Simulation of a rotor in forward flight to characterize helicopter wake signature and encounter severity

The development of safe and efficient air traffic separation policies relies on the accurate quantification of the wake turbulence generated by aircraft and rotorcraft, and on the accurate understanding of the upset experienced by a follower aircraft or rotorcraft if encountering such wakes. To address the existing gap in rotorcraft-specific studies, this paper first presents numerical simulations of the wake produced by a helicopter rotor in forward flight. The effect of the airspeed is investigated, from a high value of 100 kt to a low value of 12.5 kt. For each case, the wake is characterized at equivalent dimensionless ages, and the wake-induced hazard produced on a follower fixed-wing reference aircraft is quantified using the rolling moment coefficient (RMC) metric. At low airspeeds, the wake departs from the rolled-up wake configuration typical of fixed-wing aircraft and rotorcraft flying at high airspeed, exhibiting increased vortex spacing and higher turbulence levels. The inverse proportionality between flight speed and vortex circulation is also shown to break down; the RMC value saturates, and it also rapidly decreases downstream. In a second scenario, the rotorcraft is considered as a follower flying through a wake. For three airspeeds of the follower rotorcraft (100 kt, 50 kt, and 25 kt), simulations of the rotor flying quasi-steadily through rolled-up wake vortices are performed, and the resulting aerodynamic loads are quantified. The wake-induced pitching moment, which is the dominant perturbation, is found to scale with the airspeed of the follower rotorcraft. A pitching moment severity metric is evaluated against the numerical simulation: while it provides a useful first-order estimate, it fails to capture the observed speed dependence.