Okay, this might seem like a weird question, but basically, to make an airplane fly flat and level, the lift created by the wings must be equal to the force of Gravity pushing the airplane down.
So what is the velocity? In terms of (0,x,0), so I can make my lift the opposite.
(Eg. x = gravity and y = lift and y = -x.)
Gravity in Unity can be found and adjusted in the PhysicsManager found under Edit>Project Settings>Physics. The default force applied by Gravity is (0, -9.81, 0), so you would have to apply a constant force of (0, 9.81, 0) in order for an object to stay perfectly still while using gravity and not resting on anything.
Of course, you could just turn off gravity for that object…