Air resistance is usually calculated using the drag equation. This equation calculates the force experienced by an object moving through a fluid or gas at relatively large velocity. The result is called quadratic drag. Once the drag is calculated, you have to use a formula to calculate the power needed to overcome that drag in order propel and object. Power needs are the cube of the velocity, so if it takes 10 horsepower to go 80 kph it will take 80 horsepower to go 160 kph. As you can see a doubling of speed requires the eight times the amount of power. Knowing this is essential in calculating the amount of fuel it will take to make a journey.
There are three main types of drag in aerodynamics: lift induced, parasitic, and wave. Each affects an objects ability to stay aloft as well as the power and fuel needed to keep it there.
Lift induced(induced)drag occurs as the result of the creation of lift on a three-dimensional lifting body(wing or fuselage). It has two primary components: vortex drag and lift-induced viscous drag. The vortices derive from the turbulent mixing of air of varying pressure on the upper and lower surfaces of the body. These are needed to create lift. As the lift increases, so does the lift-induced drag. For an aircraft this means that as the angle of attack and the lift coefficient increase to the point of stall, so does the lift-induced drag.
Parasitic drag is caused by moving a solid object through a fluid. Parasitic drag is made up of multiple components including form drag and skin friction drag. In aviation, induced drag tends to be greater at lower speeds because a high angle of attack is required to maintain lift, so as speed increases this drag becomes much less, but parasitic drag increases because the fluid is flowing faster around protruding objects increasing friction. The combined overall drag curve is minimal at some airspeeds and an will be at or close to its optimal efficiency. Pilots will use this speed to maximize fuel consumption.