Flight, October 1938

British Sport and Training types

DE HAVILLAND

   For some two years the development of a successor to the famous Moth series has been the special interest of Captain Geoffrey de Havilland, and the announcement of the Civil Air Guard scheme has merely served to speed up the work on the prototypes. The D.H.94, or Moth Minor, is a two seater open or closed training and touring low-wing monoplane in which constructional, simplicity, robustness and exceptionally good flying qualities have been aimed at.
   In production form it will be fully aerobatic, and for varied reasons, not the least important of which is the provision of a good take-off and climb, the makers have not attempted to obtain economy at the expense of necessary power. The engine will be the new 90 h.p. Gipsy Minor, which may be considered as a smaller and simplified version of the well-known Major.
   The low-wing type of monoplane was chosen largely because it permits a maximum view and because the structural differences involved in the open and closed types need to be very small. In each form the Moth Minor has been designed to be flown without ballast from either seat, and the pilot or pupil will normally take that, in front. There is a simple, but efficient, form of air brake, and the wings are arranged to fold for housing economy. The structure is straightforward, but no very intimate details have yet been published.
   Provisional Moth Minor Data : - Span, 36ft. 7in.; length, 24ft. 6in.; wing-area, 162 sq. ft.; all-up weight, 1,500 lb.; weight empty, 978 lb.; wing-loading, 9.3 Ib./sq. ft.; power-loading. 16.7 lb./h.p.; maximum speed, 120 m.p.h.; cruising speed, 105 m.p.h.; cruising range, 300 miles; and price, L575.

Flight, June 1939

MOTH SUCCESSOR
The New Moth Minor Described : An Inexpensive Type for Training and Ownership : Simplicity and Standardisation

   THE object behind the particular layout of the Moth Minor, which is now in quantity production at Hatfield, may briefly be given as that of providing adequate performance at the lowest possible initial and later costs. Performance, in this case, includes not only a useful cruising speed, but also a short take-off run, reasonable comfort for the pilot and passenger and, in fact, all the features which make for practicability in a touring aeroplane. Economy both in manufacture and maintenance has been assisted by the standardisation ol parts wherever possible, and by the use of sufficient engine power to ensure that the unit is normally working well within its capabilities.
   There is little unorthodox about either the structure or the general layout, though, as far as light aeroplanes are concerned, this is the first low-wing cantilever monoplane to be produced by the De Havilland Company and, when considered as a direct successor to the original Moth family, serves to show, in the circumstances, the finality of present trends. Since De Havillands have never produced a tourer or trainer which was not essentially orthodox, the low-wing type can from now on be so described.

The Minor’s Background
   The history of the Moth Minor has been an interesting one. In the days when the Gorell Committee's report suggested that in due course all privately owned machines would be free from the necessity of possessing a normal C of A, the prototype Minor was designed to take advantage of this prospective legislation change, and so be produced at a considerably lower price than had previously been considered possible. The type was to be made for private ownership without official supervision, but still in fairly large numbers. Later on - actually before the Civil Air Guard scheme came into being - a change was made in the firm’s plans, and it was decided to produce the machine both in trainer and tourer form on a mass-production basis. Consequently the Minor's layout has all the simplicity and ruggedness which one would expect in a machine for decontrolled use, while now being fully certificated both for normal and aerobatic work. An indirect result of these circumstances in the machine’s development was that several examples of the Minor, in both open and closed form, have been extensively test flown during a period of two years or so. In its original form, when intended for non-aerobatic ownership, the test flying was almost exclusively carried out by Capt. Geoffrey de Havilland himself, and he considered the points of the machine from the owner-pilot’s angle. In due course, when it was modified for aerobatic and other training, the machine was given a further period of test flying of what may be termed a rather official nature.
   Later on we hope to describe the machine’s flying qualities. For the moment we shall concentrate on its structural and features.
   It is a conventional ply-covered low-wing monoplane and is, except for such items as engine bearers and undercarriage legs, of exclusively wood construction. In order to cheapen and simplify production, the various parts have, as already explained, been standardised so far as is practicable. For instance, the spruce lengths used are in the minimum number of sizes, while all gussets, rib cappings, angle brackets and hinges are identical.
   For purposes of description, the foundation of the machine may be taken as the centre-section, on which the fuselage is mounted and to which the folding wing extensions are attached. This centre-section is a rectangular structure made up of two box spars, braced at each end by a pair of ribs. It carries the two undercarriage legs, the two control column mountings, the front seat, which is carried on a spruce box also serving as a covering for the control unit, and the fuel tank - or tanks.

The Foundation
   The box spars are straightforward, with spruce flanges, incorporating, in the case of the front spar, packing blocks for the wing attachment fittings, and with ply webbing on either side. The steel plates of the attachment fittings are each secured by seven bolts with the usual spring washers to take up the shrinkage. The hinge wing attachment at the rear spar is made through a single steel plate inserted in the flange.
   The centre-section ribs are ply-faced, but have no cross-bracing, so that the fuel tank can be installed on the port side, and so that there can be unrestricted luggage space in the stub wing, on the starboard side. An additional tank can be fitted in the latter case if required. The whole centre-section structure is completely ply-covered on the under surface, and over the upper surface where it extends outside the fuselage.
   The perforated flap, which is, in effect, an air brake, is hinged at six points to the rear spar of the centre-section, while its trailing edge is chamfered so that, when retracted, it fairs into the contour of the fuselage. The idea of fitting an air brake rather than a true split flap was the obvious one of giving a steeper approach angle which could be adjusted without changes in the machine’s aerodynamic qualities. The holes in the flap do not affect its efficiency, but prevent violent airflow changes and so reduce the amount of trim-change when the air brake is applied. It is operated directly by means of a cable from a lever on the right-hand side of the front, or pilot’s cockpit, and a pair of springs hold it in the closed position.
   The fuselage is a straightforward box structure of spruce and plywood, with dividing bulkheads. In assembly, the sides, consisting of two spruce longerons with intermediate stiffeners and ply covering, are first mounted on the centresection and attached to the stub-wing spars by two bolts. The floor is then screwed and glued to the longerons, and to the rear spar of the centre-section. In the same way the plywood upper section of the fuselage is glued and screwed to the top longerons. Immediately behind the rear seat is a luggage well, reached through the rear cockpit, and there are inspection covers in the fuselage at two points. Another luggage tray is arranged below the dash in the front cockpit. Attached to the four longerons in the usual way are tubular steel engine bearer arms incorporating rubber shear mountings. Immediately behind is a fire-proof bulkhead of annealed mild steel sheet.
   The Gipsy Minor, which is a four-in-line air-cooled direct-drive engine, delivering 50 h.p. at 2,250 r.p.m. and 90 h.p. at 2,600 r.p.m., was fully described in Flight of November 10, 1938. In general design it follows that of the well-known Gipsy Major and it is only necessary here to deal with the details of the installation as they apply to the Moth Minor. The two sides of the cowling are hinged at the top centre line, and the third cowling piece, beneath the engine, incorporates a small sump in which any free oil can collect. The crankcase and oil tank breathers are led into this sump so that the underside of the fuselage should be free from oil in normal operation. At the same time a small pipe, incorporating a gauze filter, runs from the sump to the induction manifold, so that the engine is supplied with a small amount of incidental cylinder lubricant. The exhaust manifold is made up of two steel pressings welded together.
   When starting up there is no need to lift either of the cowling sides as the carburetter can be reached through a small opening on the port side, while the dual fuel pumps may be primed by a remote lever system which is reached through another aperture. Special ducts are built into the cowling sides to direct air on to the fuel pumps so that any possibility of vapour lock trouble in tropical countries is removed. The 1 1/2-gall. cylindrical aluminium oil tank has a cowling jacket through which air, led by a duct from the main engine scoop, is circulated. Both the suction and pressure filters in the engine are usefully accessible.
   As already explained, the standard fuel arrangement includes one 13-gall. tank in the port stub wing. This is held in place in the outer part of the centre-section by three adjustable cables. The fuel tap is on the left side of the front cockpit, and the fuel gauge is mounted directly on the tank and can be seen from either seat. The gauge is ot the flat “dial” type, the indicator needle being remotely operated through a magnetic needle which, in turn, is moved by a float on a quick-thread spindle. For long-range work a similar tank is installed in the starboard stub wing, which is normally used as a luggage container.
   Returning to the main structure, the folding extension wings are made up of two spars with transverse ribs, and plywood covering forward of the rear spar. All the ribs are of the girder type except that on the root end, which is a ply-faced spruce plate. The front spar flanges are reinforced by packing wedges and incorporate insertions of Bakelite sheet. Inboard of the ailerons, the aft portion of the wing is a fabric-covered ribbed structure which hinges at four points for wing-folding purposes. The ailerons, which extend outboard of the wing-folding flap, each have three hinges and are built up with a box spar and ribs, that on each inner extremity being ply-covered on one side for additional rigidity. Incidentally, the fabric on the folding flaps, ailerons, rudder, fin, tailplane and elevator is held in place on each rib by semi-circular-section spruce strips which are screwed in place. Needless to say, the De Havilland sprocket type of differential aileron control is employed.
   The actual wing-locking mechanism is very simple and robust. If the flap is raised a lever is uncovered which, when pulled out and to one side, releases the two wing locking bolts at the front spar. These bolts, or locking pins, move in guides which are attached to the strengthening plate on the root-end rib. It is not possible to lock the wing insecurely, since the fact that the operating lever was not fully home would be noticed as soon as an attempt was made to lower the wing flap. Nevertheless, for those who believe only what they see, there are inspection holes in the leading edge through which the position of the two pins on each side can be seen

Empennage and Undercarriage
   The tail unit is quite conventional, with a single strut braced tailplane and interconnected elevators. The tailplane itself is built up of a spruce spar with a laminated leading edge spaced by girder ribs and two diagonal ply-webbed ribs. The strut is attached to the spar, where also there are the six hinges lor the elevators, the fittings for securing the folded wings, and the necessary pick-up for the fin attachment bolts. Each side of the elevator is identical, one being interchangeable with the other, and the two are joined by bolts through a washer plate from which the elevator controls are taken. The two sides ot the fuselage terminate in a stem post to which both the tailplane and the fin are bolted. The rudder has two hinges, and its mass balance is bolted to the rib carrying the horn balance extension On either side of the fuselage, from the leading edge ol the tailplane, are anti-spin Strakes fairing.
   Ingeniously simple are the two cantilever undercarriage legs. These are mounted in electron castings which are bolted, with the wing attachment plates, to the front spar of the centre-section. Inside each leg, which is a steel tube, is another tube carrying the axle and wheel assembly. This telescopic tube bears on a fabric-based Bakelite liner and contains twenty-two rubber shock absorbers which are compressed against a fixed Bakelite piston in the main undercarriage leg. The principle will best be seen by examination of the sketch on this page; the use of plastic bearings on either side of the moving tube means that the wear should be negligible.
   Torque is taken by a forged steel link attached at its lower end by a pin to a lug welded to the telescopic tube, and at upper end by another pin to a lug on a collar welded to the fixed tube. These two pins, and that in the link hinge, are identical and interchangeable. Rebound is taken by a rubber buffer attached to the inside of the lower part of the torque link, and works against the telescopic tube - or rather, against the gaiter (a grease-filled Bazil bag) which normally covers the lower part of each undercarriage leg.
   The axle itself is directly welded to the base of the telescopic tube, the brake drum being secured by six bolts to a flange which is integral with the axle. The brakes are of the new two shoe Girling type and are cable-operated. The main leg of each undercarriage is surrounded by a simple streamline fairing of plywood
   The tail wheel, which is in a steel fork sliding in plastic bushes carried in an Elektron casting, is fully castering. Shocks are taken by a steel spring which is covered with aluminium fairing. The supporting casting is secured by four brackets to the fuselage stern post.
   In standard form the Moth Minor has the essential instruments in the front, or pilot's cockpit on the left of the board, leaving plenty of space for any additional instruments which may be ordered with the machine or required later. The machine is sold in basic form without such items as Sutton harness and duplicated instruments in the rear cockpit, but the standard equipment does, however, include cockpit covers and a fire extinguisher. Additional equipment up to the standard of that required for general, blind-flying and aerobatic training is provided to order.
   In basic form the Minor has, in the front cockpit, the usual airspeed indicator, altimeter, revolution indicator and oil pressure gauge. The hand-brake lever, throttle and altitude control and fuel tap lever are on the left, while on the right is the five-position air-brake lever. The rudder bar, which is an Elektron casting, includes foot-rests which are adjustable to suit different lengths of leg, and, as usual, operates the wheel brakes differentially when moved to its fullest extent in either direction. The seats in each cockpit are aluminium pressings which can take either normal cushions or seat-type parachutes. Non-splintering plastic material is used for the windscreens, which follow the contour of the fuselage. For blind-flying training hood, which folds sideways, has been designed, when required, is fitted over the rear cockpit.

THE DE HAVILLAND MOTH MINOR.
90 h.p. GIPSY MINOR ENGINE.

Span 36ft. 7in. (11.15 m.)
Span (folded) 12ft. (3.65 m.)
Length 24ft. 5in. (7.45 m.)
Weight empty (Touring version) 970 lb. (440 kg.)
Pilot and passenger (Touring version) 320 lb. (145 kg.)
Fuel and oil 112 lb. (50.5 kg.)
Luggage 129 lb. (58 kg.)
All-up weight 1,550 lb. (704 kg.)
Weight empty (Training version) 983 lb. (446 kg.)
Two pilots 320 lb. (145 kg.)
Fuel and oil 104 lb. (47 kg.)
Parachutes 43 lb. (20 kg.)
All-up weight 1,450 lb. (658 kg.)
Aerobatic all-up weight (Training version) 1,450 lb. (658 kg.)
Maximum speed at sea level 118 m.p.h. (190 km/hr.)
Maximum speed at 5,000 ft. 115 m.p.h. (185 km/hr.)
Cruising speed at sea level 100-105 m.p.h. (161-169 km/hr.)
Ultimate range (standard tank) 300 miles (483 km.)
Ultimate range (with extra tank) 600 miles (966 km.)
Stalling speed 43 m.p.h. (69 km/hr.)
Take-off run in 5 m.p.h. wind 190 yd. (174 m.)
Height at 500 yd. from rest in 5 m.p.h. wind 93 ft. (28 m.)
Landing run in 5 m.p.h. wind with brakes 120 yd. (110 m.)
Initial rate of climb Service ceiling 620 ft./min. (3.15 m/sec.)
Absolute ceiling 16,000ft. (4,900 m.)
Price (in standard form) 20.500ft. (6,250 m.)
Makers: The de Havilland Aircraft Co., Ltd., Hatfield Aerodrome, Herts

Flight, September 1939

To-day's Light Aeroplanes

DE HAVILLAND

   THE Moth Minor, which is now in full production, may be described as the logical successor to the D.H. Moth. It is an all-wood cantilever low-wing monoplane, fitted with the new Gipsy Minor engine of 90 h.p. The two occupants are seated in tandem, with the pilot in the front seat. The Minor is stressed for aerobatics and is intended as an inexpensive type for training and touring. In due course a cabin version will be ready for production.

Span 36ft. 7in.
Length 24ft. 5in.
Weight empty 970 lb.
All-up weight 1,550 lb.
Max. speed 118 m.p.h
Cruising speed 100 m.p.h.
Initial rate of climb 620 ft./min
Range (two tanks) 600 miles.
Price £575.