Category Archives: Forward Spar Bulkhead

More on the subject of bulkheads

The day job and a clingy head cold have been less than accommodating this last week. That is not to say I have been idle on the project, though.

A day after my last post I got back down into the workshop and was planing on diving in with the rough cut on the firewall bulkhead, then I thought maybe I should get a little further ahead on the parts ordering. The weather has been funky as of late and I wanted to be sure I have small shop work to do that can stretch out over a couple of weeks, so I dug in with building out some parts lists. Sure enough the weather got colder and wetter yesterday and today.

The next part on my build list is the Stern Post. As far as materials go, it is a very simple part. Two pieces of Douglas fir laminated together on the narrow edge to make a wider board, then sandwich the laminate between two pieces of 1/8” plywood. The complicated part is in the beveling and drilling. The biggest concern here as far as the parts is the adhesive for laminating the wood. By far the most recommended adhesive was T-88. On to the order list it went.

The next assemblies in the construction are the spar/strut bulkheads, forward and aft. It really doesn’t make any difference which one I do first. The materials lists for each are very similar, and aside from the lumber and the T-88 for laminating, the only hardware I need is four aluminum bushings for each bulkhead. While that sounds really simple, there was a complication. Well, more of a learning opportunity than a complication.

6061 T6 aluminum Bushing StockThe bushings called out for in the plans are not an item that you buy, they are something you make. You could call up a machine shop and have them made for you, but where’s the fun in that. The plans call for four different sizes of bushing. They are paired two on top and two on bottom for the forward bulkhead and two on top and two on bottom for the aft bulkhead. Between the bulkheads, the bushings on the forward one will be subjected to greater loads than the aft and between the upper and lower bushings, the upper will be subjected to the greatest loads than the lower. Sounds complicated, don’t it?

I posed the the question what if I used the same sized bushing in all eight? on a couple of aviation forums. There were some legitimate questions, for which I had answers. I’m not going to go into all of the finer points, but all boils down to this, there does not appear to be any structural reason for four different sized bushings. In fact, using the largest/strongest bushing in all eight places reduces the aircraft weight rather than add to it. A net benefit I think. Aluminum for bushing stock, added to the parts order.

Parts OrderWhile I was at it I also ordered the bolts that will be going through those bushings. I will still need to order the washers and nuts, but there are some variables with them.

The lumber still needs to be picked up. Fortunately Doug fir and white pine are not hard to come by locally, nor is the plywood I will need for those parts. Once the firewall is roughed out and the bevels cut I will head out and pick up the remaining lumber for the bulkheads.

Sadly I have not made any sawdust so far this week. Hopefully I will be able to get at least one shop day in before turning this week’s time card over.

Until next time, blue skies and tail winds.
~FlyBoyJon