Flight, July 1932

The A.W. XV Monoplane
Designed specifically for the African Section of the London-Capetown Air Route, the Armstrong Whitworth A.W. XV is a Cantilever Monoplane fitted with four Armstrong Siddeley "Double Mongoose" Engines of 340 h.p. each. The first of the eight machines being built, the "Atalanta," was illustrated in FLIGHT last week. The Machines are designed to be, as far as humanly possible, immune from hurried forced landings, and to have a good Performance in the rarefied Air of certain Sections of the African Route

   NOT for very many years has there been produced a commercial aircraft with so many interesting features as those one finds on inspecting the new A.W. XV monoplanes which are now passing through the Armstrong-Whitworth shops at Whitley, Coventry. Eight of these machines are on order for Imperial Airways, Ltd., and the first, the "Atalanta," is already flying, and visited the S.B.A.C. Display at Hendon last week, where it attracted unusual attention. The others are well on the way, and for once it looks as if the old saying that no new aircraft type, let alone a batch of large machines, is ever produced on time, will prove false. The "Atalanta" has done a deal of flying, and such minor "teething troubles," as any new type is likely to suffer from, have already been discovered, and, as far as can be ascertained, remedied. It is true, of course, that only in actual service over the route for which it was designed can it be settled quite definitely whether or not a new commercial aircraft type is entirely successful, but the few difficulties encountered have been tackled and overcome, and the first machine is now reported to be ready for its final, searching tests.
   The photographs and general arrangement drawings of the "Atalanta" illustrate very well the general appearance of the machine. What they do not show, however, is the internal structure, which is, if anything, even more interesting than the external appearance. In the space available it is quite impossible to do justice to a new type like the "Atalanta" in one issue of FLIGHT, and we, therefore, propose to divide our description into two instalments, the first, published this week, dealing with the general, or aerodynamic, design of the A.W. XV, and the second instalment, which we hope to publish next week, being devoted to the structural details.
   For a good many years the British aircraft designer has been content to follow a conservative policy in the general lay-out of his machines. The biplane type had the advantage of low structure weight, and for a very long time the British aircraft designer refused to be influenced by the vogue of the monoplane type abroad. Nor was he altogether to be blamed for this attitude. The biplane type of structure was known to be lighter, in machines of the same landing speed, than that of the monoplane, particularly the cantilever monoplane. And as it has been a guiding principle in British commercial aviation for many years not to sanction very high landing speeds, a very good case could actually be made out for the biplane.
   Progress in aero-engine design and construction during the last years has been such that a complete breakdown of an engine is now a very rare occurrence. By adopting, as the designers of the A.W. XV have done, a four-engined arrangement of the power plant, the chances of what may be termed a hurried forced landing, as distinct from a more leisurely precautionary landing on a selected site, have become still more remote. This means that a somewhat higher landing speed than hitherto adopted can be tolerated. Once that point is conceded, the relative merits of biplane and monoplane have to be reconsidered. For the same landing speed, the monoplane will be a larger machine, larger, that is, in wing span and probably overall length. While the landing speed was low, i.e., while the wing loading was low, the monoplane size as compared with the biplane, was rather prohibitive. As the wing loading is increased in conformity with the higher permissible landing speed, the difference in size becomes less, and a point is reached when it is no longer safe to say that the biplane is, on balance, the "better" type.
   In the A.W. XV we have an aircraft designed for a good deal higher performance than we have hitherto been accustomed to in British commercial aeroplanes. Exact performance figures are not yet available, but the cruising speed will probably be somewhere in the neighbourhood of 120 m.p.h. The higher performance, and the increased landing speed made permissible by a reliable power plant divided into four units, has been made the basis of a very exhaustive study by the Armstrong Whitworth engineers into the subject - biplane or monoplane? They came to the conclusion that, for the purpose in view, the monoplane offered the better solution, and the A.W. XV is their interpretation of that solution.
   We recently had the pleasure of being shown the monoplanes in various stages of completion at the works at Whitley, and of having their "points" explained to us by no less authorities than Maj. Green, the firm's chief engineer, and Mr. Lloyd, Armstrong-Whitworth's chief designer. If, therefore, we fail to do justice to the new machines, the fault will be entirely our own.
   Extremely careful streamlining is the main feature that impresses one when first confronted by the "Atalanta." The designers started by selecting the cantilever monoplane type of wing arrangement. They then decided to place the four engines in the leading edge of the wing, and to fair them into the wing surface with as gradual a change of section as possible. The fairings extend, in fact, very much farther aft on the wing surfaces than we recollect ever having seen before, and the result is a very gradual merging of the engine nacelles into the wing covering. The engines themselves have been fitted with drag-reducing rings, so that everything possible has been done to get rid of all avoidable drag. As an instance of the degree to which drag has been reduced, it may be mentioned that "interference drag," which is the term used to express the extra drag which often arises where two components of an aircraft join each other, is nil. In other words, the drag of the whole machine is the sum of the drags of its components, and not, as is very often the case, that sum plus something extra which represents interference. Much work has been done in the Armstrong-Whitworth wind tunnel (which is under the control of Mr. Reynolds) on models of the "Atalanta," and the low drag achieved is largely to be attributed to the wind tunnel work. In fact, as Air Marshal Sir John Higgins expressed it during our visit to Whitley, the wind tunnel can very quickly pay for itself on a job of this sort.
   Having disposed of their four engines in the leading edge of the wing, and having faired them as carefully as might be, the designers set to work on the undercarriage. In the average aeroplane the undercarriage accounts for anything from one-sixth to one-quarter of the total drag of the machine, and yet it is in use for a few minutes only at the beginning and end of each flight. If it could be suppressed altogether it would vastly increase the flying economy of the aircraft. Unfortunately that cannot be done at present. The alternative is to make it disappear into some other part of the aircraft when this is once well in the air. That, however, is accompanied not only by a good deal of weight, but also by considerable extra complication. In the A.W.XV the designers have adopted one of the most ingenious schemes we have seen. More will be said of the undercarriage next week. For the present it will suffice if we state that the whole of the telescopic leg, and two-thirds of the wheel axle, is housed inside the fuselage fairing. To make this possible and still retain a reasonably wide-wheel track, the wheel is carried overhung on the axle, the outer portion of which is working as a cantilever beam of some 2 ft. in length. Such an arrangement demands an axle of rather unusual construction, and in the A.W.XV the axles are very substantial members, in the form of double cones.
   On the fuselage corner is a fairing root, inside which the axle moves. On the axle is a corresponding fairing, secured to and moving with the axle. When the machine is standing on the ground, and the weight is on the wheels, the fuselage fairing root and the axle fairing are displaced relative to one another, so that there is a break in the fairing. As soon as the machine is in the air, however, and the load is off the wheels, the wheels and their axle fairings sink to the level of the root on the fuselage, and the whole thing is perfectly streamlined. The wheels themselves are partly enclosed in large streamline "spats," so that the total drag of the undercarriage should be very small indeed.
   The fuselage itself, although flat-sided, is of very good form and should have a low drag. The wing, which is in three sections, rests on top of the fuselage, and here, again, care has been taken to merge its top surface into the roof of the fuselage.
   Control surfaces of orthodox design are employed, Bristol-Frise balances being used on the ailerons, horn balances on elevator and rudder. In addition to its horn balance, the rudder is provided with a servo rudder, which forms the. trailing edge of the main rudder instead of being carried on outriggers away from the rudder. The arrangement of the servo rudder is unusual and interesting. When the "Atalanta" was first flown, it was discovered that for small angles of rudder movement the servo rudder gave too much control. The effect of this was that when the machine was flown "feet off" it had a tendency to yaw slightly, first to one side and then to the other. The manner in which Mr. Lloyd overcame the difficulty is highly ingenious.
   Our readers will probably be aware that the object of a servo rudder is to relieve the load on the pilot's rudder pedals. In large machines the "weight" of the rudder can become quite considerable, and for long flights would soon tire the pilot, even were he able to put the rudder over to its full extent for short periods. The servo rudder is an auxiliary surface, placed either on outriggers behind the main rudder, or hinged, as in this case, to the trailing edge of the rudder itself. The servo rudder moves in opposite sense to the main rudder, i.e., when it is desired to set the main rudder over to the left for a left-hand turn, the servo rudder is moved to the right. The small force on the servo rudder acts on a long "lever arm," owing to its distance from the main hinge, and thus has the power to overcome the main rudder, which, although its area is much greater, acts on a much shorter "lever arm."
   In the "Atalanta," the device adopted so successfully tor overcoming the "over-ruddering" at small angles is as follows :- The control wires from the rudder cranks (which are hinged to the rudder and not rigidly attached to it as in direct-operated rudders) to the servo rudder cranks are left just a little slack. Springs are inserted in the wires from the rudder cranks to the main rudder. For small angles of rudder movement, while the load on the main rudder is small, the springs are not extended, and the rudder is, in fact, operated direct. When the rudder angle increases to such an extent that the load on the rudder is sufficient to stretch the springs, the slack in the servo rudder wires is taken up and the servo rudder comes into operation. From then until the maximum rudder angle is reached, the operation is via the servo rudder.
   Further to smooth the rudder action and also to enable the machine to be flown without undue fatigue when one engine stops, a friction device is incorporated in the pilot's foot bar. The friction of this is adjustable, and can be set to be just sufficient to hold the rudder over against any engine combination.
   The internal accommodation of the "Atalanta" has been designed with a view to dividing the pay load approximately evenly between passengers and mails. For the African service there will be seating accommodation for nine passengers only, but the mail compartment, which is under and behind the pilot's cockpit, is large enough to accommodate something like a ton of mails.
   The cockpit is one of the roomiest we have ever seen in any heavier-than-air machine, and the view, due to the position of the cockpit in the extreme nose of the fuselage, and the absence of a central engine, is quite remarkably good. Right across the front of the cabin, in front of the pilots, is a large instrument board with a wonderful array of instruments. Those which require constant observation are placed at the port end of the instrument board, under the eyes of the chief pilot, while those which need less frequent reading are placed on the right, in front of the second pilot.
   The ailerons are operated by "kidney-shaped" wheels to give an unhindered view over the wheels, and the brake lever which operates the wheel brakes is placed centrally, within reach from both seats. Sideways movement of the brake lever applies the wheel brakes differentially.
   The four main throttle levers are placed centrally, also within reach of both pilots, while on the left side of the left seat are the four cocks which turn on and off the petrol at the carburettors. The cocks for turning the petrol on and off at the tanks are on the rear wall of the gangway over the mail compartment.
   The wireless operator occupies the space in the cockpit just behind the pilots.
   Two airspeed indicators are fitted, the pilot tubes being mounted side by side under the belly of the front portion of the fuselage. One would imagine that this position might be somewhat exposed, when the machine is being taxied on rough ground, or in long grass.
   Another mild criticism which occurs to us is the low clearance between the belly of the fuselage and the ground.
   The four "Double Mongoose" engines are, as already mentioned, mounted on the leading edge of the wings. They are spaced fairly widely apart, so that, should it at any time be decided to fit geared engines requiring larger airscrews, the space to do so is available. The petrol tanks are two in number, and are placed inside the leading edge of the wing. One tank is placed between the two engines which normally it supplies. The two systems are, however, connected together so that all four engines can be fed from either of the two tanks, which, by the way, are fitted with electric gauges for indicating the contents.
   The oil tanks are shaped to the form of the wing leading edge, and act at the same time as oil coolers. Hot oil crosses the tanks in pipes in which are drilled small holes, so that the hot oil is squirted in fine jets against the inside walls of the tanks and there cooled before being allowed to drain down into the bottom of the tanks.

Flight, November 1932

British Aircraft

Armstrong Whitworth
Whitley Aerodrome, near Coventry

The A.W.XV

   Designed specially for the African sections of the Imperial Airways' route from England to Capetown, the Armstrong-Whitworth type A.W.XV, or "Atalanta" class, is a four-engined cantilever monoplane fitted with four Armstrong-Siddeley Double Mongoose engines of 340 b.h.p. each. The very greatest care has been taken in the design of the machine to reduce drag to a minimum, and particularly has every effort been made to avoid interference drag. This has resulted in the placing of the four engines in the leading edge of the wing in a position which extensive wind tunnel tests have shown to be the most favourable, and in arranging the undercarriage in such a way that only the faired stub axles protrude, the rest of the undercarriage structure being housed inside the fuselage.
   Structurally the A.W.XV is of "mixed" construction in that its fuselage is of steel strip construction, partly covered with three-ply (the cabin portion). The cantilever monoplane wing has steel strip spars, but wooden ribs and plywood wing covering.
   The cabin of the A.W.XV is designed to accommodate nine passengers. This may appear at first sight a small number for a total engine power of 1,360 b.h.p., but the explanation is to be found in the fact that it is expected that mails will form a large proportion of the pay load, while the number of passengers is not expected, for the present at any rate, to be large.
   Normally the A.W.XV will carry fuel for a range of 400 miles (640 km.). The quantity of petrol and oil then carried is 1,600 lb. (729 kg.). If a smaller pay load is carried, the range is, of course, correspondingly increased. For example, with a pay load decreased to 3,500 lb. (1 590 kg.), the full tankage available gives a range of 600 miles (965 km.). For the 400 miles' range the pay load is 4,350 lb. (1 978 kg.). The cruising speed is in the neighbourhood of 120 m.p.h. (193 km./h.).
   The A.W.XV has an overall length of 71 ft. 0 in. (21,80 m.), and the wing span is 90 ft. (27,45 m ) . The total wing area is 1,285 sq. ft. (119,5 m.2). The certificate of airworthiness covers a gross weight of 20,000 lb. (9 100 kg.), but normally the machine will operate at a loaded weight of only 18,000 lb. (8 165 kg.). At the latter figure the wing loading becomes 14 lb./sq. ft. (68,35 kg./m.) and the power loading (normal) 13.25 lb./h.p.
   Detailed performance figures are not available, but the maximum speed is in the neighbourhood of 140 m.p.h. (225 km./h.) and the landing speed approximately 60 m.p.h. (97 km./h.).
   The considerable power reserve afforded by the four Double Mongoose engines is such that the machine should be able to maintain a relatively great height with any one of the four engines out of action, and thus freedom from forced landings should be ensured, an important consideration in a machine to be used in Africa.