Thursday, July 9, 2009

Compression Strut Dance

Compression struts are funny things - six to a wing, of which four are the same length, one is slightly shorter, and one is a good deal shorter. On the front, they take AN4-12A bolts; on the rear, AN4-11A. The first order of business is lining them up and picking out the shorter ones - they go on the butt attach and the spar attach brackets, respectively. The second is to find the barrel nuts that hold the anti-drag and drag wires, and tape the suckers so they don't fall out while you're getting the struts in place.

There are 20 barrel nuts to each wing, because the fuselage-most and wingtip-most struts only have wires attaching on their insides. Forget this and you'll come up short. While you're at this step, find the shims - those funny half-moon shaped pieces of metal. Keep an eye on the shims, or you'll be searching for them most when you really want to be doing tightening. While you're at it, make sure you have enough hardware.

Third, make sure all your ribs are positioned correctly on the wing. If you pulled your built-up ribs off an old wing, you'll note some have parts cut away. This is not accidental, or damage - this is because some ribs lie under the compression struts. (This is where shims come in handy.) Make sure your ribs are on in the correct order, or at least, the ones that are under struts are in the correct order. (Check the others while you're at it to make sure your cable guides are fine, and you haven't put any noseribs or aileron cutout ribs built to go over doubler plates on other areas, and have enough for your other wing!)

Fourth, lay out your hardware. Butt attach brackets, spar attach brackets, jury strut attach bracket, aileron control horn, hinge brackets, the works. Make sure they all fit on. Because, if they don't, now is the time to figure out what needs to be fixed. Besides, the compression struts go over one butt attach (This is also where shims come in handy), and over the spar attach brackets, so you might as well have everything ready to go on.

Do you have all required AN4-11 & 4-12 hardware, washers & nuts? AN3-11A bolts, washers, & nuts (you did note that not all holes are the same, right?) Did you miss those two AN5 holes? Because few things are as time-consuming and aggravating as stopping to go on part runs multiple times a day. (Don't ask how I know)

Now, before you have bolts in the way, take the time to run a straight line up your spars from the middle of the holes for the compression struts, and mark on top where the midpoint of the holes are. This'll come in handy when trammeling the wings later.

Ok, now you're ready to put the compression struts in place! Put them on carefully, use hand power to get them close and a retractable pen with the ballpoint not sticking out to wiggle them into final correct position on the holes. This goes faster if you stick a couple bolts in as you go to hold the strut in place. Remember your shims!

How much do you tighten your nuts? Only until they contact the wood and a quarter-turn, NO MORE. Otherwise you risk crushing the grain and getting set back in time and money while getting new spars.

Next up is trammelling your wings.

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