Drag

Drag is the aerodynamic force parallel to the relative wind, opposite to motion. Thrust must compensate it.

Drag equation

D = ½ · ρ · V² · S · CD

with:

D = drag (N)
CD = drag coefficient (dimensionless)
ρ, V, S as for lift.

Two main components

1. Parasite drag (CD₀)

Drag without lift, present even at CL = 0. Breaks down into:

Component	Cause
Form (pressure) drag	Pressure difference between front and rear of a body; streamlining minimises
Skin friction drag	Friction of flow at the surface (boundary layer)
Interference drag	Interaction at connection points (wing-fuselage, strut-wing)

Parasite drag grows with V² (linear trend in CD₀-V plot).

2. Induced drag (CDi)

Drag from lift generation, dependent on CL².

Mechanism: at the wingtip, air flows from lower to upper surface (pressure equalisation) → wingtip vortex. This vortex produces downwash over the wing → effective inflow tilts downward → lift vector tilts rearward → component opposing motion = induced drag.

Formula (for elliptical lift distribution):

CDi = CL² / (π · AR · e)

with:

AR = aspect ratio = b²/S
e = Oswald efficiency factor ≈ 0.7–0.9 (typical GA)

Consequence: induced drag grows with CL² → at high α (slow, much lift): large; at low α (fast): small.

Total drag

CD = CD₀ + CDi

and its velocity dependence:

code

CD
|
|        Parasite (∝ V²)
|        /
|       /
|      / Total
|     /   \
|    /     \___ Induced (∝ 1/V²)
|___/_________
              V
              ↑
        V for (L/D)_max
        (= "min drag speed")

Key speeds

Speed	Meaning
V_(L/D)_max	(L/D)_max — best glide ratio, best range (glider, jet range), best climb-angle ratio
V_min_power	min power required — best climb (propeller); best endurance (propeller)
V_min_sink	min sink rate (glider, idle)

(L/D)_max occurs at α where parasite drag = induced drag.

Typical GA values

Aircraft	(L/D)_max	V (L/D)_max
Cessna 172	about 9	about 65 KIAS (Vbg)
Glider	25–60	type-dependent
Boeing 747	about 17	flap-dependent

How to reduce drag?

Smooth surfaces (friction).
Streamlining (strut fairings, landing-gear fairings) — form drag.
High aspect ratio (long wings) — induced drag.
Winglets — induced drag (via reduced vortex strength).
Retracted gear — large parasite component.
Clean exterior (no unnecessary antennas, lights).

Practical consequences

Altitude: at higher altitude, lower ρ → less parasite drag → better cruise speed at same power (up to service ceiling).
Weight: higher weight → higher CL required → higher induced drag → more fuel burn.
Icing: rough surface → drastic increase in parasite drag.