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A History Of Fly By Wire

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This research report provides a historical portrait of the development and implementation of fly-by-wire flight control systems. The report explains to the reader what flight controls are. It provides an overview of major innovations in flight control systems. It then goes on to explain what a fly-by-wire flight control system is and discusses the NASA development program that made fly-by-wire a reality. It then discusses the F-16 Fighting Falcon which was the first mass produced aircraft to utilize a fly-by-wire system. The benefits of fly-by-wire flight control are discussed as is the expansion of fly-by-wire flight control systems into commercial and general aviation. Finally, a conclusion on the substance of this report is provided.

1. Introduction


This historical research report describes the development and implementation of fly-by-wire flight control systems in order to satisfy the formal report requirements outlined in the course syllabus for EGR 3350, Technical Communications for Engineers and Computer Scientists.

1.2 Background

Ever since the dawn of powered human flight was realized by Orville and Wilbur Wright in December 1903, engineers and aeronautic innovators have sought to institute more efficient and safer methods of aircraft flight control. The evolution of flight control systems from human powered mechanical linkages to fly-by-wire computer systems constitutes a marvelous display of aeronautical engineering progression. Fly-by-wire flight control systems signaled a great leap in aeronautical thinking and design from mechanical linkage and large hydraulic systems to computer-aided electrical flight control systems. An article by Gray Creech of NASA's Dryden Flight Research Center explains how [1] "these systems created enormous benefits for the aerospace industry allowing overall reduction of weight and aircraft system redundancy increasing safety of flight." NASA's fly-by-wire development program was the first program to successfully institute an electrical flight control system without a mechanical backup. This program's success led to the first mass produced fly-by-wire aircraft, General Dynamics' and Lockheed Martin's F-16 Fighting Falcon, the space shuttle's fly-by-wire flight control computer, and many other advancements in fly-by-wire flight control that are now being realized in the commercial and general aviation industries.

1.3 Scope

This report will explain to the reader what flight control is and detail a brief history of aircraft flight control and the innovations that preceded fly-by-wire system development. This report will then discuss NASA's fly-by-wire development program and the initial deployment of this technology in the F-16 Fighting Falcon. This report will explain the many benefits inherently derived from employing a fly-by-wire flight control system. Finally, this report will detail how this flight control system evolved to be used in the commercial and general aviation industry. This report will not cover future trends of fly-by-wire flight control systems.

2. Discussion

2.1 What is Flight Control?

The control of flight of an aircraft is determined by control surfaces on the aircraft body that are adjusted in coordinated movements by a flight control system that orients an aerospace vehicle around three axes of motion. These axes of motion are referred to as yaw, pitch, and roll. Figure 1 illustrates these axes.

Figure 1. Aircraft Axes of Motion

Dr. William Elliot gives a great synopsis on how these axes of motion are affected by control surfaces.

[2] "1. Normal (vertical) axis, perpendicular to the surface of the wings. Movement about the vertical axis in flight is called yaw. In most modern aircraft, stability in yaw is affected by a fixed vertical fin in the rear; active control in yaw is accomplished by a movable rudder fixed behind the vertical fin.

2. Longitudinal axis, passing through the fuselage from front to back. Movement about the longitudinal axis is called roll. Stability in roll is taken care of by wings fixed at a slightly upward angle (dihedral); active flight control in roll is done by flaps (ailerons) behind the outer wings.

3. Lateral (horizontal) axis, passing through the wings approximately from tip to tip. Movement about the horizontal axis is called pitch. Stability in pitch is conferred by a fixed horizontal tailplane; flight control in pitch is accomplished by elevators mounted behind the tailplane." In controlling these surfaces, a pilot utilizes various control mechanisms such as mechanical linkages, hydraulics, trim tabs, actuators, and, in the case of fly-by-wire systems, electricity and computers to create the desired output on the flight control surfaces based on the pilot's input.

2.2 Brief History of Flight Control System Progression

Dr. Elliot continues to explain that after [2] "Glenn Curtiss's patent of the aileron, the basics of modern flight control were firmly established, and the result was a 'standardized' cable-operated control system." [2] "In this 'standard' arrangement, a single control column (or 'stick') was used to operate both elevators and ailerons through a series of cables and pulleys; in a similar fashion, the rudder was moved by foot pedals." The physical strength of the pilot was all that was required to augment these control surfaces in flight for slow moving aircraft. The physical limitations of pilots began to be realized as aircraft became faster and heavier. [2] "This problem was initially solved by the installation of small flaps ('tabs') on primary control surfaces. These surfaces utilized the airflow acting on the tabs to move the main control surface they were attached to." The development of automatic piloting systems was also on going at this time and [2] "steady advances in autopilot technology led to the development of mechanical boosters to assist pilots in moving control surfaces of very large aircraft." [2] "Successive aircraft produced during the late 1940s and early 1950s continued to make great advances in hydro-mechanical flight control systems." [3] "During this time period hydro-mechanical control systems developed into 3000 psi hydraulic systems" as seen in Figure 2.

Figure 2. Flight Control System Innovation Timeline [3]

Technology Military Commercial

Un-Powered: 1910s 1920s

Powered Boost: 1940s 1940s

3000 psi Hydraulics: 1940s 1950s

Auto Pilots: 1950s 1950s

Fully Powered, w/*Reversion: 1950s 1960s (Boeing 727)

Fully Powered, w/out *Reversion: 1950s (B-47) 1970 (Boeing 747)

Fly-By-Wire: 1970s (F-16) 1980s (A-320)

Digital Fly-by-Wire: 1970s 1980s (A-320)

5000 psi Hydraulics: 1990s (V-22) 2005 (A-380)

Power-By-Wire: 2006 (F-35) 2005 (A-380)

*Reversion: Servo actuators unlock allowing pilot mechanical control. [3]

Figure 2 details the engineering progression of flight control systems over the last 100 years. Interestingly, prior to the institution of fly-by-wire flight control systems, [2] "artificial 'feel' systems were incorporated in flight control systems to necessitate the need for pilots to feel as though they were still mechanically connected to the aircraft flight control system even though hydraulic systems broke this connection between pilot and control surface." These advancements in flight control technology culminated in the desire for an electrical means of flight control system execution.

2.3 What is a Fly-By-Wire (FBW) Flight Control System?

[2] "Aerospace Recommended Practice (ARP) defines FBW as a flight control system wherein vehicle control information is transmitted completely by electrical means."

A FBW control system is a computer system that monitors pilot control inputs, various parameters such as airspeed, altitude and angle-of-attack, and outputs flight control surface movements with the objective of keeping the aircraft within its designated flight envelope. Literally, this computer interprets electrical signals via pilot control and sensor input and outputs electrical signals to actuate the corresponding control surface in order to achieve the desired flight orientation.

The flight envelope refers to the safe operating characteristics an aircraft is designed to fly at given different speeds, altitudes and other variables. The actuation of a fly-by-wire system is effectively the same for all such systems, namely; the system employs electrical signal inputs to create electrical signal outputs. However, these systems can be deployed with a varying array of design elements or control law algorithms that decide how the system will react in a given situation as well as what entity, human or computer, has superior control of the aircraft at a given time. This subject will be elaborated on in a later section.

2.4 NASA's Digital Fly-By-Wire (DFBW) Development Program

On May 25, 1972 at NASA's Dryden Flight Research Center, the first flight to successfully demonstrate a digital FBW flight control system without a mechanical backup was conducted. [1] "Support for the concept at NASA Headquarters came from Neil Armstrong, himself a former research pilot at Dryden. He served in NASA's Office of Advanced Research and Technology following his historic Apollo 11 lunar landing and knew electronic control systems from his days training in and operating Apollo spacecraft. Armstrong suggested that the Dryden DFBW team adapt an Apollo program digital flight control computer. It wasn't long, however, before the DFBW program developed a digital flight control computer that significantly advanced the state of the art. This was demonstrated by the fact that for the Space Shuttle, designers turned to the DFBW program for a flight computer for the Orbiters. The result was a classic case of in-house technology transfer. The original digital flight control computer development from Apollo proceeded to the DFBW program and then back again into space aboard the Shuttle." The program utilized a Navy F-8C Crusader for testing which incorporated the use of computers in making the flight control surface deflections that corresponded to the pilot input. [3] "NASA's DFBW program, consisting of 210 flights, lasted 13 years." Figure 3 [6] "shows the avionics bay of the test aircraft where the computers that managed the flight control system were installed."

Figure 3. F-8C Test Aircraft Avionics Bay

2.5 F-16 Fighting Falcon

Originally developed by General Dynamics and now produced by Lockheed Martin, the F-16 was the first mass produced aircraft to use a FBW flight control system. The F-16 has seen multiple upgrades since its service debut in the 1970s. These upgrades are typically called "blocks" and are designated by a number. In the F-16's case, [4] " the F-16 A/B model consists of blocks 1, 5, 10, 15, 15OCU, and 20 while the F-16 C/D model consists of blocks 25, 30, 32, 40, 42, 50, and 52. There also is F-16 E/F block 60 models developed for the United Arab Emirates, an F-16 MLU (Mid Life Update) block, and various other F-16 models developed for special purposes or foreign customers." These blocks signaled upgrades in areas such as avionics, engines, engine inlet area, and weapons capabilities to name a few. According to Joe Sambor, a Lockheed Martin aero field service engineer, [5] "all F-16 block designations developed prior to block 40 utilized analog flight control computers while all later blocks including block 40 utilized digital flight control computers." The difference between analog and digital computers lies in the way they process information. Analog computers work in a continuous time environment where data can take on an infinite set of values which results in no loss of transmitted data; however, its implementation is cumbersome requiring an extensive hardware configuration. Moreover, this hardware configuration is difficult to upgrade. Digital systems operate in a discrete time environment where data values are finite. Loss of data is augmented by high resolution and sampling rates which effectively renders data transmission loss negligible. The benefit in system implementation is mainly software based providing smooth transitions for system upgrades and reduction in overall system cost and maintenance. The F-16 utilizes four separate flight control computer systems which work together to select the proper flight response output at any given time. This flight control configuration is considered practically immune to failure as long as power is applied to the aircraft.

2.6 Benefits of Fly-By-Wire Flight Control Systems

One of the great benefits FBW technology brings to the aviation industry is the ability for aerospace engineers to design an aircraft to be inherently unstable allowing for increased maneuverability. Prior to FBW, aircraft had to be designed to inherently want to return to straight and level flight. This meant that maneuverability was diminished due to the fact that, in order for the aircraft to maneuver, the aircraft had to first overcome its inherently designed stability. FBW systems are able to monitor aircraft flight in real time allowing aircraft that could never fly with simply the skill of the pilot because of the aircraft's instability the ability to take to the skies. Also, [6] "aircraft weight is reduced with the removal of mechanical linkages and reduction in hydraulic system components. Enhanced safety is provided by the redundancy design of electrical circuits as well as the computer's ability to respond to an adverse flight condition much faster than a pilot. The overall cost of the system is reduced as less hardware and mechanical parts are required, fuel efficiency of the aircraft is increased, and passengers experience greater comfort derived from the increased aircraft handling characteristics. Furthermore, the system can be designed to control the flight envelope keeping the pilot from making control inputs that would put the aircraft outside its safe operating capability. Also, digital FBW control systems can accept input from any aircraft sensor reducing rigidity constraints in system design."

2.7 Expansion of Fly-By-Wire Systems in Aviation

Currently, DFBW flight control systems are available in every aspect of government, military, and commercial aviation. These systems are deployed on helicopters, fighter jets, stealth bombers, and commercial airliners. Even general aviation is starting to see the benefits that DFBW technology has to offer. Mark Tatge, a writer for Forbes magazine, explains that [7] "small-piston aircraft and business jets are undergoing a radical upgrade. Digital technology developed for combat fighters and commercial aircraft 20 years ago is finally making its way into the cockpits of small aircraft, often at a fraction of the cost of the electronics currently installed in Boeing jumbo jets."

Major airlines like Airbus and Boeing have already begun moving their fleets toward the DFBW domain. Airbus made this move with its A320 aircraft, [6] "the first commercial airliner to have DFBW technology." Boeing subsequently followed suit by employing DFBW technology on its 777 and 787 aircraft models. It is interesting to note however, that Airbus and Boeing differ in the employment of their respective DFBW flight control systems and algorithm control laws. An article written by Brian Palmer summarizes the differences between the two aircraft manufacturers. Palmer explains that [8] "Airbus employs a joystick that electrically connects the pilot's input to the flight controls where Boeing employs the standard 'yoke' that still uses cables to deliver pilot input commands. Airbus also utilizes control algorithms called 'flight envelope protection' that keeps the aircraft from flying outside its designed operating area. Boeing gives more latitude to the pilot in being able to 'push the envelope' when appropriate." Palmer also goes on to explain how [8] "it is unclear as to whether flight envelope protection makes air travel safer." Palmer cites two aircraft incidents; namely, China Airlines Flight 006 and the crash of an American Airlines jet in November 2001 where flight envelope protection could have hindered the aircraft flight control recovery or could have prevented the crash, respectively.

3. Conclusion

3.1 Summary

The employment of flight control systems over the past one hundred years has seen quantum leaps in the design and theory behind how a pilot actually controls the flight of an aerospace vehicle. DFBW technology has exponentially increased the safety of flight for millions of people. This area of engineering owes its continued refinement and evolution to countless individuals and organizations who have taken on the challenge of developing the control systems that advance the safety and efficiency of flight. It is amazing to look back in history on the evolution of aerospace vehicles and recount that within fifty years of human beings first successfully completing powered flight that autopilots were flying planes without human pilot commands from Canada to England. That physical power of flight controls was supplanted by hydraulic actuation which in turn was augmented with electrical circuits. That the advent of seemingly unrelated hardware such as microprocessors and logic circuits would have such a profound place in designing an aircraft to fly. That the innovators in this field had the vision and courage to trust their knowledge and engineering skill in putting the lives of capable pilots in the hands of a computer system. The flight control systems currently deployed in aviation constitute some the most well engineered, capable and, failure resistant electrical systems ever created. However, it should be noted that such systems seem to still be in their adolescence and much discovery and improvement is left to the next generation who endeavors to improve upon and invent the future of aerospace flight control systems.

Sources Cited

[1] Creech, Gray. "Digital Fly By Wire: Aircraft Flight Control Comes of Age." http://www.nasa.gov/vision/earth/improvingflight/fly_by_wire.html. Jim Wilson. NASA Dryden Flight Research Center, September 30, 2007. Internet. October 24, 2012.

[2] Elliot, Dr. William. The Development of Fly-By-Wire Flight Control. Air Force Material Command: Office of History, AFMC Historical Study No. 7, December 1996. Print.

[3] Greetham, Tom. Evolution of Powered Flight Controls. http://mae.osu.edu/sites/mae.web.engadmin.ohiostate.edu/files/uploads/ME888Presentations/evolution_of_powered_flight_controls_seminar.pdf, February 10, 2012. Accessed November 10, 2012. Internet.

[4] F-16.Net. Production Blocks and Experimental Versions. http://www.f-16.net/f-16_versions.html. Accessed December 2, 2012. Internet.

[5] Sambor, Joe. F-16.Net Forum. http://www.f-16.net/f-16_forum_viewtopic-t-6605.html, October 22, 2006. Accessed December 2, 2012. Internet.

[6] Philippe, Christian. "The Impact of Control Technology." T. Samad and A.M. Annaswamy (eds.), IEEE Control Systems Society, 2011. Internet. October 23, 2012.

[7] Tatge, Mark. "Fly By Wire". http://www.forbes.com/forbes/2005/1128/083.html. November 11, 2005. Accessed December 2, 2012. Internet.

[8] Palmer, Brian. "Boeing Vs. Airbus". http://www.slate.com/articles/news_and_politics/explainer/2011/07/boeing_vs_airbus.html. July 11, 2011. Accessed December 2, 2012. Internet.

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