Flight, November 1927

A NEW DE HAVILLAND AEROPLANE
The D.H.61 is Designed Specially for Dominion Conditions

   In spite of the agitation for the three-engined commercial aeroplane and the fairly extensive use made of this type during the last year or so, it would be a fallacy to suppose that the single-engined commercial machine is already a thing of the past. The question of reliability, using the word in the sense of absence of forced landings between regular 'dromes of call, is not the only one to be taken into consideration, and one may easily imagine circumstances in which the single-engined machine of medium size and power meets the case rather than a large three-engined "air liner." For instance, one may imagine a route on which the amount of traffic is not yet such as to give reasonable promise of filling a very large machine, while the country over which the route runs is such that a forced landing is not likely to result in anything more serious than a delay. Clearly under such conditions, there is little object in using a costly three-engined aeroplane which is likely to fly with half load, or less, on most of its journeys, and the immunity of which from forced landings is not an essential quality. A new machine, which is the result of considerations such as these, is now nearing completion at the Stag Lane works of the De Havilland Aircraft Company, and although a detailed description cannot yet be given, it is thought that a few notes dealing with the D.H.61 may be of interest, as well as the side elevation accompanying these notes.
   Designed and built to the order of an Australian firm for use between Broken Hill and Adelaide, the D.H.61 is a single-engine biplane fitted with a Bristol "Jupiter" engine and having cabin accommodation for 6-8 passengers, as well as ample luggage space and a considerable range at its cruising speed of 100 m.p.h. or so.
   Constructionally, the de Havilland 61 is a perfectly normal machine, with the usual ply-wood fuselage which this firm has utilised with such great success for a number of years, and with biplane wings also of standard dc Havilland type. In certain respects, however, the 61 gives evidence of the fact that it has been specially designed with Australian conditions in view.
   To begin with, a feature of the machine which is apparent at once (although the side elevation published herewith does not show it) is the divided undercarriage, i.e., without any axle or other horizontal member running across from one wheel to the other. This type, which has been relatively little used in this country, has become, one might almost say, standard in the United States, the reason for its adoption being that it is claimed to have less tendency to trip up a machine when alighting in long grass, or in a cornfield. In the D.H.61 the two bent axles cross each other in order to give a better angle and avoid too sharp a bend in the axle tube. The track is fairly wide, so that the machine should handle well on the ground.
   The cabin has two curved plywood seats running across it, one again at the forward bulkhead and one against the aft, the front passengers sitting with their backs towards the engine and the others facing the engine. If desired, reserve seats (2) can be placed midway between the permanent ones, thus increasing the passenger accommodation to eight.
   Large luggage spaces are another feature of the design, one very large compartment being situated between cabin and engine bulkhead, and extending underneath the forward seat. The second luggage compartment is placed after of the cabin, under the pilot's cockpit.
   A feature of the D.H.61 about which very possibly opinions may be divided, is the placing of the pilot's cockpit far aft in the fuselage. In this respect, the 61 resembles the famous 50. There is, however, this important difference, that the 50 is a relatively small machine, so that the forward view is not very much obstructed. In the 61, with its wide fuselage, a greater angle of vision is blanketed, but this has been overcome to some extent by raising the cockpit above the general level of the fuselage. It is admitted that there are still "blind spots," but the general weather conditions in Australia are such that good visibility can usually be counted upon, while there are advantages in placing the pilot aft. For instance, irrespective of whether the machine is flying light or fully loaded, the trim is unaffected, while in a machine with the pilot in front, it is necessary to carry ballast when only a small load is being carried.
   The engine of the D.H.61 will be a Bristol "Jupiter," Series VI. We gather that it had been the intention to install one of the geared "Jupiters," but that none could be spared at the moment for civilian work. Otherwise, there is little doubt that both take-off and climb would have been even better than those calculated for the direct-drive engine. Direct gravity feed is employed, the petrol being carried in a centre-section tank as in the 50, the "Moth," and many other de Havilland machines.
   The D.H.61 is rapidly nearing completion, and it is expected that, weather conditions permitting, the first flying tests will take place in about one month's time. Further details must be withheld until then, but it may be said that the machine is in the 6,000 lbs. class, the total loaded weight being some few hundred pounds above that figure. Performance figures are not available at present.

Flight, December 1927

THE DE HAVILLAND 61
A 6-8 Passenger Machine for Australia

   IN our issue of November 7, 1927, we published a brief description and a side elevation of a new commercial aeroplane designed and constructed by the de Havilland Aircraft Company for an Australian firm. The machine has now been completed, and has passed most of its flying tests so that a more detailed description of it has become possible. A few more tests still remain to be carried out, but already it has been definitely established that the D.H.61 has a quite remarkable performance for the paying load it carries. For instance, the paying load is no less than 1,900 lbs. (with a Bristol "Jupiter VI" engine), and with this the machine has a top speed of 126 m.p.h., and will cruise well throttled at 105-110 m.p.h., at which speed it has a still-air range of about 475 miles.
   In designing the D.H.61, Colonial requirements were kept in mind, and although the machine has been produced specially to the order of MacRobertsoh & Co., Ltd., of Adelaide, the great Australian fruit preserving company, the 61 should, by very minor alterations, be a suitable type for quite a number of regular air lines where as yet the traffic is not large enough to justify the purchase of a more powerful three-engined type. Its load-carrying capacity is such as to make it an economical machine to operate, especially bearing in mind the high performance. For instance, with fuel for nearly 500 miles at a cruising speed of well over 100 m.p.h., the paying load is 4 1/4 lb./h.p. For shorter ranges this load is, of course, correspondingly greater, and vice versa. As the Bristol "Jupiter" has a reputation for low cost of upkeep, the 61 should be a machine with many applications. The fitting of floats would probably be a comparatively simple matter, so that in districts where the seaplane type is to be preferred the 61 again would seem to meet the case. As a seaplane the paying load, might be slightly smaller, although the difference would probably not be sufficiently great to be really serious.

General Design

   There is little in the general design of the D.H.61 to indicate that its flying qualities are at all out of the ordinary. The machine is just a straightforward tractor biplane of typical de Havilland lines, and in a crowd of machines it might easily pass more or less unnoticed. In fact, those who have been beguiled by the success of a number of monoplanes during the past year might be forgiven for thinking that the 61 was merely a result of the designers having got into a rut from which they either could not, or did not bother to, escape. The monoplane looks an attractive proposition. It has an appearance of "cleanness" which is rather tempting at first. But this appearance is largely deceptive. When one goes into the subject, it is found that, aerodynamically, there is so little to choose between the biplane and the monoplane that the preference for one type or the other is more often than not due to reasons other than aerodynamic. That being so, and bearing in mind that the "Canberra," as this first "61" has been christened, is a machine of a total loaded weight of well over 6,000 lbs., it must be realised that the biplane arrangement is very much more compact, especially as the wings are designed to fold back. To design a large monoplane with folding wings is a serious problem, and the consequence is that one sees nearly all monoplanes with one-piece wings, requiring a large hangar space and presenting very serious problems in transport. The advantages of the monoplane over the biplane would have to be very considerable to make it worth while adopting the type, and as they are by no means so, it is not a difficult matter to make out a very good case for the biplane. One may be quite certain that the de Havilland Aircraft Company is fully alive to this fact, and that the biplane arrangement was not chosen merely because most of the firm's machines have been of that type.
   As to the features of the design which have resulted in such a good performance for the load carried, it is not very easy to point to any one thing and say that that is the main reason. The fuselage is of the type which has come to be somewhat derisively known as "slab-sided," and it is of relatively large cross-sectional area. Thus, superficially, there seems to be little reason to expect a very low body drag. On the other hand, the nose of the fuselage has been very carefully designed, and as the pilot's cockpit is well aft of the wings, the whole front portion of the fuselage is entirely free from excrescences (with the exception of an oil cooler and an air intake for the ventilation of the cabin). The "Jupiter" engine, in spite of a not inconsiderable diameter, looks quite small on this fuselage, and in spite of the relative absence of cowling, it seems likely that the air flow over this region is fairly free.
   The biplane wing arrangement has also been designed with a view to good aerodynamic efficiency. The wing span is 52 ft., the wing chord 6 ft. 3 in., and the gap 7 ft. The gap/span ratio is thus 7:52 = 0.135. The "Span loading” (i.e., Span2/W) is 2.704/6.200 = 0-436. At the cruising speed of 110 m.p.h., L/D, (ratio of Lift over induced Drag) is 54. This is the monoplane value, and with the biplane arrangement used, i.e., a Gap/Span of 0-135, this value is increased to 68-8, so that the induced drag at the cruising speed is only 90 lbs., corresponding to a horse-power of 26-4 only. Even allowing for propeller inefficiency, the power required to overcome induced drag at cruising speed is under 40 tip. A monoplane, to give the same value of induced drag at the same speed, would have to have a wing span of about 66 ft., and as it would probably not be fitted with folding wings, would be rather a cumbersome affair for its weight. The thin wing section of the 61, plus its bracing wires and struts, probably has no greater profile drag than that of a thick section such as would be used in a monoplane wing.

Structural Design

   Intended for use in Australia, the D.H. 61 has been designed as a very simple and robust machine, mainly of wood construction, and such few metal fittings as are unavoidable have been kept as plain and straightforward as possible. The fuselage is of the normal De Havilland type, with four longerons and a skin or planking of plywood, This again, is covered with fabric, and all joints and edges in the plywood are similarly protected. The fuselage is built in two sections, bolted together, the joint being covered with a glued-on fabric strip. The roof of the cabin forms another unit, bolted and screwed to the top longerons, which can be removed for repairs or when a thorough inspection of the fuselage structure is required.
   The installation of the "Jupiter VI" engine is of very simple type, the engine plate being of Duralumin, supported on steel tube members. The whole unit is detachable from the fuselage by undoing four bolts at the corners, the structure remaining being a complete and rigid unit without loose parts. The petrol supply is of the simple gravity feed type the centre-section of the top plane forming the petrol tank with a capacity of 80 gallons. This centre-section is situated, slightly higher than the two halves of the top plane, so that when the wings are folded the trailing edges of the top plane can pass under the centre-section tank, thus avoiding any complication in the design of the corners of the top plane.
   The oil tank is made in one with the top cowling, making one unit instead of two, and one mounting common to both. The oil cooler is mounted direct on the top of the tank. The oil temperature gauge is inserted in the sump alongside the outlet to the engine, so that the correct temperature should be recorded. A cock is provided which must be turned off when the machine is standing, and to avoid the danger of starting without turning this cock on, a magneto earthing switch is arranged to cut out the magneto when the oil is turned off.
   The wings are mainly of wood construction, but have metal drag struts. All drag wires are connected direct to the ends of the struts, so that cross-grain shrinkage does not affect their adjustment for tautness. The ribs are made a floating fit on the spars, metal tracks and leading edges being employed. Provided the wings are rigged according to instructions, there is no need for jury struts when folding the wings.
   The undercarriage of the D.H.61 is of somewhat unusual type, in that there is no axle. This type of undercarriage has become very popular in America, as it has been found that it gives less tendency for the machine to "nose over" when alighting on rough ground, in tall grass, or corn. Instead of the usual type, however, quite a different arrangement has been evolved, the general scheme of which will be clear from the sketch on page 880. The usual vee on each side, i.e., the telescopic compression "leg" and the "radius rod," are rigidly fixed together at their lower ends, the lower tube of the telescopic "leg" carrying the wheel being overhung as a cantilever from the point where it emerges from the upper tube to the wheel. The structure on each side is completed by a transverse diagonal strut running to the longeron on the opposite side. With this arrangement, the angle of the diagonal struts is very good, while a wide wheel track is provided, and the bend in the axle tube is not so sharp. Springing is by rubber blocks in compression, and the blocks are lightly loaded so as to prevent them being over-compressed, with consequent hardening. All working points are provided with "Tecalemit" grease gun nipples, with the exception of the telescopic tubes, which are lubricated by oil fed through oil cups at the top of the "legs."
   The cabin of the D.H.61 is of generous proportions, the width being no less than 4 ft., so that two passengers sit side by side very comfortably indeed, while if desired, it is possible to carry three side by side, although the accommodation is then slightly cramped. The sofa seats are arranged across the cabin against the front and rear walls, and two bucket "seats" are placed in the middle of the cabin, between the two others. There is a space between them, so that passengers can get to the front sofa seat easily.
   The pilot's cockpit, an extraordinarily roomy one, by the way, is placed well aft of the wings and cabin, and extends upward into a sort of "conning tower." It might have been thought that the view, with such a wide fuselage in front, would have been rather poor. As a matter of fact, it is very much better than one would expect, and only in dropping the tail to land is it obstructed to any serious extent. However, even then, by looking diagonally, instead of straight forward, the pilot can easily see the ground ahead. He will already have made sure, in gliding in to land, that the ground straight ahead is clear. The advantage of having the pilot aft is that the trim remains unchanged, so that it is never necessary to carry useless ballast to trim the machine. Cynics might say that also this is a very safe place for the pilot in a crash, so that he would probably live to relate what went wrong. A small window between cabin and cockpit permits of communication between pilot and passengers.
   The luggage space provided is on an unusually generous plan, one compartment being situated ahead of the cabin, large enough to hold a couple of cabin trunks, while a smaller compartment is under the pilot's cockpit. In the cabin there are spaces for light hand luggage in the forward wall, and two smaller recesses in the aft wall.
   Altogether, the de Havilland 61 is one of the most comfortable of modern aeroplanes, and it deserves to become very widely used in localities where a medium power machine suffices for the amount of traffic obtainable.
   The main characteristics of the machine are :-
   Total loaded weight 6,200 lbs. (2,280 kgs.).
   Paying Load 1,900 lbs. (864 kgs.).
   Petrol 80 gallons (4 1/2 hours' cruising).
   Wing area 613 sq. ft. (57 sq. m.).
   Wing loading 10-1 lbs./sq. ft. (49-5 kg./sq. m.).
   Power loading 13-8 lbs. (6-27 kgs.) per h.p.
   "Wing power" 0-734 h.p./sq. ft. (7-9 h.p./sq. m.).
   Span loading (biplane) 0-436.
   Top speed 126 m.p.h. (203 km./h.).
   Cruising Speed 105-110 m.p.h. (170-177 km. h.)
   Stalling speed, about 47 m.p.h. (76 km./h.).
   Rate of climb 650 ft./min. (3-3 m. per second).
   Ceiling 15,000 ft. (4,570 m.).

Everling Quantities (Metric Units).

   "High-speed figure" 19
   "Distance figure" (top speed) 4-8
   "Altitude figure" (ceiling) 8-9

   While the "Distance figure" is of about average value, the "High-speed figure" and "Altitude figure" are both unusually high, especially the former, which is far above the average.

Flight, July 1928

THE DE HAVILLAND D.H. 61 "CANBERRA
One Bristol "Jupiter" Engine

   THE "Canberra," designed by the de Havilland Aircraft Co., Ltd., is a cabin machine carrying six passengers in real comfort with a good margin for luggage, and the possibility, when occasion demands, of carrying eight passengers. A float undercarriage, interchangeable with the land chassis, adds greatly to the utility of the machine.
   Fuselage. - This is built up of complete units constructed of spruce members with three-ply covering. The cabin has been designed with a view to giving the passengers the maximum degree of comfort. The windows are arranged to slide horizontally, and the adjustment of these, and a large cowl ventilator which diffuses cold, or warm air through the roof, makes it possible to vary the temperature at will. The seats, which are quickly detachable, and can be folded into a small space, make it possible for the machine to be used for carrying freight.
   Wings. - Designed as a two-bay equal span biplane, the planes are constructed mainly of spruce with the exception of the tubular steel drag struts. The wings have been arranged to fold against the fuselage, no jury struts being required.
   Undercarriage. - The chassis is of the split-axle type, the axles being arranged to cross each other. In conjunction with the wide fuselage this undercarriage gives an ample track, and provides maximum ground clearance. Rubber-in-compression legs of streamline section take the landing shocks.
   Engine Installation. - The Bristol "Jupiter," Mark XI is mounted on a duralumin plate; this is attached to the front of the fuselage by a steel tubular structure. An oil tank carrying above it in the slipstream an oil cooler, forms the top engine cowling. A streamline tank, mounted within the top centre section of the main planes, having a capacity of 80 or 120 gallons, at the option of the purchaser, connects to the carburettors by gravity feed through petroflex tubing.