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


This Brief includes a Technical Support Package (TSP).
Fabrication and Testing of High-Speed Single-Rotor and Compound-Rotor Systems

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

Don't have an account? Sign up here.



Aerospace & Defense Technology Magazine

This article first appeared in the October, 2021 issue of Aerospace & Defense Technology Magazine.

Read more articles from this issue here.

Read more articles from the archives here.