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MODEL AIRPLANE PLANS PAGE 5

MODEL AIRPLANE PLANS: PAGE 4

The Sopwith Triplane N500

The Sopwith Pup was followed by the Triplane, which was passed by the Sopwith experimental department on May 28th, 1916. Looking back, it is hard to realize the revolutionary nature of the Triplane at the time it appeared. Nothing quite like it had ever been built for military purposes, and the best measure of its success is provided by the profusion of German and Austrian single-seat fighter triplanes which appeared after the impact made by the Sopwith Triplane had earned it a eulogy from General von Hoppner, commander of the German air service.

An astonishing variety of triplanes were built by the A.E.G., Albatros, Austrian Aviatik, Brandenburg, D.F.W., Euler, Fokker, Lohner, Oeffag, Pfalz, Roland, Sablatnig, Schutte-Lanz and W.K.F. concerns; and their very numbers hint at an almost frantic search for the elusive quality, presumably thought to be inherent in the triplane configuration, which made the Sopwith Triplane the fine fighting aeroplane that it was.

It has been said that Anthony Fokker was so anxious to produce an aircraft which would be an adequate reply to the new Sopwith fighter that he resorted to subterfuge to obtain an example of the Triplane. He contrived to arrange for the delivery to his works of the remains of a Sopwith Triplane which had been shot down, despite the fact that the aircraft should have gone to the German experimental field at Adlershof. However, the Fokker Dr. I triplane which was ultimately designed by Reinhold Platz, Fokker's chief designer, was a very different aeroplane from the Sopwith Triplane.

In the Sopwith type, the triplane layout was adopted in order to give the pilot the widest possible field of vision, and to ensure maneuverability. The central wing was level with the pilot's eyes and obscured very little of his view, and the narrow chord of all the mainplanes ensured that the top and bottom wings interfered less with his outlook than the wings of a biplane. The narrow chord aided maneuverability, for the shift of the center of pressure with changes of incidence was comparatively small; this permitted the use of a short fuselage. At the same time, the distribution of the wing area over three mainplanes kept the span short and conferred a high rate of roll.

The handling qualities of the Triplane were excellent. It is now regarded as only slightly less maneuverable than the Pup, but many pilots preferred it to the little biplane.

The fuselage and tail-unit were generally similar to those of the Pup in both appearance and construction, but the structure was stressed to take the 110 h.p. Clerget engine. The wing structure was of considerable interest. Each wing had two main spars, 15 inches apart; those of the upper mainplane were solid, but in the middle and bottom wings they were spindled out between the compression struts. The wings were internally cross-braced with wire.

The most interesting structural feature of the Sopwith Triplane was its interplane bracing. On each side there was only a single broad interplane strut which was continuous from the top wing to the bottom, and passed through a shaped slot in the appropriate compression strut of the middle wing. The center section struts were similarly continuous from the center section to the bottom longerons of the fuselage; the middle wings were secured by means of long pins to special aerofoil-shaped stubs on the center-section struts. Bracing wires were few: a single landing wire and double flying-wires were fitted on each side, and there were additional drag-wires on the middle wing. Ailerons were fitted to all three mainplanes, and were hinged to the rear spars. The shape of the wing-tips made them similar to those of the Pup, and a similar tailplane was used. Late production Triplanes had a smaller tailplane in which the leading edge was shorter than the trailing edge.

The armament consisted of a single fixed Vickers gun mounted centrally on top of the fuselage and synchronized to fire forward through the airscrew.

The first prototype Sopwith Triplane, N500, went to France in mid-June, 1916, to undergo Service trials with Naval "A" Fighting Squadron at Furnes. The Triplane was an instant success, and no time was lost in testing it in action, for it was sent up on an interception within a quarter of an hour of its arrival at Furnes.

The type was ordered by the Admiralty for the R.N.A.S., and the War Office followed suit by ordering 266 machines for the R.F.C. As with the Pup, Sopwiths were to build the R.N.A.S. Triplanes, whilst other contractors undertook production of the type for the R.F.C.

After the Battle of the Somme, air fighting increased in intensity, and the balance was not redressed by the transfer of Sopwith 1 1/2-Strutters from the R.N.A.S. to the R.F.C., nor by the loan of R.N.A.S. squadrons to the R.F.C. On November 20th, 1916, Sir Douglas Haig wrote to the War Office and asked for twenty more fighting squadrons. Major-General Trenchard amplified this request at the meeting of the Air Board held on December 11th and, so critical was the situation in France, asked for everything the R.N.A.S. could lend to be handed over to the R.F.C. His specific immediate demands were for four complete R.N.A.S. squadrons, one hundred Rolls-Royce and fifty Hispano-Suiza engines.

The Admiralty complied with as much of this demand as they reasonably could, but suggested that, instead of the fifty Hispano-Suiza engines, they should transfer to the R.F.C. sixty complete Spad S-7s out of their current contract for 120 This offer was accepted with alacrity. In February, 1917, a further agreement was made, under which the R.N.A.S. exchanged all its Spads for all the Sopwith Triplanes then on order for the R.F.C.

Thus it was that the Sopwith Triplane was used operationally by the R.N.A.S. only. In point of fact, all the 266 machines which had been ordered for the R.F.C. were not delivered: it is known that the Clayton and Shuttleworth contract was reduced by 120, and there may have been other reductions.

Deliveries of production Triplanes to the R.N.A.S. had begun late in 1916, and by mid-February, 1917, No. 1 (Naval) Squadron had received sixteen machines. The unit was one of the four R.N.A.S. squadrons which were attached to the R.F.C. in response to Major-General Trenchard's request, and on February 15th, 1917, it moved from Furnes to Chipilly, where it was attached to the I 4th (Army) Wing.

For several weeks the squadron practiced formation flying and gunnery, and made its first offensive patrol in early April, 19 17. The Battle of Arras began on the 9th of that month, and aerial fighting reached a pitch which had never previously been experienced. Between April 22nd and May 5th, 1917, the Triplanes of Naval One flew ninety-five offensive patrols, engaged 175 enemy aircraft, destroyed four of them and drove down twelve out of control.

On April 2 2nd, 19 17, two of No. 1 (Naval) Squadron's Triplanes, flown by Flight Commander R. S. Dallas and Flight Sub-Lieutenant T. G. Culling, met an enemy formation of fourteen two-seaters and single-seat fighters. The Germans were flying towards the lines at 16,000 feet. The Triplanes attacked at once, broke up the enemy formation, shot three of them down, and harried the remainder for 45 minutes until the Germans retreated eastward.

By April, 19 17, No. 8 (Naval) Squadron also had received Sopwith Triplanes as replacements for its Pups, and in the hands of such pilots as Flight Commander S. J. Goble, D.S.O., D.S.C., Squadron Commander C. Draper, D.S.C., Flight Commander C. D. Booker, D.S.C., Flight Commander R. A. Little and their squadron-mates, the Triplane acquitted itself gloriously.

It was of this period that H. A. Jones wrote in The War in the Air: "The sight of a Sopwith Triplane formation, in particular, induced the enemy pilots to dive out of range."

The performance of the Triplane had been improved by the installation of the more powerful 130 h.p. Clerget engine. Standard armament continued to be the single Vickers gun, but a small batch of six machines armed with twin Vickers were built by Clayton & Shuttleworth.

After Arras, it was decided to transfer the main Allied effort to the British front in Flanders, and preparations began for the action which became known as the Battle of Messines. On May I 5th, 19 17, the Eleventh Army Wing was reinforced by the arrival from Dunkerque of No. 10 (Naval) Squadron, equipped with fifteen Sopwith Triplanes. On June 1st, No. 1 (Naval) Squadron was transferred from the Third Army.

On June 6th, thirteen of Naval Ten's Triplanes fought fifteen enemy aeroplanes and shot down five without loss to themselves. Two of the five were Albatros scouts which fell in flames under the fire of Flight Sub-Lieutenant Raymond Collishaw.

Collishaw was probably the best known exponent of the Sopwith Triplane's superb fighting qualities. A Canadian, he was given command of "B" Flight of No. 10 (Naval) Squadron on April 1st, 1917. This was the famous "Black Flight", as redoubtable a fighting unit as took the air during the war: between May and July, 19 17, it accounted for no fewer than eighty-seven enemy aircraft. All the pilots were Canadians; the original members were Flight Sub-Lieutenant E. V. Reid, Flight Sub-Lieutenant J. E. Sharman, Flight Sub-Lieutenant G. E. Nash, and Flight Sub-Lieutenant W. M. Alexander. The Triplanes of the Black Flight were named Black Death, Black Maria, Black Roger, Black Prince and Black Sheep.

In a combat on June 26th, 19 17, Nash was wounded and forced down behind the enemy lines by Leutnant Allmenroder, a German pilot with thirty victories to his credit. Next day Collishaw avenged the loss of his friend: in a fight which began near Courtrai he shot down and killed Allmenroder, whose green-tailed Albatros crashed on the outskirts of Lille.

In twenty-seven days during June, 1917, Collishaw shot down sixteen enemy machines, of which all save three were Albatros and Halberstadt single-seat fighters.

At the end of August, 1917, No. 10 (Naval) Squadron began to re-equip with Sopwith Camels. Three of its Triplanes were then transferred to No. 1 (Naval) Squadron, which in turn gave up its beloved Triplanes on its withdrawal on November 2nd, 1917. The first Triplane squadrons to begin re-equipment with Camels were No. 8 (Naval), which had received a few Camels by the end of July, 1917, and No. 9 (Naval), which exchanged its Triplanes and Pups for Camels between mid-July and August 4th.

The Battles of Ypres were therefore the last actions over which Sopwith Triplanes flew. They fought with distinction until their final demise.

One Sopwith Triplane, N-5431, was used in Macedonia. It was on the strength of No. 2 Wing R.N.A.S., and in March, 19 17, it was allocated to the new R.N.A.S. unit known as "E" Squadron, which later combined with a Royal Flying Corps detachment to form the Composite Fighting Squadron, based at Hadzi Junas as a countermeasure to the German bomber squadron then operating from Hudova. However, N-5431 never reached Hadzi Junas. It flew first to Stavros; and, in company with four 1 1/2 Strutters, set out for Salonika on March 26th, 1917. Its pilot was Flight Lieutenant John Alcock. When landing at Salonika, Alcock made one of the few errors of judgment in his distinguished flying career: he overshot the small aerodrome and wrecked the Triplane. The wreckage was taken back to Mudros and rebuilt: it was still flying from Mudros at the end of September, 1917- On the 30th of that month it was flown by Lieutenant H. T. Mellings when he shot down an enemy single-seat fighter seaplane. By the end Of 1917 the Triplane was no longer a front-line aircraft. Its replacement by the Camel was viewed with mixed feelings by some of the units that had flown it, for it had proved to be a formidable fighter. It combined the maneuverability of the Pup with the better performance bestowed by the more powerful engine, yet despite its distinguished record it has always been neglected by historians.

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The Chance Vought TBU-1 Seawolf

In 1939, the Bureau of Aeronautics sent the industry a request for proposal on a new torpedo bomber. In April 1940, the U.S. Navy contracted with Vought for a single prototype. Designated XTBU-l and named Sea Wolf, it made its first flight 2 weeks after Pearl Harbor.

A mid-wing monoplane, the three-place Sea Wolf was a large machine powered with the big 1800-hp P&W R-2800-6 engine,a wing span of 56 ft 11 in, a length of 39 ft and a height of 18 ft 7 in. Its top speed of 3ll mph made it an exceptional performer. It was well ahead of the TBF Avenger and the U.S. Navy set about to purchase 1,000 of them.

The XTBU-1 was being flight tested at Anacostia in Washington when, during one of the arrested landings, the hook caught and tore the whole rear end off the airplane. The airplane went bouncing down the runway on just the wheels and forward portion of the fuselage. The two sections were returned to Stratford and on an expedited basis, the Experimental Department worked day and night for four solid weeks to rebuild the tail section. The day they finished, they were pushing it back across the street to the hangar when a Navy cadet lost control of his plane while taxiing in the same area. Everybody scattered as the careening trainer slammed into the rear of the XTBU-1, completely chewing up the tail that everybody had worked so hard to rebuild. If anybody came close to a lynching, it was that poor cadet.

Rebuilt again, the XTBU-1 was finally accepted by the U.S. Navy and a production contract was forthcoming. Vought was heavily committed to meet schedules on their other aircraft and arrangements were made with Consolidated-Vultee of Allentown, PA, to produce the TBU as the TBY. The first production TBY-2, as the production planes were designated, was delivered in November 1944. One hundred eighty-nine were built before the contract was canceled. Initially, the TBY-2 was issued to at least two squadrons. The squadrons were training for deployment as the war was drawing to a close. The TBY-2’s were withdrawn, however, and the squadron equipped with TBMs as it was felt there were still so many bugs in the aircraft that could not be worked out before the deployment date. Thereafter, the TBY-2’s were used in a utility configuration at various bases. Some TBY-2’s were assigned to Reserve squadrons.

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The Convair XFY-1 Pogo

The Convair XFY-1 Pogo is one of many attempts made after World War II to devise a practical Vertical Take-Off and Landing (VTOL) combat aircraft. The British finally succeeded with the Hawker-Siddeley Harrier, but before this aircraft arrived, firms around the United States experimented with various VTOL configurations. Wartime experiences had revealed how vulnerable fixed land bases could be to enemy assault from land or air. Aircraft carriers were also vulnerable as demonstrated by Allied experiences with the Japanese kamikaze threat. The U. S. Navy depended on aircraft carriers for many things, including fleet defense, but to assign a carrier task force to protect every convoy or cover every naval operation was impossible. After the U. S. Army Air Forces and the Navy demonstrated practical helicopters during World War II, naval strategists began considering the feasibility of stationing VTOL interceptors aboard non-aircraft carrier hulls.

In 1947, the U. S. Air Force and the Navy conducted design studies under Project Hummingbird. With these data and captured German material from the Focke-Wulf Triebflugel (thrust-wing) program, the Navy launched a formal VTOL fighter study in 1948. The aim was to develop an aircraft that could take off and land vertically on destroyers, LSTs (Landing Ships, Tank), fleet oilers, transports, and larger ships not otherwise equipped to handle conventional aircraft. In theory, a VTOL fighter could protect the mother ship or join other VTOL fighters to defend a task force or convoy.

After four years of study, the Navy awarded contracts to Convair and Lockheed to design, build, and fly experimental VTOL fighters in May 1951. Each company agreed to build two prototypes but in the end, they built only one Lockheed XFV-1 and one Convair XFY-1 Pogo. The XFV-1 never made a vertical takeoff and landing primarily because the Navy gave to Convair the only powerplant rated for both vertical and horizontal flight. The XFY-1 could not only take off and landing vertically, it could also transition to horizontal flight and back and did so many times. A number of interesting design features contributed to this prototype's success.

Both airplanes flew behind the same powerful Allison YT-40 turboprop engine. Allison created the YT-40 by mounting two smaller T-38 powerplants side-by-side and feeding their output into a single, massive gearbox. This brute pumped 5,850 shaft horsepower into two, 4.8 m (16 ft) diameter, counter-rotating propellers.

At rest, Pogo sat atop the trailing edges of its two wings and dorsal and ventral fins. Convair fitted a small, castering wheel onto the end of a strut several feet long and mounted four of these to form an improvised landing gear at the tips of the wings and fins. At touchdown, the struts compressed several feet, like a child's pogostick, to dampen impact forces. There were no brakes and the wheels rolled freely so flying under no-wind conditions was important. This was tolerable on a prototype but the Fleet could not have accepted a production VTOL fighter without brakes on the landing gear.

Because it launched and landed vertically but cruised horizontally, the pilot's seat had to rotate for safety and comfort in both flight regimes. Convair provided about 7.6 m (25 ft) of rope tied inside the cockpit so the pilot could dismount safely in case of off-field or emergency vertical landing. Although weapons were not tested, one proposed armament configuration consisted of either 48 folding-fin aerial rockets or up to four 20mm cannon mounted in the wing tips.

By February 1954, Convair had tested the engine in a vertical stand at Lindbergh Field, San Diego. It performed without problems and the company joined the engine to the airframe a month later. In April, Convair moved the project to Naval Air Station Moffett Field near Sunnyvale, California, for a series of tethered flight tests. It ran these experiments in the old Airship Hangar Number One built in the early 1930s to house the dirigible USS Macon.

On April 29, 1954, James F. "Skeets" Coleman, a lieutenant colonel in the Marine Reserve and a Convair engineering test pilot, made the first tethered flight in the Pogo. The XFY-1 was very much experimental. No other propeller-driven aircraft with similar size, weight, and engine power had ever attempted to take off and land vertically. The Pogo required safety lines to protect the pilot and the aircraft. Convair removed the propeller spinner and rigged a tether to a fitting in the nose. The tether streamed from a motorized reel controlled by Convair flight test engineer, Bob McGreary. McGreary could wind-up the reel and snatch the Pogo upright if Coleman lost control. Four more lines steadied each wingtip.

Coleman completed many tethered flights in the hangar, more than sixty hours of flying time, but it was dangerous work. The 4.8 m (16 ft) diameter propellers thrashed up a tremendous airflow that turned extremely turbulent as it washed against the inside of the hanger. Several times, Coleman called McGreary to "catch me, catch me" and the engineer slapped a button, spinning the reel to tighten the tether and steady the teetering Pogo.

By August, it was time to move outdoors. Coleman completed his first free flights on August 1st. He rose 6 m (20 ft) on the initial attempt but soared to 45 m (150 ft) on the second try. A short time later, Convair moved the aircraft to Naval Auxiliary Air Station Brown Field, California, to continue testing, including transition from vertical to horizontal flight. Coleman flew more than 70 additional takeoff-hover-landing flights in keeping with his conservative, safety-first approach to the XFY-1. He gained valuable experience with every flight. On November 2, 1954, Coleman finally transitioned and flew horizontally for 21 minutes. The test pilot spent seven minutes hovering. Just two days later, the aircraft made its public debut. Coleman launched and transitioned about 15 m (50 ft) above ground, thanks to tremendous engine power and a low-drag, streamlined airframe. The Pogo was fast too. Even with the throttle set at minimum power, the XFY-1 knifed through the air at well over 483 kph (300 mph). The airplane had no speed brakes or spoilers to help control airspeed and Coleman often outpaced the chase aircraft assigned to monitor him.

Trouble controlling low-speed velocity only aggravated the problems encountered during landing. Coleman's technique was interesting. He approached the field low with the engine set at flight-idle. At mid-field, he popped the control stick back into his stomach and pitched the airplane's nose straight up. The speed fell sharply but just as he reached the peak of his climb, Coleman applied power and stopped the Pogo in mid-air. With practice, the testpilot could stop the climb in a hover, reduce power and "back" down to a nice landing.

His descents often began higher than 300 m (1,000 ft). The aircraft was not stable and maintaining a hover required constant corrective action on the flight controls. Close above ground, the Pogo descended through its own, turbulent propwash, and Coleman fought the controls to get through it. With great skill and huge control inputs (stick and rudder pedal deflections), the test pilot brought this flying experiment back to earth safely, every time.

Yet another problem for the pilot made landings the most challenging part of flying the Pogo. When descending for touchdown from a high hover, Coleman found it almost impossible to judge rate-of-descent accurately with eyeballs alone. The Ryan Aeronautical Company developed a compact radar altimeter and mounted it in the left wingtip pod. Signals from the altimeter activated three lights: green signaled a stable hover or ascent, amber meant the rate of descent was safe, and red signaled an unsafe dive toward the ground at more than 10 feet per second.

Coleman climbed the airplane to 3,000 m (10,000 ft) on February 5, 1955. At this altitude during winter, temperatures can drop to freezing, yet he never closed the canopy once, during the entire time he flew the XFY-1. Convair installed an ejection seat but everyone thought it unreliable and technicians disarmed it. If serious trouble occurred in flight, Coleman's only option was to "step over the side" but it was considerably easier to leave the airplane if the canopy was already open.

No other pilot flew the airplane until May 19, 1955. John Knebel attempted to fly without tethered rig experience and the flight nearly ended in disaster. The Navy moved the tether rig from Moffett Field to Brown, and two other pilots began training in May 1956 but the end was already near. The giant gearbox had begun to wear and bits of metal were appearing in the lubricating oil. It was time for a major overhaul but the Navy was becoming enthusiastic about flying fixed-wing jets from aircraft carriers. Coleman had made his last flight on June 16, 1955. Interest in the program, and the funding, was disappearing and on August 1, 1956, the Navy closed the books on the XFY-1.

The Pogo proved that the VTOL fighter concept was theoretically possible but that much work remained to make the idea operationally practical. As it stood, flying the XFY-1 required above-average piloting skills and special training. It remained near San Diego for several more years until the Navy shipped it to Naval Air Station Norfolk, Virginia, and the Pogo sat "gate guard" there for a number of years. In 1973, the Navy transferred the aircraft to the National Air and Space Museum.

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The Douglas A-20 Havoc

In early 1939, prior to the signing of the first contract for the French DB-7, Douglas had begun a major redesign of the DB-7B to meet US Army Air Corps' requirements. On June 30, 1939, the US Army rewarded Douglas' efforts by placing an order for the DB-7 under the designation A-20.

The Air Corps version of the Douglas bomber corresponded roughly to the DB-7B version that had been ordered by Britain. As compared to the French DB-7A, the A-20 differed in using steel forgings instead of dural for its main wing attachment fittings, in having a strengthened wing and fuselage structure to cope with a 3750-pound increase in gross weight, and in being fitted with a six-inch longer nose offering 25 percent more glazed area, and in having self-sealing fuel tanks with a total capacity of 394 US gallons. It differed from the DB-7B in having American armament, which consisted of four forward-firing 0.30-inch machine guns -- mounted two each inside external side nose blisters instead of inside the lower nose section. In addition, twin flexible 0.30-inch guns were installed in an open dorsal position, and one flexible 0.30-inch machine gun was carried in a ventral tunnel position. Provision was made for a fixed rearward-firing 0.30-inch machine gun mounted in each engine nacelle. These rearward-pointing guns were fired by the rear gunner by means of a foot pedal.

Two separate versions were ordered by the Air Corps -- the A-20 which was to be a high-altitude version, and the A-20A which was intended for low- and medium-altitude operations.

Originally, 63 A-20s were ordered (serial numbers 39-735/797). The A-20 was to be powered by turbosupercharged Wright R-2600-7 radials. The large turbosupercharger was mounted internally on the outer flanks of the engine nacelle, directly under the wing. The A-20 was intended as a high-altitude light bomber with a performance sufficient to make it essentially immune from interception.

Only one aircraft was destined to be completed as an A-20. This was the first aircraft on the A-20 order (39-735). It was fitted with the turbosupercharged R-2600-7 engine which offered a power of 1700 hp at 20,000 feet. Unfortunately, the turbosupercharger installation was large and bulky, and the engines developed serious cooling problems. In the meantime, Air Corps requirements were changing, and there was no longer a perceived need for a high-altitude light bomber.

Consequently, a decision was made to convert all the other A-20s on the order to A-20A configuration with 1600 hp Wright R-2600-11 engines without turbosuperchargers.

39-735 later became the prototype of the P-70 night fighter series, and most of the A-20 order was actually completed as P-70s rather than A-20As. The remaining three aircraft on the A-20 order were delivered as reconnaissance aircraft under the designation F-3.

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The Douglas A-26 Invader

The Douglas A-26 (later B-26) Invader was the outstanding American twin-engined light bomber of the Second World War. Although it did not begin to reach combat units until the spring of 1944, it went on to provide outstanding service for the rest of the war in both the European and the Pacific theatres of action. The Invader remained in American service in substantial numbers after the war, and was a major participant in the Korean War and was even around to serve in both phases of the Vietnam conflict, first with French and then later with American units. Although the last Invaders were withdrawn from USAF service in 1972, Invaders remained in service with many smaller air forces for many years thereafter. Invaders participated in several small-scale conflicts during the 1950s and 1960s, and carried out numerous clandestine operations, including the abortive Bay of Pigs operation of 1961.

The A-26 Invader originally began as a private venture on the part of the Douglas plant at El Segundo, California. In the autumn of 1940, Douglas began a preliminary design study to develop a common successor to the Douglas A-20, Martin B-26, and North American B-25 bombers, none of which had yet entered service with the US Army Air Corps. The Bomber Branch of the Experimental Engineering Section at Wright Field, Ohio assisted in this effort by pointing out some of the deficiencies of the company's DB-7 twin-engined attack bomber that they had evaluated--lack of crew interchangeability, insufficient defensive armament, inadequate offensive firepower, and excessively long landing and takeoff distances.

At the end of January 1941, a team led by Edward Heinemann and Robert Donovan had come up with a proposal for a new twin-engined attack aircraft that would satisfy these requirements. It had a broad general family resemblance to the A-20 Havoc which was just then entering service with the Army Air Forces. It featured a mid-mounted wing with a laminar flow aerofoil and which was fitted with electrically operated double-slotted flaps. The aircraft was to be powered by a pair of 2000-hp Pratt & Whitney R-2800-77 air-cooled radials. The aircraft was to have a large internal bomb bay capable of carrying 4000 pounds of bombs or two torpedoes, and was to be fitted with external racks underneath the outer wings for additional ordnance. The defensive armament was to be provided by a pair of remotely-controlled dorsal and ventral turrets, each housing two 0.50-inch machine guns and operated by a gunner sitting in a separate compartment behind the bomb bay.

A decision was made that the aircraft would be built in two separate versions, a three-seat light bomber version with a transparent nose for a navigator/bombardier and a two-seat night-fighter version with a solid nose carrying heavy forward-firing armament and AI radar. The two versions were to be essentially identical except for their noses.

The mockup was inspected between April 11 and 22 of 1941, and on June 2 the War Department authorized the construction of two prototypes under the new designation A-26. The first aircraft was to be a three-seat attack bomber version with a transparent nose and was to be designated XA-26-DE. The second aircraft was to be a two-seat night fighter version designated XA-26A-DE. Three weeks later, the contract was amended to include a third prototype designated XA-26B-DE. It was to be a three seat ground attack aircraft with an unglazed nose housing a 75-mm cannon.

On February 28, 1941, Douglas proposed that 500 of these aircraft be built at the Santa Monica plant, with the first deliveries set to begin 20 months after contract approval. However, the Douglas bid of $142,250 per aircraft was considered excessively high by the Army's Materiel Division, and negotiations between the manufacturer and the War Department delayed approval of the project until October 21, 1941. At that time, it was planned that the three prototypes would be built at the Navy-controlled El Segundo but with production aircraft being built at the USAAF-controlled Santa Monica plant. The USAAF later decided to switch production of the A-26 to new Douglas plants that had been built in Long Beach, California and in Tulsa, Oklahoma in order not to disrupt ongoing production of A-20s, C-47s and C-54s at the Santa Monica plant.

There were some delays in the A-26 project which can be traced to USAAF indecision about what they really wanted. The AAF was initially undecided about what kind of mix that they wanted between light bombers with transparent noses, ground attack planes with solid noses housing 75-mm or 37-mm cannon, and attack bombers with solid noses housing a battery of forward-firing machine guns. The Army initially said that it wanted a 75-mm nose cannon installed in all 500 aircraft on order, but shortly thereafter changed its mind and wanted Douglas to proceed with its transparent-nosed light bomber version (which was to be designated A-26C) while continuing to develop alternative armament for the A-26B ground attack version.

The lengthy delays and slipped schedules led to a lot of mutual recrimination and finger-pointing between Douglas and the War Department, with each party blaming the other for the problems. There were late deliveries of undercarriage struts, self-sealing tanks, turrets, and other government-furnished equipment items such as engines, propellers and generators. The company was criticized for not making strong enough efforts to enroll subcontractors and for not providing enough staff to support production engineering at the Tulsa plant. The War Department accused Douglas of not really wanting to build the A-26 and of diverting too much of its efforts to transport aircraft which would promise to have a greater postwar payoff. As a result of all the delays, the A-26 took 28 months to proceed from first flight to full-scale combat operations. However, once the production line got into full swing, Douglas went on to produce 2400 Invaders during the last 20 months of the war.

The first flight of the XA-26-DE (41-19504) took place on July 10, 1942, with test pilot Ben O. Howard at the controls. The aircraft was powered by two 2000 hp Pratt & Whitney R-2800-27 radials driving three-bladed propellers with large spinners. The crew was three--a pilot, a navigator/bomb-aimer (who normally sat on a jump seat to the right of the pilot but also had a position in the transparent nose), and a defensive gunner sitting in a separate transparent compartment behind the bomb bay. For the initial phase of flight testing, defensive armament was not fitted, with dummy dorsal and ventral turrets being mounted.

The performance and handling of the prototype were excellent, but there were some problems with engine cooling which led to some cowling changes and the removal of the propeller spinners on production aircraft. The armament eventually fitted was two forward-firing 0.50-inch machine guns mounted on the starboard side of the nose and two 0.50-inch machine guns in each of the remotely-controlled dorsal and ventral turrets which were operated by a gunner in position just aft of the wing trailing edge. The upper turret was normally operated by the gunner, but it could be locked into the forward, no-elevation position and fired by the pilot. 3000 pounds of ordnance could be carried in two fuselage bays and an additional 2000 pounds on four racks underneath the outer wing panels.

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