Trim calibration

[cmarbach]

During an inspection we noticed that when the Aileron trim widget shows in the center, the mechanical trim is off to one side by about 20 degrees. We went to "Trim calibration": 0.0 volts on one side, 4.92 volts on the other. After calibration is is still off to one side.

Is there any adjustment we can do? You can EDIT the values but the manual is silent on what they do.

 

[Carl_Froehlich]

Don't confuse trim tab position with trim motor position. The trim indication is linear and driven off the trim motor - which is also linear. Most trim tabs however tend not to be linear. On the RV-10, trim neutral is 3/4 of the way on trim motor travel. I set the green line on the trim indication at the neutral trim point so I know where it is. For me, the green line is 1/4 of the way up from the bottom.

Carl

 

[swatson999]

During an inspection we noticed that when the Aileron trim widget shows in the center, the mechanical trim is off to one side by about 20 degrees. We went to "Trim calibration": 0.0 volts on one side, 4.92 volts on the other. After calibration is is still off to one side.

Is there any adjustment we can do? You can EDIT the values but the manual is silent on what they do.

What kind of plane?

 

[cmarbach]

It's a Vans RV-7. I am interested in EDITing the trim calibration in the Skyview not the airplane.

 

[dynonsupport]

The trim calibration assumes your trim is linear. You set the two ends, and it assumes the voltage that is halfway between is the middle. This is true for all the trim motors we know of, where when they are in the middle, the voltage is too.

If your servo is non-linear, this won't be true. In your case, when the servo is in the middle of it's travel, your trim tab is not centered. This could be because you don't have the various adjustments centered and have more up trim than down (or vice versa) or that the travel is non-linear it moves faster up than down.

You can edit a text file in SkyView to fix this if you really want to, but I'd first figure out what your case is. It may be as simple as lengthening a pushrod a half inch to get your left/right trim ranges balanced.

 

[swatson999]

It's a Vans RV-7. I am interested in EDITing the trim calibration in the Skyview not the airplane.

Look at the plans, p. OP-19. At half-travel of the linear actuator (the trim motor), WD-756 is NOT parallel to the fuselage, but is in fact, off to one side.

This setup works just fine, if you follow the instructions for attaching the bellcrank to the sticks, and 1/2 travel of the motor will, indeed, result in zero (or identical) tension on both ailerons.

 

[swatson999]

If your servo is non-linear, this won't be true. In your case, when the servo is in the middle of it's travel, your trim tab is not centered. This could be because you don't have the various adjustments centered and have more up trim than down (or vice versa) or that the travel is non-linear it moves faster up than down.

You can edit a text file in SkyView to fix this if you really want to, but I'd first figure out what your case is. It may be as simple as lengthening a pushrod a half inch to get your left/right trim ranges balanced.

No trim *tab* on aileron trim on an RV. It's spring bias.

 

[thibault]

Dynon,

You said, "The trim calibration assumes your trim is linear. You set the two ends, and it assumes the voltage that is halfway between is the middle."  Does it matter if this is not a correct assumption?

My trim is purposely built to have more "right" than "left" capability (due to flap asymmetry when deployed), so half the voltage is already well into the "right".  Does this mean I should delve into the edit the file solution?

What I have right now works correctly for manual trim, flaps up or down, but will not correctly auto trim at all.  It keeps over doing it to the right all by itself.

Is the auto trim algorithm designed to just zero the servo forces or does the zero voltage point of the trim position sensor also play into the equation?

 

[swatson999]

My trim is purposely built to have more "right" than "left" capability (due to flap asymmetry when deployed), so half the voltage is already well into the "right". 

Flap asymmetry? Ummm....this doesn't sound good...

 

[Dynon]

The auto-trim doesn't use the trim calibration at all - the calibration is just visual feedback of where the trim tab is at. The auto-trim feature actually uses the servo's internal force-sensing capabilities. Put another way - you could not have a roll trim indicator at all and auto-trim will still work the same way.

 

[Dynon]

As for the auto-trim not getting it right - does it keep going to the right, or does it stabilize at a different point that is slightly more right than actually being in-trim?

 

[thibault]

Steve,

It is a very slight asymmetry when the flaps are down, none when they are up.  So, it takes quite bit of right aileron and a little right rudder trim to fly slow and straight, hands off, with flaps well down.  Don't want to cut into the flap to reposition the attach.

Dynon,

It does stabilize with a large amount of right trim.  If I punch off the roll channel once that has happened, the A/C will roll sharply to the right.

 

[swatson999]

Steve,

It is a very slight asymmetry when the flaps are down, none when they are up.  So, it takes quite bit of right aileron and a little right rudder trim to fly slow and straight, hands off, with flaps well down.  Don't want to cut into the flap to reposition the attach.

Sure that's not P-factor, etc.?

I'd just set it up for normal cruise and then dial in all the roll trim I needed if necessary when going slow. I like going fast

 

[thibault]

Steve,

No.  Of course I measured the flap displacement on the ground.  The builder made a slight error in construction such that the right flap displaces slightly more than the left.  Correction would require cutting up a lot of fiberglass on the fuselage and the left flap.  Without biasing the trim slightly more to the right, it was not possible to get enough right trim to fly level hands off.

 

[thibault]

It does stabilize with a large amount of right trim.  If I punch off the roll channel once that has happened, the A/C will roll sharply to the right.

Dynon,

Any further thoughts since this?

 

[vlittle]

Hi Tom.
It looks like your aircraft is out of rig. This means that your roll trim will change with airspeed and flaps position. What could be happening is that the autopilot is running out of trim authority, so when you switch it offline, the aircraft will roll in one direction.

This is dependent on the factors I mentioned and I don't think Dynon's autotrim algorithm can correct a rigging problem this severe.

I developed an autotrim controller for the SkyView, before Dynon introduced their system, so I am very familiar with autotrim control algorithms. It was extensively tested in an aircraft will a wide speed envelope (Harmon Rocket) and both pitch and roll trim would always be perfectly neutral when the autopilot was switched offline... assuming it had enough time in level flight when engaged to converge.

It can take a few minutes for the autrotrim system to converge to neutral... if you are rapidly changing speeds or extending flaps, don't expect the autotrim to keep it perfectly neutral.

Of course, lack of trim authority is also a factor... this is an airframe issue, not a SkyView issue. I really think you need to bite the bullet and get the aircraft rigged properly.

Maybe this is not what you want to hear, but it may be the only solution.

Vern

 

[thibault]

Vern,

You said,  "It looks like your aircraft is out of rig"

Clearly true when flaps are extended.  Not true when in cruise..

However, the autopilot IS NOT running out of trim authority.  It trims, in roll, well beyond the correct in-trim condition, but it does not go "on the stops".

While in cruise, the roll and yaw trim tabs are physically in trail when flying straight and level without autopilot.  When I then engage the autopilot, with auto trim on, the roll trim will slowly drift well to the right.  It will settle out with about half right roll trim engaged.  If I then manually trim back left to what I think is the in-trim condition, sometimes I hit the point of the needle position and the roll trim doesn't change for a while.  Other times it starts to go out of trim right away.

Also,  if I manually push the roll trim well off the in-trim condition to the left, SV annunciates "trim right".  Or, when the auto trim has settled on the excessive too much right condition, if I then trim further right, SV will annunciates "trim left".  So, it seems SV is sensing the needed trim motor direction correctly, but can't find the in-trim position.

 

[vlittle]

Ok, thanks for the clarification.

One thing to check is control friction.  You can check on the ground, but it may also be necessary to disconnect your roll trim bias springs.   Any friction should be investigated, espectially if there are noticable variations as you move the controls.  Try moving the stick in all directions, looking for binding.  Try various flap positions as well.  It wouldn't take much non linear friction to interfere with the autotrim system.

You will notice the servo drag, but this should be constant throught the range of motion.  If the servo linkages are misrigged, you may notice a variation.  In general, at neutral, the servo pushrods should be about 90 degrees to the servo arms, and at 90 degrees to the bellcranks.

My understanding is that you did not build the aircraft. If this is the case, you may want to be more diligent in your investigation... Lots of things can slip through the construction process.

If you suspect a bad servo, try swapping the pitch and roll servos, recalibrate, and try again to see if the problem follows the servo.

Vern