Updrafts and Downdrafts One of the most confusing concepts for young aerodynamicists is the relative velocity between objects. Aerodynamic forces are generated by an object moving through the air, but the air itself can move also.
Aerodynamic forces depends on the square of the velocity between the object and the air.
To properly define the velocity, it is necessary to pick a fixed reference point and measure velocities relative to the fixed point. The air in which the aircraft flies can move in all three directions. In this figure, we are only considering air movement which is perpendicular to the ground. The effect of air motion along the flight path is described on another slide. On this slide, we are considering a glider which moves through the air along a flight path at some velocity called the airspeed.