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Please see the official X‑Plane Support page for help.
I am not from Laminar Research; just a flight simmer from downunder using a windows based system.
Here are the system requirements to run X-Plane 11. The link can be found at http://www.x-plane.com/kb/x-plane-11-system-requirements/
What do you want to do with X-Plane? X-Plane is not a game but a simulator that you build to replicate or setup so you can fly certain types of aircraft.
If you want to be serious then you should consider using your Mac system for every day use and move across to a windows based system. Why? The serious flight simmers go for a system that is equal to or has higher specifications than a recommended system.and upgrade components as and when they deem it necessary to do so whereas my understanding of a Mac system can only successfully upgrade the total memory.
Although X-Plane is developed on Mac systems Laminar Research staff members recommend using Windows based systems. Laminar Research has a recommended PC supplier in the USA that only deals with Windows based systems. This recommended dealer gets a mention in the X-Plane manual
It has been known for some time (XP10 and now XP11) that NVidia cards give the best performance although I understand some of the new AMD Radeon cards are getting close for performance.
It has been a known fact, and may still be an issue, that upgrades to High Sierra OS has caused problems. Plenty of comments in this forum. The problem was with Mac and not Laminar Research.
Bottom line is, download the demo version of X-Plane 11 and try it. You will be restricted to one airport and a time limit. The demo version is exactly the same as the full version other than paying the correct price for the software and then entering the assigned passwords provided at the time of purchase to unlock the software.
Good luck with the trial version and welcome to X-Plane
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