The fashions of the thirties

The thirties saw the life and death struggle between the biplane and the monoplane reach its greatest intensity. In Britain, where aeroplane designers are no less conservative than the rest of us whose first national reaction to change is one of resistance, the biplane was retained long after it had been discarded by other countries. I can well remember the ingenious apologia by which learned engineers excused and advocated their adherence to the traditional conception of an aeroplane, advancing, in some cases, the most outrageous reasons why the monoplane simply must be less efficient than the biplane!

Strongest anti-monoplane argument, at least as far as fighters were concerned, was that biplanes were more manoeuvrable . This was largely true for, with the same weight, a monoplane must have a greater span than a biplane or its rate of climb will fall and its landing speed will' rise. The reason for this is that, although the monoplane normally has a thicker, higher-lift wing section, to achieve an equivalent area to the biplane, both wing chord and span have to be greater. Fighter span in the thirties was about 30 feet for a biplane compared with 36 feet for a monoplane, with a proportionate increase in fuselage length to balance the larger wing. Since manoeuvring is generally a function of size, that is the circles one can turn are proportional to the size of the aeroplane, there was some truth in this accusation against the monoplane fighter. Span also affects rate of roll, as will be seen later.

It must be admitted that because of lack of knowledge, many of the monoplanes of the early thirties had performances little, if any, better than the contemporary biplanes. Taking speed as a comparison, since speed is the essence of air travel, it is easy now to look back twenty years and see where the faults lay. There were smart new monoplanes like the Boeing and Martin bombers of 1932, where the designers had gone to all the trouble of fitting retractable undercarriages, while leaving the pilot's and gunner's cockpits uncovered to set up air turbulence and drag. There were other designers wishing to avoid the weight and mechanical troubles of retractable landing gear who enclosed their wheels in beautiful streamlined fairings—commonly called spats—only to have these immediately removed by exasperated mechanics because of the trouble they caused in operation. Engines were mounted in front of fat fuselages, or in badly-shaped nacelles, so that their propellers spent half their energy pushing the air past excrescences instead of pulling the aeroplane smoothly forward!

The biplane/monoplane controversy raged on and the British Air Ministry tried out a way of making up its mind. About 1930 it issued a specification to the Blackburn company to make two twin-engined commercial aeroplanes, a biplane and a monoplane with identical fuselages, tails and undercarriages. The result was a pair of rather ugly aeroplanes weighing almost the same and with about one-mile-an-hour difference in speed. Admittedly the monoplane was strut-braced, but even the cleaner-looking cantilever Fokkers and Junkers seemed able to get only 10 or 20 m.p.h. edge over the biplanes.

Of course, the crux of the matter was that the speeds of those days were not sufficient for the difference in drag to show itself. Drag rises rapidly with speed, at 100 m.p.h. there is not much difference between monoplane and biplane, because frontal area is the key and only an exceptionally sleek, clean aeroplane will stand out noticeably from the crowd. At 150 m.p.h. cleanness begins to mean as much as frontal area, and it pays to fair over the corners and avoid unsightly lumps.

At 200 m.p.h. every unnecessary twist and turn given to the air extracts its penalty in drag; parts must cut into the very solid-seeming air, not push it roughly aside. At 300 m.p.h. aerodynamic cleanness, for example, the interference to air flow caused where wing and body meet, is the thing that matters. From the foregoing it is obvious that up till this date there had been no great need for the monoplane, in fact it was rather less manoeuvrable, its structure was less easy to design and make light, it tended to land rather faster and it still had some peculiar habits that were unexplained—it did not come out of spins very readily for example. No wonder there was battle between the conventionalists and the visionaries.

It is not, I think, untrue to say that Britain's 214 m.p.h. Fury biplane fighters started a run on speed that, paradoxically, led to the rapid demise of the biplane. After playing about in the 150 m.p.h. ruck for ten years the R.A.F. suddenly had the fastest service aeroplane in the world. However, although two hundred miles an hour could be reached without retractable undercarriages, it was much easier to get there with them, and so their complication became acceptable. In 1932 Lockheed built a low-wing retractable undercarriage version of their . Vega, the Orion, which carried pilot, six passengers and baggage at 224 m.p.h. on 500 h.p.

Another difficulty of flying fast had always been the problem of airscrew efficiency. A propeller is literally an air screw, and just like a wood screw, it goes forward a certain amount with each turn—if it does not do this it slips and makes a hole in the air just like a wood screw that fails to bite. It is obvious that an airscrew designed to travel at 200 m.p.h. moves a long way forward in one revolution and must be very inefficient at 50 m.p.h.—or even 100 m.p.h.—during take-off and initial climb. This low-speed inefficiency of the fixed-pitch propeller resulted in a wastage of power and very poor take-off by fast aeroplanes—the Schneider racers took something like two , miles to build up enough speed to drag themselves into the air.

In 1933 the Americans brought out a working version of what had long been the designer's dream, a variable-pitch airscrew. Made by the Hamilton-Standard company, which had long been making airscrews with detachable metal blades, it was an immediate success in the U.S.A. It was simple enough in its original form: the metal blades, two or three, had balance weights so that they could be rotated around their axes without effort and a hydraulic piston on the hub supplied the power. Movement was only a few degrees, 5 to 10 in those days, but it allowed reasonable efficiency at low and high speed. What was very infuriating was that, as has happened so often in our history, Britain had missed the boat, for the first successful variable-pitch airscrew had been British.

Almost a decade earlier Dr. Hele-Shaw invented a hydraulic v.p. airscrew which was test flown in Gloster fighter biplanes. Not only was the Hele-Shaw airscrew first, it had constantspeed control, which was later to become the universal system! Instead of the pilot selecting his pitch, a governor in the airscrew control maintained the engine revolutions at any desired figure whatever the speed or attitude of the aeroplane. The' Gloster Aircraft Co. did its best to sell the idea, pointing out the advantages to the pilots of fighters who would no longer have to watch their engine revolutions, but the authorities were apathetic and development was dropped.

But to return to the point, the v.p. airscrew was a boon to the fast monoplane and lifted everyday speeds up into the 200 m.p.h. range. Now another trouble had to be solved. These very clean monoplanes simply floated on and on when they built a low-wing retractable undercarriage version of their . Vega, the Orion, which carried pilot, six passengers and baggage at 224 m.p.h. on 500 h.p.

Another difficulty of flying fast had always been the problem of airscrew efficiency. A propeller is literally an air screw, and just like a wood screw, it goes forward a certain amount with each turn—if it does not do this it slips and makes a hole in the air just like a wood screw that fails to bite. It is obvious that an airscrew designed to travel at 200 m.p.h. moves a long way forward in one revolution and must be very inefficient at 50 m.p.h.—or even 100 m.p.h.—during take-off and initial climb. This low-speed inefficiency of the fixed-pitch propeller resulted in a wastage of power and very poor take-off by fast aeroplanes—the Schneider racers took something like two , miles to build up enough speed to drag themselves into the air.

In 1933 the Americans brought out a working version of what had long been the designer's dream, a variable-pitch airscrew. Made by the Hamilton-Standard company, which had long been making airscrews with detachable metal blades, it was an immediate success in the U.S.A. It was simple enough in its original form: the metal blades, two or three, had balance weights so that they could be rotated around their axes without effort and a hydraulic piston on the hub supplied the power. Movement was only a few degrees, 5 to 10 in those days, but it allowed reasonable efficiency at low and high speed. What was very infuriating was that, as has happened so often in our history, Britain had missed the boat, for the first successful variable-pitch airscrew had been British.

Almost a decade earlier Dr. Hele-Shaw invented a hydraulic v.p. airscrew which was test flown in Gloster fighter biplanes. Not only was the Hele-Shaw airscrew first, it had constantspeed control, which was later to become the universal system! Instead of the pilot selecting his pitch, a governor in the airscrew control maintained the engine revolutions at any desired figure whatever the speed or attitude of the aeroplane. The' Gloster Aircraft Co. did its best to sell the idea, pointing out the advantages to the pilots of fighters who would no longer have to watch their engine revolutions, but the authorities were apathetic and development was dropped.

But to return to the point, the v.p. airscrew was a boon to the fast monoplane and lifted everyday speeds up into the 200 m.p.h. range. Now another trouble had to be solved. These very clean monoplanes simply floated on and on when they

were throttled back for landing. If the nose was put down they gathered speed very quickly, if it was kept up they approached the earth in a very long, flat glide difficult to judge. Once again, although we in Britain had been sitting with the solution in our laps (since 1914 in fact) we failed to apply the variable-camber flap to this problem—not that we had many monoplanes to try it on! It remained for the Americans, through their active N.A.C.A. research centre, to evolve a less efficient method, the split flap. By lowering a transverse panel mounted under the trailing edge of the wing the frontal area of the aeroplane was increased and the higher drag made it come in more steeply. The split flaps also increase the effective camber and add some lift, but not nearly so well as do full camber flaps. The split flap is, however, easier to make and so it is more generally used to this day.

1933 saw the arrival of both the split flap and the v.p. airscrew and with them the ultimate victory of the monoplane was assured. Not that the biplane was going to die without a fight. Far from it,* in fact the spur of competition roused designers to make their greatest efforts, in many cases applying monoplane lessons to their biplanes. The military in most countries, having to account to their parliaments, tended to be conservative and to continue to order biplanes. Even in the U.S.A., where the aircraft industry had boomed after the Lindbergh Atlantic flight, the authorities were cautious about committing themselves altogether to the monoplane.

The Americans had a useful spur in their National Air Races which included the Thompson Trophy, a speed event to which much prestige attached. Some very clean biplanes were made for these races, but the monoplane began to show its superiority when speeds rose above 200 m.p.h., the advantages of one pair of wing/fuselage junctures and the absence of interplane struts doing no work other than keeping the wings apart were incontestable. Generally, these racers were radial-engined adaptations of the Schneider formula, wire-braced thin-wing monoplanes. The Gee-Bee monoplanes were, perhaps, the epitome of this breed—all engine and little else—and one, the Super-Sportster, brought the Landplane Speed Record up to 394.2 m.p.h. in 1932 piloted by Jimmy Doolittle, ex-U.S. Army Schneider pilot.

The Boeing Company made a fighter, the P-26 in 1932, less of a caricature than the Gee-Bee, but on very similar lines, with wire-braced wings and all. It was adopted by the U.S. Army Air Corps. Curiously enough, instead of a N.A.C.A. cowling, this aeroplane was fitted with the less efficient, if much simpler, British Townend ring—really a circular Handley-Page slat round the radial engine. With 600 h.p. the P-26 could reach 235 m.p.h. at 6,000 feet, which was about 20 m.p.h. faster than the original Hawker Fury biplane with a 450 h.p. Kestrel and 5 m.p.h. slower than that aeroplane with a 640 h.p. engine —which showed there was little to choose here in aerodynamic efficiency.

Some of the biplane designer's answers that hit home hard must be mentioned. Geoffrey de Havilland modified his Moth as a military trainer in 1931. While retaining the essential Moth appearance, the wooden box fuselage was replaced by a fabriccovered welded steel tube framework to withstand the heavy landings of school flying, the top plane was swept back from a centre-section ahead of the front cockpit so that both pupil and instructor could escape by parachute if necessary. The result was the trainer upon which most of the second war R.A.F. and Dominion pilots were trained and which served for over twenty years—the Tiger Moth.

De Havilland was also responsible in 1933 for the Dragon (D.H.84), a simple but clean biplane with two 130 h.p. Gipsy engines, which was a boon to operators requiring a cheap, easily maintained six to eight seater. The Dragon was characterized by a fuselage which filled the gap between the wings and by engines mounted on the lower plane. The fuselage was of generally aerofoil shape with a flat, as opposed to rounded, bottom and contributed considerably to the total lift at landing and take-off. The following year the Dragon-Rapide (D.H.89) with more streamlining, tapered wings, and 200 h.p. GipsySix engines came on the scene, raising the cruising speed from 109 to 132 m.p.h. In the same year there was also a four-engined aeroplane to this formula, the D.H.86, which carried ten to sixteen passengers at 150 m.p.h. This latter was a popular machine and was used in many parts of the world, but it did

not have the huge success of the D.H.89, which was used throughout the 1939-45 War by the R.A.F. for communications and radio-training duties, as well as by many charter companies all over the world—in fact quite a number are still flying today in the early fifties, notably for British European Airways' ambulance service to the Western Islands of Scotland.

Faireys made the Swordfish torpedo carrier in 1934. Not a lovely aeroplane by any standards, yet it was to be the mainstay of the Royal Navy's carrier strike force throughout the War. Designed first and foremost to do a particular job, the Swordfish was ideal for its work, which was getting a big load into the air with a short run. A large wing area gave this biplane plenty of lift, a long-travel robust undercarriage allowed it to be dropped on to the deck even in rough weather, a deep fuselage held the fuel and raised the pilot up to give him a good view. Folding wings, necessary for carrier stowage, meant having bracing struts near the fuselage at the extremities of the centre section. The undercarriage was arranged in two pyramids so as to leave the bottom of the fuselage clear for carrying the torpedo. As a sop to the aesthetic a Townend ring incorporating a neat nose exhaust collector ring covered the 1 600 h.p. Bristol Pegasus nine-cylinder air-cooled radial engine. The Swordfish, known to the Navy as the "Stringbag", survived because it was robust and easy to maintain. Generally, high speed was not required for its work and in the Battle of the Atlantic it could operate from the small emergency escort carriers in bad weather when faster aeroplanes were unusable. Nevertheless, when sent to attack heavily-defended ships, the slow speed of the Swordfish resulted in ghastly casualty rates—not the fault of the aeroplanes but of those who sent them to war.

Another outstanding biplane "answer" was the Gloster Gladiator, probably the epitome of the biplane fighter. The Gladiator was the last of a long line of fine fighters designed by H. P. Folland that started with the S.E.5—an outstanding success at the start and the finish. Moreover, the resemblance between the fast single-seater scout of 1914, the S.E.4(p.31) and the Gladiator of 1933 is remarkable. The success of this fighter was considerable; it was used by the R.A.F., the Fleet Air Arm and nearly a dozen foreign air services. It was still operational at the outbreak of the Second World War and distinguished itself by flying from frozen fjords in Norway and mountain aerodromes in Greece as well as in the defence of Malta, where the epic stand of Faith, Hope and Charity against the might of the Regia Aeronautica lives as one of the most gallant episodes in the history of air fighting.

The Gladiator can truly be described as the S.E.5 of its day, for it had all the characteristics of the earlier aeroplane embodied in what was then the most up-to-date manner. The layout was compact, the pilot sat behind the top plane, which had a small cut-out, and his eyes were almost level with it. The structure was a steel and duralumin version of the old wooden framework and the covering was of fabric. This was light, yet sturdy, and was easy to repair. The undercarriage had two streamlined cantilever legs with internally-sprung wheelsdeveloped from those first used on the Tiger Moth racer—which had a lower drag than the traditional type as on the Fury and yet lacked the complication of that on the new monoplane fighters. A rugged 840 h.p. Bristol Mercury radial engine and fixed-pitch airscrew were further aids to easy maintenance. A covered cockpit added to performance and the comfort of the pilot, while split trailing-edge flaps were a further compromise with the new generation. In armament the Gladiator lay between the old and the new, since it had two guns in the fuselage and two in the wings. Delightful to fly, the Gladiator was beloved by its pilots, who would vigorously defend its lack of performance and laud its manoeuvrability. The top speed was 250 m.p.h. at 15,500 ft. and it could climb to 15,000 ft. in 4.8 mins.

It was characteristic of the American's love of novelty that that country should be the cradle for the new generation of aeroplanes. Out of a nondescript and heterogeneous industry was born the low-wing monoplane air liner as we know it today. It is only fair, however, to point out that external influences were at work, conditions which existed in the U.S.A. alone in the world. In that country alone was there the combination of great distances without a frontier and a large population of wealthy business men who were always in a hurry. This led to rapid air line development once aviation had proved it could be reasonably reliable as well as fast. With many private companies operating networks across the States competition soon became fierce as each company tried to outpace its rival's timetable.

In 1932-33 The Douglas Aircraft Co. Inc. of Santa Monica, California, designed a twin-engined monoplane air liner for Transcontinental and Western Air Inc. (T.W.A.), the DC-2, which had an immediate success from the moment it started to come off the production line early in 1934. Not only was it popular in the U.S.A., seventy-five were ordered by June 1 1934, but the astute Antony Fokker acquired the exclusive sales and manufacturing rights for Europe. However, it was in 1936 that Douglas earned undying fame, for it was then that the DC-3 came on the scene. An enlarged edition of the DC-2, incorporating the lessons of two years' flying of that aeroplane, there was nothing about this straightforward transport to suggest that it was to be a history maker. Curiously the DC-3 was originally conceived as a transcontinental sleeper, known as the DST, for American Airlines, in which guise it carried 14 passengers whose seats were convertible into bunks for night travel—for it took some 24 hours to cross the American continent—but it was in its role as an ordinary 21-seater air liner that it was to achieve fame. Air .

The DC-3 and its good e.g. range

line after air line bought the DC-3, and when America was brought into the war by the attack on Pearl Harbour, the DC-3 was still in full production. It was obviously a type suitable for general knockabout duties as a trooper and general military freighter and huge orders ensued. Used by the U.S.A.A.F. as the C-47 and C-53, by the U.S. Navy as the R-4D and by the R.A.F. and the Dominion air forces as the Dakota, 10,926 in all were built by the time production ceased in 1945. Many were also built, as the PS-84, in the U.S.S.R.

After the war, hundreds of DC-3s, reconverted to commercial standards, became the mainstay of the world's short-haul and medium-length air lines. Today nobody quite knows how to replace this proven workhorse except at very great expense. Why should the DC-3 have proved so successful and so popular? In the main it was because a happy choice of design features combined a thoroughly practical economy with a good performance. Fuselage dimensions were large enough to carry a good load, without being so big that there was undue drag penalty. At take-off, 2,000 h.p. were available to give good acceleration and a reasonable climb. Because of good proportions, particularly ample tail surfaces, flying qualities were excellent. Slight sweepback on the leading edge of the tapered wing gave good stability and a reasonable c.g. range. The all-metal structure was simple, requiring little attention in service, and was easy to repair. Control surfaces were fabric covered for lightness, which also meant easy repairing. Flaps and main wheels were operated by a simple hydraulic system, and when retracted the latter still protruded sufficiently for an emergency landing. The main undercarriage was simple and robust, the tail wheel especially strong—an important feature this, since many an aeroplane has left its tail wheel behind in a rut when taxi-ing.

Originally the accommodation was for seven single seats down one side, seven double ones on the other side of an ample gangway. A fair-sized window at each seat gave a good outlook that made the aeroplane popular with passengers. Nowadays, with tourist class travel, 32 seats are often fitted, as in the B.E.A. Pionair conversion, that is pairs on each side of a central gangway.

For its day, the DC-3 was high-powered with two 1,000 h.p. radial engines, but it was designed to cruise economically on about half power with the help of its controllable-pitch airscrews . Brief particulars will help to show what made the DC-3 a Paying proposition. Span 95 ft., length 65 ft., wing area 987 sq. ft., empty weight 16,290 lb., all-up weight 24,000 lb. (nowadays with slightly more powerful engines an all-up weight of 27,000-29,000 lb. is usual); cruising speed 165-195 m.p.h., landing speed 64 m.p.h., initial climb 850 ft./min., service ceiling 20,800 ft.; range up to 2,150 miles.

Something has already been said of the impetus given to aeroplane design by competitions such as the international Schneider Trophy Contest and the Thompson Trophy Race in the U.S.A. Two other events did much for aeroplane design the MacRobertson Race to Australia in 1934 and the annual Coupe Deutsch de la Meurthe in France. The MacRobertson contest consisted of a combined speed and handicap race from England to Australia. It attracted many of the world's most famous racing and long-distance pilots and entries ranged from light aeroplanes to air liners and racers. The winner was a de Havilland Comet, piloted by C. W. A. Scott and T. Campbell Black, with a K.L.M. Douglas DC-2 in second place and also winner of the handicap section.

The Comet was certainly a landmark in British aeroplane design. Evolved specially for the race, three Comets were prepared in about seven months. Calculations were made to see just what combination of range and speed would be likely to outpace rivals on this particular route. Then a beautifully streamlined flying petrol tank was built to do the job. All available knowledge was brought to bear on the aerodynamic design. Two D.H. Gipsy-Six engines of about 230 h.p. each were chosen—small enough power for a cruising speed in excess of 200 m.p.h.—and a minimum cockpit size was then laid down. In fact the fuselage was nearly all fuel tank, the cockpit being well aft, once again a concession to keeping the' centre of gravity stationary. A thin low-drag, sharply-tapered wing of just enough area to give a reasonable landing speed using small split flaps was used. Taking something from the lines of the original Tiger Moth racer—and, incidentally, of similar wooden stressed-skin construction—the Comet was a perfect example of design for a purpose, long range with high speed.

Contemporary with the Comet were the little wooden Caudron monoplanes that culminated in the C-460 which won the Coupe Deutsch, also in 1934. This contest was a speed event in which large engines were penalized, so that both aeroplane and engine designer were forced to refine everything to the maximum. The Renault was, like the Gipsy-Six, an inverted, six-cylinder, in-line air-cooled engine, but it was highly supercharged so that 370 bii.p. were obtained from a cylinder capacity of only eight litres. The aeroplane in this case was little more than a flying engine nacelle. Behind the engine were squeezed fuel, over the wing for balance, pilot and tail. A tiny monoplane wing, with a span of only 22 feet, carried trailingedge flaps and a retractable undercarriage, although the wing was so thin that the wheels had to lie in the bottom of the fuselage. Surprisingly enough, despite its small size the C-460 had a wing loading of only 28 lb./sq. ft. The maximum speed was about 310 m.p.h.

1936 was Britain's vintage year. The R.A.F. and the Air Ministry had a year or two earlier awakened to the fact that even the Fury fighter, supreme in performance a few years before, was now surpassed in speed by such civil aeroplanes as the DC-3, the Comet and the C-460. In consequence they set out a series of specifications for fighters and bombers that resulted in the British industry producing some of the finest military aeroplanes in the world, which in one swoop put the

R.A.F. ahead of all other countries. It is worth seeing how this was done by making a detailed examination of the best halfdozen . Supreme in beauty was the Supermarine Spitfire, designed by the late R. J. Mitchell out of his Schneider Trophy ex- * perience and with the then new 1,000 h.p. Rolls-Royce Merlin liquid-cooled vee engine. The Spitfire had a graceful semielliptical wing, which is a shape giving the lowest air drag, although it is seldom used because it is difficult and expensive to make, having no straight lines. Because the air flow over an elliptical aerofoil is smooth, it also gives very good handling characteristics. The wing of the Spitfire was also thin, which made it behave well at high speeds. Fuselage lines, although more generously tailored than those of the S-6, are undoubtedly similar and no more space than was essential for pilot, instruments and controls was provided; everything was designed to fit snugly behind the slender Merlin engine. This near-perfect combination gave a speed of about 350 m.p.h. at 20,000 ft., coupled with outstanding high-altitude manoeuvrability.

Controversy between Mitchell and the R.A.F. raged over two features, the outward-retracting undercarriage and the radiator position. The service pilots were, not unnaturally, distrustful of the narrow wheel track, which they felt would lead to poor stability on the ground. In fact it did not, although this was always one of the poorer features of the Spitfire. Objection to the radiator under one wing, oil cooler under the other was mainly an instinctive dislike of asymmetry, although the vulnerability of oil and coolant pipes running into the wing was a fair criticism. Certainly these wrangles delayed ordering of the Spitfire and Mitchell died tragically of cancer while still a young man not very long after the aeroplane came into service.

On the other hand, it was an R.A.F. officer's foresight that gave the Spitfire the formidable armament of eight guns that was to do so much toward winning the Battle of Britain. Not only were eight guns novel, four and even six had been used before, but their mounting in the wing made them independent of the propeller revolutions—Constantinesco's sun had set. A new gun, the American Browning, with a rate of fire of 1,200 rounds per minute made the fire concentration terrific—albeit only 200 rounds per gun could be carried because of weight and bulk. This new conception of many guns with a rapid fire arose from the realization that at combat speeds of 300 or 350 m.p.h. the pilot would only be able to hold his target in the sights for seconds at a time, therefore as many bullets as possible must be delivered in that brief period.

Partner to the Spitfire was, of course, the Hawker Hurricane, brain child of Sidney Camm, designer of the Hunter and Fury fighters of today. With the same engine and same armament requirements, these two aeroplanes represented entirely different approaches to the same problem. While the Spitfire was a new conception based on specialized experience, the Hurricane was tradition compromising with a new world. It was, in fact, a monoplane Fury. The rising line giving the good forward view on the ground (another of the Spitfire's poorer features) the tubular girder fuselage and fabric covering, the characteristic tail shape, were all adaptations of the earlier type. Even the wing was originally fabric covered over a metal structure similar to that of the biplane—this was, however,

A ducted radiator similar in principle to that for the Merlin: 1, air flow, 2, matrix, 3, controllable outlet

replaced by a metal stressed-skin structure after about fifty had been built.

The Hurricane, despite its being rather larger than the Spitfire, could reach 335 m.p.h. at 20,000 ft, although this was reduced to about 305 m.p.h. in Battle of Britain trim. The Hurricane had a wide track undercarriage since the legs were hinged to the ends of the centre-section into which they retracted . The guns were grouped neatly in units of four in each wing. Radiator and oil cooler were mounted centrally under the fuselage in a ducted cowling.

This duct, also used on the Spitfire wing radiator, was actually the first practical application of jet propulsion. When the aeroplane was flying fast air entered the small opening, expanded and slowed down in the duct, was heated as it passed through the radiator matrix and was ejected through the narrower outlet at increased speed. This process is exactly what happens in any form of jet engine, and in this case it did add a few miles an hour at high speed.

It is noticeable that the radiators for the Merlin were little, if any, larger than those for the Kestrel of half the power. The reason for this was that with the Merlin, after an abortive effort at steam cooling, ethylene glycol was added to the water. This chemical is the same as the anti-freeze used in a motor car and it has the property of raising the boiling point and lowering the freezing point of water. In the aero-engine this was particularly useful, because it overcame the acute problem of the lowered boiling point of water at high altitudes. The cooling system was also pressurized, that is it was sealed so that if the coolant did boil steam could not escape. The result of this new cooling system was a much reduced quantity of coolant and size of radiator, which, in turn, reduced both drag and weight. It was so characteristically British that no variable-pitch airscrews were available for these two superlative fighters.

Even the first squadrons had to battle along with wooden propellers almost to the outbreak of war, and even then it was an unsuitable two-position type that became available. The very limited British variable-pitch airscrew production was wanted for the new bombers which, with their heavier loads, could scarcely have become airborne without them.

One of the causes of the disruption of Air Ministry complacency had been the "Rothermere Bomber". In 1933 the Bristol Aeroplane Company had the idea of making an allmetal eight-seater that would excel the performance of the fastest of the new American air liners. The newspaper proprietor Lord Rothermere decided in 1934 that this new aeroplane , the Bristol 142, would meet his needs if it were developed to suit his own requirements, one modification being the fitting of higher-powered engines. This new aeroplane was completed in 1935 and was christened "Britain First", an astute publicity move at a time when there was an acute outcry about the antiquated aeroplanes used by the R.A.F. and Imperial Airways. The "Britain First" proved to have such an outstanding performance when tested at the R.A.F. experimental establishment at Martlesham Heath that Lord Rothermere presented it to the nation.

From the eight passenger Bristol 142, which was a low-wing monoplane, was developed the Blenheim medium bomber, a mid-wing monoplane of generally similar appearance with two 840 h.p. Bristol Mercury radial engines. The reason for the lowered fuselage (or raised wing if you like) was to make space in the fuselage below the wing spars for the load of two 500 lb. or four 250 lb. bombs. The fuel was carried in tanks inside the fairly thick wing. The Blenheim was characterized by a curiously short nose, the reason for which was that the weight of the two engines and the pilot and navigator ahead of the

Designs of The Bristol Aeroplane Co., Ltd. The Blenheim, Beaufort and Beaufighter represent a direct line of development, one from the other

wing had only the single gunner behind the wing to balance them. It was not a very successful layout, since the pilot's sideways vision was restricted by the high-placed nacelles. Nevertheless, the Blenheim with a top speed of nearly 300 m.p.h. was a striking aeroplane—if a not very effective weapon.

Another aeroplane of this time was the Fairey Battle day bomber—an attempt to compromise between the tradition of the D.H.4 and the new style. A large, gracefully slender aeroplane , the Battle was handicapped by being almost the size of the Blenheim, yet with the engine power of the Hurricane. It was so beautifully simple in shape that it could be called almost the "pure" low-wing monoplane. The long transparent hood housed the pilot in front and either one or two gunners at the rear. The Battle was destined to fail tragically in war, not because of a fault in the aeroplane, but for the reason that the Air Staff had not seen the anomaly of ordering multi-gun fighters and a large day bomber with only two rear guns, one above, one below.

1936 also brought to light the Armstrong Whitworth Whitley, one of the most hideous aeroplanes ever designed and yet a prop and mainstay of our night bombing offensive during the first years of the War. In the Whitley you have the large wing again, with the fuselage tacked on as something of an afterthought required solely for the purpose of supporting the accessories. This is emphasized by the way the wing and the fuselage travelled along on what appeared to be different paths. When flying fast, the Whitley stuck its tail high in the air as if it were tearing down a hill, while the huge wing passed by on a ponderous level course.

The queer shape of the Whitley arose from the fact that the designer decided to have a large wing area with a moderate span and he chose a very thick, high-lift aerofoil section. The thick wing was built round an enormously strong box spar of corrugated light-alloy sheet, to which leading and trailing-edge portions were attached. In the wing, between engine nacelles and fuselage, were cells for bombs up to 500 lb. each in weight and the fuselage, under the wing, had a larger bomb bay capable of taking two 1,000 lb. bombs. The other main duties of the fuselage were to carry two pilots, two gun turrets and the tail. The size of the turrets set the depth at nose and tail, the pilots' seats decided the maximum width, plus a little height to see forward over the nose turret; and by joining these main points with a ruler the blue-print of the fuselage was evolved. This may have looked unlovely but it was practical, since

VICKERS AEROPLANES — 1918 TO 1948

elimination of curves saved weight (because a straight line is the shortest distance between two points) and also simplified manufacture.

Originally, the Whitley had a "dustbin" retractable turret under the fuselage, but since this weighed half a ton, and its lowering reduced speed at a critical time during combat, it was never used in service. The circular opening in which the dustbin was mounted proved very useful however—it was used as a dropping hatch for paratroops. The Whitley started life with two 800 h.p. Armstrong Siddeley Tiger radial engines, which were not nearly powerful enough.for it, but later the 1,030 h.p. Rolls-Royce Merlin X gave it a new lease of life and it was this version, the Mk. V, that was one of the mainstays of the Bomber Command offensive until 1942. Maximum speed of the Whitley V was rather better than 200 m.p.h. and it was usually operated at about 15,000 ft., though it crossed the Alps to raid Milan and Turin.

Companion heavy bomber to the Whitley was the Vickers- Armstrongs Wellington, behind the plump well-fed appearance of which lay an unusual method of construction, the geodetic system evolved by B. N. Wallis. The design was one of those ingenious ideas that are astoundingly simple—after someone has thought of them.

The name is derived from geodeses, imaginary geographical lines following the curvature of the earth along a straight path. By using structural members following such lines, in two directions to form a lattice and joining them at each intersection , the designer produced a very stiff and light framework. Any thin material will support a much larger load in tension than in compression, for instance you can pull hard on a sheet of cardboard without tearing it, but quite a tiny push will crumple it completely—it has "failed by buckling in com- , pression" as an engineer would say. Now the lattice of a geodetic framework is such that as a compressive load is applied in one member it bends and is supported by a tension (i.e. pulling) load in another member. To get the best and most reliable transfer of load the geodetic members must be curved and that is why the Wellington had the plump fuselage which earned it the R.A.F. nickname of "Wimpey", after Popeye's fat #am&Mrger-demolishing friend James Wellington Wimpey—a double pun.

Geodetic construction had its problems. So long as it was used to make a complete basket it was light and strong, even with its fabric covering it was a better proposition than either the old construction or the new stressed skins. However, it was not well suited to taking openings since all the loose ends had to be firmly anchored to strong frames. Any form of concentrated load had also to be catered for by special members that were more than ordinarily difficult to design.

In its day the Vickers geodetic construction served a very useful purpose, but it has now gone for all time. Other makers did not adopt it, mainly because it was very thoroughly patented and the royalties were discouraging. Vickers found that the structure was troublesome when it came to fitting bomb bays and doors, although windows were an easy matter, since it was just a matter of replacing the fabric covering by transparent sheet—which gave the fuselage a rather Olde Worlde cottage appearance! The multitudinous geodetic members were very redundant, that is if one or two were removed they were scarcely missed and this fact made the Wellington a wonderful aeroplane for taking punishment from enemy fire.

Geodetic structure, T=tension

The Wellington was Bomber Command's outstanding aeroplane (until the advent of the four-engined "heavies"), by 1941 it formed the main striking force against German arms production and its last raid was made in April 1945. Although the fuselage was divided into two long narrow bomb cells for the bombs envisaged before the War, the fat 4,000 lb. "blockbusters " were also carried, by the crude expedient of removing the bomb doors! The docility of the Wellington and its good lifting powers were also to make it useful with Coastal Command , where it first carried a huge magnetic ring for exploding mines, then the heavy early anti-submarine radar, and the Leighlight anti-submarine searchlight. The "Wimpey" was still in use as a R.A.F. air crew trainer in 1953. Most marks had

Hercules radial engines of 1,370 h.p. or more, but the lowerpowered Pegasus, Merlin and Twin-Wasp were also fitted. Naturally performance and load varied with the engines, but representative figures would be, maximum speed 250 m.p.h., normal range 1,500 miles, ceiling 20,000 ft., bomb load 4,000 to 5,000 lb.

The Wellington had a small geodetic brother, the Wellesley. This was a single-engined monoplane in the Battle category which distinguished itself by flying from Egypt to Australia, 7,162 miles non-stop in 1938 to break the World's Distance Record. The geodetic wing and fuselage of the Wellesley were beautifully hollow and could carry fuel tanks, but they had no bomb door openings and so two streamlined canisters were slung under the wing to carry the bombs—a Heath Robinson arrangement that seemed strange before the War, but now looks normal enough when so many military aeroplanes carry drop tanks.

Last of Britain's 1936 crop of military monoplanes was the Handley Page Hampden, which lay between the Wellington and the Blenheim and was classed as a "high-performance bomber". As the reader has probably guessed by now, the Air Ministry was feeling its way with bombers and was torn between taking a large load far and a small load quickly. The Hampden represented the middle course: it could cruise at 200 m.p.h., could top 250 m.p.h. in an emergency, was small enough to be highly manoeuvrable, had guns with an all-round field of fire and could deliver a ton of bombs.

The designer of the Hampden achieved his ends in an unusual manner, only attempted on a few aeroplanes. He packed his crew and bombs into a deep narrow fuselage and mounted the tail on the end of a slender boom. By doing this he maintained a reasonable streamline and, at the same time, gave his upper and lower rear gunners a good field of fire without having to resort to a tail turret or to a "dustbin". The fuselage projecting below the wing centre section made an accessible bomb bay. The bomb-aimer/navigator in the transparent nose had a Lewis gun and the pilot a fixed gun, so that the Hampden appeared well armed. In point of fact it was not, since the effectiveness of hand-trained Lewis guns against highspeed fighters was negligible, because they could seldom be brought to bear long enough to inflict serious damage.

A feature of the Hampden was a highly tapered wing; a shape giving a low drag, but usually a sharp stall, which was prevented in this case by fitting full-span Handley Page slots

ADVANCED CIVIL AEROPLANES OF THE THIRTIES.

and slotted flaps between the ailerons and the fuselage. Two 1,000 h.p. Bristol Pegasus radial engines gave a top speed of 265 m.p.h. at 15,500 feet.

It would be wrong to leave this vintage year for British monoplanes without mentioning the Bristol 138 height record machine and the Short Empire Boat.

Like most record-breaking aeroplanes, the Bristol 138 showed clearly how it was designed for a purpose: a large (span 66 feet), high-lift wing made as lightly as possible from wood; the minimum of fuselage and tail, also for lightness. Speed and streamlining were of little importance, the main requirements being engine power and wing area. The Bristol Pegasus engine had a two-stage supercharger and it carried a special radiator for cooling the air between the stages. (Supercharging , that is compressing, air heats it and therefore reduces its density, so that if the air were not cooled much of the effect

The effect of combining slots with slotted flaps

of the double supercharging would be lost.) There was a large wooden airscrew, slow turning, to give a good "grip" on the thin air of the stratosphere. There was no pressure cabin, but the pilot had a pressure suit not unlike those now being issued to the U.S.A.F. crews of high-altitude jet bombers. Piloted by S/Ldr. F. R. D. Swain the Bristol 138 gained the World's Height Record on September 28th, 1936 by reaching 49,967 ft., which was raised to 53,937 ft. the following year by F/Lt. M. J. Adam.

The Short Empire Boat was one of the great landmarks in aeroplane design. It was not by any means the first monoplane, or the first all-metal flying-boat, but it was the first to be built on thoroughly practical lines—lines that are still a general standard today. But it was not the design alone that was bold, so was the whole conception. Short Brothers Chief Designer, Arthur Gouge (now Sir Arthur) and Imperial Airways worked out the design together and the latter gave an order for no less

than 28 seaplanes "off the drawing-board". The idea was to use them on all Imperial Airways' Empire Routes—to Australia and to Africa. In those days aerodrome facilities were poor, but water was plentiful and Imperials considered they could operate more profitably using harbours, the waters of the Nile and the African lakes than by making their own aerodromes. Furthermore, the Empire Boats, despite their size had a performance comparable with the fastest landplane air liner, the DC-3, and provided much more comfortable and spacious accommodation for long trips, with promenade and smoking cabins, for 24 passengers. The large hull, necessary for buoyancy and to keep the wing high above the water was, of course, the reason why the interior was so spacious.

The performance of the Empire Boat came from three main

The Gouge extension flap

factors: engine nacelles merged into the wing, as in the fast landplanes, instead of being on struts; very careful study of the best streamline hull form consistent with water performance; and the use of a relatively high wing loading, made possible because of the unlimited stretches of water for take-off and the decelerating and shock-absorbing effect of water when alighting. The wing-tip floats were both a drag and a weight penalty (the latter largely compensated by the absence of the undercarriage), but even so the Empire Boat was a truly commercial vehicle. The wings were fitted with a special type of flap, the Gouge flap, in which a large section of the lower surface of the wing trailing edge slid back and down to increase the wing area as well as the camber.

The main figures for the Empire Boat make an interesting comparison with today's practice. Span 114 ft., length 88 ft., height 31 ft. 10 in., empty weight 23,500 lb., loaded weight 40,000 lb.; max. speed 200 m.p.h.; cruising speed 160 m.p.h.; stalling speed 69 m.p.h.; range 750 miles. This was achieved with four 790 h.p. engines, which gave 910 h.p. for take-off.

Later more Empire Boats were ordered making a total of 39, and modifications led to some changes in performance, but the foregoing figures, relating to the original boats, are representative . The data for the Empire Boat should be compared with that for the Viscount, p. 70, and the Stratocruiser, p. 214. At the time this British renaissance was taking place, there was great aeronautical activity in another country—Germany. A year earlier the Luftwaffe had come out of its disguise. Previously the German air force that was still forbidden by the Treaty of Versailles had existed in the intensely active "civil" flying and gliding schools and clubs throughout Germany. The trainers used at these schools varied from bona fide light aeroplanes to such advanced types that one was put straight into commission with the new fighter squadrons simply by fitting it with guns. Pilots of the Lufthansa state air line were all on the secret air force reserve and that company was as much a training school for air crews as it was a commercial undertaking .

Among German aeroplanes of this period there was the B.F.W. Me 108 Taifun, a fast four-seater with a 240 h.p. engine It was, although naturally somewhat more plump, very similar to the Me 109 fighter, mainstay of the Luftwaffe throughout the War. In fact, the Me 109 was flying in secret in 1936 as its design had been started in 1934.

Dornier was flying the innocent-looking Do 18 as a transAtlantic mail carrier. This cleaned-up version of the Wal was in reality the prototype for a patrol flying-boat that was destined to be very troublesome in the North Sea. Heinkel had made an exceptionally sleek low-wing monoplane , the He 70, again a mail carrier or for four passengers, which could do 222 m.p.h. on 640 h.p. This aeroplane later appeared in its true colours as a military two-seater; but in the meantime, it is amusing to record, Rolls-Royce bought an example of the civil version to use as a test bed for the Merlin— the engine which, more than any other, was to defeat the Luftwaffe.

There was also the He 111, with the same sleek lines as its single-engined relation, which a gullible aeronautical community accepted as a ten-passenger commercial monoplane—, with two 660 or 880 h.p. liquid-cooled engines and a special freight hold with doors in the floor! The slender, well-streamlined fuselage and low-drag elliptical wing and tail were so obviously designed for maximum performance rather than the comfort of its ten passengers that its lethal intent should have been obvious. A tall undercarriage made entry by passengers inconvenient, but was ideal for loading bombs into the fuselage from below and gave ample ground clearance for an under gun position. The two BMW liquid-cooled engines were also a military pointer at a time when the simpler, and by now very reliable, air-cooled radial had ousted its sleeker rival from air line use.

Junkers, whose bona fide Ju 52/31T1 air liner had turned up in the first bomber squadrons of the Luftwaffe complete with a gun ring in the roof where the emergency exit had been and a dustbin, came out with a fast new air liner, the Ju 86. For the first time the familiar corrugated skin had given place to the more modern low-drag smooth skin. As with the Heinkel, only ten passengers were carried, although it must be admitted that a few Ju 86 were actually used as air liners as well as bombers! Contrary to the British and German military authorities, both of whom believed in nose armament, the Italians took the view that speeds of 250 m.p.h. made frontal attacks by fighters unlikely. Because of this belief, several three-engined bombers were made, the most attractive being the SM-79. Despite the excellence of Italian craftsmanship, this aeroplane had a wooden wing and fabric-covered welded steel tube fuselage in the old Fokker tradition. Aerodynamically, however, it was clean and could carry over a ton of bombs at well over 250 m.p.h. The SM-79 was successful in the Spanish Civil War, where there was little fighter opposition, but it proved highly vulnerable in the last War when attacked by British multi-gun fighters.

In the U.S.A. a large monoplane, of very attractive lines, had flown, this was the Boeing Y1B-17—prototype of the Flying Fortress. As first conceived this aeroplane was intended to do about the same as our twin-engined bombers although the Americans did go for a higher operating level, 20,000 to 25,000 ft. In contrast to this very modern requirement of the U.S. Army Air Corps, none of the American single-seater fighter designs could approach the Hurricane or Spitfire in performance or fire power. The U.S. Navy went even further, clinging lovingly to braced biplane fighters, little changed from 1920 save for the rather clumsy expedient of retracting the wheels into the fuselage—which did not raise the speed above that of our own Gloster Gladiator.

Curiously, in this year of modernity the fashionable French failed to grasp the main points of the new technique. Alone among the aeroplanes of that year the Dewoitine D-620 air liner showed some modern thought. However, although it was of stressed-skin all-metal construction with a retractable undercarriage, the flat-sided fuselage, three engines and underslung nacelles savoured more of the past than the future. Three 880 h.p. radials gave a cruising speed of 168 m.p.h. with thirty passengers.