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CRJ1000 ILS approach

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CRJ1000 ILS approach

Postby tdammers » Thu Jun 28, 2018 5:04 am

I've been flying the CRJ1000 for a while now, but there's one issue I've been running into when flying ILS approaches. Everything works fine, until I reduce speed to Vref+5. For example, in a typical loadout, Vref might be 125kt, so I'm aiming to fly the last bit of the final approach at 130 KIAS (with full flaps, ofc). At airspeeds this slow, however, the autopilot seems to be unable to keep the aircraft on the glideslope, it sinks through and I get the "glideslope!" warning. The only way to recover is to disable the autopilot and fly the rest of the approach (from about 1000-1500 ft AGL) manually. Which, incidentally, works just fine as far as aerodynamics are concerned - but I'm thinking that flying down to 500 ft on autopilot should be absolutely no problem.

I've been looking at the autopilot code, and can't see anything weird, but I do suspect that the FDM is such that elevator authority is reduced quite a bit when flying slowly, and the autopilot's elevator trim control doesn't take airspeed into account, so it could be that.

Any thoughts?
tdammers
 
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Re: CRJ1000 ILS approach

Postby Thorsten » Thu Jun 28, 2018 6:02 am

Go for it - make the AP gains airspeed-dependent and see if you get better behavior out. You seem to have a fairly good idea of the problem already (and the vast majority of APs inside FG isn't overly sophisticated since AP design is a complicated art) - so likely you're in a good position to improve something.
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Re: CRJ1000 ILS approach

Postby tdammers » Thu Jun 28, 2018 2:20 pm

I could probably adopt a braindead simple solution, like just a linearly increasing boost as airspeed drops below 180 KIAS or so... I'll see what I can come up with.
tdammers
 
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Re: CRJ1000 ILS approach

Postby tdammers » Thu Jun 28, 2018 3:18 pm

Heh, upon closer inspection, it turns out there's a commented-out part in the autopilot XML which seems to be older, more complex code for the elevator control - using that instead of the newer one seems to solve the problem completely, I just flew the aircraft down to 100 ft on the glideslope at 130 KIAS, without problems.
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