Parasite & induced drag

Total drag has two parts: parasite drag and induced drag.

• Parasite drag is the resistance of the aircraft pushing through the air: form drag, skin friction and interference drag. It rises with the square of speed, so it dominates at high speed.
• Induced drag is the price of making lift: the wingtip vortices and downwash left behind. It is largest at low speed / high angle of attack and falls as speed rises.

Add them and the total-drag curve is U-shaped. The bottom of the U is minimum drag, the best lift/drag ratio (L/Dmax):

• L/Dmax gives the best glide range (most distance per foot of height lost).
• Its speed sits close to Vy, the best rate-of-climb speed.

Ground effect: within about a wingspan of the surface the ground weakens the vortices and downwash, so induced drag drops and the aircraft tends to float.

Note: parasite drag rising with the square of speed and best glide sitting near Vy are basic aerodynamics; TP 1102 states only that parasite drag rises with speed and that L/Dmax gives best glide range (it does not give the exponent or the Vy link).

Trap: mixing up which dominates. Parasite drag wins at high speed, induced drag at low speed.

Practice