There are several types of drag: form, pressure, skin friction, parasite, induced, wave and ram. However, form, pressure, skin friction, wave and ram drags are collectively known as parasite drag. Hence, there are only two types of drag: parasite and induced Parasite drag – Profile or parasite drag is caused by the airplane pushing the air out of the way as it moves forward. The parasite drag of a typical airplane consists primarily of the skin friction, roughness, and pressure drag of the major components.
Some additional parasite drag is also due to things like fuselage upsweep, control surface gaps, base areas, and other extraneous items. The basic parasite drag area for airfoil and body shapes can be computed from the following expression: f = k cf Swet, where the skin friction coefficient, cf , which is based on the exposed wetted area includes the effects of roughness, and the form factor, k, accounts for the effects of both super-velocities and pressure drag. Swet is the total wetted area of the body or surface.
Computation of the overall parasite drag requires that we compute the drag area of each of the major components (fuselage, wing, nacelles and pylons, and tail surfaces) and then evaluate the additional parasite drag components described above. Hence it is written as: CDp = S ki cfi Sweti / Sref + CDupsweep + CDgap+ CDnac_base + CDmisc, where the first term includes skin friction, and pressure drag at zero lift of the major components. cfi is the average skin friction coefficient for a rough plate with transition at flight Reynolds number. Equivalent roughness is determined from flight test data. (http://adg. stanford.
edu/aa241/drag/parasitedrag. html) Induced drag – Induced drag is the part of the force produced by the wing that is parallel to the relative wind, i. e. the lift. As it is a consequence of the vortices it is sometimes called vortex drag. Induced drag is least at minimum AOA and is greatest at the maximum AOA i. e. angle of attack. Induced drag = (k ? CL? / A) ? Q ? S where A is the wing aspect ratio. (Preston, R) The magnitude of induced drag depends on the amount of lift being generated by the wing and on the wing geometry Long, thin (chord wise) wings have low induced drag; short wings with a large chord have high induced drag.
An airplane must fight its way through both kinds of drag in order to maintain steady flight. . Total drag is a sum of Parasite and Induced drag. Total Drag = Parasite drag + Induced drag However, the total drag of an aircraft is not simply the sum of the drag of its components. When the components are combined into a complete aircraft, one component can affect the air flowing around and over the airplane, and hence, the drag of one component can affect the drag associated with another component.
These effects are called interference effects, and the change in the sum of the component drags is called interference drag. Thus, (Drag)1+2 = (Drag)1 + (Drag)2 + (Drag)interference (Johnston, D) Generally, interference drag will add to the component drags but in a few cases, for example, adding tip tanks to a wing, total drag will be less than the sum of the two component drags because of the reduction of induced drag.