Flight, June 1929

OLYMPIA AERO SHOW 1929

The Dornier Models

   By a happy coincidence (or was it design?) the first test flight of the new giant Dornier flying boat Do. X preceded the opening of the Olympia Aero Show by a few days only. As far as can be gathered, the first flight was a complete success. This only means, of course, that the machine got off and handled at least well enough in the air to be landed again safely; but that is at least something. With twelve engines developing something like 500 h.p. each, or a total of some 6,000 h.p., it may be argued that the machine should get off. While that is perfectly true, questions of controllability, behaviour on the water, and a number of other problems must be considered, and the Do. X is such a bold conception that there were many possibilities of errors in calculations and estimates. That the machine has got off and has alighted safely again is a very good beginning. Now remains to test it out with gradually increasing load. Unless the pay load per horse-power is reasonably good, and the machine can get off with the full load which corresponds with a reasonable pay load, it would remain an interesting but not entirely successful experiment. We can only await further developments. Machine's are not unknown in this country, which, although of great size and very interesting as experiments in size, were a failure regarded as useful aeroplanes.
   In order to give our readers an idea of the actual Do. X, we publish on p. 727 a photograph taken recently in the Dornier works on Lake Constance. The people walking about in front of the machine give scale to the picture and make one realise the ambitious nature of Herr Dornier's latest design. Fundamentally, a monoplane not dissimilar to previous Dornier machines, the Do. X has the same type of wing stumps projecting from the hull and serving to give lateral stability on the water. The engine arrangement is, however, unusual. Six pairs of engines, placed in tandem, are spread across a large portion of the wing. This, of course, helps to spread the load and thus reduces the stresses in the wing, the only concentrated load on which is that of the hull and contents thereof. Joining the six engine nacelles are a series of wing sections which have the effect of forming a small monoplane wing above the main wing. What exactly will be the aerodynamic effect of this arrangement seems somewhat doubtful. It is likely that the introduction of this wing is based upon structural rather than aerodynamic considerations.
   The actual Do. X will undoubtedly carry a very large number of passengers. It would have to, in order to be of any practical use. How many we do not know at the moment. Likely this will depend upon the gross weight with which it is found possible to get off the water, and at present no figures are available concerning this. The only data which we have so far succeeded in obtaining relate to dimensions, the main ones of which are: Length, 40-05 m. (131-4 ft.); wing span, 48 m. (157-5 ft.); total wing area, 170 sq. m. (1,830 sq. ft.).

Flight, February 1930

THE DORNIER Do. X
First Authentic Data and Particulars

   RARELY in the history of flying has a machine so captured the imagination as has the large Dornier flying ship, the "Do. X." The size is beyond anything previously accomplished; the lines of the machine are unusual; and the power plant arrangement is novel. Ground enough surely, for being intrigued, and for wishing "to know all about it." FLIGHT has published illustrations of the Do. X on more than one occasion, and has commented editorially on its main features. Hitherto, however, the necessary authentic data have not been forthcoming, in the absence of which it has been a little difficult to form a true picture of what the Do. X is, and what it really means. We have now received from Dornier Metallbauten of Friedrichshafen on Lake Constance, a small booklet in which is set forth an account of the underlying ideas of the Dornier engineers in producing this machine, details given of the construction, and accurate data supplied concerning such items as dimensions and areas, weight, performance, etc. Thus we feel that we are better equipped to deal with what is the most interesting design of modern times, and that FLIGHT readers will not mind - in fact, will wish us to - if we return once more to the subject of the Do. X.
   Dr. Claudius Dornier, like Dr. Rohrbach, began his aeronautical career with the Zeppelin company, and during the early days, when Rohrbach and Baumann were designing the large four-engined Zeppelin Staaken monoplane which, as a result of gross stupidity, was later destroyed because of certain clauses in the Treaty of Versailles, Dr. Dornier began to occupy himself with the subject of seaplanes and flying-boats. He produced, first as chief designer at the Zeppelin Lindau works and later as head of his own company, a series of flying-boats, the earlier types of which those sufficiently interested may find described in FLIGHT of December 16 and 23, 1920.
   Quite early in his career Dr. Dornier adopted short wing stumps springing from the sides of the boat hull for obtaining lateral stability on the water. In one early example, the Do. Rs. II, these stumps were braced by struts from below, and they must have caused the machine to be very "dirty" on the water. In those early days, too, the tail surfaces were carried either on boom outriggers or on a fuselage placed high above the hull. In the Gs. I, produced in 1919, Herr Dornier for the first time carried his tail surfaces on the main hull, which was extended right aft instead of the rather short sort of "Bat boat" hull which he had previously favoured. In the Do. Gs. I the wing stumps had become pure cantilever members; and, furthermore, the main wing lift struts were anchored to the tips of the stumps. The power plant consisted of two water-cooled engines in tandem, placed above the monoplane wing. From this machine developed as a logical outcome the Dornier Gs. II, the Wal, and the Superwal, gradually increasing in size and power, but retaining all the fundamental features of the Gs. I. From that machine, therefore, may be said to date Dr. Dornier's real programme of flying-boat development, and the Gs. I is the ancestor of the long series of boats which has now culminated in the production of the Do. X. No other flying-boat designer in the world has had the opportunities of Dornier, who, already during the war, was not expected to turn out military aircraft which must be immediately successful, but who was given more or less a free hand to prepare for the future by evolving commercial flying-boats.
   We have dealt with this phase of Dr. Dornier's work at some length, because only if one knows something of his earlier history can one obtain the correct perspective for judging his latest work. We believe we are correct in stating that altogether Dr. Dornier has produced no less than 28 types (not all flying-boats).
   Before the actual work of designing the Do. X could be started, a large number of theoretical and scientific investigations had to be made because, although it was not desired to introduce any unknown features where it could be avoided, the very size of the projected machine called for much work in determining the most economical type of structural members, and so forth. This preliminary work was begun in 1924. In December, 1927, actual construction was commenced in the new works at Altenrhein. The first test flight was made on July 12, 1929.

Aerodynamic Design

   The Do. X is a semi-cantilever monoplane flying-boat, with the engines placed above the main wing, in six tandem pairs, and a small auxiliary wing joining them. A good deal of speculation concerning this wing has occupied those interested in the Do. X, and it has even been claimed that by fitting it Herr Dornier has obtained a "sort of slot effect" and greater lift. We could never, personally, see , any reason for this supposition, and the booklet sent us by the Dornier company makes it quite clear that the auxiliary wing is a structural member first and foremost, serving to steady the engine mountings laterally.
   The boat hull is of fairly normal Dornier design as regards its external shape. Minor differences are formed on the one hand by the control cabin, which projects above the main deck and may be expected slightly to increase the drag, and on the other hand by a rounding of the stumps into the hull, which may slightly reduce the drag. The control surfaces are fairly normal and are all balanced by secondary surfaces.
   As regards the aerodynamic efficiency of the Do. X, the aspect ratio (if one may be so old-fashioned as to use this expression) is low (about 5) and the span loading is very high. This is, of course, merely another way of saying that the induced drag is high. To that the designers of the Do. X would probably reply that this is not of serious consequence, as the machine has a small speed range and a high take-off speed.
   If one now turns to the - in this particular instance, perhaps, more important - question of minimum drag coefficient, it is of considerable interest to find that this is rather surprisingly low for a multi-engined machine. For example, at the normal gross weight of 46 metric tons (101,200 lbs.) the wing loading is 19-3 lbs./sq. ft. Assuming the engines to develop a normal output of 500 h.p. each, the total power is 6,000 b.h.p. and the power loading is 16-88 lbs./h.p. The "wing power" in that case is 12-33 h.p./sq. m. (1-15 h.p./sq. ft.). For a top speed of 130 m.p.h. this corresponds to a "high-speed figure" of 13, which would appear to be as high as that attained by many smaller machines, even single-engined types. Doubtless the tandem engine arrangement has resulted in the drag being quite considerably lower than it would have been had the engines been spread out.

Constructional Features

   The boat hull of the Do. X is, in its outward shape, very similar to that of previous Dornier machines. The main step is placed rather farther aft than in British flying-boat hulls, and is formed with fore-and-aft shallow steps which gradually merge into the forward vee of the bottom. The rear step does not, as in British practice, extend laterally out to the, chines but is of narrower beam than the main bottom, and is fairly deep. The Germans term this form of step a "displacement step" (Verdrangungssporn), presumably because it acts by displacement rather than by dynamic pressure. From the main step to the stern post the chine members are straight and swept up at a fairly pronounced angle so as to get the tail well clear of the water. Forward of the step the main bottom becomes, as already indicated, of pronounced V-form, to terminate finally in a straight raked stem.
   The total length of the boat hull is 40-05 m. (131 ft. 6 in.). The beam, over the stumps, is 10 m. (32 ft. 10 in.), and the maximum beam of the hull itself is 3-5 m. (11 ft. 6 in.). The greatest depth of hull is 6-4 m. (21 ft.), and the draught empty is 0 -8 m. (2 ft. 8 in.). At a gross weight of 50 metric tons the draught is 1 -05 m. (3 ft. 5 in.), and the metacentric height 4-58 m. (15 ft.). Inclusive of the stumps, the hull has a volume of 400 cu. m. (141,200 cu. ft.), and in this connection it is interesting to record that the hull weight has been reduced in the Do. X to 21 kg./cu. m. (l-3 1b./cu. ft.), whereas in the "Wal" it was 26-2 kg./cu. m. (1-625 lb./cu. ft.) and in the little "Libelle" it was as high as 29-9 kg./cu. m. (1-85 lb./cu. ft.). The maximum cross-sectional area of the hull, exclusive of the stumps, is 17-2 sq. m. (185 sq. ft.). There are in the hull 58 main frames, spaced 0-7 m. (2 ft. 4 in.) apart. An innovation in construction as far as Dr. Dornier is concerned is the introduction of a deep keel girder which runs from the bows to the rear step and has a length of 23 a.m.,(76 ft. 5 in.) and a greatest depth of 2-12 m (6 ft. 11 in.). This fore-and-aft girder stiffens the hull very considerably. Parallel with the keel girder, and spaced from it 0-9 m. and 1-58 m. (2 ft. 11 in. and 5 ft. 2 in.), are two keelsons on each side. These, with the transverse frames and keel girder, form a very strong structure and reduce the panels (themselves of heavy gauge) of the bottom to squares of about 0-63 sq. m.2 (6-8 sq. ft.).
   The large dimensions of the Do. X have allowed of an internal arrangement unlike those of previous flying-boats, and more resembling the lay-out on board a surface vessel. The main deck is located some 4 ft. above the load water line, and forms the floor of the main accommodation for passengers. Below this deck, the hull is divided by eight watertight bulkheads into nine compartments. The side stumps themselves (of a total volume of 43-5 cu. m. (1,550 cu. ft.)) are divided each into four watertight compartments, so that altogether the hull would have to sustain very conquerable damage before the machine is likely to sink, providing the bulkheads do not give way. Below the main deck, in the watertight compartments, the main petrol tanks are mounted, their number depending upon the length of route over which the machine is to be operated. The passengers' quarters are totally above the water line, and the construction is such that the subdivision of them is reduced to a minimum, the watertight compartments finishing at the main deck. Above the passengers' quarters are the crew's quarters and the various service compartments, such as pilots' cabin, chief engineer's control station, wireless room and so forth. The forward portion of this upper deck has windows along the sides and rounded front, and is in fact, a sort of enclosed "bridge" for the pilots and navigators. The other service compartments, engineer's control station, wireless room, auxiliary engine room, etc., are, however, inside the centre of the wing, and have consequently no windows on the sides. They extend aft as far as the trailing edge of the wing.
   The wing, which has a span of 157 ft. 6 in., and a chord of 31 ft. 2 in., differs in construction from previous Dornier types in that three main spars are employed, of which the middle is situated at approximately the greatest depth of the wing section. The front and rear spars are placed 9 ft. 2 in. from the middle spar. The spars are built up of angle sections and laminated flange strips, the number of laminations in the flanges being proportional to the stresses from point to point. Box ribs are placed at distances of from 2-8 to 3-6 m. (9 ft. 2 in. to 11 ft. 10 in.), and the metal panels of the wing covering are riveted to them and to the spar flanges. This metal covering extends outward to the outer engine nacelles only, the wings from there to the tips being covered with fabric. The maximum depth of wing section is 1-28 m. (4 ft. 2 1/2 in), and the result is that almost every part of the internal wing structure can be reached for inspection by a man crawling about inside. Attachment of the wing to the hull is by a number of large bolts, situated inside the covering and offering no extra drag.
   The small auxiliary wing serves to brace and steady the engine mountings, and it is worth noting that it is so designed as to take no part of the wing stresses. In fact, in order to avoid the possibility of throwing unexpected stresses on the main wing, the auxiliary wing is arranged with flexible joints between the outer engine nacelle and the next, so that should the main wing deflect under load, the auxiliary wing can "give" to any extra loads.
   The control surfaces are of fairly normal design, and are all provided with separate surfaces acting as balances. The tail is of "sesquiplane" type in that there is a small tail plane resting direct on the stern portion of the hull, and a main tail plane higher up, braced to the lower and to the hull by struts. The rudder balances take the form of vertical surfaces placed between the top and bottom tail planes, as distinct from the horn balance or servo rudder used on large British flying-boats. The operation of the control surfaces is by steel rods, or tubes, suspended on pendulum cranks. Ball-bearings are used throughout. Trimming of elevators and rudder is achieved by a setting of the separate balance surfaces, the angle of which in relation to the main surface which they balance being adjustable from the cockpit. This setting is reported to be very easy, i.e., to require but very small forces, and the machine is stated to be as easy on the controls as are smaller aircraft.
   The power plant consists of 12 Siemens-Jupiter engines, arranged in six tandem pairs. The large number of engines required forced, it is stated, this arrangement on the designers, and it is an arrangement the advantages and disadvantages of which are familiar to the Dornier designers from more than 10 years' experience. The Dornier engineers argue that the use of tandem engines almost reduces the number of separate units to one-half, in that each tandem unit of two 9-cylinder engines is very little more complicated than one 18-cylinder engine, and is much more reliable in service. The drag of a tandem installation is, it is claimed, no greater than that of one larger engine, and the propeller diameter, for same efficiency, is smaller. The Siemens-Jupiter engines of the Do. X are of the geared type, with 2 : 1 reduction gears. Great importance has been attached to accessibility of the engines, and by mounting the nacelles on streamline supports, all the engine nacelles can be reached from the interior of the wing. Inspection doors in the covering of the streamline supports give access to the interior of the nacelles when the machine is at rest.
   As already mentioned, the quantity of petrol carried will depend upon the route operated. Normally, there are four main tanks of 3,000 litres (660 gallons) capacity each, resting on the floor of the hull, and a further two tanks of 1,700 litres (374 gallons) each, also resting on the floor, but slightly farther forward (see sectional side elevation). There are, furthermore, two small tanks of 300 litres (66 gallons) each housed in the leading edge of the wings, a total petrol capacity (normal) of 16,000 litres (3,520 gallons). The main tanks and leading edge tanks are connected to a collector (sammeltopf), and the fuel is pumped from the main tanks to the leading edge tanks. In order to avoid any possible breakdown in the petrol system, no less than three separate pump systems are provided: a windmill-driven pump, an electric pump, and a hand pump. From the leading edge tanks, the petrol is pumped by A.M. pumps to the carburettors, surplus petrol draining back into the collector. Oil tanks of 100 litres (22 gallons) capacity are housed in the engine nacelles, and there is a main oil tank of 1,000 litres (220 gallons) in the "bilge."
   The engine controls of a multi-engined flying-boat like the Do. X presented something of a problem. Obviously, the pilot cannot himself attend to all the engines, their controls, etc. On the other hand, it is essential that the pilot should have full control of all the available power. With the arrangement selected, there is a main engine control room, reigned over by the chief engineer, and in this are concentrated the individual engine controls, engine instruments, etc. To avoid confusion, all the controls and instruments appertaining to the port engines are collected on the port side of the engine control room, and all those of the starboard engines on the starboard side. From this engine control room, two sets of engine controls are taken to the cockpit, or rather pilots' control room. Thus the pilot has but two engine controls, one of which operates the 6 port engines, and the other the 6 starboard. He also has two revolution indicators, of which one shows the mean revolutions of the six port engines, the other the mean revolutions of the six starboard engines. If one of the engines is disconnected from the pilot's engine controls, a red lamp lights up, on port or starboard side, to let the pilot know that he has not available the full power of all six engines on that side. Starting of the engines is by means of compressed air worked by an auxiliary engine in the main engine control room. The average time for starting all 12 engines is 4-5 mins. They have been started in 3 mins. It is reported that the engine installation is remarkably free from vibration.
   The arrangement of the passengers' accommodation will depend upon the length of route and number of passengers carried. Normally, comfortable accommodation cannot be provided for more than 100 passengers, and it is pointed out that when the machine is used for routes so short that the pay load exceeds 10,000 kgs. (a passenger is taken to weigh, with luggage, 100 kgs., i.e., 220 lbs.), the difference between the weight of the 100 passengers and the lift available for pay load will have to be made up of mails and/or freight. The various cabins, etc., available for passengers, measure, altogether, some 24 m. (78 ft. 9 in.) in length, and have an average width of about 3 m. (10 ft.).
   A chart has been prepared which indicates the distances that can be traversed by the Do. X without refuelling, and the number of passengers, or the weight of mails and goods which can be transported over these distances. A few representative distances have been drawn in to give a better picture of what the various distances mean in practice. The chart has been based upon a take-off gross weight of 45 tons (100,000 lbs.), and in estimating the pay load available for the various distances, ideal weather conditions have been assumed, i.e., still air. By carrying the usual 30 per cent, reserve of fuel, the take-off weight would be increased. Thus, it is stated that with a 30 per cent, petrol reserve for a distance of 1,800 kms. (1,120 miles), the take-off gross weight would be increased to 49-5 tons (109,000 lbs.).