The Curtiss XP-42 was an experimental version of the P-36 that was developed in an attempt to reduce the drag caused by radial engines.
In the late 1930s inline and radial engines generated a similar amount of power. Inline liquid cooled engines were more vulnerable to battle damage, but they had a much smaller frontal area than the radial engines, and could easily contained within streamlined cowlings. In contrast high-powered radial engines needed a great deal of airflow to keep them cool, and so aircraft like the Curtiss P-36/ Hawk 75 had large flat fronts that caused enough drag to significantly lower their top speed. This is clearly demonstrated by two Curtiss Hawks. The P-36 with a 1,100hp radial engine had a top speed of 313mph, while the XP-40 of 1938, with a 1,050hp inline engine, could reach 340mph.
The XP-42 was developed in an attempt to reduce the drag imposed by the radial engine by hiding it behind a streamlined cowling. The fourth production P-36A (US Army serial number 38-004) was taken from the production line and was completed with a special Pratt & Whitney R-1830-31 Twin Wasp engine, with a long extension shaft that moved the propeller nearly two feet forward. The gap was filled with a streamlined cowling, and the propeller was given a large streamlined spinner. Cooling was provided by a large air scoop under the engine. The new aircraft was given the Curtiss designation Model 75S.
The speed of the XP-42 was disappointing. In its original configuration in March 1939 it could only reach 315mph. This was faster only 2 mph quicker than the P-36A and slower than the Allison-powered P-40. The aircraft also suffered from overheating problems.
After this initial failure the XP-42 was used to test out at least a dozen different combinations of cowlings, propellers and spinners, and eventually reached a top speed of 344mph, still slower than the P-40.
By the time the experiments finished the engine cowling had been reducing in length so often that it closely resembled a standard P-36. The aircraft retained the XP-42 specification until it was scrapped in January 1947.
One radial engined fighter would make a big contribution to the American war effort. This was the Republic P-47 Thunderbolt, which adopted a completely different brute-force approach to the problem of overcoming the drag imposed by the radial engine. The Thunderbolt had twice the engine power of the XP-42 in an aircraft that was three times heavier, but that had a top speed of 429mph.
As first built:
Engine: Pratt & Whitney R-1830-31 Twin Wasp
Wing span: 37ft 4in
Length: 30ft 3in
Empty Weight: 4,818lb
Gross Weight: 5,920lb
Max Speed: 315mph