Flying Battleships

Maintenance of Short Sunderland is Major Operation for RAF'S Fleet Air Arm Crews

Millions of miles have been flown by British airmen during the past three years. And a major share of the mileage has been built up in the Coastal Command's venerable flying battleship — the Short Sunderland, which ranks as the world's most heavily armed flying boat. In all weather, in the Arctic Ocean and China seas, these giant ships have carried out reconnaissance patrols, have successfully attacked hundreds of U-boats, have shadowed enemy fleets and planes. Sunderland pilots have been publicized and eulogized in every British newspaper. But the men behind these flying boats get little credit for their unassuming, day-to-day routine which keeps every one of the thousands of Sunderland parts in working order. Humbly known as AC2s, they perform one of the biggest maintenance jobs of the war.

Sunderland Structure

The Short Sunderland is an all-metal boat seaplane of cantilever monoplane form. Designed for long-range, overseas reconnaissance, it is powered by four 915-hp Bristol Pegasus engines equipped with either two-pitch or constant-speed propellers — airscrews to the English mechanics. The hull is divided into an upper and a lower deck. A compartment on the lower deck, in the bow, incorporates a power-driven nose turret, bomb-aiming equipment, and mooring equipment. The turret is retractable to permit rapid, safe mooring of the mammoth flying boat in all waters.

The pi1ot's compartment, located aft of the bombing station on the upper deck, is fitted with side-by-side dual controls for two pilots and is reached by means of a ladder from the lower deck. Aft of the pilot's cabin, on the port side, is the radio operator's compartment while the navigator's seat and table and engineer's station are paired on the starboard side.

Bomb Stowage

Because of hydrodynamic problems inherent in the military flying boat, bombs cannot be carried in the belly of the plane as on land-based bombers. Instead, they are stowed beneath the main crew compartment located directly aft of the galley on the lower deck. The galley connects with the engineer's compartment by means of a fixed ladder. Aft of the crew's main compartment, approximately on a level with the upper deck floor, is a mid-gun platform from which the Sunderland's hull is effectively defended. The power-driven tail turret, similar to the nose mounting, is reached by means of a catwalk continuation of the main flooring. The Sunderland's hull framework is constructed from aluminum-coated light steel alloy and is covered with a skin of the same material. The framework consists of channel-section frames interconnected by longitudinal Z-section stiffeners. Each main plane has a single spar comprised of front and rear truss braced together and covered with light alloy sheeting.

Control Surfaces

Ailerons of the Friese type are fitted to the main planes and electrically-motivated flaps are set inboard of the ailerons. The empennage has a monoplane tail and single fin and trailing portions of the elevators and rudder are fabric-covered. Rudder and elevator controls on the Sunderland are duplicated throughout but the aileron circuit consists of a single run. Spring-loaded, oil-filled dashpots are fitted to the levers of the main center surfaces to prevent sag in the control runs owing to variations in temperature, humidity encountered in marine operations. Fixed tabs are fitted to the ailerons, trimming tabs to the rudder and elevators. A balance tab is also fitted to the rudder.

Power Plant

Four Bristol radials, mounted in monocoque nacelles faired into the leading edge of the main planes, are fitted with three-bladed variable pitch propellers on some Sunderlands, constant-speed props on others. Main engine controls are operated hydraulically. For checking longitudinal and transverse levels, nine datum blocks are mounted inside the hull. When checking the hull for rigging position, straight edges are placed longitudinally and transversely on the datum blocks and a spirit level is used for checking purposes. The hull is always in proper rigging position when these straight edges are horizontal. Main planes and tail surfaces are checked periodically for squareness and symmetry by means of rigging lines run between various datum marks on the hull and airfoils. In addition, dihedral and incidence settings are frequently checked with measuring boards, in the following manner.

Tail And Flaps

Appropriate incidence boards are paired on the locating marks painted on the upper surface of the airfoil, then a clinometer is placed longitudinally on each board and the incidence reading is registered. Transverse level of the tailplane is checked in a similar manner. Checking of flaps is more complex because of the intricacy of the operating mechanisms. The torque shafts are supported by ball races at the sides of the hull and at various points inside the main plane, and are connected to a worm gear at the forward end of the flap-operating screw. This screw consists of a rotating tubular barrel driven by the worm gear with special nuts attached internally. Servo motors attached to the flying controls need special attention and extreme care must be exercised in their coupling. Before the servo units are connected to their respective circuits, the main controls are locked in neutral position and all backlash and lost motion is eliminated from all couplings so that undue wear and backlash will not develop with normal flying operations. The mid-travel position of the servo motor is matched carefully to that of the mid-travel of the corresponding control surface.

Lubrication

Correct lubrication is at once the simplest and most important of all jobs in the Sunderland maintenance routine — if any job can rate as most important. The elevator, rudder, and aileron main hinges are packed with anti-freezing grease and require no further lubrication. Owing to the inaccessibility of trimming and servo tab hinges, grouped nipples are fitted inside the hull and are connected through a junction box to the various hinges by means of small bore copper tubing. Two nipples are fitted on the forward side of the lower torque tube sleeve for the rudder and similar sleeves for the elevators are fitted on the torque shaft across the hull. The main flap drive and screw gear box sumps are regularly recharged with anti-freezing oil to the level of the filler plugs.

De-icing boots also need careful checking and to obviate sticking the airfoil de-icing system is operated at regular intervals regardless of weather. Altogether, maintenance of the versatile Sunderland is no snap nor a job for the untrained handyman. Preliminary Air Ministry maintenance bulletins on this plane contain some 19,000 words and, admittedly, cover only the essentials of the maintenance job. Apart from studying countless diagrams, Sunderland mechanics read hundreds of thousands of technical words during their military careers, then memorize failings and characteristics of every one of the parts which pass through their hands as each Sunderland returns to base. It is a war job with few cheers, no citations — but one which daily builds Allied offensive successes.

This article was originally published in the June, 1943, issue of Air News magazine. vol 2, no 6, pp 23-25.
The original article includes 6 photos.
Photos credited to Black Star, British Information Services, Acme, British Combine.