Numerical and experimental investigation into the aerodynamic benefits of rotorcraft formation flight

Abstract

The use of formation flight to achieve aerodynamic benefit applied to rotorcraft has, unlike its fixed-wing counterpart, received little attention in the literature. This document presents a proof-of-concept of rotorcraft formation flight from two independent investigations: a numerical study of a fully articulated helicopter influenced by an upstream helicopter wake and a wind-tunnel experiment featuring two small-scale helicopter models with fixed-pitch blades. Both cases feature a representation of two helicopters in a diagonal, staggered formation aligned on the advancing side of the main rotor, but do not simulate directly comparable flight conditions. The vertical and lateral alignment of the two helicopters is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yield an estimated maximum total power reduction for the secondary aircraft of approximately 24%, while the numerical models yield reductions between 20% and 34% dependent on flight velocity. Both experiments predict a higher potential for aerodynamic benefit than generally observed for fixed-wing formations, which is attributed to the asymmetric velocity profile induced by the wake of the upstream rotor. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas, rather than tip-to-tip alignment, as a result of the circular rotor disk area. Experimental data show an optimal vertical alignment of the secondary rotorcraft below the primary, due to the self-induced vertical displacement of the rotor wake, which is absent from the numerical results due to the application of a flat wake assumption. The results show a promising potential for rotorcraft formation flight, though due to the limited nature of the models used, conclusions cannot be generalized. The potential aerodynamic benefit indicated by the present study invites further research in the field of rotorcraft formation flight.