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The Douglas A2D-1 Skyshark

I just read the article on the A2D which suggested that Northrop designed the turboprop in 1939 before any successful turbojet design was in existence. The first flight of a turbojet powered plane was in Germany in 1939! The first turboprop engine, the Cs-1, was tested by a Hungarian, Gyorgy Jendrassik, in 1940, based on his 1938 design. The first turboprop engined plane went into production in 1942, but was later reengined with a piston engine to simply production for the Third Reich as Hungary was producing prop engines for the Luftwaffe. Jendrassik's centenary was celebrated by an issue of stamps in Hungrary in 1998. I hope this information is useful.
--Aaron Turner

The United States has overtaken the much publicized British lead in turbo-prop technology and there is every sign that it will not be relinquished ever! But that is only half the story, since the U.S. has also taken a commanding lead in the difficult field of turboprop-pow-ered aircraft design. For the U.S. now has not only the world's most powerful turboprop engines, but the most efficient by far; not only the world's biggest turbo-prop-powered airplane but the fastest as well.

Unlike most of the elements of U.S. aeronautical world leadership, its lead in the turboprop field has been established only in the past few months and only after a spurt of hard, determined effort. It is an astonishing fact that the turboprop engine was born right here in the United States. Way back in 1939 inventive John K. Northrop 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 diffi-culties, crystallized into the famous Northrop Turbodyne turboprop engine, easily the most powerful aircraft engine in the world. Recently purchased by the General Electric Co., the huge engine de-veloped 10,000 hp, which is, at the mo-ment anyway, more power on a single shaft than any airplane has accommo-dated.

First turboprop engine to reach the test stand was-again-a U.S. 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. Thus, these historical facts deny the impression of historical British leadership in this field, although none ques-tion their pioneering development of the straight turbojet engine, for which Sir Frank Whittle has been elaborately honored. But a combination of factors late in World War II caused the U.S. to virtually cast aside its infant turboprop and de-fault to the British, who continued to press its development with all speed. The impressive performance of the array of turbojet fighters then appearing (Lock-heed F-SO, Republic F-84, Gloster Meteor. DeHavilland Vampire, etc.) created a glamorous attraction for this engine that persists to this day in the fighter and interceptor phases of aircraft procure-ment. Fighter plane speeds crept past the 450 mph mark and pressed close to the 500 mph figure with conventional recipro-cating engines. Engineers at the Air Force's Wright Field and the Navy's Patuxent Test Center quickly pointed to the propeller as the culprit in this handi-capped piston-engine race with the tur-bojet fighter, whose speeds were already well past 600 mph and moving up. The formation of shock waves on propeller tips had been a well known fact since the 'twenties (the first supersonic research was performed on fast-turning propellers at Wright Field), and 'since it was real-ized that 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, both Air Force, Navy and industry engineers set 500 mph as the ceiling on the speed of a propeller-driven airplane. Since neither the Air Force nor the Navy was interested in any 500 mph airplanes at a time when 600 mph airplanes were com-mon, the future of the turboprop in the U.S. grew more clouded until it was almost shelved entirely! The Air Force flatly canceled all its turboprop develop-ment contracts and washed its hands of the engine.

[IMAGE] But that word "almost" is important here for, fortunately, there existed in the Navy Department a little band of engi-neer officers whose faith was unshaken in the engine. They admitted its limita-tions in the fast-climbing interceptor field but they recognized what the Air Force and even many Naval officers were over-looking: there's nothing wrong with a 500 mph bomber or attack plane (par-ticularly back in those days when the Douglas SBD Dauntless was doing only about 250 mph and the Curtiss SBC Hell-diver something less than 300 mph! These officers in the Bureau of Aeronautics Power Plant Section knew, on the basis of practi-cal tests plus numerous theoretical studies. that the turboprop engine promised a speci-fic fuel consumption of only one-half that of the turbojet engine and, if properly developed, a tower fuel consumption than the keenly-developed reciprocating engine. These promises were simply too good to ignore and these officers fought hard for the very small appropriations required to keep turboprop engine development alive. Happily they won out Despite constant criticism and heavy economy pressure. the Navy maintained its Allison T38 and T40 turboprop engine projects, culminating in successful static tests of these engines at the highest powers and lowest fuel consumption of any turboprops ever built (The new Pratt & Whitney TM is still more powerful and has even less fuel consumption than these Allison models!) 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. To simply provide a flight test bed for the engine itself, the reliable Boeing B-17 bomber was selected and the engine installed in the nose. This was a simple problem; what was really needed was a turboprop-powered combat airplane built especially for the job in order to provide actual performance figures on what a designed-for-the-purpose air-plane could do. But this wasn't so easy for there were little if any procurement funds for such a project, which could easily run into $5-10 million!

The Navy selected the Douglas AD Skyraider for its guinea pig for several reasons. First of all it was in quantity production and a tried-and-proved airplane. Secondly, 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. It therefore provided not only an economical source for testing their theories but also an excellent yardstick against which to gauge the new performance.

[IMAGE] Prior to this Navy decision, however, Douglas engineers had not been exactly idle. They, too, had seen the possibilities of both greatly increased performance and increased range in the turboprop engine and had made design studies of the AD Skyraider powered by a General Electric TG-l00 turboprop engine. Plans were com-pleted for an experimental conversion but difficulties with the engine forced eventual abandonment of the project At this early stage in turboprop history, Douglas engi-neers decided to build their own turbo-prop engine for the Skyraider through as-sembling various components. Design studies were completed on the installation of two Westinghouse 24C turbojet engines side by side with their tailpipe exhaust driving a large turbine which, in turn, drove counter-rotating propellers.

With the probability of extensive de-velopment problems on such an arrange-ment, 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. Indi-vidual air inlets were located in the nose with the jets exhausting out of exits be-low the wing. 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. It was at this point that Douglas' individual design develop-ment of the Skyraider and the Navy's decision to equip a flying tactical test air-plane with the new Allison T40 turboprop engine came together and merged into a single, joint project. The result is the Douglas XA~-1 Skyshark, our Plane of the Month. There are, of course, only two ways in which the performance of a given airplane can be increased: reducing its drag or in-creasing its power. Douglas took both courses. 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. For reasons of balance, this model of the Alli-son T40 engine is fitted with a short ex-tension shaft

But the design job turned out to be much more than the simple one of switching engines. 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 re-sulting in different stability requirements and, therefore, empennage changes. As a result there is actually little of the parent AD 5kyreido.r in the new A2) Skyshark. 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.

One of the factors often overlooked in a cursory examination of the piston-v~ turbine aircraft engine comparison, in which the lighter weight of the latter is always emphasized, is the tremendously increased length of the gas turbine engine over its compact piston rival In this case, the Wright R-3350 is only 82" long(less than 7'), wide the Allison T40 is over 26' long, more than twice as much. This increased length that must be provided for, resulted in an overall increase of about 3-1/2' in the length of the new A20 over its predecessor.

The A2D pilot who sits atop the dual drive shafts of the engine, is located well forward in the new Skyshark to provide all available vision. It will be seen that unlike the smooth blown canopy of the Skyraider, the new turboprop attack plane has an awkward-looking, slab sided enclosure. Like every other line in the air-plane, however, there is a reason for this, At the speeds of which the new plane is capable, 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 uses old4sshione'l 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 (which the owner of many of the new automobiles has discovered after breaking a windshield!) so that the A21) canopy consists 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.

The new attack plane features one of the first installations of the Air Research starting system. The Navy required that the A2D be able to get started without external power supply and the battery power required to turn the big turbine engine up to ignition Speed was extremely large. The light-weight Air Research unit is merely a small gas turbine engine pro-ducing air, which is piped to another small air turbine attached to the engine. AiResearch also used another turbine for cabin pressurizing and temperature con-trol, making the Skyshark virtually an all-turbine plane.

[IMAGE] Neither Douglas nor she Navy will discuss the performance of the new attack plane but it is certainly safe to say that it is mighty close to being a 500 mph combat craft, some even putting its top speed u p to 550 mph. It is certain, however, that its take-off run is phenomenally short. When you consider that from a dead start it has about 18,000 lb. of thrust and weight' just about 18,000 lb. you can appre-ciate that it can climb off even a tiny escort carrier in an almost vertical zoom, By shutting down one of the two separate engines making up the complete Allison T40 power plant, the A2D can cruise for better than 1,500 ml. while using only the same amount of fuel with the same bomb load as the 'AD. For the efficiency experts, Douglas says that the A2D has better performance in terms of load per mile per hour than that of the Skyraider. One of the new problems created by this high speed performance is the drag of the bombs, which are mounted external-ly under the wings. In order for the A2D to cruise at its most efficient speed, special streamlined bombs are being developed which will permit it to fly more than 50 mph faster than with ordinary bombs In-stalled.

The Navy has revealed how well pleased it has been with its experimental "flying test bed" by ordering the airplane directly into quantity production, proving the value of its original plan for the development of a tactical type in which to test its new engine. Grateful recipients of the new plane will be the Marine Corps pilots, who have been using their beloved Vought F4U Corsair for high-speed close air support. Already equipped with Grumman and McDonnell jet fighters, the A2D will round out the Marine Corps' aircraft requirements perfectly. But few should doubt that the regular Navy will see that it gets a few of the astonishing new attack planes for its own carrier air groups.

Parker Information Resources
Houston, Texas
E-mail: bparker@parkerinfo.com

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