Autopilot Push Rod Tie In

[JonJay]

I have rigged my Pitch servo directly to the Elevator Bell Crank by using a longer AN3 bolt that secures the Elevator Push Rod and some appropriate washers for spacers. Control operations seem very secure, free, and smooth. I have seen other methods of attaching the Push Rod to the Elevator Bell Crank by riveting a small tab, but my approach seems to be clean and workable. I am using the Blue Mountain Autopilot which has mulitple steps in the control arm as well as the ability to adjust sensitivity through software.
I have also rigged my Roll servo in the same manner by extending the bolt that holds the hex tube/cross over rod end. Again, free and clear and smooth.

Has anyone else done this or have any comments or concerns?

[DanH]

JJ,
Sounds ok, but a photo would show detail.

[Ironflight]

Jon,

On the surface, your solution sounds fine to me, and probably is. The only comment I would make is that since you're doing something "different" than the drawings or supplier intended, and you're dealing with the control system, make really certain that you can't get into an over center lock situation anywhere within the allowable control travel. That's what I would check for.

Paul

[JonJay]

There really is no manufactureres recomendations from Blue Mountain or install kit from Van's like I have seen for other systems. I will post some pics and see what you all think.

Thanks again for the prompt replies.

[RV7Guy]

I had mine initially set up like that. Several A&P types saw it and said this is putting undue stress on the bolt. If you got your main control pushrod on this bolt and something breaks, you're screwed.

I mounted the AP pushrod 1.5 inches below with a spacer for proper alignment with the servo.

[JonJay]

Thanks for all the responses. I am going to modify the bellcrank as I have seen done on many other RV's rather than use that push rod bolt.
My Roll Servo is another matter and I will probably need to weld a tab onto the control weldment or bracket or something like that as opposed to using the bolt that also shares the aileron push tube.
Minor set back.

[Ironflight]

Jon - one more thing you can do is go to the Tru Trak web site and download their engineering drawings for various installations - will show you exactly how they are done.

Paul

[ptrotter]

The most important thing to consider when attaching the pushrod to the bellcrank in to ensure that the geometry is correct. By that I mean that the amount of rotation of the servo arm and the bellcrank must be correct. By varying the distance from center on either the servo arm or the Bellcrank, you can change the amount of rotation of the servo arm necessary to move the bellcrank a given amount. If you get this wrong, it is possible, when manually moving the stick, to get the servo arm into an over-center condition which can cause it to jam with the cooresponding disasterous effects. You also need to ensure that the elevator hits its stops before the servo does. The manufacturer should be able to give you some guidelines on the amount of rotation is optimal for the servo and you can calculate the distance out from the center of the bellcrank to attach the pushrod to get the full bellcrank rotation with the specified servo arm rotation. On my Trio pitch servo installation, I attached the push rod to the bellcrank at the same distance out from center as the pushrod attached to the servo arm. I also made sure that all the pushrods were parrallel. This just ensures that there can be no binding due to different angles.

[JonJay]

TruTrac shows the Pitch Servo is installed with a separate hole in the bellcrank. The Roll Servo however is installed exactly as I have done, using the same bolt as the aileron push tube. I just can not see how enough stress could be introduced on that bolt shank to cause a failure through any induced loading from the ailerons in any condition, even accidental, where the autopilot servo was engaged. That weldment is also steel, so wear on the hole over time would seem to be very minimal. I could see more force on the elevator bellcrank in turbulance, and due to it's distance to center, some possible side loading, no different than using a separate hole except for the distance to the center of the bellcrank. The bellcrank is aluminum, so there might be more wear on that hole. No matter.
I will get a few more "on field" opinions. My path now will be to make the modification to the bellcrank as this makes sense and is well documented from other builders and other servo manufacturers. I intend to leave the Roll Servo installed using the aileron push tube bolt through the control weldment. The control weldment is significantly stronger than the elevator bellcrank and has a much shorter length to center of the pivot point.
The other method shown on the Tru Trac drawings is a clamp directly onto the aileron push tube. I will investigate this option too as it would shorten my push rod length for both of the Servo's

As usuall, more than one way to skin this cat.

[TSwezey]

JonJay,
Send your an email to me at IslandDes@aol.com and I will send you pictures of how we did the Blue Mountain Servo installation for the wing. I tried to put them on the forum but I can't.