**2.1 The fly-by-wire system**

The old pilot-control flight control system with mechanical links is shown in **Figure 4**. The pilot directly moves all the control surfaces using the control column (yoke) or rudder pedals with the strength of his arms or his legs. The pilot also feels the resistance to the movement of all these control surfaces.

As the flight speed of a new military aircraft increased continuously from subsonic velocities to supersonic velocities, and the aircraft was designed aerodynamically unstable to increase their maneuverability in the air, it was necessary to continuously develop new and modern flight control systems.

The first major step in the development of aeronautical technologies for flight control systems of military aircraft is the fly-by-wire (FBW) flight control designed as a multiredundant system. The command imposed by the pilot with a side stick/rudder pedal or by autopilot is converted into electrical signals sent by copper wires to a flight control computer, which interprets and sends wired electrical commands to the electrohydraulic actuators of each control surface and receives (feedback) electrical signals from the motion transducer of each control surface to provide self-corrective action, as shown in **Figure 5**. Initially, the data sent by copper wires were analog, but later these were transformed into digital signals to avoid any communication errors.

**Figure 4.** *Pilot-control flight control system with mechanical links.*

*Military Aircraft Flight Control DOI: http://dx.doi.org/10.5772/intechopen.105491*

**Figure 5.** *Fly-by-wire flight control system for a military aircraft.*

The fly-by-wire flight control system has a much lower weight than the previous flight control system because all the mechanical connections have been replaced by thin copper wires. Other advantages of this new control system are lower weight, better reliability, damage endurance, and very efficient control of a high-speed very maneuverable military aircraft designed unstable just to increase its maneuverability [2].

The fly-by-wire system is the flight control system that processes the flight control inputs made by the pilot or autopilot using flight computers and submits suitable electrical signals by copper wires to each actuator of the flight control surfaces [2]. The fly-by-wire system means that the pilot inputs do not directly move the control surfaces as explained above, but the pilot must have an effort simulator when moving the side stick/rudder pedal to feel the command. Instead, the inputs are read by a computer, which, in turn, determines how to move the control surfaces to perform the pilot's maneuvers as well as possible, controlled by the active flight envelope containing flight control laws implemented in it by specialized engineers [2, 5], as shown in **Figure 5**.

Another definition of fly-by-wire is a flight control system of an aerospace vehicle in which information is completely transmitted by electrical means via copper wires [2, 4].

The flight envelope refers to the properties of use in the safe parameters of a military airplane. The airplane is manufactured to fly at different parameters of all the kinds of different natures set exactly in advance by engineers. These parameters refer, for example, to the maximum speed, the maximum altitude, the maximum climb rate, etc [5–9].

In the past, there have been aircraft near-accidents or even crashes due to malfunctioning sensors that have transmitted incorrect data to the flight control computer. That is why it is very important to consider multiredundant sensor circuits in the design process to compare provided information. Overall, it should be noted that the introduction of automation and computers onboard aircraft has significantly reduced the possibility of human error.

The protection software included in the flight envelope automatically prevents pilots' unsafe actions and helps them stabilize the airplane. The fly-by-wire flight control system ensures the suppression of air disturbance and, consequently, reduces the fatigue loads and increases the comfort of the crew on board and ensures an optimized trim setting and, consequently, drag reduction.

In 1972, at NASA's Dryden Flight Research Center, the first digital fly-by-wire flight control system without a mechanical backup was successfully utilized.

Neil Armstrong, a former research pilot at Dryden, played an important role after his historic Apollo 11 lunar landing. NASA's DFBW program consisted of 210 flights and lasted 13 years [2, 10–15].

The Dryden DFBW program has changed the way engineers design and pilots fly commercial and military aircraft. Aircraft equipped with fly-by-wire systems are safer, more reliable, easier to fly, more maneuverable, and more fuel-efficient while having lower maintenance costs [2, 10, 14–19].

The second major step in the development of the fly-by-wire system is the F-16 Fighting Falcon, originally developed by General Dynamics (now Lockheed-Martin) and is a proven compact, single-engine, multirole fighter airplane and the World's first fly-by-wire combat airplane [14, 20, 21] presented in **Figure 6**.

**Figure 6.** *Digital fly-by-wire system [14].*

*Military Aircraft Flight Control DOI: http://dx.doi.org/10.5772/intechopen.105491*

Since the F-16A's first flight in December 1976, this highly maneuverable air-to-air combat and air-to-surface attack airplane has provided mission versatility and high performance for the U.S. and allied nations at a relatively low cost. The F-16 pilot maintains excellent flight control through the airplane's fly-by-wire system. The pilot sends electrical signals via a side stick/rudder pedal to flight computers and then to the actuators of flight control surfaces, such as ailerons and rudders. The flight computers constantly adjust the inputs to enable stability in level flight and high maneuverability in combat, inside the flight envelope. The side stick/rudder pedal allows the pilot to easily and accurately control the airplane during high G-force of combat maneuvers [14, 20, 21].

The F-16 was the first production airplane to use fly-by-wire technology. To improve maneuverability, the F-16 was designed to be aerodynamically unstable or to have relaxed static stability (RSS). To make the flight of this lightweight fighter airplane smoother, the F-16 has a flight control computer (FLCC) that manages the flight control system [14, 22].

#### **2.2 The fly-by-light system**

The fly-by-light (FBL) system installed on military aircraft, using fiber-optic cables, has multiple advantages highlighted below, which provide tactical and safety advantages for the military aircraft and its crew [23].

The structure of a fiber-optic cable [24, 25] is presented in **Figure 7**.


Owing to their qualities, fiber-optic cables are extensively used in telecommunications and data networks (Internet). In recent years, more and more countries and companies have implemented the FBL system for military and commercial aircraft [23].

The fiber-optic cables are used in fly-by-light (FBL) flight control systems of the aircraft, and they replace the copper cables previously used in fly-by-wire (FBW) flight control systems [26–28].

For this reason, the advantages of using optical fibers are highlighted, as shown in **Figure 8** and the following explanations [27, 29].

The fiber-optic cable provides a multitude of benefits and redundancy too. The flight control computer has also a flight envelope embedded in it (a computer program made by engineers) that eliminates dangerous maneuvers for the aircraft structure and the life of the crew on board while maintaining the aerodynamic stability of the aircraft in any situation or maneuvers allowed by the flight envelope.

**Figure 7.** *Fiber-optic cable structure [23].*

#### **Figure 8.**

*Advantages of using fiber-optic cables [23].*

The fiber-optic cable has a **much higher bandwidth** compared to a copper wire, meaning that it can carry multiple signals on one cable instead of a single signal on a copper wire.

The use of a fiber-optic cable to replace the copper wire will significantly reduce the weight of the new fly-by-light system, and therefore, it will reduce the weight of the entire aircraft.

Fiber-optic cables are characterized by the **very high transfer speed of multiple signals**, with the speed of light through the glass, while the copper wire can carry a single signal at a much lower speed, namely, the speed of electric current through the copper wire.

Multiple light signals can be carried by the fiber-optic cable over much longer distances, without degrading the quality of the multiple light signals, since the signal sent through the optical fiber is much less likely to be altered during transmission, compared to the copper wire.

*Military Aircraft Flight Control DOI: http://dx.doi.org/10.5772/intechopen.105491*

The core of fiber-optic cables is made of glass, which makes it **incredibly difficult to intercept the signal** without sectioning the cable, even in the case of very qualified people. This makes transmission through fiber-optic cables **very safe**, compared to the copper wire, which can be intercepted very easily, even by less qualified people.

The fiber-optic cables are **very reliable** because they only transmit light signals, without the risk of fire, while the copper wires heat up when transmitting electrical signals; in addition, the transmitted electrical signal can be altered by environmental conditions (severe weather conditions such as lightning, elevated temperature, high humidity, etc.).

**The diameter of the fiber-optic cable is smaller** than the copper wire, because the fiber-optic cable allows the transmission of multiple signals without affecting the speed or quality of the signals, while the transmission of the electrical signal through the copper wire is strictly dependent on the size of the wire.

Consequently, the weight of a flight control system using fiber-optic cables (FBL) is significantly reduced compared to the FBW system.

The fiber-optic cables do not heat up because they transmit only light signals (photons).

The fiber-optic cable is **unaffected by electromagnetic interference** (EMI) or **electromagnetic pulse** (EMP) [27] generated by nuclear detonation and, therefore, does not need protective shielding like the copper wire (which can be affected by its electromagnetic field, by the electromagnetic frequency given by military electronic jamming devices, other existing electronic devices in the aircraft or even lightning).

The fly-by-light (FBL) system installed on military aircraft, using fiber-optic cables, has multiple advantages highlighted above, which provide tactical and safety advantages for the military aircraft and its crew.

The architecture of the fly-by-light (FBL) flight control system for a modern military aircraft is presented in **Figure 9** [23], and it is like the structure of an FBW system, but there are significant differences between the two systems (FBL and FBW) [29], as presented below:


**Figure 9.** *The fly-by-light flight control system for a modern military aircraft [23].*

A list of known aircraft using the fly-by-light system is presented below.

The A-7D test aircraft, equipped with the complete fly-by-light system flew first on February 7, 1975 and then on March 24, 1982, in California, USA [23].

The Kawasaki XP-1, a Japanese maritime reconnaissance aircraft, had its first flight in September 2007, and it has the distinction of being the first operational aircraft in the world to use a fly-by-light (FBL) flight control system [23].

On March 18, 2018, Gulfstream demonstrates the fly-by-light aircraft control system, during a nearly 75-minute flight [26].

China intends to use the fly-by-light (FBL) flight control system for the sixthgeneration fighters [28].

India is developing research to use the fly-by-light (FBL) flight control system for the sixth-generation fighters for the Advanced Medium Combat Aircraft (AMCA), an Indian program to develop fifth- to sixth-generation fighter aircraft for the Indian Air Force and the Indian Navy [23].

Many companies, such as Boeing and Airbus, are interested in implementing the fly-by-light (FBL) flight control system on new aircraft or if they have the opportunity when modernize existing aircraft [23].
