Forward longeron gussets

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June 9, 2014 by Chris

So last friday I fabricated the forward top gussets that connect the upper engine mount brackets to the longerons.  The pieces were already shaped, so I just needed to drill a bunch of holes in the locations called out on the plans. Here’s where I tried to get a little too smart and had to adjust a bit.  On the plans, if you drill the aft-most hole in the gusset in the location specified, it doesn’t have enough “edge distance”.  Edge distance is the distance from the center of a rivet or fastener hole to the nearest edge of the material.  It should be at least 2D, or twice the diameter of the hole.  In order to get 2D edge distance from the aft end of the gusset, I moved all the holes forward ever so slightly, and patted myself on the back for being so smart.

I also drew a line down the longerons at 2D, so that I would be sure not to drill the holes too close to the inside edge of the longeron.  I placed the gussets in place and started match drilling them into the longerons and also the steel engine mount brackets that are nested inside the longerons.  What I noticed was that since I moved all the holes slightly forward, I had one hole that would be located right square at the aft edge of the steel bracket, whereas normally it would have been slightly aft of it.  Fortunately, it was a simple fix to peel back the skin enough to access the end of the bracket with a dremel cutting tool and take just enough off to allow clearance for the hole.  The correct number of rivets will still be in the bracket, with plenty of aft edge distance, so no big deal.  Shouldn’t have tried to be so tricky, though.

The next issue came on the right side.  After drilling two holes through the longeron and into the steel engine mount bracket, I found that I didn’t have near enough edge distance in the steel bracket.  This is because I had earlier removed some of the material on the horizontal angle of the bracket to help it nest better in the longeron; because of the way it was welded it was slightly deformed otherwise.  Unfortunately, this resulted in the holes that I drilled being too close to the outside edge of the steel bracket.  This was frustrating, but a much better issue than having inadequate edge distance on the longeron, because at this point replacing or repairing a longeron would be tough.  Instead, I will replace the engine mount bracket.  I emailed Van’s support about this, and they noted that it is a hard location to meet all the edge distance requirements, and that it is not as critical in the steel as it is in aluminum, however they agreed I might as well replace the bracket now, while it’s easy to do.

So, today, I drilled out all the rivets and removed the upper right engine mount bracket, and have ordered a new one from Van’s that should be here this week.  All in all, the mistake only cost be $40 and about an hour’s worth of work. Could be worse.

I also fit and drilled the two seatbelt cable attachment brackets to the longerons, and prepped them and the gussets for priming.

Upper right engine mount bracket, not enough edge distance on horizontal flange

Left upper gusset in place

Seatbelt cable bracket