Flight, August 1922

THE D.H.37
275 H.P. Rolls-Royce "Falcon" Engine

   To the casual observer the new de Havilland 37, built for Mr. A. S. Butler as a fast sporting and touring machine, might appear to be just a common, straightforward tractor biplane without any very pronounced novel features and offering no field for speculation as to the merits or otherwise of its peculiar points; and from his point of view the casual observer would be correct. The 37 is a very, straightforward design on orthodox lines, showing neither metal construction, cantilever wings, variable lift devices, nor semi-Diesel power plant, to mention but a few of the problems which are so prominently to the front at the present time. Nor is the machine a helicopter (although its climb is distinctly good). While thus the uninitiated might pass it by with but a cursory glance, except for its rather pleasing lines, there is much of interest to be found by those who like to look below the surface, as it were, in order to discover the ideas which were in the designer's mind, the objects at which he aimed and the methods adopted for attaining them. Viewed from this angle the new D.H.37 assumes a different aspect, and it is in this light that we propose to review its features.
   Designed in the first place as a machine on which the owner can go touring and take with him one or two friends, the 37 has accommodation for two passengers in addition to the pilot. As the owner may occasionally wish to take part in races, the machine must have a fairly good performance when flown light. In view of the fact that flights of considerable duration may occasionally be undertaken, the tankage must be generous. As duration is largely determined by the size of power plant, not to mention the effect on running costs of the size of engine, the smallest engine which will give the desired performance must be installed. And finally, the whole outfit must be as simple as possible so as to be easy to maintain and with a minimum of parts likely to get out of order. These were, briefly, the considerations with which the designers were faced when starting the design. Let us, next, examine their solution of the problems, as exemplified in the finished machine.
   An inspection of the accompanying scale drawings and photographs will reveal the fact that the D.H.37 is of very pleasing outline and of very clean design, and that it represents, perhaps, the maximum of refinement that is possible with a fiat-sided fuselage and braced wings. Simplicity of construction was, as has already been pointed out, one of the desiderata, and the flat-sided, ply-wood covered body is considered to give this simplicity to the maximum extent. It might be pointed out that a similar construction is used in the D.H.34's, one of which has just completed close upon 60,000 miles during three months' flying, so that as regards robustness and hard-wearing qualities the ply-wood covered body appears to be entirely suitable. The absence of wire bracing (with the exception of the engine bays, where cable bracing is used in the bottom panels) avoids the necessity for keeping the body trued up after prolonged use, while there is no fabric to become slack or torn.
   The simplicity of the wing bracing is attained by using a fairly thick wing section, which allows of using but one pair of interplane struts on each side. This section - which is, we believe, similar to that used on the D.H.34 - is one of the airscrew sections, with fiat lower surface giving good spar depth. Constructionally the wings are of standard D.H. type, with spruce spars (laminated) and ribs. The two halves of the top plane are secured to the centre section by vertical bolts and external fish-plates, while the lower wings attach to the sides of the fuselage by built-up sheet-steel forked plates. The top centre section is carried on raked steel struts of N formation, the sloping member of the N having threaded ends for adjustment.
   A peculiarity of the 37 as compared with previous de Havilland designs is that ailerons are fitted to the bottom plane only. This results in considerable simplification of the controls, as well as leaving the top plane solid, and if sufficient lateral control can be maintained with two large ailerons on the lower plane, the system has much to recommend it. One reason for expecting that two ailerons will suffice is that the differential movement of the ailerons, which was first tried, with success, in the D.H.29 monoplane, has been incorporated. It will be recalled that with this system of lateral control the cranks are so arranged that one aileron moves through a greater angle in going up than does the opposite one in going down. The result is that one aileron acts as a balance for the other, rendering the load on the control column small even when a large aileron angle is obtained for a small angle of control column movement. This is one advantage. Another is that with this system a spin is less likely to be precipitated when using the ailerons at large angles of incidence, i.e., near the stalling speed of the machine. The fitting of straight control cables and ball bearings in the controls further tend to give easy working of the controls.
   The controls are so arranged that the aileron cables pass straight from the fuselage to chains over sprockets in the lower wings, no pulleys being used anywhere. Small cranks on these sprockets are so set in relation to one another as to give the differential movement to the ailerons. The elevator control incorporates a transverse shaft behind the pilot, from external cranks on which cables run straight to the elevator king posts, no fairleads being required. The transverse shaft is carried on external ball-bearings, the latter being covered by streamline casings of aluminium. In the case of the rudder cables a slight change of angle is necessary, and a length of rod of ample diameter, working in a guide on the outside of the fuselage, is therefore incorporated in the cable. With regard to the rudder, it is of interest to note that the king post is not, as in the majority of machines, passed through the rudder and secured to the leading edge by an elaborate fitting, but is in two halves, bolted to a stout block of wood screwed and glued to the leading edge of the rudder. Thus when storing spare rudders the king posts can easily be removed without interfering with the fabric covering, and not only is less space then taken up by the rudder, but there is no risk of bending the king posts or puncturing with them the fabric of another rudder packed in the same case or stored in an adjacent rack, as frequently happens when the king posts are left in situ on the rudders. It may be thought that such points as these are of minor importance, but in practice they amount to a good deal, and in the 37 it is by close attention to such "minor" points that the chief merit of the design lies.
   While on the subject of control surfaces it might be mentioned that the ailerons are of somewhat unusual construction, especially as regards their leading edges. Fundamentally these consist of the usual channel section spruce spars, with their flat, closed side facing the rear spars. In order to obtain greater torsional stiffness this channel section has been reinforced by a horizontal U screwed and glued to the aileron leading edge. As the three-ply strip used is quite thin (only about 1/16 in.), the reinforcement adds but very little to the weight of the aileron, but the increase in torsional stiffness is extraordinarily great. The aileron ribs are, of course, shaped at the front to fit the curve of the three-ply strip.
   Reference has already been made to the fact that the D.H.37 has accommodation for two passengers in addition to the pilot. The latter occupies the aft cockpit, well clear of the wings, where his view is but very slightly obstructed. Just in front of him is the passengers' cockpit, which is somewhat longer, the passengers sitting tandem fashion. Dual controls are provided, but the control tube in the front cockpit can be removed by pulling it out of its socket. Both foot bars are provided with adjustable pedals, so as to enable adjustment to be made for pilots of different height. A very complete instrument board is placed in front of the pilot, and we noticed that the outfit included one of the Vickers-Reid turn indicators. It might be added that, although not actually fitted up with wireless, the machine has all the wiring necessary for its installation, should the owner desire to fit a set later on.
   The petrol system is interesting inasmuch as it combines direct gravity feed with ample tankage. The ideal would, of course, be to have all the petrol in the top centre section, but in order to do so it would be necessary to make the centre section very deep and unsightly, and, probably, bad aerodynamically. As the owner wished to have large tankage it was necessary to compromise, and consequently the top centre section has been built up as a gravity tank holding 40 gallons of petrol, while another tank of similar capacity is housed under the deck fairing of the fuselage, in front of the passengers' cockpit. Normally the machine is flown on the gravity tank, but if the supply is beginning to run low, as shown by the petrol indicator, a pump driven by the engine supplies pressure to the fuselage tank, and petrol is forced up into the gravity tank. The main objection to the pressure petrol system is that, with large main tanks and a very small header tank, the pumps have to be in operation almost continuously. With the present system this is not so, and there appears to be no reason to expect that any trouble will arise in connection with the fuel supply, the gravity tank being as large as it is. Thus, while departing from the theoretical ideal, the petrol supply of the D.H.37 will probably be found in practice as reliable as the pure gravity system. It might be mentioned incidentally that all the petrol leads are in the form of "Petroflex" tubing, which combines flexibility with imperviousness to the action of petrol.
   The engine mounting for the Rolls-Royce "Falcon" engine is very simple, the two tubular engine bearers resting on brackets bolted to the vertical struts of the fuselage, and being braced diagonally by short tubes of smaller diameter. The oil tanks are slung, one on each side of the lower portion of the crank-case, below the engine. The radiator, placed in the nose, straddles the reduction gear of the engine, and is remarkable for the fact that it is built up as a unit with its header tank and side frames. The ordinary practice of having a separate cowling is apt to give trouble, and by making the side frames and top of slightly thicker metal the whole unit is made very rigid, there is nothing loose to come adrift, and altogether a lot of trouble is avoided. The radiator itself is slung from the nose of the fuselage on trunnions, so that vibrations are not likely to affect it, while at the same time the removal of the radiator by undoing the trunnion bolts is a very quick operation.
   The undercarriage is of similar type to that used so successfully on the D.H.34, i.e., a combination of rubber shock absorbers and oleo gear mounted on the rear legs of two Vees. The undercarriage is high, so as to give a quick get-off and, especially, rapid pulling up on landing. A feature of this undercarriage, apart from the combination of rubber and oleo gear, is that if the machine is landed with a slight side drift on and the wheel on the side towards which the machine is drifting touches before the other, the axle, hinged as it is by radius rods formed by the front members of the Vees, travels in such a way as to tend to direct the wheels along the path travelled by the machine. Thus the tendency to turn over on to a wing tip is reduced. Another advantage of this type of undercarriage is, of course, that a very much longer. travel, and therefore better shock-absorbing quality, is provided.
   The main characteristics of the D.H.37 are as follows, the weights stated being estimated, as are also the figures relating to performance :- Length over all, 28 ft. 9 ins.; span, 37 ft.; chord, 5 ft. 9 ins.; gap, 5 ft. 9 ins.; wing area, 388 sq. ft.; weight of machine empty, 2,118 lbs.; petrol and oil, 330 lbs.; pilot, 170 lbs.; disposable load, 700 lbs.; weight fully loaded, 3,318 lbs.; wing loading, 8.55 lbs./sq. ft.; power loading (based upon 275 h.p.), 12 lbs./h.p.; speed at 10,000 ft., 122 m.p.h.; climb to 10,000 ft. in n minutes; service ceiling, 21,000 ft. In the Derby the D.H. 37 did not get a chance, the start being delayed by ignition trouble.