Fri Jan 2 2009

Extra fuselage stiffeners

While reading the forums on Vansairforce, I came across this post which described how Jamie Painter was experiencing oilcanning on his bottom tailcone skins. There are lots of RVs flying and none of them have had a failure to my knowledge due to oilcanning. That said, however, the RV is a monocoque design meaning that the skin is the primary load bearing member. But when the bottom skin is in compressive stress and oilcans like this, a much larger amount of load must be carried by the stringers and side skins. This makes me uncomfortable, so I decided to add some additional J-channel stringers to (hopefully) prevent the oilcanning and also provide some more load carrying members in this area.

Photo by Scott Will on 11/29/2008.

Here's the plan...I'm going to add two J-channel stiffeners along the lines where I've laid them on the bottom of the fuselage. I'll tie them into the the F-706, F-707, and F-708 bulkheads to avoid getting any skin cracks. At the aft end, I'll run them about four to six inches past the F-708 bulkhead and gradually taper the thickness down to nothing. These details are really quite important because the kind of stress that causes damage over the long or short term is known as von Mises stress (aka Young's stress or the strain energy theory). Von Mises stress is high in areas where forces transition from high force to low force over a very short distance. By keeping the structural members (stringers and bulkheads) tied together and by tapering the ends of the J-channel stringers, I can avoid these problems. Note that I'm not bothering to actually do any calculations here. I'm simply applying good practice to this modification.

I made a template to use for creating the openings for the J-channel stiffeners to pass through. The first template (bottom) didn't come out quite right, so I made the top one.

At the areas where the J-channel stiffeners intersect the bulkheads, I've got an issue to deal with. The J-channel flange and the bulkhead flange are both setup to be riveted directly to the skin. Since those two flanges obviously can't share the same space, one of them has to go. Removing a small portion of the bulkhead flange seemed like the logical choice, but that means that I won't be able to tie the bulkhead to the J-channel stringer. To get around that, I'm going to make some little doubler plates that will rivet to both the bulkhead and tie into the J-channel flange. These are the blanks of the two doublers which will attach to the F-707 bulkhead.