RV-10 roll servo fix

[Tom Martin]

I went to install the roll servo in the right wing of the RV10 and I was not comfortable about the method of attachment for the roll servo "stop". This stop is to make sure that the arm does not move so far as to lock the control system and as such it is an important part. It is held in place by two 4-40 screws that mate with matching taped holes in the servo body. as the "stop" is partially located on the bracket a standard 960 washer is used as a spacer. This leaves only one or two threads securing this important part.
This picture shows the standard setup.

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This picture is self explanatory. You could get screws that were a bit longer but there are moving parts in the servo that could contact longer screws and at the end of the day they would still be the very small 4-40 size.

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I fabricated a bracket that would attach to the unit using the same main mounting bolts. I used a scrap of .063 20-24. The stop is attached to this new bracket with #8 flush head screws and lock nuts.

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This was a very simple fix for what I consider a dangerous installation. There are no modifications required to the mount or the servo. There is an added benefit of moving the "stop" further out, relative to the servo arm so that the arm contacts the stop better. I used the same clocking as the previous mounting and it slips right into the servo and will be much easier to install then trying to get those little tiny screws in place. I hope that Tru trak will look at this option as something that should be added to their units. Feel free to copy!
Note: the bolts have not yet been safety wired as per plans and this installation may only apply to the RV10 and not other types.

 

[Mike S]

I just used longer screws............

 

[Tom Martin]

Mike, I did mention longer screws as an option

you could get screws that were a bit longer but there are moving parts in the servo that could contact longer screws and at the end of the day they would still be the very small 4-40 screws

Also the spacer that is suggested is a AN3 960 washer which has a 3/16" hole for a 4-40 screw. And the fact that the stop is cantilevered from the main bracket to those very small screws. If this unit comes loose it will jam that control. A year or so ago there was a big uproar over a small screw that held the pivot arm in place. Tru trak has fixed that issue with a sturdy and proper elastic lock nut. Holding this important bracket on with two 4-40 screws is just not good enough even if they are two threads longer. No lock washers, no safety wire, just a dab of lock-tite..... The situation is no different from removing cotter pins on all control parts and relying on lock-tite alone.
From reading some of the old archives this stop was added due to some controls going over centre. The stop is required, but the suggested mounting method is just is not strong enough to stand the test of time.
Van himself is starting a dialogue regarding the accident rate of homebuilts vs. certified aircraft; should we not expect the same standards for our equipment?

 

[rv6rick]

Hey Tom,

I've done 3 or 4 of these on 10's and I use longer screws with lock washers and loctite. I also make up my own washers that 'fit'....as you pointed out the ones called out are incorrect. I typically file off part of the 'stops' to get them where I want them as well. Some of us recognize the error and modify/change as needed to make it safer. But, that doesn't change the fact that many 1st time builders just install it per plans without giving it a second thought. Your approach looks functional and safe as well. Good job pointing it out so folks can learn.

If the weather ever gets better down here I might have to get the Rocket out and stretch it's legs a bit .

Hope all is well in the north 40 .

 

[Tom Martin]

Thanks Rick
It took longer to post the pictures to this forum then it did to make the following adaptor plate

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The perfect fix would be for tru-trak to extend the main bracket, shown, up so that the "stop" could be fastened directly to the unit with proper hardware. My belt and suspenders fix would then not be necessary.
The angles of the stop are correct for maximum throw but care must be taken that they DO not end up being the aileron stop. This is why you likely removed a bit of material from the stop tabs. If all those little holes were slotted out then adjustment would be easy at the time of rigging.

 

[johngoodman]

What brand is it? Tru-trak?

John

 

[Wayne Gillispie]

I used slightly longer screws in mine too.

Note for Pitch Servo installation: Many need to ensure that they install a .063" spacer on the pitch servo bellcrank. When the bolt gets torqued to specs it will deform the bellcrank. I made one just like the spacer that Van's supplies then drilled the hole through it. Washers would work also.

Also the pitch servo linkage will jam if the little 1/16" torque enhancer cable breaks. It would jam in the "up elevator direction"...the worst kind of jam, if there is such a thing as a "good" jam. So inspect it often.

 

[johngoodman]

Note for Pitch Servo installation:

Also the pitch servo linkage will jam if the little 1/16" torque enhancer cable breaks. It would jam in the "up elevator direction"...the worst kind of jam, if there is such a thing as a "good" jam. So inspect it often.

I'm not familiar with Tru-Trak, but wouldn't the servo always slip if it jammed so that you could always use the controls normally? Essentially making it a non-event?

John

 

[Ron B.]

The arm on the servo is directly connected to the aircraft control system. If the servo arm gets stuck , not internal to the servo, but externally as if it were to go over center and then hit the bracket securing it, stopping the arm from moving the required amount then you lose travel in your control imput.
It will be an event.
Ron