Re-Rigging the Trim Tabs
- PLUS -
Completing Van's Service Bulletin SB-8-6-1

When you rig your trim tabs, you're told to line them up at 35 degrees down, and that's the setpoint where you kind of lock everything up and call it good.  There actually IS documentation as to what the actual trim limits need to be, but I think most people actually MISS this document.  It's the Flight Testing document that goes along with the plans, and it gives the actual required angles in degrees for max and min trim tab deflections. Check out page 2 of that document for more info.  More below regarding the actual mis-alignment...



Well, the gotcha is that, depending on lots of things....builders diligence, parts drilling accuracy, rigging care, or whatever, this may not work according to what you intended.  I credit Bill DeRouchey for bringing this up on the forum a while back, and trying to get folks to listen.  I must admit, I intiially did not listen as intently as I should have, because I had never seen any negative effects to my previous state of rig.

Here's the story, as I know it:   Bill noticed one day, in a specific state of trim and loading, that when he looked back at his tail, one of the elevator horns was ABOVE the Horizontal Stabilizer by a bit (I can't remember the exact amount Bill thought it was) and one of the horns was BELOW it by about the same amount.  This confused him....why would it be so screwed up?  And knowing that in the center, both parts are bolted together with steel brackets and tough bolts, it meant that there was an incredible twisting force being applied to the tail, as one side tried to go up and the other tried to go down.  Trying to manually recreate it on the ground required LOTS of force.  So, he was naturally and rightly concerned, and tried to figure out what was going on.

This is where I personally lacked some motivation to think, because I know I've looked back at my tail and elevator horns on MANY flights, and they have always been nearly perfectly symmetric.  Never in the world would I see them with one above and one below.  So I assumed something must be drastically rigged wrong, and that it was a problem that was builder-caused.  Well...hey, maybe, but later I would come to think about it a bit more, as the topic of the "twisted tail" came up in relation to the current SB.  Van's has stated that they feel the cracks were caused by shuddering of the tail in the many many (hundreds) of demonstration flights done where stalls were completed.  Bill, however, took the initiative to call Van's, and get the measurements of their trim tab deflections, and he notes that their N220RV that had the cracking is also plagued with the same mis-alignment that he found when he checked his out.  The concern is, looking at the photos of N220RV that show up in the Service Bulletin, there are lots of forces both forward/aft, up/down acting on that bulkhead, and that the forces caused by the twisting of a mis-rigged tail could only make things worse.

So what's up with the rigging?

First, it's extremely important for you to make sure when you drill the elevator horns, that you drill them both taped neutral and that you do a good precise job.  That will at least allow you to have both elevators at the same angle and you won't be building asymmetrical elevator horns into your system.  But, there is still more to worry about with the trim tab alignment. If you lock those parts down at 35 degrees down on the trim tabs fand continue on, your tabs likely won't operate in a way that is optimal, and it MAY, in the more extreme cases, cause you to have the same effects as Bill noticed.  These errors of non-precision can be additive!  I checked my trim tabs out, and I found that even though I haven't been experiencing the twisted tail and asymmetrical elevator horns, mine weren't totally perfect, either.  What you WANT is for both tabs to come up ALL THE WAY TO NEUTRAL at the same time.  PERIOD.  In short, THAT is the alignment issue.  When you lock them down at 35 degrees nose-up trim and tighten it all up, you are almost assured that your LEFT trim tab will never get up all the way to neutral.

The way the tabs work, the LEFT tab goes DOWN (about 35 degrees), and up to Neutral.  The RIGHT tab goes down (also about 35 degrees), but also goes UP from neutral about 25 degrees.  So yes, they are mean to operate that way.  But, if built according to the plans, using the normal everyday parts that you are delivered, what happens is that Left tab when locked in down at 35 degrees down, never gets all the way to neutral, while the Right tab goes UP from neutral as you pass into that nose-down trim region.  NOW you have the problem....one tab is pushing the Horizontal Stab UP, while the other side is pushing it DOWN.  See the problem?  The problem is caused by most people TRYING to get that FULL NOSE UP trim capability by locking the parts at the maximum 35 degrees of travel.

On mine, when I initially started, the RIGHT tab would go down about 33-35 degrees, and up about 25 degrees, but the LEFT tab would go down near 35 degrees, but only come UP to about 3-3.5 degrees DOWN from neutral.  Measured between the tip of the trim tab vs the tip of the elevator, when the Right tab was neutral, the Left tab was about .18" DOWN.  If I moved the RIGHT tab UP (nose-down trim) a bit more, due to the mechanism, the LEFT tab would actually move down to about .25" below the elevator.  Yes, that's right...due to the mechanism, as you continue nose-down trim, the Left tab that was previously neutral when the right one was, will continue to have some movement.  NO BIG DEAL.  But, you can see that with a well-drilled elevator horn setup on my plane, I still had a LEFT trim tab that would have periods where it would FIGHT AGAINST the RIGHT trim tab.  Throw in a few more errors in building since these planes ARE amateur built, and throw in a little rigging variation, and I can easily see how someone elses tabs could end up worse off than mine.

So how to fix it??

VERY SIMPLE!!  When rigging the plane, once you have the entire tail on after final assembly (OR AFTER PERFORMING THE SERVICE BULLETIN!!) just FIRST ensure that your RIGHT tab goes down 32-35 degrees and up 23-25 degrees.  NEXT, NEUTRALIZE that Right Trim Tab.  Now, adjust your LEFT trim tab so that it is also NEUTRAL.  Once it is, work the trim full travel and you should find that you get maybe 32 degrees or so down out of the LEFT tab...not maybe the full 35, but within spec, and without the twisting alignment errors.  When you go full UP on the Right side, you will probably notice that the Left tab will also go up a very slight amount.  Yeah, this is NOT to the spec above where it lists Zero as the min. travel and makes no mention of up travel....but this is insignificant....it is much better to have the tabs both moving the same direction...and the deflection upwards will be very small.

Will this prevent the twisted tail?  Well, it certainly will keep the trim tabs from driving the elevators in opposite directions.  It's still up to YOU to drill all the elevator parts properly so that the rest of the things align well.

Now see below for doing the SB.

Below: Properly rigged Trim Tabs at Neutral position.  Note that both are neutral at the same time.
Below:  Properly rigged Trim Tabs at the full NOSE DOWN position.  Note only Right tab is up, spec. 23-25 degrees.
Below:  Properly rigged Trim Tabs at the full NOSE UP position.  Note that both tabs deflect downward, spec 32-35 degrees.
Note:  Previous (before completing this) Left Trim tab position when Right tab was at Neutral (0 degrees)	Left Elevator Horn In Flight at my average loading.	Right Elevator Horn In Flight at my average loading.

Tail is almost off!	No cracks, but look closely at rivets		Doubler installed

Site Home  |  N104CD Home