Side Skin Install Part 2

Posted On July 15, 2020

I was only able to log about an hour on 14 July. I started to install the long side rail angle (CF-ANG-010-R-C) that runs the length of the side skin, as well as a number of gussets that connect this rail to the uprights. The gussets share a number of holes with the skin, uprights, and the side rail, but not every hole was drilled to the correct size so I had to spend a little bit of time with a #20 reaming bit.

I happen to notice something that caused me to panic for a moment. As per my last post, I had dimpled all the holes forward of the wing spar, to match the Sling TSi instructions. The holes are drilled 3.2mm, and I used an appropriate 1/8″ dimple die.

The problem I noticed is that the manual for the TSi calls for the installation of 4.0x10mm countersunk rivets in these holes – not 3.2mm!

I’m not sure exactly why the rivets were upsized, but likely the larger engine up front on the TSi along with higher cruise speed introduces additional shearing loads through this part of the skin, which is mitigated by the larger rivets. Also, based on the part description, these are “standard” single grip-range rivets, not the multi-grip style. While the multi-grips are wonderful for fitting many different grip length needs, the compromise is about 25%/15% lower shear and tensile strength, respectively, compared to single grip-length rivets.

I’m not planning on putting a 915 engine up front, but I am motivated to adopt some of the TSi design improvements, especially if this change improves the strength of the fuselage. I have an email into Sling tech support to find out more. EDIT: I received a reply from Mike Blyth, check out this post for the details.

I previously posted a long write-up into about my research into rivets back in October. The rivets I chose (POP AK41H) are a close-end variety, which are stronger than open-end rivets of the same size. Most importantly, the AK41H rivet is comparable in strength to the 4.0mm Gesipa rivet (HW-RIV-253-X-X). So, the conclusion is this section of the fuselage will be stronger than if I had used the standard 3.2mm domed rivets, and should be very close to the strength achieved on the TSi with 4.0mm rivets.