Flight, September 1925

THE JUNKERS G.24.L
Latest German Three-Engined Passenger Aeroplane

   AN interesting visitor to the Croydon aerodrome during last week was a large three-engined Junkers monoplane; interesting partly because of the fact that the machine brought over close upon ?10,000,000 worth of bonds, and even more so because this is the first time that the latest type of Junkers passenger carrier has been seen at a British aerodrome.
   Photographs of the machine have previously been published in FLIGHT, but a considerable number of further particulars have now become available, which should be found of no inconsiderable interest.
   The Junkers monoplane, which has been built, under licence, by the A. B. Flygindustri, Limhamm, Sweden, arrived at Croydon about noon on Tuesday of last week, carrying, as already stated, Reparations Bonds to the value of ?9,660,000. Originally, it had been intended that the Junkers machine should arrive with its valuable cargo on Monday of last week, but bad weather delayed the arrival until Tuesday. The machine was piloted on this flight by Mr. Linner, the well-known Swedish pilot, and the following passengers were carried: Dr. H. H. Hagemann, who is manager of the Junkers Company, Herr Geheimrat, Hientzche, and Herr Sommerlad, officials of the German Debt Commission. These officials supervised the transfer of the sealed boxes containing the bonds from the aeroplane to motors conveying them to the Bank of England, and later the same officials attended at the breaking of the seals and the counting of the bonds. The fact that air transport should be regarded as the safest mode of conveyance for such valuable cargo is significant.

The G.24.L Aeroplane

   The three-engined Junkers commercial monoplane is produced in several types. The one permitted in Germany is known as the G.23.L, and is driven by a 195 h.p. Junkers L.2 engine, mounted in the nose of the fuselage, and two 100 h.p. Mercedes engines, mounted on the wings; the total power developed by this combination being the maximum permitted German aircraft. When built abroad, however, the machine may be fitted with more powerful engines, one combination being that found on the Swedish-built machine which visited Croydon, and which was equipped with three 230 h.p. Junkers engines of the L.2 type.
   An unusual and interesting feature of the design of the G.24 is that the machine, apart from changes in individual engine units, can be used either as a single-engined, twin-engined, or three-engined monoplane, the wings being so designed that when the two-wing engine units are removed, with their length of wing surfaces, the two end pieces of the wings can be moved inwards a corresponding amount, the wing surface being thus somewhat decreased and the machine turned into an ordinary single-engined low-wing monoplane. If desired a more powerful engine can then be fitted in the nose, and among those given as possible power plants for this purpose is the 450 h.p. Napier "Lion."
   Conversely, if the machine is preferred as a twin-engined type, the central engine unit can be removed, a streamline nose being fitted in its place, and the machine then becomes a normal twin-engined monoplane, again with a choice of engines within certain limits of power, and subject to structural considerations.
   It might have been thought that all these changes represented the maximum possible in the matter of adapting any given type of machine to a variety of purposes and conditions, but there is a further series of changes possible by which the machine is turned into a seaplane. This has actually been done, and, we understand, very successfully done, and two photographs published on another page show the machine converted in this way. Presumably, the same changes to twin and single engine could be made in the seaplane type, so that it would seem that in the G.23 and all its modifications. Professor Junkers has produced a type with almost endless possibilities.
   The following description should be taken to refer to the Swedish-built machine that visited Croydon, and which is now in use on the Amsterdam-Hamburg-Copenhagen-Malmo service of the Aero Transport Co., of Stockholm.
   In constructional design the Junkers G.24.L is similar to the smaller, single-engined Junkers machines which are already familiar to readers of FLIGHT. That is to say, all-metal construction is employed, even to the fuselage and wing covering, which is in the form of corrugated sheet Duralumin. A feature of the wing construction of all Junkers machines, and which has been retained in the G.24, is the use of tubular spars and the entire absence of wing ribs. The tubular wing spars are placed against the top and bottom wing surfaces at fairly close intervals, and are connected by lengths of corrugated strip, riveted to the sides of the tubes.
   It would appear to be impossible to calculate the strength of such a wing by ordinary methods, but we believe that the Junkers Company have, as a result of a great number of sand loading tests, succeeded in evolving empirical formulae, which enables the strength of the wing in bending and torsion to be calculated very accurately. The wing structure is, of course, to a great extent redundant, and the argument advanced against criticism of the wing design is, that a large number of the corrugated strips could probably be broken without materially decreasing the strength of the wing, as the loads would be taken up by the other members, much as in our better known biplane structures, the incidence bracing takes over the function of a front or rear lift wire, should one of the latter break.
   The wing engines are mounted on engine bearers built integral with the wing structure, the entire section of wing being built as a separate unit and removed with the engine, so that the effect of removing the two wing engines and replacing the two end sections is to shorten the wing span by an amount equal to the width of the two wing-engine sections. From the photographs it will be noticed that the wing engines are carefully cowled-in, with the fairing behind the engines merging gradually into the upper wing surface.
   In shape and construction, the fuselage of the G.24 is similar to that of the smaller, single-engined type, the curved cabin roof sweeping down towards the central engine, and the two pilots being placed between the cabin and the front engine. The view from these cockpits should be particularly good, and should not be restricted in any direction that matters greatly. Placed as they are, almost on a level with the pilots, the two wing engines can be constantly kept in view.
   In proportion to the wing span the distance from the centre line of the fuselage to the centre line of the wing engine is relatively small, and probably therefore, the turning moment produced when one wing engine is stopped, is not so great to interfere seriously with the directional control of the machine. It is stated, that the machine is able to fly level or even to climb slightly with any two engines running, either the two wing engines, or a wing engine and the central engine, and the risk of forced landings outside a suitable aerodrome should, therefore, be distinctly remote, the more so as, normally, the machine cruises at a speed corresponding to, approximately, 70 per cent, of the full engine power.
   The cabin, which has seating accommodation for ten passengers, is panelled in mahogany, and comfortable armchairs are provided, with ample leg room, so that even lengthy journeys should not be unduly fatiguing. Aft of the cabin is the usual lavatory accommodation, while behind that again is a space for luggage. It is often argued against the three-engined machine that, although, it may provide freedom from forced landings, it will not be very economical. In the Junkers G.24, the power expenditure per passenger is approximately 70 b.h.p., which is certainly rather a high figure, but it is now beginning to be realised that it is a little unfair to judge a machine on this basis, since a good power reserve is, of course, a decided advantage, and is even a necessity in order to enable the machine to get off and climb at a safe rate. Moreover, by flying normally at cruising speed with the engines throttled down, a very considerable increase in reliability is obtained, and probably the fairest basis on which to judge a commercial aeroplane would be to take the power expenditure at cruising speed per passenger carried. We have no actual figures relating to the power taken from the engines of the Junkers G.24 at cruising speed, but a rough estimate indicates that it is probably in the neighbourhood of 500 b.h.p., when the power expenditure becomes, roughly, 50 h.p. per passenger.
   The main dimensions of the Junkers G.24.L. are: Length, o.a., 15-3 m. (50 ft. 0 ins.); height, 5-4 m. (17 ft. 8 ins.); wing span, 20-85 m. (93 ft. 5 ins.); wing area, 89 sq. m. (958 sq. ft.). The weight of the machine empty is 3,600 kgs. (7,925 lbs.), and the useful load is 2,400 kgs. (5,280 lbs.), giving a total loaded weight of 6,000 kgs. (13,205 lbs.). The wing loading is high, i.e., 13-8 lbs./sq. ft., and the power loading, on a basis of normal power of 230 h.p., for each engine (i.e., 690 h.p. in all) is 19-15 lbs./h.p.
   The top speed of the Junkers G.24.L. is 175 km./h., (109 m.p.h.), and at 6,600 ft., the top speed is 100 m.p.h. The cruising speed is 150 km./h. (95-3 m.p.h.), and the landing speed for a total loaded weight of 13,205 lbs., is 105 km./h. (65-5 m.p.h.). At cruising speed the standard petrol tanks contain sufficient fuel for nine hours flying, and the petrol consumption at cruising speed is stated to be approximately 40 kg./h. (88 lbs./h.), for each engine, or a total of 264 lbs./h.
   With full load the machine gets off in 22 seconds, taking a run of 220 metres; the climb to 1,000 metres (3,300 ft.) occupies 8 minutes, and to 3,000 metres (10,000 ft.), 40 minutes.
   With full load, the ceiling is approximately 3,800 metres (12,500 ft.).

The Junkers L.2 engine

   As it is not widely known in this country, a few brief particulars of the Junkers L.2. engine may be of interest. This engine is of the six-cylinder in-line water-cooled type, which has always been so popular in Germany. The engine is not unlike the familiar Mercedes, and has an overhead camshaft of very similar type. There are but two valves per cylinder, and the overhead camshaft is driven by a vertical shaft at the back of the engine. The cylinder bore is 150 mm., and the stroke, 180 mm., and the compression ratio is 6-03 to 1. The L.2. engine develops 230 h.p. at 1,420 r.p.m., but is only rated at 195 h.p. The engine weight, (without propeller hub, air screw, water and exhaust pipes), is 290 kgs. (638 lbs.).
   The dimensions of the Junkers' L.2. engine are: length (not including propeller hub), 15-80 mm. (63 ins.); height, 10-35 mm. (41 1/2 ins.); width, 5-55 mm. (22 1/4 ins.).