Browse Topic: Fly-by-wire control systems
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.
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 propulsion, folding proprotors, and fly-by-wire transition controls to automatically manage each step of the prop-to-jet (and reverse) process. The TGTV demonstration validated the stop-fold technical approach and generated critical data to substantiate engineering models. Test results provide confidence that each step in the stop-fold transition process is thoroughly understood and that engineering tools can be used confidently for future aircraft design efforts. This paper documents the first known powered prop-to-jet and jet-to-prop transition and represents a significant milestone in vertical flight technology development.
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 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 Landing (eVTOL) aircraft accidents will be discussed, along with a comparison of powered-lift causal factors to accidents in the transport category FBW fixed-wing aircraft. From this analysis, broad observations will be offered about the trend of how accident-causal factors may evolve with greater maturity in aircraft design. This accident survey will be detailed further as part of an upcoming FAA Research Report.
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 also included the initial development of methodologies to analyze pilot workload and alternative prediction methods that better map subjective or quantitative pilot workload data to DIPES.
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 respect to the Avionics System output produced to keep adequate flightcrew awareness about the vehicle status.
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 Optimally Crewed Vehicles is presented. These efforts have evolved the concept of a digital co-pilot capable of assisting the human crew when called upon, while also acting to enhance aircraft safety through warnings and automated control input, when appropriate.
Over the past few decades, aircraft automation has progressively increased. Advances in digital computing during the 1980s eliminated the need for onboard flight engineers. Avionics systems, exemplified by FADEC for engine control and Fly-By-Wire, handle lower-level functions, reducing human error. This shift allows pilots to focus on higher-level tasks like navigation and decision-making, enhancing overall safety. Full automation and autonomous flight operations are a logical continuation of this trend. Thanks to aerospace pioneers, most functions for full autonomy are achievable with legacy technologies. Machine learning (ML), especially neural networks (NNs), will enable what Daedalean terms Situational Intelligence: the ability to understand and make sense of the current environment and situation but also anticipate and react to a future situation, including a future problem. By automating tasks traditionally limited to human pilots - like detecting airborne traffic and identifying safe landing locations - ML can raise safety levels, lower costs, and increase fleet capacity.
An advanced Vertical Rate Command (VRC) flight control augmentation is prototyped and applied to the CH-53K™ King Stallion to investigate its impact on workload and handling qualities and to further the implementation-practi-cality vertical augmentation for fly-by-wire rotorcraft. The Vertical Response Augmentation for CH-53K™ (VRAK) prototype applies a model following controller to effect a VRC response type and includes a split force-displacement (SFD) control method to address challenges to the practical, rather than theoretical, implementation of rotorcraft ver-tical augmentation. The impact of VRC on workload and handling qualities was evaluated in formal piloted evalua-tions conducted in the CH-53K™ flight simulator at NAS Patuxent River using a sequence of six maneuvers flown by US Marine Corps test pilots. Results demonstrate that VRC delivers substantial benefits to workload and handling qualities in the majority of the maneuvers evaluated. Observations from independent engineering tests show that SFD enables practical application of vertical augmentation. Altogether, VRAK is demonstrated to be a logical next step in fly-by-wire rotorcraft vertical control.
Active Inceptors in fly-by-wire aircraft offer new possibilities for control law reconfigurability, haptic cueing and system redundancies. Based on recent active inceptor research, new requirements for force-feel characteristics are proposed. Results are obtained from a simulation campaign investigating the influence of dynamic force-feel characteristics on the overall air-vehicle handling qualities (HQs). For this study, a rate-command type controller is evaluated in a pilot-in-the-loop simulation facility. Five mission task elements (MTEs) are flown with a range of force-feel characteristics (stick damping and stick natural frequency). Pilot comments on the active inceptors indicate clear differences and preferences between inceptor configurations. Generally, inceptor configurations with lower damping and higher natural frequency are preferred. Some minor differences, with respect to the MTEs flown, were noted and these may be explored in more detail in future research.
Modern vertical lift aircraft are complex and highly integrated systems that require careful design to guarantee safety and reliability. The Leonardo Helicopters AW609 Tiltrotor is one of the most complex and integrated systems in civil aviation. The AW609 relies on a full-authority, high-bandwidth, triplex-dual redundant architecture, fly-by-wire system which enables the flight crew to control transitions from helicopter to airplane mode, control the flight path of the aircraft, and manage the engine power demand. The AW609 Vehicle Management System (VMS) can manage and mitigate, in real-time, the system failures to ensure continuous safety of flight. The AW609 VMS is designed to a stringent set of requirements and objectives that meet or exceed the civil certification rules and allow the aircraft to satisfy its marketing objectives. As a result, many complex and highly integrated functions are integrated into the VMS and require rigorous methods of design assurance. This paper describes the use of hardware-in-the-loop simulation to verify qualitative safety assessments for Flight Control System (FCS) failure conditions that would be unsafe or impractical to fully evaluate in flight. Using the Simulation and Integration Laboratory (SaIL), it is possible to examine aircraft transients caused by a failure, crew recognition of the abnormal event, recoverability after the failure transient, and continued safe flight to landing in a thorough and efficient manner without presenting a safety risk to the flight test aircraft. The data illustrate that SaIL is an effective means to identify system problems and define emergency procedures that might be otherwise undetected and undefined from bench testing and stand-alone simulation. The net result of implementing SaIL methodology is an increase in level of system complexity and functionality without a corresponding increase in flight test time, safety risk, and cost.
In the early days of aviation, maintenance requirements were determined by a few experienced mechanics with assistance from the Original Equipment Manufacturer (OEM). As aircraft became more complex, a more sophisticated method of developing an aircraft maintenance program was needed. The approach aimed for a data driven maintenance philosophy. Just as sequential aircraft designs introduced new enhancements, each revision to the maintenance logic improved the maintenance approach in terms of effectiveness. The latest approach to this maintenance philosophy is known by the acronym MSG-3, for Maintenance Steering Group 3. As scheduled maintenance requirements for aircraft continue to change, the procedures need to be effective, reliable, and economically reasonable. The approach and benefits of an MSG-3 program are discussed in reference to the Bell 525, a new fly-by-wire, 16-passenger commercial helicopter under development at Bell Textron Inc. The development of the 525's MSG-3 maintenance program and its benefits to operators are discussed. The MSG-3 process schedules aircraft maintenance tasks necessary to maintain the stated levels of reliability and safety, reducing direct maintenance costs by 30% while maximizing aircraft availability (Ref 1).
The AH-64 would significantly benefit from an improved flight control system, particularly as the aircraft's requirements have evolved and continue to do so. To address this, multiple Vehicle Management System architectures are developed and presented for the AH-64 attack helicopter, each addressing several demanding and possibly conflicting future requirements, including Level 1 handling qualities, operating in degraded visual environments, autonomy, high-speed flight operation, and multiple vehicle coordinated operations. Architecture tradeoffs are performed with the understanding that the current AH-64 flight control system is mechanical with electrical partial authority augmentation, but also possesses a non-redundant full authority fly-by-wire emergency backup system. The various architectures are assessed as to how they satisfy the requirements. They are also assessed with respect to their relative costs, both for typical costs such as design, recurring, operating and sustainment, and training as well as other indirect costs such as size and weight. A methodology is developed to compare and contrast the architectures using informal qualitative-based scores assessed by subject matter experts from both industry and customer organizations, as well as scoring from Boeing program management with various operational experiences. The methodology assigns weightings to each requirement and cost criteria through the use of the Analytical Hierarchy Process. The results suggest that there are several likely cost-effective options, possibly with a higher potential return on investment than a conventional fly-by-wire (FBW) architecture. Some of these solutions are also incremental in nature, thereby providing more flexibility than the conventional FBW system.
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.
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 DVE. This paper provides details of the system design along with initial results from flight test.
Flight in icing for transport category aircraft certification presents a particularly challenging set of considerations to establish adequate safety commensurate with the associated risk while balancing design complexity and efficiency. A review highlighting important aspects of the regulatory evolution and guiding principles for flight in icing certification is presented, including the current standards and recent rulemaking activity. While historical icing certification relied on a simple yet subjective requirement to demonstrate that an aircraft is capable of operating safely within the prescribed icing envelopes, the certification requirements associated with demonstrating an adequate level of safety have progressively evolved into more explicit quantitative performance and qualitative handling qualities standards now scattered throughout the Federal Aviation Administration (FAA) Title 14 Code of Federal Regulations (CFRs) Part 25 Subpart B Flight standards which are largely harmonized with other regulatory agencies. Recent rulemaking activity, including the potential branching of the regulatory structure to address modern fly-by-wire aircraft not envisioned at the inception of the original flight standards, have firmly engrained flight in icing certification as a major design consideration with potentially large economic disparities depending on the design approach. A discussion is then presented illustrating how the regulatory standards influence the design space; while some phases of flight can integrate icing considerations into the aircraft design such that there is no perceptible operational effect, other phases of flight may not be able to fully mitigate icing considerations through the basic aircraft design and performance and are therefore susceptible to appreciable operational and associated economic impacts borne by the operator. The focus of this paper is to provide awareness for how the philosophical approach to flight in icing certification has shaped the associated design landscape and highlight the importance of flight in icing certification in the design phase.
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 handling qualities of the UH-60M. The results of the flight test provided recommended updates to the task descriptions and course cueing requirements, and helped validate the desired and adequate task performance tolerance.
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.
The development and qualification of distributed and highly safety-critical avionics systems implicate high efforts and risks. The resulting costs usually limit implementations like fly-by-wire systems to the military or commercial airliner domains. The aim of previous and ongoing research at the Institute of Aircraft Systems at University of Stuttgart is the reduction of these costs and therefore open up their benefits, inter alia, to general aviation, remotely piloted or unmanned aircraft. An approach for an efficient development is the application of a platform based development which supports the reuse of software and hardware components. The Flexible Platform adopts this approach. It is accompanied by a tool suite which automates the design and parameter instantiation, documentation generation and the generation of verification artifacts for a platform instance. This paper presents the approach for the requirement document generation compliant to ARP4754A and DO-178C. It is based on configurable requirement classes. In addition, relation classes build the base for the generation of a bidirectional traceability. The implemented system and high-level software requirement classes for the platform signal communication as well as their instantiation with the tool suite were validated with a representative platform instance model for a fly-by-wire system.
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 explicit rotor-state feedback control algorithm, where it was tested for robustness to sensor faults and effectiveness based on improvements to stability margins. The results showed that the Kalman filter was robust to rotor blade sensor spike and drop-out faults and resulted in improved stability margins and handling qualities.
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 paper.
This paper describes selected benefits of Adaptive Vehicle Management System (AVMS) tactile-cueing technologies that were demonstrated using an MH-47G Chinook equipped with an Active Parallel Actuator Subsystem (APAS). Rather than immediately proceeding to a full-authority Fly-By-Wire (FBW) system, the APAS adds tactile cueing and backdrive capabilities while retaining the existing Chinook hydromechanical primary flight controls with partial-authority augmentation. The flight-tested AVMS technologies include Carefree Maneuvering, Regime Recognition, and Task-Tailored Control Laws. Both simulation and flight test results are presented using technical performance metrics and pilot comments. Features of the APAS and AVMS control law software are described. The flight testing not only demonstrated the value of the technology, but also reduced technical risk to facilitate technology transition into a production environment. Having been successfully demonstrated in a flight environment, the AVMS and APAS technologies are transitioning into advanced development and qualification.
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.
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.
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 of takeoff and landing. This paper will summarize the development and initial evaluation of transport category performance on the AW609 tiltrotor. It includes a brief discussion of the control architecture, a summary of simulation correlation, and a summary of flight test findings.
Business Jets Bounce Back The business jet segment suffered badly from an extended economic downturn but is now seeing a new generation of airplanes becoming available, introducing features and technologies that are equal to, and in some cases superior to, jets in airline service. The business jet market has a resilience all its own. While defense spending has sharply declined, the commercial sector is over-flowing with multi-thousand order backlogs. But if business jet orders can tail off dramatically and then bounce back so quickly, what accounts for this collective long-term immunity to volatile market demands? One answer is competition. There are now so many manufacturers of business jets in the world there are niche markets within the sector that enable products to be offered for almost any need, from a private Airbus Corporate Jet (ACJ) A380 down to a 4-6 seat entry-level jet.
Advanced commercial aircraft increasingly use more composite or hybrid (metal and composite) materials in structural elements and, despite technological challenges to be overcome, composites remain the future of the aviation industry. Composite and hybrid aircraft today are equipped with digital systems such as fly by wire for reliable operations no matter what the flying environment is. These systems are however very sensitive to electromagnetic energy. During flight, aircraft can face High Intensity Radiated Fields (HIRF), static electricity, or lightning. The coupling of any of these threats with airframe structure induces electromagnetic energy that can impair the operation of avionics and navigation systems. This paper focuses on systems susceptibility in composite aircraft and concludes that the same electromagnetic rules dedicated to all metal aircraft for systems and wiring integration cannot be applied directly as such for composite aircraft.
ABSTRACT 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-generation AIS. Already proven in ground-based evaluations, flight demonstrations of APAS using a MH-47G Chinook are planned in 2015 with the intent of bringing APAS into production as a retrofit kit for the Chinook towards the end of this decade. The APAS implementation presented some specific challenges to ensure that the cues, feel, safety and performance were all acceptable while minimizing the impacts to the legacy mechanical controls installations and existing electrical interfaces. This paper presents a summary of the APAS approach, potential benefits to the Chinook helicopter, and compares some of the pros and cons to that of a full FBW AIS implementation.
ABSTRACT 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 autorotation in the event of a dual engine failure, automatic OEI compensation, load alleviation provisions, and control axis decoupling. This paper describes the process of incorporating requirements for modern handling qualities design criteria into the control system design and examines the real-time testing scheduled for the 525's first flight.
A multi-axis serially redundant, single channel, multi-path FBW (FBW) control system comprising: serially redundant flight control computers in a single channel where only one “primary” flight control computer is active and controlling at any given time; a matrix of parallel flight control surface controllers including stabilizer motor control units (SMCU) and actuator electronics control modules (AECM) define multiple control paths within the single channel, each implemented with dissimilar hardware and which each control the movement of a distributed set of flight control surfaces on the aircraft in response to flight control surface commands from the primary flight control computer, and a set of (pilot and co-pilot) controls and aircraft surface/reference/navigation sensors and systems which provide input to a primary flight control computer and are used to generate the flight control surface commands in accordance with the control law algorithms implemented in the flight control computers.
On September 30, 2011, certification authorities released Advisory Circular 20-174[1], Development of Civil Aircraft and Systems, which recognizes the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 4754A and the European equivalent ED-79A [2], in order to address “the concern of possible development errors due to the ever increasing complexity of modern aircraft and systems.” ARP4754A/ED-79A describes a process of development assurance which helps reduce the risk of design errors in the development of aircraft systems. This process is necessary for complex systems not easily comprehended by deterministic analyses or tests. This ARP was developed “in the context of Title 14 of the Code of Federal Regulations (14 CFR) part 25,” a category which includes complex systems such as full fly-by-wire flight controls. However, this paper shows that such systems are the exception to most, recent civil airplane designs. Of new airplanes designed in the last 10 years, most implement systems which are simple and easily comprehended. Many of these simpler aircraft are in the part 23 category, which the AC also associates to this ARP. This paper shows that the ARP, as written, does not consider simple systems and may unnecessarily burden the development of such systems through this lack of recognition. This paper reviews the current diversity of system complexity, discusses the regulatory and technical drivers for this diversity, and provides recommendations for incorporating such considerations in the ARP while preserving its original intent.
Active inceptors offer great potential for improving the handling qualities of fly-by-wire rotorcraft. In a cooperative research effort, the DLR Institute of Flight Systems in Germany and the U.S. Army Aero ightdynamics Directorate (AFDD) conducted several in-flight experiments to study the in uence of the dynamic characteristics (natural frequency and damping) of the cyclic stick on the overall handling qualities of a rotorcraft. Experiments were performed looking at sidestick (DLR) and centerstick (AFDD) inceptors for Rate and Attitude Command response types. The results of two different experiments are presented in this paper. The first experiment evaluated the roll handling in forward ight and was only performed with a sidestick inceptor. The task used in this experiment was designed during exercises of the Empire Test Pilots' School (ETPS) on DLR's Flying Helicopter Simulator EC135 ACT/FHS. ETPS also contributed to the optimization of the static sidestick characteristics that were used for the testing. The second, more comprehensive experiment, evaluating ADS-33 Hover and Slalom Mission Task Elements, was performed both with a sidestick and a centerstick allowing a direct comparison of both types of inceptors. Regression analyses are performed on the collected pilot ratings to gain a systematic insight into the preferred stick characteristics for the different inceptor and response types. The test results consistently suggest that the preferred characteristics are best described by a first-order response model.
Turboshaft engines of rotary-wing aircraft impose limitations on the flight envelope via Turbine Gas Temperature (TGT). Pilots are required to react on possible violations by monitoring TGT limits. For a fly-by-wire system, avoidance of the TGT limit could be accomplished using tactile cues and advanced envelope protection algorithms. In this paper, a neural-network-based adaptive limit detection algorithm is used to estimate approaching TGT violations and the corresponding control limits on the collective control. Collective limits are then used for avoidance of the TGT limit. High fidelity helicopter and engine models are used to show that TGT violations can be estimated with sufficient lead time to warn the pilot using tactile cues.
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