Abstract
This paper studies on the equations of motion for aircrafts flying in steady and unsteady wind, and represents the process for obtaining them based on the definition of inertial system. The main results are as follows. (1) There are two kinds of system moving relatively to a basic inertial system. The one moves along with observer, the other one moves along with atmosphere. (2) The basic inertial system in atmospheric and gravitational field is the atmosphere-fixed system, which is at rest or is moving at constant speed to the ground, (3) The principle of relativity in atmospheric and gravitational field requires to add the following additional condition into Galilei's principle of relativity; "the atmosphere is also moving along with the moving system at constant space"
