Flight, April 1932

The Stieger ST. 4
Built by General Aircraft, Limited, of Croydon, and incorporating the Monospar System of Construction invented by Mr. H. J. Stieger, this machine is a four-seater fitted with two Pobjoy "R" Engines. That the Stieger System does give low structure weight appears to be proved by the fact that the ratio of gross weight to tare weight of the ST. 4 reaches the high value of 1-84 so that the machine carries as normal disposable load 84 per cent, of its own weight

   IN presenting to our readers an illustrated description of the Stieger ST-4 monoplane this week, we are faced with a slight difficulty arising out of the fact that, at the moment of writing, the machine is not entirely finished. In consequence it is not possible, for one thing, to publish photographs of the machine in its completed form, and, secondly, actual performance figures, etc., are not available. However, as the machine is certainly an unusually interesting one, both from the designer's and from the user's point of view, we have thought that the best procedure would be to deal with the ST.4 this week mainly from the structural aspect, leaving the subject of the practical or operational features for a subsequent article. In order, however, to give our readers a slightly better idea of how the ST.4 will appear when finished, we have had blocks made of some photographs of a scale model, and these will be found in the description which follows.
   Before turning our attention to the structural details of the ST.4 it may be of assistance to our readers if we mention quite briefly that the machine is designed as a four-seater twin-engined monoplane, ability to maintain height with one engine out of action having been kept prominently in view during design. By clean aerodynamic design, low structure weight, and light engines, it is thought that this desirable quality has been attained, although definite proof in the form of actual test flights must still be awaited. A cruising speed of approximately 115 m.p.h. (185 km./h.) has been aimed at.
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Power Plant

   The two Pobjoy "R" engines are mounted en parte a faux from the single wing spar. Two horizontal vees whose apices meet on spar and engine frame in such a way as to form a double wedge carry the engine mounting proper, which consists of steel tubes so arranged as to give triangulation. The whole engine attachment is free to swing laterally around the vertical hinges on the spar, but are prevented from doing so by the continuation of the leading edge which crosses the engine mounting framework.
   The petrol tanks are mounted inside the wing, and straddle the wing spar. Each tank has a capacity of 21 gallons, and is made of tinned sheet steel, so that in the event of a leak during operation, anyone who can use a soldering iron can make the repair. The tanks are actually in the middle wing portions, so that to permit folding of the wings a length of flexible tubing connects with the carburetters. This placing of the petrol tanks does not give sufficient ''head'' to produce gravity feed when the machine is climbing and accelerating, and the petrol system therefore includes two A.C. pumps, the system being so arranged that either pump can supply either or both engines. All the engine controls are of the Arens remote type.

Undercarriage

   By making use of the new telescopic legs marketed recently by Aircraft Components Company (Mr. G. H. Dowty), it has been possible to design a very neat undercarriage for the ST.4. The telescopic legs incorporate coil springs for the actual aircraft load, and an oleo gear for checking bouncing. A feature of these legs is the exceedingly compact arrangement, which makes it possible to get the whole shock-absorbing mechanism into a circular tube of very small outside diameter. Each undercarriage tripod is completed by an axle and a radius rod, and further to reduce drag the wheels are partly enclosed in "spats." The wheels are Dunlops (19 x 7) of the low-pressure type (wired), and Bendix brakes are fitted. The brakes are operated by a lever centrally placed in the cabin, between the two "joy sticks." The lever applies both brakes, and for steering on the ground use is made of the fact that the engines are placed on the wings. A castoring tail wheel is fitted, the tyre being a Goodyear, 12 x 5.
   It is not our intention to deal this week with the cabin accommodation of the ST.4 in great detail. It will suffice if we state that seats for two pilots are placed in front, with a complete set of dual controls placed side by side, while the passengers are situated behind the wing spar, far enough to have very ample leg room. The seats are arranged side by side, and there is space for yet another seat on the wing spar, so that the machine could quite well accommodate five.
   The flying controls are of orthodox type, with dual "joy sticks" and parallel-motion rudder pedals. The brake lever is, as already mentioned, located centrally between the two "joy sticks," where it is within reach from either seat. The instrument board contains Smith's instruments exclusively, but is not the standard Smith board, the width of the cabin being rather greater than in most machines and the available space therefore of somewhat different proportions. A sliding map tray disappears into the instrument board when not in use.
   It is not often that one has cause to complain that a cabin is too light. In the ST.4, however, the windows in side and roof are so large that it seems quite possible that tinted blinds will have to be provided, especially if the machine is to be used outside Great Britain.
   Owing to the position of the engines in relation to the fuselage, the view from the front seats is quite remarkably good. From the passengers' seats the wing obscures the view somewhat, but even so quite a good deal of ground can be seen, and, of course, the whole upper hemisphere.
   Access to the cabin is by doors formed of combined fuselage sides and roof, the hinges being along a diagonal line on the fuselage side. Steps are provided on the trailing edge of the wing roots, although the wing is not very high above the ground when the machine is standing with its tail down. The wing root structure has been made strong enough to withstand walking on. The cabin floor is of plywood, carried on transverse fuselage members of I-section, built up from two channels of duralumin placed back to back. Light fore-and-aft stringers give the floor the necessary stiffness in the spaces between floor bearers. A comfortable feature of the cabin is that the floor is perfectly flat, i.e., there is no curvature in it in any direction. All controls, etc., pass under the floor, which is left perfectly clear.

Flight, November 1932

British Aircraft at the Paris Aero Show

General Aircraft, Limited

   THAT one of the youngest of British aircraft firms should have achieved such success with its first machine as to be in a position to exhibit at a Paris Aero Show is cause for satisfaction. General Aircraft, Ltd., has been founded by Mr. H. J. Stieger and his associates to design and build machines incorporating the Stieger monospar principle of wing construction. The machine to be exhibited at Paris will be the first type to be produced by the firm, i.e., the Monospar ST.4. This is a four-seater aircraft intended for the private owner, and is fitted with two Pobjoy "R" engines.
   The fuselage is of unusual construction in that the rear portion has a sort of monospar backbone, torsionally stiffened by pyramid bracing. The cantilever wing has the Stieger monospar construction in which a single spar is in itself strong enough to carry the pure bending loads, but is enabled to withstand torsional stresses by a system of spiral wire bracing known as pyramid bracing. The whole machine is extremely light and has an excellent ratio of gross to tare weight.
   The cabin is so laid out that one passenger sits next to the pilot in front, while the other two occupy a side-by-side seat at the back. A large luggage locker is behind the rear seat. From the pilot's seat a quite remarkably good view is obtained in all directions which matter, the absence of an engine in the nose of the fuselage making this possible, as above photograph shows.
   The main data of the Monospar ST.4 are as follows :-
   Length o.a. 26 ft. 4 in. (8,03 m.)
   Wing span 40 ft. 2 in. (12,24 m.)
   Wing area 219 sq. ft. (20,35 m.2)
   Tare weight 1,300 lb. (590 kg.)
   Disposable load 1,000 lb. (454 kg.)
   Gross weight 2,300 lb. (1 043 kg.)
   Maximum speed 132 m.p.h. (212 km./h.)
   Cruising speed 112-115 m.p.h. (180-185 km./h.)
   Range 500 miles (805 km.)
   Landing speed 46 m.p.h. (74 km./h.)
   Take-off run (still air) 84 vards (77 m.) in 7 seconds.
   Initial rate of climb 900ft./min. (4,58 m./sec.)
   Maximum ceiling 18,000 ft. (5 486 m.)
   Landing run (still air) 110 yeards (101 m.)
   The machine is capable of maintaining height on one engine, and of carrying out normal manoeuvres with one engine out of action.

Flight, November 1932

British Aircraft

General Aircraft, Limited
Air Port of London, Croydon, Surrey

   AS General Aircraft, Ltd., are exhibiting one of their Monospar St.4 monoplanes at the Paris Aero Show, and this is dealt with elsewhere in this issue, it may be preferable to deal here with the Monospar principle of wing construction rather than with any particular machine in which this system is used.
   The single-spar wing was invented by Mr. H. J. Stieger, who is now managing director and chief designer of the Monospar Company and of General Aircraft, Ltd. Mr. Stieger is a Swiss by birth, but most of his aeronautical career has been spent in England, and the headquarters of his companies are in London.
   The general principle of the Stieger Monospar wing is simplicity itself, and consists, briefly explained, of a single main spar, which may be of metal or wood construction and of any form desired, such as a box spar, an I-section beam, or any other type which will support the bending loads arising from the lift on the wing. This single spar is designed to resist bending loads only, and would be very weak in torsion. The torsional strength of a Monospar wing is supplied by a system of bracing, which may be in the form of wires, cables, tie rods, or any other member capable of resisting tension loads. This system has been termed "pyramid" bracing because it is supported at leading and trailing edges on suitable struts, the ends of which form supports for the apices of the four wires meeting at that point. Where the wires cross over the spar they are attached to the top and bottom flanges of the spar by substantial steel wiring plates.
   The "pyramid bracing" of a Monospar wing may be pictured as two spirals wrapped around the wing in opposite directions, one set resisting loads due to a forward movement of the centre of pressure and the other a rearward movement.
   It will be realised that ultimately the inner ends of the pyramid bracing must be attached to the fuselage. Also that the plain pyramid bracing is not capable of taking drag and anti-drag loads. Separate wires parallel with the wing span can be used as drag and anti-drag members, but it is also possible to make the leading edge itself strong enough to serve either as the drag wire, and to have an anti-drag wire at the rear ends of the pyramid struts, or even to make the leading edge strong enough in comparison to take both drag and anti-drag loads.
   A considerably lighter wing structure is claimed to be possible by using Monospar construction, and some time ago a Monospar wing was built for a Fokker F.VII-3m. monoplane to test out the theory. The designers claim that on the Fokker wing a weight saving of 39 per cent, was made, which resulted in an increase of 37 per cent, in pay load. That large wing is still undergoing official tests for the British Air Ministry.

Flight, July 1934

NEW AEROPLANES IN KING'S CUP RACE

GENERAL AIRCRAFT "S.T.10"

   Being built on the single-spar principle, the General Aircraft machines have naturally created a large amount of interest since they were first placed upon the market. The "S.T.10 " is the latest version of the "S.T.4," which was described in Flight of April 22, 1932. In its essential features it remains the same - that is to say, the wing has a single girder spar built up from drawn channel-section strip steel and braced for torsional rigidity by a system of king posts and bracing wires. The fuselage consists of two portions. The front portion, carrying the cabin and the wing root fittings, is a normal steel tube, rigidly braced structure, but the rear portion is built up around a girder possessing much the same characteristics as the wing spar. The "S.T.10" has, however, two Pobjoy "Niagara" engines of 90 h.p. each, as against the Pobjoy "R." type of engine, and also there are certain structural alterations which have been proved desirable since the machine was produced. For example, the nose of the fuselage has been made cleaner by sweeping it up in a straight line over the windscreen. The fuselage itself has been increased slightly, both in width and in depth, although the drag has been kept down by clean design. The fuel, instead of being housed in the wing roots, is now in a tank between the spars under the floor of the cabin, thus giving a slightly better weight distribution.
   The flying controls have been altered considerably, and now consist of a central built-up column having a swivelling head, whereby the control may be used by either occupant of the front seats. The prototype of this machine had a screwed-in front locking pin, which had to be taken out before the wing could be folded. This has now been replaced by pins working on a lever, with a consequent decrease of the time taken for folding. The wheel brakes now fitted are the latest differentially operated Palmer hydraulic type, and we understand that excellent results have been obtained. The machine in the race will be flown by Flt. Lt. H. M. Schofield, and will not have a retractable undercarriage, although the fittings allow this to be substituted for the fixed undercarriage when desired. When this is done, the machine becomes the "S.T.11."

GENERAL AIRCRAFT "S.T.10."
Two POBJOY "NIAGARA" ENGINES, 90 H.P. EACH.

   Span 40 ft. 2 in. (12,2 m)
   Aspect ratio 7.4:3 to 1
   Wing area 217 sq. ft. (20,2 m')
   Gross weight 2,550 lb. (1 156,7 kg)
   Tare weight 1,470 lb. (666,8 kg)
   Wing loading 11.75 lb./sq. ft. (57.4 kg/m2)
   Power loading 14.16 lb./h.p. (6,4 kg/hp)

Flight, July 1934

MONOSPAR "S.T.10"
New Ideas About Fuselage Design Have Greatly Increased the Speed of This Latest Monospar Aeroplane and Enabled General Aircraft to Win the King's Cup

   AERODYNAMIC interference and air stream flow over the fuselage are two factors which make an aeroplane designer's life difficult. Now that high-speed commercial aeroplanes are being built which must, if they are to be worth anything to operators, get their performance from a comparatively low total horse-power, both subjects are receiving great attention throughout the world. Mr. H. J. Stieger, Managing Director of General Aircraft, Ltd., and the designer of the Monospar series of aeroplanes, formulated certain theories about body shape some little while ago and later these were corroborated by the findings of a Russian scientist. Encouraged by the success of his first machine, Mr. Stieger has incorporated these ideas in the "S.T.10," with the result that the performance was greater than the handicappers expected and the King's Cup came into his company's possession.
   It is interesting to learn that although the cross-section of the fuselage has been increased by two square feet in the "S.T.10," the altered shape of the nose giving a better entry, particularly underneath, has resulted in an increase in speed of over 10 m.p.h. Part of this increase is due to an alteration in the fore and aft attitude of the machine during flight. These facts make the machine doubly interesting because it is one of the select few built for the ordinary user wherein the designer's ideas have resulted in beating the handicappers in a race.
   The "S.T.10," as it flew in the race, has a fixed undercarriage, but nevertheless the two Pobjoy "Niagara" engines of only 90 h.p. each pull it through the air at 144 m.p.h. That speed, for only 45 h.p. per person, is no mean achievement, but the performance of the "S.T.11" will be even more spectacular. This will be the model with the retractable undercarriage and it will have an easy cruising speed of 150 m.p.h.
   The cabin of both models is very light and airy and the windows, which extend right to the rear behind the passengers' seats, completely obviate any feeling the occupants might otherwise have of being shut in. Being a two-engined machine the outlook for the pilot is admirable, and it is worth noting that both Flt. Lt. Schofield and Mr. Gardner, who was flying an "S.T.6," were able to get through the bad weather which held up other people.
   Structurally, the "S.T.10" does not differ very greatly from the "S.T.4," which was described in Flight for April 22, 1932.
   The Monospar arrangement of both the wings and the fuselage has, of course, been retained, but the latest model has a considerably deeper cabin with the passengers sitting higher in it, and the fuel is now carried in a tank beneath the floor. As much as possible of the whole structure is built up from steel tubes or drawn steel channels of various sections, and, naturally, the whole is assembled in jigs, so that replacement of any part is a simple matter. As our sketches show, the front portion of the fuselage is rather like a "hip-bath" of steel tubes. This forms the cabin and carries both the wings and the undercarriage. The rear portion is built up round a Monospar girder braced with kingpost and wire stays for torsional rigidity. The wing stubs are structures carrying the engines and the outer struts of the undercarriage, and form the fixed hinge for the outer sections of the wings, which fold inwards when the trailing edge flaps of the stubs are folded up.
   The wings themselves are mainly built of steel and are covered with doped fabric. The single spar is a deep girder built up from strip steel drawn into channel section and tapered heavily. It is a pure cantilever and is pin-jointed, for folding, to the spar which runs right through the fuselage and wing stubs. The leading edge is locked to the triangulated structure forming the front of the wing stub and carrying the engine mounting, when the wing is extended.
   Throughout the whole machine duralumin and aluminium are used wherever weight can be saved without a decrease in strength. For example, the whole of the cabin top is of light alloy, as is the structure over the rear portion of the fuselage which carries the fabric from behind the cabin to the tail.
   Welding is not used and all joints are either riveted or bolted. This, of course, means more work in some ways, but Mr. Stieger maintains that strength for strength a lighter structure can be built with joints of this kind.

Flight, January 1935

A NEW MONOSPAR
The "S.T.12" with two 130 h.p. "Gipsy Major" Engines: A New Type in the Well-known Monospar Range, Offering a Higher All-round Performance

   IN the normal course of events, when a manufacturer produces some piece of apparatus - a car, aeroplane or fountain pen, for example - people rapidly become used to it, and then want something better. Ever since they were first brought out, the General Aircraft Company's Monospar aeroplanes have been improved from time to time, but without radical departure from their original sound layout; this design was so obviously what was wanted that there has been no demand for alterations.
   The "S.T.12" has been produced in response to those pilots who wanted a Monospar type with a better climb, a higher cruising speed, and an outstanding performance on one engine.

The Engine Mountings

   A study of the table of performance figures on page 13 will show that these requirements have been met most successfully. The two 130 h.p. "Gipsy Major" engines have been mounted in a similar fashion to the Pobjoy engines previously used; that is to say, they are carried on steel-tube mountings projecting from the wing spars at a point in the stub wing just forward of the hinge which allows the wings to be folded. The undercarriage forms part of the mounting structure, and is, in effect, the retractable undercarriage of the "S.T 11," but permanently extended. It is covered with a "trouser" form of fairing.
   Already a considerable amount of flying has been done with this new type, and it has been found that not only are the take-off and climb fully up to expectations, but that the good control when flying on one engine - a characteristic for which the earlier Monospar models were famed - has been retained. In other respects the "S.T.12" is similar to the "S.T.10," which was fully described after it had won the King's Cup Race last year in such a spectacular fashion (see description in Flight of July 19, 1934).
   The front windscreen has been modified, and, instead of the flat panes of safety glass previously used, there is now a single large curved sheet of "Rhodoid," a form of celluloid; this should not only give the pilot a better outlook but should obviate both the collection of water and reflection from internal and external lights. Lightly loaded, the "S.T.12" has a climb of 1,800 ft. /min., and an amusing story is told of an occasion upon which it outclimbed three R.A.F. single-seater fighters, although it must be admitted that these were of an obsolescent type.
   The fitting of the “Gipsy Major” engines does not in any way alter the handiness of the wing folding, nor has it resulted in the necessity for changing the interior layout of the cabin. The latest throw-over type of wheel control is used.