where g is the acceleration due to gravity at the Earth's surface. The acceleration along the trajectory is equal to the net x-wise force minus the component of weight. We should not expect significant aerodynamic derivatives to depend on the flight path angle, so only formula_84 and formula_85 need be considered. formula_84 is the drag increment with increased speed, it is negative, likewise formula_85 is the lift increment due to speed increment, it is also negative because lift acts in the opposite sense to the z-axis.