Design and developmentEdit
On 14 October 1938, Curtiss test pilot Edward Elliott flew the prototype XP-40 on its first flight in Buffalo. The XP-40 was the 10th production Curtiss P-36 Hawk, with its Pratt & Whitney R-1830 Twin Wasp 14-cylinder air-cooled radial engine replaced at the direction of Chief Engineer Don R. Berlin by a liquid-cooled, supercharged Allison V-1710 V-12 engine. The first prototype placed the glycol coolant radiator in an underbelly position on the fighter, just aft of the wing's trailing edge. USAAC Fighter Projects Officer Lieutenant Benjamin S. Kelsey flew this prototype some 300 miles in 57 minutes, approximately 315 miles per hour (507 km/h). Hiding his disappointment, he told reporters that future versions would likely go 100 miles per hour (160 km/h) faster. Kelsey was interested in the Allison engine because it was sturdy and dependable, and it had a smooth, predictable power curve. The V-12 engine offered as much power as a radial engine but had a smaller frontal area and allowed a more streamlined cowl than an aircraft with a radial engine, promising a theoretical 5% increase in top speed.
Curtiss engineers worked to improve the XP-40's speed by moving the radiator forward in steps. Seeing little gain, Kelsey ordered the aircraft to be evaluated in a NACA wind tunnel to identify solutions for better aerodynamic qualities. From 28 March to 11 April 1939, the prototype was studied by NACA. Based on the data obtained, Curtiss moved the glycol coolant radiator forward to the chin; its new air scoop also accommodated the oil cooler air intake. Other improvements to the landing gear doors and the exhaust manifold combined to give performance that was satisfactory to the USAAC. Without beneficial tail winds, Kelsey flew the XP-40 from Wright Field back to Curtiss's plant in Buffalo at an average speed of 354 mph (570 km/h).[N 2] Further tests in December 1939 proved the fighter could reach 366 mph (589 km/h).
An unusual production feature was a special truck rig to speed delivery at the main Curtiss plant in Buffalo, New York. The rig moved the newly built P-40s in two main components, the main wing and the fuselage, the eight miles from the plant to the airport where the two units were mated for flight and delivery.
The P-40 was conceived as a pursuit aircraft and was agile at low and medium altitudes but suffered from a lack of power at higher altitudes. At medium and high speeds it was one of the tightest-turning early monoplane designs of the war, and it could out turn most opponents it faced in North Africa and the Russian Front. In the Pacific Theater it was out-turned at lower speeds by the lightweight fighters A6M Zero and Nakajima Ki-43 "Oscar" which lacked the P-40's structural strength for high-speed hard turns. The American Volunteer Group Commander Claire Chennault advised against prolonged dog-fighting with the Japanese fighters due to speed reduction favouring the Japanese.
Allison's V-1710 engines produced 1,040 hp (780 kW) at sea level and 14,000 ft (4,300 m). This was not powerful compared with contemporary fighters, and the early P-40 variants' top speeds were only average. The single-stage, single-speed supercharger meant that the P-40 was a poor high-altitude fighter. Later versions, with 1,200 hp (890 kW) Allisons or more powerful 1,400 hp Packard Merlin engines were more capable. Climb performance was fair to poor, depending on the subtype. Dive acceleration was good and dive speed was excellent. The highest-scoring P-40 ace, Clive Caldwell (RAAF), who claimed 22 of his 28½ kills in the type, said that the P-40 had "almost no vices", although "it was a little difficult to control in terminal velocity". The P-40 had one of the fastest maximum dive speeds of any fighter of the early war period, and good high-speed handling.
The P-40 tolerated harsh conditions and a variety of climates. Its semi-modular design was easy to maintain in the field. It lacked innovations such as boosted ailerons or automatic leading edge slats, but its strong structure included a five-spar wing, which enabled P-40s to pull high-G turns and survive some midair collisions. Intentional ramming attacks against enemy aircraft were occasionally recorded as victories by the Desert Air Force and Soviet Air Forces. Caldwell said P-40s "would take a tremendous amount of punishment, violent aerobatics as well as enemy action". Operational range was good by early war standards and was almost double that of the Supermarine Spitfire or Messerschmitt Bf 109, although inferior to the Mitsubishi A6M Zero, Nakajima Ki-43 and Lockheed P-38 Lightning.
Caldwell found the P-40C Tomahawk's armament of two .50 in (12.7 mm) Browning AN/M2 "light-barrel" dorsal nose-mount synchronized machine guns and two .303 Browning machine guns in each wing to be inadequate. This was improved with the P-40D (Kittyhawk I) which abandoned the synchronized gun mounts and instead had two .50 in (12.7 mm) guns in each wing, although Caldwell still preferred the earlier Tomahawk in other respects. The D had armor around the engine and the cockpit, which enabled it to withstand considerable damage. This allowed Allied pilots in Asia and the Pacific to attack Japanese fighters head on, rather than try to out-turn and out-climb their opponents. Late-model P-40s were well armored. Visibility was adequate, although hampered by a complex windscreen frame, and completely blocked to the rear in early models by a raised turtledeck. Poor ground visibility and relatively narrow landing gear track caused many losses on the ground.
Curtiss tested a follow-on design, the Curtiss XP-46, but it offered little improvement over newer P-40 models and was cancelled.
- ↑ Scale Aviation Modeller International. SAM Publications. Feb 2002 Page 146