Category: Empennage

June 12, 2011  Dan and I have been struggling with the bottom nose rib on the vertical stabilizer.  Dan  fabricated a form block that matched the plans exactly.  When we installed this rib the skin would not sit right, it had a flat or shallow dish.  When I went to rivet it the stems of the pull rivet would break off meaning the ball had pulled up too much.  We are using flush rivets so the rivet holes were dimpled.  We had to Dremel tool the stainless stem off and drill out the poorly set rivet, a real pain.  I think part of the problem is that the natural curvature of the skin does into sit right on the rib, the rib pulls the skin in.  Here is a picture of the rib from the plans sitting in the frame with the skin:

Notice the gap between the rib and the skin towards the top.

Dan made some CAD layouts on Autocad increasing the radius at the front of the rib.  Dan made a new form block and made the rib.  Here is a side by side layout.  The primed one on the right is the original design from the plans.  The left one is the new rib

Here is the new rib sitting in the frame with the skin.  Notice the closer fit.

Fast Forward to Oshkosh 2011.  I walked the flight line looking at all the completed Sonex planes and noticed that almost all of them had the awkward skin layout at the top of the bottom rib.

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Not sure of the exact date March – April 2011.

We riveted the structure together and the skin on on seperate days.  Also riveted the Rudder.  Here is the assumbly test fit together.

The fiberglass tip was not straight from the factory at the trailing edge.  Dan used a heat gun to heat up the tup to bring it in.

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We decided to remake a nose rib for the vertical stabilizer.  Here is what we have learned

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Need to trim the front off and clean it up.  Basic forming time is five mintes or less.  Of course it took us a few ribs to figure the process out.

Not really sure of the specific date of this work, let’s just say it was between Feb and May 2011.  Dan I and wanted to have access to the vert stab tip in case we wanted to put a position light or an antenna later on.  Dan found some light weight plastic electrical conduit that he heated up to put a bend in to go from the bottom rib to the top rib.

Worsk great, now hoe to make the tip removable.  One method uses an upside down rib attached to the top rib so you have a flange to put nut plates on.  Dan thought of a different solution of making a sheetmetal strip that goes between the skin and the top rib and it projects up to allow a flange for the tip to attach.  I’ll try and find a better picture:

The vert stab top rib is on the bottom, the new strip is on the outside and the stip extends up under the fiberglass tip.  A great feature of this solution is that it supports the nose of the tip all the way around.  Here is the back end of the tip

Here is Dan holding the complete assembly.

So the fiberglass tip will butt up against the skin instead of going under it.  Dan did a fantastic job of mating these to edges.

Where the silver clecos are above we will have nutplates behind the stip so washer head screws will secure the tip on.

Here is the horizontal alignment and end alignment with the rudder

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With the Stabilizer skin match drilled the next thing to do before priming and final riveting is to fit the fiberglass tip to the tip rib.

Having read a few articles on Sonex web sites related to the problems with this task, I expected (and found) that it was not as simple as just slipping the fiberglass tip to the end of the stabilizer and match drilling it.

First, the overall length, particularly the length of the fiberglass that is left hanging past the trailing edge of the stabilizer, should line up with the rudder trailing edge.  While this is purely an aesthetic issue, there is at least one post where the builder installed the tip only to find out after mounting the rudder that the fiberglass tip overhung considerably farther than the rudder.  It was not a “look” I wanted to copy.  The builder ultimately  removed and refit the tip by modifying  it.  His closing comment was that he spent more time fitting  the tip than constructing the vertical stabilizer.

There is a dimension on the plans that shows the trailing edge of the tip should extend 12-5/32″ beyond the edge of the vertical stab.  Not surprisingly, this is equal to the rudder tip to the center of the hinge.  This reference mark was one that I put on the fiberglass tip to avoid it as I tried to fit it.

Below I show that if I did not put the tip rib in place I could install the fiberglass tip and push it far enough forward to obtain the requisite 12-5/32″ overhang.

The initial fitting of the fiberglass tip would slide under the stabilizer skin without the rib in place and actually fit pretty well with regard to the leading edge of the skin fitting tightly with the fiberglass while at the same time having the tip extend the required 12-5/32″.  The fiberglass tip was a bit narrower near the leading edge than the tip rib and therefore would not slide under the skin with the rib clecoed in place.

I was ready to either cut the fiberglass tip and reshape it to fit the tip or else consider building a narrower tip rib.  I mentioned my dilemma to a friend at work who had a hobby /  side business of building high performance rods and had experience with fiberglass fitting.  He told of a similar story of getting some ground effect fiberglass that fit on one side of the car but not the other and found that he could simple heat the parts with steam and then quickly mount them on the car while still hot and re-mold them to the proper shape.

This, of course, made perfect sense (I had read the issues with fiberglass airframes and the issue with summer heat).  So I used a hot air gun with the rib tip as a form pressing from the inside of the fiberglass tip I heated the outside and with a little care I was able to modify the fiberglass so the rib fit inside it and it fit into place between the skin and the rib.

Below is a shot of the inside of the reformed FG tip after heating with the rib pressed forward into the now wider tip.

Top view of fiberglass without any clecos to bring the skin tight to the fiberglass.  A little pressure will easily close the gap indicating that when riveted it will be a snug fit.

The end result was that it was relatively easy to heat the FG tip and mold it to accommodate the shape of the tip rib so that it could be slide between the skin and rib and still maintain the trailing edge overhang.  I can see that the normal variation in fiberglass dimensions could require more in-depth modifications but using heat to reshape the leading edge of the FG tip was an excellent tool to keep in mind.  While the FG tip needed only 1/8 to 13/16″ of manipulation at the leading edge, this small amount created a very large mounting error at the trailing edge.  The photos might not show this issue particularly well but the change was dramatic with regard to fitting the FG tip.

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Well, after monkeying around for quite some time we finally resolved some issues we were having with plotting the vertical skin out on paper (1:1 scale) and actually glued it (with 3M 77 spray adhesive) to a piece of 0.025 aluminum, cut, filed and drilled it.   We used a  1″step  drill  bitto create the inside corners and tin snips to cut the sheet out.  Leaving a bit of material to be filed down to the line with a vixen file on the generally straight edges and a half moon round file on the inside corners was the way to go and made it easy to get right up to the edge of the lines the paer layout provided.

Of course the main advantage in plotting the skin out in paper first is to more easliy mark the numerous drill holes and then (as we had done in the fall) wrap the paper around the frame to verify the pilot holes were something close to the center of the ribs.  This along with obtaining a very good layout of the many irregular dimensions as well as getting the layout somewhat symetrical.

The original sheet we plotted and wrapped around the frame was a bit smaller than the design dimensions and so we went through numerous attempts to generate an accurate 1:1 scale plot.  As it turns out, the problem we were having was partly due to the inaccuracy of the plotter but there was a much larger issue to deal with.   Hygroscopic expansion (and contraction) of the paper.

Hygro-what you say?  A large sheet of paper will change by a significant amount in it’s dimensions when moved from high to low humidity environments.  What’s more, it will change with a very short time constant.  Short as in minutes, not hours or days.  We found this by measuring a plot that appeared to be nearly perfect when first generated in an office environment and then after sitting it in a cool garage in a Minnesota winter over night,  found that it had shrunk by 3/16″ over a 36″ length.  We then moved the sheet back to the warm and relatively humid office environment and watched it regain nearly all of it’s lost dimension over a 20 minute period.

We verified this dimensional change with one of the guys in our research lab that happened to have ran an experiment over the last few years to identify this precise phenomenon.  What we were observing agreed very closely with the experimental information that was previously done.

So, we dug out one of the original plots that was pretty close to the correct dimensions when left in the garage overnight and just glued it to the aluminum skin.  Once glued to the aluminim the paper was constrained to the dimension of the aluminum.

Before gluing the paper to the skin, we checked the alignment of the rib frame with the paper and found it to be close in overall dimensions but some of the ribs were slightly off from their design locations.  We marked which side of the line to drill the pilot holes to accommodate the slight misalignment and then verified these adjustments before drilling the skin.

We had previously drilled out an aluminum ruler to be used for drilling hinges and used this as a guide for edge drilling the trailing edge of the skin first.  We verified the location of the holes for the hinge and used the ruler to drill out the trailing edge that would have the hinge attached. We then match drilled the other trailing edge of the skin with the first trailing edge as a pattern.

The gluing of the paper to the aluminum and adjustment of the holes were done on February 7th, 2011 with Tim and I.  It was Superbowel Sunday and we both had places to be so we had to stop work.  I (Dan) did the remainder of the drilling and skin forming shown below the following night…..

At this point the two trailing edges are clecoed together and the skin can be carefully pressed down on the table with a 2×4 (I used a 1×6 instead) to form the leading edge.  With the trailing edges clecoed together it insures that the leading edge is formed in the center of the sheet.

I clamped the sheet to each side and used a sharpie to marke through the skin to the ribs to see how well the holes lined up with the rib centers.

I examined all the ribs and found that the match drill alignment was as good as I could have hoped on all ribs and on both sides.

There still is some work to do; Clamp and match drill the ribs, dimple the skin and ribs for flush rivets, prime and assemble the frame, rivet the skin to the frame.  But to a large degree these steps are “busy work” and don’t carry the concern we had prior to bending the skin.

Even though we have the horizontal stabilizer almost to the same point as well as the rudder and elevator almost ready to rivet together, this will be the first completed frame and skin assembly and it is kind of a mile stone from the fact that it represents (when assembled with the rudder) the completion of many of the procedures for scratch building.  There are still many things to do for the first time but having completed this portion is a confidence builder.

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