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Fabrication and Testing of High-Speed Single-Rotor and Compound-Rotor Systems

Acquiring the data needed to generate a comprehensive set of measurements of the blade surface pressures, pitch link loads, hub loads, rotor wakes and performance of high-speed single-rotor and compound-rotor systems necessary to support the development of next-generation rotorcraft, such as those envisioned in the Joint Multi- Role (JMR) rotorcraft program.

Slowed rotors – traditionally associated with autogyros and gyroplanes – have long been recognized as one potential solution for high-speed helicopters (200-300 knots). During the 1950s–70s, there were several significant programs that led to the development of high-speed helicopters with thrust and lift compounding. The key technology barriers common to all were extremely high fuel consumption due to high advancing side drag and large reverse flow, complexities associated with RPM reduction, large blade motions during RPM reduction, and unexplained but catastrophic aeroelastic instabilities of rigid rotors (Cheyenne). None of these helicopters entered regular production.

Today, the CarterCopter gyroplane is the only aircraft to have demonstrated a rotor advance ratio of 1.0 in flight in 2005. With the advancement of materials, controls, and propulsion/drivetrain technologies (15-20% direct variation in RPM possible with same nominal specific fuel consumption and more dramatic reduction promised with variable drivetrain), slowed rotors have once again begun to emerge as a viable solution to high-speed, high-efficiency helicopters of the future (along with tilt-rotors and lift-offset coaxial compounds). The intent is the fundamental understanding of such rotors, using both analysis and experiment at the very high-advance ratio reverse flow conditions they are envisioned to operate in (μ~1.5-2.0 and beyond).

Compared to conventional helicopters, there are only a handful of limited experimental measurements available, which are neither sufficient for fundamental understanding of their aeromechanics nor adequate for validating high-fidelity analyses that hold promise of predicting them. The only existing data set that includes performance, pressures and loads are the recent full-scale UH-60A tests – but this data is only up to μ=1.0. Model-scale tests performed recently achieve higher advance ratios (up to μ=2.2), but with simple blades (symmetric NACA0012 airfoil, untwisted) in autorotation or lower advance ratios (up to μ=1.0) with realistic blades (asymmetric SC1095, twisted) and powered conditions – but all focused mostly on performance measurements and fall far short from being comprehensive. Similarly, discrepancies in analyses identified both in lifting-line and CFD have still not been systematically addressed due to the scarcity of reliable and comprehensive test data. Thus, both lack of experimental data and validated analyses can become significant technical barriers towards effective and efficient use of slowed rotors in the development of next-generation high-speed compound rotorcraft.

Coaxial compound has emerged as one of several potential solutions for high-speed rotorcraft - along with tilt-rotors and slowed-rotor compound – since the successful resolution of critical technology shortcomings associated with the earlier XH-59A demonstrator. These shortcomings – low efficiency/high fuel consumption, high empty weight fraction, high vibration, and challenges associated with reducing rotor speed – have now been mitigated in the X2 Technology Demonstrator by innovative use of modern technologies such as advanced airfoils (double-ended at root and super-critical at tip), advanced materials (titanium to graphite-epoxy blades), active vibration control in the fixed-frame, and advanced propulsion (high efficiency pusher propeller instead of turbojet thrust).

The potential of modern refined analytical tools that have matured over the last fifteen years – if brought to bear on this advanced coaxial rotor system – can bring about dramatic improvements in its capabilities. Some of the current technical challenges are: (1) reduced efficiency due to large reverse flow area in high-speed flight (80% of retreating side, at μ=0.8, 20% rpm reduction, 250 knots), (2) high weight penalty due to active force generators in the fixed-frame to cancel the high vibration levels due to stiff blades, and (3) weight and drag penalty due to high root stresses as well as a large hub. Recently, the X2 Technology Demonstrator achieved a level flight speed over 250 knots, proving the basic concept of the coaxial compound configuration. However, the rotor of this aircraft was designed assuming an equivalent single rotor in conjunction with blade element momentum theory. Further development of this concept and optimization of its performance requires refined analytical tools and detailed test data. It becomes necessary to generate a benchmark set of experimental hub loads; blade surface pressures; and rotor wake velocity measurements with which the analytical tools can be validated.

This work was done by Inderjit Chopra of the University of Maryland - College Park for the Army Research Office. For more information, download the Technical Support Package (free white paper) below. ARL-0242

Topics:
Aerodynamics Aeronautics Aerospace Aircraft Aircraft structures Automation Aviation Command, Control, and Navigation Systems Drag Fabrication Fluid Handling Fuel consumption Helicopters Manufacturing & Prototyping Manufacturing processes Materials Metals Motion Control Motors & Drives On-board energy sources Positioning Equipment Propellers and rotors Robotics Rotary-wing aircraft Test & Measurement Thrust Transportation Unmanned Air Vehicles/Systems (UAV/UAS) Vehicle dynamics Vehicles
This Brief includes a Technical Support Package (TSP).
Document cover
Fabrication and Testing of High-Speed Single-Rotor and Compound-Rotor Systems

(reference ARL-0242) is currently available for download from the TSP library.

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    Magazine cover
    Aerospace & Defense Technology Magazine

    This article first appeared in the October, 2021 issue of Aerospace & Defense Technology Magazine (Vol. 6 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document is a final report detailing the objectives, methodologies, and outcomes of a project focused on the fabrication and testing of high-speed single-rotor and compound-rotor systems, conducted by the Alfred Gessow Rotorcraft Center. The primary aim of the project was to enhance the center's capabilities in precision data acquisition, which is essential for generating comprehensive measurements related to rotorcraft performance.

    Key highlights of the report include the acquisition of advanced instrumentation and equipment, such as precision ultra-thin pressure transducers, miniature load cells, slip rings, rotating torque sensors, brushless DC motors, and controllers. These tools are crucial for measuring blade surface pressures, pitch link loads, hub loads, and rotor wakes, which are vital for understanding rotor dynamics and improving rotorcraft design.

    The report emphasizes the innovative technologies employed in the X2 Technology Demonstrator, which include advanced airfoils, modern materials like titanium and graphite-epoxy, active vibration control, and high-efficiency propulsion systems. These advancements aim to mitigate challenges faced in rotorcraft performance and enhance overall efficiency.

    The document also outlines the applications of the acquired equipment, which have been utilized in various experimental setups to support rotorcraft research. The report provides a detailed description of the specifications and costs associated with the new technologies, ensuring transparency and accountability in the project's execution.

    In addition to the technical aspects, the report includes information about the sponsoring and monitoring agencies, specifically the U.S. Army Research Office, which played a significant role in funding and overseeing the project. The report is structured to provide a clear understanding of the project's scope, the methodologies employed, and the anticipated impact on rotorcraft technology.

    Overall, this final report serves as a comprehensive resource for stakeholders interested in rotorcraft research and development, showcasing the Alfred Gessow Rotorcraft Center's commitment to advancing the field through innovative technologies and precise data acquisition methods. The findings and methodologies presented in this report are expected to contribute significantly to future rotorcraft designs and performance enhancements.

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