Principles of Flight - Drag (3)

The amount of Induced Drag and Parasite Drag change differently when speed increases.

Induced drag reduces when airspeed increases and vice versa. It is because at high airspeed, a smaller angle of attack is required. This results in a less pressure difference above and below the wing. The opposite is true. At slow airspeed, a larger angle of attack is needed to maintain the necessary amount of lift. Thus there will be a great difference in pressure above and below the wing which forms a stronger wingtip vortex i.e. more drag.

Parasite drag increases when airspeed increases and vice versa. It is because the faster the aircraft flies, the more amount of air is encountered with the aircraft body. 

Minimum Drag Speed

The combination of induced and parasite drags at a certain airspeed gives the total drag as shown above. The total drag curve is like a u-shape. At the point of minimum drag speed, any increase or decrease of airspeed will increase the drag. The minimum drag speed also known as the glide speed. It also gives the best L/D ratio. 

Stall Speed

The stall speed is the minimum speed required to maintain necessary lift. When the airspeed is slow, a larger angle of attack is needed to maintain the necessary lift. However, it also brings greater induced drag. When the angle of attack is too large, i.e. exceeding the critical angle, that the wing can no longer create any lift, the aircraft will stall.