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Retrofitting elevator trim

joew

Well Known Member
I’m 12 years into a four year build plan, and had figured on using manual elevator trim. Recently I decided to use electric elevator trim. I’ve searched the forums and couldn’t find a whole lot of info.

Other than the mounting bracket and the control horns, I see no real difference between the manual and the electric. Actually, the control horn in the manual sits a little higher (further from) the trim tab than does the electric trim horn.

Does anyone see an issue of sticking with the manual trim control horn? It looks like the trim control arm will not interfere with the elevator skin like the electric one does.

Thoughts?
 

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When I upgraded from manual trim to electric trim four years ago I decided to use the existing manual trim control horn on the elevator trim tab. I mounted the trim control motor on the shelf below the vertical stabilizer and purchased a push/pull cable from Aircaft Spruce.(the smallest non- custom cable I believe is 36 inches). The actual length I needed was around 28” so I have a curve in the cable which originally gave me concern but has proven not to be a problem after it was properly secured. This cable is routed through the tail into the horizontal stabilizer and out through the same holes as the old trim cable system.

I also recommend contacting Pat Hatch at PHAviation.com and install the UTRB (Ulitmate Trim Relay Board) that he sells for this conversion. It’s well worth the money and he’s real good about working with his clients via email.

Good luck.
 
Manual section 15 has the table of required travel of the control surfaces, including the trim tab. I think that as long as the range of the motor yields the correct range of trim tab motion up and down, you should be fine.
 
What I did..same scenario.

When I built my -4, I was headed in the manual trim direction until stumbling across the MAC line of trim options. I re-engineered the elevator I had already built for the electric kit. Minor changes included use of the same design horn, although I recall (20 years ago) I made it slightly larger. As you can see in the pictures, I attached the horn with 5 cherrymax rivets. It has worked flawlessly and I'm glad I did it. I also built a tab into my L/H aileron with electric trim. Electric flaps round it all out with everything on the stick grip..HOTAS, you gotta love it!
 

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I wondered the same thing when I changed to electric trim after 13 years of twisting the big black knob.
In the end, I decided to purchase the new parts from Van's and follow the plans. There is no interference between the trim rod and the elevator skin. I did widen the opening on the cover to clear the plastic clevis on the forward end of the rod assy.
 
Balancing the elevator

The electric trim adds some weight to the elevator due to the trim motor so adding provision for mass balance lead sheet plates on the inside of the elevator horn will make the balancing task much easier. There are already tooling holes on the elevator mass balance arm to add a nut plate for adding sheet lead. I put a nutplate on both elevators just in case they were needed.

KT
 
horn extension

Here's how I handled a similar upgrade with a 'horn extension' doubler.

i-XDHCNC5-M.jpg
 
Turns out, after measuring the throw and ensuring center on the motor and the trim tab, the control horn that came with the electric trim kit will need to be used.

I’m getting 25 degrees one way and 15 the other. With the longer horn, I should be able to get 25 degrees up and down at least.
 
Follow up question

Looks like a I’ll need to use pop rivets for the trim horn. Any idea which type/size pop rivets to use? I’ve searched and can’t seem to find any reference.


Any ideas?
 
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