by Alant » Mon Aug 26, 2019 11:15 pm
It is easy to to complain, but FG is a free system, with contributions from anyone who feels free.
As far as aerodynamics is concerned I can only comment on JSBSim, as this follows the same methods that I was taught at university, and after that when I worked in flight simulation. (I am now retired)
The main difficulty is finding a source of aerodynamic flight data. The aircraft manufacturers hold detailed data close to their chest, After all it cost them a small fortune to generate using wind tunnels, flight test, and more recently CFD. They can also license it to flight simulator manufacturers.
So , unless he can charm data out of the manufacturer, any JSBSim FDM developer has to fall back on alternative sources.
In order of complexity these include:
Flightgear´s own Aeromatic, which produces a generic estimate - probably equivalent to YASIM. Unlike the other methods below it requires little or no knowledge of aerodynamics.
Google will bring up lots of data, but very rarely will this cover the whole flight envelope - usually the data is valid for single case such as cruise or approach.
Datcom which only requires basic geometric data describing the aircraft and its components (wings, tail fuselage etc.) Sadly it gives no estimate of rudder effects, but this is not a great problem as a generic estimate (eg from Aeromatic) is usually sufficient . A limited knowledge of aerodynamics is required.
Panel and Vortex Lattice methods such as OpenVSP, Panukul, XFOIL, AVL which use similar geometric data to that used by Datcom, but much more care and aerodynamics knowledge is needed in setting up the problem. If we are to believe Xplane´s blurb they use a real time version of these methods, but to be real-time it must be very simplified.
CFD is the ultimate, but this requires large computer resources and a good (i.e. post graduate) understanding of fluid dynamics and mathematics. It is very easy to get, without warning, very inaccurate results as to make computer processing time acceptable there are a number of CFD methods, each of which is only relevant for limited range of geometry and speed . Pick the wrong one, or set the fluid conditions slightlynwrong and the output is garbage.
The Flightgear community does not have sufficient experienced aeronautical engineers to do this kind of work.
Many JSBSim FDMs have been fiddled with until they "feel right" - with various degrees of success. Unless these changes were done carefully, and the results checked against known aircraft performance data this technique is somewhat unreliable and usually has undesirable side effects.
Alan