Numerical and Experimental Investigation into the Aerodynamic Benefits of Rotorcraft Formation Flight

Abstract

The use of formation flight to achieve aerodynamic benefit as applied to rotorcraft is, unlike its fixed-wing counterpart, an unproven principle. This document presents a proof-of-concept of rotorcraft formation flight through a numerical research study, supported by results from an independent wind-tunnel experiment. In both cases, two helicopters are placed in an echelon formation aligned on the advancing side of the main rotor, though they do not simulate directly comparable flight conditions. The vertical and lateral alignment is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yields an estimated maximum total power reduction for the secondary aircraft of 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 observed for fixed-wing formations, which is contributed to the asymmetric upwash profile in the rotor wake. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas due to circular area effects. Experimental data shows an optimal vertical alignment of the secondary rotorcraft below the primary, due to wake displacement. This is not present in the numerical simulations as a result of the applied leader wake modeling.