In this research a study of the downwash effects for high altitudes and high speeds
using a Transient Response approach, the longitudinal equation of motion including the
effect of downwash in high altitude and speed were solved. It was found that the
downwash in at high altitude and low-speed more effected because it's increases in pitch
rate and rate of change of downwash velocity due to decreases in lift ,therefore requires
greater control surface area and largest deflection angles. It can be adopted in this
research to real requirements in the design of flying objects.