Cowls - Part 2

This blog covers the installation of the cowls and the issues when you use a different coal attachment method to the factory's.

Overview
The RV aircraft always seem to have amazing cowls and one of the prefered methods based on reading various blogs is the Skybolt attachments. What makes these different other than their amazing price is they are fully adjust table to zero clearance unlike others that have a working range. This would be important as the fiberglass on the cowls can vary a bit and the addon that were added for this project had a range of about a 1 mm [0.040''].

Note: This variation was a consequence of the modifications to extend the cowls

So it seemed like a good idea and a full kit for a RV4 was ordered.

Installation
After straightening the top cowl it was decider to start here - no reason just seem like a place to start.

The cowl was clecoed back into the jig, aligned with the air-frame and taped unto position at the bib cowl using gaffa tape. Next a light was fixed under the cowl and the edge of the air-frame transferred to the top side of the fibreglass using a 0.6 mm felt pen. A offset line was marked approximately 2 mm from that profile line and cut using a Dremal motor tool fitted with thin cutting disc.

Note: The epoxy had to be cut using a series of plunge cuts as it is more tacky compared to the factory supplied material

The next task was to install the mounts for the Skybolt fasteners and this was a reality  check. Skybolt fasteners require a minimum 20 mm [0.75''] flange from the firewall while the Tucano is finished at the firewall. 

A lot of time was now spent thinking but these is no way these could be used so another method was required.

It was decided to machine a length of 40 x 20 mm [1.5 x 0.75''] x 1.4 mm aluminium angle to provide a mount. A series of slots were cut using a 10 mm [0.38''] round nose mill and the Dremel cutting disc. 

After spending a lot of time on the first one it hit the floor as did the next one, in fact a full 3 meter length of aluminium was turned into scrap. 

Final effort ready to match drill cowl

At this point it was time to have a re-think, regardless it came back to what I was doing but with more notches. It was decided to mill a combination of wider slots and more slots especially at the tight radius. 

Attachment fitted with fasteners

With a new aluminium angle milled it was hand bent to profile, then using a 2 mm spacer positioned one attachment point at a time again, drilled, clecoed working down one side from the centre at a time. The actual process used is not describable but did involve bending, shaping and incantations of various types.

Fitting Cowl
Once cut the cowl was refitted at the same location, then a careful check was done on both sides and adjustments made as required to place it about the air-frame centre.

A series of tools were used to dress the edge in no order of importance : 80 / 120 grit aluminium open cut and timber sanding block ; 4 mm OD bastard rat tail file ; 20 mm wide half round single cut bastard file. 

Note: With the paper, replace as soon as you are doing the sanding not the paper as this provides the best control

Stared at the top at the centre and filed a slot carefully using the rat tail file, checking after each few cuts till the edge of the air-frame could just been seen, then moved down about 150 mm [6''] and repeated the process and when finished joined the two notches with a fine felt tip pen. 

Using the 80 grit paper the excess material was removed changing to 120 once close to the line. The fibreglass was pushed into position and material remove slowly until it slipped in with no displacement.

This process was repeated again and once completed worked the way down the other side.

Mounting Holes

When it goes wrong - STOP DIGGING

On the top cowl the holes were pitched at 90 mm [3.5''] recommended by Skybolt.  

Following the manufacturers direction it was decided to remove the cowl drill holes at 14 mm and finally to 9/16'' - dug a new hole and a deep one. 

Another day was lost trying to locate the center of the big hole into the cowl. A number of routes were tried and all failed with the holes filled with epoxy metal and time for a coffee and rethink.

It was decided to use a light as shown in the instructions but the lack of clearance between the hole centre and firewall created a parallax error at the transfer to the skin requiring 2 off the holes to be slotted. Not happy but the top collars cover this and the gap will be filled with epoxy metal at final installation.

The collars were primed and trial fitted and it seem that it would work so now to the side cowls.

Comment
The decision to use these fasteners is now a vexed issue but the die is cast. The drilling of the mounting holes will be different from the cowl the is no doubt