I have had a problem for the past 2 months on my RV 12 being unable to maintain RPM as set by the McFarlane Vernier throttle. My RPM has varied +/- 300 while on autopilot causing pitch oscillations and airspeed gyrations from 92 -112 knots completely uncommanded. It requires constant readjustment of the throttle. I have replaced the spark plugs, all clean, the Vernier Throttle and had the carbs rebuilt by Lockwood. The mechanical fuel pump is 2 years old. Gascolator is clean. This has occurred using avgas and 94 octane summer blend unleaded. Engine has 350 hours. Trying to fly to Oshkosh in July, but a 30 minute flight at this point is very painful. Any suggestions would certainly be appreciated!
Van's Air Force
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
RV 12 912 ULS porpoising on autopilot
- Thread starter dbenoit
- Start date
What happens when you hand fly it and maintain a constant altitude.I have had a problem for the past 2 months on my RV 12 being unable to maintain RPM as set by the McFarlane Vernier throttle. My RPM has varied +/- 300 while on autopilot causing pitch oscillations and airspeed gyrations from 92 -112 knots completely uncommanded. It requires constant readjustment of the throttle. I have replaced the spark plugs, all clean, the Vernier Throttle and had the carbs rebuilt by Lockwood. The mechanical fuel pump is 2 years old. Gascolator is clean. This has occurred using avgas and 94 octane summer blend unleaded. Engine has 350 hours. Trying to fly to Oshkosh in July, but a 30 minute flight at this point is very painful. Any suggestions would certainly be appreciated!
My RV-12 has a Dynon D-180 with autopilot. On a long cross country flight, as the plane flew under scattered clouds that caused up drafts and down drafts, the autopilot would ask for elevator trim adjustments. That became annoying. So I shut off altitude hold and let the plane go up and down with the updrafts and down drafts. It was a lot more pleasant flying. The autopilot kept the plane on course.
It sounds like an autopilot issue, not throttle. If you lock the throttle and hand fly making pitch changes the airspeed will change and engine RPM will also.
An airplane that is being lifted by an updraft will have to “dive” to maintain a constant altitude. The airspeed will increase along with rpm during that dive. The opposite is true in a downdraft.
An airplane that is being lifted by an updraft will have to “dive” to maintain a constant altitude. The airspeed will increase along with rpm during that dive. The opposite is true in a downdraft.
skydiverlv
Well Known Member
Don't want anyone to miss Osh.I have had a problem for the past 2 months on my RV 12 being unable to maintain RPM as set by the McFarlane Vernier throttle. My RPM has varied +/- 300 while on autopilot causing pitch oscillations and airspeed gyrations from 92 -112 knots completely uncommanded. It requires constant readjustment of the throttle. I have replaced the spark plugs, all clean, the Vernier Throttle and had the carbs rebuilt by Lockwood. The mechanical fuel pump is 2 years old. Gascolator is clean. This has occurred using avgas and 94 octane summer blend unleaded. Engine has 350 hours. Trying to fly to Oshkosh in July, but a 30 minute flight at this point is very painful. Any suggestions would certainly be appreciated!
Is the described issue with auto trim? Have you tried engaging the level button to experience the result of level flight?
One time I was having issues with minor porpoising and all attempts to adjust A/P settings helped none.
In a desperate move I lubricated the piano hinge on the anti servo tab with dry teflon lube. Voila!
I hand flew it today and still had variations in RPM while holding a constant altitude. The variations in RPM were lower while hand flying, but still present. On autopilot, I am getting climb and descent readings of up to 250 fpm. It was able to only hold altitude =/- 80 feet before correcting...What happens when you hand fly it and maintain a constant altitude.
I did lube the piano hinge before flying today and it seemed to help minimize the wide fluctuations a bit, but the problem persists!Don't want anyone to miss Osh.
Is the described issue with auto trim? Have you tried engaging the level button to experience the result of level flight?
One time I was having issues with minor porpoising and all attempts to adjust A/P settings helped none.
In a desperate move I lubricated the piano hinge on the anti servo tab with dry teflon lube. Voila!
That is a good data point, also flying on a perfectly calm day and flying exactly straight and level, both manually and with the auto pilot and comparing the results. Needs to be really calm and smooth. Our 12 autopilot really wants the trim perfect before initiating altitude hold. If its very rough it doesn’t really work well in altitude hold mode, we just hand fly then. Based on what I am seeing here I dont suspect the autpilot yet. If hand flown on a calm day and no RPM or altitude excursion’s then the autopilot would be next target.What happens if you hold the elevator absolutely still, no fore and aft movement. Do NOT maintain a certain altitude.
Does the RPM still fluctuate?
There's auto pilot sensitivity adjustments, gain adjustments for the auto pilot. I have a D-180 system. In the set up instructions in the D-180 manual there is default settings to start with and go from there. I've noticed the best altitude hold I can get is 10-15'...I believe the manual says it may take a few minutes to get locked in but it seems 10-15' is what I get which is better than my hand flying ability's ...
As Bob mentioned, the Dynon has a guide for tuning the autopilot (if you determine this is the issue).
If you have the skyview screen, it is under "SkyView Product Documentation" ( https://www.dynonavionics.com/skyview-documentation.php ) and then "Installation Guides" and then "SkyView Autopilot In-Flight Tuning Guide".
Doing some tuning settled down altitude oscillation when we first got our RV-12.
If you have the skyview screen, it is under "SkyView Product Documentation" ( https://www.dynonavionics.com/skyview-documentation.php ) and then "Installation Guides" and then "SkyView Autopilot In-Flight Tuning Guide".
Doing some tuning settled down altitude oscillation when we first got our RV-12.
