Drag: Parasite, Induced and Total Drag
Two families of drag
Parasite drag comprises form drag and skin-friction drag plus interference drag; it rises with the square of speed. Induced drag is the price of lift and arises from the spanwise flow and trailing vortices that reduce the effective angle of attack and tilt the lift vector rearward. Induced drag falls as speed increases because a given lift is produced at a lower lift coefficient.
Total drag is the sum, giving the familiar U-shaped curve. Its minimum occurs where parasite drag equals induced drag, which is also the speed for maximum lift-to-drag ratio and hence maximum glide range and best endurance in a propeller aircraft (approximately; endurance is at minimum power required, slightly slower).
Reducing induced drag
Aspect ratio is the key parameter: induced drag coefficient is proportional to C_L² divided by aspect ratio and by span efficiency. High aspect ratio, winglets, elliptical loading and clean tips all help. Candidates should know that induced drag depends on the square of the lift coefficient, so it dominates at low speed and high load factor, which is why manoeuvring and steep turns are costly.
Traps and worked example
Traps: saying induced drag increases with speed; confusing best glide angle with best sink rate; and forgetting that extending flaps increases parasite drag while allowing a lower speed, shifting the whole curve. Another trap is the claim that total drag is minimum at the stall.
Rule of thumb: at the speed for minimum total drag, parasite and induced drag are equal, so each is half the total.
Worked example: if at some speed parasite drag is 300 N and induced drag is 200 N, total is 500 N. Flying faster increases parasite drag and reduces induced drag; flying slower does the reverse. The minimum total drag speed lies between the two regimes, where the two components are equal at about 250 N each.