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Rocket-armed CAC Wirraway (A20-723) 1/72


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There was only ever one rocket-armed Wirraway.

 

A little history...

 

In 1949 the front-line fighter of the RAAF was the Mustang, and the first Vampires were starting to arrive (the RAAF's first jet fighter). Advanced training and refresher training for fighter pilots was being carried out using CAC Wirraway aircraft, but the Wirraways were not able to train pilots in the full syllabus required by fighter pilots, since they were not fitted with gyro-stabilised gun-sights and they could not carry rocket armament.

 

So in December of 1949 the RAAF Director of Technical Services wrote to RAAF Headquarters on behalf of the Air Member for Technical Services with a request to enable a full syllabus of training to be carried out with Wirraway aircraft by fitting the following items:

  1. An AN-N6 Cine Camera Gun (as an alternative to the G.45 gun cameras already fitted);
  2. A Gyro Gun Sight (Mark 2D Series 2 or 3) in lieu of the Reflector Sight installed by Wirraway Order No. 134 (offset to the left to enable the instructor in the rear seat to have an unobstructed view when using the Aldis sight);
  3. Rocket pods (zero length rails) to enable the carriage of 6 rockets (3 under each mainplane) in such a manner as they do not interfere with bombing and gunnery installations; and
  4. Provide remote controls in the rear cockpit for the operation of the VHF set installed by Wirraway Order No. 173 (which only specified operation from the front seat).

 

It was requested that the Air Armament School (AAS) at East Sale be instructed to carry out a prototype installation, and that Technical Services officers would visit if assistance was needed. This was seen as an urgent training requirement, and the AAS prototyping was requested as soon as possible.

 

The Mark 2D Gyro Gun Sight and the AN-N6 Camera were both designed for 24V operation, requiring voltage boosters to enable them to be operated from the Wirraway’s 12V electrical system. Tests were required to determine if the power from the Wirraway’s engine-driven generator would be sufficient for these extra items and their voltage boosters.

 

The requirement that the RP installation should not interfere with the existing bombing and gunnery installations was ambiguous, as it did not specify if the aircraft should carry under-wing gun packs (which were already fitted to all Wirraways at this time) and RP rails at the same time, or if the gun packs could be swapped for RP rails. The prototype installation carried out at AAS followed the second of these interpretations.


Records show that Wirraway A20-729 was used for a mock-up installation and A20-723 was modified for the full installation and flight trials. The first flight with the new equipment fitted was carried out on 20 July 1950 at East Sale. The first rocket firing was carried out during a flight on 24 July. Further successful firing tests were carried out in July. But the program was discontinued and no other Wirraways were fitted with this capability. 

 

I've uncovered a sketch of the proposed rocket installation by the AAS, but I've never come across photos of the aircraft or the trials. So I'm basing this build on "educated speculation".

 

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So I'm planning to model A20-723 fitted with six RP-3 rockets on zero-length rails, a gyro-stabilised gun-sight and an AN-N6 gun camera on the wing centre-section. To duplicate this aircraft, I'll need a Wirraway kit, and ideally some rockets plus a gyro-stabilised gun-sight.

 

The Wirraway will be the 1/72 Special Hobby kit (in all its flawed glory). Looking at my stash, the gun-sight and zero-length rails will come from a Tamiya F-51 kit and the RP-3 rockets will come from a Valom Bristol Buckmaster (why do I have two of those in my stash???). Here's where the fun will begin: 

 

51406975657_483ec12f93_c.jpg

 

First up is to construct the fuselage frame and detail the cockpits. Here are a couple of photos of the start of this process, alongside a scratch-built fuselage frame I've made for an MPM Wirraway I'm also building. Important to note that we must ignore the kit instructions here... Wirraways were all silver inside, no C364 "interior green" at all. None. Everything was silver, except black instrument panels, black electrical panels, and various red and yellow highlights for controls. No "interior green". 

 

51406976647_5b56260571_c.jpg 

 

And another view showing the instrument panels. You can see that the framework tubes are fairly chunky and SH have not replicated the framework too well. Several extra tubes have been added, and some real tubes are missing. Plus the foot-troughs should be discontinuous, not one long trough as the kit provides. But I'm living with it for this model... definitely changing it for my 1/48 and 1/32 builds...

 

51407994458_687d510190_c.jpg

 

Edit: Note that there are two radio sets under the rear instrument panel (R.1082 and T.1083) - these were no longer in use by the 1950s, so I've removed them from the model.

Edited by Derek_B
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Most of this build will be out of the box, apart from the rockets mounted under the wings. In the cockpit there will be a few minor changes.

 

The wings are now assembled and the fuselage halves are now joined, with a coat of Aluminium on the inside of the fuselage (NO INTERIOR GREEN!). The engine cowls provided with this kit are fairly poor, and will need some work. The injection molded version is not round, and the front opening is too large and not circular. The resin version is round, but the edge of the front opening was poorly molded, and will require remedial work to get it to an acceptable condition.

 

51414848851_a535704316_c.jpg

 

The most visible change in the cockpit is the addition of the Mark IID gyro-stabilised gun-sight. This is a huge sight, and blocks most of the pilot's forward view. This was also the case in the RAAF Mustangs which were fitted with this sight. Here are a couple of pages from the CAC Mustang Overhaul Manual showing details of the Mark IID sight:

 

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The gun-sight molding in the Tamiya F-51 kit is one of the transparent parts, so one needs to paint it carefully so that the clear sections at the top front of the sight remain clear. I successfully managed this, and then carefully carved away a section of the instrument panel coaming and glued the gun-sight in place. Since I don't have any pictures, I had to rely on a written description of how it was fitted, plus making sure that the windscreen molding still fitted to the model once the sight was in place. Here are a few views of how it looks mounted in place:

 

51415835100_bd227d0688_c.jpg

Edited by Derek_B
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  • 2 weeks later...

OK, time for some research and figuring how and where these rockets were mounted.

 

From the descriptions in the archives and the sketch plan shown previously, we know that the rockets were mounted on "zero-length rails" attached to a plate which was held to the wing by bolts that were installed in the holes for the bomb crutches. So what does that mean?

 

The underside of a Wirraway's wing is a marvel of late 1930's engineering. The RAAF wanted to mount four 250lb bombs on the outer wings, so CAC devised an ingenious method to fit four Universal Carriers inside the structure of the wings. This was not a standard capability of the NA-33 prototype which CAC had licensed from North American, in fact A20-2 could not carry any bombs under its outer wings.

 

If you're familiar with Universal Carriers, you know they are big ungainly things which are usually attached to the underside of an aircraft's wing to allow all the stuff needed to safely drop bombs to be carried along. This includes the bomb slip (the spring-loaded hook that holds the bomb in place), the electro-magnetic release (which trips the slip to drop the bomb), rails to mount the forward and rear fuses (so the bomb can be dropped in a "armed" or "un-armed" condition), front and rear steadies (to prevent the bomb from swaying in the slipstream) and wiring for all the above items.

 

Here's an example of two Universal Carriers mounted under the wing of Hawker Hind K5415 at Old Warden... 

 

51472464561_8b2ccb609f_c.jpg

 

CAC separated all these components and designed a method to mount the slip and the electro-magnetic release inside the wing skin, and attached the fuse rails to the wing skin ahead and behind the slip. Steadies were also mounted into the wing skin. Here's a drawing of what it all looked like, with the various items colour coded:

 

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And here's what it looked like on an actual aircraft:

 

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So that is two entirely flush-mounted Universal Carriers mounted in each outer wing (apart from the fuse rails on the wing skin)... quite a difference to that Hind photo above! I find that quite clever!

 

Curiously, you won't find any of this detail on Wirraway kits, as this detail was not included in the drawings on which they were based.

 

So the holes for the steadies (shown in red) are the main clues to where the rockets were mounted. If we remove the covers for the slips and the wiring, we can clearly see the 8 holes for mounting the bomb-steadies (4 per slip). It makes sense that the middle rocket of the three rockets under each wing would be centred on the centre-line of all these holes, so that's what I've shown below. I've also added the centres for the two outer rockets, based on an 11-inch spacing, which seems to be the case from photos and sketches I've studied. This places the two outer rockets almost in line with the bomb slips.

 

51464614459_07f4d7daac_c.jpg

 

To figure out the fore-aft location for the rockets, we can work with the CG range. The CG of the rockets must be within the CG range of the aircraft so that the balance is not compromised. If the rockets are too far aft, they will interfere with the flaps, so I've shown the fore-aft location of a rocket if the CG of the rocket is aligned to the forward limit of the aircraft's CG range:

 

51463117812_74e17946ca_c.jpg

 

Now that we know the location of the rockets, we can surmise the size and location of the mounting plate, based on the size of the zero-length rails. Here's how I think it would have looked:

 

51464825805_b51b441d4b_c.jpg

 

And finally a sketch showing what the whole installation (under the port wing) would have looked like:

 

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I've assumed that holes would have been cut in the mounting plate to allow access to the rear wiring holes. I've also assumed that some stiffening ribs would have been added to the plate. The CG of the rockets is almost at the forward rail hanger, so the forward rails would have taken most of the load.

 

Now to replicate this in 1/72 styrene...

Edited by Derek_B
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Here's what I'm starting with...

  • One Wirraway wing
  • Six RP-3 rockets (from the Bristol Buckmaster kit - no, Buckmasters were not fitted with rockets, the wing moldings in the Valom kit are common to the Brigand kit), and
  • Six sets of "zero-length rails" (from the F-51 kit)

The rails come attached to US 5" HVAR rockets, but I'll cut these rockets away once the "rails" are safely attached to the mounting plates which I'll glue to the under-surface of the Wirraway wing.

 

51465220685_7b8097d6a9_c.jpg

 

Edited by Derek_B
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  • 1 month later...

Next step is to cut and drill the mounting plates for the rocket rails. These were cut from 0.5mm styrene sheet. In the photo below you can see the paper template, the styrene drilling template and the two completed mounting plates (at the top):

 

51603492267_78eacae977_c.jpg

 

Then these were glued under the wings in the correct location and the (incorrect) US 5" HVAR rockets were glued in place. I only want the HVAR rockets for their zero-length "rails", so the rockets will be cut away once the glue has dried:

 

51602359948_c346354a1e_c.jpg

 

If you look closely you'll notice I glued the mounting plates in the wrong position initially, and had to cut them off and fill the two exposed holes on each wing. Oops. I also very carefully spilled half a bottle of Mr. Cement all over the wing, but it looks like it will survive after carefully draining the excess. Don't tell anybody. Sort of like a pre-clear-coat!

Edited by Derek_B
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  • 2 months later...

A brief update... having cut away the rockets from the zero-length rails, I'm ready to attach the wing to the fuselage. But first I need to add some seat-belts in the cockpit, so I'm also working on those.

 

Here is a shot of the wing underside with the zero-length "rails" in place (sans the US HVARs):

51781954484_9f5fd40a8a_c.jpg

 

Don't look too closely, or you'll see there is also some remedial work needed from my accidental dousing of the wing in styrene cement. I'll need to fill a couple of areas, and re-scribe some panel lines... not too much work I hope.

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  • Derek_B changed the title to Rocket-armed CAC Wirraway (A20-723) in 1/72 scale
  • 2 weeks later...

Before I attach the wing to the fuselage (after the remedial work on the wing), I wanted to add seat-belts to the cockpit, and this is finally done. I have some photo-etch Sutton harnesses on order, but I decided to try my hand at forming a set using the thin aluminium from the top of a sardine tin. I think they are quite acceptable, given that I'm not about to add buckles at this small scale!

 

51803183476_610e4aab95_c.jpg

 

Wirraways were fitted with D.117 Sutton-type harnesses from February 1942, replacing the D.115 lap-type belts which were originally fitted to aircraft delivered up to that time.

 

You can't tell from this angle, but I also removed the wartime R.1082 and T.1083 radios which I had installed on the rear cockpit radio shelf (under the instrument panel, visible in some of the earlier pictures), since these had been removed from Wirraways by the 1950s, and I should not have installed them in the first place!

 

This cockpit is now ready to be glued into the fuselage...

Edited by Derek_B
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The interior looks great. FWIW you can add a bit of detail to Sutton Harnesses with a fine (0.1mm) black permanent marker - a few dots for the holes and a couple of parallel lines to represent the buckles.

 

Regards,

Adrian

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Looking fantastic Derek! I appreciate all the extra info on Wirraway cockpits for when I get around to building one of my own.

 

PS a friend has kindly lent me his copy of a recent book about a certain Wirraway-derived agplane. I am beyond excited to read it.

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  • 5 months later...

Finally have the cockpit installed in the fuselage and the wings attached. Starting to look like a Wirraway. The ravages of the glue spill are apparent on the right wing, where you can see I've sanded off the surface detail. I think that some glue pooled inside the right wing, as it swelled up in a couple of places.  More sanding and filler will be required prior to priming. 

 

I've also removed the incorrect surface detail from the centre-wing section and will re-scribe that. The roll-over truss is too tall to allow the canopy to sit correctly, so that will also be adjusted...

 

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Edited by Derek_B
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  • 1 year later...

Before painting I need to attach the canopy and there were a few items I needed to add before I could do this...

 

While I was researching colour schemes for this particular aircraft, it became clear that it was one of the Wirraways which were fitted with a large triangular instrument panel in the rear cockpit in a post-war upgrade program. So I printed out one of my Wirraway drawings at 1/72 scale and made a rear instrument panel. First I cut some 0.5mm styrene sheet to the right shape:

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Then I drilled out the instruments using a combination of 0.8, 1.0 and 1.1mm drills...

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Then a coat of matt black...

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And finally the dial faces were replicated with some canopy cement before gluing the new instrument panel in place:

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I also wanted to add the head-rest for the front seat, which is attached to the front of the roll-over truss. I shaped this up from some runner material and gave it a lick of dark green. Here it is in place...

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And finally I added an Aldis sight for the rear seat occupant. Looking for colour schemes I noticed that several Wirraways operated by Central Flying School at East Sale were fitted with these sights, attached to the port side of the roll-over truss. A20-723 was operated by CFS immediately before being issued to the Air Armament School, so I'm going to assume that AAS may have wanted this extra sight fitted to assist with the rocket firing trials.

 

As an example, here's a photo of A20-742 after a forced landing at East Sale in November 1951, clearly showing the "antique" Aldis sight in place:

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In the photo above, the Aldis sight is installed on the starboard side of the roll-over truss, but various photos show it was installed on different sides on different aircraft. In my case, I went for the port side. The Aldis sight is just a piece of 1.0mm styrene rod, cut to 9.5mm length:

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So now I' ready to attach the canopy, re-scribe some panel lines on the wings and get on with painting...

Edited by Derek_B
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  • 2 weeks later...

Panel lines on the upper side of the wings have now been re-scribed... still learning how to do this, and juggling different types of scribers and straight-edges to get the right effect. I added the panel lines for the dive brakes, since this aircraft was retro-fitted with "dive bomber" wings. In actual fact all Wirraways (whether fitted with "dive bomber" wings or not) could and did perform dive-bombing for training purposes (at up to 70 degrees) and the dive brakes of the "dive bomber" wings helped keep the airspeed under control.

After correcting the panel lines on the wing centre section either side of the fuselage, I needed to re-create the fuel filler caps, since I had sanded them off getting rid of the incorrect panel lines. So I drilled some 1.2mm diameter holes (being careful not to drill all the way through the entire thickness of the plastic) and then inserted some short sections (about 0.75mm long) of 0.8mm styrene rod, to simulate the fuel filler caps. Pulled out my magnifiers to make sure they were centred in their holes as the CA glue dried - very pleased with the finished result. 

 

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Now to finish scribing the underside, and adding some missing detail under the wing centre-section...

Edited by Derek_B
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  • 2 weeks later...

Working on the underside of the wing, since that's where all the excitement will be on this build (i.e. the rockets!!). Starting to correct the lack of detail and incorrect panel lines under the wing centre section. First I've added stiffeners for the fuel tank cover panel (also known as the "stress panel" as it carried the stresses from flight loads across the underside of the wing):

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Then I've scribed the correct panel detail onto the centre-section. Still a few extra lines to add around the flaps and under the rear fuselage. And lots of holes to drill too, to complete the fuel tank sumps and the 14 bomb slips which were fitted under the centre section. I also added some very rudimentary stringer detail inside the wheel wells:

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Edited by Derek_B
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