Aeromatic
Aeromatic is a tool for inputting a small set of parameters, and will then output a JSBSim FDM.
Note that things like position of fuel tanks, gears and pilot have to be modified by hand after output.
There are 2 versions of the programs. The old online Aeromatic. And the newer C++ Aeromatic.
Online Aeromatic
Usage: Browse to the online Aeromatic website, and input a small number of inputs. It will output a simple but complete JSBSim FDM including engine.
Official readme: Link
How it works: Link
It is made by David Culp.
AeromatiC++
AeromatiC++ is an attempt to make the outputted FDM from Aeromatic more detailed by Erik Hofman. It is made in C++, so it must be downloaded and run locally.
The program is now part of JSBSim source repository, and is compiled as part of Flightgear nightlies for Win and Linux (but separate download).
I find AeromatiC++ very useful, especially for making JSBSim engines. For example it predicted the Viggen fuel consumption almost spot on. -Necolatis