Flight, March 1923

THE MARTIN U.S. NAVY "GUN-SPOTTER" MONOPLANE

   A NEW type of "gun-spotter" for directing from the air the fire of battleships has been produced under U.S. Navy specifications by the Glenn L. Martin Co., at Cleveland, Ohio, and has recently been undergoing a series of tests by a trial board. According to opinions of experts in the Bureau of Aeronautics, Navy Department, this machine gives every promise of success. We give below a few brief particulars, together with illustrations, of this machine, from our American contemporary Aviation.
   The Martin Observation, or "MO 1" as it is classed, is a three-seater cantilever monoplane, with an all-metal frame construction of aluminium alloy. It is equipped with a 375 h.p. Curtiss D12 engine. To meet the varied conditions under which a naval 'plane must operate, the MO is designed for interchangeable landing gear, which will make it adaptable for landing and taking off from the deck of an aeroplane carrier, or, in place of the wheels, pontoons may be substituted which will permit of landing and taking off from the water. It is also designed to be quickly assembled and dismantled for storage in a small space - a feature that will make it particularly suited to conditions on shipboard. The all-metal feature of construction ensures greater life and durability, and is at the same time a measure of economy in the repair and upkeep of the aircraft squadrons of the navy. This, with its attending advantages, is a direct result of investigation and research by the Bureau of Aeronautics. The design and specifications of the MO 1 were prepared by the Design Division of the Bureau of Aeronautics, Navy Department, under the direction of Commander J. C. Hunsaker. Its actual performance before the Navy Trial Board, as to high speed, low speed, climb, ceiling, load and manoeuvrability, exceeded in every case the specified requirements, whilst the complete machine is under the guaranteed weight, and over all the specified factors of safety. This is all the more creditable in view of the new features in its design and construction. Another important feature is that the horse-power required to fly level is quite low, so that the cruising radius at reduced speeds is considerably increased.
   In order to obtain good visibility below, which is the main requisite for spotting, the wings of the MO 1 are mounted on top of the fuselage. A modified U.S.A. 27 wing-curve is employed. The wings are built up in three sections - a centre section, and two outer extensions - and is believed to be the lightest metal type wing of its capacity so far developed and built in America. Under sand load before Navy experts it gave a safety factor of 8.
   The chord of the centre section is uniform - 11 ft. 6 ins. - but the outer extensions taper to a 6 ft. 6 in. chord at the tip. The main spars are of built-up Pratt-truss construction, the main longitudinal members being of 1/8 in. and 3/16 in. duralumin, formed in horse-shoe shape with a steel stiffener placed in the open part. The diagonal members are of 16- and 18-gauge duralumin tubing, all attachments being accomplished by means of heat-treated duralumin rivets and fish plates. The same material is used in the drag trussing. Wing ribs are of spruce, with ash nose strips and balsa-wood fillers in the compression members. Covering is of A-grade cotton fabric, fire-proofed, doped, enamelled and varnished.
   The outer sections are attached to the centre section by means of steel ball-and-socket hinges. In the centre section, on each side of the fuselage, are located the two petrol tanks. These are of spun aluminium, conical in shape, and provided with a novel arrangement of baffle plates. Each has a capacity of 57 gals., and they are so mounted in the wing that in the event of fire they may be released by either the pilot or observer in such a manner that they are thrown clear of the floats and tail surfaces. When released the only petrol remaining is that in the carburettor and a short length of pipe. The two ailerons, of the submerged balance type, are built up of channel section duralumin, and are controlled by cables running over pulleys within the wing structure.
   The rectangular fuselage is built up entirely of steel tubing, the joint and strut attachments being made by riveting and welding, and in the latter a new method, known as rosette welding, was effectively used at several points. For internal bracing the orthodox tie-rods are used. The engine, a 375 h.p. Curtiss D12, is mounted on ash bearers attached to the forward structure of the fuselage, and may readily be removed as a unit with the bearers attached. The engine is completely cowled in with sheet aluminium, and at the side of the engine, above the carburettor, a pressure fire extinguisher is provided, with a control from the pilot's and observor's cock-pits, which will when necessary produce an effective spray over all parts of the engine. Set in the cowling streamline below the engine is a welded aluminium oil tank, with fins for cooling the oil.
   An aluminium fire wall separates the engine section from the pilot's cockpit, which is directly back of and above the engine at the leading edge of the wing. This position provides excellent visibility for the pilot, as well as plenty of room and accessibility to controls. Immediately behind the pilot's cockpit and below the wing is the observer's cockpit, the position of which permits unobstructed vision to either side and below. An auxiliary set of controls and instruments enables the machine to be piloted from this cockpit. At the rear of the observer's cockpit, and connected by a runway, is the rear gunner's position, at the trailing edge of the wing. By means of the flexible gun here in combination with the other gun equipment, the machine is protected from all directions.
   The tail surfaces are of the internally braced type, and consist of a stabiliser, divided elevators, balanced rudder, and vertical fin. The elevators and rudder are entirely of channel, type, and horse-shoe section duralumin, with ribs of spruce. The stabiliser is adjustable, and may be trimmed during flight. The rudder and elevator controls are of the external type, and the stabiliser control cables run inside the fuselage.
   As previously stated, both land and water gear are provided. In the case of the former, the conventional two-wheel, wide-track type is employed. The struts, braces and axles are of tubular steel, with welded and riveted fittings and streamlined with sheet duralumin. The wheels have 36 by 8-in. tyres. This land type gear can be removed at the fuselage attachment fittings, and a gear, comprising two seaplane floats, with struts and braces, attached in place. So fitted the entire structural weight of the machine is 3,305 lbs., which is reduced to 2,920 lbs. in the case of the land gear.