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

Cowls - Part 1

This blog covers the preparation of the existing cowls for installation onto this project

Overview
The cowls supplied with this kit were to suit the 912 Rotax fitted with a ground adjustable propeller. The constant speed propellor selected has a 60 mm prop extension incorporated so it meant the cowl would require modification to fit.

Installation
The first task was to fit the Flying Legends stainless steel prop jig to the Rotax propeller hub creating a fixed reference for the cowl installation at all phases. After installation the correct spacing was calculated [36mm] and the front plate adjusted using the M6 threaded rods separating the two [2] [ plates.

Note: The stainless jigs were cut locally at Laser Wizard from factory supplied DXF files

The top cpwl is secured through the 3.2 mm holes laser cut in the plate
The advantage is that because of the accuracy any set of holes can be used

Installation of the top cowl showed it might have enough length if the cut allowance was used but would require modification to remove the hump at the cut line. A steel strip was clamped to the side and the cowl edge trimmed using the factory cut lines with a Dremel cutting tool fitted with a thin cutting disc. 

Repeated at both sides 

Next the lower cowls were fitted with a gap at both sides and widest at the starboard side. Again a strip fiberglass would have to be added using the cowl side as the former.

The task started with the top cowl by cutting a series of slots using the Dremel along the factory cut line and then cutting a series of perpendicular cuts to allow the area to expand when straightened.

Top cowl ready to fiberglass

A series of aluminium channels were clecoed to the cowl and then clamped to the channels for the perimeter of the cowl. Next a width of 50mm and 75mm fiberglass tape was fitted to the underside using West Systems Epoxy. With the areas fiberglassed the cowl was refitted to the aircraft and allowed to cure.

When this cured all the clamps were removed and the cowl reposition on the aircraft and a series of 50mm / 75mm tape added to the top side of the cowl in two separate applications. Finally the same was done on the underside in the same manner. The cowl was laid on the aircraft for curing at each operation.

The side cowls were covered in packaging tape allowing 3 layers of 75mm wide fiberglass tape to be laminated and allowed to cure.

Once cured they were removed and the sides scuffed and glued to the sides using the West System epoxy and sanded to profile using  a orbital sander and 80 grit discs.

Check of side cowl before sanding

Once checked the inside face of the extensions were thickend using layers of 50mm glass tape to match the factory thickness and then capped with a 75 mm wide layer bonded  between both for additional security.

Final check before starting the installation of the fasteners

Comment
It worked out even though I had no idea of what or how to do the job.

The new cowls are a total redesign with a lip at the wheel entrance, removable side cheeks with the lower cowl now beign a one piece assembly. 

Entry lip resist air entering wheel well

Joys of the early purchaser

Rudder Floor Plates Mount

This blog  covers the drilling for attaching the pilots rudder floor plate.

Overview
The floor plates on this model are secured to the fuselage side and requires match drilling.

Installation
The first task was to mark the starting reference aligned with the rivets on the fuselage sides. To do this a scrap piece of aluminium was cut into at a right angle and secured to the fuselage side with masking tape. With the aluminium aligned with the lower stringer the position of the fuselage rivet was transferred to the stringer.

Locating starting reference

The next task was to level the floor plate and establish the rivet line along the floor plate. To do this a length of 20 x 12 mm aluminium channel was secured to the side of the fuselage using spring clamps secured to the fuselage bulkheads. With the edge distance set using two cardboard stops fixed to the floor plate with double sided tape.  Next the channel was aligned to the rivet line with a cardboard template [refer photo] and finally the rivet line was marked along the face of the plate. 

O

Once this was done the hole centres were marked at 40 mm about the initial reference point and then drilled 3.2 mm [1/8'']

Note: A tapered packer was required at the side shown to fix a misalignment between the skin and the floor plate. This task may have been better done before installing the rudder pedals floor plates from a installation procedure perspective.

Once the side skin was installed the holes were matched drilled through the fuselage skin and clecoed. 

Comment
Turned out well.