Military Aircraft Flight Control

Abstract

This chapter presents major stages in the evolution of military aircraft flight control systems. As the flight speed steadily increased, it was necessary to develop new flight control systems to replace the old pilot control with mechanical connections to the control surfaces. The first major step is the pilot with a side stick/rudder pedal or an autopilot, who sends commands converted to electrical signals to a flight control computer and, in turn, 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. This stage of development of aeronautical technologies has been called the fly-by-wire flight control system. The latest major step in the evolution of military aircraft flight control systems is the replacement of copper wires with the fiber-optic cables, which have a 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 autopilot is converted into light signals to the flight control computer and to the electrical or electrohydraulic actuators of each control surface and receives light signals from the motion transducer of each control surface. The latest flight control system is called fly-by-light system.

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]:

• Yaw

• Pitch

• Roll

Figure 1.

Axes of motion of a military aircraft.

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.

Figure 2.

Main forces acting on a military aircraft.

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 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.

Figure 3.

Primary flight control surfaces of a modern military aircraft.

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).

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.

2. Flight control systems for military aircraft

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.

Figure 4.

Pilot-control flight control system with mechanical links.

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 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, 6, 7, 8, 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, 11, 12, 13, 14, 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, 15, 16, 17, 18, 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].

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.

• The fiber core is made of very high-purity optical glass or special plastic, and its thickness (9 μm/50 μm/62.5 μm), depending on the desired transmission spectrum, is less than the thickness of the human hair (about 70 μm).

• The cladding of an optical fiber has a thickness of 125 μm.

• The coating of an optical fiber has a thickness of 250 μm.

• The strengthened layer of an optical fiber has a thickness of 900 μm, which contains a tight buffer wrapped in aramid yarn.

• The outer jacket of an optical fiber has a diameter of 1.2 mm/1.6 mm/2.0 mm/3.0 mm.

Figure 7.

Fiber-optic cable structure [23].

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, 27, 28].

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

Figure 8.

Advantages of using fiber-optic cables [23].

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.

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.

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:

• The fiber-optic cable is replacing the copper wires.

• The fiber-optic cable does not heat up because it transmits only light signals (photons).

• The fiber-optic cable has a high bandwidth; therefore, the number of cables is reduced, and the weight of the flight control system is also reduced.

• The fiber-optic cable is unaffected by electromagnetic interference (EMI); therefore, the cables can be positioned near electronic devices, near weapons, or even fuel tanks in the aircraft.

• The fiber-optic cable is unaffected by electromagnetic pulse (EMP) generated by nuclear detonation, and the FBL system recovers in a few minutes after explosions that generated strong radiation; therefore, the aircraft can be used in the war zone if the mentioned explosions did not hit the aircraft directly.

• The flight control computer has a high capacity, and it is designed with open architecture for both components, that is, hardware and software, so that it can be easily adapted depending on the tactical situation, the type and quantity of weapons loaded, the type of missions, etc.

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 sixth-generation 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].

2.3 About this research

Flight control systems for military aircraft have had and still have a very rapid evolution based on the needs of the air force in each country, on the rapid scientific and technical evolution that allows new and new improvements of military flight control systems. As presented, military aircraft are designed to be aerodynamically unstable to give them superior maneuverability in training or during air combat with enemy armed forces.

During the air maneuvers, the aerodynamic forces developed on the control surfaces and the fuselage of the military aircraft are very large, which requires strong, very fast, but also very safe flight control systems, considering the huge cost of these aircraft.

To make the flight control systems very secure, they are designed as multiredundant systems, and the actuators with which the control surfaces are operated are dimensioned to exceed the aerodynamic forces in any situation.

Of all the systems presented and analyzed, the most advanced, the lightest, and with increased protection from electromagnetic interference (EMI) and electromagnetic pulse (EMP) is the fly-by-light (FBL) flight control system.

In addition, the fiber-optic cable used in the fly-by-light flight control system has a much higher bandwidth, and a very high transfer speed of multiple signals, with the speed of light, it is incredibly difficult to intercept the signal without sectioning the cable, and finally, the diameter of the fiber-optic cable is smaller, which makes it possible to design a multiredundant flight control system without significantly increasing the weight of military aircraft.

The best flight control system for military aircraft is by far the fly-by-light (FBL) system, due to its extraordinary features highlighted above.

3. Conclusions

From the creation of the first aircraft (the Wright brothers, in [30]), or even earlier, pioneer inventors used empirical mechanical flight control systems to take off, fly, and land with aircraft designed by them. Since then, flight control systems have evolved continuously, at a very fast pace, as flight speed has steadily increased and the sound barrier has been overcome several times nowadays.

The fly-by-wire flight control system is much lighter than the previous flight control system because all the mechanical connections have been replaced with thin copper wires. Other advantages of the control system are lower weight, better reliability, damage resistance, and highly efficient control of a high-speed and highly maneuverable military aircraft, unstable designed to increase its maneuverability.

The fly-by-light flight control system uses fiber-optic cables and is widely used in data and telecommunications networks. Recently, glass has been replaced with special clear plastic that helps reduce weight even more significantly. Due to its major advantages, the fly-by-light flight control system is increasingly used in military aircraft as well as in commercial aircraft [16, 31, 32, 33].

Because the fly-by-light system has low weight, high bandwidth, compact size, and resistance to electromagnetic interference (EMI) and electromagnetic pulses (EMP), it is expected to become the next generation of flight control systems as it offers immunity to new more hostile military environments.

Acknowledgments

The work was carried out within contract no. 8 N/2019, code PN 19 01 04 01, supported by the Romanian Ministry of Research, Innovation, and Digitalization.

Conflict of interest

The authors declare no conflict of interest.