Early History

The importance of training has been realised since the inception of manned flight. From the early days of gliding it was usual for "pilots" to sit in the glider, which was exposed to a strong facing wind and "feel" the controls by keeping the wings in a horizontal position. Thus, even before the glider flew, the pilot had some experience of the lateral controls.

The fliers of the first powered aeroplanes learnt by proceeding through a graded sequence of exercises on real aircraft. After passenger flights, a student would perform taxiing, where a low powered machine is driven along the ground enabling rudder control to be practised. He would then graduate to a higher powered machine and would first make short hops using elevator control. After longer hops he would eventually achieve flight. A variation of this method, known as the "penguin system", in which a reduced wingspan, landborne aeroplane was used, was developed during World War I. In this machine the student pilot could learn the feel of the controls while proceeding along the ground. This method was used at the French Ecole de Combat with a cut-down Bleriot monoplane, but was considered as early as 1910.

Other early devices attempted to achieve the same effect, especially for the testing of new aircraft prototypes, by using aircraft moving at speed supported by balloons, overhead gantries or railway bogies. Related to these ideas were the first proposals for truly ground-based trainers which were, in effect, aircraft tethered to the ground, but capable of responding to aerodynamic forces. One such device was the Sanders Teacher.

The Teacher was constructed from components which could in fact be used to build an actual flying machine, and was really an aircraft mounted on a universal joint in an exposed position and facing into the prevailing wind. In this way it was able to respond in attitude to the aileron, elevator and rudder controls as would an actual aeroplane of the type. Unfortunately, as was the case with many of these early devices, it was not a success, probably because of the unreliability of the wind. A similar device was that constructed by Eardley Billing, the brother of Noel Pemberton Billing, at about the same time, and was available for use at Brooklands Aerodrome.

Also around this period was made one of the first truly synthetic flight training devices. This photograph was published in 1910, as can be seen, it consisted of two half-sections of a barrel mounted and moved manually to represent the pitch and roll of an aeroplane. The prospective pilot sat in the top section of this device and was required to line up a reference bar with the horizon.

The need for the training of large numbers of pilots during World War I encouraged the development of the new discipline of aviation psychology and tests were introduced for aviator selection, the lead being taken in France and Italy. Many devices were invented to aid in the assessment of the aptitude of potential airmen. In 1915 such a machine was proposed for the measurement of reaction time in correcting disturbances; this consisted of a rocking fuselage fitted with controls and an electrical recording apparatus - the response of the student to tilting, manually produced by the examiner, being recorded. Further developments on this theme include the Ruggles Orientator, and the devices patented by Reid and Ocker. In all of the descriptions of these machines, it is stated that useful pilot training could also be undertaken with their use; this however, must have been of a very limited nature.

The Ruggles orientator, for example, consisted of a seat mounted within a gimbal ring assembly which enabled full rotation of the pupil in all three axes and in addition provided vertical movement. All motions were produced by electric motors controllable by the simulated sticks and rudder bars of the student and examiner. This device was stated to be useful for "developing and training the functions of the semi-circular canals and incidentally to provide such a machine for training aviators to accustom themselves to any possible position in which they may be moved by the action of an aeroplane while in flight", it must have been good fun too :-). A further optimistic claim was that the aviator could be blindfolded "so that the sense of direction may be sensitized without the assistance of the visual senses. In this way the aviator when in fog or intense darkness may be instinctively conscious of his position".

Aids were also produced for the training of other skills associated with aviation. Rolfe mentions German methods for the training of air gunners and observers, and the French are known to have used miniature painted landscapes for bomb aiming training.

The next step in the evolution of the flight trainer was the replacement of the human operator in Antoinette type machines with mechanical or electrical actuators linked to the trainer controls. The aim of these now automatic devices was to rotate the trainee pilot's fuselage into an attitude corresponding to that of the real aircraft in response to his control inputs. Provision was usually made for an instructor to introduce disturbances in attitude to simulate the effect of rough air and to present control problems to the student. An example of this technique is the family of devices described by Lender and Heidelberg, of France, in 1917. One of these consisted of a pivoted dummy fuselage with pitch, roll and yaw motions produced by compressed air motors and introduced, probably for the first time, variations of response and feel with simulated speed. Engine noise and a rudimentary visual presentation were also described.

An electrical version of this type of trainer was patented in the United States in 1929 by Buckley. This machine consisted of a small dummy fuselage mounted on a universal joint (which by now had become a common arrangement) with pitch and roll attitudes each produced by opposing motors proportionally controlled by stick movements while turning motion was provided by another motor actuated by controls on the rudder bar. Programmed disturbances could be introduced by means of a perforated tape arrangement which could also control the indications of dummy cockpit instruments; these were not, however, connected to the flying controls.

The most successful and well-known of this type of device was, the Link Trainer. Edwin Link gained his early engineering experience at his father's firm, the Link Piano and Organ Company of Binghamton, New York. The trainer was developed in the period 1927-1929 in the basement of the Link factory and made use of pneumatic mechanisms from the piano and organ business. The first trainer, touted as "an efficient aeronautical training aid - a novel, profitable amusement device" was described in a patent of 1930. Pitch, roll and yaw movements were initiated in the same manner as in its predecessors, but Pneumatic bellows were used for actuation. An electrically driven suction pump mounted in the fixed base fed the various control valves operated by the stick and rudder, while another motor-driven device produced a repeated sequence of attitude disturbances. In common with other trainers of the time, the performance was adjusted by trial and error by the designer until the correct "feel" was obtained.

The first description of the trainer made no reference to instruments and the device was therefore primarily intended to demonstrate to students the effects of the controls on the attitude of the simulated aeroplane and to train them in their operation. As with other synthetic devices of this time, the simulated effects of the ailerons, elevators and rudder were independent; they did not represent a true reproduction of the aircraft's coordinated behaviour.

However, despite twenty years of development, simulation was not seen as a substitute for actual flight. The acceptance of simulated flight as a useful training aid had to wait for further developments in the science of flying.







Instrument Flight Training

In the late 1920's the need was starting to be felt for the effective training of pilots in the skills of "blind" or instrument flying. Two methods were developed firstly the existing moving trainers, such as Link's, were fitted with dummy instruments and the means for their actuation, and secondly, non- movable devices were invented specifically for the task of instrument flight training.

Rougerie's patent of 1928 describes a simple trainer, fixed to the ground, consisting of a students seat facing an instrument panel and two sets of controls, one each for the student and instructor. The student's flight instruments are directly connected to the instructor's controls. The student, then, flies the trainer in response to commands from the instructor, who in turn modifies the instrument indications according to the students actions - the accuracy of the simulation depends entirely on the instructor. A further development of this concept can be seen in a development by W.E.P. Johnson in 1931, an instructor at the Central Flying School, Wittering, and one of the pioneers of instrument flying in Britain. He constructed his trainer from a written-off Avro 504 fuselage. In the simplest form of this invention an airspeed indicator, turn indicator, and bank indicator are directly operated by wires attached to the sticks and rudder bars of student and instructor. Further improvements included a throttle control affecting the airspeed indicator and integrating devices for the display of altitude and heading. It is interesting to note that a true simulation of accelerations due to aircraft motion was suggested. However, it seems that this idea was not to be taken seriously for more than twenty years.

Another early British instrument flight trainer is that described by Jenkins and Berlyn of Air Service Training Limited, Hamble, in their patent application of 1932. This ground-fixed apparatus used mechanisms similar to Johnson's for linking the instruments to the controls. Rotation of the magnetic compass was effected with a magnet, while transient deflections, were produced by causing a rotary movement of the compass damping fluid in response to pitch and throttle control changes.

The Link Trainers themselves were soon being fitted with instruments as standard equipment. Blind flying training was started by the Links at their flying school in the early 1930's and as the importance of this type of training was more fully appreciated, notably by the U.S. Army Air Corps, so the sales of Link Trainers increased. The newer Link Trainers were able to rotate through 360Deg which allowed a magnetic compass to be installed, while the various instruments were operated either mechanically or pneumatically. Altitude, for example, was represented by the pressure of air in a tank directly connected to an altimeter. Rudder/aileron interaction was provided in the more advanced trainers, as was a stall feature. The reproduction of aircraft behaviour and dynamics was still produced in an empirical manner.

A further increase in the usefulness of the trainers was achieved with the attachment of a course plotter. This consisted of tortoise like device, on three wheels, which was self-propelled and steerable; the course of the simulated flight was traced on a chart by means of an inked wheel. The plotter can be seen in the picture on the instructors desk. By relating the position of the student's aircraft to marks on the chart, the instructor was able to manually control the transmission of simulated radio beacon signals to the trainer. In the 1930's the device was produced in various versions and was sold to many countries, including Japan, the USSR, France and Germany. The first Link Trainer to be sold to an airline was that delivered to American Airlines in 1937. The RAF also took delivery of their first Link in the same year. By the beginning of the Second World War, many of the major air forces were doing their basic instrument training on Links, or devices derived from them. The Link Trainers continued to be manufactured into the 1950's, their principle of operation remained the same.



Two of the first electrical flight trainers, both still based on empirical designs, were Dehmel's trainer and Travis' "Aerostructor". Dr. R.C. Dehmel, an engineer with the Bell Telephone Laboratories, became interested in flight training in 1938. His first development was an automatic signal controller for the generation of synthetic radio signals for a Link Trainer, thus eliminating the need for the attendant who manually operated signal volume controls during the training session. This was an important advance in instrument flight training in that it enabled a closer match with the behaviour of actual navigational aids. Following this, Dehmel developed the "flight" portion of a trainer based on electrical circuits. This machine was never manufactured, but served as a starting point for future developments. The Aerostructor, developed by A.E. Travis and his colleagues in 1939/ 40 also in the United States, was a fixed base, electrically operated trainer with a visual rather than an instrument presentation. The visual system was based on a loop of film and simulated the effects of heading, pitch and roll movement. The trainer was widely demonstrated in the U.S., but was never commercially produced. It was however, used in large numbers by the U.S. Navy in a modified form as the "Gunairstructor".

World War II

At the start of the Second World War there was the requirement to train large numbers of people in the many individual and team skills involved in the operation of the various military aircraft. Basic pilot instruction was performed in part on Link Trainers both in the United States and Britain.

Developments in aircraft, such as variable pitch propellers, retractable undercarriage and higher speeds made training in cockpit drill essential. The mock-up fuselage was introduced as an aid to training in these procedures. One such device was the Hawarden Trainer, made from the centre section of a Spitfire fuselage, which enabled training in the procedures of a complete operational flight. The Links too, were developed to the stage where the instrument layout and performance of specific aeroplanes were duplicated; the U.S. Army-Navy Trainer, Model 18 (ANT-18), for example, was designed for training in AT-6 and SNJ flying.

In 1939 the British requested Link to design a trainer which could be used to improve the celestial navigation capabilities of their crews who were ferrying "surplus" U.S. aircraft across the Atlantic. Such a trainer could also be used to improve bombing accuracy during night raids over Europe. Ed Link, together with the aerial navigation expert, P. Weems, worked out the design of a massive trainer suitable for use by an entire bomber crew, and housed in a 45 foot high silo-shaped building. This was the Celestial Navigation Trainer. The trainers incorporated a large-size fuselage similar to that of the conventional Link Trainer, but which could accommodate the pilot, navigator, and bomber. The pilot flew the trainer, which included all the facilities and instruments of the smaller conventional Link Trainer, while a bomb aimer's station provided the appropriate sight and targets over which the trainer flew. The navigator was provided with all the radio aids and, in addition, was provided with an elaborate celestial view from which he could take his appropriate astro sights. The stars, of which enough (12) were collimated, were fixed to a dome which was given a movement to correspond with the apparent motion of the stars with time and changes in bomber latitude and longitude.

The first Celestial Navigation Trainer was completed in 1941, and the RAF placed an order for sixty of them. Unfortunately, only a limited number of these trainers were installed in Britain, such as at the Link Trainer School at Elstree, and at a number of special RAF stations. The balance were returned to the U.S. Air Force under Reverse Lease Lend, with the exception of three sets of components which were subsequently used for navigational trainers. However, hundreds of these devices were installed and operated in the United States.

Throughout the war instructors on various RAF stations were contributing their ideas to training and numerous "home-made" devices were constructed due to the long delivery times and low priority given to the manufacture of training aids. An early development was the "instructional fuselage". Such a device would consist of fuselage of the desired type mounted on stands inside a hangar. It could then be used to train air crews in the drills that they have to carry out in the particular aircraft that they are being trained on. Services like, hydraulic, electrical, and pneumatic, and their recording instruments were made to work in a normal manner, so that the various drills carried out by the crew were realistic. Bomb-dropping procedure and abandon aircraft drills by parachute and dinghy were also carried out; the bombs being released into sand trays beneath the aircraft, (duds presumably). :-)

Of particular interest are the so-called Silloth Trainers, developed by Wing Commander Iles at RAF Silloth, south of Carlisle. The picture shows one of these trainers for a Halifax bomber. The Silloth Trainer was designed for the training of all members of the crew, and was primarily a type familiarization trainer for learning drills and the handling of malfunctions. As well as the basic flying behaviour, all engine, electric and hydraulic systems were simulated. An instructor's panel, visible in the photo, was provided to enable monitoring of the crew and malfunction insertion. All computation was pneumatic, as in the Link Trainer. Silloth trainers were manufactured for 2 and 4 engined aircraft throughout the war; in mid-1945, 14 of these trainers were in existence or on order. Towards the end of the war a Wellington simulator was developed at RAF St. Athan, using contoured cams to generate the characteristics of the aircraft's flight and engines. This machine, however, did not supplant the Silloth Trainer, as all activity on these ceased at the end of the war.

In 1940 Rediffusion, whose manufacturing division later became Redifon, built a direction finding trainer for ground operators. This simulated the Bellini-Tosi goniometer DF equipment, whereby two such stations could take a fix on an aircraft transmission and pass the resulting information back to the pilot. A similar trainer was designed to train the operators of VHF stations to give fixes to fighter pilots. However, the most important member of this family of Redifon trainers was the C 100 DF and navigational trainer which was first produced in 1941 to train air crews in the skills of navigation using ground beacons.

The trainers were installed in five separate cubicles which housed the trainee pilot, navigator and radio operators, and enabled these crews under the control of an instructor, to carry out navigational exercises, plotting their track from the bearings set up by the instructor. This trainer was similar in principle to the other two Redifon trainers. Suitable decoupling was provided so that up to five receivers and goniometers could be operated from one set of transmitting goniometers enabling the instructor, at the cost of limited flexibility, to teach five crews simultaneously. The transmitting goniometers were mounted on a chart at the position of the beacon stations so that the designated north/south stator coils were aligned with the meridian passing through the particular beacon. The DF receivers were standard RAF airborne units and it was thus possible to tune them in and operate them as would be done in real life. The complete receiving goniometer stators could be physically oriented by the "pilot" of each aircraft to correspond to the aircraft heading during the flight.

The equipment had provision for the superimposition of interference such as enemy jamming. Some installations were equipped with sound effects and epidiascopes so that pictures of target areas and other landmarks of importance could be projected in front of the trainer. These installations were known as Crew Procedures Trainers. Well over 100 of the C 100 navigational trainers were built and installed on RAF Bomber Command operational training units and navigational training stations throughout the country and in Canada at the Empire Air Training Stations until the end of the war, plus the small number of trainers installed on USAF stations in this country.

In late 1942 Rediffusion were instructed to install this equipment on the first of the American 8th Air Force's stations at Bovingdon, which was known as a crew replacement centre. The American authorities quickly appreciated the benefits of this trainer and requested that it be made to operate with American equipment as installed in the B17 Flying Fortress. In 1943 Rediffusion developed for the American Air Force a Dead Reckoning Navigational Trainer to train up to ten navigators flying in formation. The production model of this trainer, the C500, utilised the C100 and provided hyperbolic Gee fixes with an existing static Gee trainer.

One of the best technological successes of the war was the part played by the Trainer Group at the Telecommunications Research Establishment (TRE) in the design of synthetic radar trainers. This group, under G.W.A. Dummer, developed trainers for all of the new radars developed during the war years. In addition to devices attached to Link Trainers, a novel flight simulator for training in AI (Aircraft Interception) was invented. This trainer, the Type 19, was a complete crew, fixed base, trainer for AI combat, which consists of four stages: following an interception course provided by a ground operator, guided by on-board radar, visual contact and the moment of firing. The type 19 provided training in the complete sequence by provision of positions for the pilot and AI operator, and instructors unit, computers for simulation of the attacking aircraft and the relative position of the "enemy", a visual projection unit and a course recorder. The flight simulation computer (known as the Type 8, Part II) was used in a number of TRE trainers, including mobile units whose function was to tour operational squadrons to train in the use of the latest versions of airborne radar. The visual projection system, designed by A.M. Uttley, was used in the larger AI training installations at RAF Operational Training Units. The image, displayed on a hemispherical cyclorama mounted in front of the pilot, consisted of a night sky and ground of controllable brightness with a tail silhouette of a bomber which moved correctly in bank, range, azimuth and elevation in response to relative movements of fighter and bomber. The first AI crew trainers went into service in 1941, while the first complete Type 19 trainer was installed in 1943. It has been estimated that the use of the TRE synthetic radar trainers saved £50,000,000 worth of aviation fuel alone.

In addition to the trainers mentioned, above many others were developed by adding extra features to the basic Link Trainer for such tasks as gunnery instruction. In Britain, the JVW Corporation Limited, formed to market and service Link Trainers, successfully produced a torpedo attack trainer for the Royal Navy, a tank trainer for the Army, and a night vision tester and glider station keeping device for the RAF. The epidiascope visual system for the Torpedo Attack Teacher was produced by Strand Electric, better known for stage lighting. Another simulator with a strong visual element was the Royal Aircraft Establishment's Fixed-Gun Trainer for fighter pilots, developed towards the end of the war, the needs for training in more specialised skills were met by the adoption of a multitude of purpose-built devices.

Electronic Flight Simulation

A major advance in simulation during the war period was the use of the analogue computer to solve the equations of motion of the aircraft. The analogue computer, or differential analyser, as it was then known, enabled simulation of the response of the vehicle to aerodynamic forces as opposed to an empirical duplication of their effects. It is difficult to make a complete separation of these two types of simulation as both may be present in the same device. However, certain devices clearly were true analogues and a number of these are the direct ancestors of the modern simulator.

The first known discussion of the computer method of flight simulation is that of Roeder in his 1929 German Patent Specification. Roeder treated the general problem of the instrument control of vehicles freely movable in space, such as airships, aeroplanes or submarines. His outlines of the requirements of a simulator for such a task could almost refer to a modern simulator. As an example of his technique he described the dynamic simulation of an airship height control system and a fluid-operated analogue computer suitable for this. No successful training devices are known to have resulted from this work. In 1939 Mueller, at MIT, described an electronic analogue computer for the faster-than- real-time simulation of aeroplane longitudinal dynamics. His interest was in aircraft design and the solution of the equations of motion, but as a postscript to his paper he mentioned the possibility of extending the time scale of the simulation and of including a man in the loop.

In 1941 an electronic simulator was designed and built at the TRE to serve as the "flying unit" for their AI radar trainers. This computer was based on the ideas of F.C. Williams, famous for his later work on digital computers, and used the velodyne, another TRE invention, for integration. The d.c. method of computing was used in the simulation of the simplified fighter aerodynamics. The first model of this computer (the Type 8 Part II) was constructed by Dynatron Radio Limited in 1941 and many were used throughout the war. Later, in 1945, a more advanced flying unit including feel forces was designed by A.M. Uttley for use in a new AI visual crew trainer. This, however, never saw service.

Also in Britain at about this time an electromechanical analogue computer for the simulation of aircraft longitudinal dynamics was proposed by G.M. Hellings, then working at the Ministry of Supply. Non-linear functions were generated with shaped cams, and it was sufficiently general to allow the characteristics of any chosen aircraft type to be represented. A mechanical version of this device, the Day Landing Trainer, was manufactured by General Aircraft Limited and used at the Empire Central Flying School. This trainer simulated longitudinal motions and had a pitch motion system with an endless belt, directly viewed visual model. Further development of the device was carried out after the war at Air Trainers Limited.

In 1941 Commander Luis de Florez, of the U.S. Navy, visited Britain and wrote his "Report on British Synthetic Training". This report was highly significant and influenced the establishing of the Special Devices Division of the Bureau of Aeronautics, the predecessor of the present Naval Training Equipment Center. Also in this year the Silloth Trainer concept was brought to the United States and one was erected at the Mohier Organ Plant at Hagerstown, Maryland. After evaluation it was decided to build an electrical version of the trainer as instability of adjustments due to humidity, temperature and ageing made the system unmanageable. The task of producing the new trainer was given to Bell Telephone Laboratories who produced an operational flight trainer for the Navy's PBM-3 aircraft. This device, completed in 1943, consisted of a replica of the PBM front fuselage and cockpit, complete with controls, instrumentation and all auxiliary equipment, together with an electronic computing device to solve the flight equations. The simulator had no motion system, visual system or variable control loading. A total of 32 of these electronic flight trainers for seven types of aeroplane were built by Bell and the Western Electric Company during the war years. It has been stated that the PBM-3 was "probably the first operational flight trainer that attempted to simulate the aerodynamic characteristics of a specific aircraft" but this is debatable.

Since the development of his electrical instrument flight trainer Dr. Dehmel had gained experience in analogue computing techniques through his work on Bell's M-9 anti-aircraft gun directors. He applied this knowledge to the design of an instrument flight simulator based on an analogue computer. He was then able to interest the Curtiss Wright Corporation in the manufacture of these devices in 1943. After the development of a prototype trainer, the U.S. Air Force ordered two trainers from Curtiss Wright for the AT-6 aeroplane; this trainer was named the Z-1 and is shown in the photo. These were followed by production examples designated the Z-2, -3 and -4.

After the war, competition from Curtiss-Wright stimulated Link to develop their own electronic simulators. Also at this time the value of the Link Trainer motion system was being called into question. The movements of the Link Trainer did not correctly simulate the forces experienced in flight, and in fact a ground-fixed trainer would more accurately locate the force vector in coordianted turning or level flight. Also, the axis of roll rotation was too far below the pilot to allow correct simulation of accelerations due to roll. It was argued that the modern pilot should not fly "by the seat of his pants", but by instruments. Ed Link disagreed and held the view that trainer motion was needed even if incorrect, since motion was present in flying. However, customer pressure caused Link to follow the trend to fixed base simulators. The company therefore developed their own electronic analogue computer which was used in their C-ll jet trainer. A contract was awarded by the U.S. Air Force in 1949, and eventually over a thousand of these types were sold.

Meanwhile, Curtiss-Wright had contracted to develop a full simulator for the Boeing 377 Stratocruisers of Pan American Airways. The simulator was installed in 1948 and was the first full aircraft simulator to be owned by an airline. No motion or visual system were installed, but in all other respects the simulator duplicated the appearance and behaviour of the Stratocruiser cockpit. The trainer was found especially useful for the practice of procedures involving the whole crew; emergency conditions could be readily introduced by the instructor on his fault insertion panel. Complete routes could be flown, as in real life, using the same navigational aids. This facility was used by other airlines, and in the words of a BOAC Captain, "From start to finish we had treated the whole exercise as if it were the real thing, and the cockpit was so complete in every detail that we soon forgot that we were not in an aeroplane'' However, there were some reservations expressed about the lack of motion in a fixed-base simulation, which caused it to feel unnatural and could even cause control problems.

In 1947 B0AC decided to buy Boeing 377 Stratocruisers, and knowing of Redifon's work on synthetic crew trainers, asked Mr Adorian if a simulator could be built for this aircraft; the simulator was to be the same as that which Curtiss-Wright were building for Pan American. In order to comply with the BOAC requirement Redifon had to enter into an agreement with Curtiss-Wright and Dr. Dehmel and obtain clearance from the U.S. State Department. Work commenced on the construction of the simulator at Redifons Wandsworth works in January 1950. The computation was analogue, using 60Hz (U.S. mains frequency) signals and servo motors, contoured potentiometers and 400Hz synchros and magnesyns for aircraft instrument drives. The control loading unit used variable levers, servo controlled as a correctly computed function of air speed, with springs to produce the necessary forces. The unit took the form of a separate frame running the whole length of the fuselage and, as today, carried the flying controls, throttle pedestal and pilot's panels and seats. The simulator was finally accepted in October 1951 with the price to BOAC being £120,000.

Prior to the final acceptance of the Stratocruiser, BOAC gave another simulator order to Redifon, this time for a Comet I. This was to become the first jet transport simulator in the world, and was designed by A.E. Cutler. Whereas the first simulator's servos had been manufactured by Curtiss, the Comet servos and potentiometers were built by Redifon. This second simulator followed similar principles to that of the first, except that a carrier frequency of 50Hz was employed and no computed control loading was necessary as the aircraft used a fixed spring-loaded control system.

The first Curtiss-Wright, Redifon and Link simulators used the a.c. carrier method of analogue computer. Air Trainers Limited however, decided to use the d.c. method - a more demanding technology, but one capable of superior precision in simulation. Their first simulator using this technology was built for the RAF's Meteor aircraft. The d.c. method was later adopted by Link in the United States. Redifon, however, developed a system using a carrier frequency of 400Hz which was very successful. Also, at this time, mechanical analogue computers were constructed for use in the simpler "type trainers" by Air Trainers Limited.

Digital Simulators

One of the restrictions in these early days was that aircraft manufacturers did not have much analytical information on the performance of their airframes and engines; the simulator manufacturers were therefore required to use ad hoc methods to achieve the desired aeroplane characteristics. This changed however, with the arrival of the large subsonic jet transport era when the aircraft manufacturers began to produce much more complete data and to perform more extensive flight development programmes. Together with requirements for driving the motion and visual systems then being introduced and pressure from the operators to improve accuracy and thereby, they hoped, better transfer of training, significant increases in the amount of analogue computer hardware became necessary to satisfy them. At this point, the law of diminishing return began to operate, the cumulative errors caused by all the additional hardware exceeded the improved accuracy which should have resulted from the more extensive aircraft data, which demanded the extra hardware.

In addition, reliability began to fall in spite of improved hardware and design technology, or at best was only maintained by the efforts of maintenance teams. At that time, the required utilisation was around 8-10 hours per day for five days per week. This was soon extended to six days per week, even then, the requirement of today, for a training utilisation of virtually 24 hours per day for seven days a week could be foreseen. It thus became obvious that the demands for increased fidelity of simulation and reliability could no longer easily be met with analogue machines even with the use of the new solid state elements which had appeared. Around this time the second generation of digital computers, started to materialise, and were able to satisfy the speed and cost requirements of flight simulation. As a consequence, there was an almost total swing to digital simulation for all but the simplest trainers.

It was realised from the earliest days of programmable electronic digital computers that a potential application would be in real-time digital simulation. The advantages of digital computers, improved flexibility, repeatability and standardisation, were approached by the U.S. Navy who initiated a research program at the University of Pennsylvania in 1950. The general purpose computers of the time could not be used directly for real-time flight simulation, due to their poor arithmetic and input-output capabilities. A special machine therefore, was designed at the University for their simulator, which was named UDOFT (Universal Digital Operational Flight Trainer). This computer was manufactured by the Sylvania Corporation and completed in 1960. The UDOFT project had demonstrated the feasibility of digital simulation and was mainly concerned with the solution of the aircraft dynamic equations. In the early 1960's Link developed a special purpose digital computer, the Link Mark I, designed for real- time simulation. This machine had three parallel processors for arithmetic, function generation, and radio station selection. In the late 1960's general purpose digital computers designed for process control applications were found to be suitable for simulation, with its large input - output requirement, and the use of special purpose machines declined. Today special purpose digital computers are only used in applications demanding very high speed processing, such as computer generated imagery.

Nearly all of the simulators produced up to the mid 1950's had no fuselage motion systems. This was justified by the statement that modern pilots did not fly "by the seat of their pants", but the fact remained that fixed-base simulators did not feel like aeroplanes to fly. It was found that a handling improvement could be made by empirical adjustment of the control loading and aircraft dynamics simulations which, in part compensated for the lack of motion. Proposals were made by the manufacturers for motion systems, but it was not until the late 1950's that the airlines decided to purchase them.

In 1958, Redifon received a contract from BOAC for the production of a pitch motion system as part of a Comet IV simulator. More complex motion systems were designed capable of producing motions in two and three degrees of freedom, and with the introduction of wide-bodied transport aircraft, such as the 747, a lateral acceleration was required which led to the introduction of four and six degrees of freedom systems. Six degree of freedom motion systems are now the most common. The perception of motion and its effect on training is one of the less understood aspects of simulation and research is still active in this area.



Systems for producing the extra-cockpit visual scene have been proposed and constructed for almost as long as flight simulators themselves. However, realistic and flexible visual attachments are a fairly recent development. Due to the large number of visual systems which have been invented, only some of the more successful ones can be mentioned here.

The point-light source projection, or shadow graph, method enjoyed popularity in the 1950's, especially for helicopter simulators. A series of simulators using this method of visual display were produced by Giravions Dorand in France including an ab initio hovering trainer produced by Shorts of Belfast in 1955. Simulators on this pattern were also built in the United States, but the shortcomings of the shadow graph system seems to have limited the success of the concept. The first visual systems achieving widespread use on civil aviation simulators were based on the scale model and television camera method, although methods based on film and anamorphic optical systems have also met with success for more restricted applications. Serious development of closed-circuit television visual systems began in the mid 1950's with monochrome systems being produced by Curtiss-Wright, Link (then the Link division of General Precision) and General Precision Systems (formerly Air Trainers and Air Trainers Link Limited). The first colour system was produced by Redifon in 1962. Television based visual systems have under gone a steady development since then, with a large part of the effort being devoted to improved methods of image display.

The first computer image generation systems for simulation were produced by the General Electric Company (USA) for the space programme. Early versions of these systems produced a patterned "ground plane" image, while later systems were able to generate images of three-dimensional objects. Progress in this technology has been rapid and closely linked to developments in digital computer hardware technology. Current systems available from major simulator manufacturers are able to produce full colour images with scene contents of several thousand polygons and point-light sources. A parallel development has taken place in night-only computer image generation systems; these use the calligraphic or stroke-writing, rather than the raster scan method of display, which enables a superior reproduction of light points. The first of these systems was produced by the McDonnell-Douglas Electronics Corporation in 1971 and called Vital II.

Current systems, can produce images of night, dusk, and daylight, they can run at two rates 60Hz for daylight, or 40Hz for dusk/night. Per second they can sustain continuous 1.8M polygons, with a 1.2Giga pixel fill rate and 360000 calligraphic lights, (approx. 30000 polygons 15000 calligraphic lights and 7500 raster lights per frame). 16 sub pixel antialiasing, texturing, Phong shading, transparency, fog, layered fog, bump mapping and depth buffering are all available simultaneously for all pixels and polygons, at the sustained rate. A typical commercial system has a viewable area of 4M pixels, spread across three projectors (1.5M pixels each) though up to 5 can be used, and a texture map memory of 1 GB.

 
 
Much effort has been devoted to improving the instructional facilities in the simulator. The use of high resolution touch screens for instructor control, and substantial increases in the number of malfunctions and radio stations which can be offered, there are also facilities for exercise recording and playback, pilot performance recording and evaluation, separate pilot and flight engineer training in the same exercise and automated training. We have now reached a point in commercial flying training, where all conversion and recurrent training can be conducted in a simulator, so that a pilot of one aircraft type, can be cross trained to another, without ever actually having flown the real target aircraft, until he or she is on board, carrying fare paying passengers.

References.
Aeroplane Maintenance & Operation Series, Vol. 8.
The Link Trainer.

50 Years of Flight Simulation.
Conference Proceedings April 1979.

Flight Simulation.
Edited by J.M.Rolfe & K.J.Staples.
ISBN. 0 521 35751 9 paperback.