A Deeper Dive into Drag

6.1 Total Drag

• Total drag is the sum of two main components:
• Total Drag = Parasite Drag + Induced Drag

6.2 Parasite 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.

6.3 Induced 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.

6.4 The Total Drag Curve

•  This curve plots the different drag components against airspeed.

•  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.

6.5 Factors Affecting the Drag Curve

•  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.

6.6 Speed Stability

• 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."