Reduction of Blade-Vortex Interaction (BVI) Noise through X-Force Control

Abstract

NASA Ames Resea~li Centec Moffen Field CA 94035-IW0Momentum theory and the longitudinal force balance equations o f a single rntor helicopter are used to develop simple expressions to describe tip-path-plane tilt and uniform inIlow to the rntor. The uniform inflow is adjusted to represent the inflow a t certain azimuthal Incations where strnng Blade-Vortex Interaction (BVI) is likely to occur. Thk theoretical mndel is then used to describe the fight conditions where BVI is likely to occur and to explore those fight variables that can he used to minimize BVI noise radiation. A separate X-force control is inhoduced to help minimize BVI noise. Several methods of generating the X-force are presented thal can be used to alter the inflow to the rntor and thus incrpsse the Likelihood of avoiding BVI during approaches to a landing.Nomenclature average profde drag coeficient of the rotor drag ofthe helicopter's fuselage and appendages, lb equivalent flat plate drag area, ft2 equivalent flat plate drag area of an aerodynamically generated x-force, ft2 constant X-force, lb gravitational acceleration, ftlsz H-force of the mtor, component of rotor resultant force parallel to the tip path plane, lb constant introduced to localize the induced velocity mass of the helicopter, slugs point on the disk where strong BVI occurs rotor radius, A thrust of the helicopter, lb rotor induced velocity, Ns nondimensional induced velocity, vN, velocity or airspeed, knots (or Ns) momentum theory hover induced velocity, fils rotor tip speed, ftls nondimensional velocity, VN, = acceleration parallel to the flight path, ftJsZ W = gross weight of the helicopter, lb aw, = tippath-plane angle of attack, deg y = flight path angle, deg V% = acceleration perpendicular to the flight path, radls Ay = equivalent change in flight path angle, deg A p p = mtor tippath-plane inflow, ftJs A = nondimensional inflow,AIvr Manuscript rcccivcd May 1996; accepted Scpt. 1997. p = advance ratio, VNT o = rotor solidity ty = blade azimuth angle, measured from downwind position in the direction of rotation, dcg

Flight-Path Management/Control Methodology to Reduce Helicopter Blade-Vortex Interaction Noise

“…1 This relationshipwas shown in an approximateway in Ref. 12 through the calculationof an average rotor in ow. When the average in ow to the rotor is near zero, strong BVI is likely.…”

“…12 Acceleration parallel to the ight path and X force simply determine a different trim location on the steady-state tip-path-plane angle vs velocity performance map shown in Fig. 3.…”

“…Although the Q-SAM method is general, as in Ref. 12, it is applied to the Bell Helicopter Textron's AH-1 Cobra helicopter for the nal calculations presented in this paper. Several constant velocity ight-path noise exposure pro les are explored.…”

“…It uses the performance modeling, developed by Schmitz,12 to approximatethe effect of acceleration and X force on the rotor tip-path-plane angle and coupling the tip-path-planeangle prediction with a variation of the RNM acoustic mapping method, 14 to project the noise to the ground. The result is a new prediction method called quasi-static acoustic mapping (Q-SAM).…”

Effects of Crosswind Flight on Rotor Harmonic Noise Radiation

“…Following Schmitz, (Ref. 26) the angle of attack of the tip-path-plane resulting from an "X-force" balance in the aerodynamic axis system can be expressed to first order as:…”