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Wednesday, March 13th 2019, 2:22pm

Research Aircraft & Prototypes

Australian Aviation AA-67




RAAF plans for the fighter fleet by 1942 included the AA-54 interceptor which although having long range, was not considered suitable for the escort fighter role. A requirement for a twin-engined escort fighter was issued in May 1942 and was soon amended to include turboprop technology to secure superior performance with acceptable fuel economy. Australian Aviation tendered an advanced design which maintained a true aerofoil section through the centre fuselage with the engine nacelles blended into the wings and large trailing edge fillets to reduce drag. Building on experience with the FA-51 programme, laminar-flow aerofoil sections would be used. The designers estimated a maximum speed of 472 mph (760 km/h) at a gross weight of 18,600 lb (8,440 kg). Armament would be up to six 20mm cannon, although 37mm and 40mm cannon were also considered.
Awarded a development contract in December 1942, work began on extensive wind tunnel tests, conducted in cooperation with the Melbourne University. Wind tunnel testing uncovered problems with engine cooling airflow through the nacelles. Normalair in Britain were sub-contracted to develop and supply the pressure cabin for the cockpit. It was decided during 1943 to order six prototypes which would test no less than three turboprop engines so a decision could be made which to select for production aircraft. Prototypes AA-67-1 and AA-67-2 were earmarked for the 750shp Rolls-Royce RB.50 Trent I, AA-67-3 and AA-67-4 for the 1,000ehp Armstrong Siddeley ASM.1 Mamba and AA-67-5 and AA-67-6 for the 2,200ehp Bristol Theseus II.

Following extensive ground trials, AA-67-1 made its maiden flight from Fisherman’s Bend on 23 October 1945. AA-67-2 did not follow into the air until 16 July 1946. Both prototypes were aerodynamic testbeds and lacked any military equipment. The RB.50 Trent engines were the same as those fitted to the Gloster Sea Meteors delivered to the RANAS around the same time which eased the spares situation and eased the groundcrew training issues. On 8 December 1945, AA-67-1 was damaged by fires in both engine nacelles caused by inadequate cooling airflow around the exhausts, but the damage was quickly repaired. Both aircraft were retained by Australian Aviation for trials work. AA-67-1 was retired in May 1948 but AA-67-2 was used for pressure cabin and aerodynamic trials until June 1949. A 1948 plan to re-engine them with the new Rolls-Royce Dart was never carried out.

AA-67-3 and AA-67-4 were delayed by the late delivery of the ASM.1 Mamba turboprops. The redesigned engine nacelles for the narrower-diameter axial-flow Mamba and Theseus turboprops cut drag further. Both prototypes were completed by summer 1946 and AA-67-3 made its maiden flight on 17 September and AA-67-4 followed on 17 October. Both had fully functioning pressure cabins and military equipment was fitted during 1947 and AA-67-4 was fitted for bomb separation and clearance trials at the Aircraft Research and Development Unit (ARDU) in February 1948. Both prototypes were assigned to ARDU in April 1947. On 6 September 1947, following failure of the starboard engine and an in-flight fire, AA-67-3 was destroyed in a crash landing and the subsequent fire, the pilot Squadron Leader Coombs escaping with minor injuries. In March 1948 AA-67-4 undertook firing trials with the 30mm ADEN cannon for the AA-20 bomber programme, but by May it had been grounded.

Originally planned for the 2,200ehp Bristol Theseus II, by the time AA-67-5 and AA-67-6 were completed they received the improved 2,500ehp Bristol Theseus III. Both were fully equipped prototypes and AA-67-5 made its maiden flight on 24 November 1946 and AA-67-6 flew on 28 January 1947. Both were assigned to ARDU in November 1947. Both were grounded in March 1948 and placed in storage.

By the summer of 1947 enough experience had been gained to make an assessment of the AA-67. Pilots judged the cockpit layout fair and ground handling satisfactory, but deemed the aircraft underpowered with the RB.50 Trent, particularly with one engine off, and the aircraft tended to be overpowered with the Theseus with large trim changes required when the power settings were altered in flight. Other flight characteristics were generally good during gentle manoeuvres; stick forces were light, roll rate was adequate and control was effective at all speeds with good longitudinal stability. However, a tendency to Dutch roll was prevalent and there were several disturbing behaviours near the stalling speed. It began to buffet well above the stall speed, becoming tail-heavy in fast turn, and its nose would tuck upwards during the stall. The problems were serious enough that test pilots declined to test the spin characteristics, fearing that a spin might be unrecoverable. Although the final flight test report was generally positive, the aircraft's manoeuvrability was deemed inferior to the FA-51. Following on from the technical problems with the AA-54, the RAAF was unwilling to take the risk of another advanced and unproven design and instead opted to stay with the AA-51C Mustang III development. The programme was formally cancelled in March 1948.

Specification: AA-67-1 & -2
Crew: one, pilot
Length: 44 ft 9¼ in (13.65 m)
Wingspan: 55 ft (16.76 m)
Height: 15 ft 9 in (4.80 m)
Wing area: 414 ft2 (38.50 m2)
Empty weight: 14,745 lb (6,688 kg)
Loaded weight: 18,114 lb (8,216 kg)
Powerplant: 2 × Rolls-Royce RB.50 Trent I turboprops, 750 shp plus 1,250 lbf residual thrust each
Maximum speed: 405 mph (650 km/h) at 25,000 ft (7,600 m)
Cruise speed: 270 mph (435 km/h)
Range: 1,800 mi (2,900 km)
Service ceiling: 36,800 ft (11,216 m)
Rate of climb: 2,600 ft/min (12.7 m/s)
Armament: none

Specification: AA-67-3 & -4
Crew: one, pilot
Length: 44 ft 9¼ in (13.65 m)
Wingspan: 55 ft (16.76 m)
Height: 15 ft 9 in (4.80 m)
Wing area: 414 ft2 (38.50 m2)
Empty weight: 16,625 lb (7,540 kg)
Loaded weight: 20,104 lb (9,119 kg)
Powerplant: 2 × Armstrong Siddeley ASM.1 Mamba 1 turboprops, 1,000 ehp each
Maximum speed: 417 mph (650 km/h) at 25,000 ft (7,600 m)
Cruise speed: 300 mph (435 km/h)
Range: 2,000 mi (3,220 km)
Service ceiling: 36,500 ft (11,125 m)
Rate of climb: 2,700 ft/min (13.7 m/s)
Armament: 4 × 20 mm Oerlikon FFB(A) cannon (250rpg); AA-67-4 later modified for 4 × 30 mm ADEN cannon and 2 × 1,000 lb or 500 lb bombs

Specification: AA-67-5 & -6
Crew: one, pilot
Length: 44 ft 9¼ in (13.65 m)
Wingspan: 55 ft (16.76 m)
Height: 15 ft 9 in (4.80 m)
Wing area: 414 ft2 (38.50 m2)
Empty weight: 17,745 lb (8,048 kg)
Loaded weight: 22,114 lb (10,031 kg)
Powerplant: 2 × Bristol Theseus II turboprops, 2,500ehp each
Maximum speed: 445 mph (716 km/h) at 25,000 ft (7,600 m)
Cruise speed: 300 mph (435 km/h)
Range: 2,385 mi (2,074 nmi, 3,837 km)
Service ceiling: 37,400 ft (11,400 m)
Rate of climb: 2,600 ft/min (12.7 m/s)
Armament: 4 × 20 mm Oerlikon FFB(A) cannon (250rpg)