Flight, June 1925

NEW DE HAVILLAND PASSENGER MACHINE

   ON Thursday of last week we were privileged to witness the first flying tests of the new De Havilland passenger biplane, the D.H. 54, which has now been finished as regards its main structure, although the cabin is not entirely finished internally in the matter of seats and other equipment. In the tests at Stag Lane Aerodrome the D.H.54 was piloted by Capt. Broad, who first took the machine up without passengers for a few trial flights, and later on went up again with several passengers in the cabin and one in the engineer's seat in front.
   The machine appeared to get off very well indeed and to have a very good climb, not only as regards rate of climb, but even more so in the way of climbing angle. Even so it should be remembered that the machine is as yet untried and that the pilot is not yet quite used to handling it, so that there is every reason to believe that later on the climb will be even better. As the flights were in the nature of preliminary flying tests, no attempt was made at obtaining other than very rough performance figures and it would scarcely be fair to the company to give these at the present time, since they may be subject to considerable alteration after properly conducted tests. There is, however, reason to believe that the new De Havilland D.H.54 will have a greater top speed than the famous '34 and a considerably lower stalling speed. This is mainly due to the fact that though the power loading is approximately the same as that of the D.H. 34, i.e., about 17 lbs. per h.p., the wing loading is a good deal less than that of the '34, while the use of the De Havilland automatic flap gear should give a considerably greater maximum unit lift. Apart from the reduction in stalling speed, another advantage of the lighter wing loading should be a considerably better angle of climb, an assumption which was indicated by the preliminary test flights to be correct. Without going into details regarding performance it may be said that the machine was designed to have a cruising speed of over 100 m.p.h. and that probably the actual cruising speed with full load will be in the neighbourhood of 103-104 m.p.h.
   In general design and construction the D.H.54 follows very closely standard De Havilland practice, and at first glance the '54 looks very much like the '34, but on a larger scale. There are however, several noticeable differences in general design as well as in details.
   One change in design is connected with the placing of the top plane in relation to the fuselage. Whereas in the '34 the top plane rests directly upon the fuselage, it is raised in the '54 a considerable distance above the top of the fuselage. This arrangement arises out of the fact that, with the larger chord of the '54 wings, a larger cap was required, and as the fuselage is of approximately the same depth as that of the '34 the raising of the top plane resulted. In general design the D.H.54 is a normal two-bay tractor biplane with straight top plane and a fairly pronounced dihedral to the lower plane. A feature which impresses one both on looking at the machine and from an examination of the accompanying general arrangement drawings, is the high aspect ratio of the wings which have a span of 68 ft. and a chord of 8 ft. giving an aspect ratio of 8-5.
   Constructionally also the D.H.54 follows normal De Havilland practice in that it has a flat-sided fuselage covered with ply-wood and having a deeply cambered roof. In view of its size the fuselage has been built in two separate sections. The joint in the fuselage occurs just aft of the cabin door, the joints being in the form of bolts through the longerons, with fish-plates inside and out. Each portion forms a complete structure in itself, and to keep out water in case of an emergency landing the small opening between the two sections is covered with fabric strip. To facilitate inspection of the rear portion of the fuselage large openings are provided in the bottom, which are covered with fabric so that when the machine has been in use for some time all that is necessary is to cut away a piece of the fabric when the interior structure can be inspected and after inspection the opening is again closed by doping a piece of fabric over it.
   The cabin portion of the fuselage is very large and roomy and seating accommodation is provided for fourteen passengers, the seats being arranged in three rows, two close together on one side and a single row on the other with a narrow gangway down between the two for the whole length of the cabin. Aft of the cabin is a large lavatory with the usual fittings, the lavatory and the last two passenger seats being actually situated in the tail section of the fuselage. Access to the cabin is by a door on the starboard side, and in the design of the door provision has been made for making a watertight joint between it and the door frame so that in case the machine should be forced to alight on the sea it will keep afloat for quite a long time. Provision has been made for heating and ventilating the cabin, the heating arrangements being in the form of a muff around one of the exhaust pipes and admitting warm air to the cabin near the floor. Fresh air is forced into the cabin from a scoop projecting out of the cabin roof, just behind the pilot's cockpit, and louvres near the back of the cabin exhaust the air so that a fresh supply of air is constantly filtering through the cabin. The rate at which the air can be changed is adjustable from within so that the passengers can alter it from time to time so as to suit any conditions obtaining at the moment. A safety measure is provided by emergency exits in the roof of the cabin, the openings normally being covered with doped fabric which, however, can easily be cut or torn off should it for any reason become necessary for the passengers to emerge from the cabin through the roof, such as might occur in a forced alighting at sea.
   The Rolls-Royce "Condor" engine is mounted on a composite structure of wood and steel, and is, of course, separated from the rest of the machine by a fire-proof bulkhead. The pilot's cockpit is in the roof and is further raised by being enclosed in a "hump," so that his view should be particularly good in almost all directions. Next to the pilot is another seat for a navigator or engineer, whichever is carried in the machine, and the view from this seat also is very good, although it is on a slightly lower level than the pilot's cockpit. The petrol system is of the simple gravity type, the main petrol tank being fitted in the top centre section, from which a “petroflex" tube runs to the engine. The capacity of the petrol tank is 148 gallons.
   The undercarriage of the D.H. 54, although of the normal V-type, incorporates certain novel features, such as the shock-absorbing gear, which is in the form of rubber blocks working in compression, a type which has been found to give excellent results in practice, and to be light and durable, and reasonably cheap. A further feature of the design is that parts of the undercarriage, i.e., the wheels, axle, and rear undercarriage struts, can be dropped should the machine be forced to come down on the sea. The front struts remain in place, but as they are free to swing loose once the rest of the undercarriage has dropped, they would presumably fly back against the bottom of the fuselage as the machine struck the water. It is expected that this arrangement will greatly reduce the tendency of the machine to nose over on striking the water, and it would therefore provide an extra safeguard.
   The main planes of the D.H. 54 are of normal De Havilland construction with box spars and spruce ribs. The latter are, however, of slightly different construction, the details of which are shown in one of our sketches. To facilitate transport the top wing is in three sections and the bottom wing in four sections. The wings are not, however, designed to fold. A novel feature in the wing design of the D.H. 54 is the incorporation of the new De Havilland automatic flap gear. This has previously been described in FLIGHT, but it may be stated briefly that it consists of trailing edge flaps extending the whole length of the wing span, but divided on each side. The inner portion of these trailing flaps lifts under air pressure against the action of springs, so that as the machine gathers speed the flaps rise to normal position. As the speed drops the flaps begin to descend, until at some pre-determined speed they are down to the maximum position, thus giving an increase in camber and incidence, accompanied by an increase in lift. The outer portion of the flaps, although moving with the main flaps, still maintain their differential movement and act as ailerons in the usual way. It is thought that by the use of this camber gear the landing speed of the machine will be reduced by quite a considerable amount, and the gear has the further advantage that it enables the machine to approach an aerodrome at low speed without having to drop its tail to any great extent, and in point of fact with the flaps down and the machine close to stalling angle the fuselage is still very nearly horizontal. This should add greatly to the feeling of safety among the passengers, since it is rather disconcerting to approach an aerodrome with the tail well down.
   As we have already said, it is not possible at the moment to give exact performance figures, but it is hoped that these will be available shortly when we expect to publish them in FLIGHT. In the meantime it may be said that the empty weight of the D.H. 54 is 7,000 lbs. and the total loaded weight 11,000 lbs. As the wing area is 1,000 sq. ft., the wing loading will be 11 lbs. per sq. foot, which is fairly high, but with the flap gear it is expected that the landing speed will only be about 55 m.p.h.