Curtiss P-36 Hawk - Design and Development

Design and Development

The Curtiss Model 75 was a private venture by the company, designed by former Northrop Aircraft Company engineer Donovan Berlin. The first prototype constructed in 1934, featured all-metal construction with fabric-covered control surfaces, a Wright XR-1670-5 radial engine developing 900 hp (670 kW), and typical United States Army Air Corps armament of one .30 in (7.62 mm) and one .50 in (12.7 mm) machine guns firing through the propeller arc. Also typical of the time was the total absence of cockpit armor or self-sealing fuel tanks. The distinctive landing gear which rotated 90° to fold the main wheels flat into the thin trailing portion of the wing, resting atop the lower ends of the maingear struts when retracted, was actually a Boeing-patented design for which Curtiss had to pay royalties.

The prototype first flew on 6 May 1935, reaching 281 mph (452 km/h) at 10,000 ft (3,000 m) during early test flights. On 27 May 1935, the prototype was flown to Wright Field, Ohio, to compete in the USAAC fly-off for a new single-seat fighter but the contest was delayed because the Seversky entry crashed on the way to the contest. Curtiss took advantage of the delay to replace the unreliable engine with a Wright XR-1820-39 Cyclone producing 950 hp (710 kW) and to rework the fuselage, adding the distinctive scalloped rear windows to improve rear visibility. The new prototype was designated Model 75B with the R-1670 version retroactively designated Model 75D. The fly-off finally took place in April 1936. Unfortunately, the new engine failed to deliver its rated power and the aircraft attained only 285 mph (459 km/h).

Although its competitor, the Seversky P-35, also underperformed and was more expensive, it was still declared the winner and awarded a contract for 77 aircraft. However, on June 16, 1936, Curtiss received an order from USAAC for three prototypes designated Y1P-36. The USAAC was concerned about political turmoil in Europe and about Seversky's ability to deliver P-35s in a timely matter, and therefore wanted a backup fighter. The Y1P-36 (Model 75E) was powered by a 900 hp Pratt & Whitney R-1830-13 Twin Wasp engine and further enlarged scalloped rear canopy. The new aircraft performed so well that it won the 1937 USAAC competition with an order for 210 P-36A fighters.

Its extremely low wing loading of just 23.9 lb/ft² gave it outstanding turning performance, and its high power-to-weight ratio of 0.186 hp/lb gave it superb climbing performance as well, especially for the time, although its lack of an engine supercharger handicapped it at high altitudes. Compared to the Allison-engined P-40, the P-36 shared the later P-40's traits of excellent high-speed handling, of roll rate that improved at high speed and of relatively light controls at high speed. However, it was underpowered affecting its acceleration and top speed and it did not accelerate in a dive as well as the P-40.