Profile – Bristol Brabazon

It was probably a very unsublime mat- ter of long-sighted self-interest that led to the setting up of the Brabazon Committee. In 1942 an agreement between the British and American governments was reached that allocated the building of fighters, heavy (night) bombers and coastal aircraft to Britain, and long-range fighters, medium bombers and transport aircraft to the USA. It was seen as a sensible division of resources, but one of its effects was to prevent any significant British development of transport aircraft during the war. Whether the dynamic Lord Beaverbrook was the first actually to perceive the consequences of this I do not know, but he certainly proposed action to combat it.

The committee’s original recommendations were for five categories of aircraft (later expanded to six, the chief of which- and not only because it was number one but because it was the hardest operational specification to satisfy-was for an aircraft able to fly London-New York non-stop with 100% regularity.

This appears to be nothing out of the ordinary today, but four decades ago it was technological frontier stuff. Gas turbines were in their infancy. Even their most dedicated protagonists could not foresee the turbine’s enormous fuel consumption coming down to economically acceptable levels for airline operation within the realisable future.

The piston engine, by contrast, was a highly efficient, reliable, well-understood piece of equipment. In both its liquid-cooled (Rolls-Royce) and air-cooled (Bristol) versions Britain stood pre-eminent. So although the Brabazon Committee did not concern itself with the power installations for the recommended aircraft, it went without saying that the power should be provided by pistons.

The London-New York non-stop service was a difficult proposition because at the cruise speeds of aircraft at that time (250 mph for the Brabazon 1) the great circle distance of 3,450 miles was increased to an equivalent still-air range of 5,500 miles when allowances for headwinds and diversion to alternates and stand-offs were taken into account. No aircraft of the period had anything like that sort of still-air range capability, let alone being able to carry a useful payload into the bargain.

The Bristol Aeroplane Company had, however, done a design study for a very long-range strategic bomber in 1937. To that extent it was somewhat ahead of the game when invited in 1942 to submit a study for a 100,000lb gross weight bomber. This led eventually to consideration of an aircraft with a range of 5,000 miles. It emerged as a design with a high wing of 225ft span, eight buried engines driving four pusher propellers, a slender fuselage, butterfly tail and an all-up weight of 225,000lb. The Air Staff, however, decided to concentrate effort on Lancasters, so nothing came of the Bristol bomber.

Until, that is, the time came to consider the realities of meeting the Brabazon Committee’s first requirement. The work already done by Bristol in this general area of design, together with the awareness in the Ministry of the advanced work in structures that Archibald Russell had pursued at Filton, resulted in the decision that Bristol should be given an order for two prototypes. There was airy talk at the time of perhaps as many as ten production aircraft to follow.

The whole concept of civilian passenger flying, in Europe at least, was then centred on civil servants and expatriates going to and fro on leave, and upon well-to-do people travelling for their private purposes and very senior businessmen going about their business. There was also the mail, but not as a primary consideration.

The airliner was seen as an aerial complement to the ocean liner, but with no counterpart to the ship’s second and third classes. It was therefore BOAC’s advice that passengers would not put up with non-stop flights of more than 18 hours and that each passenger would require a volume of 200 cu ft for comfort and 270 cu ft for “luxury” – about three Simes the interior size of a modern family car for each passenger.

The upshot of all this was that the first Brabazon design scheme (April 1944) had a 25ft diameter fuselage (nearly 5ft greater than that of the Boeing 747) divided into upper and lower decks. It provided sleeping berths for 80 passengers, a dining room, promenade and bar, or day seats for 150 people. Bristol also considered other aerodynamic and accommodation criteria, however, and these eventually resulted in the Committee recommending the adoption of the company’s proposal for a slimmer fuselage design to accommodate 50 passengers. BOAC agreed to support this but stated its preference for a smaller version for 25 passengers!

The Bristol Aeroplane Company had for some time implemented a far-sighted policy of recruiting high-flyer graduate engineers. Some of these were constituted into a project team to determine the salient characteristics of the Brabazon 1 design. These eventually coalesced (November 1944) into a 177ft fuselage of 16ft 9in diameter and superb form, with a massive wing of 230ft span (Boeing 747 span is 195 by comparison) housing eight 2,500bhp Bristol Centaurus 18-cylinder air-cooled radial engines. These were mounted in pairs to drive eight coun- ter-rotating co-axial propellers of 16ft diameter. The all-up weight was 130 tons and the total fuel capacity 13,650 gallons.

Russell’s sophisticated approach to structural design was furthered by the bright young project team. It is fair to say that the Brabazon 1 was the first aircraft to have an “optimal structure in which the form and scantlings of the structural members, including the skin panels, were both designed and disposed to meet their loadings for the minimum cost in weight. Flanges of stringers were taper machined; skin panels were rolled not only to closer tolerances than ever before, but to non-standard guages. By comparison with previous structural standards, the weight saved in the Brabazon airframe through elegant technical sophistication could be measured in tons.

The drive to keep weight down inspired the idea of using the ailerons to alleviate structural stress in the wing when encountering a gust. The system involved having a vane on a probe ahead of the aircraft’s nose which would sense the gust and signal the aileron power system to deflect the surfaces simultaneously and in the same direction to reduce the spanwise bending. Preliminary flight trials with a Lancaster gave unsatisfactory results, largely because the servo response was too slow, but the techniques were proved and were subsequently applied to the Bristol 188 supersonic research aircraft. Today they are standard practice in servo design.

(The need for gust alleviation in large, fast aircraft was diminished by the alternative practice of mounting engines far out along the wing, but with the latter-day emergence of three-engined and big twin airliners the use of powered flight surfaces for gust alleviation is making a comeback. )

The Brabazon was the first to have 100% powered flying controls, the first to have electric engine controls, the first to have high-pressure hydraulics and the first to have AC electrics. By calling for such advances in these several fields the Brabazon initiated moves toward higher technologies from which aviation has since benefitted immeasurably.

The first manufacturing drawings were issued in April 1945 and construction of the first prototype began in October. A vast eight-acre assembly hall was buit, a specially strong 8,000ft runway was constructed, and a most complex and comprehensive programme of systems and structural testing was initiated. The complete aircraft was rolled out for engine runs in December 1948 and, to the accompaniment of enormous nationwide interest, the first flight was made on 4 September 1949. It was a majestic occasion-not the least majestic part of which was the stately manner in which the huge aircraft took off, flew by and landed.

All aeroplanes have snags and development troubles, but those experienced with the Brabazon 1 were of no particular note, considering the several ways in which the technology broke new ground. This speaks volumes for the capability not only of Bristol’s aircraft and engine design teams but of their complementary fellows, the sub-contractors who produced the aircraft systems.

Everyone who flew in the aircraft was impressed by its quietness and smooth- ness and the sense of ease that comes when these qualities are allied with spaciousness. But, given the airline assumptions about the number of passengers to be carried and the constraints imposed by the operational limitations and the power/speed equation, there was no way that the aircraft could make economic sense, even by the elastic standards of the time.

Setting aside the nonsense of having an aircraft weighing nearly 300. 000lb for a handful of passengers, the Civil Airworthiness Requirements of the time called for the use of runways of no more than 2,000ft, equating with a landing speed of around 60mph. This meant that the wing loading had to be low, which in turn meant that the wing area had to be high.

Long, narrow (high aspect ratio) wings are more efficient aerodynamically than shorter, wide wings of the same area- that is, they have lower drag-but long-narrow wings of the same strength as short, wide wings are more difficult to make and are inevitably heavier. Modern structural techniques have lessened the difficulties a little, but 40 years ago in the case of the Brabazon, there was a reluctance to go for a higher aspect ratio than ten- that is, a span ten times the mean width of the wing-because of the sheer scale of the wing and the structural limitations of the time.

Burying the engines in the wing was seen to be a solution to the problem that 30% of the power of an air-cooled engine was absorbed in overcoming its own drag whereas it lost only 5% to cooling per se. The Centaurus engine was one of the most efficient piston aero engines ever made, but it measured 56. 6 inches in diameter. It was desirable for sound engineering reasons to have the engines as close as possible to the propellers, and thus the thickness/chord ratio of the wing was necessarily higher than would other- wise have been used.

It has been popularly supposed that the thickness of the Brabazon’s wing was a crippling limitation on the aircraft’s performance. “Not so,” says Sir Archibald Russell. In a subsonic aircraft the wing thickness becomes a serious matter only when the airspeed over the top surface approaches the speed of sound. ” He adds: “and thick wings are lighter and stronger. ”

Fashions in aeroplanes change as technology marches forward, but the eternal verifies that designers struggle with remain constant. Longer range means more fuel, less useful load; higher speed means more power, bigger engines, more fuel, more weight. . . compromise, compromise. But these criteria could be expressed stating the relationship between fuel weight and range as 10% of the aircraft’s gross weight for fuel for the first 1. 000 miles, 9% for the second 1,000 miles, 8% for the third, 7% for the fourth and 6% for the fifth. Thus a 5,500-mile still-air range meant a fuel load of over 40% of the take-off weight. In the case of the Brabazon 1 that left 2% for revenue-producing load.

It was on these bedrock facts of life that the project foundered.

The power installation was another matter. I have said that the Bristol Centaurus was a splendid engine; it was not only powerful, with a take-off rating of 2,700bhp, but it was also light at 3,300lb and was magnificently reliable. There was nothing wrong at all with the Brabazon’s engines. But the difficulties associated with transmitting the power to the propellers were formidable.

The layout of the power installation within the leading edge of the wing had the engines grouped in pairs with their shafts at an angle of 32° to the line of flight. The ordinary reduction gears were dispensed with and the crank shafts were instead flexibly coupled to transmission shafts with Hooke joints to the input shafts of massive bevel pinions in the unifying gearbox.

No experience existed in generating bevel gears of this size to the tolerances required, but Bristol engineers set about solving the problems and the gears were successfully made by David Brown. And, moreover, they exceeded the designed transmission efficiency of 99. 3%, inordinately high though that was. The heat generated in the meshing zones called for an oil flow of 300gal/min; only a tenth of this was needed for lubrication.

The bevel wheels transmitted drive to the co-axial airscrew shafts, the pressure and return oilways for the hydraulically actuated propellers being carried separately within the inner shafts. The whole system was designed so that, if an emergency required, any engine could be shut down and its airscrew feathered.

If the transmission itself was trouble-free, the mounting structure for the propellers was not. This took the form of a truncated cone supported from the wing structure and in which the gearbox was housed with the propeller shaft bearings in the front end wall of the cone. The “mechanicals” of this composition operated perfectly well, but the mounting structure was subjected to a complex of vibrations that were not well understood and it suffered from fatigue cracking.

It is worth remembering that the average operational life of military aircraft during World War II was around 35 hours, so aircraft manufacturers’ experience with fatigue effects was profoundly limited – as the losses of the first Comets later made tragically evident.

The second prototype Brabazon was to have been powered by eight Bristol Proteus turboprop engines. Already the inexorable march of events was overtaking earlier visions. The advent of the turboprop made a cruising speed of 330mph desirable, but such a speed would require a wholly new wing design both in its form and its structure because of the exigencies of gust loadings. Such an undertaking was increasingly seen to be impractical, not least because a much smaller aircraft-planned in the Britannia – could do the Brabazon’s job better and more economically in every respect.

By 1952 the economic validity of the whole project was being seriously questioned and in March of that year the government announced that work on the second prototype was being postponed. This was the beginning of the end of the Brabazon 1 programme and virtually no further work was done.

In October 1953, having flown fewer than 400 hours, the first prototype was broken up together with the uncompleted second prototype. It was an ignominious end to a splendid, if misguided, project. There was a lot of comment at the time about the “enormous” costs, but seen in today’s perspective those costs (even at a 10:1 ratio of equivalence) look to have been extraordinarily light. Roughly half the total (£3. 4 million) was taken by the assembly hall and runway – both of which have ever since been in constant use- and the other half was absorbed by the aircraft, engines and special development costs. It was inevitable that the decision to scrap the project should label the Brabazon as a massive failure and that, indeed, has been its melancholy epitaph. A more objective assessment would recognise that the scale and nature of the project posed a myriad problems the solv. ing of which carried forward aeronautical engineering by a quantum leap comparable only with that posed by supersonic flight.