The mission of the Subsonic Rotary Wing Project is to advance knowledge and prediction capabilities for rotorcraft to enable efficient, low-noise, multi-mission flight. Rotorcraft capabilities will be exploited to fulfill expanding civil requirements. To this end, advanced rotary wing technologies, design, and analysis techniques will be pursued, enabling new configurations and missions to be developed efficiently and confidently.

Research is focused on areas that industry and other government agencies are not pursuing or cannot pursue alone. Subsonic rotary wing research needs consist of efficiency, including aerodynamic performance and structural weight; productivity, which requires high speed, large payload, long range, and good maneuverability; and environmental acceptance, particularly noise and handling qualities. Without intending to predict where the design process will lead when truly effective design and analysis tools are available, some very promising (and very challenging) configurations can be identified to drive the required fundamental research. The challenges faced in rotary wing aviation are among the most complex and demanding of any configuration: highly complex, three-dimensional rotor and fuselage structures, unsteady flows in speed regimes from low subsonic to high transonic, dynamically stalled components, harsh operating environments, highly loaded propulsion systems, and a vehicle that is statically unstable.

The Subsonic Rotary Wing Project will focus its research efforts in the most persistent technical challenge areas, producing advances in prediction tool capability and technology in order to expand the role of rotorcraft in civil aviation.

Subsonic Rotory Wing Project Key Research Areas:

Rotorcraft Propulsion

Rotorcraft Flight Dynamics and Controls

Rotorcraft Aeromechanics and Fluid Mechanics

Rotorcraft Acoustics

Rotorcraft Structures and Advanced Materials

Rotorcraft Experimental Capability

Rotorcraft Propulsion

Rotorcraft propulsion is a critical element of the overall aircraft. Unlike fixed wing aircraft, the rotor/propulsion system is used for aircraft lift, forward flight, and maneuvering. As a result, the rotorcraft engine/gearbox system must be highly reliable and efficient. Future rotorcraft will be more versatile, efficient, and powerful aircraft that will challenge state-of-the-art propulsion system technologies. Advanced tools and methodologies must be developed that enable industry to more accurately and effectively develop and design new engine and drive system technologies. The overall approach for meeting future rotorcraft propulsion challenges is to focus on the following key research areas: variable/multi-speed drive system technologies; improved drive system analytical tools; optimized gearbox/engine system; oil-free engine technology; wide-operability engine technology; and efficient, high-power-density engine technology.

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Rotorcraft Flight Dynamics and Controls

Flight dynamics and controls for rotorcraft pose unique challenges due to the inherent instabilities of the flight vehicle, the aerodynamic and mechanical complexity of the system, and the operational environment, which is often obstacle-rich with poor visibility at low altitude. As new designs emerge, such as individual blade control (IBC), on-blade control (OBC), variable rotor speeds, and heavy-lift designs, it is essential that control of flight and the capabilities of the human pilot be integrated in the design process. Fundamental research must be conducted to address the implications of higher-bandwidth control arising from IBC and OBC concepts, mitigation of dynamic effects resulting from rotor speed changes, as well as the reduced control response inherent to larger rotorcraft. This research will improve the predictive models for aircraft dynamics and address the human performance implications of alternative human/system interface designs. A key activity in this discipline is developing an integrated, broadband rotorcraft control system incorporating a flight control system, engine control, airframe/drive-train/rotor-load control, active rotor control of vibration and noise, vehicle health management, and guidance for low-noise operation.

This research area is an integration of activities from three disciplines: flight dynamics and control, acoustics, and propulsion. Work in flight dynamics and control will emphasize developing an integrated solution of handling qualities and dynamics for problems such as a variable-speed rotor. Precision guidance, navigation, and control capabilities will also be developed to enable data collection and evaluation for rotorcraft flight experiments, including studies of acoustic properties, vehicle dynamics modeling, noise reduction, and terminal-area operations.

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Rotorcraft Aeromechanics and Fluid Mechanics

Research in rotorcraft aeromechanics comprises the study of isolated and multidisciplinary aerodynamic and dynamic phenomena, including aerodynamic performance, airloads and wakes, interactional aerodynamics, rotor loads, vibration, and aeroelastic stability. Many of these phenomena are poorly understood and remain unsolved. For example, a lack of fundamental knowledge of fluid flow phenomena limits the ability to accurately model and calculate unsteady, compressible, three-dimensional aerodynamics needed for performance and loads predictions. Particularly difficult are highly nonlinear convective wake and separated-flow phenomena. There are also important deficiencies in rotorcraft structural dynamics phenomena and multidisciplinary interactions between aerodynamics, structures, engine drive trains, and control systems. Finally, there are new concepts under development within the government and industry to improve rotorcraft aerodynamic and dynamic characteristics. These include concepts such as active-flow control on fuselages (and other fixed surfaces) for drag reduction and active on-blade control (including flow control as well as mechanical controls) for performance improvement and noise, vibration, and load alleviation. Current analysis methods are inadequate to capture the complexity of active-rotor response, and must be further validated and improved before adequate design decisions may be made.

Based on current expertise and support from government and industry partners, NASA will focus its aeromechanics research in the areas of aerodynamics, dynamics, and active controls. The primary goal of this program will be to increase the fundamental understanding of the phenomena and to develop and validate appropriate first-principle-based analysis tools. Experimental test data will be essential to help develop and validate the accuracy of the analysis methods. Existing data will be used where appropriate and new experiments will be undertaken to acquire additional test data where needed. It is anticipated that a broad range of experiments will be needed, including small- and large-scale testing. It will be necessary to obtain high-quality, accurate data of sufficient resolution to discriminate important local aerodynamic phenomena.

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Rotorcraft Acoustics

Acoustics research consists of interdependent task areas that concentrate on the development of validated prediction tools and capabilities for the rotorcraft vehicle system as well as validated prediction tools for the components, i.e., rotor, engine, interior noise, propagation, and acoustic scattering. System capabilities combine the components so that rotorcraft source noise and its propagation can be investigated for noise impact due to rotor design and/or rotorcraft operations and procedures. Tools will cover a range of fidelity, in that they will include first-principle computational fluid dynamics (CFD) and computational structural dynamics (CSD) capabilities and validated analysis tools that are also appropriate for integration into multidisciplinary predictive capabilities for development and assessment of noise technologies and noise mitigation procedures.

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Rotorcraft Structures and Advanced Materials

Work within the structures and materials discipline is focused on unique technology needs for future rotorcraft in the areas of durable propulsion materials, lightweight structure, and advanced acoustic materials/structures. Research on propulsion materials is focused on the need for materials that can withstand harsh cyclic loading and erosion conditions. This research will be coordinated with the propulsion discipline. Research on lightweight structures is focused on two areas: 1) the need for fatigue resistance and damage tolerance in extremely lightweight structures that are sometimes allowed to buckle in service; and 2) the need for reliable crash-simulation tools that will reduce reliance on expensive full-scale testing for evaluation of new materials and structural concepts to improve crashworthiness. Research on advanced acoustic materials/structures is focused on developing and evaluating materials with improved acoustic performance and integration of these materials into lightweight rotorcraft structures. This research will be coordinated with research in the acoustics discipline.

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Rotorcraft Experimental Capability

The experimental capabilities (ExCap) discipline develops and applies rotorcraft-specific measurement techniques to enable the acquisition of benchmark-quality validation data for rotorcraft analyses in aeromechanics, acoustics, flight dynamics and control, and propulsion. This research concentrates on the development and implementation of functionally independent instrumentation systems for the acquisition of experimental data in laboratory, wind tunnel, or flight tests for validation of rotorcraft predictive codes. Each area involves the development, testing, and implementation of specialized instrumentation systems that bridge technology gaps in current rotorcraft testing capability. Key challenges addressed in the discipline include measuring rotor wake, blade airloads, blade deformation, in-flight rotorcraft state, and high-temperature, high-pressure sensors for drive-train applications.

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