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Wednesday, February 1, 2006

Perspectives: How Avionics Redefined Rotorcraft Design

M.E. Rhett Flater

In the early days of rotorcraft development, avionics and systems had a limited role in aircraft design. Essentially, the helicopter was an airframe and one or more engines. Early flight control systems were mechanical, comprised of cables, push/pull rods and bell cranks connecting cockpit controls to control surfaces.

But beginning in the 1950s, helicopter designers conducted experiments to explore the application of feedback control theory to boost flight control systems (FCS) hydraulically. These efforts demonstrated that aircraft stability could be augmented by sensing aircraft states, such as body rates and attitudes and feeding these signals back through an analog computer, which conditioned the signals and fed them to the appropriate control surface. While difficult and time consuming to design, build and test, these analog systems significantly improved handling characteristics, weight and performance.

During the 1960s, designers began to experiment with the use of digital computers for FCS applications, and the benefits for helicopter stability and control became even more apparent. During the 1970s and 1980s, of course, the computer "evolution" became a "revolution." Processor speeds, performance and reliability enjoyed rapid increases, while cost and weight decreased. In recent years, helicopter designers at AgustaWestland, Bell, Boeing, Eurocopter and Sikorsky have demonstrated that they can eliminate cables, push/pull rods and bell cranks altogether and replace them with fly-by-wire flight control systems. Similar trends began appearing in other aircraft subsystem designs, such as hydraulic control, propulsion control, fuel control, power system control, communication, navigation, and cockpit controls and displays.

The industry is progressing to design concepts referred to as vehicle management systems (VMS). Here the systems required to manage and control the helicopter are collected within a single entity that optimizes interfaces and functional allocations to subsystem elements. The goal, now being realized on a few advanced military rotorcraft, is fewer components and interfaces and a simpler infrastructure.

In the military arena, avionics and systems are redefining rotorcraft capabilities. Integrated systems are facilitating new roles and missions, such as night and adverse weather attack capabilities, anti-submarine and anti-surface warfare, and day/night armed reconnaissance. The U.S. Army's Block II AH-64D Longbow Apache, designed in the 1990s, for example, handles Kbits/s data rates and can access the tactical Internet. It can transmit radar imagery to other AH-64Ds and to Air Force joint surveillance target attack radar system (JSTARS) aircraft.

But the new Block III avionics system will traffic far more data from more sources. Instead of Kbits/s, it will move Mbytes/s and interface with ground, air and space networks, including the Army's new Future Combat System network. The Block III cockpit also will give the pilot control over unmanned air vehicles. To accommodate these powerful capabilities, Block III launches a flexible open system avionics architecture with commercial off-the-shelf (COTS) hardware and partitioned software.

Consider this. The first AH-64Ds delivered to the Army featured 21 different processors. Five of them were obsolete before the end of the first multi-year contract. Now new Block III modernization replaces the three application specific integrated circuit (ASIC) processors in today's Longbow Apache with a single COTS mission processor. The lighter hardware will provide five times the processing power available in Block II aircraft. Real-time partitioned software in the Block III Longbow Apache will separate flight-critical and peripheral applications.

With more than 1,100 attack helicopters delivered so far to the U.S. Army, National Guard and the militaries of more than 10 countries, and more to follow, the Boeing facility in Mesa, Ariz., is working overtime to integrate remanufactured and new-build aircraft on the same production line. The aircraft's systems have revolutionized the approach to 21st century wars.

AHS International, meanwhile, has evolved from a vehicle-centric professional technical society to an organization focused largely on systems. When pioneers founded AHS, it focused largely on vehicle disciplines, such as aircraft design, aerodynamics, dynamics and propulsion. But during the 1990s industry leaders came to realize that avionics and systems were revolutionizing aircraft design as well as roles and missions. Avionics and systems are now a core technical committee, and the Society's Annual Forum now typically features multiple half-day sessions on topics as diverse as vehicle systems, flight control technology, glass cockpits, and health and usage monitoring systems (HUMS).

M.E. Rhett Flater is the executive director of the American Helicopter Society, AHS.

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