Principles of Flight

Chapter 6: A Deeper Dive into Drag

Technical General for Aviators — Capt. Pankaj Pahil

6.1 Total Drag

Parasite drag includes all types of drag that are not associated with the production of lift. It
increases as the square of the airspeed. If you double your speed, you quadruple your
parasite drag. It is made up of three components:
 Skin Friction Drag: Arises from the friction of the air moving over the aircraft's surfaces. It
is affected by the smoothness of the surface and the nature of the
boundary layer (the thin layer of air closest to the surface). The boundary layer can be
smooth (
laminar) or chaotic (turbulent). Turbulent flow produces more skin friction drag but is more
resistant to separation. 126126126126
 Form (Pressure) Drag: Results from the pressure differential created by the separation of
the boundary layer from the surface of a body. 127A streamlined shape (like an aerofoil) is
designed to minimize this separation and reduce form drag.
 Interference Drag: This is generated by the mixing of airflows at the junctions of aircraft
components, such as the wing root where it joins the fuselage. Fillets are used at these
junctions to smooth the airflow and reduce interference drag.

 As discussed previously, induced drag is an inherent by-product of lift generation from a
finite wing.
 It is caused by the wingtip vortices tilting the lift vector rearward.
 Induced drag is highest at low airspeeds (high AOA) and decreases as airspeed increases.

 Low Speeds (High AOA): Induced drag is the predominant force.
 High Speeds (Low AOA): Parasite drag is the predominant force.
 Vmd (Minimum Drag Speed): There is a specific airspeed where the total drag is at its
minimum. This speed is known as Vmd. It occurs at the point where the induced drag curve
and the parasite drag curve intersect (i.e., induced drag equals parasite drag).
 Flying at Vmd corresponds to the optimum angle of attack for the maximum L/D ratio.

 Aircraft Weight: An increase in weight requires more lift for level flight. This increases
induced drag at all speeds, shifting the total drag curve up and to the right, resulting in a
higher Vmd.
 Configuration: Lowering landing gear or flaps increases the frontal area and disrupts
airflow, significantly increasing parasite drag. This shifts the total drag curve upward,
increasing total drag at all speeds and decreasing the Vmd.
 Altitude: If an aircraft is flown at a constant IAS, the dynamic pressure is constant, and
therefore the drag will not vary with altitude.

Speed stability refers to an aircraft's tendency to maintain a constant airspeed. It is directly
related to the drag curve and thrust available.
 Stable Region (Front Side of the Drag Curve): At speeds above Vmd, if a disturbance
causes the speed to decrease, the drag also decreases. Since thrust is now greater than drag,
the aircraft accelerates back to its trim speed. This is a region of stable speed.
 Unstable Region (Back Side of the Drag Curve): At speeds below Vmd, if a disturbance
causes the speed to decrease, the drag increases due to the sharp rise in induced drag. Since
drag is now much greater than thrust, the aircraft will continue to decelerate unless the pilot
intervenes by adding thrust. This is a region of unstable speed, often called the "region of
reversed command."