Results of NASA/FAA ground and flight simulation experiments concerning helicopter IFR airworthiness criteria

A sequence of ground and flight simulation experiments was conducted to investigate helicopter instrument-flight-rules airworthiness criteria. The first six of these experiments and major results are summarized. Five of the experiments were conducted on large-amplitude motion base simulators. The NASA-Army V/STOLAND UH-1H variable-stability helicopter was used in the flight experiment. Artificial stability and control augmentation, longitudinal and lateral control, and in pitch and roll attitude augmentation were investigated

Techniques for designing rotorcraft control systems

This report summarizes the work that was done on the project from 1 Apr. 1992 to 31 Mar. 1993. The main goal of this research is to develop a practical tool for rotorcraft control system design based on interactive optimization tools (CONSOL-OPTCAD) and classical rotorcraft design considerations (ADOCS). This approach enables the designer to combine engineering intuition and experience with parametric optimization. The combination should make it possible to produce a better design faster than would be possible using either pure optimization or pure intuition and experience. We emphasize that the goal of this project is not to develop an algorithm. It is to develop a tool. We want to keep the human designer in the design process to take advantage of his or her experience and creativity. The role of the computer is to perform the calculation necessary to improve and to display the performance of the nominal design. Briefly, during the first year we have connected CONSOL-OPTCAD, an existing software package for optimizing parameters with respect to multiple performance criteria, to a simplified nonlinear simulation of the UH-60 rotorcraft. We have also created mathematical approximations to the Mil-specs for rotorcraft handling qualities and input them into CONSOL-OPTCAD. Finally, we have developed the additional software necessary to use CONSOL-OPTCAD for the design of rotorcraft controllers

Variable-speed rotor helicopters: Performance comparison between continuously variable and fixed-ratio transmissions

Variable speed rotor studies represent a promising research field for rotorcraft performance improvement and fuel consumption reduction. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. There are two main variable speed concepts, depending on the type of transmission employed: Fixed Ratio Transmission (FRT) and Continuously Variable Transmission (CVT) rotors. The impact of the two types of transmission upon overall helicopter performance is estimated when both are operating at their optimal speeds. This is done by using an optimization strategy able to find the optimal rotational speeds of main rotor and turboshaft engine for each flight condition. The process makes use of two different simulation tools: a turboshaft engine performance code and a helicopter trim simulation code for steady-state level flight. The first is a gas turbine performance simulator (TSHAFT) developed and validated at the University of Padova. The second is a simple tool used to evaluate the single blade forces and integrate them over the 360 degree-revolution of the main rotor, and thus to predict an average value of the power load required by the engine. The results show that the FRT does not present significant performance differences compared to the CVT for a wide range of advancing speeds. However, close to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by FRT, stating that significant fuel reductions can be obtained only by employing the CVT concept. In conclusion, FRT is a good way to reduce fuel consumption at intermediate advancing speeds; CVT advantages become relevant only near hover and high speed cruise condition

Aeronautical Engineering: A continuing bibliography, supplement 120

This bibliography contains abstracts for 297 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1980

Engine/airframe interface dynamics experience

Problems of engine/drive system torsional stability, engine and output shaft critical speeds, and engine vibration at helicopter rotor order frequencies are discussed, and test data and analyses presented. Also presented is a rotor/drive system dynamics problem not directly related to the engine

Robust nonlinear control of vectored thrust aircraft

An interdisciplinary program in robust control for nonlinear systems with applications to a variety of engineering problems is outlined. Major emphasis will be placed on flight control, with both experimental and analytical studies. This program builds on recent new results in control theory for stability, stabilization, robust stability, robust performance, synthesis, and model reduction in a unified framework using Linear Fractional Transformations (LFT's), Linear Matrix Inequalities (LMI's), and the structured singular value micron. Most of these new advances have been accomplished by the Caltech controls group independently or in collaboration with researchers in other institutions. These recent results offer a new and remarkably unified framework for all aspects of robust control, but what is particularly important for this program is that they also have important implications for system identification and control of nonlinear systems. This combines well with Caltech's expertise in nonlinear control theory, both in geometric methods and methods for systems with constraints and saturations

Aerodynamic Response of a Hovering Rotor to Ramp Changes in Pitch Input

Under transient conditions, a helicopter rotor generates a complex, time-dependent pattern of shed and trailed vorticity in its wake that has profound eects on its loading. To examine these eects, the response of a two-bladed hovering rotor to a ramp change in collective pitch is investigated using three dierent computational approaches. Solutions obtained using a Compressible Reynolds Averaged Navier{Stokes ap- proach are compared to results obtained from lifting-line theory coupled to an Eulerian Vorticity Transport Model, and from a simple single-state dynamic in ow model. The dierent numerical approaches yield very similar predictions of the thrust response of the rotor to ramp changes in collective pitch, as long as the ramp rates are small. This suggests that the basic underlying ow physics is properly represented by all the approaches. For more rapid ramp rates, an additional delay in the aerodynamic response of the rotor, that is related to the nite extent of the wake during its early history, is predicted by the Navier{Stokes and Vorticity Transport approaches. Even though the evolution of the wake of the rotor is strongly three dimensional and highly unsteady, the predictions of the Navier{Stokes and lifting-line models agree very closely as long as the blades of the rotor do not stall. In the pre-stall regime, a quasi two-dimensional representation of the blade aerodynamics thus appears adequate for predicting the performance of such systems even under highly transient conditions. When ow separation occurs, the resulting three dimen- sionality of the blade aerodynamics forces the predictions of the Navier{Stokes and lifting-line approaches to diverge, however. The characterization of the wake interactions and stall propagation mechanisms that are presented in this study oers some insight into the fundamental uid dynamic mechanisms that govern the transient aerodynamic response of a rotor to control inputs, and provides some quantication of the limits of applicability of some popular current approaches to rotor aerodynamic analysis

Aerodynamic response of a hovering rotor to ramp change in pitch input

Under transient conditions, a helicopter rotor generates a complex, time-dependent pattern of shed and trailed vorticity in its wake that has profound effects on its loading. To examine these effects, the response of a two-bladed hovering rotor to a ramp change in collective pitch is investigated using three different computational approaches. Solutions obtained using a Compressible Reynolds Averaged Navier-Stokes approach are compared to results obtained from lifting-line theory coupled to an Eulerian Vorticity Transport Model, and from a simple single-state dynamic inflow model. The different numerical approaches yield very similar predictions of the thrust response of the rotor to ramp changes in collective pitch, as long as the ramp rates are small. This suggests that the basic underlying flow physics is properly represented by all the approaches. For more rapid ramp rates, an additional delay in the aerodynamic response of the rotor, that is related to the finite extent of the wake during its early history, is predicted by the Navier-Stokes and Vorticity Transport approaches. Even though the evolution of the wake of the rotor is strongly three dimensional and highly unsteady, the predictions of the Navier-Stokes and lifting-line models agree very closely as long as the blades of the rotor do not stall. In the pre-stall regime, a quasi two-dimensional representation of the blade aerodynamics thus appears adequate for predicting the performance of such systems even under highly transient conditions. When flow separation occurs, the resulting three dimensionality of the blade aerodynamics forces the predictions of the Navier-Stokes and lifting-line approaches to diverge, however. The characterization of the wake interactions and stall propagation mechanisms that are presented in this study offers some insight into the fundamental fluid dynamic mechanisms that govern the transient aerodynamic response of a rotor to control inputs, and provides some quantication of the limits of applicability of some popular current approaches to rotor aerodynamic analysis

An exploratory investigation of the flight dynamics effects of rotor rpm variations and rotor state feedback in hover

This paper presents the results of an analytical study conducted to investigate airframe/engine interface dynamics, and the influence of rotor speed variations on the flight dynamics of the helicopter in hover, and to explore the potential benefits of using rotor states as additional feedback signals in the flight control system. The analytical investigation required the development of a parametric high-order helicopter hover model, which included heave/yaw body motion, the rotor speed degree of freedom, rotor blade motion in flapping and lead-lag, inflow dynamics, a drive train model with a flexible rotor shaft, and an engine/rpm governor. First, the model was used to gain insight into the engine/drive train/rotor system dynamics and to obtain an improved simple formula for easy estimation of the dominant first torsional mode, which is important in the dynamic integration of the engine and airframe system. Then, a linearized version of the model was used to investigate the effects of rotor speed variations and rotor state feedback on helicopter flight dynamics. Results show that, by including rotor speed variations, the effective vertical damping decreases significantly from that calculated with a constant speed assumption, thereby providing a better correlation with flight test data. Higher closed-loop bandwidths appear to be more readily achievable with rotor state feedback. The results also indicate that both aircraft and rotor flapping responses to gust disturbance are significantly attenuated when rotor state feedback is used

The application of nonlinear control theory to robust helicopter flight control.

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