Flight September 9, 1928
THE  ALBATROS L.73
A German Biplane with Two 240 h.p. B.M.W. IV Engines
With the raising of the restrictions hitherto placed by the Allies on German aircraft construction and operation, there is considerable activity of late in the German aircraft world, and several new types of commercial German machines, apart from those which took part in the recent seaplane competition at Warnemünde, have been produced, while others are rapidly coming along. A German designer with his hands tied is one thing. A German designer at liberty to do as he likes is quite another, and there is every probability that during the next few years the German Aircraft industry will make a very determined effort to secure a leading place in commercial aviation. Germany is favourably situated geographically for developing civil aviation, and it is no secret that, while they were prevented by the restrictions from doing very much actual construction, German designers and aeronautical scientists were working hard in the drawing offices and laboratories against the time when Germany should once more be able to take its place among the nations of Europe.
The results of the work thus quietly but no less seriously carried out should be felt during the next few years, and it therefore Incomes a matter of no little interest to follow as closely as may be the steps by which Germany is building up her fleet of commercial aeroplanes.
Some months ago we described and illustrated the single- engined biplane with slotted wings which was produced by the Albatroswerke of Berlin-Johannisthal   as a newspaper carrier for the Berliner Zeitung. That machine was designed m the main by Dr. Ing. C.ustav Lachmann, who has now severed lus connection with the Albatros Works and taken up a position in Japan. Herr Schubert, who has been chief designer of the Albatros firm for a number of years, and who Herr Robert Thelen, one of the earliest German pilots, the Lympne light 'plane meeting in 1924, has recently turned his attention to twin-engined commercial aeroplanes, and the first of these to be produced is the Albatros L.73 which forms the subject of the following notes and illustrations.
Requirements
In designing the Albatros L.73 the following requirements were aimed at : the machine should have a useful load of 1,586 kg. (3,500 lb.) for a total engine power of 480 h.p.. and should be able to carry this load with the greatest possible safety   and   reliability.    Consequently   the twin-engined arrangement was chosen, with the further proviso that the machine should be able to fly level and to mantruvre with onlv one engine running. At the same time it was desirable that the take-off should not exceed 260 metres (850 ft.), and the machine should pull up in not more than 200 metres (660 ft.). The tank capacity should be sufficient for a radius of action of 560 km. (350 miles), and the cruising speed should be 140 km./h. (87 m.p.h.). (This is rather low according to British ideas of modern cruising speeds.-Ed.)
The Wings
Both upper and lower planes are built in three sections, the lower centre-section running through the fuselage, and the upper centre-section being carried on struts arranged in the form of a letter W, as seen in front elevation. Both are of the same span, chord and section, and the spars of upper and lower wings lie approximately vertically above each other, thus simplifying the strut and bracing attachments. The top centre-section encloses the wing-section petrol tanks. The outboard portions of the wings are located by N-struts towards the tips and by rhomboidal struts at the wing engines. The bracing is in the form of streamline wires, both lift and anti-lift wires being duplicated. The inner bay of the wings, i.e., from engine to fuselage, is braced by sloping struts run- ning to the top longerons of the fuselage. Ailerons are fitted to both top and bottom planes, and it is of interest to record that they are of the " slot " type, although the leading edge slot is not used on this machine.
Structurally the wings arc of interest because of their all- metal construction. The main wing spars are of built-up girder form, with U-section flanges and lattice webs. The ribs, on the other hand, are of very light gauge steel tubing, arranged in the form of lattices, and have openings in the bottom flange so as to enable them to be dropped over the spars. Thus in case of damage individual ribs can easily be replaced. Fittings at the joints are of high-tensile steel. The internal drag bracing is arranged at both top and bottom of the spars in order to improve the resistance of the wings to torsion (the spars are approximately 8 in. deep). The covering is doped fabric. In connection with the wing spars it is of interest to mention that these are hinged or pin- jointed at points some 2 ft. 6 in., out from the engine struts. It is claimed that with this arrangement the bending moments in the outboard portions of the spars are reduced by something like 3D per cent.
Fuselage
The fuselage is of rectangular section, built up of welded steel tube, the bracing in the rear portion being in the form of piano wire looped over quadrants as in the Fokker machines, while in the front portion of the fuselage diagonal struts are used.   These do not, as is usually the case, run to opposite corners of the various panels, but from the bottom rear corner to a point two-thirds of the way up on the next strut in front, another and slightly shorter diagonal strut running from the top rear corner to the same point on the strut in front. The whole looks something like a series of letters thus  "KKK". The reason for this peculiar form of fuselage bracing is not obvious.
The cabin, which has seating accommodation for eight passengers, occupies, of course, the full height of the fuselage, but the nose of the fuselage is dropped somewhat, forming a slightly curved deck in front of the cockpit, which is in continuation of the cabin and is provided with front windows, in the form of wind-screens. An opening on each side allows the pilot and engineer to lean over the side so as to be able to look straight down. The cockpit is entered by a door in the front wall of the cabin, while the extreme nose of the fuselage forms the main luggage compartment, another and smaller space for luggage being situated aft of the cabin. One of the accompanying photographs shows the view inside the cabin, looking aft. The seats, which have unusually tall backs, can be tipped over to form berths, in which case there is sleeping accommodation for four passengers.
The Tail
The tail surfaces are of steel-tube construction, welding being employed for joining the rib flanges, diagonals, etc., to the spars. The tail plane is of the trimming type, and the elevator is divided to accommodate the movement of the rudder. The latter is of the horn-balanced type, and there is no fixed vertical fin.
Undercarriage
The- undercarriage is of the two-wheel tvpe, with divided axle and vees under the engines, the front legs of the Vees being the telescopic members. The shock is absorbed partly by rubber blocks in compression, arranged in the form of two columns, one in front of and one behind the under-carriage leg, and partly by an oleo damper gear in the form of a piston with a small leak-hole. The travel of the wheels is about 8 in.
Power Plant
As already mentioned, the Albatros L.73 is a twin-engined machine, the engines fitted as standard being 240 h.p. B.M.Ws of the type IV, similar to those used so extensively in the recent seaplane competition. These engines are six-cylinder verticals, and are mounted on the inner interplane struts, which are of somewhat complicated arrangement designed for perfect triangulation. The radiators are mounted between the leading edge of the top plane and the engines.
The petrol tanks are, as already indicated, housed in the top centre section, each having a capacity of 360 litres (79-25 galls.), and direct gravity feed is provided, each tank supplying its own engine.

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