What is “Fly-by-wire” technology in aviation? What benefits does it have and why is it used in modern airplanes today?
In: Engineering
An aircraft is aerodynamically stable along an axis of rotation if disturbances will tend to attenuate over time and converge on a steady position on that axis.
An aircraft is aerodynamically unstable along an axis of rotation if disturbances will tend to amplify over time, preventing the aircraft from maintaining a steady position on that axis.
An aircraft is statically unstable along an axis of rotation if steady flight tends to become unstable as a matter of course regardless of control input or disturbances. An aircraft can be statically stable and dynamically stable, statically stable and dynamically unstable, or both statically unstable and dynamically unstable; an aircraft cannot be statically unstable but dynamically unstable, that is impossible.
If the aircraft is dynamically stable, the pilot can relax the control surfaces and maintain steady flight. The impacts of turbulence and other disturbances will require only error correction
If the aircraft is unstable, the pilot has to constantly try and prevent the aircraft from wanting to go all over the place.
Intuition tells us that instability along one or more axis of rotation would be a bad thing, right?
In most cases yes, but instability allows for much better maneuverability because the airframe’s design isn’t trying to return to steady flight while the pilot is deliberately trying to depart from it. This instability is so severe that human operators cannot be expected to overcome it while still maintaining any degree of mission effectiveness. Thus, a flight computer sits between the pilot and the control surfaces. The flight computer is constantly parsing flight instrument data, pilot control data, and control surface positional data to adjust the control surfaces as needed.
The F-14 Tomcat was mechanically controlled and aerodynamically stable.
The F-15 A/B/C/D are similarly mechanically controlled and aerodynamically stable but the nearly identical F-15E is fly-by-wire.
The F-117 Nighthawk, despite not being a fighter in any sense, was aerodynamically unstable in all 3 axis and a right bastard to fly even with a digital flight system.
The F-16 is the first purpose-built fly-by-wire only combat aircraft. It was designed to be unstable from the beginning and that contributed immensely to its famed agility. The F-16 is both statically unstable and dynamically unstable; static instability means that it will tend to become turbulent even from steady and undisturbed flight, there’s no controlling it without a flight computer.
Some fighters, such as the F-22, F-35, and F-18 are unstable in some axis (mainly pitch) but stable or neutral in other axis.
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