The flow field and performance of the RAH-66 FANTAIL^ in hover and sideward flight were::alyzed using Computational Fluid Dynamic (CFD) techniques. The m u l t s were lLFed in the detailed design of the FANTAIL structure, blades, and transmission cooling system. A Navier-Stokes code developed for ducted fans by the first author was modified and employed. This analysis solves the incompressible, laminar NavierStokes equations in a n axisymmetric non-body fitted coordinate system to reduce problem set-up time. The fan was represented as fully-coupled, time-averaged momentum source terms in the momentum equations. Thii unique approach makes the code an ideal tool for the solution of detail design problems in a timely manner. Mmt exkting CFD formulations d o not have the capability to easily represent a fan installation. In this paper, the predicted RAH-66 FANTAU"' flow field and performance &re c o m p a d with test data to validate the analysis. The analysis a p p e a d to accurately caphlre the nature of the FANTAIL flow fields in all conditions including the very complex vortex ring state. Performance correlations showed good agreement with the m c a $ u d trends. Overall, the demonstrated accuracy was acceptable for analyzing many detailed design issues. A brief discussion of some specific design issues addrrssed using the CFD m u l t s is also provided.

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A new rotor blade Hp design c d e d the free-tip has been proposed as a means to improve forward night performance eharactelistir. and reduee oscillatory loads. The free-tip deslgn incorporates a tip Ulat is tke to pitch independently of the m t of the blade. Pltchiny! about an axis forward of the auarter-chord. the H a , ~~~~ ~~r weathervanes into Its I d wind, thus redueing angle if attack perturbatious and the resulting oscillatory lift loadings. A nearly constant nose-up Pitching moment is applied mechanicdv to the tin so that the tin. to . .., ~~ maintain pllchlng equilibrium, produces nearly steady positive lin around the-wirnuth. wind-tunnel test of a small-scale, 5.1 m dlamcter model rotor was conducled to obtain r~~rnparatlvc forward nlnhl wrformance and oscillatory loads data with the tips free and fixed. The free-tip was-shown to reduce ;we;in trimmed flight over a wide range of advance raHo and thrust; at an advance raHo of 0.3 and CL/v of 0.08 the reduction is 12%. Oscillatory napwise bendingmoments and oscillatory pitch linkloads are aLForeduced, hut the oscillatory in-plane bending moments increase. Notation a.c. = aerodynamic center c = reference chord, m c. = chord of the outboard end of the tip section, m c.g. = center of gravity C, , = tip drag coefficient, drag/O.SpS,Vz CF = centrifugal force of tip, or controller components, N C,, = tip lift coefficient, Iift/O.SpS,Vz C& = tip lift curve slope, per deg CL/U = rotor-lift coefficient, r~tor-liftlpS(flR)~ C,, = rate of change of pitching moment coefficient with angle of attack, per deg I = moment of inertia of the tip about the pitch axis, kg-m2

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