The FBW Concept

A main character in our story is the Gulfstream GVII, that company's second foray into fly-by-wire technology but most notable because of its revolutionary "clean sheet" design. It was the first series that fully embraced electrons over hardware. They are not alone in this. I've also flown fly-by-wire aircraft by Dassault and Embraer, which were both groundbreaking in their own rights. Airbus and Boeing also have a few fly-by-wire offerings.

All of this begs the question why? The primary answer is due to weight savings. The cables, pulleys and other replaced hardware were heavy. It takes fuel to carry all of that hardware. A second reason is for aerodynamic efficiency. If you put a computer between the pilot and the airplane, you can design an airplane that is faster and with longer range but would be too difficult for a mere human to fly.

A second question is what if? What if the computers break? Designers are required to add levels of redundancy. What if someone nefarious tries to hack into those computers and take control from the pilot? While not called this in so many words, manufacturers use fire walls to isolate the flight control systems from all other communications systems or other points of access. (As the book points out, there is yet another weak point.)

Have there been problems over the years? The few examples we have available have not been catastrophic, but fly-by-wire systems have not been entirely trouble free.

Finally, there are many complications when going from "fly by cable" to fly-by-wire. While fly-by-wire systems are lighter, less prone to breakdown, and provide efficiency not otherwise available, they are hard to design, to troubleshoot, and to train.

The evolution of flight control technology

For most of the first decades of powered flight, flight controls meant cables, pulleys, and bell cranks between the pilot's stick or yoke and the actual flight controls (ailerons, rudders, elevators, etc.) and everything was easily understood. It made intuitive sense that pulling back on the stick or yoke caused the elevator to go up, for example, due to the mechanical translation of the forces.

As aircraft became faster and heavier, however, hydraulic assistance was needed to actuate the flight control surfaces. In some cases, aerodynamic contrivances, such as Balance tabs, were needed to assist the pilot. In most cases, these systems did not replace the cables and pulleys, but added to them. Flight control systems were not only getting heavier, but they were starting to change the "feel" of aircraft control.

Partial fly-by-wire systems began to replace mechanical systems. Electronic yaw dampers and spoilers, for example, improved aircraft control and in some cases reduced the number of mechanical systems.

A true fly-by-wire airplane eliminates the mechanical systems between the pilot and flight controls. This not only greatly reduces the total weight of the system, but often improves aircraft control and allows the use of safety systems to prevent the aircraft from exceeding safe flight envelopes.

Weight savings

Replacing non-fly-by-wire mechanical systems with full fly-by-wire systems offers many weight savings:

1. Fewer mechanical componenents. Fly-by-wire systems replace many mechanical components, such as control cables, pulleys, and hydraulic systems, with electronic components like computers, sensors and wires.
2. Lighter materials. Fly-by-wire systems can use lighter materials for control surfaces since the electronic actuators used in these systems are often lighter than the mechanical systems they replace. This can lead to weight reduction in control surfaces like ailerons, elevators, and rudders.
3. Improved efficiency. Since fly-by-wire system are computer controlled, they don't need to be made as robustly to compensate for a human pilot's tendency to overcontrol.
4. Integration of functions. Fly-by-wire systems can combine functions of various components to serve multiple purposes. Some FBW aircraft, for example, use ailerons on the ground to augment ground spoilers, allow for smaller dedicated ground spoilers.
5. Elimination of backup systems. True fly-by-wire systems can eliminate heavy backup systems. The electronic and mechanical boxes of a dedicated autopilot system, for example can be replaced by software in the flight control computers.

A few words from two manufacturers

A fly-by-wire system enhances aerodynamic efficiency in several ways:

Reduced Weight and Drag: FBW systems often replace heavy mechanical components like cables, pulleys, and hydraulic systems with lightweight electronic components and sensors. This reduction in weight and drag contributes to increased aerodynamic efficiency. Optimized Control Surfaces: FBW systems allow for precise and real-time control of control surfaces (such as ailerons, elevators, and rudders) based on complex algorithms and flight conditions. This precision helps optimize the control surface positions, reducing unnecessary drag and improving overall aerodynamic efficiency. Aerodynamic Load Management: FBW systems can dynamically adjust control surface positions to distribute aerodynamic loads across the aircraft structure efficiently. This optimization minimizes stress on the airframe and contributes to improved aerodynamic performance. Tailored Flight Profiles: FBW systems can be programmed to follow specific flight profiles that are aerodynamically optimal for various phases of flight. This includes takeoff, cruise, landing, and maneuvers. Tailoring control inputs to match optimal aerodynamic conditions leads to improved fuel efficiency and overall aerodynamic performance. Automated Control and Stability Augmentation: FBW systems can provide automated control and stability augmentation, ensuring the aircraft maintains optimal aerodynamic stability and control even under challenging flight conditions. This helps minimize aerodynamic losses associated with unsteady flight. Enhanced Pilot Assistance and Feedback: FBW systems provide pilots with better feedback and assistance, allowing them to make informed decisions to optimize the aircraft's aerodynamic performance. This may include suggestions for control surface adjustments or optimizing flight parameters for improved efficiency. Adaptive Control Algorithms: FBW systems utilize adaptive control algorithms that continuously analyze flight conditions and adjust control inputs accordingly. This adaptability ensures that the aircraft is always operating at the most aerodynamically efficient state given the prevailing conditions. Overall, the advanced control and automation capabilities of FBW systems contribute to a more aerodynamically efficient aircraft, leading to benefits such as improved fuel efficiency, reduced emissions, and enhanced flight performance.

Falcon 8X Crew Operational Documentation for Dassult EASy (CODE), Dassault Aviation, Revision 04, September 10, 2020.

GVII-G500/G600 Production Aircraft Systems (PAS), Gulfstream Aerospace Corporation, Revision 9, Feb 8/22

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