3.2 Drag Components and Reduction Techniques

Aircraft drag is a crucial concept in aerospace engineering, impacting flight efficiency and performance. There are two main types: parasitic drag, unrelated to lift generation, and induced drag, a consequence of lift production. Understanding these forms is essential for aircraft design and optimization.

Reducing drag is a key focus in aerospace engineering. Techniques like streamlining, laminar flow control, winglets, and riblets are employed to minimize both parasitic and induced drag. These strategies enhance aircraft efficiency, leading to improved fuel economy and performance.

Types of Aircraft Drag

Types of aircraft drag

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• Parasitic drag occurs when drag is not associated with lift generation consists of skin friction drag and form drag
• Induced drag happens as a consequence of lift generation caused by the formation of wingtip vortices that create a pressure difference between the upper and lower surfaces of the wing

Factors in parasitic drag

• Skin friction drag results from the interaction between the fluid and the surface of the aircraft influenced by surface area, surface roughness, and Reynolds number
• Form drag is caused by the separation of flow from the surface of the aircraft influenced by the shape of the aircraft, particularly the streamlining of the body (fuselage, wings, tail)

Origin of induced drag

• Induced drag is a result of the pressure difference between the upper and lower surfaces of the wing
• As the wing generates lift, it creates a high-pressure region below the wing and a low-pressure region above the wing
• This pressure difference causes air to flow from the high-pressure region to the low-pressure region at the wingtips, creating wingtip vortices (circular patterns of rotating air)
• The energy lost in the formation of these vortices is the induced drag
• Induced drag is proportional to the square of the lift coefficient ($C_L$) and inversely proportional to the aspect ratio ($AR$) of the wing:
  • $C_{D_i} = \frac{C_L^2}{\pi AR}$

Techniques for drag reduction

• Streamlining involves designing the aircraft body to minimize flow separation and form drag using smooth, continuous surfaces and avoiding sharp edges or abrupt changes in shape (rounded nose, tapered tail)
• Laminar flow control maintains laminar flow over the surface of the aircraft to reduce skin friction drag achieved through the use of smooth surfaces, favorable pressure gradients, and boundary layer suction
• Winglets are vertical extensions at the wingtips that reduce induced drag by minimizing the formation of wingtip vortices and increase the effective aspect ratio of the wing (Boeing 737, Airbus A320)
• Riblets are microscopic grooves aligned with the flow direction on the surface of the aircraft that reduce skin friction drag by altering the near-wall turbulence structure (shark skin, golf ball dimples)