Prediction and Simulator Verification of Roll/lateral Adverse Aeroservoelastic Rotorcraft-Pilot Couplings

The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a technique to predict the adverse aeroservoelastic rotorcraft–pilot couplings, specifically between a rotorcraft’s roll motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive lead–lag mode that strongly interacts with the roll mode at a frequency within the biodynamic band of the pilots. These predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse couplings experienced while using only rigid-body models in the flight simulator verified that the observed phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the observed experimental results indicates that the techniques developed in this paper can be used to highlight the proneness of new or existing designs to pilot-assisted oscillations

Experimental study of a helicopter model in shipboard operations

The paper presents the experimental investigation of the aerodynamic interaction between a helicopter model and a ship model with a simplified geometry. In the first phase of the experiment, a series of wind tunnel tests were carried out in order to study the flow features on the flight deck for several wind conditions, without the presence of the helicopter. Pressure measurements and Particle Image Velocimetry surveys were performed to assess the effect of wind velocity and direction on the flow field in the landing region over the ship deck. Moreover, the effect of the Atmospheric Boundary Layer was investigated. In the second phase of the experimental campaign, a helicopter model was positioned in a series of points representative of a typical stern landing trajectory and a vertical descent above the landing spot. The landing maneuver was performed in three different wind conditions, including no-wind, head wind and wind blowing from port side of the deck. The rotor loads and moments were measured by means of a six-axis balance for all test points. The use of different measurement techniques in the present experiments provides a comprehensive database suitable for the study of the rotor-ship aerodynamic interaction. Additionally, the experimental results are used to develop an identification algorithm to be incorporated into the flight simulator environment to capture the effect of ship airwake on the rotor loads during shipboard operations

Prediction and Simulator Verification of Roll/Lateral Adverse Aeroservoelastic Rotorcraft–Pilot Couplings

The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a technique to predict the adverse aeroservoelastic rotorcraft–pilot couplings, specifically between a rotorcraft’s roll motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive lead–lag mode that strongly interacts with the roll modeat a frequency within the biodynamic band of the pilots. These predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse couplings experienced while using only rigid-body models in the flight simulator verified that the observed phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the observed experimental results indicates that the techniques developed in this paper can be used to highlight the proneness of new or existing designs to pilot-assisted oscillation

Instability Mechanism of Roll/Lateral Biodynamic Rotorcraft–Pilot Couplings

The paper investigates the basic mechanism of aeroservoelastic pilot-assisted oscillation about the roll axis due to the interaction with pilot's arm biomechanics. The motivation stems from the observation that a rotor imbalance may occur as a consequence of rotor cyclic lead–lag modes excitation. The work shows that the instability mechanism is analogous to air resonance, in which the pilot's involuntary action plays the role of the automatic flight control system. Using robust stability analysis, the paper shows how the pilot's biodynamics may involuntarily lead to a roll/lateral instability. The mechanism of instability proves that the pilot biodynamics is participating in the destabilization of the system by transferring energy, i.e., by producing forces that do work for the energetically conjugated displacement, directly into the flapping mode. This destabilizes the airframe roll motion, which, in turn, causes lead–lag motion imbalance. It is found that, depending on the value of the time delay involved in the lateral cyclic control, the body couples with rotor motion in a different way. In the presence of small or no time delays, body roll couples with the rotor through the lead–lag degrees of freedom. The increase of the time delay above a certain threshold modifies this coupling: The body no longer couples with the rotor through lead–lag but directly through flap motion

Structural Coupling and Whirl-Flutter Stability with Pilot-in-the-Loop

This paper investigates structural coupling problems for tiltrotors, considering not only the interaction of the flight control system with the flexible structure but also the potentially adverse effects on the aeroservoelastic stability that may be caused by the pilot's involuntary, high-frequency, biodynamic response. The investigation is focused on the analysis of the side effects that could appear at high speed in the airplane flight regime, where the whirl flutter boundaries may be significantly reduced. A detailed tiltrotor model, representative of the Bell XV-15 and of a flight control system has been built and joined with a pilot biodynamic model acting on the power-lever and on the center stick, available in the literature. Additionally, a modified version of the XV-15 using differential collective pitch for yaw control in airplane mode instead of rudder has been investigated to show the effect of different yaw control designs. The stability analyses presented demonstrate that the structural coupling analysis and the flutter boundaries for tiltrotors must be evaluated not only considering the closed loop created by the flight control system but also the effect of involuntary pilot response. Sensitivity analyses examine the most critical parameters impacting tiltrotor aeroservoelastic stability. Finally, the employment of notch filters as a means of prevention is discussed

Helicopter Collective Bounce Proneness: Which are the Good, the Bad (and the Ugly!) Pilot Biometrics?

The interaction between the helicopter vibrations and the pilot involuntary control input can lead to the emergence of adverse, possibly even unstable, feedback loops. These phenomena are called Pilot-Assisted Oscillations (PAO). One of the most important is the "Collective Bounce", caused by vertical vibrations of the cockpit inducing an unwanted collective control input. On the rotorcraft side, the main rotor coning mode excitation has been shown to produce a phase margin reduction in the collective pitch-heave loop transfer function. On the pilot's side, biometrics such as stature, weight, age and sex are known to play a major role, but relatively limited effort has been placed in exploring the effects of their variability. This work represents a first attempt at filling the gap. A pseudo-random population of pilots, exhibiting different biometrics, is generated and the corresponding multibody biomechanical models are derived. The population is then simulated in a feedback loop with the rotorcraft dynamics and allowed to evolve, through a genetic (de-)optimization algorithm, towards the individuals most likely to be prone to instability. The result of the (de-)optimization process is the identification of the worst possible pilot biometrics with regard to collective bounce proneness on the modeled rotorcraft

Wing–Pilot Vertical Bounce in Tiltrotors

The basic mechanism of the vertical bounce in tiltrotors in hovering flight is discussed. This rotorcraft–pilot coupling phenomenon arises when the pilot’s biomechanics interact with the airframe elastic modes, in particular with the first symmetric wing bending mode. For this reason, it can be referred to as wing–pilot vertical bounce. This work proposes a simple mathematical model to predict the phenomenon. The XV-15 tiltrotor is used as benchmark. The closed-loop pilot–vehicle system shows that the direct effect of a change in collective input, through a vertical power lever, results in a nearly immediate change in thrust, which accelerates the aircraft, exciting the symmetric wing bending mode and, in turn, the pilot biomechanics, leading to a feedback path that could easily become unstable. Robust stability analyses are performed to take into account the large variability of some influential parameters. The tiltrotor shows a significant proneness to this rotorcraft–pilot coupling problem, which must be considered from the earliest phases of the design, especially when fly-by-wire architectures are considered. Means of prevention, considering both active and passive devices, are investigated and compared