Experimental Investigation and Fundamental Understanding of a Full-Scale Slowed Rotor at High Advance Ratios

Abstract

This paper describes and analyzes the measurements from a full-scale, slowed revolutions per minute (rpm), UH-60A rotor tested at the National Full-Scale Aerodynamics Complex 40-by 80-ft wind tunnel up to an advance ratio of 1.0. A comprehensive set of measurements that includes performance, blade loads, hub loads, and pressures/airloads makes this data set unique. The measurements reveal new and rich aeromechanical phenomena that are unique to this exotic regime. These include reverse chord dynamic stall, retreating side impulse in torsion load, large inboard-outboard elastic twist differential, diminishing rotor forces and yet a dramatic buildup of blade loads, and high blade loads and yet benign levels of vibratory hub loads. The objective of this research is the fundamental understanding of these unique aeromechanical phenomena. The intent is to provide useful knowledge for the design of high-speed, high-efficiency, slowed rpm rotors of the future and a database for validation of advanced analyses. Nomenclature

Vortex Shedding from Airfoils in Reverse Flow

“…An elliptical airfoil (featuring a blunt geometric trailing edge) was also shown to delay deep stall to jα rev j 14 deg, but it produced a large downwardacting lift force and pitching moment about the geometric quarterchord. Recent experimental and computational studies on a full-scale UH-60A at high advance ratios have characterized some of the threedimensional unsteady flow features encountered in the reverse flow region that are responsible for rapid variations in blade airloads [1,33]. These studies focused on the influence of a dynamic stall vortex in reverse flow.…”

“…Additionally, the transition of a rotor blade from forward flow to reverse flow results in a pitching moment impulse as the center of pressure shifts from close to the geometric quarter-chord toward the three-quarter-chord. This imposes large unsteady torsional blade loads that increase fatigue of the blades, hub, and pitch links [1].…”

NASA Technology for Next Generation Vertical Lift Vehicles

“…The significant findings that are reported in Ref. 43 include that the dynamic blade loads are very high at advance ratios above 0.8 although rotor forces are low; even with high blade loads, vibratory hub loads were lower than expected. The retreating side exhibited reverse chord dynamic stall and a retreating side impulse in torsion load.…”

“…43 The study was the first test conducted on a full-scale rotor in the NFAC for advance ratios up to 1.0, and Reference 43 describes the significant results. The objective of the work was to obtain a comprehensive set of validation data for rotors operating in a high-advance ratio environment to support the development of future high-speed rotorcraft with variable RPM rotors.…”

“…The retreating side exhibited reverse chord dynamic stall and a retreating side impulse in torsion load. 43 Initial steps to investigate the feasibility of Active Flow Control (AFC) as a future advanced rotor technology are underway in partnership with United Technology Research Center, Sikorsky Aircraft and the Georgia Institute of Technology through a NASA Research Announcement. 44 Combustion powered actuation will be installed in an oscillating airfoil model and tested through a range of Mach numbers (0.2 to 0.5) and reduced frequencies to determine the effectiveness of the AFC approach on dynamic stall.…”