Flight, May 1929

A.B.C. "ROBIN”
A New Cabin Single-Seater with "Scorpion" Engine

   SLOWLY but steadily the little "Scorpion" engine, designed and built by A.B.C. Motors, Ltd., of Walton-on-Thames, has been gaining popularity for use in low-power light 'planes, until at the present time it is to be found in most European countries, such as France, Germany, Austria, Italy, Poland, etc., not to mention places as far afield as Australia and New Zealand. At home, owing to the absence so far of machines specially designed for really low power, the market has been more restricted, but there is ample evidence that during the next few months quite a large number of machines will make their appearance, fitted with this popular power plant. Some of these will be exhibited at Olympia, while others may not be ready in time for the show but will come out later in the year. Already it is obvious that 1929 will in future be remembered as the year in which the cheap, low-power, single-seater, light 'plane was first seriously put on the market in England. The light 'plane of 30 h.p, or so is, of course, no novelty, and was produced in a variety of versions several years ago, but that was as a result of competitions intended to encourage the production of machines of this class. The light 'plane development in this country has followed lines very different from those originally contemplated, but we are now definitely returning to the real light 'plane via a somewhat roundabout road. This time, the revival of the type is not caused by competitions but as a result of the realisation of a demand which undoubtedly exists. That we in this country shall ever go to the same extreme as have the Germans, and design for a maximum of 20 h.p. or so is very unlikely. A spectacular performance is not required, but a useful turn of speed will without doubt be demanded, and the machine which is capable of making good over the ground a speed of 50 m.p.h. only against quite an average head wind will never, we feel sure, attain any degree of popularity in this country. Nor is there any very logical reason for such a machine. The difference in production cost between an engine developing a maximum of some 20 h.p. and one capable of close upon twice that power is relatively small. And as for running costs, the 40-h.p. machine win be very little more expensive to run than the 20 h.p. Yet the extra power is sufficient to enable a really useful performance to be attained.
   Mr. T. A. Dennis, managing director of A.B.C. Motors, has long realised the truth of what FLIGHT has been "preaching" concerning the need for a low-power single-seater machine, and becoming somewhat impatient with the slow progress made, he decided some time ago to enter the field of aircraft construction and to build machines to utilise the two types of engines which he was already producing: the 30-40-h.p. "Scorpion" and the 60-70 h.p. "Hornet." Looking around for a designer, Mr. Dennis secured the services of Mr. A. A. Fletcher, whose name will be familiar at least to the older readers of FLIGHT. "Tony" Fletcher was, back in the "dark ages," connected with the Martinsyde firm, and later with the L. & P. school at Hendon and the Central Aircraft Company at Kilburn and Northolt. Afterwards he joined the Whitehead company, and some years ago he went to Japan whence he was driven by the earthquake that wrought havoc with so much property. Mr. Fletcher has thus had a very long experience of aircraft design under varying conditions, and the first machine which he has designed for A.B.C. Motors is the little monoplane that forms the subject of the present notes and illustrations.
   The A.B.C. "Robin," as the first machine is to be named, is of more than ordinary interest in that it is the first low-power single-seater with conduite interieure to be produced in this country. The comfort of the pilot (who will in most instances be the owner) has been seriously studied, and in the "Robin" we have a machine which the owner can use all the year round without having to don special flying kit. The little cabin has its floor so low above the ground that the pilot (one feels he ought to be called the driver) can step in without the use of any built-on steps. Large windows on each side give a good view outwards and, as the windows can be opened, backwards during flight as well as forward while taking off and landing. In fact, as "Tony" Fletcher put it "if he is a sufficiently bad pilot he can watch his wheels until the actual moment of touching the ground." A V-shaped windscreen closes the cabin in front, and as the engine mounting is placed quite low, the view forward is also excellent. A hinged skylight forms the roof of the cabin, and can be raised to facilitate getting in and out, as well as forming an emergency exit through the roof in case of a crash. The cabin door itself is on the starboard side.
   Just behind the cabin, and with a hatch in the roof of the fuselage, is a luggage compartment large enough to take two large suitcases, so that the owner of a "Robin" will be able to go on a prolonged tour carrying with him changes of suits, linen, etc., in addition to the proverbial toothbrush.
   That the "Robin" will be a really useful and serviceable machine, and not merely a toy fit only for "tootling around an aerodrome," will be realised when we state that the estimated top speed is 105 m.p.h., while the machine should cruise very comfortably at 85 m.p.h., taking but 24 h.p. from the engine. The fuel consumption at cruising speed is but 2 gallons per hour or so, giving a mileage of more than 40 miles per gallon, which must be regarded as extremely economical. Taking the price of petrol as 1s. 7d. per gallon, the "Robin" would, in still air, do about 85 miles for a fuel cost of 3s. 2d., or, in other words, at the rate of less than 1/2 d. per mile! We believe that large operating companies reckon the fuel cost as being approximately one-third of the total running cost. Whether the same applies to a small machine like this is, perhaps, open to doubt, but assuming that it does, the total running cost of a "Robin" should be less than 1 1/2 d. per mile! Flying threatens to become cheaper than third-class railway travel. When small machines of this class become really popular, as they are bound to become very soon, the most serious item of expense is likely to be the insurance.

Structural Design

   Simplicity has been aimed at in planning the structural design of the "Robin," and wood is the material most extensively used, the employment of metal being confined to fittings and a few highly-stressed parts.
   The fuselage is a wooden "box" having four spruce longerons in the corners, and top, bottom and sides covered with a thin ply-wood veneer. Light formers are placed at intervals to retain the rectangular cross-sectional shape, these formers consisting of panels with very light spruce frames and thin ply-wood walls lightened by large cut-outs.
   Up to the cabin the fuselage is deep and the deck forms a roof except for the skylight already referred to. In front of the cabin, however, the deck drops considerably so as to provide the forward opening of the front window or windscreen. The lower longerons extend right up to the engine plate, but owing to the sudden drop in the deck in front of the cabin, the top longerons of the engine mounting are short separate lengths, stopping short at the front wall of the cabin. The engine plate itself is a multi-plywood former, and is attached to the longerons by rather neat steel fittings, which are simply short lengths of square-section steel tubes, split for a distance along the corners, and the free ends thus formed being turned out at right angles. In this way a flanged socket is formed without the use of welding. The diagonal bracing of the forward part of the fuselage, i.e., from cabin to engine plate, takes the form of steel tubes. The petrol tank, with a capacity of 8 gallons, is housed in the forward deck fairing, above the pilot's legs, a position which still gives sufficient "head" to enable direct gravity feed to the engine to be employed.
   The monoplane wing is built in two halves, hinged to the fuselage top corners, and braced each by a pair of struts arranged in the form of a Vee. The upper ends of the wing bracing struts are attached to the wing spars by steel straps, while at the fuselage end the two tubular struts have fork ends fitting over a sheet steel fitting bolted to the fuselage. As the lift is taken from this point, a steel strap bolted to the corner fitting runs right across the bottom of the fuselage to the corresponding fitting on the opposite side, while inside the fuselage a tie rod transmits the tension. This tie rod lies along a plane just above the top of the lower longerons, and as the external steel strap is just below the lower longerons, the whole structure is so stabilised that no twisting stresses are imposed upon the lower longerons.
   Across the top of the fuselage runs a Duralumin tube, which terminates at each end in a sheet steel fork, between the jaws of which a Duralumin block is swivelled. This block, which is vertical, has a horizontal hole drilled through it for the reception, in the case of the front spar fittings, of the locking pin used in connection with the wing-folding arrangement. This pin is rather in the form of a sort of key, as when the wing is erected and the pin pushed home and turned, it cannot accidentally come out of its socket, being held in by a coil spring and having a small projection or cam which fits into a corresponding recess in the block.
   The wing spars are of solid spruce, spindled out to an I-section. This form of spar is rather cheaper to make than a box-section spar, and as there is no difficulty in obtaining good spruce in such short lengths, the spindled I-section spar was chosen. The ribs are light girders, of square-section spruce, attached to the spars by corner strips. The rib flange strips are joined to the rib tie strips by thin three-ply wood gussets. The covering is doped fabric. The wing section used is a bi-convex one, R.A.F. 34, which has an almost stationary centre of pressure, and has been found to work excellently on full scale.
   The tail surfaces are also of wood construction, and perfectly normal, both in aerodynamic and structural design. The tail plane incidence can be adjusted while the machine is on the ground, the rear spar of the tail plane being supported from the fuselage by two threaded bolts, lock nuts on which locate the tail plane at the desired angle by being tightened up against a flat sheet steel fitting.
   The undercarriage is of the "split" type, the wheel on each side being supported on a tent axle, which is in turn located in a fore and aft direction by a radius tube, and in a vertical direction by the telescopic "leg," the upper end of which is bolted to the side of the fuselage. Endless rubber cord rings form the shock-absorbing medium, and the correct amount of springing is very simply determined by the number of rings employed. There is no damping device for checking bouncing. The wheel track obtained with this form of undercarriage is very wide, and there should be little danger of the machine turning over when taxying in a strong cross wind.
   The main dimensions, etc., are shown on the general, arrangement drawings. The estimated tare weight of the "Robin" is 415 lbs., but actually, the machine promises to come out quite a good deal lighter than that. The permissible gross weight for the certificate of airworthiness is 680 lbs., which will enable a fairly heavy pilot and a considerable load of luggage to be carried, in addition to fuel for 4 hrs. at cruising speed, or a range of about 340 miles.
   Assuming a maximum power of the "Scorpion" of 40 b.h.p., the power loading is 17 lbs./h.p., while on normal power (35) the power loading is 19-4 lbs./h.p. The wing loading is 7 lbs./sq. ft., which gives a landing speed of approximately 40 m.p.h.
   As the "Robin" is not yet finished, actual results of flying tests cannot be given, but the high placing of the wing, in conjunction with a wing section with stationary centre of pressure, should make for great stability. R.A.F. 34 shows no violent stall, and the machine should merely sink slowly when flying beyond the stall, and should show little tendency to go into a spin.