Sunday, November 14, 2010

A clever two-fer

There are any number of subtle benefits that accrue from the decision to build an E-LSA airplane as opposed to a more traditional "figure it out for yourself" experimental. One of those benefits comes from the fact that the designer of the kit knows exactly what components will eventually be installed in the airplane. I ran into a very good example of this when I built the combination battery box and oil tank holder.

Because I will be required to use a specific size of battery, and because the designer also know that I would be installing a dry-sump Rotax 912, it was possible to design a clever little box that will hold both the battery and the external oil tank that is the definitive feature of a dry-sump engine.

There are only a handful of parts involved. I started out by clecoing it all together in order to get a clear idea of how it's supposed to look.

The battery will fit into the slot behind the curved arms that will hold the oil tank. It's a deliberately tight fit, so the rivets used to attach the battery hold-down brace need to be flush to the interior. That means two things: the plates need to be dimpled (easy), and the flanges of the hold-down brace need to be countersunk to make room for the other side of the dimples. If you remember a similar operation just a few days ago when I was building the canopy ribs, you will remember that it was not all that easy. It's a stressful operation, requiring a careful balance between digging the countersink holes deep enough to absorb the entire dimple yet shallow enough to not end up increasing the diameter of the rivet hole.

I think I got it just right, but it took a lengthy series of trial fits to reach the final fit.

A quick trial fit to the firewall to make sure everything was correctly aligned.

Then a decision had to me made: rivet front-to-back through the firewall, or from back-to-front from inside the airplane. Van's provided no clue whatsoever as to their personal preference, so I opted for the easier of the two. Again, note that "easier" is not to be conflated with "easy." While it would have been a royal pain to get the rivet puller up tight against the flanges if riveting from the outside, it was no picnic to endure the contortions required to rivet from the inside out.

Here's what happens when you don't have any choice but to work in the close confines of a flange. That broken mandrel stub is going to have to be filed down.

With everything finally riveted to the firewall upper panel, I had another decision to make. The plans had been very clear that I was to rivet to the firewall upper, but had made no mention as to whether or not I should also rivet to the firewall ledge. I could see that I would have to do so from inside the plane, and that's an area that is going to get ever more crowded as I progress through the installation of the rudder pedals, etc. I opted to go ahead and do it now.

Like I said: easier, but not easy. It took painstaking effort for each and every rivet since I had to lean around to the front while fishing around with the rivet puller trying to find rivet holes.

It took awhile, but I got it done. I can only imagine what it would have been like to have to design and fabricate my own mounts - I love this kit!!

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