Aerodynamic Optimization of the Sizing and Blade Designs of Hovering Corotating Coaxial Rotors

Corotating coaxial rotors are seeing renewed interest in distributed electric propulsion systems and electric vertical take-off and landing (eVTOL) aircraft. The recent literature reports many interesting investigations, using prescribed rotor blades, into the flow phenomena as well as aerodynamic and aeroacoustic benefits of corotating rotors. However, the subject of the design of blade geometries, optimized to a design goal, for corotating rotors is currently lacking in the literature. This paper is written from such a design perspective, by extending a previous generalized approach to the aerodynamic optimization of counterrotating rotors to corotating rotors. The previous requirement for upper and lower counterrotating rotor torques to be equal can now be lifted in the case of corotating rotors, enabling improved versatility in the optimization of corotating blade designs. The optimization is demonstrated on an application example to address the conflicting conditions that index angles (high) for aeroacoustic benefits of reduced noise are at odds with those (low) for aerodynamic efficiency. The approach demonstrated in this paper is to set the index angle for reduced noise and then recover back the aerodynamic efficiency by using the newly developed aerodynamic optimization technique.

Nonlinear Aeroelastic Analysis for Highly Flexible Flapping Wing in Hover

Aeromechanics of highly flexible flapping wings is a complex nonlinear fluid–structure interaction problem and, therefore, cannot be analyzed using conventional linear aeroelasticity methods. This paper presents a standalone coupled aeroelastic framework for highly flexible flapping wings in hover for micro air vehicle (MAV) applications. The MAV-scale flapping wing structure is modeled using fully nonlinear beam and shell finite elements. A potential-flow-based unsteady aerodynamic model is then coupled with the structural model to generate the coupled aeroelastic framework. Both the structural and aerodynamic models are validated independently before coupling. Instantaneous lift force and wing deflection predictions from the coupled aeroelastic simulations are compared with the force and deflection measurements (using digital image correlation) obtained from in-house flapping wing experiments at both moderate (13 Hz) and high (20 Hz) flapping frequencies. Coupled trim analysis is then performed by simultaneously solving wing response equations and vehicle trim equations until trim controls, wing elastic response, inflow and circulation converge all together. The dependence of control inputs on weight and center of gravity (cg) location of the vehicle is studied for the hovering flight case.

UH-60A Airloads Workshop—Setting the Stage for the Rotorcraft CFD/CSD Revolution, Part I: Background and Initial Success

The UH-60A Airloads Workshop was a unique collaboration of aeromechanics experts from the U.S. Government, industry, and academia to address technical issues that hindered accurate rotor loads predictions. The Airloads Workshop leveraged the NASA/Army UH-60A Airloads flight test and NFAC wind tunnel test data. It functioned continuously for 17 years, from 2001 to 2018, and brought about one of the most important advancements in rotorcraft aeromechanics prediction capabilities by successfully demonstrating high-fidelity coupled computational fluid dynamics (CFD) and computational structural dynamics (CSD) analyses for both steady and maneuvering flight. The article is divided into two parts. Part I surveys the background of rotorcraft CFD/CSD development difficulties, the origins of the Airloads Workshop, and the rapid success achieved during the first phase that consisted of eight Workshops. Part II describes ongoing development during the subsequent two phases of the Airloads Workshop, the Ninth through the 13th, and the 14th through the 31st Workshops; the impact of the Airloads Workshop; and the lessons learned. Part I surveys the technical activities that led to a breakthrough for CFD/CSD coupling to successfully predict rotor blade airloads in trimmed steady-level flight conditions. This success illustrated the importance of collaboration among key experts with diverse backgrounds focused on a common objective to advance rotorcraft prediction methods.

Rotorcraft Design: The Crucial Influence of Safety from Concept to Fleet Support The 41st Alexander A. Nikolsky Honorary Lecture

It is an immense honor to have been selected to hold the prestigious 41st Nikolsky Lecture and to have the opportunity to synthesize my experiences with regards to the most important principle that permeates aeronautical engineering—“the concept of safety.” Having worked in the rotary-wing field for 39 years, with growing levels of involvement and responsibilities, I have been involved in the design, development, and certification of many helicopter models at the Leonardo Helicopters Division (LHD; formerly Agusta and then AgustaWestland), such as A109, A119, EH101, A129, NH90, AW609. More recently, I had the full responsibility of design, development, certification, and entry into service of three new helicopter types within the “AW Family concept”, specifically the AW139, AW189, and AW169. I am profoundly grateful for the mentors encountered in my professional life—Bruno Lovera and Santino Pancotti, both of whom were also honored with the Nikolsky Lectureship. In working with them, not a single day passed where the word “safety” was not mentioned. They taught me that “safety” shall be the mantra of every aeronautical engineer because it is our principal duty and responsibility, towards those who travel in, work on, and work with our products and entrust their lives to our work and professionalism daily. I have tried hard never to forget this lesson, and to convey this to the young engineers that I have had the chance and pleasure to work with. If I have been able to pass on this lesson successfully, through my work with others through this lectureship, it would be the greatest achievement of my life. In this vein, this paper is organized in three parts: (i) definitions and principles, along with some “philosophical” concepts; (ii) the application of these principles at Leonardo in the design of the latest generation of helicopters, and finally (iii) a discussion of emerging “safety technologies” that promise to improve the safety of future helicopters and operations.

Rotorcraft Lateral-Directional Oscillations: The Anatomy of a Nuisance Mode

High-fidelity rotorcraft flight simulation relies on the availability of a quality flight model that further demands a good level of understanding of the complexities arising from aerodynamic couplings and interference effects. One such example is the difficulty in the prediction of the characteristics of the rotorcraft lateral-directional oscillation (LDO) mode in simulation. Achieving an acceptable level of the damping of this mode is a design challenge requiring simulation models with sufficient fidelity that reveal sources of destabilizing effects. This paper is focused on using System Identification to highlight such fidelity issues using Liverpool's FLIGHTLAB Bell 412 simulation model and in-flight LDO measurements from the bare airframe National Research Council's (Canada) Advanced Systems Research Aircraft. The simulation model was renovated to improve the fidelity of the model. The results show a close match between the identified models and flight test for the LDO mode frequency and damping. Comparison of identified stability and control derivatives with those predicted by the simulation model highlight areas of good and poor fidelity.

On the Influence of Inflow Model Selection for Time-Domain Tiltrotor Aeroelastic Analysis

The methods of using the viscous vortex particle method, dynamic inflow, and uniform inflow to conduct whirl-flutter stability analysis are evaluated on a four-bladed, soft-inplane tiltrotor model using the Rotorcraft Comprehensive Analysis System. For the first time, coupled transient simulations between comprehensive analysis and a vortex particle method inflow model are used to predict whirl-flutter stability. Resolution studies are performed for both spatial and temporal resolution in the transient solution. Stability in transient analysis is noted to be influenced by both. As the particle resolution is refined, a reduction in simulation time-step size must also be performed. An azimuthal time step size of 0.3 deg is used to consider a range of particle resolutions to understand the influence on whirl-flutter stability predictions. Comparisons are made between uniform inflow, dynamic inflow, and the vortex particle method with respect to prediction capabilities when compared to wing beam-bending frequency and damping experimental data. Challenges in assessing the most accurate inflow model are noted due to uncertainty in experimental data; however, a consistent trend of increasing damping with additional levels of fidelity in the inflow model is observed. Excellent correlation is observed between the dynamic inflow predictions and the vortex particle method predictions in which the wing is not part of the inflow model, indicating that the dynamic inflow model is adequate for capturing damping due to the induced velocity on the rotor disk. Additional damping is noted in the full vortex particle method model, with the wing included, which is attributed to either an interactional aerodynamic effect between the rotor and the wing or a more accurate representation of the unsteady loading on the wing due to induced velocities.

Performance and Loads of a Lift Offset Rotor, Part II: Prediction Validations with Measurements

The predictions of an upgraded UMARC comprehensive analysis are compared to experimental lift offset rotor results. The experiments cover a range of collective pitch angles (θ°) from 2° to 10°, advance ratios (μ) from 0.21 to 0.53, and lift offset from 0% to 20%. The experimental model rotors are from a system of coaxial hingeless rotors, with two blades each, and a first flap frequency of approximately 1.6/rev. The simulation is compared with isolated rotor performance and controls with lift offset, loads, and pitch link forces. Increasing efficiency with increasing lift offset, the impact of lift offset on different loads, and the dependence of pitch link loads on pitch bearing damping are identified in the experiment and correlated with the simulation.

Wind Tunnel Test on a Slowed Mach-Scaled Hingeless Rotor with Lift Compounding

A single main rotor helicopter's maximum forward speed is limited due to the compressibility effects on the advancing side and reverse flow and dynamic stall on the retreating side. Compound helicopters can address these issues with a slowed rotor and lift compounding. There is a scarcity of test data on compound helicopters, and the present research focuses on a systematic wind tunnel test on lift compounding. Slowing down the rotor increases the advance ratio and, hence, the reverse flow region, which does not produce much lift. The lift is augmented with a wing on the retreating side. A hingeless rotor hub helps to balance the rolling moment with lift offset. Wind tunnel tests were carried out on this configuration up to advance ratios of 0.7 at two different wing incidence angles. Rotor performance, controls, blade structural loads, and hub vibratory loads were measured and compared with in-house comprehensive analysis, UMARC. A comparison between different wing incidences at constant total lift provided many insights into the lift compounding. It increased the vehicle efficiency and reduced peak-to-peak lag bending moment and in-plane 4/rev hub vibratory loads. The only trade-off was steady rotor hub loads and rolling moment at the wing root carried by the fuselage.

Normalized Adaptive Hybrid Control Algorithms for Helicopter Vibration with Variable Rotor Speed

Variable rotor speed technology implemented in a helicopter can improve the flight performance, reduce the required power, and increase the flight speed. However, variable rotor speed changes the frequencies of rotor vibratory loads and may produce helicopter fuselage resonance under the excitation of the rotor vibratory loads. Active vibration control (AVC) has been effectively used in vibration reduction of helicopter fuselages. However, the frequency domain control algorithms that are currently used have poor adaptability in controlling vibration with variable frequencies (i.e., during time varying rotor speeds). In order to effectively improve control convergence, adaptability, and effectiveness, the normalized adaptive hybrid control algorithms containing both the normalized adaptive harmonic control algorithm and the normalized frequency tracking algorithm have been presented in this paper. Simulations of AVC with variable frequencies on a dynamically similar frame structure of a helicopter fuselage driven by piezoelectric stack actuators installed on the gearbox support struts show that the normalized adaptive hybrid control algorithms can accurately track the changes in rotor load frequencies and can be effectively used in the AVC of a helicopter with variable rotor speed.

Simplified Analysis of an Emergency Flare Maneuver

This technical note describes a simplified study of the flare maneuver of a helicopter at the end of an autorotation descent. Energy methods and momentum theory were utilized for the analysis of the parameters involved in the flare maneuver. A simulation was run on the basis of the data available for a commercial helicopter. The results are discussed, and a series of parameters are defined for the performance analysis of the flare maneuver.