North American A-36 Apache - Design and Development

Design and Development

With the introduction of the North American Mustang I(P-51) with the RAF Army Co-operation Squadrons in February 1942, the new fighter began combat missions as a low-altitude reconnaissance and ground-support aircraft. Supplementing the Curtiss P-40 Tomahawks already in service, Mustang Is were first supplied to No. 26 Squadron RAF, then rapidly deployed to 10 additional squadrons by June 1942. First used in combat over the Dieppe Raid on 19 August 1942, a Mustang of No. 414 (RCAF) Squadron downed one of the formidable Focke-Wulf Fw 190, the first victory for a Mustang. Despite the limited high-altitude performance of the Allison V-1710 engine, the RAF was enthusiastic about its new mount which "performed magnificently."

During the Mustang I's successful combat initiation, North American's president Howard "Dutch" Kindelberger pressed the newly redesignated U.S. Army Air Forces (USAAF) for a fighter contract for the essentially similar P-51, 93 of which had passed into the USAAF when the Lend-Lease contract with Britain ran out of funds. The Mustang IA/P-51 used four 20 mm Hispano wing cannon in place of the original four .30 in (7.62 mm) M1919 Browning machine guns and two .50 in (12.7 mm) M2 Browning machine guns mounted in the wings and two .50 in (12.7 mm) "chin" machine guns. No funds were available for new fighter contracts in fiscal year 1942, but General Oliver P. Echols and Fighter Project Officer Benjamin S. Kelsey wanted to ensure the P-51 remained in production.

Since appropriations were available for an attack aircraft, Echols specified modifications to the P-51 to turn it into a dive bomber. The contract for 500 A-36A aircraft fitted with bomb racks, dive brakes, and heavier-duty wing, was signed by Kelsey on 16 April 1942, even before the first flight of the first production P-51 in May 1942. With orders on the books, North American Aviation (NAA) began modifying the P-51 to accept the bomb shackles which had already been tested in a "long-range ferry" program RAF had stipulated. Engineering studies totaling 40,000 hours and wind tunnel testing with a ⅛-scale model was completed in June 1942. Utilizing the basic P-51 airframe and Allison engine, structural reinforcing "beefed up" several high stress areas and "a set of hydraulically operated dive brakes were installed in each main wing plane." Due to the slightly inboard placement of the bomb racks and unique installation of four cast aluminum dive brakes, a complete redesign of the P-51 wing was required.

The first A-36A (42-83663) was rolled out of the NAA Inglewood plant in September 1942, rapidly going through flight testing with the first flight in October, with deliveries commencing soon after of the first production machines emerged. The A-36A continued the use of nose-mounted .50 in (12.7 mm) machine guns along with wing armament of four .50 in (12.7 mm) caliber machine guns (this was the heaviest armament package of all the Allison-power Mustangs). The USAAF envisaged that the dive bomber would operate mainly at altitudes below 12,000 ft (3,658 m) and specified the use of a sea level-rated Allison V-1710-87, driving a 10 ft 9 in (3.28 m)-diameter three bladed Curtiss-Electric propeller and delivering 1,325 hp (988 kW) at 3,000 ft (914 m). The main air scoop inlet was redesigned to become a fixed unit with a larger opening, replacing the earlier scoop which could be lowered into the airstream. In addition the A-36 carburetor air intake was later fitted with a tropical air filter to stop sand and grit being ingested into the engine.

The USAAF later ordered 310 P-51As which were essentially A-36s without the dive-brakes and nose mounted weapons, leaving an armament of four wing-mounted 0.50 in (12.7 mm) Browning machine guns. An Allison V-1710-81 1,200 hp (895 kW) was fitted and used the same radiator and air intake as the A-36A. The P-51A was still fitted with bomb racks although it was not intended to be used primarily as a fighter-bomber and the racks were mainly used to carry drop tanks.