Exhaust - Cat Whisker

This blog covers the manufacture and installation of the faux Shorts T1 exhausts

Overview
The selection of the Battle of Britain paint scheme set the task to replicate the British Shorts T1 Tucano with the general approach that for a stand-off scale RC model but just bigger.!

One of its distinctive features is the cats whisker exhaust required to shoe horn in the 1100 hp turbine in place of the factory 750 hp.

This builders are styled to capture the feel not the overall size for drag reasons.

Manufacture
After looking at options it was decided to design a stylised exhaust and have it manufactured as two halves and then fit onto a flat board allowing it to be moulded over.

The model was created with Albire 3D software, exported and manufactured by Shapeways. On arrival they were inspected then sanded with 120 / 240 grit aluminium open cut paper .

Next they were covered with packaging tape before fixing them to a board covered with tape and a coat of Mr. Sheen.

Assembled exhaust and one half on former
The white former is to mark the cutout to
allow mounting to the underside of the fuselage

Seven [7] layers of 8 oz fiberglass was placed over a mold then coated with West System 105 epoxy / slow cure catalyst. Once all layers were wetted through, a cardboard squeeze was used to remove excess resin. This was allowed to pool at the base then cure for removal later. Once this shell was created before preparing the other side and once cured repeated the process.

The finished components were clipped together and once aligned the gap created by the radius at the edge's was filled with a mixture of epoxy / milled glass with a length of 50 mm fiberglass tape applied over the mixture and allowed to cure. Finally the waste was removed and joint sanded and any gaps filled with epoxy/milled glass, allowed to cure and sanded to final profile. 

The profile slip was fitted, profile marked and the stacks trimmed to suit the side profile and set aside.

A piece of styrene sheet was fixed to the underside of the fuselage to create a flange for stack fitment, covered with packaging tape and a spray of Mr Sheen. Seven [7] layers of cloth were applied to represent the flange, set aside to cure and another done for the opposite side.

Held for checking with carpet tape

The stack was positioned, aligned, perimeter marked then a mixture of 5 minute epoxy/milled glass applied to the top and bottom edge and the stack offered up to the flange.  With the stack held in position the excess was filleted and held in position for 5 minutes utill the mixture set up. The assembly was removed then a mixture of epoxy/milled glass/micro-balloons applied to the inside face via a hole cut through the backing plate. The mixture was filleted and set aside to cure. Finally  the flange was trimmed and sanded into its final profile. 

When completed the whole assembly was sprayed with plastic primer filler then coated with blade putty and allowed to dry. When dry it was sanded with a orbital sander fitted with a 80 grit disc then finished by hand with 120 grit. Once happy with the shape it was sprayed with spray putty and finish sanded.

Comment
Happy dance..!

Spinner

This blog covers the installation pf the spinner.

Overview
The factory fiberglass spinner and backing plate have to be prepared to be installed in the Aviprop 4 blade.

Installation
The first task is to locate the center of the backing plate to allow the drilling template to be installed to allow the preparation of the backing plate.

Drilling jiging holes
This process was recommended by Flying Legend

The drill was checked for square to the drilling table before starting any work. With this complete the jig plate that was on the engine was centered on the back of the spinner backing plate. This was done with the fingers and once it felt centered it was clamped and the four [4] holes drilled to mount the propellor drilling jig.

Note: Do not use center marked on backing plate as it may not be a true center

Filing jig is bolted to the four[4] holes
drilled from the backing plate jig above

Additional dressing was required to allow a neat fit over the boss of the propeller with the eight mounting holes lining up with the holes in the propeller hub.
 

Center hole was dressed to the edge of the S/S plate [drill jig]
Eight [8] mounting holes drilled using the jig

Backing plate fitted

With the backing plate mounted the thorny issue of the cutouts for the propellor. A check showed that the base of blade aligned with the spinner face meaning the cutout would have to be more than a circular cutout.

Cutout profiles were trialed allowing a template to be developed premitting the blade to move though its rated range but this will be tested later and may require further modifications.

Cutout template

The next task was to mark four [4] equispaced points on the spinner rim and then locate the center for the initial 63 mm holes.

Marking out hole locations - marking center line [70 mm from back]
Drilled hole 63 mm OD

With the holes drilled the angle from the template was created and a parallel line marked for the opposite side. A length of S/S bar was used as a cutting jig with the scrap removed using a Dremel plastic cutting disc. Each cut out was numbered to its individual location for later reinstallation at the rear of the blade.

With the cutout center on the blade three [3] mounting points was marked at each segment and a S/S  #6 floating nut used as a drilling template. After drilling the center hole was enlarged to 4 mm and the two mounting holes countersunk to accept a 3/32'' countersunk pop rivet. 

The floating nuts were installed on the underside of the backing plate and spinner secured with 5/32'' clecos through the nuts. The spinner will be fixed using #6 titanium button head screws for their appearance.

Comments
Easier that I thought it would be.

Cowls - Part 5

This blog cover the installation of the fasteners

Overview
It was decided to use the Skybolt fasteners based on there success with the RV aircraft with a RV4 kit used as it was felt to be the best option but in the end were were about 12 fasteners short.

Installation
The installation on the firewall was covered in a previous blog on the cowls with the method to locate the holes proving to be a problematic on the Tucano because of the inability to use the mounting brackets supplied.

Note: Using a light to locate a hole then wriggling the step drill to line it up holes is not a good engineering approach.

It was decided to build the whole assembly off the initial cleco holes with these holes enlarged to a 1/2'' using a step drill. To keep the cowl in align as the clecos were removed, lengths of 1/2'' fuel tube were dressed to fit and placed in each hole as drilled.

Drilling jig and location pins

The fastener receptacles mounting holes were drilled using an aluminium jig and 1/2'' fuel tube located in the pre-drilled holes. With the mounting holes drilled the center hole was enlarged to 9/16'' and both mounting holes countersunk with a 120 degree tool. This was undertaken at the rear first so that all the Skybolt fasteners could be fitted. 

Note: Would use a hole larger than 9/16"" if doing it again

All were fixed using 3.2 mm  [1/8''] countersunk pop rivets not the solid rivets supplied.

With lower cowl now fixed between the firewall and front jig the top cowl could now be addressed. The top side of the cowl had the 32 x 20 mm aluminium angle installed in the previous blog linked above so with the top cowl trimmed to fit, it was mounted onto the firewall and fixed using the fasteners. [see photo below].

The cut has been marked in this photo
see below

Before installation a length of blue painters tape was added at the mating edge on the lower cowl for contrast.

The next task was to determine how much to remove at the front. 

With the cowl in place the distance from the top of the jig plate to the cowl was determined and this was transferred to the side with the rear marked from the lower cowl rear.

With the edge rough trimmed the same technique used previously to locate the final edge was applied with the edge trimmed and sanded as required.

After trimming the edge to profile with a small amount left for final adjustment the cowl was fitted using the rear fasteners and clecos pitched as required over the length of the top cowl.

Note: At all points the cowls must be fitted to the front jig

Cowl ready for fastener installation

Drilling pop rivet holes - fuel tube is used to locate jig

The technique that was used on the rear was applied to the side rails. Will all the holes now a 1/2'' the top cowl was removed and the mounting holes for the Skybolt fasteners and the receptacles mounted as described above.

Finished job - all 41 off...!

Off the rails

It can go wrong and did so in the starboard side with the hole running to the base of the angle with a new rail manufactured and fitted as well as filling all the holes. 

How?  - Simple in all the work the angle across the cowl went from 0.5 to 1.0 degrees and this raised that side and a quick check would have revealed this - just to gung ho.

Sandbag a must have

Comment

The method as described works and removes all the variability of the Skybolt method and is quicker.

Assembled Cowl - no visible support

The spinner will be covered in the next blog but was much easier that I expected.

Cowls - Part 4

Thus blog covers the gluing of the cheek cowls into a single assembly.

Overview
Time to make the lower cowl one piece.

Installation
The first task was to assemble the two lower halves with these installed on the fuselage using clecos fitted to the aluminium mounting angles. 

Next lengths of 25 x 12 aluminium channel were clecoed to the underside of the cowl to hold the two [2] pieces flat and in alignment for gluing.

Brackets to align the halves for gluing

Front alignment jig

Ready for gluing

With the jigs prepared the cowl was removed and the factory joints coated with epoxy and milled glass mixture and then reassembled with all the jigs and finally set aside to dry.

In the setup cutting the gear openings to the factory marks created a large error at assembly so it was decided to repair the error with a 1.0 mm carbon fibre panel. The panel was cut, drilled for cleco's and more aluminium jigs used to hold it flat and in alignment.

Glued 

Carbon fibre insert
This stuff eats hard edged tools in short order

With all this done the belly was now coated with a mixture of epoxy and micro balloons as a dry macro. After drying it was sanded with the orbital sander starting with 80 grit and finishing with 120 grit.

First sanding

Marking flange for dressing to final profile

End game

Comment
Next blog covers the installation of the fasteners.