Simulator structure Flight simulators are an example of a
human-in-the-loop system, in which interaction with a human user is constantly happening. From perspective of the device, the inputs are primary
flight controls, instrument panel buttons and switches and the instructor's station, if present. Based on these, the internal state is updated, and equations of motion solved for the new time step. The new state of the simulated aircraft is shown to the user through visual, auditory, motion and touch channels. To simulate cooperative tasks, the simulator can be suited for multiple users, as is the case with
multi-crew cooperation simulators. Alternatively, more simulators can be connected, what is known as "parallel simulation" or "distributed simulation". As military aircraft often need to cooperate with other craft or military personnel,
wargames are a common use for distributed simulation. Because of that, numerous standards for distributed simulation including aircraft have been developed with military organisations. Some examples include
SIMNET,
DIS and
HLA .
Simulation models The central element of a
simulation model are the equations of motion for the aircraft. The regulations place a limit on maximum
latency between pilot input and aircraft reaction. Because of that, tradeoffs are made to reach the required level of realism with a lower computational cost. Flight simulators typically don't include full
computational fluid dynamics models for forces or weather, but use databases of prepared results from calculations and data acquired in real flights. As an example, instead of simulating flow over the wings,
lift coefficient may be defined in terms of motion parameters like
angle of attack. While different models need to exchange data, most often they can be separated into a modular architecture, for better organisation and ease of development. Typically, gear model for ground handling would be separate input to the main equations of motion. Each engine and avionics instrument is also a self-contained system with well-defined inputs and outputs.
Instruments All classes of require some form of replicating the cockpit. As they are the primary means of interaction between the pilot and the aircraft special importance is assigned to
cockpit controls. To achieve good transfer of skills, there are very specific requirements in the flight simulator regulations
Visual system Outside view from the aircraft is an important cue for flying the aircraft, and is the primary means of navigation for
visual flight rules operation. One of the primary characteristics of a visual system is the
field of view. Depending on the simulator type it may be sufficient to provide only a view forward using a flat display. However, some types of craft, e.g.
fighter aircraft, require a very large field of view, preferably almost full sphere, due to the manoeuvres that are performed during air combat. Similarly, since
helicopters can perform hover flight in any direction, some classes of helicopter flight simulators require even 180 degrees of horizontal field of view. There are many parameters in visual system design. For a narrow field of view, a single display may be sufficient, however typically multiple projectors are required. This arrangement needs additional calibration, both in terms of distortion from not projecting on a flat surface, as well as brightness in regions with overlapping projections. There are also different shapes of screens used, including cylindrical, spherical Because the screen is much closer than objects outside aircraft, the most advanced flight simulators employ
cross-cockpit collimated displays that eliminate the
parallax effect between the pilots' point of view, and provide a more realistic view of distant objects. An alternative to large-scale displays are
virtual reality simulators using a
head-mounted display. This approach allows for a complete field of view, and makes the simulator size considerably smaller. There are examples of use in research,
Contribution to modern computer graphics Visual simulation science applied from the visual systems developed in flight simulators were also an important precursor to three dimensional computer graphics and
Computer Generated Imagery (CGI) systems today. Namely because the object of flight simulation is to reproduce on the ground the behavior of an aircraft in flight. Much of this reproduction had to do with believable visual synthesis that mimicked reality. Combined with the need to pair virtual synthesis with military level training requirements, graphics technologies applied in flight simulation were often years ahead of what would have been available in commercial products. When CGI was first used to train pilots, early systems proved effective for certain simple training missions but needed further development for sophisticated training tasks as terrain following and other tactical maneuvers. Early CGI systems could depict only objects consisting of planar polygons. Advances in algorithms and electronics in flight simulator visual systems and CGI in the 1970s and 1980s influenced many technologies still used in modern graphics. Over time CGI systems were able to superimpose texture over the surfaces and transition from one level of image detail to the next one in a smooth manner. Real-time
computer graphics visualization of virtual worlds makes some aspects of flight simulator visual systems very similar to
game engines, sharing some techniques like
different levels of details or libraries like
OpenGL. Many computer graphics visionaries began their careers at Evans & Sutherland and Link Flight Simulation, Division of Singer Company, two leading companies in flight simulation before today's modern computing era. For example, the Singer Link Digital Image Generator (DIG) created in 1978 was considered one of the worlds first CGI system.
Motion system Initially, the motion systems used separate axes of movement, similar to a
gimbal. After the invention of
Stewart platform simultaneous operation of all actuators became the preferred choice, with some regulations specifically requiring "synergistic"
6 degrees of freedom motion. In contrast to real aircraft, the simulated motion system has a limited range in which it is able to move. That especially affects the ability to simulate sustained accelerations, and requires a separate model to approximate the cues to the human
vestibular system within the given constraints. Motion system is a major contributor to overall simulator cost, but assessments of skill transfer based on training on a simulator and leading to handling an actual aircraft are difficult to make, particularly where motion cues are concerned. Large samples of pilot opinion are required and many subjective opinions tend to be aired, particularly by pilots not used to making objective assessments and responding to a structured test schedule. For many years, it was believed that 6 DOF motion-based simulation gave the pilot closer fidelity to flight control operations and aircraft responses to control inputs and external forces and gave a better training outcome for students than non-motion-based simulation. This is described as "handling fidelity", which can be assessed by test flight standards such as the numerical Cooper-Harper rating scale for handling qualities. Recent scientific studies have shown that the use of technology such as vibration or
dynamic seats within flight simulators can be equally effective in the delivery of training as large and expensive 6-DOF FFS devices. == Modern high-end flight simulators ==