Types of Drag: PHAK Chapter 4

Handbook Reference

PHAK Ch 4

4.drag-types. Types of Drag

Drag is the aerodynamic force that opposes an aircraft's motion through the air. It acts parallel to the relative wind and rearward, directly opposing thrust. Total drag on an airplane is the sum of two principal categories: parasite drag and induced drag.

Parasite Drag

Parasite drag is produced by any part of the airplane that does not contribute to lift. It increases as airspeed increases — roughly with the square of velocity (drag varies with V²). Parasite drag is further divided into three subtypes:

Form drag — caused by the shape of a body moving through the air. Blunt objects (a flat plate held perpendicular to the airflow) produce large wakes of turbulent, separated flow and high form drag. Streamlining components (fairings on landing gear, teardrop antennas, cowlings around engines) reduces form drag dramatically. A flat plate may have 10–20 times the form drag of a streamlined shape of equal frontal area.
Skin friction drag — caused by the viscous resistance of air molecules sliding along the aircraft's surface in the boundary layer. Even a polished surface has microscopic roughness; rivets, dirt, bug splatter, frost, and peeling paint all increase skin friction. This is why pilots wax wings and why wing leading-edge cleanliness matters before flight.
Interference drag — created when airflows from different parts of the airframe meet and mix turbulently. The wing-fuselage junction, strut-wing junction, and engine nacelles are classic offenders. Designers add fillets (curved fairings) at these intersections to smooth the merging flows. Interference drag is not simply additive; the total can exceed the sum of the individual components' drag.

Induced Drag

Induced drag is the unavoidable byproduct of producing lift. As a wing generates lift, higher-pressure air beneath the wing spills around the wingtip toward the lower-pressure air on top, creating wingtip vortices. These vortices deflect the relative wind downward behind the wing — a phenomenon called downwash. The local relative wind is tilted downward, which tilts the lift vector aft. The rearward component of that tilted lift vector is induced drag.

Key relationships:

Induced drag is greatest at low airspeeds and high angles of attack, because at slow speeds the wing must operate at a high AOA to produce the lift required to support the airplane.
Induced drag varies inversely with the square of airspeed (1/V²).
Induced drag is reduced by higher aspect ratio wings (long, narrow wings like a sailplane's), by winglets that disrupt tip vortex formation, and by tapered or elliptical planforms.
Induced drag is the dominant drag during slow flight, takeoff, and the approach to landing — and it is the main reason airplanes have a minimum power required speed.

Total Drag and L/D Max

When parasite drag and induced drag are plotted against airspeed, parasite drag rises steeply with speed while induced drag falls. The two curves cross at a single airspeed, and total drag (their sum) reaches a minimum at that point. This minimum-total-drag airspeed is L/D max — the speed of maximum lift-to-drag ratio.

At L/D max:

The airplane achieves its maximum glide range (engine out).
The airplane achieves its maximum range in a propeller airplane (because thrust required equals total drag and is minimized).
Best-glide speed (V_BG) and best-endurance / max-range speeds are referenced from this point.

For a typical training airplane such as a Cessna 172, L/D max occurs around 65 KIAS at gross weight; published best-glide and best-range speeds are based on this aerodynamic relationship.

The Region of Reversed Command

Because induced drag dominates at low airspeeds, an airplane flown slower than L/D max actually requires more thrust (or power) to maintain altitude as it slows down. This is the back side of the power curve, or region of reversed command. Operating in this region — common during a short-field approach or a slow-flight maneuver — requires careful power and pitch coordination to avoid sinking into the runway or stalling.

Practical Implications for the Pilot

Keep the airplane clean. Frost, ice, mud, dead bugs, and unsealed gaps add measurable parasite drag and reduce climb performance.
Retract flaps and gear (in a retract) on schedule to minimize parasite drag during climb.
Fly published best-glide speed in an engine-out emergency — that speed corresponds to L/D max.
Recognize that adding flaps, lowering the gear, and slowing toward stall AOA all increase total drag and require additional power to maintain level flight.
Wingtip vortices (induced-drag byproducts) create wake turbulence; heavy, slow, clean (no flaps/gear) airplanes generate the strongest wake.

Oral Exam Questions a DPE Might Ask

Q1What are the two main types of drag, and how does each behave with airspeed?

Parasite drag (form, skin friction, and interference) increases with the square of airspeed, so it dominates at high speeds. Induced drag is a byproduct of lift and varies inversely with the square of airspeed, so it dominates at low speeds and high angles of attack.

Q2What is L/D max and why does it matter to a pilot?

L/D max is the airspeed where total drag is minimized and the lift-to-drag ratio is greatest. It corresponds to maximum glide range with the engine out and is the basis for the airplane's published best-glide speed.

Q3What is the region of reversed command?

It's the range of airspeeds below L/D max where slower flight requires more power, not less, because induced drag rises sharply as the wing operates at a higher AOA. Pilots encounter it during slow flight and short-field approaches.