Flight, September 1927

THE DE HAVILLAND "TIGER MOTH”
130 H.P. De Havilland Engine

   WITH two such outstanding performances to its credit as a speed of 186-5 m.p.h. and an altitude (which does not by any means represent the ceiling) of 20,000 ft., more than usual interest attaches to the little "Tiger Moth" monoplane produced by the de Havilland Company just before the race for the King's Cup. The machine has previously been illustrated in FLIGHT, photographs of it on the ground and in flight having been published from time to time. It is thought, however, that the general arrangement drawings, detail sketches, and notes dealing with various technical points may assist those who desire a more than superficial knowledge of a machine which represents a distinct advance over anything of the same power which has been produced in modern times. As we said when first referring, in FLIGHT, to the new “Tiger Moth,” the machine is a racer pure and simple. By no means is it intended for the man who has just learnt to fly. It is decidedly a machine for the experienced pilot. But it is something more than that. Those who are at all familiar with the history of the Schneider Trophy seaplane race of the last few years will know that, apart from technical difficulties, the cost involved in producing machines of this type is exceedingly great. In fact, so great has the expense become that it is quite beyond the capacity of any individual, or even of any aircraft firm, to carry out the work without the financial assistance of the government. As a result, the great race which is to be flown at Venice on Sunday, September 25, will really represent a race between nations and not between firms or individuals. In the "Tiger Moth" however, the de Havilland Aircraft Company sees a possibility of carrying out high-speed research at a reasonable cost, and it is for that reason that the machine is entitled to rather closer scrutiny than if it were merely a freak machine built to win races. It is, we think, essential that these considerations should be kept in mind in examining the design and construction of the "Tiger Moth."
   Curiously enough, in the general arrangement of the ' Tiger Moth'' there is considerable similarity to the Supermarine S.5 and Macchi M.52 Schneider Trophy racers. The low wing monoplane position, the reduction of strut bracing to the absolute minimum by having all members tension members except the undercarriage struts, the location of the whole depending upon the attachment of the anti-lift wires to the top of the fuselage, and so on; all are features which the "Tiger Moth" has in common with the Schneider machines. It is even conceivable that the machine could be turned into a seaplane, although with its short length it would presumably require an absolutely smooth sea. In this connection it is not without interest to recall that the Schneider Trophy Race was won at Cowes in 1923 by an American Curtiss racer at the average speed of 177-38 m.p.h. That machine had a Curtiss engine developing close upon 500 h.p. It is not suggested that the "Tiger Moth" as a seaplane would equal that performance, although it might come very close to doing so, but perhaps this illustration may at least serve to bring home the merits of having attained more than 186 m.p.h. with an engine of only 130 h.p.
   Constructionally the de Havilland "Tiger Moth" follows normal de Havilland practice fairly closely, i.e., the fuselage is a plywood-covered structure, generally speaking very similar to the fuselage of the standard club "Moth." The cross-section is, however, different in that the sides have a pronounced outward slope towards the top longerons, while the rounded deck of the normal machine has given place to a narrow fairing extending right back to the tail. Roughly, the cross-section conforms to the shape of a man seated with his legs stretched out, the greatest width occurring where the shoulders are, and the top fairing being in line with the head. The usual flat bottom of the standard "Moth" fuselage has been supplanted by a rounded belly, and the work of getting the rounded covering of top and bottom to merge smoothly into the flat sloping sides has been extremely neatly carried out, the joints being quite invisible.
   Substantial transverse bulkheads or formers occur where the wing spars are attached, and the fuselage terminates in front in a fire-proof bulkhead. The de Havilland engine, designed by Major Halford, continues the deck fairing in a forward direction, being a 4-cylinder, in-line, air-cooled engine. The engine accessories are very neatly placed, so that there are no excrescences on the sides of the engine. Consequently, the engine cowling is very neat, and the whole engine installation offers a minimum of resistance. The crankcase is ribbed at the bottom, the ribs projecting through the bottom of the fuselage, thus providing cooling of the oil without offering any appreciable extra resistance.
   The air intake is on the right-hand side, and the tube crosses over to the left where it bends forward and joins the inlet ports. Immediately above the induction pipe are the exhaust ports with their short stubs. The overhead valve gear is enclosed in a cowling, as is also the starboard side of the engine, but the port sides of the cylinders are left uncovered. The petrol tank is mounted in the deck fairing in front of the pilot.
   The cockpit is necessarily small, but although the machine was designed specially for Captain Broad, a pilot of larger dimensions can be accommodated without undue cramping. The fairing in front of the pilot is provided with celluloid windows, and is made in two halves joined by quick-release catches on the centre line and hinged on the top longerons. Thus, when the pilot is in his seat, the two halves of the cockpit coaming are closed, and only the pilot's head projects. The view is quite good, taking into consideration that the pilot is almost enclosed, since by leaning his head slightly to one side or the other he can look forward past the engine.
   Owing to the small space available in the cockpit, the controls are somewhat unusual. The stick is universally mounted as usual, but the lateral control is somewhat less direct than normal, owing to the fact that the crank on the stick operates a T-shaped piece, which in turn actuates the ailerons via push and pull rods and torque tubes. The principle is illustrated by a sketch.
   Concerning the monoplane wings of the "Tiger Moth" little need be said, as they are of normal construction, with I-section spruce spars and normal wood ribs. The wing is built in two halves, the spars forming a butt joint on the centre line, where they are secured with steel plates and bolts, the spar roots, of course, passing through holes in the sides, of the fuselage. The wing bracing is by streamline wires in duplicate. The ailerons, as already mentioned, are operated by torque tubes and cranks. A very neat arrangement has been adopted for closing the gap between main rear spar and aileron leading edge. This space is filled with a strip of sponge rubber, and incidentally the use of this material allows of a certain amount of damping, since working the aileron necessitates compressing the rubber at one edge or other. Thus, by choosing an appropriate thickness of rubber strip, any desired "stiffness" in the lateral control can be obtained. A machine like the "Tiger Moth" is naturally very sensitive on the controls, and so the amount of damping employed is fairly considerable. A similar form of damping and gap-closing is employed between elevator and tail plane.
   The undercarriage is chiefly remarkable for the fact that the shock-absorbing gear is enclosed in the wheels, there being no axle. The central discs of the wheels are attached to the apices of the undercarriage vees, and streamline wires run across from one wheel centre to the other. Details of the wheel construction are shown in some of our sketches.
   Concerning performance, there is little need to say more than that the "Tiger Moth" has covered the 100-km. course at an average speed of 186-45 m.p.h., and has reached a height of 20,000 ft. The latter figure can certainly be improved upon, since during the flight the high-speed propeller was used, and there is little doubt that in the next attempt Captain Broad will reach a height of between 25,000 and 30,000 ft. On the recent altitude attempt, the machine, at 20,000 ft., was still climbing at the rate of about 1,000 ft. per minute.
   Following are the main characteristics of the "Tiger Moth" :-
   Wing span, 225 ft. (6-86 m.).
   Wing area, 76-5 sq. ft. (712 m2).
   Weight, empty, 618 lbs. (281 kgs.).
   Petrol (1675 gals.), 124 lbs. (56-4 kgs.).
   Oil (2 gals), 20 lbs. (9-1 kgs.).
   Pilot, 143 lbs. (65-0 kgs.).
   Total load, 287 lbs. (130-5 kgs.).
   Total loaded weight, 905 lbs. (411-5 kgs.).
   Wing loading, 11-83 lbs. /sq. ft. (578 kgs./m2).
   Power Loading, 696 lbs./h.p. (317 kgs./h.p.).

"Everling Quantities"
   "Wing Power" 183 h.p./m2 (1-7 h.p./sq. ft.).
   "High-speed Figure," 26.
   "Altitude Figure," 4-5.