Model aircraft, driven by petrol engines, are generally considered to be out of the scope of the average aeromodellist, and the construction of such a machine is rarely attempted, chiefly because it appears, apart from it having to be considerably larger than the general run of models, the weight has to be expressed in pounds instead of ounces, and the enthusiastic modellist, who is striving to knock half-an-ounce off his latest bus, will appreciate the magnitude of the difficulties that are presented by an un-avoidable additional one pound, not to mention the several additional pounds that are involved in a petrol driven machine.
The wonderful improvements that have been made in miniature internal-combustion engines in the last few years, (for which we have to thank the power boat men) have, given us a power unit, developing nearly half horse-power, the weight of which can be held down as low as 2 lb., excluding the flywheel, which, of course, is unnecessary in aircraft.
The advantages of the I.C. engine applied to model aircraft are numerous, the more important being (1) the time of flight can be increased out of all proportion to the size of the machine, (2) the pull of the propeller is uniform throughout the complete length of run. this not being the case with rubber or compressed air plants, the power of which is constantly decreasing, and (3) the increase in speed permits the machine to keep head on to a wind which would send its smaller brother either scurrying round in a semi-circle, or floating gracefully backwards.
The principal disadvantages are: (1) considerably more space is necessary for flying, (2) the initial cost of building is much greater, and (3) the damage resulting from a crash is far more serious than in a machine weighing only a few ounces.
In the opinion of the writer, the advantages easily over-shadow the disadvantages, and this opinion resulted in the design and construction of the ” Hawk-Special,” described and illustrated in the following notes. With regard to the cost of building. this machine stands in the neighbourhood of go, but there is no doubt that this price could be cut down considerably, as several alterations were made during construction, each necessitating further outlay.
The writer is looking forward to the day when power-driven machines will be the rule rather than the exception, and will willingly give all the assistance in his power to any-one intending to build a machine of this type.
The following is a description of the Dowsett ” Hawk-Special ” monoplane, general arrangement drawings of which are shown in Fig. 1.
The fuselage is of the usual box type, the longerons and cross-members being of 1/2 in. by 3/4 in. birch, and the formers of 1/16 in. 3-ply. Formers and cross-members are placed at intervals of 8 in. throughout the length of the fuselage, and all joints are glued and screwed, with the exception of the vertical members at the nose of the machine. To give these sufficient strength to carry the engine bearings, they are of 1/2 in. sq. birch, halved, glued and bolted to the ends of the longerons. The second and fourth sections of the fuselage are diagonally braced with 1/2 in. by 1/4 in. birch, and although originally the remaining sections were braced with wire, this was found to be unnecessary and was sub-sequently removed.
The wing housing, which is built direct on to the fuselage, is the next point for consideration. This was originally designed with the petrol tank built in, but during tests with a float controlled carburettor, it was found necessary to move the tank down into the nose of the machine to improve the feed.
The housing is constructed with sides of 1/4 in. birch, 9 in. long, and of airfoil section, with cross-members of 1/4 sq. spruce. Two eyebolts are fixed through each side, such that when in position, the eyebolts are level with the spars in the main planes, and the leading edge of the housing in a straight line with the leading edge of the planes. Four streamline struts fix the housing to the fuselage, the upper and lower surfaces then being covered with 1 nm. 3-ply.
As will be seen in the illustrations, the undercarriage is practically a replica of that fitted to the Westland Widgeon machine, but is of rather novel construction. The two main members, incorporating the shock absorbers, were built up from two thin cycle pumps, which were treated in the following manner: The plunger rod was first removed, and the leather cup-washer reversed. A length (2 and 1/2 in.) was then cut 0ff the barrel, connection end, and the open end tapped to take the original bush. A coil spring was inserted, followed by the plunger, and the bush screwed home. The connection hole of the pump was sealed by means of a bolt, which also secures a hinged bracket to the top for attachment to the fuselage. It will now be clearly seen that when the spring is suddenly compressed, the air passes freely out of the barrel past the reversed cup-washer, but its efforts to return normal are checked by the suction, which was found to be s0 great that it was necessary to drill a small hole to permit of a slow inlet of air. Fig. 2 shows a section through the completed shock-absorber.
The cross-members of the undercarriage are in two sections, the top half, jointed to the fuselage, being a piece of steel into the end of which is fitted a length of steel rod, 3/16 in. in diameter. This rod is attached to the shock-absorber leg by means of a flexible joint; where it is bent almost to the horizontal, and continues for a further 1 and 1/2 in., this section being the axle on which the wheel is mounted. The undercarriage is completed by the two rods running back from the extremities of the shock-absorber legs to the lower longerons of the fuselage, to which they are firmly bolted.
The wheels are cut nut of 9-ply wood, and are 8 in. in diameter, a brass bush being fixed into the centre hole as a bearing for the axle.
The main plane is in two parts, each 4 ft. by 1 ft. A section through the wing is shown in Fig. 3. The ribs are of 1/4 in. white wood, and are spaced at intervals of 6 in.; the two end ribs of each section, after being fretted out, were faced with 1 mm. 3-ply, this greatly increasing the strength, and at the same time facilitating the glueing down of the silk covering.
The three main spars, A, B and C, are of 1/4 in. sq. spruce, and the leading and trailing edges of 3/8 in. by 1/8 in. birch. The wing is diagonally braced from the spar A to the trailing edge by means of strips of birch 1/4 in. by 1/16 in., these strips being glued and pinned in alternate directions between each pair of ribs.
The ends of the V struts are attached to the wing by means of eyebolts passing through the leading and trailing edges, the lower ends of the struts running together into a metal cup. Attached to this cup is a small brass lip, which fits between two eyebolts in the lower longerons of the fuselage, and is locked in position by means of a small steel wire clip.
The wing is attached to the housing by means of two steel wire hooks, which are bound to, and project from the spars marked A and C in the wing section (Fig. 3), these hooks fitting into the eyebolts on the side of the housing.
A section through the tail plane is shown in Fig. 4. The ribs are of 1/16 in. 3-ply, the spars, leading and trailing edges of birch.
The tail plane is fitted between the upper and lower longerons of the fuselage, the fabric covering of the latter finishing 6 in. from the stern post to permit the adjustment of the incidence angle, which is accomplished in the following manner. The centre of the rear spar of the tail plane is hinged to a block screwed to the upper longerons of the fuselage, the centre section of the wing behind the rear spar being cut away to allow the trailing edge to overlap the stern post. The leading edge is fitted with a small brass hush through which passes a length of threaded rod. A flat brass plate is screwed across the upper longerons of the fuselage, the end of the rod passing through a hole in the centre, and having a fixed bush on the underside of the plate, and a knurled knob on the upper. By turning the knob, the leading edge of the plane is either lifted or lowered as desired.
The fin is of steel wire construction, and is fiat in section. It is attached by means of a straight length of wire (secured to the fin by binding and soldering) which passes through a small copper tube bound to the stern post of the fuselage. At the front of the fin is a curved wire extension which passes through a small terminal also attached to the fuselage. The fin is then adjustable on the balanced principle, and can be locked in any desired position by means of the screw in the terminal. On preliminary tests, the fin showed signs of serious flutter, and it was therefore necessary to brace it by means of wires from the top to the ends of the tail plane. Main planes, tail plane and fin are all covered with jap silk and coated with aluminium dope.
The engine is an air-cooled two-stroke, having a bore and stroke of 1 and 1/8 in. The cylinder is machined in one piece, the top being drilled and tapped to take a standard size rotary engine sparking plug. The piston has a deflector head, and is fitted with two rings, spaced 1/8 in. apart. The crankcase is of aluminium, and is made in two parts, bolted together, the front section carrying the single crankshaft bearing. On either side of the crankcase is a projecting fin, the engine being mounted to the fuselage by bolting each of these fins between two steel brackets, the brackets in turn being bolted to the vertical members at the nose of the fuselage. The crankshaft which is balanced, carries a bush, from which project a number of steel pins corresponding with holes in an aluminium bush set in the propeller boss, the propeller being secured by means of a nut screwed on the end of the crankshaft. The make and break which is the usual brush and pin contact, is immediately behind the propeller boss, the fitting carrying the brush being attached in a manner which permits of it being rotated, thus making an efficient ignition adjustment.
The ignition is in the form of a trembler coil, and a standard size pocket torch battery. The coil, which was purchased from Messrs. Carnage’s, measures 2 and 1/2 in. by 1 and 1/2 in. by I in., and gives an efficient spark from a small input. The coil and battery are located on a hinged flap of 3-ply wood on the under side of the fuselage, slightly behind the main plane. Leads to plug, etc., are carried through the fuselage, and come out immediately behind the engine.
As has been previously stated, the original arrangement for petrol supply was to have the tank in the wing housing, the carburettor being at that time merely an atomizing tube with a needle valve operated by gravity only. On test, however, the vibration seriously upset the flow of petrol from the tank, and air bubbles passed into the tube, causing a miss-fire which, with only a light propeller as flywheel, frequently caused the engine to stop altogether. This fault was remedied by fitting a miniature float controlled carburettor bolted direct to the cylinder, and moving the tank to a position immediately above the float chamber.
The petrol tank, built up from sheet brass, measures 3 in. by 1 and 1/2 in. by 1 in. (this holding sufficient petrol to run the engine full out for about 25 minutes) and is fixed to the fuselage by means of brass brackets soldered to the tank, and screwed to the longerons.
The engine cowling is built up from l mm 3-ply wood and aluminium, and four screws secure it to the cross-members at the front of the fuselage.
A coarse pitch four-bladed propeller, of 18 in. diameter was originally fitted to the machine, the maximum speed of the engine with this being about 900 r.p.m. A 24 in. two bladed propeller of fairly fine pitch was then substituted, which increased the engine speed to 1,250 r,p.m., and considerably improved the all-round performance of the machine.
| SPECIFICATION | |
|---|---|
| Type | parasol monoplane |
| Power | 2 stroke IC engine rated 1/2 HP at 1200 r.p.m. |
| Wingspan | 8 ft. 4 in. |
| Chord | 12 in. |
| Length | 4 ft. 11 in. |
| Total wing area | 1306 sq. in. |
| Fin area | 79,6 sq. in. |
| Total weight | 7 lbs |
| Wing loading | 12,4 oz/sq.ft. |
| Capacity | 25 minutes |
Resources:
- FLIGHT magazine, issue 28.3.1930
- internet www.antiquemodelaircraft.co.uk