros have on speed-tothrottle feedback. It is concluded that there are fundamental differences between thrust/speed effects in subsonic and supersonic flight. It also is stated that "thrust variations with speed (or Mach number) have practically no effect on the phugoid in supersonic flight," then that "the phugoid is not influenced at all in supersonic flight." The purpose of this Comment is to show that thrust/speed effects are indeed small but not zero in supersonic flight and that this is a kinematic effect of increasing speed rather than an aerodynamic effect of Mach number. In the process, a simplified analytical model is offered for further study. Furthermore, it is noted that pitching moment/speed sensitivity due to thrust-axis offset is likely to have important long-period effect even when the direct thrust/speed effects are small.