Parasitic drag

Drag curve for a body in steady flight

Parasitic drag is drag that results when an object is moved through a fluid medium (in the case of aerodynamic drag, a gaseous medium, more specifically, the atmosphere). Parasitic drag is a combination of form drag, skin friction drag and interference drag. (The other components, induced drag and wave drag, are separate components of total drag, and are NOT components of parasitic drag.)

Description

In flight, induced drag results from the need to maintain lift. It is greater at lower speeds where a high angle of attack is required. As speed increases, the induced drag decreases, but parasitic drag increases because the fluid is striking the object with greater force, and is moving across the object's surfaces at higher speed. As speed continues to increase into the transonic and supersonic regimes, wave drag enters the picture. Each of these drag components changes in proportion to the others based on speed. The combined overall drag curve therefore shows a minimum at some airspeed; an aircraft flying at this speed will be close to its optimal efficiency. Pilots will use this speed to maximize the gliding range in case of an engine failure. However, to maximize the gliding endurance, the aircraft's speed would have to be at the point of minimum power, which occurs at lower speeds than minimum drag.

At the point of minimum drag, C_D,o (drag coefficient of aircraft when lift equals zero) is equal to C_D,i (induced drag coefficient, or coefficient of drag created by lift). At the point of minimum power, C_D,o is equal to one third times C_D,i. This can be proven by deriving the following equations:

$F_{drag} = \frac{1}{2} \rho V^2 A_s C_D$

and

$C_{D} = C_{D,o} + C_{D,i}$

where

$C_{D,i} = K C_L^2$

Form drag

Form drag or pressure drag arises because of the shape of the object. The general size and shape of the body are the most important factors in form drag; bodies with a larger presented cross-section will have a higher drag than thinner bodies; sleek ("streamlined") objects have lower form drag. Form drag follows the drag equation, meaning that it increases with velocity, and thus becomes more important for high-speed aircraft.

Form drag depends on the longitudinal section of the body. A prudent choice of body profile is essential for a low drag coefficient. Streamlines should be continuous, and separation of the boundary layer with its attendant vortices should be avoided.

Profile drag

Profile drag is usually defined as the sum of form drag and skin friction. However, the term is often used synonymously with form drag.

Interference drag

Interference drag results when airflow around one part of an object (such as a fuselage) must occupy the same space as the airflow around another part (such as a wing). The two competing airflows must speed up in order to pass through the restricted area; this speeding-up process requires extra energy and creates turbulence, resulting in a measurable increase in the form drag. This velocity increase is present at all airspeeds, but becomes even more important in the transonic range when the resulting velocity becomes sonic, producing shock waves.

Interference drag plays a role throughout the entire aircraft (e.g., nacelles, pylons, empennage) and its detrimental effect is always kept in mind by designers. Ideally, the pressure distributions on the intersecting bodies should complement each other’s pressure distribution. If one body locally displays a negative pressure coefficient, the intersecting body should have a positive pressure coefficient. In reality, however, this is not always possible. Particular geometric characteristics on aircraft often show how designers have dealt with the issue of interference drag. A prime example is the wing-body fairing, which smooths the sharp angle between the wing and the fuselage. Another example is the junction between the horizontal and vertical tailplane in a T-tail. Often, an additional fairing (acorn) is positioned to reduce the added supervelocities. The position of the nacelle with respect to the wing is a third example of how interference-drag considerations dominate this geometric feature. For nacelles that are positioned beneath the wing, the lateral and longitudinal distance from the wing is dominated by interference-drag considerations. If there is little vertical space available between the wing and the nacelle (because of ground clearance) the nacelle is usually positioned much more in front of the wing. The NACA area rule is one approach to reducing transonic interference drag.

Skin friction

Skin friction drag arises from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the wetted surface, the area of the surface of the body that is in contact with the fluid. As with other components of parasitic drag, skin friction follows the drag equation and rises with the square of the velocity.

The skin friction coefficient, $C_f$, is defined by

$C_f \equiv \frac{\tau_w}{\frac{1}{2} \, \rho \, U_\infty^2},$

where $\tau_w$ is the local wall shear stress, $\rho$ is the fluid density, and $U_\infty$ is the free-stream velocity (usually taken outside the boundary layer or at the inlet). For boundary layers without a pressure gradient in the x direction, it is related to the momentum thickness as

$C_f = 2 \frac{d \theta}{d x}.$

For comparison, the turbulent empirical relation known as the 1/7 Power Law (derived by Theodore von Kármán) is:

$C_f = \frac{0.074}{Re^{0.2} },$

where $Re$ is the Reynolds number.

Skin friction is caused by viscous drag in the boundary layer around the object. The boundary layer at the front of the object is usually laminar and relatively thin, but becomes turbulent and thicker towards the rear. The position of the transition point depends on the shape of the object. There are two ways to decrease friction drag: the first is to shape the moving body so that laminar flow is possible, like an airfoil. The second method is to decrease the length and cross-section of the moving object as much as practicable. To do so, a designer can consider the fineness ratio, which is the length of the aircraft divided by its diameter at the widest point (L/D).

See also

• NACA duct
• Skin friction line