Browse Topic: Attitude control
This paper presents an initial handling qualities analysis of an Electric Vertical Take-Off and Landing (eVTOL) hexacopter. The analysis uses the Distributed Electric Propulsion Simulation (DEPSim), developed by Penn State University (PSU) and the Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM), developed by Continuum Dynamics, Inc. (CDI). The study focuses on evaluating a generic AAM hexacopter performing Handling Qualities Task Elements (HQTE) as defined by the DOT / FAA. A trajectory controller was developed to enable simulation of prescribed flight paths, allowing automated simulation of four HQTEs: Heliport Approach, Hovering Turn and Hold, Pirouette, Lateral Reposition and Hold. Design modifications incorporating lateral mast tilt and Direct Side Force Control (DSFC) were implemented to enhance yaw control and ride qualities. Piloted simulations were conducted at the PSU rotorcraft flight simulation facility using DEPSim, employing an Attitude Command Attitude Hold (ACAH) architecture with mode switching to Translational Rate Command / Position Hold (TRC / PH) and TRC plus DSFC modes. Two of the four HQTEs were tested in piloted simulations. Though formal ratings were not collected at this time, pilot commands and performance indicated that TRC / PH and TRC plus DSFC modes enhance handling qualities over ACAH mode. The DSFC control law was found to have substantially reduced roll attitude, which could potentially enhance visual cueing, pilot comfort, and pilot-perceived handling qualities.
This study investigates the fault tolerance of a large-scale coaxial quadrotor Electric Vertical Takeoff and Landing (eVTOL) under motor failure through high-fidelity software-in-the-loop (SIL) simulations using PX4-Gazebo environment. The objective is to evaluate the vehicle's ability to maintain flight stability and complete critical missions under various propulsion failure scenarios, without the control system being explicitly aware of which motors have failed. Four motor failure cases-single, two adjacent, two diagonally opposite, and three distributed motor failures-were introduced during takeoff, hover, cruise, and hover under crosswind missions. Results show that the eVTOL maintained controllability and mission completion under all scenarios, with increasing levels of performance degradation under more severe failures. Notably, considerable yaw instabilities of about 10 degrees occurred under two diagonally opposite motor failures. The highest thrust demands after motor failures were observed during cruise mission, with some motors demanding about 80% to 90% of their maximum throttle. Hover under crosswind revealed compounded challenges in attitude control during descent under severe failure cases compared to calm weather. These findings underscore the robustness of the integrated control system and vehicle configuration in managing motor failure scenarios.
The transition phase of eVTOL aircraft poses a challenge in balancing energy efficiency and stability. This study presents the development and evaluation of an automatic flight control system for eVTOL transition phases, focusing on minimizing energy consumption while ensuring robust performance. The control architecture implements a hybrid response type combining Translational Rate Command below 5 knots and Acceleration Command Speed Hold above 5 knots, with control allocation dynamically adjusted based on airspeed and rotor shaft angle. Stability analysis reveals surge mode instability at high shaft angles due to negative speed stability derivatives, stabilized through carefully tuned feedback control. The system demonstrates Level 1 handling qualities against bandwidth, quickness, and disturbance rejection criteria when evaluated against MIL-DTL-32742 and MIL-STD-1797B standards. Simulation results verify the control system's ability to maintain precise acceleration/deceleration rates and attitude control while ensuring passenger comfort through limited pitch excursions. The control strategy achieves minimum energy transitions by locking rotor shaft angles to optimal schedules while avoiding excessive hub moments. Flight test maneuvers developed specifically for conversion phases confirm the system's capability to execute efficient transitions within defined performance boundaries. This research establishes a framework for certifiable eVTOL flight control systems that balance energy efficiency with robust performance across diverse flight regimes.
In application, the Aeronautical Design Standard for the handling qualities of military rotorcraft, ADS-33E-PRF, provides the means to effectively predict rotorcraft handling qualities via validated criteria and demonstrate actual handling qualities in flight test using mission task elements. Besides a definition, a note that rotorcraft shall have no tendencies, and a note regarding Attitude Command Response-Types and gain bandwidth frequency, the topic of pilotinduced oscillations (PIO) is not addressed via specific criteria or flight test techniques. As the use of full authority fly-by-wire flight control continues to expand in Vertical Takeoff and Landing (VTOL) aircraft, the likelihood of encountering PIO will also expand. In the fixed wing world where PIO has been commonplace, at least in developmental test if not operations, predictive analytical methods that can also be used for detection of PIO in realtime have been developed, which can also be applied to rotorcraft. Furthermore, recent time-frequency domain methods that have been developed to differentiate VTOL piloting techniques are directly applicable to PIO scenarios. Using a flight test database generated with the UH-60L in-flight simulators at the U.S. Naval Test Pilot School (USNTPS), this paper explores the utility of these methods for identifying rotorcraft PIO tendencies.
To achieve Level 1 Handling Qualities, Aeronautical Design Standard ADS-33E-PRF requires an Attitude Command Attitude Hold or Translational Rate Command response-type in Degraded Visual Environments while allowing a rate response in Good Visual Environments. The authors describe the design and analysis of a Blended Command Model that may offer the precision of the former and the aggressiveness of the latter. The command model, comprised of a single flexible transfer function and its parameter scheduling functions, produces attitude, rate, and blended responsetypes as functions of cyclic stick position. The authors explain the command model’s design considerations, test considerations, and its behavior through time domain and frequency domain perspectives. This effort precedes a handling qualities assessment aboard the U.S. Army’s JUH-60A RASCAL aircraft.
NASA is preparing for the next generation of CubeSats that are propelled and will make directional maneuvers. The new gimbal mount provides a seat for the motor, and controls the position of the thrusters that propel the CubeSat as it moves about and/or changes orbits.
The vast majority of the U.S. Army's helicopter fleet consists of aircraft initially developed in the 1960s and 1970s and which were designed based on the handling qualities and flight control requirements of the time for flight in good visual environments (GVE). The Army today uses helicopters at night and in brownout and other degraded visual environment (DVE) conditions but with the same control laws of the original models; the major exception being the CH-47F and MH-47G DAFCS, which have been highlighted as a successful partial authority flight control system upgrade to provide improved handling qualities. The U.S. Army Aviation Development Directorate–AFDD has partnered with the U.S. Army Utility Helicopter Program Office's Futures Team and the RDECOM DVE Mitigation Program to further develop and test the UH-60 Modernized Control Laws (MCLAWS). Previous work implemented a model following control system architecture which provided an attitude command/attitude hold response-type for hover and low speed flight. This system demonstrated improved handling qualities as compared to the UH-60L SAS/FPS rate command response-type. This paper documents work to integrate an outer-loop position hold with velocity command mode into the MCLAWS. Flight testing of the MCLAWS with position hold demonstrated Level 1 Cooper-Harper handling qualities ratings in simulated DVE conditions. Finally, landing logic has been integrated into the MCLAWS to support DVE landing flight testing.
We give a comprehensive illustration of a new approach to rotorcraft noise abatement carried out in the framework of the Clean Sky collaborative project MANOEUVRES. This approach is based on technologies and tools for real-time, in-flight monitoring of the emitted noise. By means of a new cockpit instrumentation, the Pilot Acoustic Indicator (PAI), the current noise impact is presented to the pilot in a condensed, practical form as an aid in performing quieter maneuvers. The PAI algorithm makes use of several ingredients that have been implemented and tested within the project, including offline steady and unsteady acoustic predictions, and estimation of flight mechanics parameters based on the measurements derived from a new contactless rotor state measurement system. The latter is capable to accurately acquire the motion of the rotor blades, allowing the computation of non-directly-measurable quantities such as tip-path-plane angle of attack and thrust coefficient, and offering a reliable support for the development of enhanced attitude control laws based on the Rotor State Feedback approach. The paper reports on the current state of the project, which is close to its completion.
This paper describes the design, development and flight testing of a meso-scale cyclocopter. Weighing only 29 grams, the present vehicle is the smallest cycloidal rotor based aircraft ever built. Unlike the previous cyclocopters, the current prototype utilizes a novel, light weight (3 grams) cycloidal rotor design, with cantilevered blades, having semi-elliptical planform shape and no exposed rotor shaft. To minimize bending deflections the blades use a unique, lightweight (0.15 grams each) but high strength-to-weight ratio unidirectional carbon-fiber based structural design and are fabricated using a specialized manufacturing process. The cycloidal rotor design was chosen through systematic performance measurements conducted using a custom-built miniature three-component force balance. Based on experimental parametric studies, a 4-bladed rotor and symmetric blade kinematics with pitch amplitude of 45° provided the highest thrust and power loading (thrust/power) and was used in the final rotor design. The airframe is fabricated using a combination of carbon-fiber and state-of-the-art 3D printing techniques. The attitude control strategy utilizes a combination of rpm-control of the two cycloidal-rotors/tail-rotor and thrust vectoring of the cycloidal rotors. The control strategy is implemented on a custom-built 1.3 gram autopilot, which uses a closed-loop proportional-derivative controller for hover stability. The vehicle has been systematically flight tested by tuning the feedback gains and has demonstrated stable hovering flight.
The present contribution aims at providing a comprehensive illustration of a structured approach to the design, implementation and testing of a new rotor state measurement system for rotorcraft applications. This effort has been carried out in the framework of a Clean Sky collaborative project in which the novel sensor system plays a fundamental role by enabling the real-time estimation of non-measurable quantities that govern the rotorcraft running acoustic emission, in view of external noise alleviation. Furthermore, the availability of the new sensor system capable to accurately capture rotor blade motion allows the derivation of enhanced attitude control laws based on rotor state feedback. We detail the complete process that led to the full-scale development of a stereoscopic vision-based measurement system mounted on the rotor head, which has been fully integrated on board a prototype helicopter for ground and flight testing.
A joint research project (2010-2014) between Delft University of Technology and Boeing Mesa was conducted in SIMONA Research Simulator (SRS) at Delft University with the goal to develop advanced flight control laws for handling qualities (HQs) improvements of the Apache AH-64 helicopter. The goal of the present paper is to concentrate on implementation and simulator testing of modern control laws for Apache's AH-64D Longbow helicopter to provide improved handling qualities for hover and low speed flight in degraded visual environment. The paper will implement an "Incremental Nonlinear Dynamic Inversion (INDI)" controller into the Boeing's FlyRT AH-64 Apache baseline model for the existing partial authority stability augmentation system (SAS). The INDI will be used to provide both attitude command attitude hold and translational rate command response types based on the requirements in ADS-33E. Implementation of the INDI into Apache's FlyRT proved to be challenging because the model was not affine in the control inputs. Actuator dynamics and rotor dynamics (most likely flapping dynamics) caused the designed INDI controller to overcontrol the helicopter. The paper proposes three adaptations of the INDI methodology that prove able to stabilize the helicopter and can be used as an alternative to a full authority, fly-bywire, control system upgrade. Handling qualities evaluations conducted in SIMONA simulator for the ADS-33 hover and pirouette maneuvers demonstrated that the INDI resulted in improving from Level 2 to Level 1 HQs, reducing the pilot workload when compared to the Apache's legacy flight control system.
Preliminary data was recently provided for a reaction sphere prototype on NASA’s zero-gravity parabolic flight vehicle. Gyroscope telemetry indicates that reaction spheres were successfully commanded at 10- to 20-ms pulses during a handful of parabolas in each flight. This is the first publicly disclosed validation of a freely rotating reaction sphere in a standalone compact package. At dimensions of
The MAI-400SS Space Sextant is a turnkey Attitude Determination And Control System (ADACS) for CubeSats and nanosatellites. It is an enhanced version of the MAI-400, which is a precision CubeSat ADACS incorporating three reaction wheels, three electromagnets, and an ADACS computer in a ½-U module. This Space Sextant version incorporates two star trackers to improve overall pointing knowledge to 0.02° or better. The star trackers feature “Lost In Space” attitude determination requiring no a priori information.
ABSTRACT A previous investigation studied the use of advanced response types and non-linear dynamic inversion (NLDI) control to improve handling qualities for shipboard operations from a moving ship deck with unsteady airwake. The results showed potential for ship-relative Translation Rate Command (TRC) control modes to significantly reduce pilot workload, at least in mild sea states. The paper extends the investigation to better understand the bandwidth and the disturbance rejection requirements of the NLDI controller (and for rotorcraft control characteristics in general), when operating in a range of sea states and airwake conditions. The US Army's rotorcraft handling quality specification, ADS-33E-PRF, provides no specific design guidance on bandwidth or disturbance rejection properties for maritime operations. A family of controllers was developed to test varying levels of bandwidth and disturbance rejection proper-ties of Attitude Command / Attitude Hold (ACAH) and TRC control modes. The controllers gain sets were baselined at the minimum Level 1 ADS-33 requirements for response to pilot inputs and the 45° / 6-dB stability margins recommended by standard flight control design specifications. Piloted simulation tests were conducted to evaluate the handling qualities of the family of controllers in the midst of higher sea states and a Computational Fluid Dynamics (CFD) model for airwake turbulence. Simulations used the GENHEL-PSU UH-60 model integrated with the Penn State rotorcraft flight simulator. A maritime mission task element (MTE) was flown to evaluate handling qualities ratings (HQR) using the various response types and gain parameters. Results indicate that ACAH can improve HQRs over the conventional rate command mechanical control. HQRs with ACAH were still Level 2, and results indicate that the required ACAH bandwidth is significantly higher than that currently specified in ADS-33E-PRF. Results indicated that level 1 HQR could be achieved with ship-relative TRC in sea state 5, but that handling qualities were sensitive to rise time, with the required rise time at the low end of the range recommended by ADS-33E-PRF.
Multicopter type helicopters have become prevalent in the past ten years with applications in military, academia, commercial, and recreational use. These aircraft benefit from their inherent simplicity, robust design, and ease of control. This paper discusses the design of an attitude control system for a large-scale gas powered multicopter and the challenges overcome in the development of the system. A nonlinear model was developed for simulation purposes and a simplified linear model was developed for control system design. The resulting control system architecture and design was implemented and successfully tested on an eight-engine prototype aircraft with 880 horsepower and a max gross weight of 4,400 lb.
Piloted simulation tests were conducted to develop and evaluate advanced control laws and optimal response types for ship-based rotorcraft. Simulations used the GENHEL-PSU model integrated with the Penn State rotorcraft flight simulator. The simulation includes ship motion, a visual model of a FFG-7 frigate, and the Control Equivalent Turbulence Input (CETI) model for airwake turbulence. The controller uses a Non-Linear Dynamic Inversion scheme to accurately track a variety of response types. An Attitude Command / Attitude Hold (ACAH) control mode was used as the baseline control law. Different variants of Acceleration Command / Velocity Hold (ACVH) and Translational Rate Command / Position Hold (TRC/PH) response types were designed to make use of ship deck motion measurements. Filtered deck states are fed into the control laws to command velocity and position relative to the landing spot. Piloted simulation tests were performed for a variety of control configurations with and without ship motion and airwake turbulence effects using a maritime MTE. Pilot comments and preliminary handling qualities ratings indicated that the best performance was achieved using an ACVH response type for the pitch axis on approach, which then automatically transitions to TRC/PH over the ship deck. A 3.5 HQR improvement over the baseline ACAH control mode was achieved when using the optimized ship-relative ACVH/TRC/PH control mode. Simulation results indicated that it is best to filter out most of the dynamic ship deck motion (primarily ship roll) and to maximize the lateral axis TRC bandwidth.
In June 2013, NASA and the U.S. Army jointly conducted a simulation experiment in the NASA-Ames Vertical Motion Simulator that examined and quantified the effects of limited-authority control system augmentation on handling qualities and task performance in both good and degraded visual environments. The vehicle model used for the experiment was the OH-58D with similar size, weight and performance, and the same 4-blade rotor system as the Bell 407 civilian helicopter that is commonly used for medical evacuation and emergency medical services. The control systems investigated as part of this study included the baseline aircraft Rate Command system, a short-term Attitude Command/Attitude Hold system that uses lagged-rate feedback to provide a short-term attitude response, Modernized Control Laws that provide an Attitude Command/Attitude Hold control response type, and Modernized Control Laws with an additional Position Hold function. Evaluation tasks included the ADS-33 Hover, Sidestep, Acceleration/Deceleration, and Pirouette Mission Task Elements, as well as a new proposed Emergency Medical Services task that includes an approach and landing at a minimally prepared remote landing site. Degraded visual environments were simulated with night vision goggles and an unaided night scene. A total of nine experimental test pilots participated in the four-week simulation experiment. Data recorded during the evaluation included Cooper-Harper handling qualities ratings, Bedford Workload scale ratings, and task performance. The Usable Cue Environment (UCE) was measured for this simulation experiment, and found to be UCE=1 in good visual environments and UCE=2 in degraded visual environments with night vision goggles. Results showed that handling qualities ratings were improved with a control system providing short-term attitude response over a rate command system, although the improvements were not sufficient to produce Level 1 handling qualities in degraded visual environments. Results for an Attitude Command/Attitude Hold control system showed that borderline Level 1 handling qualities could be achieved in degraded visual environments, and the 10% authority stability augmentation system was adequate to obtain these handling qualities ratings.
Until the time of this reporting, when a space vehicle required a reference signal for inertial pointing, the choices were a signal beacon from an Earth location, the Earth radiance in the visible spectrum, or a star tracker. However, limitations can arise from using these techniques. For example, the signal beacon suffers from limited signal power (either in RF or optical) and will constrain the application to limited ranges, errors due to stray-light and centroiding limit the accuracy of a star tracker, and the spatial/temporal variability of the Earth’s albedo and its illumination by the Sun introduces limitations when used in the visible or near infrared light.
This checklist is to be used by project personnel to assure that factors required for adequate system electromagnetic compatibility are considered and incorporated into a program. It provides a ready reference of EMC management and documentation requirements for a particular program from preproposal thru acquisition. When considered with individual equipments comprising the system and the electromagnetic operational environment in which the system will operate, the checklist will aid in the preparation of an EMC analysis. The analysis will facilitate the development of system-dependent EMC criteria and detailed system, subsystem, and equipment design requirements ensuring electromagnetic compatibility.
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