Trio Wing Servo install question

I just mounted my roll servo in my right wing. Since the bottom skin is not riveted on yet, I built a mounting plate for the servo and attached it to the rib on the outboard side of the aileron bellcrank.

I have 2 questions about this setup:

1. Is it generally bad-practice to install a servo on a rib? I used a .060" thick piece of 6061 as a doubler and extended it about an inch longer on each side of the servo (for extra surface area to keep the rib from flexing).

2. I mounted the doubler plate to the side of the rib that was flat. Unfortunately, the flat side is not in the bay with an access plate. I won't have enough room to get my hand in there after the skin is riveted on, let alone work on the servo if needed. Can I cut out another access plate in the bottom skin? Again, would this be bad form?

Thanks.

Update: added a picture.

Sonny,
That installation sure looks robust enough. I mounted mine on the same rib on the opposite side (bellcrank side). I made a smaller diameter pushrod from the same stock as the pushrod tube if I remember correctly. To improve access, you could mount the servo on the bellcrank side using some spacers for clearance. I haven't flown, but have operated the servo, slightly loading the aileron and could not detect any flexing.

I would not mount any mechanical device in a place where it can not be serviced, and preferrable easily serviced. I recommend you cleco the skin on and completely remove and install the servo just to try out the access. My guess it after a few cuts and bruises, you will move the servo to a location where it can easily be serviced.

Best,

lostpilot28 said:

I mounted the doubler plate to the side of the rib that was flat. Unfortunately, the flat side is not in the bay with an access plate. I won't have enough room to get my hand in there after the skin is riveted on, let alone work on the servo if needed. Can I cut out another access plate in the bottom skin? Again, would this be bad form?

Click to expand...

Sonny, you need to move the servo. It is indeed bad form to "build yourself into a corner". The Trio servo is a reliable device, but if you put it in an unaccessible location you can rest assured the maintenance gremlins are going to make a point of using you for an object lesson.!

Either move the servo so you can access it through the bay or move it to another accessible location in the wing (different access bay or the outboard wing-tip rib/spar). Adding another access plate would be your least desirable option.

By the way, your mount looks fine, just needs to be somewhere else.

You will really like your EZ-Pilot!

I used nutplates

Sonny, I used a couple pieces of alum angle mounted on the side of the rib your servo is on and installed a couple of nutplates on the angles. I mounted the Trio servo on the bellcrank side of the rib............I'm with the gang......you will regret boxing yourself into a corner with the servo where it is presently mounted. My thinking is it would be much easier to reposition the servo than cut a new inspection panel.

Ditto what the other guys said. I think the easiest installation for maintenence purposes is in the otherboard most bay. I used the kit from SafeAir1.

Servo Installation


Maintenance. You can easily get in there with the wingtip removed. I completely installed this servo with the wing buttoned up and it was no big deal whatsoever.

Thanks guys. It dawned on me after I had installed it that access would be a problem. I'll drill out the rivets and move the plate to the other side of the rib.

I agree that either you need to move the servo or put a new inspection panel in. I didn't put my servo there, but I did mount my Dynon pitot and mast just inboard of the bellcrank. Per tech counselor recommendation, and OK by Ken at Van's (after I showed him pictures), I added another inspection panel so that I could easily access the pitot fittings. Cutting a new access panel was very easy to do. I think I used an 0.32 or .040 doubler and modeled it on the other access panels except that I made it narrower.

Sometimes you can get different answers talking to different people at Van's, so you might run it past them just to see what they say this time around.

<<...servo on a rib?>>

The maximum force applied to the rib is equal to the force necessary to slip the servo clutch. It is not much, but you can test by wiring the servo to be operational, engaging it, and then overriding it with force on the big primary control pushrod. If you don't see any flex you'll feel better <g> For the test you may want to support the rib flange as the skin would do.

Minor notes. Your mount rivets are in tension, so at least use a strip doubler on the opposite side of the rib. And turn the servo pushrod bolts around.

Do-overs suck

OK fellas, thanks for the feedback. I was quite certain that I could've done a better job after I finished last night (hindsite being 20/20, etc). So, I rebuilt the mounting plate since I screwed the original one up removing it today. I ended up using screws and nuts to attach it instead of rivets because I had to drill the holes in the ribs oversize (since I screwed them up, too). Needless to say, removing rivets with a 90-degree angle drill doesn't allow for perfect work.

Below is my new-and-improved mounting location. I am using a shorter control arm, and it's in the same bay as the bell crank. No need for a new access panel. Thanks for your help.

lostpilot28 said:

OK fellas, thanks for the feedback. I was quite certain that I could've done a better job after I finished last night (hindsite being 20/20, etc). So, I rebuilt the mounting plate since I screwed the original one up removing it today. I ended up using screws and nuts to attach it instead of rivets because I had to drill the holes in the ribs oversize (since I screwed them up, too). Needless to say, removing rivets with a 90-degree angle drill doesn't allow for perfect work.

Below is my new-and-improved mounting location. I am using a shorter control arm, and it's in the same bay as the bell crank. No need for a new access panel. Thanks for your help.

Click to expand...

Much better.

I can't tell by the photo, but did you attach the servo to the mount plate with platenuts or regular 'ol nuts? If you didn't use platenuts you might still have a bear of a time removing the servo (how would you get a wrench in there to hold the nuts?).

Also, if you will turn the pushrod bolts around to where you can see the nuts it will greatly facilitate inspection in order to assure the fasteners haven't loosened. I'm also assuming the aileron bellcrank is in the neutral position since the servo arm is neutral, and that the servo pushrod can't jam at full travel against the two bellcranks (looks like you have a larger diameter pushrod than standard).

Oh...one more thing....make sure you have at least 1/2 of the threads of each rod end bearing inserted into the pushrod ends. This will eliminate the possibility of the pushrod coming apart if the lock nuts somehow loosened.....still a little concerned about that big pushrod jamming at full travel. That is the primary reason why the standard pushrod is smaller (or is your pushrod what is being supplied with the servo now?).

Believe us, getting this right will pay huge dividends down the road once the plane is in service.

Hey Sam

You are good!

Sam Buchanan said:

Much better.

I can't tell by the photo, but did you attach the servo to the mount plate with platenuts or regular 'ol nuts? If you didn't use platenuts you might still have a bear of a time removing the servo (how would you get a wrench in there to hold the nuts?).

Also, if you will turn the pushrod bolts around to where you can see the nuts it will greatly facilitate inspection in order to assure the fasteners haven't loosened. I'm also assuming the aileron bellcrank is in the neutral position since the servo arm is neutral, and that the servo pushrod can't jam at full travel against the two bellcranks (looks like you have a larger diameter pushrod than standard).

Oh...one more thing....make sure you have at least 1/2 of the threads of each rod end bearing inserted into the pushrod ends. This will eliminate the possibility of the pushrod coming apart if the lock nuts somehow loosened.....still a little concerned about that big pushrod jamming at full travel. That is the primary reason why the standard pushrod is smaller (or is your pushrod what is being supplied with the servo now?).

Believe us, getting this right will pay huge dividends down the road once the plane is in service.

Click to expand...

Thanks Sam,
I took your advice and turned the bolts on the pushrod end bearings around. The pushrods are made from the materials sent with the kit direct from Trio. I did add a washer on the bellcrank side to allow for a bit more spacing to prevent the pushrod from touching the bellcrank...but I don't think it'll ever rotate that far.

The mounting plate is held on with nylon lock nuts. I made sure that I could remove the nuts by reaching on the far side of the servo and fitting my hand through the lightening hole to get a wrench on the nuts. I know this is less ideal than using nutplates, but I don't plan on removing the entire servo, and if I do it will (hopefully) be only once.

The bearings are threaded about 2/3 in on each side.

Lastly, the aileron bellcrank is at the neutral position...I have the bellcrank lock plate in place in the photo (you can see it's still covered in blue plastic).

Thanks again for everyone's help!

good job

Sonny, I saw your post late last night. I thought to myself...he's going to want to swing that into the inside bay.
Funny, I didn't want to be the bearer of bad news. Your install looks (virtually) identical to mine now. Nice job Now,
where will you install your adel clamp for your harness? Hint, hint...prompt, prompt

Just a note, I discovered after I mounted mine just like this, that when the wing is mounted, the main aileron pushrod tube cannot be inserted(or removed) through the wingtip area without taking out the servo and mount bracket, I elected to mount my wings then push the pushrod into the wing and fuselage from the wingtip rather than trying to fight the pushrod alignment to the fuselage while trying to install the wing to the fuselage. I used nutplates on the ribs so I didn't have to get at the nuts to take the bracket out. It wasn't too much trouble to remove it like this.

Sam is right of course; placing the servo pushrod nuts on the bottom will make them a lot easier to check. However, there's a structural reason to turn them around.

We tend to find a lot of rod ends installed in single shear. They're really a crime against nature. The load is applied to the joint eccentrically; the load path includes a lever arm equal to half the width of the ball. The result is a bending force on the bolt.

You might argue that the shoulder of the ball provides some support against bending. Ok, fine....in which case it is a shear joint.

Here's the kicker. The weakest point in any bolt is the root thread (red in the following illustration). If you install the bolt with the head located at the free end of the ball, you guarantee the root thread will be in the area of the greatest bending or shear load. Yes, if you're very careful with bolt length and washer arrangement you can get a reasonable result, as in the "ok" illustration. If you're not so careful you'll build in a disaster waiting to happen. The difference is a matter of a few 1/16th's in bolt length and washer placement given the thin material in aircraft control arms. Are you that careful?

Ok, flip the bolt around. Now the root thread is in a nice safe place where it can never see shear or bending loads.

Yeah, I know, that servo pushrod doesn't see much load, and yes, it will probably be fine forever. I'm simply encouraging a good habit. You'll find more highly loaded single shear rod ends in other designs. A little thought about how to install the bolt is sure to pay a dividend somewhere, sometime.

DanH said:

Sam is right of course; placing the servo pushrod nuts on the bottom will make them a lot easier to check. However, there's a structural reason to turn them around.

Click to expand...

Great info, Dan! Thanks for sharing.

lorne green said:

Sonny, I saw your post late last night. I thought to myself...he's going to want to swing that into the inside bay.
Funny, I didn't want to be the bearer of bad news. Your install looks (virtually) identical to mine now. Nice job Now,
where will you install your adel clamp for your harness? Hint, hint...prompt, prompt

Click to expand...

Hi Lorne,
Thanks...it's nice to know I'm not the only guy with this setup! I installed a small nylon clamp on the backside of the mounting plate. I wrapped the wires with a thick layer of insulating tape where they pass through the lightening hole in case they chafe, but the clamp holds them very still, and I doubt they'll move. I hope it works out!

DanH said:

Sam is right of course; placing the servo pushrod nuts on the bottom will make them a lot easier to check. However, there's a structural reason to turn them around.

Click to expand...

Dan, you're a plethora of useful information. Keep it coming! I like it when I learn something new.