Answer:
Velocity
Step-by-step explanation:
As we know that for the equilibrium of flight in air the force due to air pressure must counterbalance the weight of flight.
Now here the force due to air pressure is generated by the condition of air flow above the wings
Now in order to maintain a sufficient pressure on the wings so that it will counterbalance the weight it requires a fixed flow rate of air above and below the wings
Now in order to maintain this flow speed of air the velocity of flight must have to maintain at a fixed value.
So here we can say for any flight to move from one point to other its velocity in air must remains constant throughout the motion.