February 14-18, 2010

More on the Elevator Pushrods, Cabin Frame continued - 3.0 hrs.

After being sick for the better part of a week, I'm finally getting to the point where I feel like working on the project again. I was very discouraged by the rubbing pushrod problem. I'm not sure what made me feel the most sick; being sick, or being sick about this problem. But I motivated myself enough to get out there and dismantle the flap motor housing so I could see underneath and try to at least diagnose what was going on. Then I took some pictures. Then I wrote to Vans and posted some questions on the online forums. The tremendous response by my online friends really lifted my spirits. I never cease to be amazed by the RV community. What a great bunch of people! So anyway, here's what was going on.

The forward pushtube was rubbing on the bottom of the hole through the F-705 bulkhead. When you study how this works, you see that as the pushtube moves forward and aft, it also goes up and down as it swings through it's motion. This is because of the circular movement of the bellcrank in the rear and the pushing of the arm on the forward end. The tube reaches it's maximum "up" motion at the extreme ends of the travel - forward or aft all the way. It reaches the bottoming of it's motion at the middle, or neutral positioning of it's movement. So unfortunately, the stick was rubbing where it would be spending most of it's time... near the middle or neutral position. Here's a shot of the forward pushtube at maximum forward or aft, in it's highest "up" position:

There's about 3/8" of clearance above the top of the tube. Much more than it will ever need. In the neutral stick position, however, seen below (sorry for the blurry picture) it rubs on the bottom of the hole:

I put an inspection mirror under the tube in the bottom of the tunnel and took this picture. You can see how the rubbing of the tube is scraping the primer off the tube.

Now, how is it that there can be too much clearance at the top of the tube, but not enough at the bottom? This is the question I posed to Vans and to the online forums. I think it's because the short descending arm on the control stick weldment is too long. If it were about 3/16" shorter, it would position the pushtube in the middle of the space and you would have about 1/8" clearance on top and bottom. So did I get a bad part, or is there just that much variance? How long is this arm supposed to be? I measured and asked about it. I didn't get an answer to that question. So what am I to do? Take the weldment out and take it to a welding shop to shorten it a tiny bit? Or just file the hole open a bit more to provide the necessary clearance? I was very nervous about the idea of taking a file to this bulkhead. The thick stiffener passing underneath the bottom of this opening is the rear wing spar carry-through stiffener. This is a rather critical structural part of the airplane, and I didn't want to risk weakening it.

Both the technical support at Vans and my friends online said I should just file the hole in the bulkhead open a little more so it won't rub. Vans said as long as it's less than 1/8" and properly radiused and smoothed, it would be okay. I also learned that this is a very common problem. I didn't know that.

So, long story short, I dismantled everything and took the pushrods out. Then using my dremmel tool with a small sanding drum, I filed away a tiny bit of the bottom of the hole. Less than 1/16" was enough. Here's a picture after I smoothed it with my die grinder and a fine scotchbrite wheel:

Then I used my Q-tip method, with a Q-tip soaked in primer, to apply primer to the area:

Looks good to go!

Then I took some time to spray a new coat of primer on the pushrod where it had rubbed off. While all this was drying, I went back to the cabin frame and picked up where I left off with the angles and pieces that will hold it to the fuselage.

These angles are clamped to the roll bar and to each other to set the spacing, and then these two holes are drilled for a pair of keeper rivets. These rivets will hold them together at the proper spacing until it all gets drilled and bolted to the F-705 bulkhead.

After the usual deburring, countersinking and prep work, the keeper rivets are squeezed in place.

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Contact me: swayze "at" europa.com (replace "at" with the @ sign... no spaces... you know the deal)