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Tag Archives: Digital Computer Systems

FLIGHT SIM

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While watching the recent movie Sully, I was amazed at the sophistication of current flight simulators available to the major aircraft producers.  During the course of this blog, we will trace the development of flight simulators from mere mechanical devices to the virtual reality electronics of today.

A flight simulator is a mechanical or electronic device, which attempts to duplicate both aircraft flight and the environment in which it flies.  Current simulators can replicate factors such as flight controls, wind, moisture and electronic system interaction.  While flight simulation is used primarily for pilot training, it may also be used to design aircraft, as well as identify effects of aircraft properties.

The earliest flight simulators were used during World War I to teach gunnery techniques.  This involved a static simulator with a model aircraft passing in front to aid both pilots and gunners to develop correct lead angles to the target.  This was the only form of flight simulation for nearly ten years.  The Link Trainer, developed by Edwin Link in the late 1920′s, capitalized on the use of pneumatic devices from player pianos and organs from the family musical instrument business. The first trainer was patented in 1930 with an electrical suction pump boosting the various control valves operated by stick and rudder action while another motor simulated the effects of wind and other external disturbances.  These actions could be manually adjusted to provide a variety of flight characteristics.

While the Link Trainer provided a quantum leap in capability over previous flight simulators, many in both the military and civil aviation communities believed the live flight experience offered a better training environment.  However, by the early 1930′s, the United States Army Air Corps had a need for flight simulator applications which could train mail pilots to fly by instruments for long distances.  An enhancement to the Link Trainer was a device called the course plotter, in which a self-propelled  tracker could remotely trace the trainer position from an inked wheel with communications between pilot and instructor facilitated by the use of simulated radio beacons.

It was during the late 1930′s, when flight simulation began to be based on electronic applications.  The Dehmel Trainer, developed by Dr. R. C. Dehmel of Southwestern Bell, coupled a Link Trainer with an advanced radio simulation system, which could accurately duplicate navigation signals transmitted to a receiving aircraft, providing a state of art simulation of radio navigation aids.  The Aerostructor, developed by A. E. Travis, utilized a fixed base trainer with a moving visual presentation, as opposed to radio and electronic signals.  This presentation was based on a loop of film which depicted the effects of course changes, pitch and roll.  While the Aerostructor was never mass produced, a modified version of it was in service with the US Navy.

During World War II advances in aircraft design such as retractable landing gear, variable pitch propellers and higher speeds created a demand for more realistic forms of flight simulation.  In response to this, the Hawarden Trainer was developed, which used a cutaway center section of a Spitfire fuselage, which allowed training in all aspects of operational flight.  In 1939, the British were in need of a simulator which could train it’s navigators who were ferrying US aircraft across the Atlantic.  The navigator was supported by a number of radio aids, as well as a celestial dome corresponding to changes in the position of the stars relative to changes in time, longitude and latitude.  The Celestial Trainer, designed by Ed Link and P. Weems was also modified to train bomber crews, in which simulated landscapes gave the bomb aimer target sightings as they would appear from a moving aircraft.  Redifussion (Redifon) produced a navigation device in 1940, which simulated existing radio direction equipment allowing two stations to take a fix on an aircraft’s position.  By the end of the war, aircraft crews were trained by the simulation of radar signals to acquaint them with new types of radar developed during the war.

While the science of flight simulation had progressed dramatically over the past thirty years, they were unable to accurately duplicate performance characteristics of a plane.  This changed with the arrival of subsonic jetliners in the 1950′s.  Aircraft manufacturers began to produce more complete data and extensive flight testing.  This data was stored on analogue computers, making the data transferable, but requiring more hardware as aircraft testing became more sophisticated.  By the early 1960′s, digital computers began to replace the aging analogue units due to the increased data capacity and speed of the digital units. The most successful of these, the Link Mark I, operated with three parallel processors functional, arithmetic and radio selection, using a drum memory for data storage.  By the 1970′s the majority of computer systems could be adapted for flight simulation.

During that decade computer image generation or CGI technology became available for flight simulation models.  This technology, adapted from the space program, used a ground plane image, supplemented by three dimensional graphics. This technology became more sophisticated in recent years, mating it to advances in digital computers – a far cry from the rolling ground plane pictures of the 1940′s.  Today, flight simulation is a colossal industry, spanning the globe with a wide range of high tech applications for both aircraft users and producers,  enhancing the safety of both crew and passengers.

 

FLY BY WIRE

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Ever since powered flight was first achieved by Wilbur and Orville Wright in 1903, the aviation community has sought both safer and more efficient methods of aircraft flight control.  During the course of this blog, we will trace the development of fly by wire control systems from the basic electrical controls of the 1930s to the enhanced computerized systems of today.

For about thirty years after the Wright Brothers first flight, pilots controlled an aircraft by direct force, by moving control wheels, sticks and rudder pedals linked to cables and pushrods , which pivoted control surfaces on the wings and tails. However, as engine power, speeds and aircraft weight increased, more force was required to effectively direct aircraft control surfaces.   Mechanical and hydraulic control systems were introduced to compensate for the increased power needs upon control surfaces.  Such systems were relatively heavy and necessitated a careful routing of flight control cables through the plane by systems of cranks, pulleys, hydraulic pipes and tension cables.  Although the mechanical and hydraulic systems provided a substantial boost to aircraft controls, they required multiple backup systems in the event of failures, further increasing weight in the design of the aircraft.  Another problem of the hydro/mechanical systems was their insensitivity to outside aerodynamic forces such as spinning, stalling and vibrations during flight.

Electrical transmission to a plane’s control surfaces was first accomplished in 1934 on the Soviet ANT-20, the Maxim Gorky.  The series of mechanical and hydraulic connections were replaced with electrical ones.  This was an extremely large aircraft for its day and the electrical connections worked flawlessly until the collision of the aircraft the following year, proving the potential of electrical flight control.  However, a dedicated electronic signal avionics control system was not tested until 1958 on the Avro Canada CF-105 Arrow.  Ironically, the first vehicle to utilize an electronic flight control system without mechanical or hydraulic backup was the Lunar Landing Research Vehicle or LLRV, which flew successfully in 1964 as part of the Apollo moon program.

A fly by wire control system is a computer system, which monitors pilot control commands and related factors such as altitude, airspeed and angle-of- attack.  The FBW system then relays these pilot inputs to the flight control surfaces in order to keep the aircraft within its designated flight envelope, or safe flight parameters of the aircraft at various speeds, altitudes and other flight conditions.  The fly by wire computer employs electrical signal inputs to create electrical signal outputs which affect the flight control surfaces to produce the desired aircraft attitude.  FBW computers utilize both analog and digital processing with digital units first appearing in quantity in the late 1970s.  The essential difference between digital and analog units lies in how they process information.  Analog computers work in a continuous cycle in which data can accept an infinite set of values, resulting in no loss of data.  The primary limitation of analog units is the time required to initially configure the hardware to the aircraft, in addition to the difficulty of upgrading existing hardware.  Digital systems operate in a designated time environment, in which values are finite.  Any loss of data is supplemented by relatively high resolution and sampling rates, which minimize data loss.  Upgrading a digital unit is merely a matter of downloading current software, achieving a smooth transition coupled with reduced software and maintenance costs.  The flight control systems offer both redundant computer processing and circuitry in case of failure of the primary unit.

Fly by wire technology offers a number of advantages.  Aircraft weight is greatly reduced since mechanical and hydraulic linkages are no longer necessary.  Safety is enhanced due to both redundancy of electrical circuits as well as a quick response and processing time from the FBW unit, supplanting the skill of the pilot.  Fly by wire systems benefit military aviation by allowing engineers the latitude to design an aircraft which may be inherently unstable, but yet be able to attain superior maneuverability under the parameters of the fly by wire computer.  FBW systems require fewer parts and less fuel usage while providing more comfort for passengers because of more precise handling characteristics.  Fly by wire control systems provide for greater safety by establishing control parameters within the capability of the plane with digital units compatible with the entire range of aircraft sensors.