Flight control design for rotorcraft is challenging due to high-order dynamics, cross-coupling effects, and inherent instability of the flight dynamics. Dynamic inversion design offers a desirable solution to rotorcraft flight control as it effectively decouples the plant model and effectively handles non-linearity. However, the method has limitations for rotorcraft due to the requirement for full-state feedback and issues with non-minimum phase zeros. A control design study is performed using dynamic inversion with reduced order models of the rotorcraft dynamics, which alleviates the full-state feedback requirement. The design is analyzed using full order linear analysis and non-linear simulations of a utility helicopter. Simulation results show desired command tracking when the controller is applied to the full-order system. Classical stability margin analysis is used to achieve desired tradeoffs in robust stability and disturbance rejection. Results indicate the feasibility of applying dynamic inversion to rotorcraft control design, as long as full order linear analysis is applied to ensure stability and adequate modelling of low-frequency dynamics.

This paper examines the additional noise produced by a helicopter rotor during a short-time maneuver, which we call “transient maneuver noise.” A coupled flight simulation/noise prediction system was developed to predict the low-frequency noise generated during transient maneuvers. The system is partially validated for steady noise predictions in this paper. A simulation of a complex 80-second maneuver flight is performed and transient maneuver noise is identified both in the entry and exit of a coordinated turn. The characteristics of the transient maneuver noise were examined in detail by studying the turn entry. The noise generated during short-duration maneuvers is due to multiple sources: aperiodic blade motions, aircraft attitude changes, and transient aerodynamic loading. More aggressive maneuvers produce higher noise levels. The primary effect of a transient maneuver on thickness noise is to change the acoustic directivity, while both loading noise directivity and levels change significantly during aggressive maneuvers.

This article investigates vibrational stabilization effects in rotorcraft flight dynamics. This study is motivated by recent results in flapping-wing flight, which showed that the time-varying aerodynamic and inertial loads due to the insect wing periodic motion induce a vibrational stabilization mechanism in hover. The dynamics of flapping-wing flyers and rotary-wing vehicles are both described by time-periodic systems so vibrational stabilization mechanisms can also have an effect on stability characteristics of rotary-wing vehicles. The article extends the use of the harmonic decomposition method to vibrational stability analysis of rotorcraft. Two cases are considered: vibrational stability due blade imbalance at hover, and vibrational stability due to number-of-blades-per-rotor-revolution ( $$N_b$$N b /rev) in high-speed forward flight. Results show that while vibrations induced by rotor blade imbalance do not stabilize the hovering dynamics of a helicopter, these vibrations can still have a significant effect on the hovering dynamics. Rotor blade imbalance results in a symmetric effect on the roll and pitch axes, in that it tends to decrease the frequency of the subsidence modes of the hovering cubic, while the unstable oscillatory modes tend to increase in frequency and decrease in damping (destabilizing effect). On the other hand, the yaw/heave dynamics are relatively unaffected compared to the lateral and longitudinal axes. Moreover, $$N_b$$N b /rev rotor loads in forward flight are shown to reduce the damping of the coupled roll/pitch oscillation mode.

A marine hydrokinetic (MHK) cycloturbine is a renewable electric power generation system used in rivers or tidal environments to address the need for electricity in remote regions. MHK cycloturbines have hydrofoils oriented perpendicular to the flow in a paddlewheel configuration, and use lift generated from these foils to produce power. Due to the high cost associated with its operation and maintenance, an MHK system with four stacked counter-rotating turbines that can self-deploy and with propulsion and control mechanisms similar to a cycloturbine aircraft has been designed. A detailed turbine simulation model is necessary to understand the vehicle dynamics and assist in the design of the vehicle controllers. The simulation model solves the six degree-of-freedom rigid body equations of motion for the maneuvering MHK system subject to the hydrodynamic lift and drag forces, hydrostatic forces, and the propulsive forces from the turbines. The turbine propulsive force model is matched to computational fluid dynamics analysis and experimental data. Experimental work includes investigation at 1/5.56 scale of a single turbine rapid prototype device and a subscale demonstrator in a reverberant tank.

This brief paper proposes a dynamic data-driven method for stability monitoring of rotorcraft systems, where the underlying concept is built upon the principles of symbolic dynamics. The stability monitoring algorithm involves wavelet-packet-based preprocessing to remove spurious disturbances and to improve the signal-to-noise ratio (SNR) of the sensor time series. A quantified measure, called Instability Measure, is constructed from the processed time series data to obtain an estimate of the relative instability of the dynamic modes of interest on the rotorcraft system. The efficacy of the proposed method has been established with numerical simulations where correlations between the instability measure and the damping parameter(s) of selected dynamic mode(s) of the rotor blade are established.

Marine Hydrokinetic (MHK) cycloturbines generate sustainable power by exploiting tidal currents. By powering the turbines and using pitching foils for control, a vehicle comprised of MHK cycloturbines also has the ability to station keep and maneuver. The vehicle consists of four counter-rotating cycloturbines, with hydrofoils oriented perpendicular to the flow in a paddlewheel configuration. Lift and drag generated from these foils sum together to produce thrust. An experimentally tuned simulation model that solves the six-degrees-of-freedom rigid body equations of motion for the MHK vehicle subject to hydrodynamic, hydrostatic, and propulsive forces is used to aid the design of vehicle controllers. Global feedback controllers are initially designed by applying classical control methods to an approximate linear model of the system dynamics. A higher performing nonlinear controller is designed using the nonlinear dynamic inversion (NDI) method. NDI accounts for the nonlinearities of the MHK system and therefore is suitable for a wide range of operating conditions. The response of the classical and NDI controllers to speed, depth, roll, pitch, and yaw commands are evaluated and compared in simulation. The classical controller outperforms the NDI controller for small amplitude maneuvers, although the degradation with NDI is minor. However, in the nonlinear operating regime the NDI controller outperforms the classical controller and the classical controller exhibits instability.