Install tail-skid

When doing final clecoing of the rear side skins, I noticed that the manual specified the right skin overlaying the left, in contrast to a number of production aircraft I have seen photos of, or inspected directly. Sling Have confirmed it can go either way, and so I will follow the manual as it is the better approach in my view, for the narrow band of material alongside the tail hook slot in the left hand rear skin is protected by the overlaying right hand skin.

The 4.1mm reference holes established earlier for the tail-skid rivnuts were enlarged to 6.1mm. A couple of the tabs in rib 801 were slight;ly out of round even after enlarging. To reduce the risk of a rivnut pull-through I created shims of 0.4mm 6061-T6 to lay on the tail-side of these and two adjacent rivetted tabs.

I created two other shims, both of 1mm 6061-T6. One shim is freestanding, thickening up the material for three of the rivnuts on the right hand side. The other shim overlies the full length of RF-CHL-004 so as to build its surface level to match the adjacent longeron 801. This will improve the flatness of the skins in this area.

I did not use the M4 large flange rivnuts supplied, and used just about everywhere else, as I noticed the required grip range (3.2mm) is often in excess of the maximum grip range of these rivnuts (0.25-3mm). I noticed that at this upper limit, only a very small amount of material was available for swelling. Instead I use 2-4mm grip range M4 rivnuts. These were much more successful, forming a much larger swelled region on the tail. This should amount to a more secure connection. Rivnuts were dipped in JB-Weld.

The longer grip range also helped to reliably pull the layers together, as sometimes - despite clecoing and bolting - a gap would be present between layers, only to be closed up by setting the rivnut.

I used a AN4 bolt cut down and thinned to hold the skins in place on the centre flange hole on rib 901, and also in other locations to help pull the layers together along RF-ANG-007

Some small amount of hole elongation was required for a good fit of the tail-skid, in part caused by the rivnuts not forming fully on-centre. The tail-skid holes were reamed to 4.5mm for an easy fit.

Rivnutting two of the locations alongside the tiedown bracket required a contrivance made from of the rivnut tool, as the width of the fully assembled tool was too great to fit alongside the bracket.

I initially used a 0.25-3mm rivnuts in the side holes of rib901. A combination of ill-judged force on the tool and insufficient grip range caused a pull-through of the rivnut. This was rectified, but on the left hand side the narrow band of metal alongside the 6.1mm hole on the rib901 flange cracked, however the skin layers were unaffected. All subsequent rivnutting was done with an endoscope viewing the tail formation. This small crack does not pose any issue apart from irritation. This observation inspired the move to use 2-4mm grip range rivnuts in all location and not just the centre hole of rib901 as originally intended. This single location is the only one in which the total thickness of material exceeds 3mm.

Frankly, I could not imagine easily fitting this tail skid on an existing aircraft, lying on my back, with anything like the results obtained here.