Intact airframes do not free fall in the atmosphere
Just a honest question.
What do you calculate the average vertical G to be in order to start at zero vertical speed at 6000 feet and then reach the ground in 18sec?
Drag, thrust and lift work in three dimensions. Only gravity works in a single dimension.
Etkin's Dynamics of Atmospheric Flight devotes 75 pages of serious math to General Equations of Unsteady Motion