In Italy the Train goes often faster:
ok yeah, you're following super high speed trains. Fair enough.
How does it handle when something happened outside of there parameter? That way I want use 3x Units who must confirm the other 2x or the Data get thrown away to reduce the chance for an error.
I've not looked into majority-rules fail-safe flight controllers for models.
I'm not aware of any iNAV feature to do that. The issue would be the PID controllers and feedback.
Without some careful design you'd likely get compounding differences between the output of each flight controller. Caused by either the fact one has control and the others don't or caused by the fact that all have control and each is detecting the input from the other as an error to be corrected.
You would need to do the majority-rules calculations on the inputs to each flight controller (GPS, Accel, air speed, etc). Only then could you do a majority-rules decision on the outputs of each flight controller and expect them to all be identical (or within a few %).
So what is major problem with? Frame, Structure, Wings,...? The good thing with an Autopilot System the can react much fast and can overwatch all parameter at the same time. That mean for example if one of two Engines Vibrate to much, get to hot,... the can immediately abort the flight and land on an designate alternative landing strip (aka Grass) where someone from a Recovery Team wait if there is an emergency landing. An Model who is fly by Hand have much more trouble to over watch all parameter and decide to land before something happening.
Yes, to fly at 250kph it needs to be aerodynamic. My delta wing RC plane is quite aerodynamic and it can get up to 150kph on a good day, but the flight time at that speed would be ~3 minutes vs ~25min if flying at 60kph instead.
The power required to fly through the air at 250kph is huge. A lot of the energy is being used just to overcome air resistance.
So not only do you need a very aerodynamic craft but you burn through a massive amount of energy/fuel compared to flying slower.
Electric power wont work for sustained 250kph over many kilometers. It would need to be ICE or gas turbine powered.
Big Aircraft = room for Fuel
Yes but material strength doesn't scale, a smaller aircraft is stronger for less material needed.
A smaller aircraft can devote more of its mass to fuel vs construction materials and so get longer range.
So you only make it as big as you need to carry the payloads you require.