32-03: Initial Fit of Baggage Skins / Baggage Tie Downs

Initial Fit of Baggage Skins

Moving on, i thought it was a good idea to test fit the baggage floor skins to see how they fitted. However, no matter what i tried i could not work out how to get these to go in! They simply would not fit over the seatbelt lugs. The skins felt like they were being snagged by the fuselage side structure - i could not slide them forward enough to get them to slip over the ends of the seatbelt lugs. 

I was so annoyed by it - i asked for more help on the forums! However, i needn't have bothered asking. It was just a case of me not thinking things through clearly enough!

It turns out, i had under set the rivets holding the 2 halves of the seatbelt lugs together. The hole in the skins for the seatbelt lug was hanging up on the rivet tail, preventing me sliding the skin forward enough to get it to slip over the ends of the seatbelt lugs. I had thought it was hanging up on the fuse side structure. 

Once i squeezed the rivets a little more, the forward part of the skins could be bent upward, and this allowed the whole skin to slide forward and pop over the end of the seatbelt lugs. They fit very easily actually.

Oops. 

Baggage Tie Down Rings

Van's sells baggage tie down rings but i was able to source some BOAB OLTD5 - Tie Down Ring and Base. These take a 1/4" countersunk screw, so i purchased some AN509-416R11 structural screws, and corresponding nutplates - MS21047L4K. 

The nutplates have a 1" space between rivets, and the tie down ring bases also have a 1" space between the mounting holes. The idea is that i will make up some skin doublers using some sheet i have the same thickness as the skin. The doublers and the nutplates will be backriveted to the skin before the skin is installed with pull rivets. Then i can just screw the bases in after the cockpit is painted (and they can be removed at a later date if required). 

I started by laying the baggage skins in place and working out a plan for where the tiedowns could go, and where i was able to fit a doubler under the skin which would not interfere with any of the underlying structure. 

I elected to place a pair of tie downs just next to the seatbelt lugs, as these would end up out of the way of where i wanted to mount the smoke tank, and another pair in the very after outboard corners of the floors. 

Baggage floors finally in place! (headslap)

The nutplates show the approximate location of where the tiedown will end up.

This is where the forward bases will go - adjacent to the seatbelt lugs. Also shown is the first go at laying out a doubler, by just marking it out with a pen.

Tie Down Doublers

The process to accurately make the doublers was interesting. Initially, i just marked out where the holes needed to be and drilled on my marks. I did not back drill using a nutplate etc. What i ended up with was a set of holes very slightly out, such that the rivets did no go in straight. I needed a better system! 

What i ended up doing was following the process below:

• I cut a piece of sheet the same thickness as the skins, to act as a drilling template and marked where i wanted the 2 screw holes to end up using a #40, and drilled these. These were marked to be 1" apart, using calipers.
• I laid out and drilled 2 other rivet holes, which will serve as reference holes in the initial stages, but will end up being rivets. 
• I then drilled these 4 holes into another sheet of aluminium the same size, which will act as a sacrificial template piece. 
• I took the sacrificial template piece and up-drilled the 2 holes where the screws go to 1/4". 
• I used a spare AN4 bolt and screwed this into a nutplate. I then cut the head off the bolt with a dremel, cleaned it up, and added a slot in the head for screwing it back out later. 
• I was then able to use this as a nutplate jig in my sacrificial template piece, to drill the rivet holes for each of the 2 nutplates. 
• Once i had the sacrificial template piece made, i then clecod it to the first template piece using the 2 reference holes. I was then able to accurately transfer the nutplate rivet hole locations

What i ended up with was a drilling template that only had #40 holes, but laid out each of the other required holes. The advantage of only having #40 holes is that they can be accurately match drilled, more so that a 1/4" hole in the thin sheet.  The template includes the screw hole (which gets up-drilled to 1/4" later), the nutplate attach holes, and other additional rivet holes to add strength to the doubler when it attaches to the skins.

To test that the drill template was accurate, i match drilled a scrap piece, dimpled it, riveted on some nutplates and screwed a base into position. It worked great! 

I removed the bolt from the 'nutplate jig' and drilled out the rivets from the test piece as i only ordered the minimum number of nutplates i needed (they are $1.86 each). 

Here is the drill template on the bottom, and the 2 doublers on the top. They have the 2 reference holes in the middle vertically, and the 2 screw holes already drilled, but no nutplate attach holes.

This was the home made nutplate lig

The nutplate jig was used to match drill the nutplate attach holes to a sacrificial template piece. 

Here the sacrificial test piece is on the top, and you can see the drill template on the bottom. I was then able to use the sacrificial template to transfer the nutplate attach rivet holes to the drill template. Alignment is maintained by the 2 centre reference holes (clecod)

Here a piece of scrap was drilled using the drill template, dimpled and nutplates riveted on. The holes on the left where where i did my initial test based on a template just marked out and drilled. The rivets were slightly off and this was not a good approach.

A test fit shows the drill template works great. 

Once the doublers were made up, i match drilled them to the skins in their correct locations.

One screw hole was drilled in the skin in the required location, then the drill template was clecod on and match drilled. 

I made up doublers for the rear baggage tie down locations, and was able to use the forward doubler drill template to accurately mark out the 1" spaced screw holes, then rotate this template to match drill where the nutplate attach holes will go.

The aft tie down doublers were cut to fit in the small area between the ribs in the aft outboard corners of the baggage area. They were marked out, and i used the drill template to match drill them. I added additional rivets for extra strength. I then placed these in position and match drilled the skins. 

The doublers were clecod to the skins, and the skins test fit into the aircraft to make sure the doublers would not interfere with any structure below the floors. 

The positions of the clecos show the locations of the tie down ring bases.

These doublers will eventually be primed and back-riveted to the skins with AD3 flush rivets, including riveting on the nutplates. Then after the cockpit is painted, i can simply screw the tie down bases in place.