^Time of flight

From point F.5. b (ii) we know v_y = zero at

projectile reaches its maximum height from point of projection. In the absence of air friction, wind & upthrust (actually all forces other than gravity) projectile takes same time to return back from maximum height to level plane of projection i.e. to travel path OPMA will be twice of this time, the total time to travel path OPMA is called time of flight (T), thus

.  Let a is the retardation due to air friction,