Improved flap pushrod attachment

I had two problems with the flap pushrod attachment to the flaps on my -8. The main problem was that the geometry was such that I would have had to cut a bigger hole in the fuselage than I wanted. I tried spacing the rod end out (inboard actually) from the face of the end rib on the flap, and that helped, but I was running out of length on the threaded post of the integral rod-end and post. It also meant that the threaded portion of the post was bearing on the flap rib.

The second problem is that the integral rod-end and post arrangement does not have any positive capture feature that rod end bearings are supposed to have in case the bearing fails. This is probably very rare, but what the heck, follow AC 43-13 when you can. So here is what I did:



I used a normal rod-end bearing, and made a spacer sleave to fit over the AN3 bolt, plus the appropriate washers, including an AN970 washer to act as a positive retainer in case the bearing fails. The spacer is roughly 0.300 long, so it improves the geometry of the pushrod passage through the fuselage, allowing a smaller hole.

In case someone is worried about the signs of paint wear on the pushrod, don't worry, its not touching the fuselage during flap operation, I triple-checked. Those are paint scrapes from the initial installation, which had to be done a few times to get the fuselage hole right. I need to touch up that paint, because these are 4130 steel pushrods.

I wouldn't worry too much about the paint on the push rods Steve - they'll eventually have a nice fine coating of engine oil, and be just fine....

Paul

Safety Issue?

scsmith said:

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I admit I'm a neophyte in the area of plane building/maintenance but it seems to me from this photo there might be a risk that the bolt could back-out with potentially catastrophic results. May I suggest drilling the bolt-head for safety wire or at least adding it to your preflight checklist to ensure its security?

Did the same

I have made a similar modification using the heavier hexagonal pre-threaded flap push rod found in Van?s catalog as part number VA-256. The modification not only provides improved strength and security but was in my case necessitated by the geometry of the actuator weldment. The integral stud on the supplied rod end bearing was not long enough to allow sufficient clearance between the rod and fuselage and still have the required number of threads engaging the nut.

Speaking of the nut. My quick built flaps were of the older style requiring the use of a free nut on the stud, a difficult install maneuver at best. I managed to install self locking single leg nut plates similar to the updated design found on later model flaps. I used CCR264CS3-4 pulled rivets available from GAHco to attach them. Very secure and I will be able to check for appropriate torque easily.

I agree that a check of control linkage integrity including flap linkage should be a part of every preflight and is something I do before each flight in my Cardinal.

Don't worry...

...That bolt has a AN 365 lock nut on the back side. It is pretty tricky to get a wrench on the nut, as most RV builders can tell you. I made a special open-end wrench to do it.

Another poster described putting a nut plate in place of the elastic lock nut. As long as the nut plate has good lock-nut friction, that would be fine, and yes, way easier to assemble.

FredMagare said:

I admit I'm a neophyte in the area of plane building/maintenance but it seems to me from this photo there might be a risk that the bolt could back-out with potentially catastrophic results. May I suggest drilling the bolt-head for safety wire or at least adding it to your preflight checklist to ensure its security?

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Flap rod rod end bearing

Steve,
Going through archives and found this post. I?m installing my flap rods and have run into the same issues with the rod end bearing running out of enough tread after washering out from the flap end rib. This is a RV4. I?m sending my drawing, I?m having trouble understanding the ?spacer? added to get the rod spaced further inboard. Did you use a different rod bearing than specified? Thanks for any pics drawings or info you can share. I could make this work, it?s just a lot more hole running up the fuse than I wanted.
Mike

After I made these I was shocked at how thin the material is after threading, if ONE ever failed on final close to the ground, you would have your hands full with an unexpected roll, I went with the hex rod, more beef!

Posted this recently in the F-4 Raider thread concerning my conversion to the RV-8 style flaps, but seems applicable here.



As with the -8, the torque tube runs behind the back seat and the arms go forward. The torque tube itself is a simple weldment using stock I had in the shop. As stated above, you can see the bearing brackets I made from angle extrusion. The axle is a Hi-lok fastener. The brackets bolt to the lower longerons, and the spherical bearings allow the splay. I had to make new link arms from steel tubing, and despite the fact that they are longer than the solid aluminum rods that were there before, are slightly lighter. I drilled out and pressed the integrated stud out of the lower link bearings so I could use a larger diameter (0.750) spacer and through bolts for better load carying ability. The stock actuator was set up for the motor to mount horizontally, so I fabbed up a link to change orientation. The upper mount bolts to the rear seat bulkhead next to the pasenger's left shoulder, and a simple clip ties it into the upper longeron behind the bulkhead.

Like I said: less parts, much beter geometry, stronger, lighter, ....much gooder!

bret said:

After I made these I was shocked at how thin the material is after threading, if ONE ever failed on final close to the ground, you would have your hands full with an unexpected roll, I went with the hex rod, more beef!

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It's never happened and there are many examples of flaps dropped accidently and overspeeds that show the "tapped tubes" to be more than adequate, well past the designed loads. Somebody analyzed this many years ago and it was surprising how much load/stress the "per plans" rod could take. Your flaps would probably sustain unrepairable damage before the rods go. However, the pretapped hex tubes are readily available so if that makes you feel better, but a scenario as you described isn't realistic.

bret said:

After I made these I was shocked at how thin the material is after threading, if ONE ever failed on final close to the ground, you would have your hands full with an unexpected roll, I went with the hex rod, more beef!

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A standard (built per the kit/plans) flap push rod has been previously statically load tested in compression (the only high load mode that these parts ever see).

The test push rod failed by buckling at the mid point of the tube (expected), at a load value wayyyy above the calculated load value induced by the flap at Vfe.

I believe your statement about the strength of the plans rod, my problem was trying to tap the threads in the tube, if I had access to a lathe maybe, but I did not trust my hand tap method and they did not come out as good as I would have liked, so I went with the ready made option.