Flight, November 1930

THE SPARTAN "ARROW"
A New Two-Seater with Good Climb

   WHEN, some years ago, Mr. O. E. Simmonds established his own aircraft firm, he did so on the strength of a very ingenious scheme whereby was attained a degree of interchangeability never before attempted. By this scheme, use was made of wings of symmetrical aerofoil section, which made it possible to use one spare wing in any of four positions: Top right-hand, bottom right-hand, top left-hand and bottom left-hand plane of a biplane. Struts, bracing wires and fittings were so designed that they could be attached to either side of the wing, and the same system of interchangeability was extended to ailerons, elevator flaps and rudder. For example, a portion of the elevator flap was identical with the horn-balanced rudder. Similarly, in the undercarriage the right-hand half could be turned around and used on the left-hand side and vice versa. One spare wing bracing wire could be used in any position in the wing structure.
   The reduction in the number of spare parts which a club or a private owner needed to stock was. therefore, very considerable, and it might have been thought that such a system would have a strong appeal. While this was undoubtedly so, the fact that the whole scheme was based upon the use of a symmetrical aerofoil section has, in practice, proved to be something of an obstacle to complete success. Certain machines using symmetrical wing sections have been found to be rather vicious in a stall, flicking into a spin as soon as the stalling angle is reached. It is by no means certain that this is an unavoidable feature of the symmetrical section wing, but a good deal of prejudice undoubtedly grew up, and as a result, there were many who looked with suspicion on a machine with such a wing section. The fitting of Handley Page automatic slots would cure any tendency a machine might have to go into a spin, but some purchasers prefer aircraft without slots, and it is for such as these that Spartan Aircraft, Ltd., have produced the new version of their machine, the "Arrow."
   Apart from the fact that in the Spartan "Arrow" a non-symmetrical wing section of greater lift has been adopted, the performance has been further improved by increasing the wing span and thus reducing the induced drag at low speeds. The combination has resulted in a considerable improvement in take-off and climb, both of which are very good in the “Arrow," as will be seen from the fact that, at aerobatic gross weight, and with the "Gipsy II" engine, the take-off run is only 60-80 yards, while the initial rate of climb is 830 ft./min. Even when the machine is loaded up to its somewhat remarkable "normal" gross weight of 1,750 lb., the take-off is only 100-130 yards, while the initial rate of climb is 700 ft./min.
   It might have been thought that the use of a non-symmetrical wing section would preclude the features of interchangeability, which the earlier Spartan machines possessed, but actually the majority of interchangeable parts have been retained in the "Arrow," although in a slightly modified form. To those who knew Mr. Simmonds when he first started manufacture, this will not come as a surprise, and the writer of these notes recollects that when discussing the subject some years ago with Mr. Simmonds the possibility was mentioned of using one spare wing, not of symmetrical section, in any of the four positions in a biplane. Thus, the arrangement adopted in the "Arrow" is not, as it were, an afterthought on the part of Mr. Simmonds. On the contrary, it was foreseen several years ago, and is, we believe we are right in saying, a part of the original patent.
   As far as the wings are concerned, the system adopted in the Spartan "Arrow" consists in slightly modifying the shape of a well-known aerofoil section in such a manner that the rear portion, from the rear spar to the trailing edge, is symmetrical, and in using a wing tip which forms a detachable unit, so that it can be placed at either end of the rectangular portion of the wing. By this means the spare or "key" wing can be used in either of the four positions on the biplane. To do so it is not, of course, "turned upside-down," as was the spare wing of the earlier Spartans, but the end which is nearest to the fuselage or top centre-section on one side of the machine becomes the outer end when the spare wing is used on the other side. The complication is just a little greater, but as the detachable wing tip is a small light structure costing, figuratively, a few shillings, this is no great drawback. That the interchangeability is not obtained in the "Arrow" at the expense of a great increase in structure weight seems to be proved by the fact that the tare weight is, with the "Gipsy II" engine, 965 lb., while the maximum permissible gross weight for the "Normal" category of the Certificate of Airworthiness is 1,750 lb. This gives a ratio of gross to tare weight of 1-814, which seems to point to very efficient structural design.

General Description

   The Spartan "Arrow" is, in its general design, a normal single-bay two-seater biplane, and a superficial examination does not reveal any very unorthodox features. When, however, one has the opportunity to see the machines being manufactured and assembled at the Weston works of Spartan Aircraft, Ltd., the ingenious way in which many details have been thought out becomes apparent.
   The fuselage is of all-wood construction, i.e., a light skeleton of longerons and struts, covered with three-ply planking. The "lines" of the fuselage are pleasing to the eye, and convey an impression of slimness which is a little surprising after one has sat in the cockpits and found them particularly roomy. This impression of roominess is probably a result of careful lay-out rather than of actual dimensions, but the comfort is there, and is enhanced by the fact that the coamings and windscreens have been so shaped that both cockpits are particularly free from draught. Large side doors make access to the cockpits very easy, and as there is a very large luggage locker behind the rear cockpit, the owner of an "Arrow" is in a position to tour in comfort carrying with him all necessities for quite a prolonged tour.
   The nose of the "Arrow" has been so designed that quite a variety of engines can be fitted, and the purchaser has a choice, including the "Gipsy I," the "Gipsy II," the “Cirrus-Hermes" or any other engine of similar type and of approximately the same power. Presumably it would be possible to install the inverted Gipsy ("Gipsy III") should a purchaser so desire, although as far as we are aware this engine has not so far been used in the "Arrow." The petrol system is of the usual gravity type, with a tank mounted in the top centre-section. This tank does not project beyond the wing contour of the centre-section, but is flush with it. A sketch on this page shows how the tank is neatly mounted and housed, and yet capable of being removed in a very short time.
   Normally a tank of 22 gallons (100 litres) capacity is fitted as standard, this giving a range at cruising speed of approximately 300 miles, more with certain engines and a little less with others. This range is usually sufficient, at any rate for touring in the British Isles and to the nearer portions of the Continent. If, however, an owner desires greater range for tours to the distant parts of Europe or beyond, another size tank has been standardised, with a capacity of 34 gallons (155 litres). With most of the power plants likely to be used, this capacity will give a range of from 400 miles to something over 500 miles, according to the engine and gross weight of the machine.
   The undercarriage of the "Arrow" is generally similar to the type used so successfully on the earlier Spartans. The two sides are interchangeable, and the telescopic leg is of long stroke, so as to give good shock-absorption qualities. The telescopic leg is attached at its upper end to an eyebolt through the front spar of the wing root, and a short strut runs from the top of the spar to the top longeron. The eyebolt through the spar has a sleeve around it which relieves the actual spar of compression stresses. In a minor crash, or a very bad landing, it is only the eyebolt which suffers, and possibly the steel plate on the under side of the spar, and repairs are confined to the replacement of the eyebolt and plate.

Wing Interchangeability

   It is in the wing structure of the "Arrow" that the main difference between this machine and earlier types is found. As already mentioned, the use of a non-symmetrical wing section has made the usual Spartan system inapplicable, but the difficulty has been overcome in a most ingenious way. The wing structure as such has not been altered, i.e. the spars are I-section spruce beams, and the ribs are Warren girders. But the trailing edge and wing tips have been designed with a view to making an interchange possible. The sketch on the next page, purely a diagrammatic one, indicates the system. The trailing edge of the wing section has been very slightly modified from the standard so as to make the portion from the rear spar to the trailing edge symmetrical. This trailing portion of the wing is made in two halves of identical length, and the two halves are secured to the rear spar by the same type of hinge, the exception being that the leading edge of the fixed trailing edge portion is flat and vertical, while the aileron leading edge is bevelled to allow of aileron movement. The "key" wing, exclusive of trailing portion, is of plain rectangular plan form, and the strut fittings are so designed that they can provide strut anchorage on top or bottom of the wing at will. If, for example, the spare wing is to replace a broken bottom right-hand wing, the two trailing edge portions are put on as shown in the sketch, and the wing tip (a separate unit) is attached to the right-hand end of the wing. The strut fittings are placed on top. If the wing is wanted, on the other hand, to replace a top left-hand (wing, the fittings are placed under the wing, the fixed trailing edge portion A is turned upside-down (hence the need for its being symmetrical) and bolted in place at the right-hand end of the wing (adjoining the top centre-section), and the aileron is turned upside-down and hinged to the left-hand end. The wing tip, also of symmetrical section, is also reversed and put on the left-hand tip. In this way the interchangeability is very nearly as thorough as in the Spartans, but achieved in a slightly different manner.
   In view of the increasing popularity of Handley Page automatic slots, provision is made in the "Arrow" for fitting these, if desired by the purchaser. The interchangeability is extended even to the slots, and Mr. Simmonds has evolved a very neat scheme for slotting the "Arrow." The entire slot mechanism is made up as a separate unit, in the manner shown in the sketch and photograph, on page 1208. To avoid one end of the slot opening before the other a torque tube is incorporated in the system, joined by cranks and links to the link members of the slot, as shown in the sketch. In the unslotted machine a plain leading edge unit is attached in the gap in the wing, and if the owner of an unslotted machine later decides to fit slots, he does not require new top planes, but merely removes the plain leading edge units and substitutes the slot units.
   Apart from the thought given to interchangeability, the designers of the Spartan "Arrow" have devoted much attention to the subject of maintenance and general ease of handling and upkeep in service. The machine is of robust construction, with few and simple parts, and such parts as are at all likely to need renewal periodically are designed for easy replacement. One-man handling has not been overlooked either, and the manner of locking the wings to the fuselage for folding has been so arranged that the pilot can, without assistance, perform the operation of folding. The catch which secures the lower wing to the fuselage is provided with a cable running forward where it can easily be reached, and the pilot can fold and spread the wings without leaving the vicinity of the undercarriage.

Various Engines

   It has been mentioned that a range of power plants are available for the "Arrow." In the table of data on page 1208, the figures relate to the machine with "Gipsy II" engine. If the "Gipsy I" is fitted, the tare weight is reduced to 950 lb. (432 kg.), the useful load becomes 608 lb. (276 kg.) in the "normal" and 418 lb. (190 kg.) in the "aerobatic" category, while the gross weight in the two categories remains as before, 1,750 lb. and 1,560 lb. respectively. At aerobatic gross weight the top speed is 100-102 m.p.h. (163 km./h.), the cruising speed 85-88 m.p.h. (140 km./h.), and the initial rato of climb 670 ft./min. (3-4 m./sec.). At normal gross weight the figures become : Top speed, 98-100 m.p.h. (160 km./h.). Cruising speed, 80-83 m.p.h. (133 km./h.). Initial rate of climb, 530 ft./min. (2-7 m/sec).
   If the "Cirrus-Hermes" engine is fitted, the tare weight becomes 975 lb. (443 kg.) and the useful load, 583 lb (265 kg.) and 393 lb. (180 kg.) in the normal and aerobatic categories respectively. The top speeds in the two categories are 101-103 m.p.h. (165 km./h.) and 103-105 m.p.h. (167 km./h) respectively, and the corresponding cruising speeds, 82-85 m.p.h. (135 km./h.) and 87-90 m.p.h. (142 km./h.). The corresponding rates of climb are 630 ft./min. (3-2 m./sec.) and 770 ft./min. (3-9 m./sec.).
   The ranges, with normal tank of 22 gallons (100 litres) capacity: “Gipsy I" engine, normal gross weight, 330 miles (530 km.). Aerobatic gross weight, 350 miles (565 km). "Cirrus-Hermes" engine, normal gross weight, 302 miles (485 km.). Aerobatic gross weight, 320 miles (515 km.).
   If desired, a larger tank of 34 gallons (155 litres) capacity can be fitted, the useful load being then, of course, correspondingly decreased. For all three types of engine, and in both categories of gross weight, the ranges are then increased by approximately 50 per cent.
   The price of the Spartan "Arrow" varies according to the engine fitted. With the "Cirrus III," the price is ?650. With "Gipsy I" it is ?675. With “Hermes" ?685 and with "Gipsy II" ?710.