- Email: [email protected]
Facilitated Airplane – project and preliminary in-flight experiments
Aerospace Science and Technology 8 (2004) 469–477 www.elsevier.com/locate/aescte
Facilitated Airplane – project and preliminary in-flight experiments Andrzej Tomczyk Department of Avionics and Control, Rzeszów University of Technology, W. Pola 2, 35-959 Rzeszów, Poland Received 17 November 2003; received in revised form 22 April 2004; accepted 15 June 2004 Available online 29 July 2004
Abstract The main goal of the presented project is to improve safety and Handling Qualities of general aviation aircraft so that airplane becomes pilot-friendly, “almost unmanned” aircraft. In this paper, a proposal of employing an intuitive, human-centered, simplified software-based flight control system in general aviation aircraft has been presented. Airplanes equipped with such a flight control system belong to a new class of general aviation aircraft with improved safety and efficiency handling properties – Facilitated Airplane (FA) or Simple Flying Airplane (SFA). The basic idea of the project is to employ an indirect (Fly-by-Wire) software-based flight control system characterized by high degree of automatization, leading to an almost “unmanned” general aviation aircraft. “Unmanned” does not mean eliminating humans from the control process but changing their role in the system. User-friendly control system should shape handling qualities of an aircraft in such a way that control becomes easy and safe. Pilot retains the crucial role of decision-maker, and control system takes appropriate steps to fulfill his requirements, or suggests optimal methods of implementing his decisions. Such a system may be considered to be pilot’s electronic assistant as it integrates simplified handling flight controls and autopilot functions, and reduces the complexity of interactions between aircraft attitudes, power settings, and rate of motion, and in conclusion limits the possibility of loss of control. 2004 Elsevier SAS. All rights reserved. Streszczenie Głównym celem prezentowanego projektu jest zwi˛ekszenie bezpiecze´nstwa i modyfikacja wła´sciwo´sci pilota˙zowych samolotów ogólnego przeznaczenia w taki sposób, aby zapewni´c mo˙zliwo´sc´ bezpiecznego pilotowania samolotu osobom z niewielkim do´swiadczeniem lotniczym. W pracy przedstawiono projekt perspektywicznego układu sterowania, pozwalajacego ˛ na intuicyjne sterowanie samolotem. Tak projektowane samoloty mo˙zna zaliczy´c do klasy ułatwionych samolotów lub łatwo sterowanych samolotów. Podstawowa˛ idea˛ tego rozwiazania ˛ jest zastosowanie po´sredniego systemu sterowania (Fly-by-Wire), a jego istotna˛ cecha˛ b˛edzie wysoki poziom automatyzacji, prowadzacy ˛ do budowy “prawie bezpilotowego” samolotu ogólnego przeznaczenia. Nie oznacza to wyeliminowania człowieka z procesu sterowania, lecz zmian˛e jego roli w systemie. Przyjazny u˙zytkownikowi układ sterowania powinien tak ukształtowa´c wła´sciwo´sci pilota˙zowe samolotu, aby sterowanie było łatwe i bezpieczne. Pilot przede wszystkim podejmuje decyzje, a system b˛edzie je realizował lub sugerował najlepsze sposoby ich wykonania. System taki mo˙zna równie˙z nazwa´c elektronicznym asystentem pilota, który integrujac ˛ funkcje r˛ecznego i automatycznego sterowania samolotem, oraz redukujac ˛ zło˙zone wzajemne zale˙zno´sci pomi˛edzy sterowaniem orientacja˛ przestrzenna,˛ moca˛ zespołu nap˛edowego i pr˛edko´sciami katowymi ˛ samolotu, zminimalizuje prawdopodobie´nstwo utraty kontroli nad samolotem. 2004 Elsevier SAS. All rights reserved. Keywords: Facilitated Airplane; Flight control systems; Handling qualities; General aviation Słowa kluczowe: Ułatwiony samolot; System sterowania samolotem; Wła´sciwo´sci pilota˙zowe; Lotnictwo ogólnego przeznaczenia
E-mail address: [email protected] (A. Tomczyk). 1270-9638/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.ast.2004.06.003
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1. Introduction In recent years, a growing interest in light airplanes (which constitute the general aviation sector) has been observed. This trend has been particularly visible in the USA where AGATE (Advanced General Aviation Transport Experiment) had been started in 1994 and continued, from 2000, as SATS (Small Aircraft Transportation System). These projects unite several federal institutions, universities and aircraft manufacturers for the purpose of establishing a general aircraft transportation system [1–3]. Air traffic is to be organized in a way similar to the highway system (Free Flight, Air Highway System, Highway in the Sky) with small airplanes operating from 5400 public airports and 18000 local landing airfields. Time of travel is to decrease by half within 10 years and by 2/3 within 25 years [4–7]. There is no such research program in Europe, but theoretical and experimental research of similar nature has been conducted in several Universities and research centers [8– 10]. Light planes do not require extensive aviation training, but do require certain psychological and physical predisposition and proper piloting skills. In order to facilitate training for safe handling of light planes, design properties that do not require the pilot to have exceptional manual skills and specialized aviation knowledge are searched for. For this purpose, efforts are undertaken to design aircraft exhibiting required handling qualities, and automate many navigating and piloting activities [10–13]. Specialized systems of handling and navigation data visualization, which facilitate interpretation of instrument readings (for example, tunnel display), have been designed [10,14,15]. Integrated navigation systems, based on satellite systems are used; global aircraft Automatic Depended Surveillance (ADS) systems are designed [15,16]. As far as piloting technique is concerned, limits are set by aircraft characteristics, which result from aerodynamics laws, and flight mechanics and dynamics. An aircraft’s complex dynamic properties are the reason that attitude stabilization, and especially takeoff and landing procedures, require the pilot to have proper coordination of manual flying controls displacement and engine power management, as well as acting-ahead abilities, made necessary by aircraft’s inertia. A significant difficulty in piloting is the necessity to abide by operating and safety limitations. Simple devices which help pilots and warn them when limits are being exceeded have been designed, among them: proper mass and aerodynamic balance, or devices controlling aircraft’s stall, such as stick-shaker and stick-pusher, for example. The next stage of control automatization is fly-by-wire systems. This development resulted from experiments with the F-8 aircraft digital flight control system [17,18] and was first applied in the F-16 aircraft [19]. In this case, there is no direct connection between stick displacement and control surface deflection. The aircraft is automatically controlled, and the pilot, by displacing the stick, enters information
about desired flight parameters to the flight control computer [20–23]. This flight control method allows for almost unlimited choice of designing the aircraft’s handling qualities and also makes it possible to control statically unstable aircraft. Light aircraft equipped with classic, mechanical flight control systems require full aviation training and comprehensive theoretical knowledge from the pilot. Measuring and navigation systems require extensive observation, detailed analysis of instrument indications, and complex interpretation. In military aircraft (F-16, F/A-18, Saab/BAe Gripen, etc.) and state-of-the-art jetliners (A-320, A-340, B-777, etc.), integrated systems of information collection, processing and presenting, along with the fly-by-wire flight control systems have been implemented [23–25]. Current technological level allows undertaking efforts to design an automated and integrated flight control system in light aircraft. New design concepts allow improvements in operating properties of already existing and newly designed executive aircraft, as well as increase flight safety.
2. Structure of flight control system Research team of the Department of Avionics and Control, Rzeszów University of Technology, has been working on a long-term project of designing a small aircraft flight control system, which would allow pilots with limited aviation experience to establish a safe control of the aircraft. The project draws from experiences and solutions employed in earlier designs of our team: digital flight control system for general aviation and commuter aircraft (1990) [26–28] and navigation and control system for Unmanned Air Vehicles (1999) [29,30]. The general structure of the fly-by-wire flight control system is presented in Fig. 1. The system is based on three independent flight control computers (CCx) that control double electromechanical actuators (ARx). Pilot selects a control option (system’s mode) using a control mode selector panel (SP) and then controls the aircraft by the means of side-stick (SS) and throttle lever (TL) which plays a role of the speed lever during flight. The main source of information concerning angular orientation of aircraft is the Attitude and Heading Reference System (AHRS). Parameters of aircraft’s movements in relation to air are measured by Air Data Computer (ADC). Navigation system, consisting of integrated receiver GNS530 (GPS, VOR, ILS, comm) and backup receiver GPS-35, assures proper navigation and instrument-assisted landing. “Other measurements” block presents remaining sensors and measurement systems, engine instruments included. Measurement systems are multiplied (hardware redundancy). Integration of the system is established by a triple digital, low cost, high speed, bi-directional databus network CAN-2 (C1, C2, C3) that connects all system elements with controlling computers [31]. A PWM (Pulse Duration Modulation) signal standard has been used for controlling the actuators –
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Fig. 1. General structure of the fly-by-wire flight control system for general aviation aircraft.
independent lines P1, P2 and P3. In case of malfunction of all three controlling computers or all three CAN-2 network lines, or total breakdown of measurement systems, an emergency direct control of actuators is possible by the means of independent PWM signal (line P0) generated directly in side-stick module. Maintenance computer (MC) may be connected in order to complete service tasks, full testing or adjustment of the system and it will be used during flight testing of the control system, as well. Software installed on flight control computers performs analytical redundancy function, carries on diagnostic procedures, and reconfigures the system in case of malfunction. The following general design rule has been adhered to: the probability of simultaneous malfunction of two complementary elements is not greater that in case of classic mechanical flight control system (of the order of 10−9 per hour of flight) [21]. Operation of an indirect flight control system must be characterized by high degree of reliability. Diagnostic systems that evaluate state of control system and on-board instruments have been employed. Method of controlling aircraft is determined on the basis of rules established for the following levels: • Level I – normal control, all the properties of indirect flight control system are employed, • Level II – simplified control, only the simple CAS (Control Augmentation System) or forming filters are used, • Level III – emergency control, displacement of aerodynamic control surfaces depends directly on side-stick displacement.
Change of control method is realized by on-board supervisory subsystem if imperfect operation of crucial elements of control system is discovered. However, we can also consider an option of manual switching to Level II operation in case of any problem with autopilot or for “more manual” handling a plane. Level III should be used in emergency only for reaching a safe area where airplane-parachute type lifesaving system could be used. General functional properties of the system have been presented in earlier publications [32–40]. Main characteristics of the system may be described as continued stabilization of attitude orientation of an aircraft and guidance (for example, flight along a selected path on constant altitude). However, pilot can influence the flight state at any moment by displacing the side-stick. It is not a classic automatic control but rather a manual control following the general rule: if pilot does not take action (side-stick remains in neutral placement), previously planned flight plan is realized, or, previous safe flight conditions are maintained. Using a Control Mode Selector Panel, pilot selects a desired mode of aircraft flight control system: TEST TAXI
checking correctness of system’s operation, taxiing with limited speed adjusted to the type of aircraft, TAKE-OFF take-off performed by the automatic flight control system in cooperation with the pilot, FLIGHT stabilization of an aircraft’s attitude, SAFE FLIGHT stabilization of attitude of an aircraft assuring the lowest probability of crash landing or loss of aircraft’s attitude: straight line flight with zero
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bank, maximum power of engine, speed VIAS = V2 – the best climbing speed, NAVIGATION flight following a trajectory determined by selected navigation system, APPROACH manual (performed by pilot) or automatic guidance of aircraft along the instrument landing path, LANDING flare and touchdown performed by the automatic flight control system in cooperation with the pilot, landing run. Selecting a control option determines employing appropriate aircraft control laws, based on the modified modelfollowing control method [34]. The properties of the flight control system are described in the form of control algorithms with specific restrictions (constrains). For example, from pilot’s point of view, the main mode of operation (takeoff, flight and landing) can be presented as follows:
• throttle lever is used for selection of the IAS value from the interval between minimal safe speed and maximum speed for a given flight state, • side-stick displacement causes a change in aircraft bank angle and flight path angle (change in vertical speed), according to predefined control laws, with constant IAS airspeed. The flight control system automatically ensures: • sideslip angle reduction (symmetric flight), • damping aircraft’s own and external forced oscillations, • adjusting flaps’ and airbrakes’ positions to selected flight conditions, • minimizing hinge moments by aerodynamic balancing of controls. LANDING
TAKE-OFF Throttle lever position limits the power of the engine(s); for example, maximum power or only power necessary for take-off in given configuration. Automatically, flaps are moved into take-off position, on-board systems, plane and power plant are checked. Displacement of throttle controls into the extreme front position results in brake release and take-off run start. The pilot maintains direction of take-off by side-stick rotation. Up to VT2 speed, the aircraft is controlled by front wheel rotation and rudder displacement; for speed V > VT2 , only rudder deflection is used. After reaching rotation speed VR , the elevator is automatically moved into position, which ensure obtaining pitch angle necessary for pull-off. Simultaneously, the pilot can correct the position of aircraft if necessary by control sidestick displacement. After the front wheel lifts from runway, the control system automatically switches into a mode of stabilizing current heading using the rudder surface. Lifting aircraft’s main wheels from the ground signifies beginning the task of stabilizing desired predefined pitch angle and current heading by coordinated ailerons and rudder deflections. Take-off phase continues until safe climbing speed (V2 ) is reached, and then the system automatically switches to the FLIGHT mode. Current heading and aircraft’s pitch angle will continue to be stabilized. FLIGHT It is a basic flight control option. Manual control elements perform the following functions: • throttle lever position limits maximum power of power plant, extreme minimal position determines power necessary to continue flight with safe speed in current aircraft configuration,
Final approach, flare, and touch down are executed using manual control, assisted by the control augmentation system. Selecting the LANDING option on the control mode selector panel activates a button on the side-stick, which activates the landing control augmentation system. When the pilot presses this button at chosen altitude (based on visual observation), an automatic change in aircraft’s attitude is executed. The second solution analyzed is measuring the real height by sonar ranging transducer and changing the pitch angle without pilot’s intervention. New attitude ensures safe touchdown because: • aircraft’s heading consistent with selected landing direction (drift angle is eliminated if necessary) is determined, • proper pitch angle appropriate for contact between aircraft and runway is selected, • engine power, which ensures proper vertical speed for a particular aircraft during touchdown, is determined, • in this phase of flight, zero bank angle is sustained by use of ailerons, heading stabilization is accomplished by rudder, • at all times, pilot has an option of correcting aircraft position and flight speed using side-stick and throttle lever, • when main wheels touch the runway, engine power is minimized and nose wheel is lowered, • when nose wheel touches the runway, automatic switch to taxiing is executed; pilot sustains landing run direction and controls braking intensity by using the throttle lever and brakes. All modes of operation were prepared and tested at the laboratory stand. So far, only FLIGHT mode was used in flight.
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Fig. 2. Simplified diagram of model-following aircraft control.
Fig. 3. Example of a forming filter characteristic in aileron channel.
3. Control algorithm The basic idea of control system being designed is appropriate stabilization of angular orientation of aircraft without pilot’s intervention, and timely following of pilot’s instructions that change attitude and flight parameters. This problem has been solved by employing a model following control system. Its simplified diagram is presented in Fig. 2. If pilot does not displace the side-stick, aircraft is stable (current state vector X is maintained) and influence of external factors (N ) is minimized. By displacing the side-stick, pilot generates directional signal UP . Expected reaction of aircraft is described by change in vector YM following the properties of the model of ideal aircraft (MoIA) employed in the particular option of aircraft control. Forming filter FF allows initial transformation of the UP signal into information about intended change in angular orientation of aircraft (UPF ). For example, in bank angle control channel, a forming filter presented in Fig. 3 is tested. Filter’s properties take into consideration expected efficiency of control and restrictions placed on bank angle (φ) and roll rate (p). An example presents one of the variants of control system properties in which, for relative side-stick displacements smaller than some value a, the roll rate proportional to stick displacement is obtained, and bank angle is limited to φ1 , while for large stick displacement (aggressive control), roll rate is constant (pmax ), and maximum bank angle is proportional to stick displacement. For practical purposes, introducing a dead band (UP ) around the central (neutral) side-stick placement is planned. Basic properties of flight control systems are shaped by selection of ideal aircraft model. Selection of the model is a result of extensive theoretical analysis as well as practi-
Fig. 4. 3D-surface describing Rogalski’s forming filter, where: x = UP , output1 = UPF .
cal experiences, including flight simulator tests and flight tests [22,23]. Output signal of the model (YM ) is compared to realize properties of the aircraft (observation matrix C maps measured vector of output signals Y from measurement subsystem (ME) to signals YC with structure compatible with model’s output vector YM ) and difference Y is minimized. Flight computer (FC) controls aircraft (A/C) by actuator (AR). Blocks encircled with a dotted line form a control augmentation system, which may be described as Handling Qualities Augmentation System, or Handling Augmentation System (HAS). An example of HAS synthesis in case of the landing augmentation system has been presented in Refs. [35,36,39]. In case of malfunction of measurement systems, control level II or III should be used when deflection of aerodynamics control surfaces depends directly on stick displacement. In such situation, dynamic properties of actuators significantly influence quality of manual control. Significant improvement of flight control quality may be obtained by employing appropriate forming filter. Very good results have been obtained with filter described by Rogalski’s formula [37,38]: • • |f (U ) + k · U P P | · sign f (U ) + k · U , δD = min P P UP max 2 UP , f (UP ) = √ UP max where δD – required displacement of aerodynamics control surface, UP – stick displacement, k – weight factor of sidestick rate of movement.
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Properties of forming filter are graphically presented by 3D-surface in Fig. 4. The experimental fly by-wire control system was designed, built, and tested [40]. All basic modules of the system were designed by research team, build and tested in laboratory conditions. There are two main goals of the project. At first, we are going to conduct research in the area of small aircraft Handling Qualities, PIO problem, and develop diagnostic and reconfiguration flight control system. The second goal is different: we would like to use the experimental aircraft for didactic purposes. There will be a kind of flying laboratory for students. They’ll be able to execute the experimental part of their diploma thesis, in the area of flight dynamics, control systems, Flying Qualities, and flight tests. The main properties of the control system are described by algorithms. Basic software modules were prepared in C++ language and different versions of algorithms and compilers were applied. Databus CAN-2 is not aviation standard; so, the original protocol of communication was prepared and tested. All data sending by on-board network are monitored in real time and recorded at the laptop computer during laboratory and flight tests. The last step of this project – so far – were flight tests in July 2003. The small aircraft “Koliber” was used as a test platform (Fig. 5). Test flights took place at Rzeszów University Aviation Training Center located in the south-eastern
part of Poland. Polish Aviation Authority was monitoring the activity of the research team from flight safety point of view, and they have given permission for flight tests. The cockpit of the “Koliber” aircraft was equipped with selector panel, side stick and GNS-530 navigation receiver. All electronic modules were fastened on the plate and located on the passenger seat. There are three AHRS, two ADC, three flight computers, and another modules (Fig. 6). During the first testing flight, the Level III mode of operation was tested. A proportional flight control system has been used, when expected displacement of control surface is proportional to side-stick movement (analogously to direct mechanical linkage, but electromechanical actuator was used in the control loop). This version of aircraft has been found by pilots to be difficult to control. Necessary deflections of the side-stick are big and frequency of them is too high (Fig. 7). The second example (Fig. 8) shows the some horizontal turns, but the Level II mode of operation was used, with forming filter. Pitch and bank angle stabilization is easy – there were no problems. Picture in Fig. 9 presents results of the horizontal turns with normal operation mode of the flight control system. Piloting is very easy because aircraft attitude is automatically stabilized; side-stick deflection causes the bank and pitch angle changing. The leveler system is used as well.
4. Final remarks
Fig. 5. PZL-110 “Koliber” aircraft as a test platform.
Flight control system constitutes only a part of on-board equipment, but it determines, to a large extent, overall usage properties of an aircraft. There are no published data on properties of fly-by-wire control systems installed in small aircraft. Designed system was employed in general aviation aircraft PZL-110 Koliber which will allow to verify the correctness of assumptions concerning system and design solutions as well as to design a method of synthesis and validation of control laws for this class of aircraft. It will hopefully
Fig. 6. Electronics modules and additional cockpit equipment.
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Fig. 7. Attitude stabilization – emergency control (Level III).
Fig. 8. Attitude stabilization – simplified control (Level II).
Fig. 9. Attitude stabilization – normal control (Level I).
lead to establishing an intuitive, easy-to-follow flight path guidance. In general, the first preliminary results of the flight tests are positive. Hardware solutions are accepted, system worked well, any malfunctions were not observed. During
next flights, we would like to make more experiments concerning Handling Qualities and aircraft dynamic properties modification. The experiments should show the role of human-aiding automation in creating single-crew safety flights. This re-
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search will also aid the design of new types of pilot training and certification process because new piloting skills will be required. Favorable research results allow to propose creation of a new class of general aviation aircraft – Facilitated Airplane (FA) or Simple Flying Airplane (SFA).1 Further experiments will shape requirements and design recommendations for this class of aircraft. Term “carefree handling” which is used in military aviation, may also be employed for the proposed class of small aircraft [21,23]. Simultaneously, a future FA aircraft should be equipped with recently developed navigation systems (EFIS, Tunel-in-the-sky display, GPS, EVS, TAWS, ADS-B) as well as an airplane-parachute type lifesaving system (like SR20 Cirrus aircraft). Should further experiments confirm favorable properties of such equipped aircraft, it will be possible to propose a new Facilitated Airplane Pilot License (FAPL) as fast and safe operation of a FA aircraft will be easy to learn.
Acknowledgements This paper was prepared within the framework of the project “Fly-by-wire control system for general aviation aircraft” financed by The Polish State Committee for Scientific Research in the years 2003–2005.
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1 The reviewer of the paper has suggested the term: Safe Flying Airplane. Thank you.
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Facilitated Airplane – project and preliminary in-flight experiments Andrzej Tomczyk Department of Avionics and Control, Rzeszów University of Technology, W. Pola 2, 35-959 Rzeszów, Poland Received 17 November 2003; received in revised form 22 April 2004; accepted 15 June 2004 Available online 29 July 2004
Abstract The main goal of the presented project is to improve safety and Handling Qualities of general aviation aircraft so that airplane becomes pilot-friendly, “almost unmanned” aircraft. In this paper, a proposal of employing an intuitive, human-centered, simplified software-based flight control system in general aviation aircraft has been presented. Airplanes equipped with such a flight control system belong to a new class of general aviation aircraft with improved safety and efficiency handling properties – Facilitated Airplane (FA) or Simple Flying Airplane (SFA). The basic idea of the project is to employ an indirect (Fly-by-Wire) software-based flight control system characterized by high degree of automatization, leading to an almost “unmanned” general aviation aircraft. “Unmanned” does not mean eliminating humans from the control process but changing their role in the system. User-friendly control system should shape handling qualities of an aircraft in such a way that control becomes easy and safe. Pilot retains the crucial role of decision-maker, and control system takes appropriate steps to fulfill his requirements, or suggests optimal methods of implementing his decisions. Such a system may be considered to be pilot’s electronic assistant as it integrates simplified handling flight controls and autopilot functions, and reduces the complexity of interactions between aircraft attitudes, power settings, and rate of motion, and in conclusion limits the possibility of loss of control. 2004 Elsevier SAS. All rights reserved. Streszczenie Głównym celem prezentowanego projektu jest zwi˛ekszenie bezpiecze´nstwa i modyfikacja wła´sciwo´sci pilota˙zowych samolotów ogólnego przeznaczenia w taki sposób, aby zapewni´c mo˙zliwo´sc´ bezpiecznego pilotowania samolotu osobom z niewielkim do´swiadczeniem lotniczym. W pracy przedstawiono projekt perspektywicznego układu sterowania, pozwalajacego ˛ na intuicyjne sterowanie samolotem. Tak projektowane samoloty mo˙zna zaliczy´c do klasy ułatwionych samolotów lub łatwo sterowanych samolotów. Podstawowa˛ idea˛ tego rozwiazania ˛ jest zastosowanie po´sredniego systemu sterowania (Fly-by-Wire), a jego istotna˛ cecha˛ b˛edzie wysoki poziom automatyzacji, prowadzacy ˛ do budowy “prawie bezpilotowego” samolotu ogólnego przeznaczenia. Nie oznacza to wyeliminowania człowieka z procesu sterowania, lecz zmian˛e jego roli w systemie. Przyjazny u˙zytkownikowi układ sterowania powinien tak ukształtowa´c wła´sciwo´sci pilota˙zowe samolotu, aby sterowanie było łatwe i bezpieczne. Pilot przede wszystkim podejmuje decyzje, a system b˛edzie je realizował lub sugerował najlepsze sposoby ich wykonania. System taki mo˙zna równie˙z nazwa´c elektronicznym asystentem pilota, który integrujac ˛ funkcje r˛ecznego i automatycznego sterowania samolotem, oraz redukujac ˛ zło˙zone wzajemne zale˙zno´sci pomi˛edzy sterowaniem orientacja˛ przestrzenna,˛ moca˛ zespołu nap˛edowego i pr˛edko´sciami katowymi ˛ samolotu, zminimalizuje prawdopodobie´nstwo utraty kontroli nad samolotem. 2004 Elsevier SAS. All rights reserved. Keywords: Facilitated Airplane; Flight control systems; Handling qualities; General aviation Słowa kluczowe: Ułatwiony samolot; System sterowania samolotem; Wła´sciwo´sci pilota˙zowe; Lotnictwo ogólnego przeznaczenia
E-mail address: [email protected] (A. Tomczyk). 1270-9638/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.ast.2004.06.003
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1. Introduction In recent years, a growing interest in light airplanes (which constitute the general aviation sector) has been observed. This trend has been particularly visible in the USA where AGATE (Advanced General Aviation Transport Experiment) had been started in 1994 and continued, from 2000, as SATS (Small Aircraft Transportation System). These projects unite several federal institutions, universities and aircraft manufacturers for the purpose of establishing a general aircraft transportation system [1–3]. Air traffic is to be organized in a way similar to the highway system (Free Flight, Air Highway System, Highway in the Sky) with small airplanes operating from 5400 public airports and 18000 local landing airfields. Time of travel is to decrease by half within 10 years and by 2/3 within 25 years [4–7]. There is no such research program in Europe, but theoretical and experimental research of similar nature has been conducted in several Universities and research centers [8– 10]. Light planes do not require extensive aviation training, but do require certain psychological and physical predisposition and proper piloting skills. In order to facilitate training for safe handling of light planes, design properties that do not require the pilot to have exceptional manual skills and specialized aviation knowledge are searched for. For this purpose, efforts are undertaken to design aircraft exhibiting required handling qualities, and automate many navigating and piloting activities [10–13]. Specialized systems of handling and navigation data visualization, which facilitate interpretation of instrument readings (for example, tunnel display), have been designed [10,14,15]. Integrated navigation systems, based on satellite systems are used; global aircraft Automatic Depended Surveillance (ADS) systems are designed [15,16]. As far as piloting technique is concerned, limits are set by aircraft characteristics, which result from aerodynamics laws, and flight mechanics and dynamics. An aircraft’s complex dynamic properties are the reason that attitude stabilization, and especially takeoff and landing procedures, require the pilot to have proper coordination of manual flying controls displacement and engine power management, as well as acting-ahead abilities, made necessary by aircraft’s inertia. A significant difficulty in piloting is the necessity to abide by operating and safety limitations. Simple devices which help pilots and warn them when limits are being exceeded have been designed, among them: proper mass and aerodynamic balance, or devices controlling aircraft’s stall, such as stick-shaker and stick-pusher, for example. The next stage of control automatization is fly-by-wire systems. This development resulted from experiments with the F-8 aircraft digital flight control system [17,18] and was first applied in the F-16 aircraft [19]. In this case, there is no direct connection between stick displacement and control surface deflection. The aircraft is automatically controlled, and the pilot, by displacing the stick, enters information
about desired flight parameters to the flight control computer [20–23]. This flight control method allows for almost unlimited choice of designing the aircraft’s handling qualities and also makes it possible to control statically unstable aircraft. Light aircraft equipped with classic, mechanical flight control systems require full aviation training and comprehensive theoretical knowledge from the pilot. Measuring and navigation systems require extensive observation, detailed analysis of instrument indications, and complex interpretation. In military aircraft (F-16, F/A-18, Saab/BAe Gripen, etc.) and state-of-the-art jetliners (A-320, A-340, B-777, etc.), integrated systems of information collection, processing and presenting, along with the fly-by-wire flight control systems have been implemented [23–25]. Current technological level allows undertaking efforts to design an automated and integrated flight control system in light aircraft. New design concepts allow improvements in operating properties of already existing and newly designed executive aircraft, as well as increase flight safety.
2. Structure of flight control system Research team of the Department of Avionics and Control, Rzeszów University of Technology, has been working on a long-term project of designing a small aircraft flight control system, which would allow pilots with limited aviation experience to establish a safe control of the aircraft. The project draws from experiences and solutions employed in earlier designs of our team: digital flight control system for general aviation and commuter aircraft (1990) [26–28] and navigation and control system for Unmanned Air Vehicles (1999) [29,30]. The general structure of the fly-by-wire flight control system is presented in Fig. 1. The system is based on three independent flight control computers (CCx) that control double electromechanical actuators (ARx). Pilot selects a control option (system’s mode) using a control mode selector panel (SP) and then controls the aircraft by the means of side-stick (SS) and throttle lever (TL) which plays a role of the speed lever during flight. The main source of information concerning angular orientation of aircraft is the Attitude and Heading Reference System (AHRS). Parameters of aircraft’s movements in relation to air are measured by Air Data Computer (ADC). Navigation system, consisting of integrated receiver GNS530 (GPS, VOR, ILS, comm) and backup receiver GPS-35, assures proper navigation and instrument-assisted landing. “Other measurements” block presents remaining sensors and measurement systems, engine instruments included. Measurement systems are multiplied (hardware redundancy). Integration of the system is established by a triple digital, low cost, high speed, bi-directional databus network CAN-2 (C1, C2, C3) that connects all system elements with controlling computers [31]. A PWM (Pulse Duration Modulation) signal standard has been used for controlling the actuators –
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Fig. 1. General structure of the fly-by-wire flight control system for general aviation aircraft.
independent lines P1, P2 and P3. In case of malfunction of all three controlling computers or all three CAN-2 network lines, or total breakdown of measurement systems, an emergency direct control of actuators is possible by the means of independent PWM signal (line P0) generated directly in side-stick module. Maintenance computer (MC) may be connected in order to complete service tasks, full testing or adjustment of the system and it will be used during flight testing of the control system, as well. Software installed on flight control computers performs analytical redundancy function, carries on diagnostic procedures, and reconfigures the system in case of malfunction. The following general design rule has been adhered to: the probability of simultaneous malfunction of two complementary elements is not greater that in case of classic mechanical flight control system (of the order of 10−9 per hour of flight) [21]. Operation of an indirect flight control system must be characterized by high degree of reliability. Diagnostic systems that evaluate state of control system and on-board instruments have been employed. Method of controlling aircraft is determined on the basis of rules established for the following levels: • Level I – normal control, all the properties of indirect flight control system are employed, • Level II – simplified control, only the simple CAS (Control Augmentation System) or forming filters are used, • Level III – emergency control, displacement of aerodynamic control surfaces depends directly on side-stick displacement.
Change of control method is realized by on-board supervisory subsystem if imperfect operation of crucial elements of control system is discovered. However, we can also consider an option of manual switching to Level II operation in case of any problem with autopilot or for “more manual” handling a plane. Level III should be used in emergency only for reaching a safe area where airplane-parachute type lifesaving system could be used. General functional properties of the system have been presented in earlier publications [32–40]. Main characteristics of the system may be described as continued stabilization of attitude orientation of an aircraft and guidance (for example, flight along a selected path on constant altitude). However, pilot can influence the flight state at any moment by displacing the side-stick. It is not a classic automatic control but rather a manual control following the general rule: if pilot does not take action (side-stick remains in neutral placement), previously planned flight plan is realized, or, previous safe flight conditions are maintained. Using a Control Mode Selector Panel, pilot selects a desired mode of aircraft flight control system: TEST TAXI
checking correctness of system’s operation, taxiing with limited speed adjusted to the type of aircraft, TAKE-OFF take-off performed by the automatic flight control system in cooperation with the pilot, FLIGHT stabilization of an aircraft’s attitude, SAFE FLIGHT stabilization of attitude of an aircraft assuring the lowest probability of crash landing or loss of aircraft’s attitude: straight line flight with zero
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bank, maximum power of engine, speed VIAS = V2 – the best climbing speed, NAVIGATION flight following a trajectory determined by selected navigation system, APPROACH manual (performed by pilot) or automatic guidance of aircraft along the instrument landing path, LANDING flare and touchdown performed by the automatic flight control system in cooperation with the pilot, landing run. Selecting a control option determines employing appropriate aircraft control laws, based on the modified modelfollowing control method [34]. The properties of the flight control system are described in the form of control algorithms with specific restrictions (constrains). For example, from pilot’s point of view, the main mode of operation (takeoff, flight and landing) can be presented as follows:
• throttle lever is used for selection of the IAS value from the interval between minimal safe speed and maximum speed for a given flight state, • side-stick displacement causes a change in aircraft bank angle and flight path angle (change in vertical speed), according to predefined control laws, with constant IAS airspeed. The flight control system automatically ensures: • sideslip angle reduction (symmetric flight), • damping aircraft’s own and external forced oscillations, • adjusting flaps’ and airbrakes’ positions to selected flight conditions, • minimizing hinge moments by aerodynamic balancing of controls. LANDING
TAKE-OFF Throttle lever position limits the power of the engine(s); for example, maximum power or only power necessary for take-off in given configuration. Automatically, flaps are moved into take-off position, on-board systems, plane and power plant are checked. Displacement of throttle controls into the extreme front position results in brake release and take-off run start. The pilot maintains direction of take-off by side-stick rotation. Up to VT2 speed, the aircraft is controlled by front wheel rotation and rudder displacement; for speed V > VT2 , only rudder deflection is used. After reaching rotation speed VR , the elevator is automatically moved into position, which ensure obtaining pitch angle necessary for pull-off. Simultaneously, the pilot can correct the position of aircraft if necessary by control sidestick displacement. After the front wheel lifts from runway, the control system automatically switches into a mode of stabilizing current heading using the rudder surface. Lifting aircraft’s main wheels from the ground signifies beginning the task of stabilizing desired predefined pitch angle and current heading by coordinated ailerons and rudder deflections. Take-off phase continues until safe climbing speed (V2 ) is reached, and then the system automatically switches to the FLIGHT mode. Current heading and aircraft’s pitch angle will continue to be stabilized. FLIGHT It is a basic flight control option. Manual control elements perform the following functions: • throttle lever position limits maximum power of power plant, extreme minimal position determines power necessary to continue flight with safe speed in current aircraft configuration,
Final approach, flare, and touch down are executed using manual control, assisted by the control augmentation system. Selecting the LANDING option on the control mode selector panel activates a button on the side-stick, which activates the landing control augmentation system. When the pilot presses this button at chosen altitude (based on visual observation), an automatic change in aircraft’s attitude is executed. The second solution analyzed is measuring the real height by sonar ranging transducer and changing the pitch angle without pilot’s intervention. New attitude ensures safe touchdown because: • aircraft’s heading consistent with selected landing direction (drift angle is eliminated if necessary) is determined, • proper pitch angle appropriate for contact between aircraft and runway is selected, • engine power, which ensures proper vertical speed for a particular aircraft during touchdown, is determined, • in this phase of flight, zero bank angle is sustained by use of ailerons, heading stabilization is accomplished by rudder, • at all times, pilot has an option of correcting aircraft position and flight speed using side-stick and throttle lever, • when main wheels touch the runway, engine power is minimized and nose wheel is lowered, • when nose wheel touches the runway, automatic switch to taxiing is executed; pilot sustains landing run direction and controls braking intensity by using the throttle lever and brakes. All modes of operation were prepared and tested at the laboratory stand. So far, only FLIGHT mode was used in flight.
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Fig. 2. Simplified diagram of model-following aircraft control.
Fig. 3. Example of a forming filter characteristic in aileron channel.
3. Control algorithm The basic idea of control system being designed is appropriate stabilization of angular orientation of aircraft without pilot’s intervention, and timely following of pilot’s instructions that change attitude and flight parameters. This problem has been solved by employing a model following control system. Its simplified diagram is presented in Fig. 2. If pilot does not displace the side-stick, aircraft is stable (current state vector X is maintained) and influence of external factors (N ) is minimized. By displacing the side-stick, pilot generates directional signal UP . Expected reaction of aircraft is described by change in vector YM following the properties of the model of ideal aircraft (MoIA) employed in the particular option of aircraft control. Forming filter FF allows initial transformation of the UP signal into information about intended change in angular orientation of aircraft (UPF ). For example, in bank angle control channel, a forming filter presented in Fig. 3 is tested. Filter’s properties take into consideration expected efficiency of control and restrictions placed on bank angle (φ) and roll rate (p). An example presents one of the variants of control system properties in which, for relative side-stick displacements smaller than some value a, the roll rate proportional to stick displacement is obtained, and bank angle is limited to φ1 , while for large stick displacement (aggressive control), roll rate is constant (pmax ), and maximum bank angle is proportional to stick displacement. For practical purposes, introducing a dead band (UP ) around the central (neutral) side-stick placement is planned. Basic properties of flight control systems are shaped by selection of ideal aircraft model. Selection of the model is a result of extensive theoretical analysis as well as practi-
Fig. 4. 3D-surface describing Rogalski’s forming filter, where: x = UP , output1 = UPF .
cal experiences, including flight simulator tests and flight tests [22,23]. Output signal of the model (YM ) is compared to realize properties of the aircraft (observation matrix C maps measured vector of output signals Y from measurement subsystem (ME) to signals YC with structure compatible with model’s output vector YM ) and difference Y is minimized. Flight computer (FC) controls aircraft (A/C) by actuator (AR). Blocks encircled with a dotted line form a control augmentation system, which may be described as Handling Qualities Augmentation System, or Handling Augmentation System (HAS). An example of HAS synthesis in case of the landing augmentation system has been presented in Refs. [35,36,39]. In case of malfunction of measurement systems, control level II or III should be used when deflection of aerodynamics control surfaces depends directly on stick displacement. In such situation, dynamic properties of actuators significantly influence quality of manual control. Significant improvement of flight control quality may be obtained by employing appropriate forming filter. Very good results have been obtained with filter described by Rogalski’s formula [37,38]: • • |f (U ) + k · U P P | · sign f (U ) + k · U , δD = min P P UP max 2 UP , f (UP ) = √ UP max where δD – required displacement of aerodynamics control surface, UP – stick displacement, k – weight factor of sidestick rate of movement.
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Properties of forming filter are graphically presented by 3D-surface in Fig. 4. The experimental fly by-wire control system was designed, built, and tested [40]. All basic modules of the system were designed by research team, build and tested in laboratory conditions. There are two main goals of the project. At first, we are going to conduct research in the area of small aircraft Handling Qualities, PIO problem, and develop diagnostic and reconfiguration flight control system. The second goal is different: we would like to use the experimental aircraft for didactic purposes. There will be a kind of flying laboratory for students. They’ll be able to execute the experimental part of their diploma thesis, in the area of flight dynamics, control systems, Flying Qualities, and flight tests. The main properties of the control system are described by algorithms. Basic software modules were prepared in C++ language and different versions of algorithms and compilers were applied. Databus CAN-2 is not aviation standard; so, the original protocol of communication was prepared and tested. All data sending by on-board network are monitored in real time and recorded at the laptop computer during laboratory and flight tests. The last step of this project – so far – were flight tests in July 2003. The small aircraft “Koliber” was used as a test platform (Fig. 5). Test flights took place at Rzeszów University Aviation Training Center located in the south-eastern
part of Poland. Polish Aviation Authority was monitoring the activity of the research team from flight safety point of view, and they have given permission for flight tests. The cockpit of the “Koliber” aircraft was equipped with selector panel, side stick and GNS-530 navigation receiver. All electronic modules were fastened on the plate and located on the passenger seat. There are three AHRS, two ADC, three flight computers, and another modules (Fig. 6). During the first testing flight, the Level III mode of operation was tested. A proportional flight control system has been used, when expected displacement of control surface is proportional to side-stick movement (analogously to direct mechanical linkage, but electromechanical actuator was used in the control loop). This version of aircraft has been found by pilots to be difficult to control. Necessary deflections of the side-stick are big and frequency of them is too high (Fig. 7). The second example (Fig. 8) shows the some horizontal turns, but the Level II mode of operation was used, with forming filter. Pitch and bank angle stabilization is easy – there were no problems. Picture in Fig. 9 presents results of the horizontal turns with normal operation mode of the flight control system. Piloting is very easy because aircraft attitude is automatically stabilized; side-stick deflection causes the bank and pitch angle changing. The leveler system is used as well.
4. Final remarks
Fig. 5. PZL-110 “Koliber” aircraft as a test platform.
Flight control system constitutes only a part of on-board equipment, but it determines, to a large extent, overall usage properties of an aircraft. There are no published data on properties of fly-by-wire control systems installed in small aircraft. Designed system was employed in general aviation aircraft PZL-110 Koliber which will allow to verify the correctness of assumptions concerning system and design solutions as well as to design a method of synthesis and validation of control laws for this class of aircraft. It will hopefully
Fig. 6. Electronics modules and additional cockpit equipment.
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Fig. 7. Attitude stabilization – emergency control (Level III).
Fig. 8. Attitude stabilization – simplified control (Level II).
Fig. 9. Attitude stabilization – normal control (Level I).
lead to establishing an intuitive, easy-to-follow flight path guidance. In general, the first preliminary results of the flight tests are positive. Hardware solutions are accepted, system worked well, any malfunctions were not observed. During
next flights, we would like to make more experiments concerning Handling Qualities and aircraft dynamic properties modification. The experiments should show the role of human-aiding automation in creating single-crew safety flights. This re-
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search will also aid the design of new types of pilot training and certification process because new piloting skills will be required. Favorable research results allow to propose creation of a new class of general aviation aircraft – Facilitated Airplane (FA) or Simple Flying Airplane (SFA).1 Further experiments will shape requirements and design recommendations for this class of aircraft. Term “carefree handling” which is used in military aviation, may also be employed for the proposed class of small aircraft [21,23]. Simultaneously, a future FA aircraft should be equipped with recently developed navigation systems (EFIS, Tunel-in-the-sky display, GPS, EVS, TAWS, ADS-B) as well as an airplane-parachute type lifesaving system (like SR20 Cirrus aircraft). Should further experiments confirm favorable properties of such equipped aircraft, it will be possible to propose a new Facilitated Airplane Pilot License (FAPL) as fast and safe operation of a FA aircraft will be easy to learn.
Acknowledgements This paper was prepared within the framework of the project “Fly-by-wire control system for general aviation aircraft” financed by The Polish State Committee for Scientific Research in the years 2003–2005.
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