this postwar airliner project was on Greg's list and to be worked on sooner or later. Well, more later, as I deliver slowly...
There is a Wikipedia article about it, but like in most of them only a rough description is given. Fortunately,
delivers a nice story - but in Russian. The Google translation is a bit difficult. On the first glance it looks perfect, but it often twists the meaning. I hope I made not too much mistakes - especially about the engineering sections I am not quite sure if I got it right. The source given there is English: Oliver Tapper: «Armstrong-Whitworth Aircraft since 1913». So, whoever may own this book may be able to check if my re-translation from Russian to English is correct. Now please read about
The Armstrong Whitworth AW 55 Apollo
At the end of the Second World War, Britain had at its disposal a highly qualified, technically-advanced aircraft industry, which for five years or more extensively thought about the problems of aircraft projects. On the other hand, with the continuation of the war air transport quickly became of special interest. From a handful of civilian aircraft, which the UK had in 1939, there grew a large armada of military transport aircraft, which, after all, played a decisive role in the defeat of Germany and Japan. With few exceptions, the growing demand for transport aircraft, by agreement, was satisfied by the aviation industry of the United States, which thus was in a dominant position to meet the large demand for post-war airliners. In this situation, with one side of the British aviation industry had been hampered by the lack of continuity in the development of transport aircraft, and on the other hand, as a valuable asset Englishmen had turbine engines, in the development of which the UK was ahead of all other countries, including the United States.
The first jets were usually purely military power plants, and particularly in America, considered unsuitable for commercial use because of the high specific fuel consumption. However, in the UK, some designers have thought otherwise, but at first glance, their preferences were divided between the merits of single-circuit turbojet engines and gas turbines, rotating conventional screws. Since the latter combination could promise improvement in specific fuel consumption, it is the general view, to be more appropriate for civilian use, although it could not provide high speed, which was possible with single-circuit turbojet engines. History shows that the victory was for turbojet engines, which became predominant since the 1960s, while the turboprops filled supporting roles. The subsequent history paid tribute to the two points of view: engines with high bypass ratio, or turbofans, began to dominate the airplanes from 1970, representing a logical combination of the best features of turbo-jet and turbo-prop engines.
Although during the war, Britain could not spare production capacity to transport aircraft, for the future needs of the British civil aviation some theoretical studies had been given. Already in 1942, the government appointed an expert body, which became known as the Brabazon Committee. This Committee had developed proposals for different types of aircraft that may be required in peacetime. Among the Committee's recommendations was Brabazon Type II - 24 aircraft for the transport of passengers, suitable for European airlines of short and medium-haul. Two categories were proposed: one with piston and the other with turboprop engines. For the last, Vickers and Armstrong Whitworth presented projects, and by order of the spring of 1946, the supply of the Ministry had been ordered for two prototypes, each eventually embodied in Vickers Viscount and AW 55 Apollo.
The Armstrong Whitworth Company's aircraft was developed basing on the C.16 / 46 specification, which demanded an airliner that could carry 24-30 passengers at a distance of 1,000 miles (1,609 km) at a cruising speed of 300 miles / hour (483 km / h). At this early stage of the development of gas turbines opinions about the merits of the two types of engine compressor divided. Centrifugal compressors were used in the Rolls-Royce Derwent and de Havilland Ghost engine, while the compressors of axial type at that time were not as well developed, but promised a much higher degree of efficiency than the rough, but more reliable, centrifugal supercharger. In the turboprop engines of those years there were the engines of the two Rolls-Royce Dart types with centrifugal compressor and the Armstrong Siddeley Mamba, with its long, slender, axial compressor. The last one was an engine with a large potential for development, and not surprisingly, Armstrong Whitworth, with their strong sense of preference in relation to its sister engine-building company, chose the Mamba engine as the powerplant for the AW 55. On the other hand, Vickers chose the Dart for the Viscount project, doing a bet on security. In the case of Armstrong Whitworth failure has occurred, and the lack of the Apollo's success, to a very large extent, was due to the Mamba engine failure, while the success of Viscount was largely basing on the outstanding qualities of the Dart engine. Besides it is interesting to note that with turbojet engines everything was exactly the opposite: the first Comets used engines with a centrifugal compressor, which were soon replaced by engines with axial compressors. These engines, in the end, completely replaced their opponents with centrifugal blowers.
For the first set of the Apollo, the Mamba engines were designed to develop a shaft power of 1010 hp plus 307 pounds (139 kg) of jet thrust at takeoff. It was also expected that by the time the aircraft is in production, the engine will be able to develop a shaft power of 1270 hp. However, when the Apollo's first flight took place, the Mamba engines could develop on the shaft about 800 hp and thrust about 780 pounds (354 kg). An attractive feature of the engine was its small diameter, which mesured only 31 inches (0.787 m). The Mamba engine's surrounding gear housing had an annular air inlet, from which air enters a ten-step compressor, and then six combustion chambers followed, then the two-stage turbine came, and finally, a jet nozzle was mounted. The speed reducer on the turbine shaft reduced the speed of 15 000 rev / min to 1450 rev / min on the propeller. The Mamba engine was first launched in April 1946, and being installed in the nose of an Avro Lancaster, it first flew on 14 October 1947. In May 1948, it was installed in the trainer aircraft Boulton Paul Balliol, and later the engine was in the experimental Avro Athena and Handley Page Marathon. In February 1948, the Mamba engine completed 150-hour ministerial civilian and military standard tests (Ministry Civil and Military Type Test), and on 25 August 1948 under the supervision of the Aviation Register (Air Registration Board) it successfully completed a 500-hour endurance test. These early tests seemed to bode well for the future of the motor, but this did not materialize: when installed in the Apollo airliner various difficulties occurred, many of which were not resolved during the life of the aircraft.
Early figures show the plane with a wingspan of 92 feet (28.04 m) and a relatively short fuselage of total length of 64½ feet (19.66 m). At a later stage the development of the technical design length was increased to 68 feet (20.73 m) and, finally, as built, gave a further increase in the overall length of 71½ feet (21.79 m). During 1946 simultaneously with the development of AW 55, equipped with Mamba engines, there was an airliner project presented with turbojet engines, better known as the AW 55 Mk.II. The machine had to get four turbojet engines Rolls-Royce Derwent V, developing 3500 lb (1588 kg) static thrust each. This power unit at an altitude of 25,000 feet (7620 m) provided the economic plane cruising speed of 375 miles / hour (603 km / h) and a range of 1000 miles (1609 km). The fuselage had to be the same as in the Mk.I, but the new wing would have a slightly smaller scale of a redesigned chassis. Another proposed design was a version with an additional insert in the fuselage length of 6 feet 8 inches (2.03 m) to increase passenger capacity to 45 people or more, but this option, as well as a version with THD, went not out of the design stage. For carriers, the companies preferred piston engines - there were many who had not yet been convinced of the merits of a gas turbine - there was a version of the standard AW55 designed, offered with engines Rolls-Royce Merlin 35 or Pratt & Whitney Twin Wasp R-1830.
When the AW 55 was finally built, it was first named Achilles, Avon and then, Apollo. The aircraft had a total gross weight of 45,000 pounds (20,412 kg) with accommodation from 26 to 31 passengers. Outstanding features of the design included sealing and air conditioning for crew and passengers, the thermal removal of ice from wing and stabilizer and a constant speed propeller with a reverse pitch and automatically changing the azimuth angle of the blade unit. The Apollo's fuselage had a circular cross-section with an inner diameter of 10 feet 2 inches (3.10 m) and was designed for the working pressure drop 5½ lbs / dym² (3867 kg / m², 0,387 kg / cm²), which allowed in the cabin at a height of 25000 ft (7620 m) to maintain the pressure altitude of 8,000 feet (2,438 m). The fuselage was made of duralumin sheets, riveted Z-shaped stringers and frames with the profile box section. The wing was built around an extremely lightweight and durable spar box consisting of two welded sheet metal girders, to which the wing skin was attached. In turn, this lining was reinforced with a corrugated inner shell, riveted to the usual external panels of the outer skin of the wing. Six fuel tanks, three on each side, were arranged between the walls of the spar. At the trailing edge, throughout between the fuselage and the ailerons, Fowler flaps were installed. To avoid wing turbulence, the stabilizer was mounted high on the tailfin; elevators were constructed as aerodynamically balanced closed casing system "Irving ". It consisted of a plate projecting forward from the hinge of the elevator in a high-pressure chamber located within the stabilizer opening channel thickness in the upper and lower tail surfaces; such a system had been used on the flying wing aircraft AW 52. Initially, the Apollo's steering device was divided into two parts - the front half comes into effect only after the rear half has reached the total angular displacement; the object of the arrangement was to ensure sufficient power steering to cope with two disabled engines on one side. In the case of Apollo this was particularly difficult because of the high engine power and a considerable distance from the axial line of the aircraft. In fact, the split rudder was not set, as well as yet not been established in a recent design feature - for reducing the intensity impulse device through which the ailerons can deviate upward under the influence of wind gusts, reducing thereby the load on the wing. On each main landing-gear leg twin wheels were mounted; fighter-like they receded on raising in the direction of the fuselage center section under the aircraft body. This cleaning method was necessary to be chosen for the small diameter of the Mamba engine, rendering a conventional placement of legs in the engine nacelles impossible. The nose gear was retracted into the fuselage in the usual way - by turning back. In early specifications and brochures describing the Apollo, there was mention of the chassis with a longer suspension travel, allowing the aircraft "... to touch the ground without respecting the landing way from a normal glide path ... ... with the right fit."
This design was allegedly inspired by earlier experiments with the Albemarle, but those works had not been continued.
The Apollo's construction began in early 1948, and two aircraft together with a third fuselage, designed for ground tests, were set to work. Pressurized airplanes in 1948 were still a novelty to the UK, and additional testing of the fuselage was mostly related to the pressure test. This test fuselage followed the prototype in a single fuselage bench equipment in Baginton, thereby delaying the second full assembly of the aircraft. However, the importance attached to the pressure test was considered to be a sufficient basis for the adoption of such a policy. During the test, while setting the test pressure, there was always the likelihood that the fuselage could explode so that in addition to the destruction of the control sample important evidence could be lost as well, indicating where the initial failure took place. It is this consideration which led to the invention of pressure tests according to the method of the water tank. This method was based on the premise that water, being practically incompressible, does not store energy (as opposed to compressed air) and therefore does not lead to a catastrophic explosion occurred in the case of rupture of the fuselage skin. Apollo first tested the control sample consisting of the front part of the fuselage - the most critical section because of its irregular shape and the presence of large areas of glazing around the cockpit.
The problem of providing an appropriate water tank was solved in Baginton, when someone thought of the hospital emergency room, built during the war in a concrete dugout. Subsequently, it was filled with earth, but then the dugout was excavated, the roof was removed, and the sides were made watertight. Thus, the tank was able to hold the 22-foot (6.7 m) fuselage section and quickly provided 27,000 gallons (122,744 liters) of water. What was needed was a permanent source of supply for pumping water into the fuselage (which meant that the direct supply from the network was not acceptable), and it was obtained by placing the pressure tank on the roof of a neighboring factory building. The first tests included increasing the internal pressure in the fuselage to the maximum permissible load, that is 1.33 from the normal operating pressure, which in the case of Apollo was 7.33 lb. / dym² (5154 kg / m², 0.52 kg / cm²). The instructions required that this pressure is maintained for 2½ minutes, without causing any displacement or irreversible deformation of the structure. This standard was easily achieved, and in the subsequent test, the internal pressure rose to 13 lb. / dym² (9140 kg / m², 0.91 kg / cm²), 2-3 times higher than the normal pressure, without any damage to the fuselage. These tests of the pressurized fuselage conducted in Baginton were believed to be the first that used a water tank. However, it is worth noting that this first use of this method was not then due to the problems of structural fatigue caused by pressure cycles. This aspect of the cab sealing was not considered a problem until 1954, when the Comet disaster made it a well-known phenomenon. Later, when the Apollo prototype completed its flight operation, it was returned to Baginton, subsequently dismantled and used for cyclical pressure tests as part of the program of study of this aspect of metal fatigue. Later, during these tests, the Apollo fuselage underwent 38,000 reverses (pressure increment sign changes to the opposite), pressure equivalent of at least 60,000 hours, or more than 20 years of flights on routes - amazingly enough evidence of the reliability of the aircraft structure.
The Apollo prototype, carrying Royal Air Force roundels and serial number VX220, was ready for the run in engines in March 1949. After the usual taxiing and high-speed runs along the ground the plane on April 10, 1949, made its first flight. From the beginning there were problems, and, above all, the motors Mamba engines, still were in an undelivering state. It soon became apparent that a successful trial run on a test bench did not give any guarantee that the engine will be equally good in the air. In order to avoid excessive heat coming from the turbine engine gas the Apollo's shaft power was limited to 800 hp.. In addition, the Mamba compressor suffered from a tendency to speed loss (stall). As a result, most of the early Apollo test flights were devoted almost exclusively to fine-tuning the engines, but in spite of intensive efforts, these and other theater Mamba problems have not been resolved, unable to save the reputation of the aircraft. As it turned out, installed in the Apollo Mamba engines reached the promised shaft power of 1,000 hp only for a short period at the end of the flight test program, before their performance was again reduced to 970 hp on the shaft due to breakage of the compressor blades. Calculations have shown that, if driven engines ever been able to develop on the shaft a power of 1270 hp, the Apollo's economic cruising speed and range would amount to 280 miles / hour (451 km / h) and 1260 miles (2027 km) compared to 270 miles / hour (434 km / h) and 1,130 miles (1818 km) when the engine power remained of 1000 hp.. On the other hand, the power for take-off of larger engines has led to a significant increase in speed for a safe running engine with little loss of longitudinal stability. This factor arose, because the Mamba engine had advanced length screws, far forward relative to the aircraft center of gravity, resulting in some degree of destabilization effect aggravated by increasing power.
In addition to problems due the engines, the plane itself was creating accompanying problems as well. Mainly because of the relatively short fuselage the lever arm had created a certain instability, both in the longitudinal and in the azimuthal direction. There were also insufficient sizes of elevator rod beds, on the other hand forces on the the rudder pedals were too the high. Problems have been partially overcome by the increase in scope of the stabilizer, the reduction of the rudder chord clause and tailfin area increase. These changes were done in the first months of 1950 after having completed about one hundred hours of flight tests. Another modifications were made to get rid of of the periodic vibrations in the cabin, including the replacement of the three-bladed propellers of the inner engines installed to four-bladed ones; later the four-bladed screws were installed on all engines. After these changes the Apollo was registered as civil aircraft, having the registration number the G-AIYN. On October 30, 1950, the Apollo received a certificate of airworthiness to a restricted directive category, which allowed it to the carry passengers "without payment for travel to." By the time the Mamba engines were allowed to work with the take-off power on shaft of 920 hp, while the permitted total weight of flight was 45,000 pounds (20,412 kg).
On March 12, 1951 the Apollo flew to Paris, performing the first of a series of test flights, provided with a contract of the Ministry of Supply. The flight was made from Baginton directly to Orly airport at a cruising altitude of 11,500 feet (3505 m). Flight time was 86 minutes, which resulted in saving of 60 minutes compared to the current schedule of the British European Airlines flights Birmingham-Paris. Way back to Baginton the flight took 78 minutes at an altitude of 12,000 feet (3658 m). Plans for further test flights were postponed until to the completion of the test program and the provision of a full certificate of airworthiness, the receipt of the which, however, did not take place. In July 1951 enforced Mk.504 engines were installed, which had delivered the take-off shaft horsepower of 1,000 hp, but at the end of the same year the engine compressor failure put an end to flights until the spring of 1952, when the new engines with modified blades were installed. Initially, these new engines were designed for to have a take-off shaft horsepower of 970 hp ..
In 1950, an energetic commercial campaign aimed primarily at European airlines was launched; also for potential customers a number of flights on certain routes were to be experienced. On the base on the aircraft's cost of £ 200,000 and an annual rate of 3000 hours of use the amortization needed eight years. Solution: For these and other assumptions, the operating costs (aircraft × nautical miles) were calculated with £ 96.16 for the the distance of 260 nautical miles (482 km). Unfortunately, no immediate sales did happen and, as the test program showed various shortcomings of of the aircraft and its engines, the company's efforts about enterprise, organizing and sales promotion has lost much of its impetus. By 1952, it became quite clear that the Apollo had no commercial future, and in June it was decided to discontinue the development of this type of aircraft. At this time, the second plane had not yet been completed. However, the work was continued and eventually, on December 12, 1952 the plane, carrying the RAF serial number VX224, made its first flight. After two subsequent flights in December the aircraft was returned to the shop for retrofitting and, finally, in the finished state, flew in September 1953.
Both the Apollo aircraft were paid by the Ministry of the Supply and the in end of the development work program they were transferred to the Ministry and delivered to the research center of aviation and armament (Aeroplane and Armament Experimental Establishment - A & AEE) at Boscombe Down. The first plane, which by then was returned to registration VX220, was delivered September 24, 1952 after it had flown a total of about 300 hours, while the VX224, which never wore its number of civil registration G-AMCH, was handed over on October 15, 1953. In Boscombe Down the VX220 was used as experimental aircraft for testing the radio navigation system Decca ; for these tests continued until April 1953, when, after a total of about 400 landings, there was a breakdown of the the chassis. Design changes were made to the VX224's chassis, but the prototype was not restored, and in December 1954 it was dismantled and then returned to Armstrong Whitworth, where, as it was told earlier, it was used for further investigations of metal fatigue. At the same time, in October 1953 VX224 was taken to Boscombe Down for general studies of testing and handling, at the end of which it was transferred to the the Imperial School of Test Pilots at Farnborough airport. We cannot say that Farnborough embraced it with enthusiasm as the engines continued to act up, and within nine months from March to December 1954, when the plane was in the ETPS, the aircraft performed a total of less than 20 hours of flight. The VX224 's last flight happened on 14 December 1954, after which it was transferred to a department of the Royal Aeronautical Research Institute (Royal Aircraft Establishment - RAE) at Farnborough, where the fuselage was used for another series of pressure tests within a water tank.
Inevitably, the question arises whether the Apollo, equipped with reliable engines, had been able to successfully compete with the Viscount? The Apollo had to get a longer a fuselage, as its rival had, to heal the resident problems of stability and controllability. On the other hand, the Viscount had the advantage, and it is difficult to avoid the suspicion that the sinking profitability of the Armstrong Whitworth trust from military subcontracting could distract energy and initiative for the Apollo, which would habe been essential to the make the aircraft technically and commercially successful.
After the text, I would like to show you some pictures, as usual (the internet source above has some more). There are no real good pictures of the aircraft on the net, only #2 can be found in a slightly smaller version. I have these already since a longer period of time and cannot tell the sources. #3 shows the aircraft together with others at the exhibition in Farnborough 1949.
Hope you enjoyed,
and best regards, RT