FLY-BY-WIRE OVERVIEW


Fly-by-wire (FBW) is the generally accepted term for aircraft where the pilot's control commands are processed by a computer and sent to the flight control surface actuators with electrical rather than mechanical signals. In other words, Fly-by-wire is where all mechanical linkages between the controls and movable surfaces are removed and instead carried out electronically.

As technology pushed aircraft speeds and gross weights higher and higher, flight control designers replaced cables and gearing with hydraulic control surface actuators. As improved stability and handling precision were needed, fly-by-wire technology provided the solution. FBW greatly improved aircraft performance and flying qualities.

Briefly stated below, FBW technology primarily includes Feedback Control Systems and Control Laws.

Feedback compensation is accomplished by measuring an aircraft motion parameter, conditioning the signal, amplifying it and summing it with the input command. Feedback control regulates the system by comparing the output to the input signal. The error between the two becomes a command to the flight control surface until output equals input.

Control laws describe the basic feedback strategy of a feedback control system. Common feedbacks are vertical load factor (Nz or just "G"), pitch rate (q), pitch angle (0 or attitude), and angle-of-attack (a or AOA or "alphaftedback"). Common lateral feedbacks are bank angle (p) and roll rate (p). Typical directional feedbacks are yaw rate (r), sideslip angle ($ or "beta feedback"), and rate of change of sideslip angle ("beta dot feedback"). Control laws are commonly named after the primary feedback parameter as "feedback." or "command."

G command, desired in many fighter designs, means for a particular amount of stick force, you get the same "G" regardless of airspeed (energy permitting). For a pitch rate command system, you get the same amount of pitch rate for a given stick force regardless of speed. Pitch rate feedback and its effects are presented in detail; the concepts apply to any feedback control law.

The pilot applies a certain control force, demanding pitch rate, which becomes the flight control computer's command for a particular pitch rate. Since the pilot's control "demands" a certain maneuver parameter, such an arrangement is often termed a maneuver demand system. The computer, not the pilot, then moves the control surfaces, often rapidly, as required to meet the pilot's demand.

To provide immediate response to pilot input, a direct path to the elevator is provided via the proportional line. For precision over time, an integrator produces a control surface command until the feedback signal is equal to the pilot's command signal. Pure integral control, or too much integrator gain (K), causes excessive lag in the aircraft response; hence the use of the proportional circuit. This arrangement is called "proportional plus integral" control and is found in most fly-by-wire designs.

Incorporating FBW technology into our Aeros 40B has made it one of the most desirable to fly by Lighter-than-air pilots. This technology greatly reduces pilot exertion, enabling longer flight durations by a single pilot rather than the typical two required in other airships. Additionally, FBW technology allows for great ease in maneuverability enabling the Aeros 40B superb handling and response. Fly-by-wire technology in our airships has revolutionized lighter-than-air flight.