Check Your Autopilot Servo Arm Screws!

[dynonsupport]

With the off the shelf TT and Dynon design the SHAFT and SCREW always rotate together. Or at least they are supposed to, since Pat's whole problem is that they didn't and the screw came loose. The screw goes into the shaft. If the shear pin breaks, the ARM can rotate around the shaft, but cannot fall of the end of the shaft due to the screw.

Safety wiring the screw to the arm means that now the arm and the screw are one. Now as the shaft rotates, the screw will unscrew or tighten. This is bad. It's the exact opposite of what is supposed to happen when the shear screw breaks.

On the other note, there are two screws, the arm screw and the shear screw. What we are discussing here is the arm screw. But the shear screw is what keeps the arm in relation to the shaft. Failure or loss of a shear screw should be no big deal.

Breaking a shear screw does happen, but it normal situations, all it causes is lost use of that servo (like it did to Paul/Louise today). But if you safety wire the screw to the arm, then it becomes a very serious situation.

[rv72004]

Originally Posted by dynonsupport

With the off the shelf TT and Dynon design the SHAFT and SCREW always rotate together. Or at least they are supposed to, since Pat's whole problem is that they didn't and the screw came loose. The screw goes into the shaft. If the shear pin breaks, the ARM can rotate around the shaft, but cannot fall of the end of the shaft due to the screw.

Safety wiring the screw to the arm means that now the arm and the screw are one. Now as the shaft rotates, the screw will unscrew or tighten. This is bad. It's the exact opposite of what is supposed to happen when the shear screw breaks.

On the other note, there are two screws, the arm screw and the shear screw. What we are discussing here is the arm screw. But the shear screw is what keeps the arm in relation to the shaft. Failure or loss of a shear screw should be no big deal.

Breaking a shear screw does happen, but it normal situations, all it causes is lost use of that servo (like it did to Paul/Louise today). But if you safety wire the screw to the arm, then it becomes a very serious situation.

Should the arm screw just be locked in place with thread lok ? There doesnt seem much more one can do. Maybe a bracket attached to the servo which acts as a gate to the arm and screw is the only solution. Or TT can send out shafts with a castelated setup.

[dynonsupport]

Only TT can answer what they believe is the proper fix in this case. When we designed our servo we didn't feel there was any other way to make the setup completely safe except for the castle nut solution.

[sneedrv-6]

Where do I find the drawings above dates 8-29-08? I had a pitch servo installed about a week ago. I just took a look at the servo and noticed there is NO safety used on the install and on the rod linkage there is not a nut on both sides of the shaft. The drawings on Trutrak's website do not show these items. I need to know where the drawings for RV-6 Pitch servo install as show below are located.

Thanks,

Jason

Originally Posted by rvmills

Dan,

I understand what you're saying (place the bolt so that the pressure from the servo arm and rod are applied to the shank, not the threads, correct?), and I see that Pat has his TT servo installed as you are suggesting.

I do appreciate you watching my six, and after reading this, I went back to the install guides to see if I had followed them correctly. The diagrams actually show it as I have them installed, with the bolt head on the side of the connection rod bearings, so if I may, let me bounce a Q off you and the group, as I do want to do this right (best practices), and really respect your opinion:

The way the diagram has it, the bolt head is on the rod bearing side, which is captured by the large washer. Then there is a small washer on either side of the servo arm, followed by the nylok fastener. In looking at it, that puts the bolt shank under the push rod. It shows the same assembly at the bellcrank.

I guess my question is, where do you think the most pressure is applied to the bolt, at the connecting rod bearing, or at the servo arm and bellcrank? Since the servo arm is the driver, I sure can see your point. What do you think after looking at the diagram, posted below?



The diagram for the Dynon roll servo shows the same orientation for the servo arm connection (bolt head on the rod bearing side, bolt shank under the rod bearing), but the opposite orientation at the stick attach point (bolt shank under the aileron push tube connection, nylok at the rod bearing side). Here's a pic of that diagram:



Any thoughts after seeing the diagrams? Like I said, I respect your opinion, and would like to have this put together in the strongest and most reliable way! Thanks again very much!

Cheers,
Bob

[Dave Cole]

Jason, the drawing you asked about for the servo installation that includes a castle nut and cotter pin on the output shaft of the servo is a Dynon servo, not a TruTrak.

[cbo111]

Do most of you guys with TT's or trio's engage the autopilot prior to takeoff and move the controls to ensure you can overcome the autopilot? This procedure was drilled in to me when learning to fly Cessnas many years ago. I've looked over some of the POH's listed on this website but don't see any reference to checking the autopilot.

Chuck Olsen
Tehachapi
RV-7A

[dynonsupport]

Jason,
As Dave mentioned, those drawings you see are Dynon install drawings, not TruTrak. But if you want to have a look at them, they are all on our website. Obviously, they assume you are using our servo and hardware, so they may differ quite a bit from what TruTrak gives you.

http://dynonavionics.com/docs/suppor...mentation.html

[Sam Buchanan]

Originally Posted by cbo111

Do most of you guys with TT's or trio's engage the autopilot prior to takeoff and move the controls to ensure you can overcome the autopilot? This procedure was drilled in to me when learning to fly Cessnas many years ago. I've looked over some of the POH's listed on this website but don't see any reference to checking the autopilot.

Chuck Olsen
Tehachapi
RV-7A

Chuck,

I can speak concerning the Trio servos. The pre-flight check is certainly good practice but not particularly critical due to the design of the Trio servo. If you remove power from the Trio control head (flip the power switch) a solenoid in the servo drops out and completely disconnects the gear train from the aircraft controls. At that point the servo has "disappeared" from the control system. Even if the servo isn't released (sudden evasive action needed) the servo has a clutch that is very easy to overpower, and no damage is suffered by the servo. In testing the Trio devices I have intentionally hand-flown the plane through aggressive maneuvers with the servo engaged to make sure I can easily fly the plane in that mode if necessary.

The Trio servo drive system is similar in design (but much beefier in execution) to the old Navaid servo which no doubt has the safest failure mode of all the current servos. Another safety feature of the Trio system is how the autopilot will automatically disengage once the aircraft starts moving if you inadvertently leave it engaged following a pre-flight check.

[Bevan]

In the previous Dynon diagram and in the photos, you have large penny washers on the rod end bearing. My Trutrak drawings don't show this "large" washer. Why such a large washer?

Bevan
RV7A wiring

[CESSNADON]

The "fender washer" is often used on ball rod ends to maintain the integrity of the connection if the the ball drops out of the rod end.