Flight, August 1923

A NEW DE HAVILLAND COMMERCIAL AEROPLANE
The D.H.50 with 240 H.P. Siddeley "Puma" Engine

   "THIS is not a good machine." Thus commenced the report of one of the de Havilland test pilots after a trial flight on the first of the new D.H.50's just completed at the Stag Lane works of the de Havilland Aircraft Company. It should be pointed out, however, that the continuation of the pilot's report made it clear that the machine, in his opinion, was not merely a good machine, but that from the pilot's point of view it was something quite out of the ordinary, quite extraordinarily good. As the pilot is a man who has done many thousands of hours in the air on a number of different types, he may be assumed to have become somewhat blase, and to be with difficulty moved to enthusiasm. When, therefore, he expressed himself, in no uncertain terms, delighted with the D.H.50 it may be accepted that the machine really does possess features which mark a definite improvement upon previous types.
   Yet on looking at the new D.H.50 in a casual way, one might be forgiven for failing to detect anything very remarkable in its design. Resembling in general appearance other de Havilland machines that have become famous, the D.H.50 looks, and is, a clean, straightforward piece of work, but certainly not remarkable for originality. It is not until one begins to go into detail, and looks into the figures of weight, useful load, etc., that the real merits of the type become apparent. When it is stated that the D.H.50, although possessing all modern features such as oleo undercarriage, damped tail skid, gravity petrol feed, etc., carries four passengers and yet weighs something like 100 lbs. less than the D.H.9 from which it has descended, one begins to get an idea of the progress that has been made. And even so, it seems quite possible that it will not be the economy, the performance, nor the general excellence of the design that will make the type most popular during the years to come. That popularity will most likely be based upon the delightful manner in which the D.H.50 responds to her controls. This applies not only at full speed and cruising speed, but also, and what is, perhaps, even more important, at or near the stalling speed. Although complete tests have not yet been carried out, especially on performance, etc., sufficient has been found out to be able to state that the D.H.50 is very definitely faster than the D.H.9 - how much faster cannot as yet be stated with certainty - and also has a greater speed range. From the preliminary tests it appears that the L/D of the machine is very good indeed, especially is this noticeable at large angles, i.e., at low speeds. In gliding in to land, the machine "floats" along, and even at about 50 m.p.h. banked "S" turns can be carried out with the greatest ease, the controllability being, apparently, just as good as at higher speeds. Thus, even if it is granted, in the absence of complete data, that on a basis of wing loading the D.H.50 should land as fast as the D.H.9, the practical landing speed at which it can be put down will be considerably lower, owing to the excellent controllability at large angles. Now, it is a well-known fact that it is not possible to land the majority of machines at their absolute minimum speed, owing to the controls becoming "sloppy" at low speeds. In other words, lack of controllability prevents full use being made of the speed range of which the machine is capable. In the D.H.50 it appears that a very marked improvement in this respect has been effected, and that this type will come much nearer to the ideal than even its designers had expected.
   The natural question that will be asked, as the general appearance of the machine does not at once indicate the reasons for this all-round improvement, is: Which are the causes that have contributed towards making the D.H.50 not merely a good machine, but something considerably more than that? The answer must be that no one feature of the design is responsible, but that the improvement is due to the cumulative effect of a number of things. For instance, as regards the reduction of 100 lbs. in weight which has been effected, this is the result of the most painstaking care in detail structural design. Thus, by way of an example, it may be mentioned that the fuselage has "bulges" on the sides to give "elbow room." By making the fuselage itself narrower by the extent of the "bulges" something like 4 ins. in the width has been saved, with consequent reduction in fuselage weight. This is only one item; a number of others could be mentioned but this will serve as an example of the attention which has been given to detail, and which has resulted in such a considerable saving in structure weight.
   As regards the aerodynamic reasons for the improved performance, and the vastly improved controllability., it is more difficult to get at the facts. To begin with, the fuselage, with its small nose radiator and enclosed cabin top, is much "cleaner" than that of the D.H.9, giving the air, one may assume, a more undisturbed flow. Then there is no cut-out in the trailing edge of the top plane, nor any cables running along the leading edge, nor any wing flap cranks projecting upwards on the upper surface, so that here again the air flow is probably greatly improved, with resultant improvement in the aerodynamic efficiency of the machine.
   The improvement in controllability is due, in the first place, to the de Havilland patented type of differential ailerons, in which system the upward-moving flap travels through a greater angle than that moved through by the corresponding downward-moving flap on the opposite side. This system of aileron control is not, however, in itself sufficient to account entirely for the better controllability, which must be partly due to other causes, connected, probably, with the generally cleaner airflow over the wings. The degree of differential movement, and the area and shape of the ailerons, play an important part, and perhaps the best one can do is to say that here is one of those cases where a machine has come out "right" from the first. Any designer knows that this occasionally happens, and that one can never be quite certain beforehand whether it will or will not happen. In this case there is not the slightest doubt that the machine is exactly "right," and one can but congratulate the designers on their achievement.
   Considered as a commercial machine, the D.H.50 is intended for use on "feeder" lines, where the employment of a larger machine is not justified by the volume of traffic, or as a "taxi-plane" in place of the D.H.9's, which have hitherto been employed for this purpose with such remarkable success. One can also envisage its usefulness in opening out new air routes where, at first, the number of passengers, or the amount of goods, cannot be expected to be sufficient to fill machines with larger capacity. For all such work the D.H.50 should be particularly suitable, not only on account of its economy, but also because of its good performance. As already mentioned, the makers have not yet completed their tests, and do not, therefore, wish performance figures to be quoted, but that both the maximum speed and the speed range are greater than those of the D.H.9 has already been definitely established. It seems safe to state that the cruising speed of the machine will be somewhere very close to the 100 m.p.h. figure. The fuel economy at this speed, owing to the high value of the L/D of the whole machine, should be very good indeed, and the criticism that the machine is using, on a basis of full power, close upon 60 h.p. per paying passenger, is met by calling attention to the fact that the "power-required" curve is very far below the "power-available" curve at cruising speed, and that consequently there is a large excess of power, which means that the machine has an ample reserve of power for emergencies, while at the same time the engine can be run normally at only 60 per cent, or so of its maximum, thus ensuring not only a long life but also good reliability.
   The general lines of the D.H.50 are well shown in the accompanying general arrangement drawings and photographs. The "clean" lines and absence of projections should be noted. The fuselage is of usual de Havilland construction, i.e., it is a ply-wood covered structure without wire bracing. The sides and bottom are fiat, for ease and cheapness of construction, while the cabin top, roof of engine housing, and deck fairing aft of the cabin are cambered.
   The 240 h.p. Siddeley "Puma" engine is very accessibly mounted, but there is no separate engine mounting unit such as that fitted in some of the larger machines. Nor is this probably necessary in this case. There is ample space left around the engine, and all parts and accessories likely to need attention are easily accessible, either through inspection doors or through the large openings left on each side of the upper part of the engine. Although the top of the engine housing is covered by a cambered roof, and the fuselage floor under the engine by a sheet beaten up to form a rounded belly, the space from the top longerons to the "eaves" of the roof is left entirely open, with the exception of curved deflectors near the back, serving to send the air which has passed through the radiator and into the engine house clear of the cabin sides. A special arrangement has been adopted for dealing with oil leak problems, with the consequence that the D.H.50 should be exceptionally clean and free from oil thrown back over the walls of the cabin. This feature is of importance not only because of the general cleanliness of the machine, but also, and even more so, on account of the smaller amount of work required to keep the machine spick and span. By leaving the sides of the engine-house open a considerable amount of heat is carried off by the air direct from the water jacket walls, and a smaller nose radiator can be employed, with consequent gain to the appearance of the machine. At the same time the upper portions of the cylinders, with sparking plugs and high tension cables, are immediately accessible, without inspection doors to undo. Needless to say, a fireproof bulkhead separates the engine from the cabin space, which follows immediately aft of the engine.
   The cabin, although its size has been kept down to a minimum, is not at all cramped. In fact, on sitting down inside one is surprised at the amount of leg-room and the general airy impression which the cabin gives. Seating accommodation is provided for four passengers, all facing forward. The front seat is of the hammock type, the two cross bars for which rest in trunnions on the sides of the cabin. The after seat is fixed, and is of three-ply construction, but provided with .a comfortable back-rest and air cushions. An air-speed indicator and an altimeter are mounted on the front wall of the cabin, and a map of England, Northern France, north-western portion of Germany, and the Scandinavian countries was provided on the machine which we had an opportunity of inspecting at Stag Lane the other day. This was because the machine was that evening flying to Rotterdam in connection with the Rotterdam-Copenhagen-Gothenburg "Arrival Competition." The machine was piloted by Mr. Alan Cobham, who carried as passengers Mrs. Cobham, Admiral Mark Kerr, and Mr. C. C. Walker, chief engineer and a director of the de Havilland Aircraft Company.
   By so arranging the seats that they can be easily unshipped the D.H.50 can very rapidly be converted into a goods carrier, in which role its carrying capacity is extraordinarily good - approximately 1,100 lbs. of paving load being possible without exceeding the total loaded weight for which the machine's airworthiness certificate is issued. This amount of paying load is additional to the weight of fuel carried, which is in the neighbourhood of 400 lbs. weight, or sufficient for more than 3 1/2 hours' flying. Assuming a range of about 400 miles, the paying load of the D.H.50 is therefore approximately 4-6 lbs. per horse-power, and that at a cruising speed of 100 m.p.h. As a passenger-carrier we believe it is intended to hire out the D.H.50 at the usual fee charged for the D.H.9's, i.e., 2s. per mile, so that the cost per passenger works out at 6d. per mile, which is certainly very reasonable indeed. What it amounts to is this: Assuming that four people wish to travel from London to Paris in a private aeroplane. The distance is approximately 240 miles, which, at 2s. per mile, works out at L24, or L6 per passenger, or about the same fare as that charged on the regular air lines. If goods be carried, and the full load of 1,100 lbs. assumed, the cost from London to Paris works out at 5 1/4d. per lb. These figures are, of course, based upon the charges made when the machine is hired out by the de Havilland Hire Service, and may therefore be assumed to show a profit on the actual cost, which profit would be gained by any firm operating the machines on their own lines.
   Mention has already been made of the seating accommodation in the cabin. Access is gained from the starboard side, the roof of the cabin being hinged along the port side. The passengers step up on the trailing edge of the lower right-hand plane, which is reinforced with three-ply for the purpose, and from there to a small step permanently mounted on the side of the fuselage. When the passengers have embarked the roof is closed down, the fastenings being within easy reach from inside as well, although a notice warns the occupants not to open the cabin while the machine is in flight. Ventilation is ensured in the simplest way possible, i.e., by leaving the window on the starboard side without glass panes. Owing to the direction of the slip stream from the propeller this has been found quite feasible, and, in order to get rid of the very small amount of draught noticeable in the back seat, a small deflector is mounted outside the starboard window, adjustable by the passengers. From the cabin it is quite easy to converse with the pilot, whose cockpit is placed aft of the cabin, through a large-diameter tube of three-ply running aft from the cabin. The long exhaust pipe on the port side effectively silences the engine, and conversation in the cabin is easily possible during flight.
   From the pilot's cockpit an exceptionally good view is obtained, and the coaming has been so shaped as to restrict his view as little as possible, while at the same time "streamlining" his head. The controls are of usual type, but fitted with ball bearings everywhere, as in all recent de Havilland machines, and the cables are in all instances arranged "straight through" so as to avoid taking them over pulleys. The consequence is that all control surfaces work remarkably easy, and even during flight the pilot can operate the joy-stick with his little finger.
   The petrol tank is mounted above the top centre-section, and is given the shape of a thick, high-lift wing section. From it the fuel flows by gravity to the carburettors, through "Petroflex" tubing, and the tank is divided by a partition, two petrol cocks being, of course, provided, so that, although a petrol gauge is fitted the pilot need not worry much about its extreme accuracy, as he knows that when the main supply runs out he has, by turning on the supply in the smaller tank, sufficient petrol for one half-hour's flying. The two cocks are operated by cables from the port side of the cockpit.
   A certain amount of unorthodoxy is found in the wing bracing, which at first sight looks rather incomplete. To begin with, there are no anti-lift wires in the inner bays. At first this looks alarming, but the reason for the omission at once becomes evident: by leaving out this wire access to the cabin is greatly facilitated, and as these particular wires are only loaded to any great extent on landing the incidence wires between the first pair of struts are probably well capable of transmitting the loads. Again, in the bracing of the centre section struts the right-hand set of wires has been omitted, the left-hand set being duplicated. As in the case of the anti-lift wires the reason is found in the problem of access to the cabin. The top centre-section has been covered with thick three-ply so as to transmit the stresses, and the upper ends of the centre-section rear struts are attached behind the rear spar, to specially strong beams used to reinforce the structure. Thus the omission of certain wires need cause no misgivings, although looking at first somewhat alarming.
   The undercarriage is of the usual V-type, but with oleo gear. The first 6 ins. or so of the travel is taken by the oil, and is then assisted by rubber shock absorbers for the rest of the stroke. The tail skid is sprung and damped, as shown in the sketch, by rubber compression blocks. It is steerable and works in conjunction with the rudder, although springs are incorporated so that, should the skid jam, the rudder can still be operated.
   The tail is of usual de Havilland form, and the tail plane is of the trimming type. It is operated by a horizontal T, as shown in a sketch, by cables and a lever in the cockpit. The action is, of course, much quicker than in the case of a worm type of trimming gear, and it has actually been found possible to control the machine on the trimming gear instead of on the elevator, although the latter is, of course, more effective. Thus it will be seen that the machine can be very quickly trimmed to meet any emergencies.
   The main dimensions of the D.H.50 are shown on the accompanying scale drawings. The empty weight of the machine is 2,253 lbs. and the weight of fuel 394 lbs. The useful load, including pilot, is 1,253 lbs., giving a maximum permissible total loaded weight of 3,900 lbs., which is the weight covered by the machine's air-worthiness certificate. The wing loading is approximately the same as that of the D.H.9, and the power loading is 16.3 lbs. per h.p.

Flight, May 1924

THE NEW D.H. AUTOMATIC CAMBER GEAR

   SOME weeks ago we announced that the de Havilland Aircraft Company, Ltd., had developed, and were experimenting with, a new device for altering, automatically, the camber of the wings during flight. The device has since then been further perfected, and on Thursday of last week we were invited by Capt. Geoffrey de Havilland to witness actual flying tests with the new camber gear. Unfortunately, for reasons connected with the granting of foreign patents, it is not possible to give, this week, a detailed description of the actual mechanism employed, and details must be reserved for a future occasion. The general principle may, however, be outlined, and an account given of the results and impressions of the flying tests.
   Briefly explained, the de Havilland automatic camber device consists of wing flaps extending over the whole length of the wings, and working upwardly against the action of springs. On the ground, or at very low speeds, all the flaps are right down to the limit of their travel. Once the machine gets into the air the flaps begin to move up against the action of the springs. According to the spring tension the speed at which the flaps are right up varies. In the D.H. 50, with 240 h.p. Siddeley "Puma" engine, this speed is about 70 m.p.h. At about this speed, and at all speeds above it, the flaps are lying flush with the rest of the wing section, i.e., the wing section becomes normal. At lower speeds the flaps gradually move down, until at about 45 to 50 m.p.h. they are right down. It will thus be seen that the top speed of the machine is not in any way affected (except, of course, lo such small extent as is caused by the weight of the cambering device, which is quite light). The landing speed is, however, considerably reduced.
   It is not, perhaps, in the actual reduction in landing speed attained that the chief merit of the D.H. camber gear lies, although this is not inconsiderable. (In the D.H.50 it amounts to something like 7 or 8 miles per hour.) Rather is it in the improvement in the angle of climb, and in the reduction in speed on approaching an aerodrome.
   On the occasion of the demonstration last week we took our seat in the D.H. 50, with Capt. de Havilland himself as passenger and Capt. Broad as pilot, and were taken for a flight around the district surrounding the Stag Lane aerodrome. The machine got off after a very short run, and the climb appeared extremely good. By this we do not necessarily mean that the rate of climb was better than that of the standard machine. It may have been, but we do not know. What we mean by "better climb" is better angle of climb.
   During the flight referred to Capt. Broad gradually slowed down the machine, the airspeed indicator needle drooping back slowly to 60, 50, 45 and 40 m.p.h. When the needle touched the 40 m.p.h. figure the machine gently stalled. The recovery was extremely rapid, and it was estimated that the drop from stall until the machine was again flying level (without opening the throttle) could not have been more than about 60 ft. It would appear that in recovering from a stall the spring-loaded trailing edge flaps assist materially.
   Another thing that was particularly noticeable during the flight was that whatever the speed shown on the indicator, the level of the fuselage altered but little. This was, of course, to be expected, as the fact of the flaps coming down virtually increases the angle of incidence as well as the camber, and is thus equivalent to dropping the tail. It was, however, rather startling to have the fact brought home to one by actual experience.
   Even on approaching the aerodrome when about to land, the tail remained nearly horizontal, and the speed at which the aerodrome was approached seemed ridiculously low, especially when taken in conjunction with the level fuselage. As a matter of fact, the feeling of floating into an aerodrome at about 45 m.p.h. air speed with the tail still well up is wholly delightful, and gives a sense of security that must be experienced to be appreciated. On dropping to the ground the machine stopped its forward speed almost in its own length, the flaps, then right down and the whole wing at a large angle, acting as a very efficient air brake.
   There is, of course, the risk that a pilot unaccustomed to this flap gear might accidentally stall the machine, being accustomed to dropping the tail far landing, but this is purely a question of the pilot getting used to the particular machine, and is hardly worth serious consideration.
   It is not difficult to foresee a line of development by which the Handley Page front slot and permanently slotted ailerons are combined with the de Havilland automatic cambering device, and, as a matter of fact, some such combination should add very greatly to the speed range of a machine, especially by using the new type H.P. metal auxiliary aerofoil, which does not affect the wing at all when closed. The disadvantage of the large angles necessitated by the H.P. slotted wing should be to a great extent counterbalanced by the D.H. camber gear.
   In the meantime we understand that the de Havilland Aircraft Company are prepared to consider the granting of licences to any constructor who desires to employ the camber gear, much as several constructors are already incorporating the de Havilland differential aileron gear, which also constitutes a de Havilland Patent.