# Interference Aerodynamics

#### Summary

Aerodynamic interference arises from the fact that aerodynamic bodies are generally not stand-alone devices in unbounded flow, but come to interact with each other, or find themselves in marginal regions of the flow domain, or both. Examples include: multi-element wings, biplanes, cascades, wing-body, wing-nacelles, wing-tail in aircraft, wing-propeller, ducted propellers, counter-rotating propellers, between propeller blades, ground effect, wind tunnel boundaries, and more. Some of these aspects are treated in this section.

A general treatment is not straightforward. Interference at low speeds is of elliptic type (perturbations radiate in all directions), while at supersonic speeds it can only be unidirectional (law of the forbidden signals, von Karman).

The problem is always to find the perturbation quantities from one aerodynamic body onto the others, in order to be able to evaluate the change in pressure, lift, drag, moments, thrust, etc. All methods that can be reduced to a numerical solution are effective for this purpose.

Interference produces the so-called interference drag, whose effects can be negative, as in aircraft technology, or beneficial, as in wing newar ground (see below), and birds formation flight.

#### Methods for Aerodynamic Interference

Lifting surface methods of various kind (vortex lattice, for example) and panel methods are the most effective at subsonic speeds. Computation is relatively easy, since the methods lead to the formulation of a system of equations that contain influence coefficients from the bodies onto themselves and from each body onto the other bodies. The system is solved for all unknowns all all bodies.

For example, a wing in ground effect will be equivalent to a two-wing problem, where the second wing is the mirror image of the first; Wing-body combinations are solved simultanously.

The most complicated cases are those where there is a time variation of one body with respect to another (wing- propeller, ducted propellers, etc.), because the influnce of one body onto the other changes with time. This may require the computation of the influence coefficients at every time step (unsteady analysis.)

Linearized theory, based on some theorems (Munk’s stagger theorem, etc.) are effective to determine the aerodynamics of an arbitrary arrangement of lifting lines.

Related Material

#### Selected References

• Cone CD. The Theory of Induced Lift and Minimum Induced Drag of Nonplanar Lifting Systems, NASA Report R-139, 1962.

• Ashley H, Landahl M. Aerodynamics of Wings and Bodies, Addison-Wesley Publ. Company, Reading, Mass. 1965.

• Schlichting H, Truckenbrodt E. Aerodynamics of the Airplane, McGraw-Hill, New York, 1979.

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