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ronschreck
02-17-2008, 04:35 AM
I have been flying my RV-8 with the Trio altitude hold for 400 hours and have never been completely satisfied with the ability of the unit to hold altitude steadily. It seems to wander most at higher altitudes and with an aft CG. I have sent the unit back to Trio for a software upgrade, braced the servo tray to eliminate any and all movement and have tried every combination of servo gain I can think of. I can usually get it to work fine if I am solo, at lower altitudes (about 7000 feet and below) and if I trim it just a bit nose-up before I engage the servo. At higher altitudes or with a passenger in back the altitude will wander plus or minus 20 or 30 feet and when it makes a correction back to altitude it jerks the elevator rather than making a smooth input.
I don't know any of my flying buddies with the Trio altitude hold so I am hoping someone out there can give me some trouble shooting tips that I haven't thought of. Thanks.

captainron
02-17-2008, 07:27 AM
Static system in good shape=no leaks?

Sam Buchanan
02-17-2008, 07:33 AM
I have been flying my RV-8 with the Trio altitude hold for 400 hours and have never been completely satisfied with the ability of the unit to hold altitude steadily. It seems to wander most at higher altitudes and with an aft CG. I have sent the unit back to Trio for a software upgrade, braced the servo tray to eliminate any and all movement and have tried every combination of servo gain I can think of. I can usually get it to work fine if I am solo, at lower altitudes (about 7000 feet and below) and if I trim it just a bit nose-up before I engage the servo. At higher altitudes or with a passenger in back the altitude will wander plus or minus 20 or 30 feet and when it makes a correction back to altitude it jerks the elevator rather than making a smooth input.
I don't know any of my flying buddies with the Trio altitude hold so I am hoping someone out there can give me some trouble shooting tips that I haven't thought of. Thanks.

First of all Ron, you might consider changing the title of your thread to something that suggests the possibility that your Trio unit is not set up properly. :)

In my experience (RV-6 which is known for being very light in pitch, especially loaded) the EZ-Hold will hold altitude very tightly regardless of how the plane is trimmed (unless it is waaaay out of trim, then the "Clutch Slip" message will appear). The fact that you are having to mess around with trim and that the unit makes abrupt corrections leads me to believe there is something not quite right in the installation. Does your elevator control have any stiction in the system? From your message it sounds like the autopilot is having to overcome friction or hysteresis in the elevator controls. You mentioned trying different combinations of gain settings, but your description leads to conflicting conclusions on that point. Higher gain will result in more abrupt corrections, but would not allow the plane to deviate 20-30 feet before the corrections occur. Low gain will result in soft corrections but may allow the plane to deviate a little in less than smooth air.

But you are describing altitude deviations with abrupt corrections. This is counter to customary gain adjustments and EZ-Hold behavior and leads me to believe the system is working against inconsistent forces in the elevator controls. Just a tiny amount of hysteresis near neutral would cause the servo to hunt. Your efforts to secure the servo were mentioned so I assume the servo isn't moving in its mount and all the attachments are tight.

Here is something you might try. Disconnect the elevators from the pushrod at the back of the fuse. Make sure the elevators have absolutely no friction around the neutral position. If the elevator hinges aren't aligned properly there can be some stiffness as the elevator pass through neutral. This probably wouldn't be very apparent while handflying the plane, but to a servo that is trying to make extremely minute adjustments, it could be significant.

One more significant item to check; Is your EZ-Hold plumbed to the plane's static system? It needs to be connected to outside static and even the tiniest leak can skew the system.

Your experience isn't consistent with many other EZ-Hold users I am familiar with and that is why I am suggesting taking another look at your installation. Hopefully this will lead to something productive because the EZ-Hold is a great product and you should be able to enjoy outstanding performance with it.

(I don't work for Trio, but I have quite a bit of experience with their systems.)

mrreddick
02-17-2008, 08:17 AM
FYI,

I have to agree with Sam. I've been flying with the Trio altitude hold shortly after it was introduced. On many cross countries in our 6A it has performed spectacularly.
When we get in an area of turbulence, the unit maintains our altitude within 20 to 30 feet, +/-. I think that's pretty close to the advertised performance.

One person's opinion does not make it a bad product! Please check and recheck your setup before condemning a fine product.

Thanks!

rocketbob
02-17-2008, 08:28 AM
Keep in mind that the RV-8 trim sensitivity varies greatly, much more so than the SBS RV's. An autopilot is simply a PID controller and in this case the tuning of the PID contoller will be affected by trim sensitivity. Its possible that the tuning of the A/P can work well in one and not the other.

ronschreck
02-17-2008, 09:15 AM
First of all Ron, you might consider changing the title of your thread to something that suggests the possibility that your Trio unit is not set up properly. :)

In my experience (RV-6 which is known for being very light in pitch, especially loaded) the EZ-Hold will hold altitude very tightly regardless of how the plane is trimmed (unless it is waaaay out of trim, then the "Clutch Slip" message will appear). The fact that you are having to mess around with trim and that the unit makes abrupt corrections leads me to believe there is something not quite right in the installation. Does your elevator control have any stiction in the system? From your message it sounds like the autopilot is having to overcome friction or hysteresis in the elevator controls. You mentioned trying different combinations of gain settings, but your description leads to conflicting conclusions on that point. Higher gain will result in more abrupt corrections, but would not allow the plane to deviate 20-30 feet before the corrections occur. Low gain will result in soft corrections but may allow the plane to deviate a little in less than smooth air.

But you are describing altitude deviations with abrupt corrections. This is counter to customary gain adjustments and EZ-Hold behavior and leads me to believe the system is working against inconsistent forces in the elevator controls. Just a tiny amount of hysteresis near neutral would cause the servo to hunt. Your efforts to secure the servo were mentioned so I assume the servo isn't moving in its mount and all the attachments are tight.

Here is something you might try. Disconnect the elevators from the pushrod at the back of the fuse. Make sure the elevators have absolutely no friction around the neutral position. If the elevator hinges aren't aligned properly there can be some stiffness as the elevator pass through neutral. This probably wouldn't be very apparent while handflying the plane, but to a servo that is trying to make extremely minute adjustments, it could be significant.

One more significant item to check; Is your EZ-Hold plumbed to the plane's static system? It needs to be connected to outside static and even the tiniest leak can skew the system.

Your experience isn't consistent with many other EZ-Hold users I am familiar with and that is why I am suggesting taking another look at your installation. Hopefully this will lead to something productive because the EZ-Hold is a great product and you should be able to enjoy outstanding performance with it.

(I don't work for Trio, but I have quite a bit of experience with their systems.)


Funny how a plea for help can be construed as slamming a product! Let me set the record straight: I really like my Trio autopilot. It is a first class product and the support from the factory has been superb. If I knew how to change the title of this thread I would change it to "Trio Altitude Hold - help this idiot make it work as advertised and proven." Sorry if I have offended anyone and thanks for all the great suggestions.

(Wow! I just noticed that the Thread Police changed the thread title while I was composing this message. Ain't technology wonderful?)

Sam Buchanan
02-17-2008, 12:17 PM
Keep in mind that the RV-8 trim sensitivity varies greatly, much more so than the SBS RV's. An autopilot is simply a PID controller and in this case the tuning of the PID contoller will be affected by trim sensitivity. Its possible that the tuning of the A/P can work well in one and not the other.

That may be the case, but I don't think it would fully explain the behavior of Ron's installation. If it is, the gain can be adjusted quickly at the control head (but Ron said he tried that). After some acclimation, it would be pretty easy to learn a couple of gain settings that could be dialed in that would be appropriate for the particular CG condition.

Matter of fact, maybe the Trio guys might want to consider a tweak in future software versions that would allow a pilot to save two gain configurations...one for solo flight and the other for a bubba in the back seat and a light fuel load! :D

I will add that the CG on my RV-6 can shift considerably from quite stable when solo and full fuel to "Wow!" squirrelly when loaded heavily and light on fuel. I've never felt the need to adjust autopilot gain but I can see where adjusting the gain in various conditions might optimize the system.

Kahuna
02-17-2008, 05:57 PM
Smokey,
I used to have this trouble way back in the early days of development of the unit, but has since been fixed with firmware upgrades.

Are you sure its not pilot error?:p

I have done a lot of testing with these units. Next time we are flying together, Ill take yours for a spin and see whats up.

Yes of course you can chase me in mine:)

You going to Kentucky next weekend for the fly in?

Best

till3
02-18-2008, 05:10 PM
Please let the forum know how this turns out. I have a similiar problem with my TruTrak. I do have a question about solders, could a cold solder cause erratic movements as described?

ronschreck
02-21-2008, 09:09 AM
Please let the forum know how this turns out. I have a similiar problem with my TruTrak. I do have a question about solders, could a cold solder cause erratic movements as described?


Found the problem. :) As I suspected, there was a tiny bit of flex in the mounting tray for the servo. It was just enough to flex slightly when the servo moved and then when it rebounded I got a "jump" in the autopilot. I stiffened it up with a triangular brace and "voila" it tracks the altitude setting perfectly. Interestingly, the flex was not even big enough to detect visually. I could only feel it with my hand on the servo.

Thanks to all for the suggestions.

IowaRV9Dreamer
02-21-2008, 11:16 AM
I stiffened it up with a triangular brace and "voila" it tracks the altitude setting perfectly.
Thanks great news! To help us who are about to mount our own trays, how about posting "before" and "after" pictures.

Thanks a lot,

ronschreck
02-21-2008, 01:06 PM
Thanks great news! To help us who are about to mount our own trays, how about posting "before" and "after" pictures.

Thanks a lot,

Wish you mentioned that before I re-installed the baggage bulkhead! Actually, pictures of my fix may not do you much good. I have aft mounted batteries in my RV-8 so the servo for the ailtitude hold autopilot is installed aft of the battery tray and behind the elevator bellcrank. There is not much solid material to mount the servo to in that space. If you have your battery mounted foreward then the servo has a great mounting position forward of the elevator bellcrank where my battery tray now sits.

Whatever you do, make it solid for best performance.

DanH
02-21-2008, 03:50 PM
<<I have aft mounted batteries in my RV-8 so the servo for the ailtitude hold autopilot is installed aft of the battery tray and behind the elevator bellcrank. There is not much solid material to mount the servo to in that space.>>

Dave, drop a line to Jerry at Trio. He should be able to supply a detailed drawing for an "aft battery" RV-8 servo mount. It was checked for flex at the clutch slip setting. I'll try to remember to post a photo here later.

(later) Here it is.

http://img120.imageshack.us/img120/2257/triomountstiffnesscheckoc0.jpg (http://imageshack.us)