Understanding of FBW Benefits
By the early 1970s, the full range of benefits that could be possible by the use of fly-by-wire flight control had become ever more apparent to aircraft designers and pilots. Relevant technologies were rapidly maturing, and various forms of fly-by-wire flight control had successfully been implemented in missiles, aircraft, and spacecraft. Fly-by-wire had many advantages over more conventional flight control systems, in addition to those made possible from the elimination of mechanical linkages. A computer-controlled fly-by-wire flight control system could generate integrated pitch, yaw, and roll control instructions at very high rates to maintain the directed flight path. It would automatically provide artificial stability by constantly compensating for any flight path deviations. When the pilot moved his cockpit controls, commands would be automatically be generated to modify the artificial stability enough to enable the desired maneuvers to be accomplished. It could also prevent the pilot from commanding maneuvers that would exceed established aircraft limits in either acceleration or angle of attack. Additionally, the
flight control system could automatically extend high-lift devices, such as flaps, to improve maneuverability.
Conceptual design studies indicated that active fly-by-wire flight control systems could enable new aircraft to be developed that featured smaller aerodynamic control surfaces. This was possible by reducing the inherent static stability traditionally designed into conventional aircraft. The ability to relax stability while maintaining good handling qualities could also lead to improved agility. Agility is a measure of an aircraft’s ability to rapidly change its position. In the 1960s, a concept known as energy maneuverability was developed within the Air Force in an attempt to quantify agility. This concept states that the energy state of a maneuvering aircraft can be expressed as the sum of its kinetic energy and its potential energy. An aircraft that possesses higher overall energy inherently has higher agility than another aircraft with lower energy. The ability to retain a high-energy state while maneuvering requires high excess thrust and low drag at high-lift maneuvering conditions. Aircraft designers began synthesizing unique conceptual fighter designs using energy maneuver theory along with exploiting an aerodynamic phenomenon known as vortex lift. This approach, coupled with computer-controlled fly-by-wire flight control systems, was felt to be a key to unique new fighter aircraft with very high agility levels.
Neutrally stable or even unstable aircraft appeared to be within the realm of practical reality and were the subject of ever increasing interest and widespread study in NASA and the Air Force, as well as in foreign governments and the aerospace industry. Often referred to at the time as Control Configured Vehicles, such aircraft could be optimized for specific missions with fly-by-wire flight control system characteristics
designed to improve aerodynamic performance, maneuverability, and agility while reducing airframe weight. Other CCV possibilities included the ability to control structural loads while maneuvering (maneuver load control) and the potential for implementation of unconventional control modes. Maneuver load control could allow new designs to be optimized, for example, by using automated control surface deflections to actively modify the spanwise lift distribution to alleviate wing bending loads on larger aircraft. Unconventional or decoupled control modes would be possible by using various combinations of direct-force flight controls to change the aircraft flight path without changing its attitude or, alternatively, to point the aircraft without changing the aircraft flight path. These unconventional flight control modes were felt at the time to provide an improved ability to point and fire weapons during air combat.
In summary, the full range of benefits possible through the application of active fly-by-wire flight control in properly tailored aircraft design applications was understood to include:
• Enhanced performance and improved mission effectiveness made possible by the incorporation of relaxed static stability and automatically activated high-lift devices into mission-optimized aircraft designs to reduce drag, optimize lift, and improve agility and handling qualities throughout the flight and maneuvering envelope.
• New approaches to aircraft control, such as the use of automatically controlled thrust modulation and thrust vectoring fully integrated with the movement of the aircraft’s aerodynamic flight control surfaces and activation of its high-lift devices.
• Increased safety provided by automatic angle-of-attack and angle-of-sideslip suppression as well as automatic limiting of normal acceleration and roll rates. These measures protect from stall and/or loss of control, prevent inadvertent overstressing of the airframe, and give the pilot maximum freedom to focus on effectively maneuvering the aircraft.
• Improved survivability made possible by the elimination
of highly vulnerable hydraulic lines and incorporation of fault tolerant flight control system designs and components.
• Greatly improved flight control system reliability and lower maintenance costs resulting from less mechanical complexity and automated built-in system test and diagnostic capabilities.
• Automatic flight control system reconfiguration to allow safe flight, recovery, and landing following battle damage or system failures.