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20 Jan 2018, 20:43 [ UTC - 5; DST ]


Greenwich AeroGroup



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 Post subject: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 09:34 
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Joined: 10/21/09
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Location: New Orleans LA and Durango CO
Aircraft: E55
My E55 has benefited from re-rigging and rod end replacements, and probably other items which have just about eliminated wing wag in cruise. This makes the pitch oscillation in autopilot mode more apparent. Speed variation is about 5 knots plus or minus, over about a five minute period, and altitude excursions are about 20 feet up and down. The C-IV has received lots of attention over the years and works perfectly in all modes. Is the failure to damp pitch oscillations something I need to live with or is there an adjustment or part or board that could be examined to improve pitch hold?


Last edited on 13 Dec 2017, 10:01, edited 1 time in total.

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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 09:52 
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Joined: 01/18/11
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Location: Lake Marion, SC KOGB
Aircraft: B60 Duke
It can be cured. Autopilot Central can do it.


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 10:11 
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Location: Loveland, CO
Aircraft: 35C-33
Try it in ATTitude mode. If it doesn't do it there, it's the altitude sensor. BH


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 10:39 
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Joined: 05/09/16
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Company: Bevan-Rabell Inc.
Location: Wichita, KS 67209, USA
I would look at pitch servo start-up voltage (should be under 2 VDC unloaded) and bridle cable tension. The only adjustments for alt hold are pitch centering on the system coupler and a null pot on the 1D496 flight computer. Pitch centering pot is used to adjust the transition into alt hold at target alt in level flight.

Depending on the dash number of the 1D496 you could have a electronic transducer that is failing or possible problems with the old pressure canister. The dash number should be something like 1D496-01002. If the first number after the - is a 0 it's the old pressure canister, if it is a 1 then it's the electronic transducer.

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Jacob Kinsey
Head Autopilot Tech
Bevan-Rabell Inc.


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 18:42 
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Joined: 04/01/13
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Location: Overland Park, KS (KOJC)
Aircraft: 1975 Bonanza F33A
Twenty feet up and down over a 5-minute period? That sounds pretty good to me.

I'm not sure I could differentiate between mild up and down drafts and an autopilot problem with only 20 feet of variance. In perfectly smooth air my autopilot will hold a nearly exact altitude, but if there is any turbulence, thermal or wave activity it will vary by 20'. I consider it to be working perfectly (I have a Century IV). I don't see plus or minus 5 knots, more like two or three.

I consider this normal, it's been this way for the 14 years I've owned the plane. Perhaps my expectations are too low, but it's not something that I think needs fixing.

Jack


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 19:08 
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Joined: 11/15/09
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Location: Red Deer, Alberta
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The altitude hold on the Century IV in my Aerostar was almost perfect. It would quickly drift down about 50' from where ever I set it but never varied from there.

Glenn


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 20:16 
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Location: KMMU / Morristown, NJ
Aircraft: Baron 58P
Not to steal the thread but I've recently started having a similar issue with my Century 41. Attitude mode is fine but in Altitude Hold it works for a minute or so before slowly pitching up (trim wheel moves up as well) and gaining altitude. If not disconnected it will just continue a slow climb. Interested if anyone has had a similar issue and what the remedy could be.


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 13 Dec 2017, 20:45 
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Location: $ilicon Vall€y
Aircraft: Columbia 400
This "hunting" in pitch is really annoying.

First step is the aircraft rigging and tension of the elevator cables. Incorrect tension, excess drag, missing springs or weights or imbalance will cause this kind of hunting. The bridle cable that attaches to the main elevator cable has to also be set to its correct tension.

Ditto the pitch trim system - make sure there's no slop, no worn bushings or hinges and no excess play in the trim tabs.

Once you're sure that's all 100%, the next most likely culprit are the motors themselves. If the starting voltage is high on the DC motor in either the pitch or trim servo, it will hunt. The starting voltage increases as the brushes and commutator in the motor wear.

The system will usually try to correct the pitch with a small motor drive voltage and increases the voltage if it doesn't respond. If the motors are sluggish, or the mechanics of your flight controls have slop, play, excess friction in them, results in lag, then an over-correction, which cycles over and over.

I chased this for a long time on my old Seneca II. I could just about stop it by putting my finger on the trim wheel and just applying some pressure to "help" it as the pitch meter needle would start to move off the center mark. Got all the play out of the trim tab, got the exact right lubricant on the trim jack-screw and eventually, overhauled the attitude indicator to get the last little bit of it to go away.


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 Post subject: Re: Centuryy IV pitch oscillation, damping
PostPosted: 15 Dec 2017, 00:47 
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Joined: 12/21/16
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Location: Springfield, IL
Aircraft: Bonanza A36
Username Protected wrote:
Not to steal the thread but I've recently started having a similar issue with my Century 41. Attitude mode is fine but in Altitude Hold it works for a minute or so before slowly pitching up (trim wheel moves up as well) and gaining altitude. If not disconnected it will just continue a slow climb. Interested if anyone has had a similar issue and what the remedy could be.


Mine didn’t the same thing and it turned out to be a vacuum failure. I’m having issues now that it won’t follow my heading bug or nav. It’ll stay pretty much where it’s at I’m heading mode and drifts back and forth in nav mode.


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