Trim investigation for coaxial rigid rotor helicopters using an improved aerodynamic interference model

摘要

The coaxial rigid rotor helicopter has been proposed as a future high-performance rotorcraft concept. However, the aerodynamic interference of this helicopter is complicated because it couples with the unique flapping feature of the rigid rotor, which further alters the trim characteristics of coaxial rigid rotor helicopters. Thus, a multi-point vortex ring element (MVRE) model is developed to simulate the aerodynamic interference between rotors. The method for establishing this MVRE model is illustrated, and a wind tunnel experimental dataset is used to assess its precision hover and forward flight states. Next, a flight dynamics model of the coaxial rigid rotor helicopter is built based on the MVRE aerodynamic interference model and the flapping feature of the rigid rotor. The influence of the rotor wake on the fuselage and the horizontal and vertical tails can also be calculated using this model. The trim characteristics of this helicopter are evaluated with flight test data for speeds ranging from 0 m/s to 80 mks, and the results confirm that this model can reflect the trim characteristics with satisfactory precision. In addition, the calculation process demonstrates that the MVRE model provides a much faster computing rate. Considering the aerodynamic interference and rigid rotor characteristics, the trim results of the coaxial rigid rotor helicopter present unique features: aerodynamic interference in the coaxial rotor system not only increases the collective pitch and the collective differential but also adds a negative gradient under the forward speed in the longitudinal cyclic pitch in the low-speed forward flight range. Moreover, the rotor wake effect on the other parts of the helicopter is distinct from the corresponding effects on conventional helicopters in terms of the trim characteristics. (C) 2018 Elsevier Masson SAS. All rights reserved.