Bruce's Technical Papers

The past half century has seen the evolution of rotorcraft flight controls from early mechanical systems to modern redundant hydraulic systems with full-authority digital flight control computers. Despite the recent cancellation of the RAH-66 Comanche program, the future of rotorcraft fly-by-wire technology looks brighter than ever, with fly-by-wire in production or development on the V-22, NH-90, BA-609, UH-60, AH-64, Mi-38, and S-92, among others. Drawing on the personal experiences of two project test pilots and several design engineers, the paper describes the development of the important elements of successful fly-by-wire systems.

A description is presented of an active controller designed to augment the air resonance stability of the Boeing-Sikorsky RAH-66 Comanche. The controller was successfully flight-tested on the RAH-66 demonstrator aircraft. An active feedback scheme was simple and cost effective to implement as flight control software using existing flight control hardware: Body pitch and roll rate signals were used to calculate swashplate control inputs that enhance rotor damping. The RCAS computer program (Rotorcraft Comprehensive Analysis System) was used to perform the detailed design of the active feedback. Extensive correlation was first performed between RCAS and the baseline aircraft for key flight conditions. Several feedback designs were developed and the best were selected for flight-testing. The flight test approach is described and RCAS predictions are compared with flight test results with and without active control.

This paper summarizes initial results from the integration of the CHARM free-wake module into two nonlinear flight dynamics simulations. It is shown that for flight dynamics applications, the first-principles free-wake model captures all of the known important results of the widely used Pitt-Peters model for the effects of the rotor induced velocity on the rotor itself. Preliminary results also indicate the potential of the free-wake to accurately represent other important effects which are beyond the capability of current flight dynamics models, including interactions of the main rotor wake with the empennage. Fruitful avenues for ongoing development are also discussed.

In late 2002 and early 2003, the RAH-66 Comanche first flew with the Altitude Hold and Velocity Stabilization/Hover Hold (“VELSTAB”) selectable control modes. These successful milestones are the result of more than ten years of developmental effort, much of which was performed by the Boeing/Sikorsky team with the aid of Boeing Philadelphia’s simulation and linear models. This paper presents some of the Comanche flight test data that both validate the disturbance rejection and stability robustness of the selectable modes design and verify the fidelity of the linear and simulation models.

Despite decades of very successful yaw-control and anti-torque applications, the aerodynamics of ducted rotors in low-power, near-edgewise flow conditions are not well understood. Motivated by phenomena discovered during the development of the RAH-66 Comanche’s directional axis control laws, a research program was initiated to use CFD to improve the understanding of the dynamic relationship between ducted rotor thrust and applied collective pitch, especially when the rotor is operating in near-edgewise flight conditions. This paper is a presentation of the initial results of this study. Numerical solutions of the inviscid Euler equations were obtained for the flow over the Comanche fuselage with a uniform actuator disk and blade element models for the FANTAIL TM ; the main rotor is excluded in this study. The solutions were obtained by running a modified PUMA 2 (Parallel Unstructured Maritime Aerodynamics) computational fluid dynamics code with an unstructured grid with 2.8 million tetrahedral cells. Excellent correlation between the calculations and a variety of static test data are presented and discussed. Subsequent efforts will examine the important aspects of the dynamics of the thrust response, and allow further comparisons with flight test data.

An updated and expanded version of the results presented in this paper were also published as a 2-part series in the AIAA Journal of Aircraft in 2004, Volume 41, Numbers 5 and 6.

This paper describes the development of the Core Automatic Flight Control System (CAFCS) on the RAH-66 Comanche during the first five years of prototype flight-testing. Much of the control law development occurred within a strongly analytical framework provided by the combination of the explicit model-following architecture of the CAFCS and the quantitative requirements of ADS-33D, Section 3. Nevertheless, direct pilot involvement was also found to be invaluable, and numerous updates to the system parameters and control-law architecture were implemented in direct response to issues associated with qualitative piloted evaluations. The important elements of the control law changes and their relationships to the ADS-33D requirements and to pilot comments are explored in detail.

Tethered hover is a standard test technique used to establish the variation of hover power with gross weight, and was an important aspect of the recent successful completion of Milestone II exit criteria for the RAH-66 Comanche program. Using theory, numerical analysis, piloted simulation, and flight test data, this paper considers a variety of flight mechanics issues related to the conducting of the tethered hover testing on Comanche. The paper begins with a brief review of the requirements and procedures for conducting tethered hover. The development of a simple but representative model of the cable, for use in both the linear stability analysis and the nonlinear real-time simulation, is then presented. Finally, a survey of flight mechanics issues is presented, including cable engagement loads, vehicle trim characteristics, low-frequency “pendular” modes, verticalbounce dynamics, rotor-body coupling (regressing lag mode), and elastic-fuselage effects.

This thesis considers high-frequency modelling of the directional axis dynamics of single-rotor helicopters, with particular attention to those vehicles which achieve antitorque and directional control using a ducted tail rotor. Although the work was initiated as part of the development of the first Boeing-Sikorsky RAH-66 Comanche prototype aircraft, the thesis presents insights into problems of general relevance to the design and analysis of high-bandwidth rotorcraft directional-axis feedback systems. A new analysis of the unsteady thrust response of a ducted tail rotor in an arbitrary flight condition is developed and correlated to the RAH-66 Comanche’s FANTAIL behavior using a variety of flight and ground test data. The general problems of airframe elastic response modelling and identification from flight data are considered and applied to Comanche flight data. A simplified analytical model which captures the salient effects of a flexible fuselage is developed and applied to the prediction of closed-loop stability robustness in the presence of these effects. Analytical insights into the three-dimensional nature of a coupled elastic drive system model are discussed. Finally, the implications of the new results for control law design are discussed.

Air resonance has been observed on the RAH-66 Comanche prototype aircraft in the form of a lightly damped lateral axis dynamic response. The frequency of lateral axis oscillations is coincident with the Bearingless Main Rotor (BMR) regressing lag mode. This paper describes the mechanisms of the rotor-body coupling and predicts the effects of body roll rate and roll acceleration feedback on rotor stability and lateral axis response. A body roll feedback design concept is presented which uses roll rate filtered to resemble roll acceleration near the regressive lag mode frequency. This design predicts a reduction in rotor-body coupled roll oscillation with no predicted degradation in rotor stability. Results of Comanche flight testing of the feedback design are presented which show improved roll damping and no adverse stability impact. Flight testing techniques for monitoring dynamic stability and incrementally testing the feedback design change are described.

The RAH-66 Comanche linear aeroservoelastic (ASE) model provides concurrent handling qualities and closed-loop, high-frequency stability analyses. Using analytical models, flight and ground test data, and numerical model predictions, this paper describes new insights into problems of general relevance to the design and analysis of high-bandwidth rotorcraft which have come about as a result of the ASE model development effort. Empirical adjustments of the pitch-lag coupling of the bearingless main rotor to improve correlation with test data are presented. A new analysis of the unsteady thrust response of a ducted rotor in an arbitrary flight condition is developed and correlated to the Comanche’s FANTAIL™ behavior using a variety of test data. Improvements to the prediction of the fuselage elastic response based on flight data are discussed and the impact on predicted system stability margins is presented. Analytical insights into the three- dimensional nature of a coupled elastic drivesystem model are discussed.

It has been known for some time that the prediction of rotor forces and moment for flight dynamics applications is sensitive to the specific assumptions employed in modelling aerodynamics. This paper reviews several critical aspects of the most widely used models and the suggested extensions to them, with an emphasis on the prediction of helicopter control response.