THE RUMPLER TWIN-HULL FLYING BOAT
Is The Giant Transocean 'Plane Coming?
By EDWIN P. A. HEINZE
FOR years past, German aeronautical circles have been discussing the plans of Dr. Rumpler for the construction of a giant all-metal flying boat, meant for transocean service. All details of the design have been completed in ten years of consistent work, and innumerable have been the model tests and calculations car; ed out simultaneously with the design work at the famous Aerodynamical
Institute of Gottingen, and at the Shipbuilding Institute at Hamburg. Dr. Rumpler has had the somewhat unusual satisfaction of leading German aeronautical experts of international repute giving excellent opinions on his new design after carefully studying it from all aspects. In spite of the decidedly good reception of Dr. Rumpler's ideas, it was not possible to find the necessary financial support in Germany for the construction of the 'plane.
Dr. Rumpler therefore turned to America for aid, and now appears likely to receive it there, for he has been invited to the United States for preliminary negotiations by a group of financiers. If these negotiations are successful, Dr Rumpler proposes to construct the aeroplane in Germany, where the building costs would be lower than in America, and the company which is to be founded for the construction is also to operate the 'plane between Europe and North America.
The design of Dr. Rumpler's giant 'plane is protected by far-reaching patents in all countries, its main characteristic being that the load and all heavy parts, such as the engines, are distributed evenly over the whole wing surface, so that each sector of the wing carries directly a portion of the entire weight. That is, the load is decentralised. It is a well-known fact that it is impossible to increase the size of 'planes of normal type without sacrificing a disproportionate amount of the carrying capacity. The reason is that the wing spars have to be made exceedingly strong to withstand the bending strains arising from a centralised load. This is of tremendous importance, and German experts have calculated that a 'plane of the same size as the one planned by Dr. Rumpler, but built along lines to-day considered normal, would weigh anything between 8 and 10 tons more. The machine is of the cantilever monoplane type, and the wing, resting on two hulls, contains the engines and the accommodation for the passengers and crew, whilst the petrol is located in the hulls. The adoption of two of the latter secures great lateral stability on a rough sea. and with a side-wind blowing.
The wing is built up of steel tubes braced by flat aluminium rods, and constitutes a single rigid structure, which is covered with a skin of duralumin. The total span amounts to 289 ft., with a chord of 41 ft. The tips, to a distance of approximately 58 ft. at each end, are slightly turned up and carry inside some auxiliary petrol tanks, which, however, are right away from the engines, as also from the passengers'..and crew's quarters. The middle section of the wing is perfectly level, and has a length of 173 ft. The plan shape is rectangular, whilst the leading edges of the wing tip sections retreat. The wing  tips are rounded off and merge into the ailerons, the trailing edges of which are level with the rear edge of the central wing section. The ailerons are as long as the upturned part of the wing ends. The maximum height of the central section is 8£ ft. This tapers towards the tips from the upward bend onwards.
The tubular framework of the wing is so arranged that there are two especially strong girder spars passing down the entire span of the wing. The rear one lies 21 ft. back from the leading edge, and between the two are mounted 10 engines, with an output each of 1,000 h.p. These engines are distributed evenly over the 173-ft. length of the central wing section, and are connected by means of long shafts with 10 four-bladed propellers arranged along the trailing edge.
In front of the engines is a gangway 3 ft. broad and having 7-ft. headroom. On each side of this gangway, extending over the whole span of the wing, is a partition with noise damping lining, and located in front of the forward partition are the passenger cabins. These are commodious, and contain six seats each, the measurements coinciding with the German railway standards for first-class accommodation. The crew's quarters, kitchen, etc., are located left and right of the engine room, while the navigation cabin projects forward from the centre of the leading edge. Adjoining behind are the chart and wireless rooms, as also the officers' quarters. Altogether, the wing  holds accommodation for 135 passengers and 35 officers and men. besides ample room for cargo, mail and luggage.
The two hulls each have a length of 160 ft., of which 9J ft. are submerged when the 'plane is in the water. The hulls are attached to the wing by means of streamlined turrets with doors leading out on the hull decks and having stairs inside leading up to the wing.
The shape of the hulls has been carefully worked out with the aid of innumerable towing tests of models at the Shipbuilding Institute, of Hamburg. The bottom transverse section of the hulls in their forepart has a flat V-form and gradually develops into two successive steps towards the rear, where, coming out of the water, the hull sweeps up assuming an oval section, which constantly grows flatter and at the end forms a large vertical fin carrying the rudder.
The keels of the two hulls are 59 ft. apart, and their tails are joined by a large stabilising surface extending left and right of the rudder fins and carrying on the rear edge three elevator surfaces. Like the wing, the hulls are constructed of steel and duralumin. As already mentioned, the hulls carry the main petrol tanks, which are arranged so that they can be inspected, gangways for this purpose being provided. The hulls also carry such equipment and accessories as anchors, ropes, etc.
The whole 'plane fully laden ready for flight is calculated to weigh 250,000 lb., of which 143,000 lb. are useful and 44,0001b. paying load. Besides 170 persons, it will be able to transport 13,000 lb. of mail or cargo. It is hoped to attain a cruising speed of 185 m.p.h. with it, which appears very probable in view of the fact that air resistance is reduced to a very great extent by the engines being located inside the wing and no struts and wheels being exposed to the air current. The take-off speed, it is estimated, while the ship will come down on the water with a speed of approximately 55 to 56 m.p.h. It is intended to fly the 'plane normallv at an altitude between 8,000 and ' 13,000 ft. The peak will probably be somewhere around 16,500 ft. The total lifting area of the wing is 10,764 sq. ft., so that fully-laden the wing loading would amount to 23 lb./sq. ft.
The 'plane will be able to fly a non-stop distance of 3,700 miles, the fuel consumption being assumed as that now normal in standard aero engines. Dr. Rumpler, by the way, hopes ultimately to be able to use Diesel engines, as these work more economically, while at the same time the heavy fuel means reduced fire risks. Experience with propellershaft transmission for such high-power outputs is lacking in aeroplane work. The Junkers G 38 has such a type of transmission.
But the shafts are very short and the engine output is much lower. The propeller-shafts of Dr. Rumpler's ship will have to be about 20 ft. long and this appears the only real difficulty in the whole design. Dr. Rumpler has designed a special transmission, incorporating large diameter hollow shafts running in self-aligning ball-bearings, and having flexible couplings at each end. Their weight is of no very great account when it is considered that engine trestles above the wing or nacelles below it fall away in this design so that their weight is saved.
The 10,000-h.p. output of the engines comprises a big reserve, for the 'plane is capable of flying with less than 7,000 h.p., even when fully laden, at an altitude of 13,000 ft. At the beginning of the flight the power reserve amounts to 31 per cent., so that the 'plane could continue flying even if three engines fail. That is at 13,000 ft., and when going down to 3,000 ft., the power reserve would amount to as much as 40 per cent. As the machine becomes lighter the longer it flies, owing to the fuel consumption, the degree of safety constantly increases and reaches 60 per cent, towards the end of the flight. Dr. Rumpler has made up an interesting chart, demonstrating the behaviour of the 'plane in the event of a number of engines failing in succession. Supposing the 'plane is flying along'at 13,000 ft., and no fewer than four engines stop work before the first 300 miles have been traversed- surely, very pessimistic !-the machine will then sink down to 8,000 ft., and again gradually climb up to 11,500 ft. before a distance of 620 miles is reached. If now a fifth engine were to fail, the 'plane would sink to 5,000 ft. and again rise to 11,500 ft. by the time it has traversed 1,500 miles, and now even a sixth engine could stop without impairing the safety of the 'plane or necessitating its going down on the water. After once more sinking to 5,000 ft., it could again climb to 8,000 ft., working with only the remaining four engines. Such a large number of engine failures are scarcely ever to be expected since it is possible to repair the engines, which are very well accessible on all sides inside the wing, during flight. Thus, the matter of safety appears very well cared for.

SOME COMMENTS ON THE RUMPLER
THIS is a day of large aircraft, and with negotiations in progress here, there and everywhere, for transoceanic air mail services, the problem of the really large and really seaworthy marine aircraft becomes more and more urgent. The Do.X has been produced and flown, but, although an extremely interesting experiment in size, that machine, as it stands, is not likely to represent the solution. We have a good deal more faith in certain large machines (although of modest weights as compared with the Do.X) now being built by such British firms as Blackburns, Shorts, and Supermarines. Great Britain has established a distinct lead in the matter of seaworthy marine aircraft, and while it would be unwise to rest content with what we have already achieved, we should not be stampeded into attempting to run before we have learnt to walk. The British flying-boat policy has been one of steady development, each step being no greater than justified by past experience. FLIGHT has consistently refused to be dazzled by mere size, and has done its best to plead for a true appreciation of the real problems involved. After all is said and done, the criterion of an aircraft is not how large a machine we can get into the air, but what is the largest size to which we can go (in our search for seaworthiness) before the useful load shrinks to a point where the resulting machine is of no practical value as a commercial transport vehicle.
In publishing our German correspondent's article on the proposed Rumpler twin-hull flying boat, we have had in mind to let FLIGHT readers know what others are doing or proposing to do. It must not necessarily be taken for granted that we believe in the success of the Rumpler scheme. In fact, we have very serious doubts on a number of points. But as it seems likely that Dr. Rumpler will find in the United States the necessary capital to realise his ambitious project, it would be well if we in this country kept in mind that Great Britain will, in the very near future, have to face serious competition in the flying-boat field. We have gained a good lead on our competitors. It is up to us to see that the advantage is not lost. Turning to the figures quoted by Mr. Heinze for the Rumpler twin-hull flying-boat, one should bear in mind that Dr. Rumpler is one of the pioneers of German aviation, and that he would not lend his name to any project of the feasibility of which he was not personally convinced. Moreover, our correspondent points out that he has been permitted to examine expressions of opinion by a number of German scientists, Profs. Prandtl and von Karman among them, who pronounce in favour of the Rumpler design. Thus, it is with considerable diffidence that we offer the following comments on Dr. Rumpler's proposed giant.
The magnitude of the Rumpler flying-boat is little short of staggering. For example, the wing span is given as 289 ft., or very nearly twice the span of the Dornier Do.X, and the estimated gross weight is 250,000 lb., or 2J times the greatest gross weight at which the Do.X has taken the air. The wing area is 10,764 sq. ft., as compared with 5,225 sq. ft. of the Do.X.
The criterion of an aircraft's usefulness is the amount of disposable load it will carry, taking into consideration the power of its engines. Our German correspondent states that the " useful load " is 143,000 lb. This we take to mean disposable load, as he gives 44,000 lb. as pay load. Assuming this to be correct, the tare weight of the machine is evidently 250,000 lb.^143,000 lb. or 107,000 lb. This would give a ratio of gross to tare weight of 2-338, a figure little short of staggering when we remember that an average ratio for British machines of all classes is about 1 -6. Dr. Rumpler has done his best to " cheat " the cube law by spreading his weights out over the wing, and that by doing so he has saved a very considerable amount of structure weight is not to be doubted. But that he should be able to attain such a ratio of gross weight to tare weight seems incredible. By spreading his engines along the wing, and placing the passengers inside the wing, Dr. Rumpler has doubtless been able to reduce his flying stresses in the wing, but in spite of the twin-hull arrangement the landing stresses must still be considerable. That they are smaller than would be the case in a single-hull machine may be granted. If one examines the figures for wing loading and power loading it is found that both are high, very much higher than any British designer would be prepared to go to. The wing loading is 23-2 lb./sq. ft. and the power loading 25 Ib./h.p., assuming that Dr. Rumpler can get engines of 1,000 h.p. each. While it is likely that the very size of the machine may help to make such loadings less intolerable than they would be in a smaller machine, one cannot regard them as other than prohibitive. If, on the other hand, the designer were content with either a smaller pay load or a shorter range, or both, the loading figures might come down to reasonable proportions. It is likely that the machine, if it is ever built, will be intended for trans-atlantic flying, and in that case considerable ranges will be required. The large size and twin-hull arrangement should make for seaworthiness, but it should not be forgotten that the use of two hulls will cause wracking stresses to be set up in the wing when the machine is running diagonally across the waves. There is considerable wing depth in Dr. Rumpler's machine, to be sure, but the dimensions of the hulls are correspondingly great.
Apart from these considerations of what may be regarded as the fundamentals of the Rumpler design, it is not difficult to find other somewhat problematical features. For example, the very long propeller shafts transmitting each 1,000 h.p. to airscrews placed on the trailing edge of the wing is a somewhat speculative undertaking. Resonance troubles have been encountered before now in much less ambitious lay-outs, and certainly a good deal of experimentation will be necessary before such shafts can be safely assumed to be feasible.
The distribution of the weights across the wing will doubtless reduce wing stresses and structure weights, as calculated by Dr. Rumpler, but they will also, obviously, increase the lateral moment of inertia of the machine, and the question of controllability is not one that can be ignored.
Altogether, it is difficult not to feel a certain amount of misgivings concerning the new giant Rumpler machine. But as we said at the beginning of these comments, we are informed that the subject has been extremely carefully studied by some of the best brains in Germany, and such criticisms as we have offered are presented with all diffidence. They merely express the reaction of minds accustomed to thinking in terms of British sizes and loadings.

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