**1. Introduction**

The flight control system of a military aircraft is determined by the control surfaces installed on the airplane body that are balanced movements coordinated by a flight control system that drives an aircraft around the three axes of motion, as shown in **Figure 1** [1, 2]:


Main forces acting on a military aircraft in straight and level flight or any other type of aircraft in straight and level flight [3] are shown in **Figure 2**.

To take off and to keep in flight, a military aircraft must meet the following conditions: the **lift** forces must be bigger than the **weight** of the aircraft and the **trust** must

**Figure 1.** *Axes of motion of a military aircraft.*

be bigger than the **drag** forces (the aerodynamic forces that oppose a military aircraft's movement through the air). If the lift is less than the weight, then the aircraft falls, and if the trust is less than the drag, the aircraft slows down, especially when the aircraft maintains the same altitude [3].

Primary flight control surfaces of a modern military aircraft are shown in **Figure 3**.

Flaperons are flight control surfaces on the rear wing of a military aircraft used as flaps during takeoff and landing maneuvers when the aircraft has a low speed. Flaperons are also used as ailerons to roll aircraft; therefore, the flaperons combine the functions of flaps and ailerons.

Leading-edge slats are used to increase the aircraft lift during takeoff and landing maneuvers when the aircraft has a low speed.

The horizontal stabilizer provides stability for the military aircraft, and it can be slowly rotated to act as an elevator (both for pitch control).

**Figure 3.** *Primary flight control surfaces of a modern military aircraft.*

The two vertical stabilizers provide the stability of the military aircraft around the vertical axis. The two rudders ensure the control of the yaw movement of the military aircraft.

As the flight speed of military aircraft has increased continuously, it was necessary to develop new flight control systems. The old flight control system with mechanical links from the pilot control column (yoke) and rudder pedals to the control surfaces is using the power of the pilot's arms and legs to directly move the control surfaces.

The first major step in the development of flight control systems for military aircraft is the fly-by-wire (FBW) flight control system [2, 4], which is 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 to a flight control computer (FLCC), which interprets and sends wired electrical commands to the electrohydraulic actuators of each control surface and receives electrical signals from the motion transducer of each control surface. To increase flight safety, each flight control computer has a flight envelope embedded in it (a computer program made by specialized 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.

The latest major step in the evolution of military aircraft flight control systems is the fly-by-light (FBL) flight control system consisting of the replacement of copper wires with fiber-optic cables, which have an even much lower weight and a much higher capacity to carry digital information (light or photons). The command imposed by the pilot with a side stick/rudder pedal or by autopilot is converted into light signals to the flight control computer and from here to the electrical or electrohydraulic actuators of each control surface and receives light signals as feedback from the motion transducer

of each control surface. The flight computer of the fly-by-light flight control system has a flight envelope embedded in it, which 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 [2, 5]. The Fly-by-Light flight control system is designed as a multi redundant system.
