A review of recent developments in helicopter noise reduction is presented. The paper includes a discussion of the present state of understanding of helicopter noise radiation mechanisms and a review of actual noise reductions achieved by the manufacturers over the past decade. An analysis of progress achieved in meeting certification limits is given, together with an evaluation of the impact on design. Future trends in helicopter noise reduction based on current noise research programmes are discussed and suggestions advanced for possible additional areas of study.

The application of the unstructured mesh system FLITE3D.0 to the numerical analysis of the inviscid Euler flow past installed aero-engine nacelle is described. The main features of the system are detailed and the approach is demonstrated by simulating the flow over a Rolls-Royce model of a twin-engined civil transport aircraft with long-cowl nacelle powerplants. The computed pressure distributions are compared with the measurements produced by transonic wind tunnel experiments.

A theoretical model had been previously developed to predict the behaviour of a semi-rigid helicopter rotor blade during rotor engagement and disengagement under high wind conditions. This has now been extended to cater for an articulated rotor by modelling the droop and anti-flap stop mechanisms by high rate linear springs.. Much increased blade deflections are predicted, as anticipated, along with the possibility of blade tip/tail boom contact at a high enough wind speed. The importance of the higher modes in the description of the phenomenon is demonstrated. A comparison of the theoretical model with an experiment using a full size Puma aircraft is presented.

Some of the experimental features of the dynamic response of a cantilever subjected to two streams of air at right angles to each other are reported. The cantilever is mounted across a mainstream flow while subjected to another jet injected parallel to the test cylinder.

The dimensionless vibration displacement amplitude was measured in the transverse direction (lift force direction) as well as in the streamwise direction (drag force direction) while varying the secondary jet flow velocity. It was found that in the vicinity of the cylinder a secondary jet can be used to reduce the dimensionless transverse vibration displacement amplitude in the lock-in region and increase it outside the lock-in region. The effect of jet injection on the streamwise direction vibration is to increase it for all reduced velocities considered.