Re: Why Doesn't The Autopilot Do Its Job Properly?
Posted: Tue Jan 11, 2022 3:05 pm
If your AHRS is operating normally then it is not the cause of any issue.
The AHRS is used to detect CHANGES in attitude. Anything else is secondary to the AP function. The AP does not care if if you are flying left wing low or pitch up - it's only interested where the aircraft is going.
Practically all issues are related to coupling of the servos to the control surfaces in our experience.
The first thing to check is the shear screw on the servo arm - that needs to be tight. Once installation is near completion the screw should be fitted with a drop of locktight. If this is not done it WILL come loose and the AP has no chance of controlling the aircraft properly.
Similar should be done to the large grub screw in the servo boss - this needs to be tight and a drop of locktight needs to be applied so it stays that way. The forces acting on the servo shaft can be pretty big.
The AP relies on small corrections on the servo arm making it though to the control surfaces. If this does not happen you will get oscillation as the AP will forever be behind the aircraft - no possible setting you can do on the EFIS will fix that.
I would also like to mention another example: A sinus came to my attention where the pitch control was really bad. The installation was done well. When I got to see the aircraft and tried the servo test the entire floor of the aircraft would flex - the servo was mounted on a structure that was connected to the fuselage floor - and this was pretty thin to save weight. It was easier for the servo to flex the floor than move the elevator !
Look at your entire control linkage scheme - make sure a small movement of the arm WILL feed to the control surfaces and don't let gravity on a surface fool you - a small amount of play somewhere can have big consequences once it gets to the control surface. Make sure your mounting brackets etc do not flex under load.
In most cases it is pitch control that needs attention as roll is much more stable than pitch on most aircraft.
The AP is programmed to hold the aircraft within 50ft using a "herd mentality" - it will not force the altitude to 0ft but rather try gently. In calm conditions you can get there but in bumpy stuff it will hold off and correct only for the big stuff with some determination. This is to avoid unnecessary control inputs - much like you would do as a pilot.
The AHRS is used to detect CHANGES in attitude. Anything else is secondary to the AP function. The AP does not care if if you are flying left wing low or pitch up - it's only interested where the aircraft is going.
Practically all issues are related to coupling of the servos to the control surfaces in our experience.
The first thing to check is the shear screw on the servo arm - that needs to be tight. Once installation is near completion the screw should be fitted with a drop of locktight. If this is not done it WILL come loose and the AP has no chance of controlling the aircraft properly.
Similar should be done to the large grub screw in the servo boss - this needs to be tight and a drop of locktight needs to be applied so it stays that way. The forces acting on the servo shaft can be pretty big.
The AP relies on small corrections on the servo arm making it though to the control surfaces. If this does not happen you will get oscillation as the AP will forever be behind the aircraft - no possible setting you can do on the EFIS will fix that.
I would also like to mention another example: A sinus came to my attention where the pitch control was really bad. The installation was done well. When I got to see the aircraft and tried the servo test the entire floor of the aircraft would flex - the servo was mounted on a structure that was connected to the fuselage floor - and this was pretty thin to save weight. It was easier for the servo to flex the floor than move the elevator !
Look at your entire control linkage scheme - make sure a small movement of the arm WILL feed to the control surfaces and don't let gravity on a surface fool you - a small amount of play somewhere can have big consequences once it gets to the control surface. Make sure your mounting brackets etc do not flex under load.
In most cases it is pitch control that needs attention as roll is much more stable than pitch on most aircraft.
The AP is programmed to hold the aircraft within 50ft using a "herd mentality" - it will not force the altitude to 0ft but rather try gently. In calm conditions you can get there but in bumpy stuff it will hold off and correct only for the big stuff with some determination. This is to avoid unnecessary control inputs - much like you would do as a pilot.