Originally designated the XF4U-6, the AU-1 was a dedicated low-level ground attack variant developed from the F4U-5. It retained the metal skinned outer wing panels of the FU4-5 series, and it had the blown canopy with the fairing directly behind it. Its service was for the U. S. Marine Corps and the Marines. 25 Marine AU-1s were turned over to the French for use in Indochina, while others did serve with the U. S. Navy.
F4U-5N, BuNo. 124665, was converted to the XF4U-6 prototype, and it first flew on January 31, 1952, after having been redesignated the XAU-1. Deliveries of the 111 production AU-1 s were made between February 7 and October 10, 1952. AU-1 s were used by several Marine squadrons in Korea during the second half of the war. They remairied in Marine service until 1957, a year after they had been retired by the Navy Reserves. A few served with the French Navy until 1964.
A ground attack aircraft is not expected to fly at high altitudes, the two-stage, variabIe-speed super charger used on the F4U-5 was not needed. Instead, the AU-1 was fitted with an R-28OQ-83W which had a single stage, manually controlled supercharger instead. The longer fuselage of the F4U-5 was retained, and even the bulges on the si des of the forward fuselage remained where they had been on the F4U-5. The R-2800-83WA (also designated the R-2800-83W), with its single-stage supercharger, did not require the two cheek scoops.
The number of outer wing racks was increased from eight to ten, Five pylons were instalIed under each outer wing panel, and 5-inch rockets, 100-pound bombs, or 250- pound bombs could be carried on each. If only the inner, middle, and outer pylon on each wing was used, 500- pound bombs could be loaded. The AU-1 also retained the two pylons under the center wing section as weIl as the centerline station as found on the F4U-5. The gun armament remained the four 20-mm cannon ,with 231 rounds each ,were installed in the wings, used in all Corsairs from the F4U-4B on, but they received some modification for use in the AU-1 . This allowed them to all be fired together or in pairs. By using only one cannon in each wing at a time, firepower was cut in half, but firing time was doubled.
To make the aircraft more survivable jn the hostile environment at low altitudes, 25 pieces of armor were added to protect the engine, fuel tanks, and cockpit. The oil coolers were repositioned so that they no longer faced forward.They were mounted in the wing roots facing inboard. The air entered the wing scoops as be fore, but routed to the oil coolers. This design change protected the oil coolers from ground fire coming from the front of the aircraft and also increased protection from fire coming from below.
Performance decreased. The handling had suffered even more, and the AU-1 was not pleasant to fly.