Yes that is a good point.
If the minimum speed setting is too high for whatever reason - and you are flying at less than that speed - the AP is going to forget all about holding altitude and will drop the nose no matter what. That has absolute highest priority.
This will of course also happen if your airspeed is not plumbed in our not working for whatever reason - the screen shots offered here don't seem to show airspeed so we don't know.
This is also an effect you will get if your pitot tube ices up and airspeed readout drops below minimum.
Why Doesn't The Autopilot Do Its Job Properly?
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Re: Why Doesn't The Autopilot Do Its Job Properly?
I installed an all MGL System with Challenger lite in my RV-12iS. I am not happy eather. In calm air with the rotax at 5200 upm it works acceptable. If it gets a little bumpy and I reduce power the AP is useless. The AP will nose dive without warning even if it is below target altitude. Also the ride gets very uncompfortable with AP on. Some days the artificial horizon is unreliable too. I have seen the ball wander from left to right and back for no reason. There are hundreds of Dynon and Garmin in RV-12's, so install should be o.k. I wish I would have spend 10.000 bucks more.
Re: Why Doesn't The Autopilot Do Its Job Properly?
I think you may have supplied your own answer.
The "ball" is simply the output of the roll axis accelerometer.
You need an external AHRS - then it's also a more fair comparison with the other systems.
The internal AHRS in the Lite's are not comparable in performance to an external system. This is simply due to the location. In many instances they work fine but if your panel suffers from vibration - tat feeds right into the AHRS and severely compromises it.
Newer "lite" systems made in about the last 12-18 months or so have an improved AHRS that includes new algorithms to handle vibrations better (These can create false gravity direction measurements). Typically this is very dependent on engine RPM - as well as effects related to harmonic vibrations which are a result of prop and engine RPM not the same - as in the case of the Rotax.
I think you will do yourself a great favor installing an SP-7 - and that would be my recomendation.
Generally the performance of the AP is excellent in very turbulent conditions - but be sure to give it enough control ability - i.e. the servo "magnitude" setting. If that is too low it will work fine in calm air but is restricted from giving enough control input in turbulent conditions.
If it nose-dives when you slow down - please lower your minimum speed setting in the pitch control setup.
The "ball" is simply the output of the roll axis accelerometer.
You need an external AHRS - then it's also a more fair comparison with the other systems.
The internal AHRS in the Lite's are not comparable in performance to an external system. This is simply due to the location. In many instances they work fine but if your panel suffers from vibration - tat feeds right into the AHRS and severely compromises it.
Newer "lite" systems made in about the last 12-18 months or so have an improved AHRS that includes new algorithms to handle vibrations better (These can create false gravity direction measurements). Typically this is very dependent on engine RPM - as well as effects related to harmonic vibrations which are a result of prop and engine RPM not the same - as in the case of the Rotax.
I think you will do yourself a great favor installing an SP-7 - and that would be my recomendation.
Generally the performance of the AP is excellent in very turbulent conditions - but be sure to give it enough control ability - i.e. the servo "magnitude" setting. If that is too low it will work fine in calm air but is restricted from giving enough control input in turbulent conditions.
If it nose-dives when you slow down - please lower your minimum speed setting in the pitch control setup.
Re: Why Doesn't The Autopilot Do Its Job Properly?
Sorry, forgot the A/P engagement screen but the A/P was in HDG (Track) Hold and Alt Hold.Would you be so kind as to show:
A screen shot of your pitch control setup menus contents.
A screen shot of the autopilot engagement screen.
Today, it was so calm I think I would have had better altitude control if I'd have left the pitch channel disengaged and just left it trimmed level (as I did before engaging the A/P).
Photos below show it's great at ignoring the selected altitude both high and low
It's just all not very good at all and if you think I'd spend another £500 fitting an SP-7 you're having a laugh. Another EuroFOX owner I've spoken to did exactly that and said it made no difference whatsoever to the vagaries of attitudes displayed nor the abilities of his A/P.
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Re: Why Doesn't The Autopilot Do Its Job Properly?
Have some more photos 
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Re: Why Doesn't The Autopilot Do Its Job Properly?
Your pitch magnitude setting is unusually high. Not saying it is wrong but usually you would be around a 7 or so.
I can't see the flight director on your screen. Can you please enable it in your 3D setup menu. The flight director will show us what the AP wants to do.
Please also a screen shot of the AP popup screen where you normally engage the AP - I would like to see the Vertical mode selected.
Your servo torque settings are pretty low - I hope the servos actually have enough force to actually control your stick with those settings. In a similar aircraft to yours I tend to have settings of 80% for both.
Please just have a look at your servo arms and the big round thing the servo arm mounts to - we call that the servo boss - there is a big grub screw in there you can stick an allan key into from the side - make sure that thing is really tight - it should be fitted with a drop of blue locktite after installation. If this is not done it WILL come loose and then what you get is the servo shaft rotating without the arm actually moving or not moving as much as it should.
The next thing to look for is the little shear screw on the arm itself - that will quickly work itself loose if there is no locktite. This will allow the servo arm to move a small amount - it may seem like nothing but it magnifies itself along the controls greatly - making it impossible for the AP to feed in small corrections - and those are really needed for pitch control.
I can't see the flight director on your screen. Can you please enable it in your 3D setup menu. The flight director will show us what the AP wants to do.
Please also a screen shot of the AP popup screen where you normally engage the AP - I would like to see the Vertical mode selected.
Your servo torque settings are pretty low - I hope the servos actually have enough force to actually control your stick with those settings. In a similar aircraft to yours I tend to have settings of 80% for both.
Please just have a look at your servo arms and the big round thing the servo arm mounts to - we call that the servo boss - there is a big grub screw in there you can stick an allan key into from the side - make sure that thing is really tight - it should be fitted with a drop of blue locktite after installation. If this is not done it WILL come loose and then what you get is the servo shaft rotating without the arm actually moving or not moving as much as it should.
The next thing to look for is the little shear screw on the arm itself - that will quickly work itself loose if there is no locktite. This will allow the servo arm to move a small amount - it may seem like nothing but it magnifies itself along the controls greatly - making it impossible for the AP to feed in small corrections - and those are really needed for pitch control.
Re: Why Doesn't The Autopilot Do Its Job Properly?
Your pitch magnitude setting is unusually high. Not saying it is wrong but usually you would be around a 7 or so. I have had it set on various settings, both higher and lower and found it makes little difference. I'll put it back down to 7 and see what happens
I can't see the flight director on your screen. Can you please enable it in your 3D setup menu. The flight director will show us what the AP wants to do. I don't ever recall seeing a FD option. I'm sure it is there, just I rarely go into that setting and, if I do, it's normally to turn the SV on or off. I'll see if I can find the FD option and turn it on. As you say, at least it'll tell us what the A/P is being told to do.
Please also a screen shot of the AP popup screen where you normally engage the AP - I would like to see the Vertical mode selected. WIll do when I fly next, although the FMAs (as blatantly copied from my day job) at the top of the screen, show the engaged modes.
Your servo torque settings are pretty low - I hope the servos actually have enough force to actually control your stick with those settings. In a similar aircraft to yours I tend to have settings of 80% for both. Not a snag, I'll whack them up. I've tried various settings on those as well but didn't make any difference.
Please just have a look at your servo arms and the big round thing the servo arm mounts to - we call that the servo boss - there is a big grub screw in there you can stick an allan key into from the side - make sure that thing is really tight - it should be fitted with a drop of blue locktite after installation. If this is not done it WILL come loose and then what you get is the servo shaft rotating without the arm actually moving or not moving as much as it should.
The next thing to look for is the little shear screw on the arm itself - that will quickly work itself loose if there is no locktite. This will allow the servo arm to move a small amount - it may seem like nothing but it magnifies itself along the controls greatly - making it impossible for the AP to feed in small corrections - and those are really needed for pitch control. I'll double-check those but only recently had the seats out and had a good look around when I did
I can't see the flight director on your screen. Can you please enable it in your 3D setup menu. The flight director will show us what the AP wants to do. I don't ever recall seeing a FD option. I'm sure it is there, just I rarely go into that setting and, if I do, it's normally to turn the SV on or off. I'll see if I can find the FD option and turn it on. As you say, at least it'll tell us what the A/P is being told to do.
Please also a screen shot of the AP popup screen where you normally engage the AP - I would like to see the Vertical mode selected. WIll do when I fly next, although the FMAs (as blatantly copied from my day job) at the top of the screen, show the engaged modes.
Your servo torque settings are pretty low - I hope the servos actually have enough force to actually control your stick with those settings. In a similar aircraft to yours I tend to have settings of 80% for both. Not a snag, I'll whack them up. I've tried various settings on those as well but didn't make any difference.
Please just have a look at your servo arms and the big round thing the servo arm mounts to - we call that the servo boss - there is a big grub screw in there you can stick an allan key into from the side - make sure that thing is really tight - it should be fitted with a drop of blue locktite after installation. If this is not done it WILL come loose and then what you get is the servo shaft rotating without the arm actually moving or not moving as much as it should.
The next thing to look for is the little shear screw on the arm itself - that will quickly work itself loose if there is no locktite. This will allow the servo arm to move a small amount - it may seem like nothing but it magnifies itself along the controls greatly - making it impossible for the AP to feed in small corrections - and those are really needed for pitch control. I'll double-check those but only recently had the seats out and had a good look around when I did
Re: Why Doesn't The Autopilot Do Its Job Properly?
O.K. I will do as advised and report. Thanks RainierI think you will do yourself a great favor installing an SP-7 - and that would be my recomendation.
Re: Why Doesn't The Autopilot Do Its Job Properly?
O.k. I did as Rainer told me to do and installed an external SP-7 to my lite system. All my AP issues are gone. Also horizon and slip indicator are stable as one would expect them to be. Thanks
Re: Why Doesn't The Autopilot Do Its Job Properly?
Thats exactly what I did this week, and the problems I had also disappeared.
Mike