DFC90/DFC100 Digital Autopilot - DFC90 and Reported Pitch Porpoising

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DFC90 and Reported Pitch Porpoising

Printed From: Avidyne
Category: Avidyne General
Forum Name: DFC90/DFC100 Digital Autopilot
Forum Description: Topics on the Avidyne DFC Digital Autopilots
URL: http://forums.avidyne.com/forum_posts.asp?TID=124
Printed Date: 21 Nov 2024 at 9:02pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com

Topic: DFC90 and Reported Pitch Porpoising

Posted By: AviJake
Subject: DFC90 and Reported Pitch Porpoising
Date Posted: 22 Jan 2011 at 5:46pm

Like similar reports of roll oscillations in aircraft that install the DFC90, there are some reports of "pitch porpoising" in DFC90 equipped aircraft when the autopilot is engaged. The typical report notes that in cruise flight, sometimes not noticed until about 30-60 minutes into flight, the aircraft will start a gentle, slow period (10-30 sec from peak to peak) oscillation in pitch. There is usually a resultant 20-80 foot altitude deviation in the process. Disconnecting and then reconnecting the autopilot often clears the issue for a while or the rest of the duration of the flight.

And, like in the case of the reported roll oscillations, these reports have all been traced to an out-of-spec pitch trim motor. Repairing (by cleaning the built up carbon deposits on the internal brushes) or replacing the pitch trim motor has solved the reported behavior in all cases that Avidyne is aware of.

As noted in another thread in this forum, the DFC90 (and DFC100) system will actively use both the pitch trim and pitch servos to fly the airplane. This results in a very responsive autopilot and solves the "deadband" challenges that would faced otherwise. This has the affect of relying a lot on the pitch trim motor for behind-the-scenes fine movements to precisely track the autopilot loops. So, in aircraft with "dirty" pitch trim motors, a form of slow motion "AIO" (autopilot induced oscillation) occurs because it slowly ramps up the input voltage required to start the pitch trim motor moving. The "out-of-spec" pitch trim motor results in higher than spec startup voltages required to get the motor turning. That means that instead of the expected 1.0 or 1.5 volts required to get the motor turning, it may now need 3 or 4 or 5 volts to get it moving. The autopilot will slowly ramp up the voltage until it senses the motor beginning to turn. This lag in motor turning, in turn results in the "AIO" and hence the observed "pitch porpoising".

We do not view this as a "broken" autopilot but instead, the DFC system is less tolerant of sloppy trim motors than predecessor autopilots may have been, hence the much improved performance with "in spec" aircraft.

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Steve Jacobson
sjacobson@avidyne.com

Replies:

Posted By: AviJake
Date Posted: 09 Mar 2011 at 6:29am

9 March 2011 Update: Reports from our Tech Support crew, phone calls I've received from the field, and posts on COPA all remain consistent that any observed pitch porpoising in DFC90 equipped Cirrus aircraft is due to some degradation to either the pitch trim motor (located in tail) or the pitch servo (located under a floor board in the cabin). The two common types of degradation observed are internal carbon build up inside the motors or lose mounting and/or bearings.

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Steve Jacobson
sjacobson@avidyne.com

Posted By: robrecord
Date Posted: 15 Apr 2011 at 11:27am

What are you recomending to "clean" the servos with?

Thanks Rob

Posted By: AviJake
Date Posted: 23 Apr 2011 at 9:45am

Hi Rob,

I am aware of a few shops who are capable and confident in removing, disassembling, cleaning, re-assembling and reinstalling the motors and can put you in contact with them to help here. Avidyne doesn't have a specific recommendation for cleaning methods but others will. Where are you located?

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Steve Jacobson
sjacobson@avidyne.com