Someone else will eventually point this out, so, I might as well do it now. If you're talking about an actual airplane that can carry one, or more people, the regulatory bureaucracy is going to ground you permanently, or at least for a very long time. As it is, they have to currently close down airspace for at least 15 km around an airport doing this sort of testing in order to prevent interference with aircraft carrying innocent bystanders (i.e., pilots and passengers). Modifying a real aircraft in any way must be done by people certified to do so if it's a commercially-built aircraft. If it's an experimental aircraft, it has to be at least inspected by qualified inspectors, and they're going to want to see a very detailed technical plan, along with all of the equipment designs, schematics, as-built drawings, etc. For starters, WiFi isn't going to be anywhere near as reliable as you're going to need to do this properly for lots of reasons, starting with range/orientation and electromagnetic interference. Plus, what happens when the aircraft inevitably does fly out of range? How does it autonomously fly back to within range? Obviously, a lot of GPS-based software would be needed, but, what happens when, not if, the GPS receiver fails? What happens when, not if, anything fails, and especially more than one thing fails?
You would be much better off experimenting with a radio-controlled, electric-powered model aircraft, although it would have to be big enough to carry the R-Pi, batteries, and any other ancillary circuitry. You would still have all of the "What happens when ... ?" scenarios listed above, but, at least you wouldn't have a real airplane falling out of the sky from thousands of feet when, not if, a problem occurs. BTW, I wouldn't even begin with a model aircraft, I would start with a ground model vehicle and get that working in two dimensions, before embarking on three dimensions. You could do a lot of troubleshooting even with an aircraft model mounted on a ground model vehicle, thereby saving money on crashing model airxraft, which is going to happen. Going another level of abstraction, why not just do it all in virtual reality, since the R-Pi has such a great GPU and HDMI video output? Once you get the simulation working, then you can start spending money crashing model aircraft.
I don't intend to be a wet blanket, I'm just pointing out some things you're going to find out about soon enough. I do hope you pursue this, just do so safely. Good luck!
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!