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The Rohrbach flying boat Type Ro. III.
Beyond the scope of the previous publication, a detailed description of the Rohrbach flying boat Type Ro. III.
In his lecture given in 1922 on the occasion of the W. G. L. conference in Bremen, Dr.-Ing. Rohrbach proved that the
development of the large economic aircraft led to an increase in wing loading with increasing aircraft size. The smaller wing area
caused by the higher wing loading also results in a reduction of all other dimensions of the airframe. Accordingly, the weight of the
airframe increases much more slowly with increasing aircraft size than with the same wing loading, and the proportion of the paying payload reaches higher values.
How far the possible payload can be increased by this new approach cannot yet be stated with certainty, since
the power system and the design of the airframe have not yet been developed to the limit of the possible weight savings.
Judging by what has been achieved so far, the limit is about an aircraft full weight of 16 tons, while with the same wing loading under the same design conditions
, the limit is already reached at about 9 tons full weight.
In addition to the advantage of higher wing loading just discussed, there are others that come into play in their favour. These include: greater manoeuvrability,
easier controllability and greater insensitivity in flight to disturbance caused by weather influences. With regard to manoeuvrability, Dr. Rohrbach stated in the above-mentioned
lecture that in the case of large aircraft, the necessary turning time for a full circle increases strongly with the size of the aircraft with low wing loading,
whereas it increases only slowly with an increase in wing loading. The easier controllability was readily apparent from the smaller flight dimensions.
The disadvantage of the higher wing loading, the higher landing speed, which is to be accepted, is above all a question of airfield, while the difficulty of landing as a
direct consequence of the higher speed can probably be mastered by training the pilot. In the case of a large flying boat, which as an ocean-going aircraft always has large landing sites available , the disadvantage of the high landing speed is therefore not significant.
In pursuit of these guiding principles, Rohrbach Metall-Flugzeugbau G. m. b. H., Berlin, has built the flying boat Ro. III. The successes of this type have fully met expectations
The dimensions, power and weight data are as follows:
10 seater sedan
2 engines Rolls-Royce "Eagle IX" 2X360 hp.Total
length 17.2 m, wingspan 29 m, wing 73.4 m2,
empty weight 3 600 kg payload 2 400 kg
fuel
for 2 flight hours 300 kg
total. Weight 6300 kg
Wing load 85.8 kg/m2
Power load 8.75 kg/hp
Speed on the ground 200 km p. h Climbing ability 1500 m/13 min.
Maximum altitude 4000 m
When looking at the flying boat, the first thing that strikes you is the peculiar position of the two engines. The arrangement was based on good maneuverability on the water
and good flying with an engine. The two Rolls-Royce "Eagle IX" engines are therefore, as can be seen from the illustrations, mounted on trestles made of
tubular steel profiles next to each other on the wing in order to have the propellers sufficiently high above the water. The arrangement of the engines next to each other has the advantage for manoeuvring on the water, that by accelerating on an engine, a sufficiently strong torque can be generated for a quick turn, whereas with the
arrangement of the engines one after the other, the flying boat would first have to pick up greater speed for fast turning. As a result, more water is taken over in heavy seas, and maneuvering in busy harbors and waterways becomes more difficult. As the flight tests have shown, despite the side-by-side arrangement of the engines, it is easy to keep course and turn against the running engine in the event of an engine failure thanks to the purposeful design of the vertical stabilizer.
Another feature of the Rohrbach design is the narrow hull. The small width of the boat means a very considerable saving in the weight of the ground bracing, especially in an aircraft with a high wing load, where he ground stresses due to water pressure are quite unusually high due to the high interception speed. With a wide boat, the weight of all parts to be measured with regard to water stress would have been so great that most of the weight saved by the high wing loading on the wing would have been lost again.
The construction of the boat is very simple. The outer skin is stiffened by internally riveted profiles in such a way that it can transmit all
stresses together with the profiles. Furthermore, full buoyancy is guaranteed by bulkhead partitions, even if two compartments are leaking at the same time.
The lateral stability required by the oceanic capability is achieved by two side floats arranged next to the hull. Through these side-floats, the advantages of the
flying boat, as they were previously known, are to a certain extent combined with those of the normal two-float flying train.
The floats are also divided into watertight sections. This subdivision was provided to make a start safe even with a damaged swimmer.
The floats are supported by two tubular steel struts laterally towards the flying boat and four further struts upwards to the wing.
The wing arrangement has the unusually large V-position of 6 degrees. This large V-position initially appeared to be a certain risk for such a large and new metal machine, since all newer machines had no or only very small V-position. On the one hand, however, the V-position is very desirable with regard to seaworthiness
, as it brings the wing tips high above water, and on the other hand, it leads to a significant improvement in flight characteristics. In fact, the
aircraft showed excellent flight characteristics during the test flights. manoeuvrability and controllability, e.g., it can be done very easily by the operation of the rudder alone, while
releasing the rest of the rudder into the curve and likewise it can be brought back into straight flight simply by operating the rudder.
The wing, built entirely of duralumin, consists of three main parts: the hollow box girder, which absorbs all loads, and the two extremely light nose and end pieces attached to it at the front and rear only for shaping. These nose and end pieces are usually stiffened by ribs and covered with thin sheet metal. The hollow box girder is formed by two longitudinal webs and upper and lower cutaneous plates connected to them by longitudinal angles. Transverse walls riveted to the upper and lower skins, as well as to the bars, ensure the correct cross-sectional shape of the entire box girder. The longitudinal webs are left out in such a way that only the necessary diagonals and posts are available. The skin plates are protected from local kinking by riveted profiles. Since, in addition, all material cross-sections of this hollow girder are very far outwards, and all parts form a firmer whole than in other Holrn designs, it should be clear that the wing will be so light that it can be built completely self-supporting with an ^aspect ratio of 1:10, with a strength such as is required for loop flight and rolling.
With the steep nosedives and curves that are associated with the tubular bath; flying boat, the wing did not show the slightest tendency to flutter or even to
tremble in parts.
Furthermore, as the wing is made entirely of duralumin, and only the main fittings used to assemble the whole aircraft are made of steel, it
is less susceptible to corrosion than any other type of wing. For complete protection against corrosion, all parts are well painted inside and outside with paint.
The hinged nose and end boxes (see Fig. 2) allow all parts of the wing to be easily inspected during operation and, if necessary, a new
coat of protective paint. The nose and end boxes are removed by loosening a few screws that are exposed on the outside. The advantage of the detachability of the
nose and end boxes is further increased by the fact that they are divided into sections that are the same and independent of each other. In the event of damage to
nose and end boxes, they can therefore be easily replaced by parts held in stock.
A special recovery facility is provided for bringing the aircraft to the water and to land. Recovery vehicles are attached to the wing on both sides of the boatTThe attachment and removal of the recovery vehicles can easily be done by two unskilled workers in 4 minutes. The aircraft rolls onto
these recovery vehicles from the hall on land into the water under its own power and vice versa back to land.
Furthermore, it should be mentioned that the Rohrbach flying boat can also be equipped with an already tried and tested sail (cf. Fig. 3). The operational safety, which is actually increased by the use of two engines, is further increased by this sailing equipment, as the flying boat is able to continue on its way as a sailing ship if both
engines fai