Commonwealth Aircraft Corporation CA-12 / CA-13 / CA-14 / CA-19 Boomerang
CA-13 Boomerang Mk.II
CAC Boomerang A46-63
The standard production CA-12/13/19 Boomerang’s Pratt & Whitney R-1830 Twin Wasp power plant was not supercharged sufficiently to permit adequate high-altitude operations. Full-throttle height, in high blower at military power, being only 13,500 feet. To overcome this deficiency, a turbo-supercharger installation was engineered and fitted into A46-1001, the suffix – 1001 allotted to all Australian military prototypes.
Initial layout, under the name, ‘Turbo Boomerang Interceptor’, was shown on a GA drawing dated 8 January 1942, so that early thought was being given to the altitude deficiency inherent in the use of the standard Twin Wasp. A CA-13 airframe was taken off the assembly line at an early stage of production and converted to suit the installation, (attempting to fit the supercharger and ancillaries to a completed aircraft being too demanding).
Engineering commenced in July 1942 with Ian Ring, Ian Fleming, and Alan Bolton working on it. Under Tom Air, Bolton was Project Engineer for the design with CSIR doing the research work on the ducting. A mock-up was built in the No.2 Hangar of the Aircraft Factory. A CAC-built R-1830 S1C3G single-stage Twin Wasp on a longer engine mount, with General Electric B2 exhaust driven supercharger and ancillary equipment from a Boeing B-17E, were made available by the USAAF. Their fitment into the aircraft necessitated considerable redesign.
A Harrison intercooler was fitted into the fuselage behind the cockpit, and an intake for engine and intercooler air was positioned on the fuselage port side. Intercooler air was exhausted over the top of the fuselage through a temperature-controlled shutter behind the cockpit. The supercharger was mounted in the starboard fuselage, the normal exhaust tailpipe being extended down the fuselage to feed the turbine. The standard down-draught carburetor air intake was dispensed with giving it a cleaner top cowl line, while high-altitude magnetos were also fitted. A panel of test instruments was mounted in the rear fuselage, with a 35mm remotely-controlled camera to record in-flight data.
As in production of the Boomerang, compromises had to be tolerated, due to lack of optimum equipment. The best propeller available was a three-blade, eleven-foot Curtiss Electric, with a blade activity factor of 87, which limited climb performance and was 50 pounds heavier than that on the standard Boomerang. The B2 supercharger (as the only one obtainable) had a diffuser design which limited critical altitude by 4,000 to 5,000 feet when used with the R-1830 engine, and a further 500 feet in critical altitude was lost because of a 0.50 in-Hg greater pressure drop across the intercooler, compared with an Airesearch cooler which became available after first flights.
Rate of climb was also restricted because of the necessary use of large opening, conventional cowl gills to induce sufficient cooling flow, and resultant large increase in drag.
While the project was being developed, CAC design engineers had the use of the six RAAF Republic P-43 Lancers at Fishermans Bend for study and evaluation, examination of which was of material assistance.
Through October, work progressed and target date for completion was set at the end of the year. Some trouble was experienced with rudder and elevator control-cable runs which had to be duplicated to meet revised AP970 requirements; bends in the system to clear the ducting compounded this modification.
Although Lawrence Wackett requested Air Board approval, on 4 January 1943, for test pilot Jim Carter to be given some flying in an RAAF P-43B Lancer before flying the CA-14, it was advised that there was not one ‘conveniently available’. Instead, Flt Lt John Holden, the RTO at CAC, who had both Lancer and Boomerang experience, was nominated. (six P-43B reconnaissance fighters were allotted for Australian use, as PRU aircraft).
Ian Ring wrote, on 29 November 1986, ‘I was responsible for the turbo-supercharger installation in the CA-14A. I seem to recall seeing an American installation, but I do not remember getting any significant help from it, other than a general picture. Our installation and turbine performance data came from Pratt & Whitney. The problem was mainly one of accommodating the turbine in the appropriate position in the exhaust system, with suitable heat and fire isolation, and designing the inlet manifolds. The result was a spectacular rate of climb.’
The initial CA-14 flight was made on 13 January 1943 by Holden (promoted to Squadron Leader in that period). He not only proved the turbo installation, but also carried out comprehensive performance trials, and was commended by Aircraft-Superintendent Hugh Francis in a letter to Wackett, then in the US.
Even if Carter was to have taken over the further test work, as would have been the norm, it was not to be, as he was killed in the CA-4 Woomera crash on 15 January. Subsequently the new company test pilot, Greg Board, who arrived on 10 March, took over the task, and in the letter to Wackett, Francis told of Board’s appointment, stating that he had so far flown at very high pressure, and had been satisfactory in every way.
Precedence was given to assessing the engine installation and aircraft performance, and, with approximately 17 hours of flying by 29 March, the only other aspects noted were that ground handling, take-off, and the stall, were similar to the Boomerang, with better forward visibility resulting from removal of the carburetor air intake on top of the cowl.
At that time, engine overheating was occurring with the use of continuous climb outlet to 20,000 feet, but it was felt that fan cooling, then being planned, would cure this. Turbine exhaust temperature was at 780C, 35C below the maximum permissible, while turbine cooling and lubrication were satisfactory. Initial intercooling deficiency was overcome by removing the two lower cowl gills on the port side, thereby obviating engine cooling efflux inflow to the intercooler. Ignition breakdown at high altitude was overcome by replacing the Scintilla magnetos with Bosch high-altitude types. Further flights revealed that the aircraft was neutral stable in the lateral plane, directionally unstable, and rudder-trim changes with speed were excessive.
To overcome these latter two faults, the fin area was increased and redesigned, while lateral stability was improved by sweeping the wing tips up, as had been done on the Boomerangs. The supercharger installation was considered to be very satisfactory, but, due to the restricted climb, a cable was sent to Wackett on 15 April recommending that he obtain a four-blade eleven-foot propeller for the aircraft, similar to that fitted to the P-51B.
Performance and handling tests, at which time it was fitted with a large spinner, were completed by CAC (including flying to 35,000 feet) and it was delivered to the RAAF in April. On its return from Laverton in May, a new fin and rudder were fitted to further increase vertical area, and these improved the aircraft’s handling qualities as reported by the company test pilot.
It was returned to the RAAF for further evaluation. Board had flown it to 41,000 feet, its upper limit, and found that controllability was then almost non-existent, and that it fell away out of control very easily, (as was to be expected). From the lessons learnt with the CA-14, the same aircraft was developed into the CA-14A, still as A46-1001. Design calculations showed it to have a better all-round performance than the Thunderbolt and the Spitfire VC, comparative operational ceilings (at which rate of climb falls below 1,000fpm) were 34,400 feet, 32,000 feet, and 29,700 feet respectively. With 30 hours of CAC test flying and ten hours of RAAF evaluation on the CA-14, the conversion was carried out through June and July, with the following changes and new equipment scheduled for incorporation.
1) CAC-built R-2000 Twin Wasp replacing the R-1830. (not achieved.)
2) A B9 turbo-supercharger (actually a B13 was fitted).
3) The four-blade 11 foot Hamilton Standard propeller with blade-activity-factor of 113.5.
4) Ten-bladed cooling fan mounted behind the propeller, and driven through the reduction gearing at three times propeller speed.
5) Sliding cowl gills in place of the conventional hinged type.
6) Retractable tail wheel (not achieved).
7) The intake for the intercooler and engine air was fed from directly behind the fan, gaining considerably from ram effect, and also further increasing critical altitude. The ducting was set nearer the fuselage centre line and did not protrude as far out from the fuselage as it did on the CA-14.
8) The Airesearch intercooler. (40 pounds lighter than the Harrison)
9) A bulkhead in the rear fuselage (not achieved, it is believed, unless the auto observer panel was so described).
10) Changes in the oil-cooler installation and ducting.
With the original hinged gills opened, there was a tremendous increase in drag and buffeting and a reduction in controllability and forward vision. To overcome these liabilities, sliding gills were developed on A46-157 and a variable exit orifice, which was capable of controlling the cooling airflow within laid-down limits, was achieved. Operated hydraulically, as were the standard hinged gills, these slid backwards and forwards along a series of rails to alter the exit.
For some reason, the hinged gills were reinstated at some later stage, and were still on the aircraft when it was eventually broken-up. (It has not been possible to find detail of the changes in item 10, necessary because the exit ducting from the oil-coolers could not be allowed to feed the exhausted air into the extended intercooler/engine intake in the CA-14A, on thermodynamic grounds, and so as not to detract from maximum power).
The first graduate from the Sydney University Aeronautical Engineering Course, completed in 1941, was Alan Bolton, who went straight to CAC on Boomerang stressing, and then worked on the CA-14/14A design - he was able to advise in January 1994 that, to the best of his recollection, the outlet had been relocated to the starboard side into the duct which carried the exhaust tailpipe.
However, it is not known how oil-temperature control was effected in such a position, seemingly without the pilot-controlled exit flap, which had been on the port-side of the CA-14 and all production Boomerangs. During the conversion, a further increase in fin area was added, and the centre-section leading edges were straightened and swept forward, thus cutting out the wheel well fairings. During high-speed dive tests, up to 400 mph IAS, on A46-27, shock waves at the fuselage wheel-well junction occurred at about 360 mph by obviating this junction, wind-tunnel tests, showed the shock waves disappeared unti1 420mph was reached.
A limiting dive speed of 410mph was then placed on the CA-14A. Weight increase with these modifications was 100 pounds, making the all-up weight 8,132 pounds. Wackett wrote to DAP Secretary Letcher on 29 June 1943 to recommend production of the CA-14A before CA-17 manufacture could be started. With almost all of the detail parts for the 200 Boomerangs completed, various departments would be out of work within the next two months, and the assembly task would be completed in October.
With at least six months to the start of detail work on the CA-17, dependent on the arrival of information from the USA, and manufacture of the necessary tooling to begin fabrication, at least 200 more Boomerangs could be built, and such was necessary to keep the organisation intact, pending the start of CA-17 work. As there was need for an urgent decision, for either standard Boomerangs or the CA-14A development, he recommended the latter.
It used fully 75% of the tooling for the standard Boomerang, and it was therefore possible to issue production orders for much of the detail parts immediately, while producing tooling for the new parts necessary to suit the supercharger installation. He summarised the project and company hopes in a report which contained the following key points:
The CA-14A’s performance would better the Spitfire Vc:
• it is sound reasoning to produce a version which can continue until the CA-17 becomes available;
• installation of the CA-14A’s turbo-supercharger has been carried out successfully;
• the 2000 cubic inch conversion of the 1830 engine, conducted locally on a Lidcombe built engine, has much promise;
• after a long series of trials the 40-degree propeller with wooden blades functions perfectly in flight;
• the development of the geared cooling fan is sufficiently advanced to enable this feature to be incorporated in the CA-14A. The success of the Focke Wulf 190 is largely due to this feature;
• the CA-14A will incorporate a larger fin and rudder and an improved fairing at the leading edge of the centre section. Both of these features have proved advantageous;
• the U.S. Army Air Force in Australia has hundreds of superchargers in store, and it should be possible to make a strong case for favourable consideration that sufficient of these be allotted for CA-14A production.
At the 5 July 1943 meeting of the AAC, Wackett emphasised that it was necessary to give immediate consideration to the production to be carried out between the thencurrent Boomerang contracts and the beginning of the Mustang, re-iterating the points made in his letter to DAP, and pointing out that the CA-14A was definitely the best possible aircraft to build until the CA-17 was under way.
Production of 120 further Boomerangs out of a DAP proposed 325 was recommended by the Minister for Aircraft Production on 8 July, with these to have the R-2000 and turbo-supercharger installed as soon as it could be done. However, the War Cabinet authorised only 50; this was not considered worth the extra effort involved for such a small number, and accordingly the CA-14A proposal was dropped. These 50 were built as standard Boomerangs, under the CA-19 contract. The CA-14A was actually completed and flown on about 26 July with the R-1830 sans fan, and a three-blade Hamilton Standard propeller. The R-2000 engine had suffered from bearing failure on test, and was not ready. Although the necessary enquiries had been made overseas for a suitable four-blade propeller, none was procured in time.
This would have resulted in much greater efficiency at climbing speed at all heights above about 15,000 feet, and at maximum speed. A letter from CSIR to Dept of Air (dated 20 August 1943), stated that the CA-14A incorporated many experimental features, in the design of which the CSIR had been actively concerned, and for this reason the RAAF was requested to carry out further detailed tests beyond the Type Trials.
These were to cover intercooler flows and temperatures, engine cooling, fuel consumption,duct pressures, etc, in order to confirm the theoretical figures arrived at. Gp Capt Walter Armstrong (DTS) answered on 27 August, by saying that, although the CA-14A was of great technical interest, alterations in policy and pressure of other work placed further testing of the aircraft in a low-priority bracket. Turbo-supercharging for fighter aircraft had fallen out of favour here and overseas, mainly because of the slow response to throttle opening, andconsequent lack of acceleration during combat.
However, if opportunity occurred, tests on the CA-14A were to continue. Both the CA-14A and A46-157 (and A46-27?) were used by CAC as test beds for the prototypes of a CAC-designed-and-built four-blade propeller with high-density wooden blades, a 38 degrees pitch range and 130 pounds lighter than similar metal propellers. Although developed with some success, and intended for, among others, the Mustang, it did not go into production.
A forced landing was made by A46-157 on 5 November 1943, believed to be due to the loss of about six inches off the tip of a blade, the resultant vibration loosened all but one tailplane attachment bolt before the landing could be effected. Just one week later, the CA-14A made a forced landing at Geelong when a bolt on the spinner backplate fractured.
Both of these aircraft were finally handed over to the RAAF in June 1944, fitted respectively with the first production fan and the first experimental one. The original allotment for the CA-14 project was £15,000 and the actual costs incurred until final delivery totalled £16,418/11/5, the engineering hours expended on it amounting to 15,160. The CA-14A was scrapped at the surplus aircraft park at Laverton during 1947 however, its mainplanes were used by ARL in its structural-fatigue research program.
Commonwealth CA 12 Boomerang Mk.I