A2D Skyshark

The A2D Skyshark was a turboprop, attack aircraft built by the Douglas Aircraft Company for the United States Navy. Reduction gear problems plagued the aircraft and, as a result, only seven were built. The Douglas XA2D Skyshark was a radical reconfiguration of the company's AD Skyraider that was then proving itself in combat in Korea. The AD was a basic and versatile design that proved capable of many improvements and upgrades. Two decades later, the Skyraider would cross over to the Air Force and a new generation of warriors would know it as the A-1 "Spad."

In 1939 the inventive John K. Northrop had conceived the idea of a gas turbine driven aircraft engine, this at a time long before the turbojet en-gine had been successfully developed. This idea, through a long series of difficulties, crystallized into the famous Northrop Turbodyne turboprop engine, easily the most powerful aircraft engine in the world. The huge engine developed 10,000 hp, which was more power on a single shaft than any airplane had accommodated. The first turboprop engine to reach the test stand was -- again -- a US product, the General Electric TG-100, which made its first test run on May 15, 1943. It was almost two years later that the first British turboprop made its first run; this was the Rolls-Royce Trent in March, 1945.

During the late 1940's and early 1950's, the Navy would try just about anything to increase the performance and capability of its aircraft. The turboprop engine seemed to offer higher speeds, while still offering low-speed controllability that would allow a turboprop-powered aircraft to operate off existing carrier decks.

The propeller was the culprit that handicapped piston-engines in the race for speed with the tur-bojet fighters. The formation of shock waves on propeller tips had been a well known fact since the 1920s. The speed of the air moving past a propeller is made up not only by the rotation of the blade but by its forward motion through the air. As the compo-nents of these two speeds increase, that tip portion of the blade undergoing shock losses spreads toward the hub with the result that at 500 mph nearly one third of the propeller blade length is suffering shock losses and its overall efficiency has dropped to as little as 50%. Thus engineers set 500 mph as the ceiling on the speed of a propeller-driven airplane.

The Navy's Bureau of Aeronautics Power Plant Section knew that the turboprop engine promised a specific fuel consumption of one-half that of the turbojet engine and, if properly developed, a lower fuel consumption than even reciprocating engine. Despite constant criticism, and heavy economy pressure, the Navy maintained its Allison T38 and T40 turboprop engine projects. As soon as the eventual success of the big Allison T40 double unit seemed assured, these Naval officers began searching for an air-plane in which their theories could be given the acid test.

The Navy selected the Douglas AD Skyraider for several reasons. It was in quantity pro duction and a tried-and-proved airplane. And it was the Navy's standard attack plane and, consequently, the one the new turboprop-powered craft would replace, if and when it was proven successful.

Work started in 1945, at which time the Navy did not have faith in jet-powered aircraft for carrier-based service. In June 1945, barely weeks after the XBT2D-1 (aka the A-1 Skyraider) had flown, Douglas was asked to submit a proposal for a similar aircraft, but this time powered by a turboprop engine. At first it was thought that a Skyraider modification would suffice, however the power of the XT-40 was nearly twice that of the R-3350 in the SPAD, so an entirely new airframe was developed. The Skyshark possessed a modified cell, a completely new tail unit and a strengthened chassis.

Douglas engineers had made design studies of the AD Skyraider powered by a General Electric TG-l00 turboprop engine, but difficulties with the engine forced abandonment of the project. Douglas engineers then decided to build their own turbo-prop engine for the Skyraider, and studies were completed on the installation of two Westinghouse 24C turbojet engines side by side. Their tailpipe exhaust would driving a large turbine which, in turn, drove counter-rotating propellers. Discouraged by the extensive development problems on such an arrangement, the Skyraider design crew next turned to a side by side installation of the two Westinghouse 24C turbojet engines without the turbine. resulting in a pure jet version of the big attack plane. This arrangement gave the airplane a good high speed but, like, all turbojet arrangements, presented serious take-off and endurance requirements for an attack-type airplane.

The Navy's decision to equip a flying tactical test airplane with the new Allison T40 turboprop engine resulted is the Skyshark. With an aerodynamically cleaned-up airframe most notable in a much thinner wing, and with the 3,035 shaft hosepower with 1,225-lb exhaust thrust of the Allison T-40 turboprop, it was estimated the "Skyraider successor" would be capable of nearly an additional 200 mph top speed, while maintaining the range capability, maneuverability, and carrier qualification of the existing aircraft. The 17% wing root thickness on the standard AD series was reduced to less than 12% on the new model, with a corre-sponding reduction in tail thickness, Not only does this reduced thickness lower the parasite drag of the wing on the AD but it also delays the onset of compressibility difficulties.

The power was better than doubled: from the 2,500 hp of the standard Wright R-3350-24W to the 5,500 hp of the Allison XT4O-A-2 turboprop engine. Douglas engineers replaced the original Wright R-3350 "Cyclone" radial engine with a Allison XT40-A-2 turboprop, driving a pair of contra-rotating propellers by means of a large gearbox. The XT40-A-2 was basically two T-38 turbojets linked by this main bearing to provide power to the contra-rotating propellers. It was possible to shut down one engine and have the aircraft fly on the other. The Allisons drew in air through a pair of ducts beneath the "chin," and their exhaust exited through a pair of large ports behind the cockpit.

The Skyshark began as a turboprop version of the AD Skyraider, but eventually was an entirely different airplane. The Skyshark was to be the most drastic transformation of the AD's basic airframe. As the design progressed, snore and more standard Skyraider parts proved unusable until there remained little of the original airplane. These changes were required due to the fact that the tremen-dous increase in power and performance brought with it much greater loads on the airplane and the necessity for increased structural strength. The location of the engine weight back over the wing, instead of forward in the nose as in the standard installation, meant balance changes resulting in different stability requirements and, therefore, empennage changes. The wing planform remained the same and the same landing gear and cockpit arrangement is used but even these are not the original parts, since the landing gear, for instance, had to be strengthened and its stroke increased.

The A2D pilot sat atop the dual drive shafts of the engine, located well forward in the new Skyshark to provide all available vision. Unlike the smooth blown canopy of the Skyraider, the turboprop attack plane had an awkward-looking, slab sided enclosure. Like every other line in the air-plane, however, there was a reason for this. At the speeds of which the plane was capable, the friction of the air generates' as high as 500 F of temperature over the ambient air which, on a warm day, is enough to soften the familiar blown plastic canopy materials. The A2D canopy used old fashioned glass, which can take plenty of heat before losing its strength or shape. Such glass is very difficult and expensive to form into compound curves, so that the A2D canopy consisted simply of fiat panels, with the top surface curved in one plane only. Location of the engine drive shafts below the pilot prohibited use of the escape tunnel used on the Douglas FJD Sky Knight fighter and the A2D is fitted with the conventional upward-firing ejection seat. With its streamlined canopy, high fin, and midnight blue paint job, the Skyshark looked as dangerous as its namesake. Dangerous it proved to be.

The first flew on 26 May 1950, with the second several months later. The two Skysharks were able early on to demonstrate that the estimated performance figures Douglas had quoted to the Navy were achievable; an order for 331 A2D-1s was made that fall as Chinese Communists armies overran the UN forces in North Korea and the Skyraider was proving its worth in the crucible of war.

During the third week of December 1950, Lt. Cmdr. Hugh Wood made the Navy's first preliminary evaluation flight of a new attack plane. Woods' initial flights went well, but the airplane was subject to gearbox problems that ultimately proved to be insolvable. On 19 December 1950, his XA2D lost an engine and the commander was killed during an emergency landing on the south lakebed.

Due largely to the failure of the Allison T40 program to produce a reliable engine, the Skyshark never entered operational service. Jet powered aircraft proved to be better suited to the tasks intended for the A2D and the project was shelved in the early stages of prototype flight testing. With the advent of what would become the Douglas A4D-1 Skyhawk, the hulking Skyshark was cut back to five production aircraft, which only made limited flights. Though ten aircraft were built, the last four did not fly.

The Navy tested the Hi-Bred Allison XT-40 not only in the A2D, but also in the XA2J-1, and several others. Even though all of the XT-40 programs were eventually ended in cancellation, the lessons learned by Allison allowed them to built the reliable powerplants still being used on Grumman E-2/C-2 and Lockheed P-3 and C-130 aircraft.