Editing Flight simulator (section)

== Technology ==

=== Simulator structure ===
[[File:Flight simulator block diagram.svg|thumb|right|500px|Flight simulator block diagram]]

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 [[Aircraft flight control system#Cockpit controls|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.<ref name="baarspul1990">Baarspul, M. (1990) A review of flight simulation techniques. Progress in Aerospace Sciences, 22, 1–20.</ref> 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".<ref name="fujimotoParallelDistributedSimulation2015">{{cite Q|Q63321790}}</ref> As military aircraft often need to cooperate with other craft or military personnel, [[wargame]]s 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]], [[Distributed Interactive Simulation|DIS]] and [[High Level Architecture|HLA]] .

=== Simulation models ===

The central element of a [[simulation]] model are the equations of motion for the aircraft.<ref name="baarspul1990" /> As the aircraft moves through atmosphere it can exhibit both [[six degrees of freedom|translational and rotational degrees of freedom]]. To achieve perception of fluent movement, these equations are solved 50 or 60 times per second.{{r|allerton2009|p=16}} The forces for motion are calculated from aerodynamical models, which in turn depend on state of control surfaces, driven by specific systems, with their avionics, etc. As is the case with modelling, depending on the required level of realism, there are different levels of detail, with some sub-models omitted in simpler simulators.

If a human user is part of the simulator, which might not be the case for some engineering simulators, there is a need to perform the simulation in real-time. Low refresh rates not only reduce realism of simulation, but they have also been linked with increase in [[simulator sickness]].<ref name="pauschLiteratureSurveyVirtual1992">{{cite Q|Q112822678}}</ref> The regulations place a limit on maximum [[Latency (engineering)|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]].{{r|allerton2009|p=17}}

While different models need to exchange data, most often they can be separated into a modular architecture, for better organisation and ease of development.<ref name="ippolitoSoftwareArchitectureReconfigurable2000">{{cite Q|Q112822781}}</ref><ref name="oberhauserVirtualRealityFlight2017">{{cite Q|Q112822831}}</ref> 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 ===
[[File:Simpic.jpg|thumb|Simulator with primary flight instruments replicated with flat displays]]

All classes of {{abbr|FSTD|Flight Simulator Training Device}} 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 [[Aircraft flight control system#Cockpit controls|cockpit controls]]. To achieve good transfer of skills, there are very specific requirements in the flight simulator regulations<ref name="CS-FSTD(A)" /> that determine how closely they must match the real aircraft. These requirements in case of full flight simulators are so detailed, that it may be cost-effective to use the real part certified to fly, rather than manufacture a dedicated replica.{{r|allerton2009|p=18}} Lower classes of simulators may use springs to mimic forces felt when moving the controls. When there is a need to better replicate the control forces or dynamic response, many simulators are equipped with actively driven [[Haptic technology#Force feedback|force feedback]] systems. Vibration actuators may also be included, either due to helicopter simulation requirements, or for aircraft equipped with a [[stick shaker]].

Another form of tactile input from the pilot are instruments located on the panels in the cockpit. As they are used to interact with various aircraft systems, just that may be sufficient for some forms of procedure training. Displaying them on a screen is sufficient for the most basic {{abbr|BITD|Basic Instrument Training Device}} simulators<ref name="CS FSTD(A).200" /> and [[amateur flight simulation]], however most classes of certified simulators need all buttons, switches and other inputs to be operated in the same way as in the aircraft cockpit. The necessity for a physical copy of a cockpit contributes to the cost of simulator construction, and ties the hardware to a specific aircraft type. Because of these reasons, there is ongoing research on interactions in [[virtual reality]], however lack of tactile feedback negatively affects users' performance when using this technology.<ref name="aslandereVirtualHandbuttonInteraction2015">{{cite Q|Q112826446}}</ref><ref name="tatzgernExploringInputApproximations2021">{{cite Q|Q112826551}}</ref>

=== Visual system ===
[[File:DA42-Simulator at Horizon SFA.jpg|thumb|A wide angle cylindrical display]]

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.<ref>Section 91.155 14 CFR Part 91 - General Operating and Flight Rules - FAA</ref> 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.<ref name="barretteModernAirCombat1985">{{cite Q|Q112840484}}</ref> Similarly, since [[helicopter]]s can perform hover flight in any direction, some classes of helicopter flight simulators require even 180 degrees of horizontal field of view.<ref>Appendix 1 to CS FSTD(H).300, 1.3 Visual system, requirement b.3</ref>

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.<ref name="renoFullFieldView1989">{{Cite Q|Q112790735}}</ref> There are also different shapes of screens used, including cylindrical,<ref name="cameronDevelopmentImplementationCosteffective2016">{{cite Q|Q112812641}}</ref> spherical<ref name="renoFullFieldView1989" /> or ellipsoidal. The image can be projected on the viewing side of the [[projection screen]], or alternatively "back-projection" onto a translucent screen.<ref name="bestM2DARTRealImage1999">{{cite Q|Q112840621}}</ref> Because the screen is much closer than objects outside aircraft, the most advanced flight simulators employ [[cross-cockpit collimated display]]s that eliminate the [[parallax]] effect between the pilots' point of view, and provide a more realistic view of distant objects.<ref name="pierceImplicationsImageCollimation1998">{{Cite Q|Q112793062}}</ref>

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,<ref name="oberhauserVirtualRealityFlight2017" /> as well as certified {{abbr|FSTD|Flight Simulator Training Device}}.<ref name="easavr2021">{{Cite press release |title=EASA approves the first Virtual Reality (VR) based Flight Simulation Training Device |date=2021-04-26 |publisher=[[European Union Aviation Safety Agency]] |url=https://www.easa.europa.eu/en/newsroom-and-events/press-releases/easa-approves-first-virtual-reality-vr-based-flight-simulation |access-date=2025-02-10}}</ref>

==== 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.<ref>{{cite book |last1=Rolfe|first1= JM |last2= Staples|first2= KJ |title=Flight Simulation Cambridge Aerospace Series No 1 |date=May 27, 1988 |publisher=Cambridge University Press |isbn=978-0521357517}}</ref> 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.<ref>{{Cite journal |last=Yan |first=Johnson |date=August 1985 |title=Advances in Computer-Generated Imagery for Flight Simulation |url=https://ieeexplore.ieee.org/document/4056245 |journal=[[IEEE Computer Graphics and Applications]] |volume=5 |issue=8 |pages=37–51 |doi=10.1109/MCG.1985.276213 |s2cid=15309937 |via=|url-access=subscription }}</ref>  Real-time [[computer graphics]] visualization of virtual worlds makes some aspects of flight simulator visual systems very similar to [[game engine]]s, sharing some techniques like [[Level of detail (computer graphics)|different levels of details]] or libraries like [[OpenGL]].{{r|allerton2009|p=343}}  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.<ref>{{cite web |last1=Carlson |first1=Wayne |title=Computer Graphics and Animation: a retrospective review |date=20 June 2017 |url=https://ohiostate.pressbooks.pub/graphicshistory/chapter/13-2-singer-link/ |page=13.2}}</ref>

=== Motion system ===
[[File:Hexapod general Anim.gif|thumb|upright|[[Stewart platform]]]]

Initially, the motion systems used separate axes of movement, similar to a [[gimbal]]. After the invention of [[Stewart platform]]<ref name="Stewart, 1965">{{cite journal |first=D. |last=Stewart |title=A Platform with Six Degrees of Freedom |journal=Proceedings of the Institution of Mechanical Engineers |year=1965–1966 |volume= 180 |issue=1, No 15 |pages=371–386 |doi=10.1243/pime_proc_1965_180_029_02}}</ref> simultaneous operation of all actuators became the preferred choice, with some {{abbr|FFS|Full Flight Simulator}} regulations specifically requiring "synergistic" [[Degrees of freedom (mechanics)|6 degrees of freedom]] motion.<ref>Appendix 1 to CS FSTD(H).300, 1.2 Motion system, requirement b.1</ref> 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.{{r|allerton2009|p=451}}
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Motion system is a major contributor to overall simulator cost,{{r|allerton2009|p=423}} 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 [[G-seat|dynamic seats]] within flight simulators can be equally effective in the delivery of training as large and expensive 6-DOF FFS devices.<ref>{{cite conference | url=https://rosap.ntl.bts.gov/view/dot/8949 | title=Transfer of Training from a Full-Flight Simulator vs. a High Level Flight Training Device with a Dynamic Seat| author=Andrea L. Sparko|author2=Judith Bürki-Cohen|author3=Tiauw H. Go | year=2010 | conference=AIAA Modeling and Simulation Technologies Conference | doi=10.2514/6.2010-8218| url-access=subscription}}</ref><ref>{{cite web | url=https://www.aviationfocus.aero/wp-content/uploads/2018/02/Aviation-Focus-A-Summary-of-Studies-on-the-Effect-of-Motion-in-Flight-Simulators-Pilot-Training.pdf | title=A summary of studies conducted on the effect of motion in flight simulator pilot training | publisher=MPL Simulator Solutions | access-date=12 November 2019 | author=Peter John Davison}}</ref>