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.

Electronic Simulators and the War

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.

British Synthetic Training

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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.

Post-war Electronic Simulators

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.

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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.

Stratocruiser Synthetic Trainers

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.

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.

This is one part of an article by Kevin Moore. If you would like to read more about the history of airplane flight simulation, go to http://homepage.ntlworld.com/bleep/index.html


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