Flight, January 1925
THE AEROMARINE EO SPORT FLYING-BOAT
A LITTLE while back the Aeromarine Plane and Motor Co., of Key Port, N.J., completed a small metal-hull flying-boat possessing several very interesting features, and we give below a description, together
with illustrations, of this machine.
This flying-boat has been specially designed for Mr. E. D. Osborn, who was formerly associated with Aeromarine Airways, to take the place of his older Aeromarine Model 44 flying-boat. While the new model EO was built around a smaller engine in order to reduce petrol consumption and give improved maintenance, it was required to carry the same useful load and have the same - if not better – performance as the older boat. It was also required that special consideration should be given to the question of safety, as this was rightly held to be an important factor should this model be put into production (as it probably will), not only for sport purposes, where the owner will also be the pilot, but for training work as well.
Special attention was to be given to the matter of balance with and without passengers, and with power on and off - a somewhat troublesome problem as far as flying-boats are concerned. Designs for the EO, embodying these requirements, were completed early last year, and the first tests were made, Mr. Osborn piloting himself, in June last. During these tests, the machine fulfilled all the requirements called for, especially as regards balance. Tests were made with pilot only, with one and with two passengers, and the balance was found satisfactory in each case, while the flying qualities seemed to be but little affected by the amount of load carried. In all tests the boat was on step in four or five seconds after opening of throttle, and with pilot only was off the water in 15 seconds, and with two passengers in 20 seconds. As a large-size air-screw was fitted, the engine speed did not exceed 1,390 r.p.m. (corresponding to 72-5 h.p., according to maker's power curve).
Tests over the speed course, with two up, showed a high speed of 73-5 m.p.h., and the speed at minimum throttle setting (1,010 r.p.m.) was 40-5 m.p.h.
The Aeromarine EO is a tractor biplane having a lower plane of considerably smaller chord staggered some way back of the top plane - below the trailing edge of the latter. The wings are of wood and fabric construction, while the engine, petrol and oil tanks, with all accessories, are mounted in an aluminium-alloy nacelle supported on struts above the hull, to which struts are also hinged the top wings, All nacelle, interplane and tail struts are of streamline-steel tubing.
Several advantages are claimed for the tractor-screw arrangement, most important of which being the question of safety. In this respect there is little danger of anything from the cockpit fouling the air-screw, while in the event of a crash the danger of the crew being pinned under the engine is considerably minimised. Incidentally, the position of the cockpit far back in the EO also makes for safety in the event of a crash. Another advantage is that in the case of any leakage in the petrol system there is less danger of the petrol falling on the hot exhaust or engine, and thus causing a fire.
Apart from the question of safety, however, the tractor-screw arrangement allows for better cooling and improved efficiency and reliability in consequence, while it also enables the pilot and passengers to be located at the centre of gravity, thus improving and simplifying the balance of the machine. In this connection, also, the tail surfaces get more pronounced downwash, a feature considered to be desirable in the case of flying-boats.
For the present a six-cylinder Anzani type 6A3 engine is fitted, as there are not yet any engines of this type made in America, but provision has been made in the design of the engine installation for the fitting of any radial engine of about the same weight and horse-power.
In general construction and design - apart from the arrangement of the power plant and the pronounced stagger - the model EO follows Aeromarine practice. The wings are built up of spruce I-section spars, with spruce trussed ribs and spruce drag struts. All the fittings, of which there are comparatively few, are of steel. Owing to the large stagger the drag bracing in the top plane is double as far as the strut fitting. The lower plane has a single spruce and plywood box spar, which is attached to a steel tube passing through the hull and interconnecting the port and starboard panels. The attachments at this tube and at the outer V-struts are designed so as to resist any torque that may be produced in the wing panel at high speeds or during a dive. Provision is also made in the strut attachments for adjusting the angle of incidence of the lower plane, either for varying the fore-and-aft balance of the machine or for balancing the propeller torque. Aeromarine No.2B wing section is employed for the top plane, and Aeromarine No.6 for the bottom.
Long and narrow ailerons are fitted to the top plane, and the control is so designed that pulleys are entirely eliminated, and all cables run in straight lines without any guides or fairleads. The control stick is connected by push-and-pull rod with the countershaft located at the rear of the seat, while the lever on the countershaft is connected by cables with the control horns on the elevators. The stick itself is mounted on a torque tube running fore and aft in the hull, and carrying the lever for operating the ailerons. The latter are provided with extension torque tubes, at the inner ends of which, alongside the engine nacelle, are crank levers, which in turn are connected by streamline push-and-pull rods to the torque tube in the hull. All the joints of this control gear are readily accessible, as is also the countershaft in the hull, which is protected by a hinged cover from interference with the passengers.
Wing-tip floats, made of 17s aluminium alloy and weighing 7 1/2 lbs. each, are fitted to the lower plane.
In designing the hull the main considerations were good seaworthiness and take off, and easy landing. Its lines are based in general on those of the Aeromarine model AMC hull (described in FLIGHT for Nov. 22, 1923), except that certain modifications and improvements gained from experience with the AMC hull have been incorporated. The most important of these is the pronounced V-bottom - the angle with the horizontal on each side of the keel being 18° at the step and 28° halfway from the step to the bow; from the step to the stern the angle is uniformly 18°. The rear portion of the hull has a slight upward sweep in order to bring the tail surfaces clear of the water and also to shorten the tail bracing. The large reserve of buoyancy provided and the upward sweep of the rear bottom give a rather short waterline, resulting in exceptional manoeuvrability. It has been found that the minimum turning radius in taxying is about its own span (38 ft.), and that in turning, instead of burying its outer wing-tip float in the water - as often occurs with flying boats - it banks itself towards the centre of the turn owing to the reaction effect on the sharp V-bottom.
In construction the hull is built up of 17s aluminium alloy sheets and stamped shapes. The framework consists of a series of frames and fore-and-aft stiffeners spaced to give the correct support to the sheet-metal covering, which is riveted to the framework. All the stiffeners and frames in the cockpit are of equal U-shape section, while the frames in the enclosed compartments are of Z section. The hull is divided into four watertight compartments, excluding the cockpit, and aluminium alloy tubes passing through these compartments carry the control cables without affecting their watertightness.
The seats are built in the hull permanently, and provide excellent comfort for pilot and two passengers. The pilot's seat is slightly forward of the passengers' seats. The view from the seats downwards and all round is excellent, although the top plane, naturally, somewhat restricts the view directly upwards.
In order to facilitate handling the boat on shore a watertight tube is provided across the hull just forward of the step to receive an axle and wheels, which may be seen in situ in the accompanying illustrations.
As previously stated, the complete power plant is assembled as one unit in a streamline nacelle. The latter is built up of sheet 17s alloy, and carries the fitting for the attachments of the nacelle support struts, for lifting the machine, and for the attachment of the upper wings. A combined petrol and oil tank is located between two bulkheads in this nacelle, one of which is situated at each of the wing spar attachments. The oil section of this tank is placed immediately behind the front bulkhead, and serves to isolate the engine from the petrol supply. The petrol tank is divided into two parts, a service and an emergency section, and has a capacity of 25 gals., or sufficient for more than four hours' flying at full throttle.
The engine itself is bolted to the front bulkhead of the nacelle, which is of 5/32-in. thick 17s alloy. The magneto, oil pump, and tachometer drive project inside the nacelle, where they are well protected from the elements. Short petrol piping under the nacelle connect the carburettor and petrol tank, while the cocks under the tank can be reached from the pilot's seat during flight. The tachometer, mounted on the side of the nacelle, and the oil gauge are both in full view of the pilot.
The following is a specification of the Aeromarine Model EO :-
Span (top) 38 ft.
Span (bottom) 34 ft.
Chord (top) 5 ft.
Chord (bottom) 3 ft.
O.A. length 25 ft. 2 ins.
Angle of incidence 3-5° - 5°
Dihedral (bottom) 3-5°
Area of main planes 266 sq. ft.
Area of tail plane 20-5 sq.ft.
Area of elevators 12-1 sq. ft.
Area of rudder 10 sq. ft.
Area of fin 5-5 sq.ft.
Stagger (leading edges) 3 ft. 5 ins.
Weight (empty) 1,040 lbs.
Useful load 710 lbs.
Weight loaded 1,750 lbs.
Weight per h.p 23-3 lbs.
Weight per sq. ft 6-58 lbs.