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Electric trim pic's

Maxrate

Well Known Member
After connecting the Ray Allen Electric trim motor and following the construction manual from Vans I have zero clearance between the pushrod and the skin. The rod was cut to the suggested 3 3/8", and the tab is perfectly faired with the elevator. It seems at this point that the only option is to file the skin back almost to the rear elevator spar.

I have inserted several pics and a copy of the construction manual pics for the trim motor section. Vans shows the opening to be approx where I have it now. Also how much up and down travel are most tabs able to achieve. Vans says the max should be between 25 and 35 deg up and down. Im no where near that much.

Thanks gents,

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Hi Mark,
This appears to be a common issue. When I built my -7A I did a search on this topic and found that a very slight bend in the threaded rod was acceptable. The alternative is to extend the cutout area of the sheet metal where it's making contact. I would think that the latter is preferred, however I just simply bent the rod slightly and it works just fine.
 
Hi Mark,

I just installed my trim servo and had the same issue. To fix it, I used a dremel w/sanding drum to extend the cutout and I also put a slight bend in the rod. It seems that this is a fairly common issue.
 
Hi Mark,

I just installed my trim servo and had the same issue. To fix it, I used a dremel w/sanding drum to extend the cutout and I also put a slight bend in the rod. It seems that this is a fairly common issue.

Ethan, Was your rod a full threaded rod like the one in the pics. The literature from Ray Allen says not to make any bends in the rod. I have seen different rods used in different pics. Also do you remember how much throw you were able to achieve in the trim tab.

Thanks,
 
Rod bend

I take that back. After looking at the Ray Allen material a little closer it actually states not to make any "sharp" bends as that could cause rod failure.

Thank's all
 
It's hard to bend threaded rod uniformly. Bending around a large-diameter jar or similar works best; you're just trying to achieve a 'bow' to clear that edge. The other problem is then you have to fix the orientation of the rod. Common practice in RC models is to use one threaded and one soldered clevis. The soldered clevis fixes the orientation while the threaded clevis allows adjustment. Keep the bow as minimal as possible; the more bow the more flex you will have, which will only make consistent trimming difficult.
 
Mark,
The Ray Allen literature is vague and had me concerned about bending the rod. Searching the forums and some builder's websites showed that bending the rod is a common practice and the bend only needs to be 5-10 degrees. I can't remember my exact numbers for trim tab travel, but it was close to what is listed in the plans. I figure I will fine tune it when I'm close to flying and when I have all the indicators/switches in place. For now, I'm just hapy that I'm done with the trim tab!
 
I have fitted two electric pitch trim systems. In both cases, I did not feel the need to even slightly bend the threaded rod. I simply removed enough material until positive clearance throughout full up and down travel was achieved.

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Trim rod adjustment

Here are some updated pics. I like Rick's policy of not bending the trim rod. I still have enough from the original length to make a whole new rod. My big concern now is flex in the skin and removing too much material from the E615PP Trim access reinf. plate. Rick, how much trim back toward the trailing edge is acceptable. The pics show where Im at right now with the material that has been removed, and the rod bend measured a 4 deg bend in it.

Thank's guys.

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.....Rick, how much trim back toward the trailing edge is acceptable.....
As shown in these photos, the trim tab is set at the max limit of deflection. You should get an idea of how much material I removed. In real life, nowhere near that much trim is ever going to be needed.

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Rick,

Thanks for the detailed pics. It looks like filing some more material shouldn't be a problem. I like this idea more than bending the trim rod.
 
Another suggestion would be to make a different control horn. You could make one a slight bit larger...ie more distance between the trim tab and the control horn pivot point where the control rod connects. This would put more of an angle on the control rod and should completely miss the back end of your skin where its rubbing.. I would think a 1/2" to 3.4" might do it......

Just a suggestion...
 
I had the same issues with the 9A trim tab. I ended up just using a rat tail file to enlarge the slot.

I wonder if it would have worked to just use the other (non-electric option) position on the tab of the control horn. I had already trimmed mine to the electric option size.

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A slight bend worked for me. Build on.

After 660 hrs. aloft, I'm still amazed at how well my -7A holds altitude after I take the time to get it trimmed at cruise. It keeps me from taking the leap to add altitude hold to my TruTrak autopilot.

Mike
 
Elevator cut out

All,

Thank's for the input. I ended up using the dremmel tool to remove the material that the rod was rubbing on. It feels like I still have about 3/4" of reinforcement plate left. It looks like I have approx the same amount of skin left that Rick has in his pics. The deflection up and down is now approx 25deg as Vans says.

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The rod was cut to the suggested 3 3/8", and the tab is perfectly faired with the elevator.

Question to anyone who's recently done their electric trim: Where in Van's (or Ray Allen's) documentation is it suggested to cut the pushrod to 3 3/8"?
 
Cutting the Pushrod

I measured the pushrod after I centered the servo using a 9V battery and cut it to fit based on the position of the servo arm and the trim tab. This is the similar process I used when I was building my RC airplanes. There are various plays in the servo mount and the servo internals so I cut it after all the mechanical components are in place.
 
I’m just double-checking control displacements. For the electrical elevator trim, I’m only able to get about 22 degrees up and down. Van’s say 25-35 degrees. Is it common to get less? Any ideas on how to increase those (without a lot of work)?
 
I’m just double-checking control displacements. For the electrical elevator trim, I’m only able to get about 22 degrees up and down. Van’s say 25-35 degrees. Is it common to get less? Any ideas on how to increase those (without a lot of work)?

Yes, you often have to elongate the slot as the pushrod usually hits the skin before reaching it's range of motion.

Also, up elevator is where you want the most if you have a heavy front end (constant speed). I pretty much never use down elevator trim other than a tiny amount to level off and an equally tiny amount when I start a decent. In slow flight for landing though, massive up trim needed.
 
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