The growth of the transport aeroplane
It Is difficult to assess how much the Second World War affected the evolution of the air liner, what is certain is that the whole approach to it has altered completely. This change has been one based more upon passenger accommodation than 1 upon aerodynamic advances. In point of fact, several pre-War air liners and one in particular, the de Havilland Albatross, were of far better shape than are those of today.
Aviation is, and always has been, an extravagant industry. There have been two principal reasons for this: in the first place, soon after its birth it became the tool of military necessity on which money was lavished without careful assessment of the returns; and secondly, the essentially greater dangers of flying compared with other modes of travel demanded a standard of manufacture and maintenance that could only be expensive. The second of these reasons is the one most affecting air transport, because it is behind the elaborate systems of inspection and recording during the whole process of manufac- . ture, whereby one faulty bolt can be traced to the very cast of steel from which it was made. In maintenance also, every repair, every replacement, is logged and if there is an error both the mechanic and the inspector involved can be identified. In all this, it is not only the checking of the actual jobs, but also the vast quantity of paper work that costs money.
This background of the high cost of flying is essential to the understanding of air liner development. Before the War, , almost all European air lines were subsidized in some way by the various governments. Probably greater efforts to make civil aviation "fly by itself" were made by Great Britain than by any other country, but the War nipped these in the bud just as the first fruits were forming. The U.S.A. was, even then, beginning to establish a lead with its inestimable advantages of great distances, no customs delays and a wealthy population -' —though even here adjustable air mail rates were used to meet deficits on certain routes.
When War came in 1939 all development, even all manufacture , of air liners stopped in Europe, but the Americans had nearly two years of grace in which to continue their work. However, the great future of the American air liner was truly founded on a point in the Atlantic Pact, when under the division of industrial war effort between Britain and the U.S.A. we undertook to concentrate upon fighters and bombers, leaving the provision of military transports in the hands of the Americans. Money poured out for the establishment of the vast airborne forces fleets for the invasion of Europe served to cover the high development costs of the four-engined air liners that were to form the backbone of the post-war long-range fleets of the World's air lines.
In the years immediately preceding the War, the "main liner" was generally twin-engined and carried about twenty passengers. To keep down head resistance, there were usually single seats on each side of a central gangway, a tradition broken by the Douglas DC-3 with its two-and-one layout. Even the highly successful series of twin-engined air liners made by the Lockheed Aircraft Corp., the Electra, "12", and "14", had the single seat arrangement, although the lastnamed type was one of the first transports to have the fuselage deepened to make room for baggage under the floor—now an almost universal feature. As an indication of pre-War American air transport activity, the Lockheed output in 1939 was 35 to 40 aeroplanes a month—the majority for the home market.
Britain had for many years been tied to the rather "safe but slow" policy of Imperial Airways, which led to the very tardy introduction of clean, fast monoplanes. Imperials had, too, in a way handicapped themselves by setting a standard of comfort in their biplanes that was hard to incorporate in a monoplane. Four seats abreast in a hundred mile-an-hour biplane cost little in engine power, but in a 200 m.p.h. monoplane the price was considerable, either in terms of lost speed or of higher fuel consumption—it was a matter of making the choice. Imperial Airways had also adopted a policy of fourengined safety as early as the twenties and it was hard to go back on a principle—particularly when it had been well advertised!
As a result of all this, a difficult compromise had to be made. There were two answers, the beautiful de Havilland Albatross, which carried from twenty to thirty passengers, and the larger, less attractive, but more capacious forty-passenger Armstrong Whitworth Ensign. These aeroplanes had two features in common, which at the time seemed excellent, but have since been proved dead-ends in evolution. The fuselages were well streamlined, which means that space was lost where the nose and tail tapered off. It is the parallel, tubular fuselage which gives the best capacity for structural weight in a transport. The second feature was general in the larger air liners of all countries before the War, that was the splitting up of the passenger accommodation into several cabins.
Regarded purely as an aeroplane, the Albatross was one of the most perfectly streamlined that has ever been built and its best cruising speed was 210 m.p.h. on a total of 1,280 h.p. (less than 60 per cent full power) at 11,000 ft. Larger and heavier than the DC-3, because of the four engines, the Albatross carried a comparable load on similar cruising power some 20 m.p.h. faster. The extra drag of four nacelles was overcome by extremely careful design and special reverse-flow cooling for the inverted vee-twelve air-cooled engines involving wing intakes. The whole aeroplane was beautifully proportioned, each part matching—it, in fact, was a stage between the Comet racer and the Mosquito. It was, too, on the Albatross that first use was made of the moulded ply and balsa sandwich fuselage and the high-strength wooden wing which were the structural highlights of the Mosquito.
A special version of the Albatross, with extra fuselage tanks, was bought by the Air Ministry for experiments in a trans-Atlantic mail service. This was also an abortive line of thought and today nobody would dream of an all-mail service —it is far better to let mail revenue support the normal services.
The Ensign was an effort to combine the comforts of the Hannibal with the performance of the DC-3. The result was a very large aeroplane, but not a very big payload for the power used, originally four 850 h.p. Armstrong Siddeley Tiger radials, later 1,100 h.p. Wright Cyclones. One of the troubles was the retention of a very low biplane figure for the wing loading of only about 20 lb./sq. ft. which put up the drag. Like the Whitley, the Ensign flew very much nose down and the cabins were all on different levels, which added structural weight where the floor changed. The high wing is, literally, an excellent feature from the passenger's viewpoint, since it gives a perfect outlook to all and the wing provides welcome shade and relief from glare. Economically, however, the high wing has been proved time and again to be unprofitable.
On relatively small aeroplanes with fixed undercarriages, the high wing could be used to advantage, but on the new and
faster types it had too many snags—a fact tacitly admitted by the astute Antony Fokker when he dropped his own designs in favour of the DC-3 licence. Retraction of the long undercarriage involves both weight and complication, but the real trouble is that, for a given fuselage size baggage stowage is reduced. The high wing cuts into the top of the fuselage and the floor has to be lowered so that there can be no holds below it. Instead, passenger baggage has to be distributed in nose and tail, with consequent increase in c.g. sensitivity. The Ensign, because its fuselage was large, had a hold above the I cabin, but the awkwardness of loading would make this uneconomical with today's heavy traffic and emphasis on 1 rapid turn-round.
Shortly before the War de Havilland's designed a particularly smart all-metal twin for Jersey Airways Ltd., the Flamingo—which was adopted for the R.A.F. under the name Hertfordshire. With a roomy cabin for from 12 to 20 passengers, the Flamingo was in the same group as the Lockheed types and had been designed to operate from the restricted Channel Island aerodromes, so that it had rather high power in the form of two 890 h.p. Bristol Perseus radials. A high-wing monoplane, the cabin headroom problem was solved by taking the wing loads across the fuselage by a single beam. Baggage was stowed under the cockpit and aft of the cabin. When War = came the Flamingo was dropped before the pros and cons of the high wing could be weighed.
It is difficult to decide whether two pre-War German four-engined prototypes, the Fw 200 and the Ju 90, were bona fide air liners or not, since they were both developed into long-range convoy shadowers, the Fw 200C and the Ju 290. In appearance they very much resembled the modern fourengined low-wing air liner, save that they had tail-wheel'' undercarriages. Quite probably they were considered as quasimilitary transports and when the U-boat campaign was intensified they were adapted to their new duties as being the only airframes available.
In the U.S.A. demands for more seats led by 1939 to the! building of the four-engined Boeing Stratoliner and the I Douglas DC-4 for 33 and 40 passengers respectively. In both of these the fuselages were fat and streamlined. Each included an innovation, the Stratoliner was pressurized, the DC-4 had a tricycle landing gear.
As a result of the wartime agreement many thousands of DC-3s were built as transports for the Allies and the DC-4 was developed as a military transport. Also out of this agreement came the Lockheed Constellation and the Boeing Stratocruiser—certainly a flying start for American mercantile aviation.
The history of the DC-4 is quite curious. The prototype had a span of 138 ft. but in the production version this was , reduced by some twenty feet and the plump fuselage was replaced by an elongated version of that on the DC-3, to the considerable improvement of the tare weight. Adopted by U.S. Troop Carrier Command as the C-54, it was developed with large cargo doors and heavy-duty floors, to the great advantage of the post-War version of the DC-4. Thereafter the fuselage has been lengthened successively to give more and more capacity in the DC-6, DC-6A, and DC-6B and, eventually, the DC-7, with the gross weight rising from 65,000 lb. to 122,000 lb. and the seating capacity from 44 to 66 — 95 in the tourist, or coach class version. This mounting weight and wing loading are symptomatic of today's trend and undoubtedly arise from the practice of overloading established during the War. Throughout, the wing and the general appearance have remained unaltered apart from minor details and, of course, structural strengthening. The circular fuselage, parallel over most of its length, lends itself to lengthening by the simple operation of adding a few more frames at front and rear. This shape is also easy to pressurize and with the floor level in line with the top of the spar of the low wing there is ample hold space, evenly distributed about the c.g. and easily accessible from the ground.
The DC-4 can De truly called the model for today's air liners.
Also developed at government expense after having been taken over from its civil sponsors, T.W.A. and P.A.A., the Lockheed Constellation first flew as the C-69 service transport on January 9th, 1943, and received its civil certificate on December nth, 1945. Larger than the DC-4 the Constellation started commercial life at a gross weight of 96,000 lb., which after strengthening modifications was progressively raised to 107,000 lb. Then 18 ft. 5 in. were added to the middle of the fuselage, undercarriage and wing again strengthened, more powerful engines fitted, and the weight raised to 120,000 lb. The Constellation is a beautiful aeroplane, one of the few successful air liners with a gracefully curved fuselage—it is, in fact, adapted from an aerofoil section. By very careful choice of sizes, the fuselage capacity is ample for both passengers and baggage, above and below the floor. With four 2,200 h.p. engines the Constellation was one of the first really high-powered air liners and was also the first to give 300 m.p.h. cruising.
The DC-4 and Constellation were both air liners in original conception, diverted to military roles only to return to their designed duties. The Boeing C-97, however, was a military adaptation of the B-29. The principal change from the bomber lies in the fuselage, which has a figure 8 section, being really two fuselages one above the other and with the main floor lying along the join. The chief virtue of this layout is that the stresses caused by pressurization are taken by the two cylinders, tied together by the floor. There is an internal domed bulkhead at the rear of the pressurized volume. In the original military version clamshell doors opened under the tail and vehicles or freight could be loaded up a ramp. The lower "fuselage" gives extra freight space. In its post-War civil guise the Stratocruiser is the luxury trans-Atlantic air liner and is the mainstay of B.O.A.C.'s first-class traffic, as well as being operated by P.A.A., U.A.L. and N.W.A. In the B.O.A.C. version there are 59 seats on the main deck and the lower deck is used for a cocktail lounge aft of the wing with freight ahead of it.
Despite the very ugly fuselage—incidentally the pilot's view is exceptionally good for an air liner because of the military origin—four 2,800 h.p. engines give a maximum cruising speed of 340 m.p.h. Water-methanol injection boosts take-off power to 3,500 h.p., which is fully needed to lift 65 tons into the air. The days of such high piston-engined power are numbered, but with steadily increasing traffic the Stratocruiser fuselage may well be the shape of things to come. Late in the War it was realized that something had to be done about British transports. For one thing, the C-54 and C-69 were a long time in coming and would obviously not be available to overseas buyers for several years. Avro's were asked by the M.A.P. to make a conversion of the Lancaster, using as many of the same components as possible, for V.I.P. and Airborne Forces use. This conversion, the York, was achieved by making a large rectangular fuselage and fitting it under a Lancaster wing. The Lancaster tail was also used with the addition of a central fin to compensate for the extra area of the long deep nose. Since the fuselage was not pressurized , many close-pitched, circular windows of ample size
could be fitted, so giving the passengers an excellent view from beneath the high wing.
Considering the conditions under which it was designed, with too much urgency for the development of the pressure cabin that would have allowed high-level operation, the York was a good aeroplane. For several years after the War it did duty with B.O.A.C. and Transport Command, until superseded by more modern equipment, and is still in wide use with charter companies. The liquid-cooled Merlin engines were, however, unpopular with civil operators who, working at higher pressure than the services, had cast their lot for the simplicity of the air-cooled radial a generation before.
Realizing the need for ensuring Britain's place in civil aviation, the Government set up a committee late in the War under the Chairmanship of Lord Brabazon of Tara. Several specifications were evolved, but on the whole the committee was handicapped by lack of up-to-date knowledge of air line requirements because of Britain's current preoccupation with winning the War. Suffice it to say that among these specifications were those from which the Dove, the Viscount and the Comet were evolved. Another led to the mighty Brabazon, a wonderful aeroplane about which controversy still rages as to whether it might have been a wonderful trans-Atlantic air finer. Unfortunately, the very government that sanctioned the building of an unnecessarily elaborate assembly hall and quite unnecessary runway, cancelled as an economy the construction of the extra aeroplanes necessary to make an economical Brabazon operating unit.
The Brabazon is a well proportioned aeroplane, unlike the Lockheed Constitution and the Convair XC-99. Of these, the latter has a slab-sided tubular fuselage of exceptional stowage capacity, but lacking any aerodynamic shape, while the former was an attempt to combine a figure 8 fuselage with some aerodynamic refinement.
In France two gallant efforts were made by a War-shattered industry to design large transport aeroplanes. The S.N.C.A. du Sud-Est, led by Pierre Satre, under the occupation, started work in secret on an air liner which eventually flew on January 12th, 1949. This aeroplane, the Armagnac, was an attempt to make a hundred-passenger, trans-Atlantic air liner. The pressurized fuselage is of large diameter (15 ft.), is very capacious and yet the outline is pleasing. The sharply tapered wing crosses the fuselage just below the maximum diameter, leaving a large hold space below the floor. The curious-looking Breguet
76 Deux Ponts has, so the makers explain, the same lines as' the Constellation, only the top and bottom outlines are some ten feet farther apart! The deep fuselage gives ample stowage space on two decks which can carry some fourteen tons as freight or passengers. The underpart of the rear fuselage opens and vehicles can be driven up ramps to either deck. The designer of the Deux Ponts took the choice of streamlining his huge fuselage so that a reasonable cruising speed could be attained at moderate altitude, rather than venture among the pitfalls of pressurization—where the thin air at 20,000 ft. makes streamlining of rather less importance. Like many Continental aeroplanes the Breguet originally had too small a tail, hence the added central fin.
To back the large long-range air liners, the World has first of all relied on the DC-3—mainly reconditioned from military models. Attempts to design a "DC-3 replacement" are con- , tinually being made, and as frequently fail to materialize. The DC-3 was just right for the job and present-day costs prevent a replacement being made with just the right amount of improvement all-round at a reasonable price. Instead, several manufacturers have made the next size up. The plump Vickers Viking, based on the Wellington, was the first post-War air liner to fly. It has an ample under-floor stowage and a pleasant cabin originally for 27 passengers, which B.E.A. have converted to take 40.
The modern look of the inter-city air liner is, however, epitomized in the forty-seater Convairliner, Martin 2-0-2 (and 4-0-4). The first two have the "tube look" and are smaller twin-engined applications of the DC-4 principle. The sharp taper of the rear fuselage has led to the need for very large dorsal fins for directional stability.
The Airspeed Ambassador (B.E.A. Elizabethan class) is in contrast to these two aeroplanes. It is an enlarged, modernized edition of the Flamingo, with a high wing and an . exceptionally graceful fuselage. As an aeroplane the Ambassador is outstanding, with a very high performance on low cruising power, achieved by careful streamlining and a happy choice , of wing aspect ratio, and exceptionally pleasant handling characteristics. The Ambassador passenger gets unusually good treatment with the large windows giving a magnificent view and the plump central part of the fuselage providing an exceptionally spacious and pleasant cabin. The operator, however, must be less happy, since there are the usual highwing drawbacks, lack of under-floor stowage and the balancing of baggage weight between front and rear holds.
Outstanding post-War development has been the emergence of an entirely new breed, the British turbine-engined air liners. Most of the British effort has been concentrated on these and, in 1952, seven years after the end of the War, the faith of the visionaries was proved—for it had required stout hearts to persist in this revolutionary development when all the experienced heads of the U.S.A. were being shaken in doubt. Convinced that the vibrationless turbine engine would provide the complete answer to the main criticisms of air travel, noise and fatigue, the Ministry of Supply, under the direction of Sir Alec Coryton, strongly supported the development of special engines for these new air liners and also paid for the prototypes.
The characteristic feature of the three most important types is, once again, the tubular fuselage, easily pressurized and giving good accommodation for most of its length. There is no space here to detail the many contributory experiments, such as the Dart-Wellington and the Nene-Viking, that led to the three superb air liners, the de Havilland Comet, Vickers Viscount and Bristol Britannia. The reason why 1952 was a crowning year of achievement in this work is that the Comet entered service with B.O.A.C., the first scheduled jet air line operation in the World, deliveries of the Viscount to B.E.A. started and the Britannia made its first flight.
The Comet, which first flew in July 1949, has been the subject of much criticism, not as an aeroplane but as a commercial vehicle. Its capacity and range are said to be too small for economical operation, but in its first six months use with B.O.A.C. a clear operating profit was made—which to the simple mind appears conclusive! The first Comet, the Mark 1, was built to gain experience, to use the engines available, and to establish the practicability of the turbine for civil operation . The larger, higher-powered Marks 2 and 3 are the real targets.
In order to achieve economy, a jet aeroplane must climb high at once and must fly fast in the thin air of the stratosphere where resistance is low. During the climb and when delayed at low level, as for instance by Air Traffic Control ordering a holding pattern at a height below about 18,000 ft., fuel consumption is very high, since unlike the piston engine a turbine does not greatly reduce fuel consumption when throttled. Foreseeing this the Comet's designer, R. E. Bishop, grouped his four engines close to the fuselage so that flight on any two would not require heavy rudder trim correction. Although preceding the British four-jet bombers in design, the Comet has the same general characteristics: large wing area, sweepback and widened, thickened wing root. The fuselage is tubular to give capacity, but is finished off with clean-run nose and tail. A dihedral tail plane adds considerably to the effective fin area, so ensuring the extra directional stability so essential for the comfort and safety of passengers.
In a jet air liner, the low wing has an extra advantage over those already emphasized; it permits easy access to all refuelling and inspection points without the aid of trestles or ladders. This is vitally important, since to take full advantage of the 450-600 m.p.h. cruising speed offered, between-flight servicing must be very rapid—particularly with the relatively poor range of the Comet 1. This restriction on range is caused by the large fuel reserve carried in case of diversion at low level, not by the actual point-to-point consumption at high altitude. In the Vickers 1000 air finer (VC-7 troop transport) which is the civil counterpart of the Valiant, the wing is lowered as much for accessibility as for increasing fuselage volume.
Apart from faith in the pure-jet air liner, Britain also took the long view with the propeller-turbine. This type of engine is a means of harnessing the energy of the exhaust, through a turbine, shaft and gearbox, to a propeller. In this way the thrust at take-off and during climb is greatly increased, because a far larger mass of air can be accelerated backward by the propeller than can be passed through the engine. This means that the propeller-turbine engine, while retaining the smoothrunning , light-weight advantages of the turbine, also makes use of the best properties of the airscrew. It serves, because of reduced installation and cooling drag, to give a higher cruising speed than the piston engine, but is slower than the pure jet because of slipstream drag at speeds above about 400 m.p.h. These facts lead to two distinct applications for the propeller-turbine: the medium-range inter-city transport, where distances are too short for a turbo-jet to climb economically to 40,000 ft., and the long-range weight carrier, where low fuel consumption comes into its own.
The Vickers Viscount is the epitome of the first group. Originally a 32-seater to a Brabazon specification, the present Viscount has been evolved as a result of close co-operation
between the maker and the customer—Vickers Armstrongs and British European Airways. Being a tubular fuselage, lengthening to take 40/48 passengers was not very difficult and the wing span was also increased—a novel virtue in these days. Enlarging the Viscount was the result of the great rise in air traffic and it was made possible by a higher power output of the Rolls-Royce Dart engines. Early in 1953 a further "pulling out" of the Viscount was announced, which gives capacity for up to 60 passengers—again thanks to extra power from the Darts.
The shape of the Viscount is one dictated by practical considerations, although the result is pleasing to the eye as is so often the case with good design for a purpose. There is ample under-floor stowage within the pressurized fuselage volume, while the cabin is pleasantly spacious. A unique feature is the large oval window shape which gives each passenger a fine outlook and also an individual emergency exit. A high aspect ratio wing is used to reduce cruising drag and, despite the propellers, the wing is low enough for all refuelling to be done from the ground. The small diameter and considerable length of the Darts lead to pencil-slim nacelles. The cowlings for these swing open and every part of the engine is accessible without trestles.
The Bristol Britannia, although double the size, follows the same general design trends. This fine aeroplane was made to British Overseas Airways Corporation requirements and upon it they are pinning their faith for the Atlantic "tourist" traffic, seating ninety passengers. The sweeping curve of the Britannia's dorsal fin belies the fact that it, too, has a tubular fuselage, while the good proportions of the whole aeroplane tend to conceal its vast size. External design features of the Britannia are logical, but there are many internal structural refinements which, it is hoped, will make it a very profitable type to operate.
One unique feature is worth noting. After experience with the Brabazon, A. E. Russell, the designer, decided to use pure aerodynamic servo-tab controls, rather than embark upon the complications of power-operation or boosting. Along the entire trailing edge of each main surface are mounted tabs controlled directly by the pilot. These tabs are miniature control surfaces, complete with mass balances, and their deflexion is the sole means of operating the ailerons, elevators and rudder. Such a * system is very much lighter than would be the direct control runs for the main surfaces and requires much less maintenance than would hydraulics or electrics. It is, however, curious to see the control surfaces of the Britannia take up various independent attitudes when the aeroplane is standing on the ground with them unlocked.
Although the day of the turbine-engined transport is dawning, the sun of the piston-engined types is very far from setting. Many will remain in use for twenty years yet, while for some duties the turbine may never be used, just as the compound steam engine remains in use despite the virtues of the steam turbine and the diesel.
The final stages of the War, the invasion of Europe and the Burma campaign, showed that the air-lifting of supplies was essential to success. It also showed that the highly refined air liner was of less use than the rugged, general-duty aeroplane with the emphasis on easy and rapid loading of bulky equipment . Fortunately, high flying is in general unnecessary and if it is oxygen can be provided, neither military passengers nor freight being very delicate. Necessity dictates large stowage space for jeeps, guns and even light tanks, giving little hope of streamlining the fuselage.
Several lines of thought were pursued: nose-opening doors with the crew sitting on an upper deck; "pods-and-booms", where the fuselage was cut short, the tail fairing used as a door and the tail itself mounted on one or two booms; and the "clamshell" belly door with an entry ramp. All were tried during the War and similar arrangements are in use today. The C-97 and the Deux Ponts are examples of belly loading, the former military and the latter civil. This layout probably gives the best opportunity of streamlining. Nose doors are well exemplified by the Bristol 170 Freighter and the Douglas C-124 Globemaster. The former was one of Britain's first post-War types and has been very successful in a quiet way. The large, slab-sided fuselage has room for any ordinary car, bulldozers, and even dismantled aeroplanes and helicopters. Since the rather brutal nose necessary for bulky loads limited performance in any case, a fixed undercarriage was adopted to ease maintenance. With the high wing goes a very low fuselage ground clearance and loads are easily run in and out. The Bristol 170 is commercially economic and it has established a high reputation for reliability with Silver City Airways crosschannel car ferry service, as well as with bush operators in many parts of the world.
The Douglas Globemaster is an entirely different proposition: it is a multi-purpose aeroplane capable of taking large loads or of having a long range. It is virtually a two-deck version of the DC-4 formula, but because of the enormously deep fuselage, lacks all pretence to grace. With four highpowered engines and having been developed regardless of cost, it would probably be far too expensive for a commercial firm either to buy or to operate.
Two good examples of booms-and-pods are the Fairchild Packet and the Blackburn and General Aircraft Universal Freighter—both, be it noted, high-wing monoplanes. The former is, if you like, a scaled-up freight edition of the Lightning formula. It is now very successful and several versions are in use, but there were severe teething troubles with the booms and the whole aeroplane is designed on a rather extravagant basis, with very powerful engines and retractable undercarriage . Capacity is large and, since it can be flown without the rear doors, anything that can be put in can be thrown out by parachute—a useful feature in military operations * where landing may be impossible.
The Universal Freighter gives twice the capacity of the Packet, but its price is the same, which makes it a commercial as well as militarily practical proposition. It has been adopted by the R.A.F., as the Beverley, and by Silver City Airways who can use it to take six cars and 18 passengers across the Channel. Once again, beauty is the last feature to be designed into such an aeroplane, it is literally a flying pantechnicon. There are various arguments for and against the different loading methods, but they seem to be heavily flavoured with enthusiast's bias. It may, however, be fairly argued from war- • time experience with gliders, that it is better to avoid nose loading for a military transport, since the doors jam if there is a crash landing—often unavoidable in advanced-base operations—and there is also the aspect of dumping parachuted equipment from the rear.
It seems likely that, in the smaller feeder-liner class too, the piston engine has quite a few years of life. The modern descendants of the Dragon, the feeder liners with the need to take off from small grass aerodromes, require the power and thrust of piston engine and airscrew.
One of the most amazing aeroplanes in this class is "Faithful Annie"-—the Avro Anson. The Anson has been used in so many ways that it was impossible to omit from and yet was difficult to place in this book. Starting life as a six-seater feeder liner, the Avro 652, the same basic aeroplane was used for coastal
reconnaissance, for crew training, as a ferry-crew transport (the writer has been one of fourteen, with parachutes and kit), a post-War air taxi and light freighter (most of the daily papers flown to Paris travel in Ansons) and as a radio trainer. Originally built on "Fokker" principles (fabric-covered, welded steel-tube fuselage, plywood wing and tail), the Anson was made in Canada with a plastic fuselage, while at the end of the War it was redesigned with a light-alloy wing and tail and had two 420 h.p. Armstrong Siddeley Cheetah engines in place of the original 310 h.p. Cheetahs. The Avro 652 was built for Imperial Airways in 1933, the Anson went into service with the R.A.F. in 1936 and the last was completed at Manchester in 1952.
The reason for the Anson's long life, like that of the DC-3, was a happy choice of features that gave excellent handling qualities both in the air and for maintenance. Despite the low power and the ample fuselage size, the Anson has a good performance.
Today, probably the most successful feeder liner is the de Havilland Dove. Built to a Brabazon specification, the Dove was brought on to the market in 1946, before even the Americans had anything in the same class to sell, and by the end of 1952 nearly 350 had been built. The parallel cabin section of the fuselage holds eight to eleven passengers, one on each side of a gangway, with two pilots in the cockpit. The streamlining of fuselage, wing, tail and engine nacelles is excellent, so that with a total of only 610 h.p. the maximum speed is 210 m.p.h. Much of the performance springs from careful weight control, which kept the tare weight down to 5,625 lb. To the passenger the Dove gives a very pleasant ride because of the unusually large and closely spaced windows. Another approach to the same problem is the high-wing .
Percival Prince. Here, two higher-powered Alvis Leonides radials increase the speed, and improve single-engined flight, as well as making possible a larger fuselage volume. Structure weight and fuel load go up with the higher power, so that the actual payload is little more than that of the Dove. It is, once again, the question of compromise, the operator chooses the aeroplane best suiting his needs.
Yet a third variation on the feeder-liner theme is the Heron, an aeroplane developed for the operator who wants simplicity. Using as many Dove items as possible—wings, with a new centre section, front and rear fuselage, tail—a longer cabin taking 14/17 passengers is the basis of the design.
The highly refined supercharged Gipsy Queen 70 engines are replaced by four of the earlier Mark 30, unsupercharged ones with direct drive. A fixed undercarriage reduces tare weight as well as simplifying maintenance. For longer ranges, where higher cruising speed compensates for the extra weight, operators who need the extra speed to get back to base at night, and those who fly over swampland, where the wheels would trip the aeroplane in a forced landing, a Dove retractable landing gear is available. Incidentally, the main shock absorbers of both Dove and Heron have rubber blocks akin to those of the Moth.