autopilot rudder trim lock?

[cj8vet]

apparently the stec setup on maules is different than regular in that it only connects to the rudder---my AP seems to wander about 6degrees back and fourth- was questioning an stec guy about it --he has only put 2 on maules--but was wondering about how the rudder trim lock works ..Would it be better to let it off when using the AP?
thought Id get your comments before I fly and try it out
thanks
mark
mxt-180

[Green Hornet]

Hi Doc,

I got this reply from Jeremy when I asked about rudder trim:

The T handle is the rudder trim control and is connected via a spring to the right pedal. All it does is take some of the load from your right foot when operating the plane in acceleration, climb and cruise and you set it where it gives the most comfort.
The 'servo' tab in the trailing edge of the rudder is not anything to do with the rudder trimming control. It is a separate system and is driven in concert with the aileron, to reduce adverse yaw which takes place when an aileron is manipulated.

My take on this is you should not have the rudder trim locked while using AP but that is an interpretation since I don't have AP. It seems to me that if the rudder trim is locked it would be counter productive since you are not using your feet unless you have AP that does not control rudder inputs.

[cj8vet]

Thanks, I wonder If I hurt anything leaving it on?
mark

[Green Hornet]

I don't think so I left mine on for a long time but I did not have AP. I would fly it around with the AP engaged and see if it is performing accurately without the rudder trim locked.. The data input may have tried to compensate for the amount of trim. Hence the wandering back and forth.

[a64pilot]

My feeling is that you do with the rudder trim whatever is necessary, to obtain trim, then engage the autopilot. Otherwise the autopilot will have to trim the airplane. Autopilots have limited authority, and some of it will be used just to get to an in trim condition. It's best to trim whatever axis the auto pilot uses prior to engagement. You wouldn't engage altitude hold while trimmed for a climb would you?

[a64pilot]

When it's wandering, what is it slaved to? Does it wander in heading hold as well?

[cj8vet]

A64--IT seems to wander worse in heading mode than in nav mode--sometimes it seems to not move hardly at all others worse--Sorry I am still a low timer in the maule(all planes for that matter) so I dont always get to "play" observe things as often or as clearly as I would like--
It seems I read we should just set the rudder trim where we need it for takeoff then just leave it--maybe I should adjust it on every stage of flight?
thanks for the help
mark

[a64pilot]

Normally, the higher the power setting and the lower the airspeed, the greater the need for right pedal. In a descent at high speeds and low power it may be that you need a little left pedal even with the trim pushed all of the way in.
I would do whatever feels best for you. Some people set it and forget it, and some retrim at each phase of flight. Both ways are correct. A whole lot of airplanes don't have rudder trim.
I have honestly not experienced an autopilot that used rudder for heading control. In my limited experience, I don't see how that would work very well. Look in your manual to see if their is a gain adjustment, or ask an avionics person. If the gain (sensitivity for you and me) is set too high, it will constantly seek and wander back and forth slowly like a drunk.

[Green Hornet]

My feeling is that you do with the rudder trim whatever is necessary, to obtain trim, then engage the autopilot. Otherwise the autopilot will have to trim the airplane. Autopilots have limited authority, and some of it will be used just to get to an in trim condition. It's best to trim whatever axis the auto pilot uses prior to engagement. You wouldn't engage altitude hold while trimmed for a climb would you?

Jody,
If the auto pilot adjust the trim but it is locked in to what the pilot feels is trim over a period of time the situation would change therefore needing trim adjustment. The computer would send data instruction to adjust the trim but the trim is physically locked to a specific position. Would this not be like the brain sending messages to remove your arm from underneath you while you are sleeping on top of your torso? The brain dreams it is moving the arm but it remains locked in position. Eventually you actually move your arm a little waking you to find a numb arm still underneath your torso? Unfortunately computers don't dream. I would think the computer would be more efficient if the trim was not locked. As for the question about altitude control I agree you would not engage altitude hold while trim for a climb. So why would you physically lock trim before engaging automatic trim? Isn't that the same thing? Just trying to understand

[a64pilot]

The trim does not "lock". It is a KISS system. All it is is a spring that pulls on the cable attached to the right pedal. I think it pulls on the cable, but it might pull on the pedal. So having the handle out just enough for the ball to be centered doesn't lock anything. One ball width out of trim in either direction requires the same amount of travel. As long as the autopilot is engaged in the center of trim it will have the same amount of travel either way.
If your STEC is like mine, it will start beeping when it commands a rate that doesn't happen. If that happens you either recenter trim or hold constant pressure on the control. I don't have one on a Maule, it's on a 210, but I assume it's similar. I think it's an STEC 20 or 30.
I think your not holding a heading is not related to rudder trim. If that were it, and if it's like mine, it would be beeping. I think either whatever is giving heading info is "wacked" or more likely the sensitivity is turned up too high. It could also be the cables to the servo are loose.

[maules.com]

A64, correct re the trim. Set it for cruise to keep aircraft going straight, then, activate the Stec. First set the level wing balance by rotating the knob left or right as needed, BUT this only works if the plane is rigged correctly, then activate Hdg or Nav, then trim for altitude and activate Alt. If the torque is set wrong or set for a different model on the clutches, or if any lube or anti corrosion fogging has got to them, the clutches will slip.
Amphib floatplanes, where the floats protrude forward of the prop do have a tendency to hunt left and right while trying to track and/or hold alt.

[cj8vet]

ok I'm back with more AP questions as mine is ?"dead" and off at Stec and I finally flew a plane with a different AP--a c-T206--its pre flight ck required you to watch the yoke move when adjusting DG etc , which is what the S-Tec POH says also BUT since mine/ours(all maules?) have them hooked to the rudder, how are you suppose to ck AP function and keep your brakes on?
2ndly if it comes back from the shop and still "wanders" a bit would you help it stay straight with rudder or aileron? or should you not have to "help" an AP?
mark the dummy

[maules.com]

Set the brakes with the brake lock, then rest feet 'lightly' on pedals and you will feel the rudder attempting to move the pedal. It cannot fully move them on a tri-gear because of the weight on the nosewheel.
Or, test it before starting the engine and you don't need the brakes locked.....unless on a hill.
The airplane needs to be properly rigged and the rudder trim set for the speed you will cruise at. Properly rigged means you can fly it straight and level with hands and feet off the controls. Only then can you expect an autopilot to work nicely.
Loose control cables, stiff nosewheel steering, loose flaps, can all cause the autopilot to try to find a place to work from. Stec clutch settings are also important. The Stec people are helpful and have helped certify many models of the Maule. There are dozens of Maules flying successfully with the yaw control Stec.
The M5 and M6 era, Maule used the Century 2B and Century 21 roll control autopilots but there was no altitude control.