Editing FlightGear (section)

=== Physics ===
Forces experienced by a flying aircraft depend on the time-varying state of atmospheric [[Fluid dynamics|fluid flow]] along the flight path - the atmosphere being a fluid that can exchange energy, exchange moisture or [[Particulates#Sources of atmospheric particulate matter|particles]], change [[Water vapor#Properties|phase]] or [[Atmospheric entry#Shock layer gas physics|other]] state, and exert force with [[Boundary conditions in fluid dynamics|boundaries]] formed by surfaces. Fluid behaviour is often characterised by [[eddies]]<sup>(Videos:[https://www.youtube.com/watch?v=-D5N_OnZ_Tg aircraft], [https://www.youtube.com/watch?v=4JNkaVEXKqU terrain])</sup> or [[Vortex|vortices]] on varying scales down to the [[No-slip condition|microscopic]], but is harder to observe as the air is clear except for moisture phase changes like [[Contrail|condensation trails]] or clouds. The atmosphere-terrain boundary [[:File:Vortex-street-1.jpg|interaction]] follows fluid dynamics, just with processes on hugely [[Meteorology#Spatial scales|varying scales]] and 'weather' is the [[planetary boundary layer]]. The aircraft surface interaction works with the [[:File:Airplane vortex edit.jpg|same dynamics]], but on a limited [[Wind tunnel#Flow visualization|range]] of scales. Forces experienced at any point along a [[flight path]], therefore, are the result of complicated atmospheric processes on varying spatial scales, and complex flow along the craft's surface. Craft also experience varying [[gravitational force]] based on the 3d shape of the potential well and the [[Figure of the Earth#Ellipsoid of revolution|non-spherical shape]] of the Earth.

==== Atmospheric and environmental physics ====
''FlightGear'' can simulate the atmosphere ranging from [[Earth's energy budget#Energy budget|energy inputs/outputs]] to the system, like energy from the Sun or volcanic sources, through to fluid flow on various scales and changes of state. ''FlightGear'' is able to model different surface characteristics such as heating or cooling, and the exchange of heat and moisture with the atmosphere depending on factors like windflow or dew point. ''FlightGear'' models the continuously evolving life-cycle of phenomena on various scales, driven by interaction of fluid with terrain. They range from turbulence on different scales to, individual thermals, thunderstorms, through to moving air layers, and depicting air-masses on the scale of thousands of kilometers. Atmospheric water is modeled by ''FlightGear'' ranging from state changes such as condensing into cloud or haze layers, along with energy provided from latent heat to drive convective fluid flow, through to precipitation as rain droplets, snow, or hail.<ref name=":022">{{Cite web|last=|first=|date=June 25, 2013|title=The art of cloud and weather rendering – FlightGear Flight Simulator|url=https://www.flightgear.org/tours/the-art-of-cloud-and-weather-rendering/|url-status=live|archive-url=https://web.archive.org/web/20201031172624/https://www.flightgear.org/tours/the-art-of-cloud-and-weather-rendering/|archive-date=31 October 2020|access-date=2019-07-23|website=FlightGear.org|language=en-US}}</ref><ref name=":232">{{Cite web|last=|first=|date=December 17, 2014|title=The magic of light and haze – FlightGear Flight Simulator|url=https://www.flightgear.org/tours/the-magic-of-light-and-haze/|url-status=live|archive-url=https://web.archive.org/web/20201205092108/https://www.flightgear.org/tours/the-magic-of-light-and-haze/|archive-date=5 December 2020|access-date=2019-08-31|website=FlightGear.org|language=en-US}}</ref><ref name=":32">{{Cite web|last=|first=|date=November 30, 2014|title=A preview of features for Flightgear 3.4 – FlightGear Flight Simulator|url=https://www.flightgear.org/info/a-preview-of-features-for-flightgear-3-4/|url-status=live|archive-url=https://web.archive.org/web/20210119222757/https://www.flightgear.org/info/a-preview-of-features-for-flightgear-3-4/|archive-date=19 January 2021|access-date=2019-08-31|website=FlightGear.org|language=en-US}}</ref><ref name=":03">{{Cite web|last=|first=|date=February 24, 2012|title=Advanced Weather v1.4 in Flightgear 2.6+ – FlightGear Flight Simulator|url=https://www.flightgear.org/tours/advanced-weather-v1-4-in-flightgear-2-6/|url-status=live|archive-url=https://web.archive.org/web/20190831105119/https://www.flightgear.org/tours/advanced-weather-v1-4-in-flightgear-2-6/|archive-date=31 August 2019|access-date=2019-08-31|website=FlightGear.org|language=en-US}}</ref>

The process of generating lift creates turbulence with vortices, and ''FlightGear'' models wake turbulence with [[Vortex shedding|shedding]] of wingtip vortices by flown craft as well as AI craft.<ref>{{Cite web|title=AI wake turbulence - FlightGear wiki|url=http://wiki.flightgear.org/AI_wake_turbulence|access-date=2019-09-04|website=FlightGear wiki}}</ref><ref>{{Cite web|title=Changelog 2017.3 - FlightGear wiki|url=https://wiki.flightgear.org/Changelog_2017.3#JSBSim|url-status=live|archive-url=https://web.archive.org/web/20200705062057/https://wiki.flightgear.org/Changelog_2017.3#JSBSim|archive-date=5 July 2020|access-date=2021-03-15|website=FlightGear wiki}}</ref>

FlightGear also has a less physically accurate model that uses [[METAR]] weather updates of differing frequency, designed for safe operation of [[aerodromes]], to [[Classification of discontinuities#Jump discontinuity|dis-continuously]] force atmosphere based on attempted guesses of processes that are fundamentally constrained by the closeness or density of observation stations, as well as the [[Meteorology#Spatial scales|small-scale]], limited, rounded off, [[Quantization (signal processing)#Types|non-smoothly varying]], and need-to-know precision of information.<ref>{{Cite web|title=Weather - FlightGear wiki|url=http://wiki.flightgear.org/Weather#Scenarios_and_METAR|access-date=2019-09-04|website=FlightGear wiki}}</ref> Aloft waypoint settings modelling high altitude behaviors of wind can be synced to updates from Jeppeson.<ref>{{Cite web|last=|first=|date=|title=Weather - FlightGear wiki|url=http://wiki.flightgear.org/Howto:Fetch_live_aloft_data|url-status=live|archive-url=https://web.archive.org/web/20190904073627/http://wiki.flightgear.org/Howto:Fetch_live_aloft_data|archive-date=2019-09-04|access-date=2019-07-23|website=FlightGear wiki}}</ref>

''Flightgear'' has a simulation of planetary bodies in the [[Solar System]] which is used for purposes like driving latitude dependent weather from solar radiation, as well as the brightness and position of stars for [[celestial navigation]]. There is a model of gravity based on a non-spherical Earth, and craft can even experience differing gravity across their bodies which will exert [[Torque|twisting force]].<ref name=":732">{{Cite web|last=|first=|date=December 18, 2015|title=An experience like no other… – FlightGear Flight Simulator|url=https://www.flightgear.org/tours/an-experience-like-no-other/|url-status=live|archive-url=https://web.archive.org/web/20210315092603/https://www.flightgear.org/tours/an-experience-like-no-other/|archive-date=15 March 2021|access-date=2019-08-31|website=FlightGear.org|language=en-US}}</ref> A model of the observed [[Magnetic declination#Air navigation|variation]] in the Earth's [[Dynamo theory|complex]] magnetic field, and the option to simulate, to an extent, the propagation of radio wave signals due to interaction with different types of terrain, also exists in ''FlightGear''.<ref>{{Cite web|title=Avionics and instruments - FlightGear wiki|url=http://wiki.flightgear.org/Avionics_and_instruments#Compass|access-date=2019-09-05|website=wiki.flightgear.org}}</ref><ref>{{Cite web|title=Radio propagation - FlightGear wiki|url=http://wiki.flightgear.org/Radio_propagation#Rationale|access-date=2019-09-05|website=wiki.flightgear.org}}</ref>

''FlightGear'' uses an exact, [[Figure of the Earth|non-spherical]], model of Earth, and is also able to simulate flight in [[Polar regions of Earth|polar regions]] and airports ([[:Category:Airports in the Arctic|arctic]] or [[List of airports in Antarctica|antarctic]]) without simulator errors due to issues with coordinate systems.

==== Flight Dynamics ====
''FlightGear'' supports multiple [[flight dynamics]] engines with differing approaches, and external sources such as [[MATLAB]]/[[Simulink]], as well as custom flight models for hot air balloons and spacecraft.<ref name=":122">{{Cite web|title=Flight Dynamics Model - FlightGear wiki|url=http://wiki.flightgear.org/Flight_Dynamics_Model|access-date=2019-07-23|website=wiki.flightgear.org}}</ref><ref>{{Cite web|title=Flight Simulator Interface - MATLAB & Simulink|url=https://www.mathworks.com/help/aeroblks/introducing-the-flight-simulator-interface.html|archive-url=https://web.archive.org/web/20130704083155/http://www.mathworks.com/help/aeroblks/introducing-the-flight-simulator-interface.html|archive-date=2013-07-04|url-status=live|access-date=2021-03-15|website=Mathworks - Makers of MATLAB and Simulink - MATLAB & Simulink}}</ref>

===== JSBSim =====
''JSBSim'' is a data driven flight dynamics engine with a C++ core built to the needs of the FlightGear project from 1996 to replace NASA's ''LaRCSim'', and integrated into ''FlightGear'' as the default from 1999.<ref name=":14">{{Citation|last=Berndt|first=Jon|title=JSBSim: An Open Source Flight Dynamics Model in C++|url=https://arc.aiaa.org/doi/abs/10.2514/6.2004-4923|work=AIAA Modeling and Simulation Technologies Conference and Exhibit|year=2004|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.2004-4923|isbn=978-1-62410-074-1|access-date=2019-09-01|url-access=subscription}}</ref> Flight characteristics are preserved despite low frame rate, as JSBSim physics are decoupled from rendering and tick at 120&nbsp;Hz by default.<ref>{{Cite web|last=|first=|date=|title=Howto:Methods to replace the NASAL code with JSBSim code - FlightGear wiki|url=http://wiki.flightgear.org/Howto:Methods_to_replace_the_NASAL_code_with_JSBSim_code#Performance_of_JSBSim_.28or_data-driven_languages.29_in_a_real-time_.28RT.29_context|url-status=live|archive-url=https://web.archive.org/web/20190903092718/http://wiki.flightgear.org/Howto:Methods_to_replace_the_NASAL_code_with_JSBSim_code|archive-date=2019-09-03|access-date=2019-09-03|website=FlightGear wiki}}</ref> This also supports high time-acceleration as rendering does not have to be done faster causing the [[Graphics processing unit|GPU]] to be a bottleneck.

Mass balance, ground reactions, propulsion, aerodynamics, buoyant forces, external forces, atmospheric forces, and gravitational forces can be utilized by ''JSBSim'', the current default flight dynamics engine supported by ''FlightGear'', to determine flight characteristics.<ref>{{Cite web|title=JSBSim - FlightGear wiki|url=http://wiki.flightgear.org/JSBSim|access-date=2019-07-23|website=wiki.flightgear.org}}</ref> ''JSBSim'' supports non-terrestrial atmospheres and has been used to model unmanned flight in the Martian atmosphere by NASA.<ref>{{Cite web|last=|first=|date=|title=JSBSim Open Source Flight Dynamics Model|url=http://jsbsim.sourceforge.net/links.html|url-status=live|archive-url=https://web.archive.org/web/20190901135809/http://jsbsim.sourceforge.net/links.html|archive-date=2019-09-01|access-date=2019-09-01|website=jsbsim.sourceforge.net}}</ref><ref>{{Citation|last1=Kenney|first1=P. Sean|title=Simulating The ARES Aircraft In The Mars Environment|url=https://arc.aiaa.org/doi/abs/10.2514/6.2003-6579|work=2nd AIAA "Unmanned Unlimited" Conf. and Workshop & Exhibit|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.2003-6579|access-date=2019-09-01|last2=Croom|first2=Mark|year=2003|isbn=978-1-62410-094-9|hdl=2060/20040034718|s2cid=13269363 |hdl-access=free}}</ref><ref name=":14" />

====== Benchmark testing by NASA ======
JSBSim was used by NASA in 2015 with other space industry simulation code, both to establish a ruler to judge future code for the requirements and standards of the space industry, as well as check agreement. The verification tested both atmospheric and orbital flight in [[Six degrees of freedom|6-degrees-of-freedom]] for simulations like JSBSim<ref>{{Cite web|date=January 2015|title=Check-Cases for Verification of 6-Degree-of-Freedom Flight Vehicle Simulations - Volume II|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150001264.pdf|url-status=live|archive-url=https://web.archive.org/web/20170227123110/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150001264.pdf|archive-date=2017-02-27|access-date=|website=NASA Technical Reports Server|at=See Section B.6.7 JSBSim|last1=Murri|first1=Daniel G.|last2=Jackson|first2=E. Bruce|last3=Shelton|first3=Robert O.}}</ref> that supported both. The results from 6 participants consisting of NASA Ames Research Center (VMSRTE), Armstrong Flight Research Center (Core), Johnson Space Center (JEOD), Langley Research Center (LaSRS++, POST-II), Marshall Space Flight Center (MAVERIC), and JSBSim<ref name=":83">{{Citation|last1=Murri|first1=Daniel G.|title=Check-Cases for Verification of 6-DOF Flight Vehicle Simulations - Volume I|date=2015|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150001264.pdf|volume=|pages=|publisher=NASA|language=en|doi=|access-date=2019-09-03|last2=E. Bruce Jackson|last3=Shelton|first3=Robert O.}}</ref><ref name=":62">{{Cite web|last=|first=|date=|title=Check-Cases for Verification of 6-Degree-of-Freedom Flight Vehicle Simulations - Volume I|url=https://nescacademy.nasa.gov/src/flightsim/Reports/NASA-TM-2015-218675-EOM_checkcase_summary.pdf|access-date=|website=NASA Engineering and Safety Center Academy|at=See Section 7.4 - Summary of Comparisons}}</ref> were anonymous<ref>{{Cite web|last=|first=|date=2015|title=Further Development of Verification Check-cases for Six-Degree-of-Freedom Flight Vehicle Simulations|url=https://nescacademy.nasa.gov/src/flightsim/Reports/aiaa-15-1810-EOM_chkcases-II.pdf|url-status=live|archive-url=https://web.archive.org/web/20210310134926/https://nescacademy.nasa.gov/src/flightsim/Reports/aiaa-15-1810-EOM_chkcases-II.pdf|archive-date=10 March 2021|access-date=|website=NASA Engineering and Safety Center|at=See Section II G}}</ref> as NASA wanted to encourage participation. However, the assessment found agreement for all test cases between the majority of participants, with the differences being explainable and reducible for the rest, and with the orbital tests agreeing "quite well" for all participants.<ref name=":62" /><ref name=":732"/>

===== YASim =====
YASim's approach to flight dynamics uses the geometry of the aircraft present in the 3D model at startup, conceptually similar to [[Blade element theory]] used by some software, to calculate a rough approximation of fluid dynamics - with the conceptual problems that each "element" is considered in isolation therefore missing affecting fluid flow to other elements, and the approximation breaking down for craft in [[Mach number#Classification of Mach regimes|transonic to hypersonic regimes]].<ref>{{Cite web|last=Neely|first=Gary|date=|title=What is YASim?|url=http://www.buckarooshangar.com/flightgear/yasimtut_introduction.html|url-status=live|archive-url=https://web.archive.org/web/20190902085249/http://www.buckarooshangar.com/flightgear/yasimtut_introduction.html|archive-date=2019-09-02|access-date=2019-09-02|website=www.buckarooshangar.com}}</ref> By contrast, offline approaches like JSBSim can incorporate [[Wind tunnel|windtunnel]] data. They can also incorporate the results of [[computational fluid dynamics]] which can reach computable accuracy only [[Numerical methods in fluid mechanics|limited]] by the nature of the problem and present day [[Supercomputer#Performance measurement|computational resources]].

''FlightGear'' also supports LaRCsim and UIUC.<ref name=":132">{{Cite web|title=Flight Dynamics Model - FlightGear wiki|url=http://wiki.flightgear.org/Flight_Dynamics_Model|access-date=2019-07-23|website=FlightGear wiki}}</ref><ref>{{Cite book|last1=Zhang Jingsha|last2=Geng Qingbo|last3=Fei Qing|title=International Conference on Automatic Control and Artificial Intelligence (ACAI 2012) |chapter=UAV Flight Control System Modeling and Simulation Based on FlightGear |date=2012 |url=http://mr.crossref.org/iPage?doi=10.1049/cp.2012.1443 |pages=2231–2234|publisher=Institution of Engineering and Technology|doi=10.1049/cp.2012.1443|isbn=978-1-84919-537-9}}</ref>

==== Time acceleration ====
''FlightGear'' is able to accelerate and decelerate time, speeding up or slowing down the simulation. Time acceleration is a critical feature for simulating longer flights and space missions. For all interactions with the simulator, it allows people to speed up uneventful parts, and gain more experience (decisions and problem solving). It also means automated simulations used for research finish faster - this is helped by ''FlightGear's'' [[Headless software|headless]] mode.

''FlightGear'' is able to support high time accelerations by allowing parts of the simulation to run at different rates. This allows saving of CPU and GPU resources by letting unimportant parts of the simulation, like visuals or less time-sensitive aircraft systems, run at slower rates. It also improves performance. Separate clocks are available for JSBSim physics, different parts of aircraft systems, as well as environment simulations at large scale (celestial simulation) and small scale (weather physics).