Although the Chinook was basically a good design, the Anglo-Canadian design team under Sir Sydney Camm were disappointed with the wing which proved to be too thick in its cross section; this created problems with the airflow and inhibited the performance of the aircraft, especially at higher altitudes. In addition, the Chinook had the narrow fuselage associated with the liquid-cooled Merlin engine and the transition to the wider radial engine caused discontinuities and drag. A nthinner wing and a fuselage optimized for a radial engine seemed to offer potential for a much superior aircraft. The new laminar flow wing adopted for the Williwaw had a maximum thickness to chord ratio of 14.5% at the root tapering to 10% at the tip. By comparison the Chinook's wing, using a NACA 4 digit series wing section, was substantially thicker - 19.5% (root) to 12% (tip). The maximum thickness of the Williwaw wing was set further back at 37.5% of the chord versus 30% for the Chinook's wing. The wingspan was originally greater than that of the Chinook at 43 ft, but the wingtips were later "clipped" and the wing became shorter; 41 ft versus 41 ft 7 in. The wing planform was changed to an elliptical shape to accommodate the 800 rounds of ammunition for the four 20 mm cannons, which were moved back further into the wing. The new elliptical wing had greater area than the Chinook.
The thinner wing also displaced fuel tanks that had been fitted into the leading edge of the Chinook's wing. This greatly reduced fuel capacity but Hawker engineers added a new 21 in cm) bay ahead of the cockpit accommodating a 76 gallon fuel tank, giving a maximum of 360 gallons and an operational radius of 500 miles.
A further improvement of the Williwaw wing over that of the Chinook was the exceptional, flush riveted surface finish, essential on a high performance laminar flow airfoil. The redesigned main undercarriage legs were longer and had a wider track (16 ft) to improve stability at the high landing speed of 110 mph and to allow tip clearance for a new four-blade, 14 ft diameter propeller. The main undercarriage units were designed to incorporate a system of trunnions which shortened the legs as they retracted. The main wheels also needed new thin tyres in order to fit within the wing. Finally, the retractable tailwheel was fully enclosed by small doors. The most important change was the engine. The Williwaw received the R-2800-CB16 engine rated at 2,400 hp. This gave the aircraft a maximum speed of over 450 mph.
The first Williwaw flew on 28 June 1944. This was followed by the second, which first flew on 18 September 1944 and was assigned to engine development. The first production Williwaw was rolled off the line on 4 October 1944 with the type seeing service in Russian from February 1945 onwards.
During World War Two, exports of the Williwaw were limited to Lend-Lease supplies to Russia. Postwar, the very large numbers of American aircraft being dmped on the market at firesale prices depressed the demand for new-built aircraft. India bought a small number of aircraft in 1948/49 as an interim measure while its own programs were developed.
The main production version of the Williwaw with details essentially the same as those of the prototypes.
Ground attack version of the Williwaw powered by the R-2800CB-17 giving 2,500 hp. Wings were strengthened and provided with two extra pylons for a total bombload of 3,000 pounds in addition to its six or eight rockets. Additional armor installed under the engine and to protect the oil coolers.
Radar-equipped nightfighter version of the F.1.
Length: 33 ft 8 in
Wingspan: 41 ft 0 in
Height: 16 ft 1 in
Wing area: 302 ft²
Empty weight: 9,250 lb
Loaded weight: 11,400 lb
Max takeoff weight: 13,640 lb
Maximum speed: 452 mph at 18,400 ft
Range: 740 miles, 1,530 miles with 90 gallon drop tanks
Service ceiling: 36,500 ft
Rate of climb: 4,700 ft/min
Wing loading: 37.75 lb/ft²
Power/mass: 0.21 hp/lb
4 × 20 mm Mark II Hispano cannons, 200 rpg
2 × 500 lb or 1,000 lb (454 kg) bombs
8 × 3 in RP-3 rockets
Provision for 2 × 45 galon or 2 × 90 gallon drop tanks.