Flight, September 1934

THE DE HAVILLAND "COMET"
Boat-building practice has been extensively adopted in the construction of the machines for the MacRobertson Race

   STRESSED-SKIN construction has been used extensively in the new de Havilland machines specially designed and built for the England-Australia Race. For the benefit of the less technically-minded of our readers it may be pointed out that this expression is applied to a form of wing or fuselage covering which, in addition to giving the component the desired external form, helps also to give it strength. In an ordinary biplane wing, the strength is provided by the primary structure: Spars and ribs; the doped fabric is not taken into account, although it may under certain conditions contribute something to the strength. The "stressed-skin" covering may, of course, be of any material capable of resisting compressive loads, such as wood or metal. When metal is used, it is of necessity applied in very thin panels in order to reduce weight. This means that unless stiffened by some internal framework of stringers or the like, the metal skin will crinkle. When wood is used for the skin, less stiffening is usually necessary because the wood, being much lighter than metal, can be and is used in much greater thicknesses.
   In the case of the de Havilland "Comet" the use of a stressed skin was, once wooden construction had been decided upon, forced upon the designers by the fact that two spars of a size which could be housed in the thin wing-section employed would not have provided sufficient strength, even if they were of solid wood. To obtain the requisite strength the wing covering had to be made of a form which would reinforce the spars. The system finally adopted was one in which certain members having the appearance of orthodox wing spars, and of similar construction, i.e., box-section, are used for taking the shear loads and for transmitting the loads to the covering. Bending and torsional loads are taken by the skin, which is in the form of a spruce planking laid on after the manner of "double diagonal" planking of many lifeboats. That is to say, there is an inner and an outer layer, each composed of spruce strips some two inches wide, the strips of inner and outer layers crossing each other at approximately right angles. The thickness of this skin or planking is reinforced where the stresses are high, such as on the upper surface near the root of the wing, by a third and even a fourth layer, reaching in places a thickness of more than half an inch. At other places, such as at the tips where the loads are small, the planking is only about one-eighth of an inch thick.

Fuselage Construction

   For the fuselage a somewhat similar construction has been adopted. The shape is almost a perfect streamline, but not quite. The sides, which are not of great depth, are flat, and there would be no point in using the double-diagonal type of planking for them. Consequently they are covered with plywood in the ordinary way. The top and bottom of the fuselage, however, have a double curvature, and are planked with spruce strip in the same manner as the wing. This is necessitated by the fact that sheet material cannot be bent to a double curvature. It can, to put it in a different way, be bent into the form of a cylinder or cone, but not into the shape of a barrel. The use of diagonal strip planking makes it easy to get a smooth double curvature. The nose and tail fairings of the fuselage are of beaten Electron sheet.
   From an aerodynamic point of view the de Havilland "Comet" is interesting on account of the trouble taken to reduce drag to a minimum. The fuselage is, as already mentioned, of nearly perfect streamline form. Its maximum cross-sectional area has been kept down to the minimum which would house the crew, and by placing the cockpit well aft, increasing the fuselage depth slightly behind the cockpit, and using a flatly-sloping windscreen, the break in the airflow caused by this necessary excrescence has probably been reduced to a minimum. The view obviously cannot be as good as if the cockpit were in the nose of the fuselage, but in a racing machine something must always be sacrificed for speed.

The Retracting Undercarriage

   The fact that the machine is a twin-engined monoplane has brought with it the almost inevitable use of retractile undercarriages, the wheels of which draw up into the tail fairings behind the engines. A mudguard over the front of each wheel prevents stones, etc., from being flung into the airscrews, and when the wheels are raised this same mudguard forms part of the bottom of the engine nacelle, leaving merely an opening large enough to let the air escape from inside the engine cowling.
   Sheet metal fillets are used where the wing joins the fuselage in order to reduce interference and keep the airflow as smooth as possible. The same system has been employed on the tail, fillets being used to run the fin surface gradually into that of the fuselage and tailplane. The rudder extends down to the top of the fuselage only so that in straight flight and for small rudder movements, there is an easy path for the air to follow and eddying is reduced to a minimum. By giving wings and fuselage several coats of paint, rubbing down between coats, and repeating the process, a remarkably smooth surface has been obtained, which by reducing skin friction in such a fast machine should add materially to the speed.
   A machine with the aerodynamic “cleanness” of the "Comet" will obviously have a very flat gliding angle, and when most of the fuel has been consumed, so that the wing loading is reduced, it might be a matter of some difficulty to bring the machine into a small aerodrome. In order to make this possible air brake flaps have been fitted. These take the form of. split trailing edge haps over the central portion of the wing, extending from one engine nacelle to the other. When closed, the flaps lie snugly against the main wing surface. They are operated by a simple torque tube and levers.
   Designed for long-range flying (the distance from London to Baghdad is about 2,550 miles), the "Comet" is provided with very large petrol capacity. The three tanks are all carried in the fuselage, one of 128 gallons capacity in the nose, another of 110 gallons capacity approximately over the centre of gravity, and a third, of 20 gallons, just aft of the cockpit. The latter tank is used for trimming purposes. As petrol is consumed the forward tank begins to empty, and to keep the machine from being tail-heavy a small quantity is taken from the rear tank, which is farther from the e.g., and therefore works on a longer "leverage." In addition, the elevators are provided with a spring-loading device for trimming purposes. The tailplane itself cannot be adjusted for incidence.

The "Office"

   Accommodation is provided in the cockpit for a crew of two. They are seated one behind the other, and have dual controls so that they can take turns at piloting. A well-equipped instrument board is placed in front of the forward seat, and can be seen, by craning slightly, from the back seat also. The flying controls are of the usual type, with a plain "stick" for elevator and ailerons. A large wheel to the right of the front seat operates the undercarriage retracting gear. On the left is a lever which operates the air brake flaps fitted under the central trailing edge portion of the wing. Wheel brakes are connected to the rudder bar for steering on the ground. A transparent roof over the cockpit encloses the occupants.
   Flying control surfaces are of usual type, with a form of Frise aileron operated by the patented de Havilland differential method. The ailerons are mass-balanced by lead distributed along the leading edge of the aileron. Rudder and elevators have the usual bob-weight mass balances. In the rudder and elevator controls a mechanism has been inserted to give a very low gear ratio at small angles of movement of the control surfaces and an increasingly greater ratio for larger movements. This has, of course, been done in order to provide lightness of control and eliminate violent response to small movements of the controls at high speed.
   Engine mountings and undercarriage attachments form almost the only metal parts in the "Comet." Welded steel tube construction is used for the engine supports, and the undercarriage wheels are carried en steel forks with telescopic limbs. The wheels are raised and lowered by a worm gear, or rather by the use of “overgrown” bolts and nuts, the "nuts" forming cable drums for the operation of the gear. Thus when the “nuts” are rotated by cables from the cockpit, they draw the “bolts” upwards, shorten one member, which forms one side of a triangle, and thereby raise the triangle and with it the wheel. The movement is explained in two diagrams. As the worm gear is self-locking, no danger arises if the machine should land with the wheels not quite in the “fully down” position. The Dunlop wheels are provided with brakes, which can, as already mentioned, be operated separately via the foot bar. They can be locked "on" together for parking. Instead of a tail wheel there is the usual tail skid, which is thought to offer less air resistance and which helps to pull the machine up when landing.
   The power plants fitted in the "Comet" are de Havilland "Gipsy Six" engines, specially modified for the purpose of the MacRobertson Race. By using a modified cylinder head, valve rocker gear and piston, the overall height of the engine has been slightly reduced, the compression has been raised from 5.25 to 6.5, and the engine works satisfactorily on standard service fuel to D.T.D. 224 specification.
   In order to take advantage of the use of the Hamilton controllable pitch airscrews fitted, the normal speed of the engine has been increased to 2,350 r.p.m. At the maximum of 2,400 r.p.m. the engine develops 224 b.h.p. on the bench. In the air this is slightly increased owing to the small degree of "boost" obtained by the high forward speed of the machine. A new crankshaft has had to be made to get an appropriate hub fixing for the Hamilton airscrew. The high pressure required (about 100 lb. /sq.in.) to operate the pitch control of the propeller is obtained from the usual engine pumps through duplicate oil relief valves. It does not affect in any way the normal lubrication system. The standard arrangement of alternative hot or cold air supply for the carburetters has been retained in case adverse weather conditions are encountered during the race. Warm air, if required, is taken from the vicinity of the cylinders through a flame trap.
   A special coupling is provided on the rear end of the crankshaft for driving a rotary vacuum pump used to operate the Sperry gyro compass.

Data

   But few data relating to the "Comet" are available. The dimensions and areas are shown on the general arrangement drawings. The gross weight is in the neighbourhood of 5,250 lb. As the quantity of fuel carried is 258 gallons, a little "guessing" puts the tare weight, fully equipped, at something like 3,000 lb. Should this be approximately correct, the ratio of gross to tare weight is 1.75. In other words, the machine carries 75 per cent, of its own weight.

Flight, October 1934

THE SUCCESSFUL MACHINES
Points of Interest in the Specifications of the De Havilland "Comet,” Douglas D.C.2 and Boeing Transport

   WHEN the details of the route over which the England-Australia Race was to be flown were made known, there were many who held that it was impossible to produce a machine capable of a flight of more than 2,500 miles non-stop and yet capable of passing the I.C.A.N. take-off requirement, which demands that an aeroplane in the "normal" category, i.e., not stressed for aerobatics, shall be able to clear, from standing start, a barrier 20 metres (66ft.) high in a horizontal run not exceeding 600 metres (656 yards). The De Havilland Aircraft Company, Ltd., designed and built the "Comet," and in a test flight Capt. H. Broad, the firm's chief test pilot, cleared the barrier by as much as 120ft., carrying full load.
   It still remained to be proved whether or not the machine would do the flight from Mildenhall to Baghdad non-stop. The Mollisons provided the proof by leaving Mildenhall at 6.30 a.m. last Saturday and landing at Baghdad at 7.10 p.m. the same day without having landed en route. Moreover, they covered the distance at an average speed of 200 m.p.h.
   The De Havilland "Comet" is a very small low-wing cantilever monoplane of all-wood construction. One of its most interesting features is the wing construction. A very thin wing section was chosen because of its low drag, but the problem of providing the necessary strength in so small a thickness was a serious one. It was solved by planking the wing with spruce strips some 2in. wide, the strips of one layer crossing those of another layer at approximately right angles. A similar form of construction is often used in boat building, and is known as the “double-diagonal” type of planking. Near the wing roots of the “Comet” there are several layers, and the thickness of the planking is more than half an inch thick.
   The streamline fuselage is of somewhat similar construction, and by placing the crew far back the “break” in the lines which is caused by the windscreen has been reduced to a minimum.
   With two special high-compression "Gipsy Six" engines placed outboard on the wings, it was logical to fit retractable undercarriages, as these would go nicely into the engine fairings. This was done, and the combination of a small fuselage of small cross-sectional area and nearly perfect streamline form, with a cantilever wing of very small thickness, and streamlined engine nacelles housing also the wheels, has produced what is probably the most efficient aeroplane ever built.
   Accommodation in the tiny cabin is of necessity somewhat cramped, as there is no room for the occupants to get up and stretch their legs, nor is it possible for them to change places. In a racing machine something has to be sacrificed, and in this case it had to be comfort. As far as one can ascertain, Scott and Campbell Black were no more tired than other competitors who were flying larger machines in which it was possible to change seats and alter position, so that the strain of sitting in one position for ten or twelve hours cannot have been altogether excessive.
   Although built as a racing machine, it is obvious that by taking out the large petrol tanks and fitting smaller ones, reducing the range to, perhaps, 1,000 miles, and possibly transferring the cockpit to the nose of the fuselage for better view, the "Comet" could be turned into a very useful mailplane with quite a good payload and relatively economical in operation.
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Flight, June 1935

NEW and EXPERIMENTAL TYPES at HENDON

   It is known that the de Havilland "Comet" which won the air race to Australia last October will be seen at the Display. It may or may not be included among the new machines in the aircraft park. This machine, a low-wing cantilever monoplane with two de Havilland "Gipsy Six R." engines of about 230 h.p. each, has been acquired by the Air Ministry for research purposes. Flown by C. W. A. Scott and T. Campbell Black in the Australia race, the machine has a maximum speed of about 230 m.p.h., and at Hendon there will be but one military type as fast - the Gloster "Gauntlet" single-seater fighter. The "Comet" is, of course, a two-seater.