Resolution Of Tail Buffet Phenomena For AH-64D™ Longbow Apache™: A Case Study On The Application Of A Modern CFD Analysis Tool For Improved Helicopter Aerodynamic Design

A cooperative numerical investigation involving researchers from McDonnell Douglas Helicopter Systems (MDHS), the US Army Aeroflightdynamics Directorate (AFDD) and the NASA Ames Research Center (ARC) was conducted to identify the cause, as well as the solution(s), to a pilot-reported horizontal and vertical tail buffeting problem on the Longbow Apache AH-64D helicopter in low speed descent flight. The investigation was conducted using the NASA Ames steady, three-dimensional, compressible Euler code, "TIGER". With the exception of the rotor, all geometric details of the flight test aircraft were modeled. Grid adaption of a cartesian computational grid near and on the surface of the configuration was used to preserve geometric integrity and solution accuracy. Flowfield diagnostics were performed using a combination of streamwise particle trajectories and cross flow vorticity plots. Numerical results indicated that two potential contributors, namely the blunt bases of the two side-mounted EFABS and the bases of the two channel-like grooves created as a result of the attachment of the EFABS to the fuselage, were responsible for the shedding of high levels of vorticity which convected downstream and impacted the surfaces of the empenage. To reduce the observed levels of shed vorticity, three geometric modifications to the baseline helicopter fuselage configuration were made. A flight test program for the helicopter with a number of suggested modifications was conducted to verify their effectiveness on the handling qualities of the aircraft. Numerical results and supporting flight test data are presented.