Large excrescences on transport aircraft: a cautionary tale

Extract

In recent years the increasing importance of electronic warfare has resulted in a proliferation of assorted antennae, large fairings and other foreign bodies attached to transport aircraft airframes. In many cases the size, shape and location of these excrescences are fixed by the requirements of the relevant electronic devices and it is then left to the engineers and aerodynamicists to attach them to the airframe in the least adverse manner.

This note describes an installation which was extensively tested in a large-scale commercial windtunnel and was found to give apparently well-behaved steady-state aerodynamic characteristics but which, when flight tested, exhibited unacceptable levels of airframe buffeting and rather unusual directional stability problems. A re-examination of the fuselage flowfield using a simplified model, but with attention focused on the unsteady aspects of the flow, rapidly identified the underlying cause and enabled a successful aerodynamic fix to be developed. The lessons learnt from this experience are presented as a warning to those contemplating the addition of large excrescences to any airframe.