Browse Topic: Fly-by-wire control systems
Future military missions for Agile Combat Employment (ACE) and next generation Special Operations Forces need an aircraft with effective hover and the ability to operate in transonic cruise. Hover requires significant power that can only be mitigated by larger diameter rotors, but large diameter rotors become a detriment to achieving transonic flight. The stop-fold rotor configuration can “make the rotor disappear” in cruise and stands out as the most viable option for meeting these next-generation air vehicle requirements. This paper discusses the progress Bell has made in developing enabling technologies for a practical and scalable high-speed VTOL (HSVTOL) based on the stop-fold configuration. To this end, a unique Track-Guided Test Vehicle (TGTV) was developed at Bell and tested at the 10-mile High Speed Test Track at Holloman Air Force Base. The test vehicle integrates all subsystems required to demonstrate the key technologies in a representative environment, including multi-mode
This paper outlines observations from an FAA-sponsored research project that examined aviation Fly-By-Wire (FBW) accidents. The goal was to identify risk areas that will help guide a focus for FAA certification testing. Part of this study specifically focused on current powered-lift tiltrotors, identifying six general categories of causal factors for accidents, which will be discussed in detail regarding how they influenced flight control designs. The results of this survey, along with extrapolation to current designs, will be discussed and will illustrate why manufacturers are moving toward state-based flight control designs. In a state-based flight control scheme, the pilot does not have direct control over aircraft attitudes and motor tilt angles. Instead, the pilot requests a speed and or flight path with inceptor input, and the commanded attitudes and motor tilts are scheduled by the flight control computer. Additionally, recent lessons learned from electric Vertical Takeoff and
A robust velocity stability augmentation system was developed for the CoAX 600/2D coaxial-rotor helicopter to enable safe testing of a fly-by-wire system on an optionally piloted variant of the aircraft, developed by Piasecki Aircraft Corporation. The control law design and subsequent stability analysis were based on a validated nonlinear model of the CoAX 600 rotorcraft. A subset of helicopter handling qualities were evaluated through both analytical methods and piloted simulations, conducted with and without the stability augmentation system. Additionally, flight test data contributed to the analysis, albeit to a limited extent.
In April of 2024, Sikorsky flight tested an open loop Higher Harmonic Control system on an S-97® helicopter. The S-97® helicopter is a prototype aircraft, based on Sikorsky's X2 Technology™, that first flew in May 2015. It has contra-rotating, stiff in-plane main rotors with fly-by-wire controls, and a pusher propeller. This paper describes the HHC design, how it was implemented on the aircraft, how it was tested, and what the test results were.
This paper describes the methodology, involving testing and simulation activities, to assess malfunction conditions of complex systems installed on fly-by-wire vehicles, including the evaluation of their effects. This paper provides also a description about how the system malfunction tests are designed, driven by input requirements and systems capability and behavior. With respect to prior publications, this paper includes some practical test examples, based on systems monitoring, logics and alerting functions. The case study described here comes from a portion of multiple laboratory certification tests done for AW609 Tiltrotor, focused on Avionics System malfunctions. These tests and simulations are a valuable Means of Compliance with respect to applicable airworthiness rules, and a suitable means to verify the design safety requirements. Three relevant examples are presented, grouped by input requirement and safety conditions. The effect of such malfunctions is evaluated, with
The integration of automation and autonomy into modern aircraft has significant potential to simplify many piloting tasks. On the other hand, poor integration of automation and autonomy systems with the human crew has sometimes led to unintended consequences. With the goal of improving human-machine integration in piloting tasks, Bell Textron has conducted several autonomy demonstrations in both the simulator and aircraft. The team assessed automated terminal operations, enhanced station keeping, and maneuver tactile limit cueing in a flight simulator. Additionally, the V-280 technology demonstrator conducted autonomous flight profiles to explore these systems in an airborne environment. To mature autonomy systems for integration on future platforms, a Bell 429 was converted into the Aircraft Laboratory for Future Autonomy, completing its first flight last year with fly-by-wire controls at the evaluation pilot station. The influence of Bell autonomy demonstrations on the evolution of
The National Research Council of Canada (NRC) has recently developed an Integrated Reality In-flight Simulator (IRIS) that allows helicopter pilots to fly the NRC's Bell 412 Advanced Systems Research Aircraft (ASRA) while wearing a commercial off-the-shelf (COTS) virtual reality headset. IRIS is the first airborne simulator of its kind that combines COTS virtual reality and Fly-By-Wire (FBW) synthetic turbulence for helicopter operations. Simulations are not exact replications of actual environments; therefore, a methodology of comparing pilot workload with respect to an analysis of the differences between the simulated and actual environments is required. During a recent flight trial, NRC validated the effectiveness of IRIS to replicate a pilot's workload during ship landing tasks using these workload scales. During the analysis, NRC took initial steps in developing methodologies to examine environmental characteristics and then correlate them to an associated pilot workload. The work
A core mission of the CH-53K involves flying in severe brownout conditions, which increases pilot workload and can reduce mission success rates. With state of the art Fly by Wire capability, the CH-53K leverages the computational power of a flight control computer to provide higher order control modes which reduce pilot workload in all degraded visual environments such as brownout. The preliminary design of the flight control system included the inceptor system and low speed control architecture, which created an expansive design space. High fidelity simulations, cockpit mockup, and use of the NRC Bell 412 in-flight simulation Advanced Systems Research Aircraft surrogate aircraft allowed for a comprehensive development environment to narrow down to final control system design. The final design of the low speed maneuvering provided a command strategy similar to translational rate commend yet provided an approach profile that more closely replicated a piloted approach to a landing zone
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
In this paper, previously developed flight envelope protection algorithms and an active control system are integrated in a simulator environment and used as a limit avoidance evaluation framework for fly-by-wire helicopters. A force feedback map is developed and used in a new simulator environment to cue pilots against load factor limits. Adaptive models are developed online and used to calculate allowable control travels on the cyclic controls due to approaching load factor limits. The developed framework is demonstrated for load factor limit avoidance in different simulation scenarios.
Since it was first adopted in 1987, Aeronautical Design Standard ADS-33 has been through four major revisions, and the Mission Task Elements (MTEs) used to qualitatively assess aircraft handling qualities have been expanded to cover scout, attack, utility, and cargo missions. However, even the current version of ADS-33 (ADS-33E-PRF) focuses on the hover/low-speed flight regime with limited coverage of high speed (140-150 kts) and conventional rotorcraft configurations. The ADS-33E MTEs are based on legacy vehicles and were developed at an early stage of rotorcraft fly-by-wire technology. The U.S. Army National Rotorcraft Technology Center recently completed a multi-year project to develop MTEs for future high-speed configurations and missions using a series of simulation studies. This paper documents a flight test assessment of two high-speed MTEs—Break Turn and High-Speed Acceleration/Deceleration—using a UH-60M Black Hawk. The MTEs were deemed suitable for assessing high speed
ABSTRACT Flight testing of explicit rotor-state feedback (RSF) fly-by-wire control laws showed that measuring rotor tip-path-plane (TPP) flapping, via a laser measurement system, provided additional lead to the control system. This resulted in superior handling qualities in turbulence and heavy winds and improved stability margins. However, a significant impediment to the adoption of explicitly measured RSF has been the difficulty in extracting reliable rotor measurements. Therefore, this paper describes the development of a Kalman filter that was designed to estimate rotor TPP coordinates, and remove noise from the flapping signals while retaining the useful information without introducing large time delay, as would be the case for conventional low pass filtering. A new method for the design of the process noise covariance matrix using optimization of frequency domain specifications was implemented using flight test data from the UH-60 Black Hawk. The design was integrated into an
Advances in technology have made rotorcraft more comfortable, more capable, and more complex. With these advances, operators rely more on automated systems to reduce flight crew workload and to elevate safety. Correspondingly, flight-critical systems must remain operational at all times. Mechanical vibration or impact shocks such as bird strikes must not lead to a system failure. The approach for making the Bell 525 as safe as possible uses the guidelines of ARP4754A. The 525 is a commercial entry in the new super medium class of helicopters, and is the only commercial helicopter with fly-by-wire (FBW) flight control technology with state-of-the-art avionics. Safety requirements and certification regulations mandate the ability for continued safe flight following a bird strike incident. Improvements in computer simulation capabilities enable relating mechanical shock qualification tests with in-flight impact threats.
Rotor-state feedback (RSF) technology uses tip-path-plane measurements of the rotor to improve the tracking response of the aircraft in winds and turbulence, and provide improved stability margins. Three fly-by-wire control systems were designed and flight tested on the RASCAL JUH-60A aircraft to determine the benefits of RSF. A Baseline control system that used only conventional fuselage feedback but was optimized for Level 1 performance was compared to two control systems that used both rotor-state and fuselage feedback (and were also optimized for Level 1). The Implicit RSF control system implemented fuselage feedback and estimated (implicit) rotor-state feedback. The Explicit RSF control system implemented fuselage feedback and measured (explicit) rotor-state feedback via a laser measurement system installed on the aircraft. The sensor characteristics, frequency response validation, handling qualities ratings, and MTE tracking performance for hover/low-speed are discussed in this
ABSTRACT Development of the first fly-by-wire (FBW) commercial helicopter requires an integrated approach to design, testing, validation, and verification. The Bell 525 Relentless Advanced Systems Integration Lab (RASIL) - or "Aircraft Zero" - provides a platform for Vehicle Management System (VMS) hardware integration and associated validation and verification testing, including certification testing. The utility of the 525 RASIL is the ability to perform system testing ahead of and in support of the 525 flight test, envelope expansion, and certification program. RASIL testing is supporting the development of the 525 FBW control laws through initial design, open loop, closed loop, failure and certification related testing. The RASIL has seen advances in efficiency through automated testing and results verification, scripted failure insertion, and streamlined lab reconfiguration. Leveraging RASIL functionality and the wide array of testing conducted there, the 525 program achieves
ABSTRACT Vertical speed is a critical limit for rotorcraft at low height above terrain and low speed flight conditions. In this paper an adaptive estimation algorithm is proposed to estimate allowable control travel on the collective axis at the onset of pre-defined vertical speed limits. A concurrent learning neural network based framework is used to model vertical speed online and used to predict future values of the vertical speed for given collective inputs. The generated online model is used to estimate the control sensitivity of the collective axis to formulate the allowable control travel. A generic nonlinear utility helicopter model is used to show estimation and avoidance of vertical speed limits.
ABSTRACT Transport Category Certification requires the ability to safely land or continue flight after an engine failure during all phases of flight. The maximum transport category gross weight is a key parameter that can strongly influence the success of an aircraft. Due to its unique configuration, the tiltrotor offers unique challenges and abilities for surviving an engine failure during its critical mission phase. Challenges include energy management and thrust maintenance for a low inertia, high disk-loading rotor. Unique capabilities include rapid acceleration due to tilting of the thrust vector via the nacelles, which is a powerful method of improving takeoff performance and ground clearance for continued flight after a single engine failure. With its fly by wire flight control system and integrated engine controls architecture, the AW609 offers an unparalleled ability to evaluate, tune, and improve transport category performance, specifically during the critical flight phases
After supplying a first-generation Active Inceptor System (AIS) for the Boeing Vehicle Management Systems Integration Technology for Affordable Lifecycle cost (VITAL) AH-64 Apache helicopter, BAE Systems has produced a range of flightworthy AIS products for several military and commercial aircraft. The latest fifth-generation AIS is in development for the civil marketplace. Most recently, BAE Systems worked collaboratively with Boeing Helicopters to define a new type of tactile cueing solution, the Active Parallel Actuation Subsystem (APAS) that provides the benefits of tactile cueing to vehicles that are not Fly-By-Wire (FBW). Initially aimed at the H-47 Chinook platforms, the APAS equipment provides synthetic force feedback and tactile cues to pilots of any aircraft having mechanically interconnected pilot stations and displacement-trim flight controls. The solution leverages Commercial-Off-The-Shelf (COTS) components, software, and civil certification artifacts from the fifth
Since its formal launch in 2012, designers of the Bell 525 Relentless have sought feedback and utilized input from a Customer Advisory Panel (CAP) comprised of industry-leading global helicopter operators. The top initiative of this group is to adopt the features of a next generation helicopter that provides enhanced margins of flight safety. In response, the Bell 525 will be the first commercial helicopter to offer full authority fly-by-wire digital flight controls. Helicopter safety reviews have consistently found that human factors and situational awareness are the leading cause of helicopter accidents. With this history, safety clearly points to the need for careful consideration of pilot workload, especially in demanding situations such as hovering near multiple obstacles or performing in Degraded Visual Environments (DVE). Design-for-safety enhancements include Translational Rate Command / Position Hold control laws at low speed, automatic computer assisted entry into
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