-7A elevator electric trim rod length

My trim rod is about 7 or 8 inches long and I believe it needs to be shortened (unless I'm not looking at it right). My guess is that the factory just throws a length of rod in the box and lets the builder figure it out.

Can anyone tell me the best way to figure out the rod lenght? My concern is that I cut it to short and it isn't strong enough in the plastic clevis, or that it's too long and cracks the clevis. How far in does it need to be threaded?

Muchas gracias.

Can't say from any flying experience since I am just finishing my emp, but I just estimated the length plus a half inch or so, cut it with a hack saw, smoothed the burr with scotch brite wheel, screwed on the two plastic ends without forcing, then attached it to the trim tab and servo. Initially it was a little long, so I cut off some more. Then I used the battery on my cordless drill to cycle the trim servo to both extremes. In the process, I had to shorten it a little more. When all was said and done, it seems right, plus I still have the other half of the original that is longer in case I discover later that it needs to be longer for some as yet unknown reason.

hope that helps.

What length to cut the threaded rod for the trim servo?

I guess I am going to ressurect this thread from the dead, since I cannot seem to find a suitable answer on the forum archives, and the mathematical solution for this question works rather well. I am building an RV-8, but I am fairly certain that the empennage kits for both the 7 and the 8 are identical as they pertain to the installation of the electric trim servo.

As for what length to cut the threaded rod included in the trim servo kit, here was my solution, which is bit more precise than simply trimming and fitting and trimming some more:

My plans for the left elevator show a drawing with the trim tab in its level position on the elevator, with the trim servo and rod assembly also shown. If you look closely you can see where they drew in the presumed ends of the rod about half way inside each clevis. This appears to be right at the point where the clevis begins to change the direction of the flair in the shaft. In other words, about half way into the clevis shaft, but not all the way to the very end.

The drawing is in 3/8 scale. If we know the measurement of the distance of the rod as shown in the plans, and we know the scale factor of the drawing, we can obtain the full size measurement by applying the correct scale conversion factor to that measurement.

Measure the distance of the rod in the drawing. Mine measured approximately 1 3/8 inches.
1 3/8 inches is the same as 1.375 inches
3/8 scale is the same as .375 scale.
To get to full size from 3/8 scale you determine the inverse of the scale and apply that to the scale measurement from the plans.
So 8/3 is the inverse of 3/8, which equals a scale factor of 2.67.
Multiply 1.375 inches (length of rod in the plans) by 2.67, and you get a value of 3.67125, or just over 3 5/8 inches or so.

One other note about this is that when you check this measurement on your own airplane, make certain that the trim tab is in the level position, and the trim tab servo output arm is also at its level or centered position. I checked this measurement against my assembly, and the measurement looks to be just about right.

This at least takes some of the guess work out of this process, and hopefully reduces the amount of trimming and fitting to a single event. Minor adjustments can be completed by screwing each clevis in or out the required amount.

I hope this helps somebody....

Flyin'Bryan said:

I guess I am going to ressurect this thread from the dead, since I cannot seem to find a suitable answer on the forum archives, and the mathematical solution for this question works rather well. I am building an RV-8, but I am fairly certain that the empennage kits for both the 7 and the 8 are identical as they pertain to the installation of the electric trim servo.

As for what length to cut the threaded rod included in the trim servo kit, here was my solution, which is bit more precise than simply trimming and fitting and trimming some more:

My plans for the left elevator show a drawing with the trim tab in its level position on the elevator, with the trim servo and rod assembly also shown. If you look closely you can see where they drew in the presumed ends of the rod about half way inside each clevis. This appears to be right at the point where the clevis begins to change the direction of the flair in the shaft. In other words, about half way into the clevis shaft, but not all the way to the very end.

The drawing is in 3/8 scale. If we know the measurement of the distance of the rod as shown in the plans, and we know the scale factor of the drawing, we can obtain the full size measurement by applying the correct scale conversion factor to that measurement.

Measure the distance of the rod in the drawing. Mine measured approximately 1 3/8 inches.
1 3/8 inches is the same as 1.375 inches
3/8 scale is the same as .375 scale.
To get to full size from 3/8 scale you determine the inverse of the scale and apply that to the scale measurement from the plans.
So 8/3 is the inverse of 3/8, which equals a scale factor of 2.67.
Multiply 1.375 inches (length of rod in the plans) by 2.67, and you get a value of 3.67125, or just over 3 5/8 inches or so.

One other note about this is that when you check this measurement on your own airplane, make certain that the trim tab is in the level position, and the trim tab servo output arm is also at its level position. I checked this measurement against my assembly, and the measurement looks to be just about right.

This at least takes some of the guess work out of this process, and hopefully reduces the amount of trimming and fitting to a single event. Minor adjustments can be completed by screwing each clevis in or out the required amount.

I hope this helps somebody....

Click to expand...

Nice one Bryan. Another one for the "Tips"" section I think. What do you say Moderators?

I hooked mine up to a 9 V. battery on the table, found nutral, installed on elevator and cut to length.

What he said.

bret said:

I hooked mine up to a 9 V. battery on the table, found nutral, installed on elevator and cut to length.

Click to expand...

Me too.
Don

Is there any documentation that states how many degrees of trim tab deflection up and down is ideal? Should one assume it should be equal?

Maybe I'm thinking too much and should just set the servo to neutral, set the tab to neutral and cut the rod as appropriate as mentioned above.

Chapter 15 of the manual I believe, but...

grayforge said:

Is there any documentation that states how many degrees of trim tab deflection up and down is ideal? Should one assume it should be equal?

Maybe I'm thinking too much and should just set the servo to neutral, set the tab to neutral and cut the rod as appropriate as mentioned above.

Click to expand...

Russ, the chart with the max throws for the trim tab is in Chapter 15 of the manual. However, if you do some searching of the forums you will also find that just about everyone that I am aware of does not get the max throw defined in the chart when using the electric trim servo from Ray Allen. Reports from many others already flying with this system are that there is more than enough trim range for actual flight conditions.

If memory serves, I ended up with about 27 and 29 degrees on mine - can't remember which way off the top of my head, but I know it was less than described in the chart. it's posted in my blog. Also keep in mind that the chart states that these are max values, and not that they MUST match the values published, at least for the trim tab.

Just as you stated, set the trim tab flush with the rest of the elevator with the pin installed in the hinge. Set the Trim servo to neutral and mount it in the elevator so that it is secure. Tape the trim tab in place if you wish, then cut the rod. With my method described in the previous posts, I think I had to shorten things up with only a couple of turns in each clevis, and it worked out great. I only needed to trim a small amount of material away from the rod exit hole on the elevator for clearance, and no additional bends in the rod were required at all.

Exactly what I was looking for. I dug through the manual a bit last night, but not well enough, it seems. I was expecting it to talk about elevator trim setup in a crazy place like the Left Elevator section.

Thanks Bryan!

I used the above method to measure the length of rod I needed. I measured with the rod all the way into each clevis minus 3/16 on each clevis. IE, the rod can move further into each clevis by 3/16, if needed. The rod ended up close to perfect the 1st try (needed just a couple turns to get the trim tab flush). Thanks!

Glad it worked!

Would you believe Ray Allen's son told me that their pull test of the rod and clevis assembly was accomplished by screwing the rod in to each clevis only about 1/8 of an inch, and it took some enormous amount of pressure before it failed (4000 pounds I think). Sounds like yours turned out about like mine, with pently of rod to spare in each clevis.

Now, if you haven't already done so, get yourself a length of bundled mil spec wire (26 gage I think) and solder an extension to the leads of the trim servo - about 12 inches should work. I ordered a 20 foot roll from Vans. That way you have some extra wire to work with later on when you determine how you want to fasten it to the fuselage wiring. They don't give you much extra wire to work with on the trim servo from the factory. Don't forget the heat shrink tubing. HD has a set of tubing from GB that includes the smaller size tubing for these very tiny wires.

OR.... you can save that till later and get out of Tail assembly mode, and get to work on those Wings!