Flying-boats and their future
There have been three main lines of thought in the design of flying-boats since the demise of the biplane; the Empire Boat style, the parasol, and the short hull. Of these the first named is the only one to survive. It is, perhaps, therefore best to deal briefly with the other two before following the principal line of development.
One of the most widely used and successful flying-boats of the Second World War was the Consolidated Catalina. It was a parasol monoplane with the wing carried high above the hull on a streamlined neck and braced on either side by a pair of struts. This layout was the final and logical end of a process of American flying-boat evolution that had continued since the end of the First World War. In the days of the biplane the lower wing was gradually reduced in span to make a sesquiplane , then it was replaced by an open framework of struts carrying the stabilizing floats, leaving only the upper wing. Finally the engine nacelles were lifted from their mid position to the wing and the array of struts eliminated. This process took place in a succession of military designs, notably by Hall and by Consolidated and in the civil Sikorsky Clippers.
The principal advantage of the Catalina layout was a low structure weight, since the minimum-size hull was joined to the wing by much less metal than if the hull had been the full depth from wing to keel. The drag of the superstructure, due to interference, was probably higher than that of a clean hull and, to compensate for this the weight of retractable wingtip floats was accepted. The graceful lines of the Catalina's hull were somewhat spoiled by the two large observation blisters— each inadequately armed with a hand-operated machine-gun. The relatively broad but shallow hull had bare space for the crew and its offensive load was carried externally under the wing.
Cramped, virtually defenceless and having a poor bomb load, the Catalina was useful because it had the very long still air range of 3,100 miles at the low speed of 130 m.p.h. This slow cruising speed gave the crew plenty of time to make observations and it also meant that the engines were at low power, making life quiet for the crew. This last is an important
point in patrols lasting twelve hours and more where noise can cause great fatigue. High placing of the engines also helped to make the hull quiet.
The low structure weight and general layout of the Cat allowed it to be adapted as an amphibian without a too drastic sacrifice in load. A hydraulically retracted tricycle gear was fitted, main wheels each side of the step and the nose wheel under the bow. The amphibian is a tempting proposition because of its adaptability, but its having to carry two complete sets of alighting gear, neither of which helps the other, always results in an uncompetitive payload.
The Dornier Do 18 and Do 24 reconnaissance flying-boats were on a similar formula, but using the characteristic stub wings for water stability. The Do 18 is of interest because it was one of the few aeroplanes in service with Diesel engines, two 600 h.p. Jumo 205s. These were six-cylinder, opposedpiston units, very high and narrow, which gave a distinctive shape to the tandem nacelle in the centre section. The Diesel engines were chosen for this boat because of their low fuel consumption.
Short hulled flying-boats have appeared from time to time since 1912. The idea behind them lay in a water-drag formula, which showed that length rather than beam led to high water resistance. It seemed natural to several designers that if, instead of atenuating the hull to carry the tail, they concentrated on making it an efficient boat they would gain overall. In such designs the tail had, of course, to be carried on booms, with all their attendant troubles.
Undoubtedly Sir Arthur Gouge's Short Empire Boat established the practical layout for large flying-boats and he was flattered by being copied by designers in every country. An examination of flying-boats built between 1937 and 1952 shows that the majority deviated remarkably little from the original. In some cases there is a gull wing to lift the engines a , little higher without deepening the hull, tail shapes vary considerably , and not a few have retractable floats, but the general shape, deep flat-sided hull and planing bottom, is fundamentally the same. Even Consolidated and Dornier, companies with a long flying-boat history of their own, turned to the new shape.
Boeing made a magnificent boat, the Type 314-A Clipper,' just before the War, as a rival to the new Short G Class transAtlantic boats being built for Imperial Airways. The original feature of the Clipper was the use of Dornier-style sponsons instead of floats, otherwise it was much like a large Empire Boat. The military version of the Empire Boat, the Short Sunderland , has been the mainstay of Coastal Command's flying-boat squadrons since just before the War. Every time a replacement specification has been evolved, it has been shelved—partly because of expense and partly because of the difficulty of improving on the Sunderland. For convoy protection, high speed is of little import, what is required is long endurance, a reasonable load of anti-submarine equipment and comfortable accommodation. The latter is very important, since most of the observation has to be done from open hatches, to ensure the best view, and is very tiring. A novel military feature of the Sunderland is that the "stores", bombs, depth-charges and sono-buoys are carried in the hull, the racks being run out under the wing on rails prior to an attack. This is a very useful feature, since changes can be made in the load to suit the target from the extra weapons carried in the hull.
So well, in fact, does the original Sunderland fit its purpose that when a new version, strengthened to carry a gross weight of 75,000 lb. and fitted with four 1,600 h.p. Hercules engines, was built it was never adopted. This boat, the Seaford, had the same basic structure, apart from the strengthening, but the planing bottom was increased in length and beam to support the extra weight and, as a result of water tank tests, the main step was faired. The extra power required a larger tail to cater for an engine failure at take-off. The higher power and slight aerodynamic cleaning made the Seaford fifty miles an hour faster than the Sunderland, despite its extra weight.
The Seaford was "civilized" and became the post-War Solent of B.O.A.C. and Tasman Empire Air Lines. Further cleaning, by removing the turrets, and the fitting of 2,000 h.p. Hercules engines raised the maximum speed of the Mark IV to 284 m.p.h. at 3,400 ft. and gave an economical cruising speed of 200 m.p.h.—remarkable figures for a boat.
When Imperial Airways introduced the Empire Boats on the Australian and African routes they overcame the lack of aerodrome facilities. They also gave the passengers a standard of air travel never before offered. After the War the scene was different, aerodromes with runways were available everywhere and the modern four-engined air liner was commodious and comfortable. Furthermore, it was pressurized, which allowed it to fly high, where reduced air resistance gave a fast cruising speed. On the other hand, the spacious hull of the flying-boat could not be pressurized and this meant limiting the operating ceiling to 10,000 ft.—and on long ranges this means a lot of fuel. Again, as sole users of flying-boats, B.O.A.C. had to carry all the overheads (moorings, launches, etc.) themselves, whereas rival landplane operators were only faced with landing fees and office rentals. Those are the arguments that led to the disappearance of the flying-boat from the Empire routes, apart from the T.E.A.L. service between Australia and New Zealand. One other operator, Aquila Airways Ltd., flies the Short boats to and from Madeira by way of Lisbon.
At the end of the War Sir Arthur Gouge left Short Brothers and went to Saunders-Roe, Britain's second flying-boat firm, and this led to the Princess project, jointly with the Ministry of Supply and British South American Airways. To explain the many vicissitudes of the Princess—political, technical and even strategic—would take a whole chapter and at the time of writing eight years after the conception of the project the answer is still unknown.
In the view of its designer, Henry Knowler, and many others, the Saro Princess represents the practical amalgamation of the virtues of the flying-bcat and the pressurized air liner. The hull has two genuine decks, running almost the whole length, and is made on the figure eight principle to ease pressurization stresses. The planing bottom is built on to the lower pressure cylinder and is of very refined shape, with a faired, vee-shaped step. The cockpit is completely submerged in the nose and the only excrescence is a radome for the cloud-andcollision warning radar scanner. The wing has the fairly high aspect ratio of 9-5 and is intended to give low cruising drag. Because of the low c.g. no dihedral is necessary since there is ample pendulum stability. The tail is of typical Gouge shape, with a dihedral tail plane to add effective fin area to the already tall fin—the dihedral also keeps the tail plane clear of the jets. The floats retract outward on to the wingtips.
The Princess has ten engines completely buried in the wing so that they cause practically no drag. The four inner engines on each side are coupled in pairs to counter-rotating airscrews, while the outer engines each drive a single airscrew. The engines are Bristol Proteus propeller-turbines. In the prototype they are the Mark 2, a de-rated engine giving 3,200 h.p. plus 800 lb. jet thrust. If, however, the three boats go into service they will have the Mark 3, a lighter engine with a maximum output of 3,320 h.p. plus 1,200 lb. thrust, and, what is most important, a higher cruising output. The designed cruising
speed of the Princess is 380 m.p.h. and under trooping conditions it could carry 200 men from, say, England to the Canal Zone without a stop and return the same day. The U.S. Navy operates an efficient trans-Pacific service with their four Martin Mars boats, smaller and of inferior performance.
An entirely different line in flying-boat development is presented by the small amphibian. For this, too, the Empire Boat has set the pattern. The long series of successful Grumman amphibians, mainly used by the U.S. Navy and Coast Guard, look like large models of the Short designs, although they are quite independent studies. Shorts have made their own contribution in this field with the Sealand.
The small amphibian has an appeal for bush operators, where its versatility overcomes its drawbacks. In the small sizes the cabin has to be close up under the wing where the engine noise is greatest and retraction of the undercarriage means a large piece out of the cabin on each side. These inconveniences , coupled with a low payload, make amphibians unattractive to most air line operators, while the high initial cost puts them out of reach of all but the wealthiest private owners. A neat four/five-seater amphibian is the Piaggio P-136, but even this failed to sell commercially and was taken over by the Italian Air Force as a seaplane trainer. A feature of the P-136 is that pusher engines keep the noise well away from the cabin, and allows this to be ahead of the wing so that the wheels retract into the hull behind the cabin.
A special amphibian role, which arose out of the War, is that of air /sea rescue, since the aeroplane can be based at an aerodrome and is yet capable of alighting on the sea to pick up survivors.
The flying-boat has been almost the last class of aeroplane to be influenced by the turbo-jet engine. Saunders-Roe were the first to use the jets in the SR/A1 single-seater fighter flyingboat . This looked very much like a tiny Empire boat without any engines! Actually two axial units were housed in the hull with a common intake in the nose and outlets just aft of the wing. The necessary hull width somewhat restricted the pilot's downward view. The floats retracted under the wing. This project was dropped because no use could be found for it, rather than for any fault in the design.
The single-seater fighter jet flying-boat was taken up with energy in the U.S.A. by Convair. From a long dynamic model
test programme a revolutionary flying-boat configuration was evolved. Hull and wing were blended and the wing itself was used for buoyancy, so greatly reducing frontal area. The hull was given a very sharp vee bottom, what is called a "deadrise" cf 60 deg., to soften the entry on alighting. Model tests were excellent, but whether the raft-like wing would be practical in full scale is another matter.
This Convair design proved, in fact, to be a prelude to their Skate fighter design. This is a delta-wing jet aeroplane using fuselage and wing for floatation. Hydroplaning is achieved not by a vee bottom, but by vanes, called hydro-skis, that are extended below the fuselage for take-off and alighting. Hydrodynamic "lift" occurs at quite low speeds of about 15 knots. This idea is not new, it has been tried for fast surface craft, and it was actually tried full scale in the Piaggio P.9 Schneider Trophy racer of 1927. In this case difficulties with the clutch used to transfer power from the water screw to the airscrew as the aeroplane lifted prevented flight trials. Saro have also been making model tests with hydro-skis and have had promising results. If successful, this system promises to bridge the gap between the landplane and the flying-boat, now that there are jet engines to provide the means of propulsion. It is, however, too early as yet to say if this is a theoretical ideal or a practical method of flotation. It seems reasonable in the smaller sizes, but is a terrifying thought for big boats. Experiments by the Glenn L. Martin company in the U.S.A.
suggest a more immediately practical advance for large boats. Model tests gave this company the idea that very long narrow hulls might be the best of all for seaworthiness. Sure enough, further tests proved that after a certain value, the hull characteristics improved as the length was increased. In a fullscale experimental boat a bottom with a length to beam ratio of 15 to 1 has proved very satisfactory. Furthermore, a fully faired step and steep deadrise on the afterbody suggest considerably improved aerodynamic drag. This new hull criterion makes possible for the first time planing bottoms as slender as, or even less than, the aerodynamic requirements demand.
There are possibilities for the high-speed, long-range, jetpropelled boat. With a slender hull and swept wing, engines could be mounted on top of the hull clear of spray, the flyingboat could be as clean as a landplane and almost as light. Sheltered areas of water exist all over the world and provide runways of unlimited length—there are take-off waterways.
within short distances of almost all the world's great cities for their waterways were the basis of their rise to greatness. One day, airline operators may suddenly realize this and they will seize upon it to increase their wing loadings to an unheard-of figure! There may well be a commercial repetition of the Schneider Trophy story, with the sea once again coming to the rescue when the land could no longer provide landing space.