Subsonic Aerodynamics | Drag | PPL(A) Principles of Flight

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What is Drag?

Drag is the aerodynamic force that opposes the motion of the aircraft through the air. Total drag acting on a wing and aircraft has two fundamentally different origins, and understanding each is essential for managing aircraft performance.

Total Drag = Parasite Drag + Induced Drag

These two components behave in opposite ways with respect to airspeed — which is why their relationship is central to understanding aircraft performance, range, and endurance.

Another way to frame the same distinction is:

• Two-dimensional drag — friction and pressure drag along the wing surface, arising from airflow, viscosity, and the boundary layer
• Three-dimensional drag — induced drag from lift generation, influenced by wingspan, aspect ratio, and wingtip vortices

Both framings describe the same physics. The parasite/induced split is the more common exam terminology.

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Parasite Drag

Parasite drag is drag produced by non-lifting parts of the aircraft — everything that creates resistance without contributing to lift. It has three components:

Form Drag

Caused by pressure differences around the aircraft’s shape. As air flows around a body, it separates from the surface and creates a low-pressure wake behind it. The pressure difference between the high-pressure front and the low-pressure wake produces a rearward force — form drag. Streamlined shapes minimise the wake and reduce form drag significantly.

Skin Friction Drag

Caused by air rubbing against the aircraft’s surface. A boundary layer forms where shear forces slow the air adjacent to the surface. The friction between this slow air and the surface produces a rearward drag force. Surface finish, contamination (ice, bugs, dirt), and boundary layer type all affect skin friction drag.

Interference Drag

Caused when airflow from different parts of the aircraft meets and disrupts smooth flow — for example, where the wing joins the fuselage, or where an antenna protrudes from the skin. The interaction of these airflows creates additional turbulence and drag beyond what each component would produce in isolation.

Reducing Parasite Drag

Parasite drag is reduced by streamlining (reducing form drag), smooth surface finish (reducing skin friction), and careful junction design with fairings (reducing interference drag). Retractable landing gear is one of the most effective parasite drag reduction measures — extended gear creates significant form and interference drag; retracting it streamlines the aircraft, improving speed, fuel efficiency, and climb performance.

Parasite Drag Formula

D_P = ½ ρ V² S C_DP

Symbol	Meaning
D_P	Parasite drag force (N)
ρ	Air density (kg/m³)
V	Airspeed (m/s)
S	Wing surface area (m²)
C_DP	Parasite drag coefficient

Because airspeed is squared in the formula, parasite drag increases very rapidly with speed:

• Double the airspeed → V² increases by a factor of 4 → parasite drag quadruples
• The parasite drag curve is parabolic — it rises steeply at high speeds

Parasite Drag and High Speed

At high airspeeds, parasite drag dominates total drag. Landing at excessive speed significantly increases the energy that must be dissipated — requiring a longer runway. Smooth shapes, retracted gear, and clean surfaces are essential for managing parasite drag in cruise.

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Induced Drag

Induced drag is an unavoidable consequence of lift generation by a finite wing. It does not exist on a theoretical infinite wing — it is purely a three-dimensional effect.

Cause — Wingtip Vortices

A wing generating lift has higher pressure below and lower pressure above. At the wingtip, where there is no physical barrier, air from the high-pressure lower surface spills around the tip to the low-pressure upper surface. This spanwise flow rolls up into wingtip vortices — rotating columns of air trailing behind each wingtip.

These vortices induce a downward component of airflow (downwash) across the entire wing. Downwash tilts the local relative airflow downward, which:

• Reduces the effective angle of attack seen by the wing
• Tilts the lift vector rearward — the rearward component of this tilted lift vector is induced drag

More lift required → stronger vortices → more downwash → more induced drag

Induced Drag Formula

D_I = C_L² / (π × AR)

Symbol	Meaning
D_I	Induced drag (proportional value)
C_L	Lift coefficient
AR	Wing aspect ratio (span² ÷ area)
π	Mathematical constant (~3.14159)

Key relationships from the formula:

• C_L² — induced drag increases with the square of the lift coefficient. Flying slowly requires a high C_L → high induced drag
• AR — a higher aspect ratio (longer, narrower wing) reduces induced drag. This is why gliders have very long, narrow wings

Induced Drag and Airspeed

Induced drag behaves in the opposite way to parasite drag with respect to airspeed:

• At low speed, a high angle of attack and high C_L are needed → strong vortices → high induced drag
• At high speed, a lower angle of attack and lower C_L are needed → weaker vortices → low induced drag
• Induced drag ∝ 1 / V² — doubling airspeed reduces induced drag to one quarter of its original value
• The induced drag curve is hyperbolic — it falls steeply as speed increases from low values

Aspect Ratio and Induced Drag

A high aspect ratio wing (long and narrow) reduces induced drag because the wingtip vortices affect a smaller proportion of the total span. Gliders use very high aspect ratio wings to minimise induced drag and maximise glide performance. Most light training aircraft use moderate aspect ratios as a compromise between induced drag, structural weight, and handling.

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Total Drag and the Drag Curve

Total drag is the sum of parasite drag and induced drag at every airspeed. Because the two components vary in opposite directions with speed, their sum produces a characteristic bucket-shaped curve:

Speed	Parasite Drag	Induced Drag	Total Drag
Very low	Low	Very high	High
Moderate	Moderate	Moderate	Minimum
High	Very high	Low	High

Minimum Drag Speed — V_MD

Total drag is minimised when parasite drag equals induced drag. The airspeed at which this occurs is called V_MD (minimum drag speed). This is also the speed for maximum lift-to-drag ratio (best glide speed in a glider, or best range speed in a powered aircraft). Flying faster or slower than V_MD increases total drag and reduces efficiency.

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Environmental Factors Affecting Drag

Heavy Rain

Heavy rain primarily increases drag and reduces performance:

• Raindrops disrupt the boundary layer, raising profile drag
• Water on the surface increases skin friction drag
• Can reduce visibility and affect sensors or avionics
• Does not directly cause major lift loss, jam control surfaces, or significantly shift CG

Ice and Contamination

Surface contamination — ice, frost, snow, insects — increases both form drag and skin friction drag by disrupting the smooth boundary layer and altering the wing’s profile. Even a thin layer of frost on the upper surface can significantly increase drag and reduce cLmax.