The best open source alternative to X-Plane is FlightGear. If that doesn't suit you, our users have ranked more than 10 alternatives to X-Plane and three of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to X-Plane are Rigs of Rods and YS Flight Simulator.
The FlightGear flight simulator project is an open-source, multi-platform, cooperative flight simulator development project. It also simulates space flight in Earth's orbit.
Rigs of Rods is an open source vehicle simulator licensed under the GNU General Public License version 3. What makes Rigs of Rods different to most simulators is its unique soft-body physics: vehicles, machines, objects, etc.
YS Flight Simulator is a free flight simulator specifically designed for computers without powerful graphics rendering capabilities.
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What it offers in here it is a certain basis for a very complete flying development>.