Copyright © A. Filippone (1996-2001). All Rights Reserved.

Aerodynamic Drag

Interference Drag in Aerodynamics


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.

Wing-Nacelle Interference
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.

Roughness Drag

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.

Junction Drag

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.


Figure 2: common types of interfering imperfections

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.

Related Material

Selected References

  • AGARD. Special Course on Concepts for Drag Reduction, AGARD Report R-654, 1977.

  • AGARD. Special Course on Subsonic/Transonic Aerodynamic Intereference for Aircraft, AGARD Report R-712, 1983.

  • AGARD. Aircraft Drag Prediction and Reduction, AGARD Report R-723, 1985.

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Copyright © A. Filippone (1996-2001). All Rights Reserved.