During the course of their 20 years of service with the RAAF, CAC Wirraway aircraft were fitted with several different types of engine cowls.

There has been some confusion on this topic, particularly among scale modellers, which has been added to by a well-known kit manufacturer.

This short article is intended to remove any confusion about the types of cowls used and when they came into use.

Background

First, let’s look at the basic design of the engine cowling. The cowling wraps tightly around the outside ends of the cylinder heads of the radial Pratt & Whitney Wasp engine, working in combination with baffles between the cylinder heads to both cool the engine and provide forward thrust.

As with other members of the NA-16 family, the engine cowl on the NA-32 (NA-16-1A) demonstrator was constructed in two sections for easy installation and removal. The joint lines were roughly at 10 o’clock and four o’clock when viewing the aircraft from the front. The halves were officially described as “upper” and “lower” halves.

Clamps inside the cowl allowed it to be tightened against the cylinder heads, with heat-resistant felt pads between the cowl and the cylinder heads. The lower half of the cowl incorporated an intake for cold carburettor air (just inside the lower lip of the cowling) and a duct taking the cold carburettor air to the rear of the cowl. The duct protruded slightly below the underside of the cowl. Oil cooler air was drawn from the inside of the cowl via a duct between cylinders 2 and 3. This duct is sometimes visible in photos showing the port side of the aircraft.

The engine cowl on the NA-33 (NA-16-2K) demonstrator was longer than the cowl on the NA-32, since the NA-33 was fitted with a geared engine, as was the Wirraway. The engine cowling was of the same design, but extended by 4 inches to accommodate the longer engine. The lower cowl included the cold carburettor air intake and duct. Oil cooler air was taken from inside the cowl, but on the NA-33 two ducts lead to the oil cooler, one between cylinders 2 and 3 and the other between cylinders 3 and 4. The NA-33 also featured more louvres in the fuselage cowling to allow airflow out of the oil cooler.

CA-1 Cowling

The adaptation of the NA-33 design to production as the Wirraway Mk. I included the addition an extra fixed gun in front of the pilot. This necessitated relocating the high-mounted oil cooler to a lower position, aft of the underside of the engine cowl, between the wheel wells. Therefore the cold carburettor air intake on the lower half of the cowl was modified to feed both the oil cooler as well as the cold carburettor air[1].

The front opening was enlarged and positioned below the cowl. When viewed from below, the intake duct was parallel for the first 11 inches and then tapered to match the diameter of the oil cooler. At the point of tapering, an internal duct led some of the airflow upwards to feed cold air to the carburettor via a mixing chamber, where the proportions of cold and hot air fed to the carburettor could be controlled by a butterfly valve actuated from inside the cockpit. A simple wire-mesh screen was fitted across the front of the air intake to prevent the ingress of foreign objects such as stones or leaves.

All Mk. I Wirraways produced under the CA-1 contract (A20-3 to A20-42) were fitted with this initial air intake design. Mk. II Wirraways produced under the CA-3, CA-5 contracts and the first 202 aircraft under the CA-7 contract (A20-43 to A20-203, delivered between 16/02/1940 and 24/12/1940) also featured this single air intake design.

Riverina Dust and Cylinder Wear

The air intake on the “CA-1 cowling” was adequate for engine cooling and air induction under most conditions, but the decision to locate several Service Flying Training Schools in the Riverina district of Southern New South Wales introduced an issue which the “CA-1 cowling” was not designed to accommodate. The runways at these Schools were unpaved paddocks, and dust quickly became a major problem for Wirraways operating in these conditions. This was particularly the case for No 5 SFTS at Uranquinty, where the largest number of Wirraways were based at one school. Fine particles of dust sucked into the carburettor made their way inside the cylinders, causing rapid wear of the pistons and cylinder liners.

CAC Service Engineer Ewart Chennery visited a number of RAAF units operating Wirraways to check on their performance and at the end of January 1942 he reported that only 14% of Wirraways at 5 SFTS were serviceable, mostly due to engine wear[2]. An engine overhaul workshop was also established at No. 5 Aircraft Depot, RAAF Forest Hill, near Wagga, to service worn engines. A similar problem arose with the Cheetah engines on Avro Ansons.

Numerous trials were carried out to solve this dust-ingestion problem, and several design changes were made to the air intakes and air cleaners in subsequent Wirraway production models.

Order 87 Twin Air Scoop

The first change to the air intakes in an attempt to resolve the issue of dust ingestion came in late 1940 with a redesigned lower cowl air scoop that incorporated a second air intake below the main intake. The second scoop fed air to the oil cooler, leaving the main scoop to supply only the cold carburettor air, and a filter element was added at the front of the main air scoop in an attempt to remove dust from air going to the engine induction system.

This modified air scoop design was fitted at the factory to the last 30 aircraft of the CA-7 contract (A20-204 to 234, delivered between 28/12/1940 and 26/3/1941).

This change was officially promulgated with the release of Wirraway Technical Order Number 87 Engine Cowling – Bottom Half – Fitting of Air Cleaners[3] on 12 December 1941. This Order briefly described “a modified type of bottom-half engine cowling in which an air cleaner is incorporated in the air scoop”. A filter element (“Cleaner Assembly”, part number 08-31016) was added behind the mesh screen on the main intake[4], which fed only the cold carburettor air, to remove dust from the incoming air.

This modified “Order 87 scoop” was also fitted at the factory to Mk. II aircraft delivered under the CA-8 and CA-9 contracts (A20-235 to A20-622, deliveries from 30/03/1941 to 21/06/1942).
The modified air scoop assembly was also fitted all aircraft already in service (A20-3 to 203) with replacement lower cowls shipped to units via No. 1 Aircraft Depot, Laverton.

Improvements did not stop with the “Order 87 Scoop”. An experimental air intake with filter was shown in the undated RAAF drawing A2554[5]. This was a modification of the “Order 87 Scoop”, with cold carburettor air taken from inside the lower lip of the cowling, rather than an external scoop. This was similar to the arrangement on the NA-32 and NA-33 cowls. Air for the oil cooler was still drawn in via the second (lower) intake.

Order 108 Lower Cowling

By October 1942 a new design for the lower cowl was in progress following discussions between DTS, Lawrence Wackett and Ellis Wackett (AMEM). This design featured a raised intake and duct for carburettor air. The urgency of moving ahead with the installation of this new type of lower cowl resulted in a discussion by the Air Board on 19 November 1942[6]. ??? (AMSE) explained that the cylinder wear due to dust ingestion was costing in the vicinity of £5,000 per week for 8 months of the year, and 20-25% of this amount could be saved with the new type of filter. It was estimated that 500 sets would be required, at a total cost of ￡17,500 (£35 per aircraft). It was agreed by the Air Board that Ministerial approval be sought under War Cabinet minute No. 1573, and that DAP Demand 1034 for ￡17,500 be approved. Air Minister Arthur Drakeford approved the Air Board’s request on 26 November 1943. CAC modification 01-M3110 “Addition of Upper Scoop and Cleaner” provided the details of the modification.

Wirraway Order 108 Engine Lower Cowl and Air Cleaner – Modification was issued on 26 March 1943 and called for the lower engine cowl on all aircraft to be replaced with a new lower cowl incorporating a raised intake for cold carburettor air. The order applied to all Mk. I and II aircraft in service (A20-3 to A20-622 inclusive), with aircraft A20-623 and later (Mk. III aircraft delivered under the CA-16 contract) to be modified by the manufacturer as they were produced. New lower cowls were shipped direct to RAAF units from the supplier, Wunderlich Limited, in Sydney. Redundant lower cowls with the “CA-8 scoop” were packed in the delivery crates and shipped back to Wunderlich Limited.

The first Mk. III aircraft delivered under the CA-16 contract (A20-623) was delivered to the RAAF on 23/11/1943, fitted with an Order 108 lower cowling during final assembly.

The design of the Order 108 lower cowling incorporated several changes, relating to both improved carburettor air filtering and reducing accidental engine starting fires. The raised intake for cold carburettor air fed through a duct along the leading edge of the lower cowling down to a large bulbous chamber surrounding the air cleaner element. The bulbous chamber also served as a flame damper in the case of a back-fire during starting, often caused if too much fuel was primed into the engine prior to starting.

References:
[1] CAC drawing 03-31095 Assembly, Air Intake Scoop 23/05/1939
[2] Meggs, Australian-Built Aircraft and the Industry, Volume 2 Book 1, Echelon Starboard, 2020, p. 85 and p. 356
[3] NAA: A705, 150/4/1647, 1119874
[4] CAC drawing 08-31095 Assembly, Air Intake Scoop 29/11/1940
[5] NAA 440026
[6] NAA 24488837 Air Board Agenda 4416 (RAAF) Modification of lower half engine cowls on Wirraway aircraft – Department of Aircraft Production No. 1034

Between August 1935 and April 1938 a “battle” was waged between several newspapers, arguing about the merits of producing an American-designed aircraft in an Australian factory, for use by the Royal Australian Air Force.

Here’s an example of some of the negative headlines which appeared during this period:

Click on the link below to read my article in the Australia ICOMOS journal Historic Environment about the “newspaper war”:

Historic Environment Volume 33 Number 3 – 2021 The Wirraway ‘Newspaper War’

For reference purposes I’ve collated a list of production batch numbers for the CAC Wirraway aircraft delivered to the RAAF between July 1939 and July 1946. This page contains the batch numbers for Mark III Wirraways, which were delivered under the CA-16 production “contract”. A summary of Mark III production is given in the table below:

DAP = Department of Aircraft Production

Batch numbers
Wirraway Mark III production was planned in batches of 11 aircraft. But there was one exception:

• The final batch of production, batch “N” consisted of only 3 aircraft to make up 135 aircraft

Batch numbers consisted of a letter for each batch followed by a number for each aircraft in that batch (e.g. A1, A2, A3, A4, etc)..The letters I, O, Q and V were generally not used as batch numbers, as they could be confused for numbers. The CA-16 contract started with batch “A”.

The definitive source for batch numbers is a ledger book kept by Mr Gordon Parker, Supervisor in the Sales Department, and one of CAC’s earliest employees from 1936. This book is now held in the collection of the Australian National Aviation Museum at Moorabbin Airport.

Unfortunately there are two discrepancies in the last 25 entries in the ledger. Firstly, Parker skipped batch “L” for A20-733 to A20-743 and went straight from “K” to “M”. We know that batch “L” was not skipped in reality, as two surviving data plates reclaimed from airframes at Tocumwal show L2 for A20-734 (c/n 1186) and M5 for A20-748 (c/n 1200). Secondly, Parker stopped recording batch numbers at A20-740, so the batch codes shown in the list below are “extrapolated”. The batch “M” extrapolation is supported by the evidence of the M5 batch number on the A20-748 data plate. This leaves a question as to the last three aircraft – were they N1, N2 and N3, or was batch M continued for three more aircraft, as M12, M13 and M14. I have postulated that a new batch “N” was commenced, since CAC did not expect to end production at 755 aircraft. There were many debates between CAC and DAP over how many aircraft to supply to the RAAF (the original order was for 150 aircraft, and another order had been received for 100 additional “Dive Bomber” aircraft planned as CA-20, but only 135 were delivered under the CA-16 order), so it is logical to presume that the last three aircraft were N1, N2 and N3.

During production, batch numbers were temporarily attached to the firewall and then the rudder (once the engine was mounted). These temporary numbers are sometimes visible in factory photos.

Constructor’s numbers
These were a sequence of numbers allocated to each aircraft in the sequence that they came off the line. Wirraways delivered under the CA-16 contract were given constructor’s numbers from 1075 to 1209.

RAAF stores number
All equipment in the RAAF was allocated a stores number for tracking and accounting purposes. Aircraft were tracked under the category “A” and each aircraft was given a specific number, Wirraway aircraft being allocated the category A20. These numbers are often referred to as the “serial number” of the aircraft in RAAF service.

Linking batch numbers, stores numbers and constructor’s numbers
Here are several photos showing Mark III Wirraways under construction, allowing us to link the batch codes to RAAF stores/serial numbers.

Below we can see a group of camouflaged Mark III Wirraways on the production line. Batch numbers visible include D10 (furthest away, on the left) and E1 to E5 (closest, on the right). In the original photograph, D11 is also visible, sitting between D10 and E1, but pointing tail-on to the camera, so its rudder cannot be seen. These aircraft are A20-665 to A20-671, as evidenced by the three-digit number after the two-digit batch codes. 

Later in the CA-16 contract, below is a view of H8 (A20-707, c/n 1159) attached to the lifting sling. The batch label taped to the rudder showing H8-707 is repeated in the inset at lower right.

The photo below shows late CA-16 production, with the aircraft now painted overall Trainer Yellow.  K7 (A20-728) is visible sitting on dolly wheels, and the rudder of K8 (A20-729) is visible behind the aircraft (shown enlarged in the inset at lower left).

The table below lists batch numbers and construction numbers for all Mark III Wirraway aircraft, delivered under the CA-16 contract. The batch numbers are transcribed from Gordon Parker’s delivery records (with the last 25 entries corrected), and have been cross-checked with photographic evidence (some of which is shown above) and several frames and ID tags held in museums or private collections.

I welcome feedback on the table above, with supporting evidence of any possible inaccuracies.

References:

Commonwealth Aircraft Corporation. Sales Department Aircraft Delivery Ledger; Kept by Gordon Parker. CAC Collection, Australian National Aviation Museum.

Commonwealth Aircraft Corporation. Aircraft Drawing Office Manual. Incorporating up to Amendment List No. 2, Melbourne, Australia, 1946.