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Old 4th Apr 2019, 00:17
  #3018 (permalink)  
ecto1
 
Join Date: Nov 2018
Location: madrid
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Gums,

Yeah, simple FBW gives you that. But each philosofy has pros and cons, as you know.

You cited a big pro of simple (closed loop control of pitch rate, for instance) FBW. It will follow whichever control force feedback law you want. That's true, regardless of aerodinamic characteristic of the plane.

I'll give you some cons. In such FBW, a lot of feedback is lost. For instance, it's much harder to tell how authority you have left. 50% deflection at the stick will give you whatever deg/s roll regardless of the plane needing 5% aileron or 95% aileron. Controls do not get more sensible or harder at higher airspeed, so your subconscious mind do not have that clue to compute the actual airspeed. And you cannot feel trouble: sticky control surfaces, vibrations, hard spots or friction, all gets lost. Plane is trying to keep wings level if you don't touch the controls, which is nice, but it is also nice to know how hard it's trying. Otherwise it may surprise you when/if authority is depleted.

Of course you can make FBW with force feedback proportional to hydraulic pressure and stick deflection proportional to surface position. Only use FBW to "cheat" a little bit when you want to place protections (eg: if you are going fast and go for 90% stick deflection, force feedback will artificially ramp up a lot to keep you from easily ripping your wings out) or with dutch roll or overall damping (opposing a bit of control surface to a given pitch or roll rate). That would be my ideal system. I think it is not given sufficient thought most of the time because at first it seems a lot less fancy to directly correlate stick to control surfaces position instead of something with a much better sound like "target AOA that progressively changes to a target g when you accelerate". But it will have many advantages, you would be able to see and feel what autopilot does, feel the most subtle abnormality in the plane (cofg, drag, you name it)...
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