Aircraft fly when they generate more lift than drag. You can calculate lift using the aircraft's velocity, its wings' surface area and the density of air.
Square the aircraft's velocity. For instance, if it's traveling at 60 meters per second:
60 * 60 = 3,600
Multiply your answer by the density of air. For instance, if the density is 1.2 kilograms per cubic meter:
3,600 * 1.2 = 4,320
Multiply by the combined upper surface area of both wings. If the surface area is 30 square meters:
4,320 * 30 = 129,600
Divide by 2.
129,600 / 2
Multiply by the lift coefficient. If your lift coefficient is 0.2:
64,800 * 0.2 = 12,960 newtons.
A newton is the force required to accelerate a kilogram 1 meter per second per second.