29-06 -> 08: Fuselage Longerons and Cowl Hinges

After the failed first attempt at riveting the bottom skins, it was going to be a few days before i could get Dad to come back over to help. So i decided to move on with things and prepare the longerons. The plans call for firstly prepping the upper longeron, including trimming, bending and match drilling the engine mount bracket, then riveting the engine brackets to the longerons. Before riveting i wanted to prime these components. However, if you read ahead, on Page 29-09 you are then required to countersink the longeron to accept the side skins - meaning i would have to touch up and re-prime all those countersinks if i follow the plans steps in order. 

I elected instead, to cleco the engine mount brackets to the longerons and continue with the plans steps up to and including the countersinking on Page 29-09, then prime all the components. Seemed like a good way to keep the project moving while waiting for Dad to be available again. 

Upper Longeron

 Work starts by marking and trimming the upper longeron. 

Once trimmed, i made up some wooden blocks as per the plans and clamped the longeron into the vice at the required location. I clamped on my digital level and bent the longerons the required 10 degrees. To get this amount of bend, i had to bend the longeron almost 180 degrees! 

Twisty Twisty!

I then placed the engine mount into the longeron and match drilled. The holes in the longeron were #40, so i match drilled to #40 first, then reamed to #30. There are a few holes which were supposed to be drilled to #20, however i did not have a #20 drill bit; all i had was a #21!. So i drilled a test hole and found the #21 to be within the required tolerance for a -5 CherryMax rivet. Indeed, these rivets fit in the holes fine. We will see how we go once things are primed and the actual riveting commences. 

Match drilling the holes to #40, then #30. 

For a -5 cherry, the hole must be 0.160 to 0.164...

A #21 drill gave me a 0.161 hole (within tolerances)

On the small side of the range, but within the acceptable limits. Better than waiting 2 weeks for a #20 drillbit.

The upper longeron was then bolted in place to see how it fit and check the bend. 

The plans bend angle is very close. 

I was surprised the upper longeron just sits above the 2 angles attached to the bulkheads and the 2 flanges don't have any physical attachment to the longeron. I suppose the wide skin is the connected between the 2. 

Lower longeron

The lower longeron is also trimmed and the doubler is match drilled. The plans are a little confusing as to which way the flange on the doubler is supposed to align - after doing some research and coming up with conflicting build logs, i asked the question on VAF. The flange of the lower doubler is supposed to go DOWN! (Note, that this makes it hard to get the side skins on if the longerons are clecod to the skins first - more later)

Preparing the Cowl Hinges

Next up, i cut some 1/8" hinge material to the length specified in the plans, and fabricated the hinge pairs (left and right). The plans ask you to use "1/8 piano hinge" - rather than a specific part number. Thankfully i only had one piece of hinge material which was 7/8" wide (it was clear anodised aluminium) so i used that. It was marked "HINGE PIANO 1/8X6'" which was as per my inventory.  

I numbered the eyelets to as not to make a mistake!

I cut the required eyelets off using a dremel.

Clecoing on the Side Skins

Page 29-08 says to cleco the upper longeron, lower longeron and the F-01470 side frame to the side skins on Step 1. I went ahead and did this and found i could not get a bolt to go into the lower longeron. As it turns out, the fuselage side skins have a 90 degree flange which slips under the bottom fuselage skin. While the lower longeron has the doubler attached to the bottom (with the flange facing DOWN) - this flange interferes by almost 1/4" with the upward pointing flange of the lower skins. 

To get the side skins clecod into position, i had to leave the F-01470 side frame off, and just lay the lower longeron assembly in place in the fuselage. I then began to cleco on the skins and included the lower longeron assembly and the F-01470 side frame in to the cleco progression as i moved forward from the side plate. Eventually, i was able to temporarily bolt both longerons to the firewall. 

Here you can see the lower flange of the longeron doubler is hitting the upward pointing flange on the bottom fuselage skins.

The flange on the longeron doubler is almost 1/4" too long to slide past the flange of the bottom fuselage skins.

You can see here how if this longeron was clecod to the skins, there is no way to slide the skin lower flange under the bottom skins, without the longeron doubler interfering with the upward pointing flange of the lower fuselage forward skins. 

The blue line shows where the bottom fuselage skins upward flange lies.

Drilling the Cowl Hinge

Next, i followed the plans and drilled the cowl hinge. Note that the plans ask you drill a pilot hole in the hinge and initially cleco this into position on the firewall flange using the pilot hole. Then, at Step 7 you drill the hinge above the pilot hole first, then work your way down from the pilot hole, removing clecos which are in the way as you go. Step 7 asks you to begin drilling above the pilot hole - however there is an erroneous callout on Figure 1 which says "begin drilling here" at a hole way below the pilot hole. I did it as per the written instructions, and ignored the callout on Figure 1. 

I worked upward from the pilot hole first.

Drilling the upper drag fitting

Since i had the side skins in place, i decided to also complete Step 1 on Page 29-09 and match drill the side skins and side plates with the upper drag fitting to #30. This will allow me to countersink the side plate to be primed along with the side skins once they are prepared. 

Some beefy structure here!

Countersinking

As mentioned above, i was then able to pull everything apart and deburr all the holes i had drilled. I then countersunk the upper and lower longerons as per the plans, and also countersunk the cowl hinges for the dimples which will go in the cowl hinge shim later on. I was then able to clean etch and prime all these components. 

Note - i did not remove the fuselage side plate or upper drag fitting to countersink these - i did not want to enlarge the holes etc yet before i had riveted the bottom skins together, as i imagine these parts are ensuring alignment. Once the bottom skins are riveted, the side plates and upper drag fitting will be removed, countersunk and primed etc. 

Longeron parts ready for priming

Once all this work was done, i went back through the steps on Page 29-04 including attaching the wooden support leg once again. I wanted to make sure i had not missed any critical steps before turning the fuselage on its side and attempting to rivet the bottom skins for a second time!