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Design and Development:
The aircraft is of conventional post-World War II design, with a low-mounted delta wing, tricycle undercarriage, and a single turbojet engine in the rear fuselage, with intakes on the fuselage sides. The tail is of cruciform design, with the horizontal stabilizer mounted above the fuselage. Armament consisted of two 20 mm Colt Mk 12 cannon, one in each wing root, with 200 rounds per gun, plus a large variety of bombs, rockets and missiles carried on a hardpoint under the fuselage centerline and hardpoints under each wing (originally one per wing, later two).
The design of the A-4 is a good example of the virtues of simplicity. The choice of a delta wing, for example, combined speed and maneuverability with a large fuel capacity and small overall size, thus not requiring folding wings, albeit at the expense of cruising efficiency. The leading edge slats are designed to drop automatically at the appropriate speed by gravity and air pressure, saving weight and space by omitting actuation motors and switches. Similarly the main undercarriage did not penetrate the main wing spar, designed so that when retracted only the wheel itself was inside the wing and the undercarriage struts were housed in a fairing below the wing. The wing structure itself could be lighter with the same overall strength and the absence of a wing folding mechanism further reduced weight. This is the opposite of what can often happen in aircraft design where a small weight increase in one area leads to a compounding increase in weight in other areas to compensate, leading to the need for more powerful, heavier engines and so on in a cycle.
The A-4 pioneered the concept of "buddy" self air-to-air refueling. This allows the aircraft to supply others of the same type, eliminating the need of dedicated tanker aircraft - a particular advantage for small air arms or when operating in remote locations. A designated supply A-4 would mount a center-mounted "buddy store", a large external fuel tank with a hose reel in the aft section and an extensible drogue refueling bucket. This aircraft was fuelled up without armament and launched first. Attack aircraft would be armed to the maximum and given just enough fuel allowable by maximum take-off weight limits. Once airborne, they would then proceed to top up their fuel tanks from the tanker using the A-4's fixed re-fueling probe on the starboard side of the aircraft nose. They could then sortie with both full armament and fuel loads. While rarely used in US service since the KA-3 Skywarrior tanker became available, the F/A-18E/F Super Hornet includes this capability, with a view to the imminent retirement of dedicated tankers.
The A-4 was also designed to be able to make an emergency landing, in the event of a hydraulic failure, on the two drop tanks nearly always carried by these planes. Such landings resulted in only minor damage to the nose of the aircraft which could be repaired in less than an hour. Ed Heinemann is credited with having a large "K.I.S.S." sign put up on the wall of the drawing office when the aircraft was being designed. Whether this is true, the A-4 certainly is a shining example of the application of that principle to aircraft design.
The Navy issued a contract for the type on 12 June 1952, and the first prototype first flew on 22 June 1954. Deliveries to Navy and US Marine Corps squadrons commenced in late 1956.
The Skyhawk remained in production until 1979, with a total of 2,960 aircraft built, including 555 two-seat trainers.
Sources:
Wikipedia: A-4 Skyhawk
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