What Is Parasitic Drag?
Parasitic drag is a fundamental force that resists the motion of any object moving through a fluid, such as air or water. It encompasses all forms of drag not associated with the generation of lift. This resistance is present for anything from a car driving down a highway to an airplane flying through the sky. It is often called “parasitic” because it generally provides no benefit and reduces performance. Understanding parasitic drag is important for designing more efficient vehicles and objects that move through fluids. It is distinct from induced drag, which is directly related to the lift produced by wings.
Understanding Its Components
Parasitic drag is composed of three primary components: form drag, skin friction drag, and interference drag. Each type arises from different interactions between the object and the fluid it moves through.
Form Drag
Form drag, also known as pressure or profile drag, is caused by the object’s shape and its resistance to airflow. Blunt objects create a large turbulent wake behind them, leading to significant pressure differences between the front and rear, which generates substantial form drag. Conversely, streamlined shapes allow air to flow smoothly around them, minimizing this pressure difference and reducing drag. For example, a flat plate held perpendicular to the wind experiences much more form drag than a teardrop shape.
Skin Friction Drag
Skin friction drag results from the friction between the fluid (like air) and the surface of the moving object. Air molecules in direct contact with the surface adhere to it due to viscosity, and these slowed molecules then impede the layers of air above them. This force is directly proportional to the surface area in contact with the fluid.
Interference Drag
Interference drag occurs when airflows around different parts of an object interact and create turbulence. This is particularly noticeable at junctions where components meet, such as the wing and fuselage of an aircraft. The mixing of these airflows creates localized eddies and disturbed air, leading to additional drag that is greater than the sum of the drag from the individual components. Sharp angles at these junctions can significantly increase interference drag.
Factors Affecting Parasitic Drag
Several factors influence the amount of parasitic drag an object experiences, with speed, shape, surface area, and surface smoothness being prominent.
Speed
Speed has a significant impact on parasitic drag; it increases proportionally to the square of the velocity. This means that if an object’s speed doubles, its parasitic drag quadruples.
Shape
The shape of an object directly determines its form drag. Streamlined designs, characterized by smooth, continuous contours, allow air to flow efficiently around them, minimizing turbulent separation and reducing drag. In contrast, objects with blunt or irregular shapes disrupt airflow more severely, leading to higher drag. Aircraft designers prioritize aerodynamic shapes to reduce this resistance.
Surface Area
The total surface area exposed to the fluid also affects parasitic drag. Objects with a larger frontal area or a greater wetted surface area generally experience more drag. For instance, an extended landing gear significantly increases the exposed surface area of an aircraft, leading to a notable increase in drag. This is why designers aim to minimize the overall external surface area where possible.
Surface Smoothness
Surface smoothness plays a role in skin friction drag. A rougher surface increases the friction between the fluid and the object, leading to greater skin friction drag. Contaminants like dirt or ice on a surface can also increase its roughness, thereby increasing this type of drag. Maintaining smooth surfaces is therefore important for reducing resistance.
Reducing Its Impact
Minimizing parasitic drag is a primary goal in the design of vehicles and other objects that move through fluids. Various design principles and strategies are employed to achieve this reduction, focusing on each component of parasitic drag.
Streamlining
Streamlining is a fundamental approach to reducing form drag. This involves designing objects with smooth, aerodynamic shapes that allow air to flow over them with minimal disturbance. The ideal streamlined shape often resembles a teardrop, facilitating smooth airflow and minimizing the turbulent wake. This principle is applied to aircraft fuselages and car bodies to enhance efficiency.
Surface Smoothing
Surface smoothing is employed to reduce skin friction drag. This strategy involves using smooth materials and finishes, such as polishing or specialized coatings, to minimize the friction between the fluid and the object’s surface. Even seemingly smooth surfaces can be further refined to decrease microscopic roughness, thereby promoting laminar flow and reducing drag.
Retracting Components
Retracting components is an effective method to reduce both form and interference drag. In aircraft, for example, retractable landing gear is pulled into the fuselage after takeoff, significantly reducing the exposed surface area and improving the overall aerodynamic shape. This action drastically cuts down the drag generated by these non-lifting components.
Fairings and Fillets
Fairings and fillets are design elements specifically used to reduce interference drag. Fairings are smooth structures that cover junctions, such as where a wing meets the fuselage, to create a gradual transition for airflow. Fillets are curved surfaces that smooth out sharp angles, preventing turbulent mixing of airflows at these intersections. These additions help guide the air smoothly, preventing the formation of eddies and reducing drag.