By Thierry Dubois
Fly-by-wire controls on civil turbine helicopters may be in the offing at last, as all major manufacturers—Sikorsky, Bell, Eurocopter and AgustaWestland—have some sort of research and development (R&D) work under way to have such technology certified, although they hardly admit it. Russian Helicopters has already delivered some 20 copies of the Ansat, a light utility helicopter, with fly-by-wire controls. Such technology is hoped to make flying easier and maintenance lighter. Fly-by-wire is to conventional controls what a glass cockpit is to steam and gauges. Controls are not connected mechanically to the control surfaces. There are no more rods, cables or pulleys. Instead, controls are connected electrically to the flight control computer. Based on designed flight control laws, the computer sends commands to actuators that move control surfaces.
Fly-by-wire controls on helicopters have been studied and tested for almost half a century. This was when the National Research Council of Canada (NRC) developed its first “variable stability” helicopter. In 1960, NRC acquired a Bell 47 and modified it into a fly-by-wire aircraft. There was full authority control on three axes.
Yet, today, only two Western rotorcraft—the Eurocopter/AgustaWestland/Fokker NH90 military transport and the Bell Boeing V-22 Osprey military tiltrotor—are in service with such controls. The next one in the military—with possible civil offshoots—may be the Sikorsky CH148 Cyclone, derived from the S-92 medium transport. “The CH148 is in the last stages of its development,” Kenneth Arifian, Sikorsky’s technical fellow for flight controls, told Rotor & Wing. Two aircraft are flying and a third one was ready to fly, as of mid-May. One will be used for sea trials aboard some of the Canadian Forces’ ships. The CH148 program calls for first deliveries to take place in late 2010.
Benefits, for the pilot, are improved handling qualities and a reduced workload, emphasized Teresa Carleton, vice president for mission systems integration. “A FBW control system helps the crew pay more attention outside the aircraft; in other words, the pilots do less housekeeping inside,” said James Kagdis, program manager for advanced programs. In terms of maintenance, the aircraft gets rid of a number of mechanical parts that needed servicing. Moreover, such a system has built-in data recording and diagnostic capabilities. Finally, a FBW control system weighs less than a conventional one, Carleton pointed out.
Specific to the CH148 is the flight director mode, closely linked to the flight control system. For example, the Canadian Forces will be able to automate search patterns.
For the CH148’s fly-by-wire controls, BAE Systems supplies the computers and the operating system, Carleton explained. Sikorsky provides flight-control laws. The U.S. manufacturer has never produced a fly-by-wire helicopter. However, it may claim to have some knowledge, since the Comanche attack helicopter was to have such controls, before the U.S. Army canceled the program in 2004.
The CH148’s system was designed to support a lot of aircraft. It would be a good baseline for FBW on other helicopters, Carleton said. Moreover, although it will enter into service in the military field, it was designed with civil certification in mind. This could help certification on a civil helicopter to take place within a few years, should Sikorsky launch such a program. Differently from Eurocopter, Sikorsky says replacing hydraulic control actuators is not on the agenda yet. Power density (in kilowatt per pound) is still too low with electric actuators, Arifian said.
|Russian Helicopters is working on fly-by-wire systems on different types of rotorcraft, building on the FBW controls for the in-service Kazan Ansat. Russian Helicopters
On the X2 high-speed compound prototype, development of the fly-by-wire system has been complete for months. It was first tested in 2005 on a Schweizer 333. Sikorsky is using fly-by-wire to develop the X2’s flying capabilities to high speeds—which is the purpose of the aircraft.
Compared to the CH148’s fly-by-wire controls, a major difference is that the X2’s system was not designed to become part of a product. Rather, it supports a research demonstration effort, Arifian clarified. The fly-by-wire system also helps make the flight simulator more realistic, which in turn helps refine the actual aircraft, Kagdis added.
For its future helicopters, Russian Helicopters is to build on the fly-by-wire controls developed for the in-service Kazan Ansat. A 7,300-pound MTOW twin, it is powered by Pratt & Whitney Canada PW207K turboshafts. “We are working on the possible installation of fly-by-wire systems on different types of helicopters, keeping in mind that these systems are meant to be the future of helicopters,” a spokesman told Rotor & Wing. He sees fly-by-wire being beneficial, for civil operators, in terms of simplicity and reliability (see box on certification). Whether the Ansat is still being produced, however, is unclear.
The KSU-A flight control system, developed by MRPC Avionica in Moscow for the Ansat, is said to meet Russian AP-29 requirements for stability and control-ability in all flight phases. According to MRPC Avionica, it automatically stabilizes the aircraft’s attitude, speed and altitude. It also monitors its own technical condition to make flying safer and ground maintenance easier.
KSU-A designers describe it as a four-channel (pitch, roll, heading and altitude) digital integrated system. Each control lever (cyclic-pitch lever, pedals and collective-pitch lever) has a quadruple redundant position sensor. The deflection signals from these sensors are transmitted to a quadruple redundant computer. Control algorithms determine electro-hydraulic actuator deflection on control surfaces. The electric and hydraulic parts of the actuators feature quadruple and double redundancy, respectively.
Four sensors receive information on angular rate and acceleration. Information sources for altitude, speed and attitude are the air data system, radio altimeter, barometric altimeter, attitude indicator and the compass system. The computer unit has four analog backup channels.
The KSU-A control system weighs about 77 pounds and maximum power use is estimated at 700 watts.
Eurocopter declined to give an update on its fly-by-wire research. A spokesperson told Rotor & Wing it was “too secret.” Yet, three years ago, Eurocopter revealed it was considering all-electric flight controls. R&W now understands the lead time needed to develop fly-by-wire controls for a production aircraft is compatible with the X4 program. A Dauphin replacement in the 20,000- to 24,000-pound category, the X4 was to be launched in June.
The only helicopter in service with fly-by-wire controls, the NH90 military transport has hydraulic actuators. In 2007, Eurocopter was studying an all-electric system, with fly- and power-by-wire controls. This means both command and actuation are electric, as opposed to mechanic or hydraulic.
Eurocopter expected fly-by-wire to bring better function integration to the pilot. This allows the autopilot to cooperate more closely with the controls. Such controls also were hoped to offer improved handling qualities and more precise flying. For the passenger, the expectation was a smoother ride.
Also, Eurocopter says, the maintenance side of the operation becomes significantly simpler. Using FBW, there is no longer a need for scheduled checks, as with hydraulics. The system monitors itself permanently.
Bell Helicopter did not answer Rotor & Wing’s questions but provided a paper that was presented at the annual American Helicopter Society forum in May. Last year, the company flight tested fly-by-wire control laws on a testbed Bell 412. An innovative rapid prototyping process helped develop the control laws directly on the aircraft.
In 2009, Bell started research on the “next generation of advanced flight control laws” at its XworX facilities in Arlington, Texas. The focus was to enhance safety through reduced pilot workload and automated flight. The advanced control laws are said to offer exceptional capabilities in robustness, gust rejection and control decoupling.
In addition, the control laws are designed to automatically switch to the desired flight mode. This eliminates cockpit control mode switches. The philosophy is that the aircraft holds the current flight condition until the pilot commands a change. Features of these control laws include automatic hover hold with single-control-input landings, speed hold, heading hold and altitude hold.
The NRC provided the Bell 412 testbed. It features both conventional and fly-by-wire flight control systems. The safety pilot in the right seat flies the aircraft with the production flight controls. In the left seat, the evaluation pilot can fly the aircraft through an experimental fly-by-wire flight control system. In case the evaluation pilot loses control, the safety pilot can take control of the aircraft at any time.
A unique component of the development and testing was the use of a laptop computer running the control laws, using Matlab/Simulink in “quasi real-time.” With this method, code generation, compilation, linking and deployment to the host computing system were not required. This allowed the control law designer to disengage the computer, make changes and then re-engage the new control laws. All this could be done in a few minutes.
In addition to the two pilots, the Bell 412 was carrying a NRC operator engineer and two Bell engineers for control law evaluation and laptop computer operation. In this configuration, the development engineers can make modifications in flight.
Bell’s advanced control laws were developed much more quickly than in a typical program. The program started in the fourth quarter of 2008.
Only the core three-loop control law design was in place from previous work. The first development flight test on the NRC’s aircraft took place in April 2009. Then, for example, it just took two weeks for low-speed SCAS (stability and control augmentation system) and attitude modes to be operational.
The aircraft was flown by “over 30 Bell employees and military and commercial customers.” Most of them had no prior flight training. In every case, the pilot, with minimal instruction on how to operate the controls, was able to land the aircraft in 20–25 knot gusts, Bell reported.
The program totaled 60 flight hours. Bell said it took very little time to develop, flight test, evaluate and demonstrate new control laws. Development goes on in 2010. Planned is the introduction of new automated features that are possible once the core control laws are in place.
An AgustaWestland spokesman confirmed to Rotor & Wing that it is “keeping on working on fly-by-wire technology,” but declined to provide any information.
AgustaWestland has experience in fly-by-wire thanks to its joint venture with Bell on the civil BA609 tiltrotor. The earlier XV-15 tiltrotor’s flight controls were conventional. They were so complex and heavy that the only payload available was the two pilots flying it. Thanks to the much lighter fly-by-wire system, the BA609 will be able to carry nine passengers, test pilots pointed out in 2008. However, Bell/Agusta has not given a detailed update on the program since.