Interference Drag in Aerodynamics
Summary
Interference is the effect of an aerodynamic component on another: wing-body, wing-nacelle (Fig. 1), vertical-horizontal tail, junctions in general, biplane, ground effect, freee surface problems in hydro- dynamics, and more.
When Interference Occurs
Interference occurs when the sum of the drag forces of the single components is larger that the drag of the composite system. In general, interference is a reciprocal effect, although in some cases (such as supersonic flows) it can be unidirectional (downstream propagation only.) Interference at supersonic speeds can be excessively high.
Figure 1: Wing-nacelle-pilon interference at transonic speeds.
Aerodynamic interference on wing-body combinations has been widely investigated at subsonic, transonic and supersonic speeds. Recent research on supersonic aircraft (Kharitonov, 1998) has allowed to determine the optimal position of the wing by systematic analysis of the interference coefficients.
Interference at Low Speeds
Interference at low speed can be computed with some approximate yet powerful
methods: Munk’s stagger theorem gives the value of the induced drag for an
arbitrary system of lifting lines; Prandtl’s theory allows to compute the biplane
configuration; the inviscid flow models resulting in the panel methods allow the
computation of quite general multi-body configurations, including ducted propellers.
The most common interference effects arise from imperfections, small scale bumps,
holes and other irregularities (Fig. 4 below), due to surface finish, accumulated
dirt, etc. Roughness/excrescence drag can be virtually eliminated when the surface is
hydraucally smooth, e.g. the excrescence height is less than the boundary
layer sublayer thickness. Some typical drag values are the following: cylinder
excrescence CD=0.76, semi-sphere CD=0.32.
Important data on junction drag have been compiled by Hoerner (1965). Particularly
important is the T-strut configuration, for which some technical solutions with
fairings yield as much as 94 % drag saving.
Methods for reducing the interference effects include accurate streamlining, and
proper system design.
Sometimes it is possible to take advantage of the interference effects (using
appropriate strakes, for example) to reduce the system drag to values below the sum
of the single components. A typical example taken from the natural world is the birds
formation flight.
Roughness Drag
Junction Drag
Related Material
Selected References