Flight, May 1926

THE BLACKBURN "SPRAT"
A Training Machine Convertible into Landplane or Seaplane

   IN external appearance, no less than in detail construction, the Blackburn ''Sprat'' bears a very strong ''family resemblance'' to the famous Blackburn "Swift" and "Dart" machines, which are already familiar to our readers. The "Sprat" has, however, been designed for use as a training machine, and consequently a power unit of considerably lower power is fitted. The engine fitted as standard is the Rolls-Royce "Falcon," of 275 h.p., although the makers point out that if desired, the machine can easily be adapted for other engines of approximately the same power. As illustrated in the accompanying photographs and general arrangement drawings, the "Sprat" is shown with the "Falcon.”
   In general design the Blackburn "Sprat" is of the normal biplane type, with top and bottom wings of equal span. A feature of the design is that the machine can be changed rapidly from a landplane to a seaplane and vice versa, the two types of undercarriage being so designed that each is a complete unit in itself, detachable at the fuselage and wing root joints. As a seaplane the "Sprat" is of the twin-float type, the float undercarriage being so designed that no cross tubes or struts are required. As a landplane the "Sprat" has an undercarriage similar in design to that of the well known "Swift." In the general arrangement drawings the machine is shown with the land undercarriage drawn in full lines and the float chassis in dotted lines.
   Controllability in the air and on the water, low landing speed, good view from both cockpits, and economy in operation and maintenance were the objects aimed at in the design of the Blackburn "Sprat." Concerning the manoeuvrability and general behaviour on the water, the form and dimensions of the floats are based upon those fitted to the Blackburn "Dart," which has been in use for a long time at the Blackburn training school at Brough. These floats have been tested in the tank in model form, and it was found that they ran very cleanly, had no tendency to “porpoising," and led to stable conditions when trimmed to take off. Full scale tests have confirmed these results. The "Dart" seaplane takes off with the pilot's hands off all controls. In order to facilitate manoeuvring at low speeds, the floats are fitted with water rudders. While on the subject of floats, it should be pointed out that the "Sprat" can be supplied with either wood floats or all-Duralumin floats. One of our sketches shows the metal floats, with some of their constructional details. It will be seen that the floats are of the vee-bottom rounded deck type with single steps, and the keelson extends from the keel to the deck, the chassis struts being attached to the keelson and bedding down on the keel. These sketches also indicate the attachments on the floats for wheeled trestles used in changing one type of undercarriage for the other, and as there are no cross tubes or axles in either undercarriage, each half of either can be detached or attached independently of the other half, which naturally facilitates the operation.
   Constructionally the "Sprat" follows the "Swift" and "Dart" types, the fuselage being built up in such a manner that components are grouped in units easy of access and removal with a minimum of disturbance of the main structure. There are three such units, the engine unit, the central unit or backbone (which is built entirely of steel tubing) to which all the main components are attached, and the tail portion, which is of wood construction. The engine unit is quickly detachable by means of four bolts, just forward of the fireproof bulkhead, and is so designed that when disconnected from the fuselage it forms its own engine bed and can be set down on a flat bench or on the floor.
   The accommodation for pupil and instructor (the latter occupying the aft cockpit) is such that not only is communication between them made as easy as possible, but both obtain an excellent view, as they are situated aft of the trailing edge of the top plane, which is cut away near the centre. In point of fact, the view is equal to that of the "Swift" torpedo carrier, which has proved exceptionally good for deck landing work, where a good view is essential. A neat feature is the placing of the instruments on the rear top spar, where they can be seen by both occupants.
   The biplane wings are of normal construction, and are designed to fold, so that in this state the machine occupies a very small space.
   The land undercarriage, as already mentioned, is similar to that of the "Swift," and has two independent halves, unconnected by axles. The shock-absorbing gear is in the form of rubber blocks in compression, a form which is becoming increasingly popular, and which has been used by the Blackburn company for several years. With the sloping centre-section struts typical of the torpedo plane, the chassis attachments are situated a fair distance out from the centre, thus giving a very wide wheel track and making the machine particularly stable on the ground. Moreover, the absence of any cross-members renders the machine much less likely to "nose over" if a landing in tall grass or corn should be necessary. In the illustrations the grapples used for deck landing may be seen.

Item Weights

   We have been able to obtain from the Blackburn company a very full schedule of item weights, which should be of interest. The weights are as follows :-
   Structure Seaplane Landplane lbs.
   Float or land undercarriage 380 130
   Fuselage (seats, floors and flotation bags) 290 290
   Chassis 80 -
   Engine mountings, cowls, etc. 80 80
   Tail skid 12 12
   Flight controls 65 65
   Main planes 480 480
   Tail unit 70 70
   Total structure weight 1,457 (41 per cent.) 1,127 (35 per cent.)
   The weight of the power unit is, of course, the same for the seaplane and the landplane, and is made up as follows: Engine (dry), 725 lbs.; water in engine, 25 lbs.; radiator and water, 160 lbs.; propeller, 50 lbs.; engine accessories and piping, 90 lbs.; gas starter, 65 lbs. Total weight of engine unit, 1,115 lbs. This represents a percentage weight of 31-4 for the seaplane and 34-65 for the landplane.
   The items connected with the fuel, tanks, etc., are also identical for both types, and are as follows: Fuel (56 gallons), 425 lbs.; oil (4 gallons), 40 lbs.; water (1 1/2 gallons), 15 lbs.; fuel tanks, 60 lbs.; oil tank, 10 lbs.; water tanks, 6 lbs. Total, 556 lbs., which represents 15-7 per cent, in the case of the seaplane and 17-25 per cent, for the landplane.
   The following military load is carried in both types of machine: Crew (2), 360 lbs.; instruments, 55 lbs.; Very pistol and 12 cartridges, 7 lbs. Total, 422 lbs., or 11-9 per cent, of the total loaded weight of the seaplane and 13-1 per cent, of the landplane. The total loaded weight of the seaplane is 3,500 lbs., and of the landplane 3,220 lbs.

Performance

   Following are the estimated performances of the machine in its two forms :-
   Seaplane Landplane
   Top speed at sea level 98 knots 100 knots
   Top speed at 5,000 ft. 95 knots 100 knots
   Climb (sea level) 937 ft./min. 1,100 ft./min.
   Time to 5,000 ft. 6 1/2 mins 5 1/2 mins.
   Time to 10,000 ft. 16 mins. 13 mins.
   Service ceiling 15,700 ft. 17,500 ft.
   Landing speed 40 knots 39 knots