Read an Excerpt
The Fighting FLYING BOATA HISTORY OF THE MARTIN PBM MARINER
By RICHARD ALDEN HOFFMAN
Naval Institute PressCopyright © 2004 Richard Alden Hoffman
All right reserved.
Chapter OneDESIGN, DEVELOPMENT, AND PRODUCTION
Development of the Glenn L. Martin Company's Mariner flying boat began in the 1930s. In 1935 the U.S. Navy designated its new Catalina patrol plane a "patrol bomber" and promulgated a requirement for follow-on heavily armed flying boats that would be capable of bombing naval shore bases as well as performing the traditional naval missions of locating and attacking warships. Consolidated, Sikorsky, and the Martin Company submitted four-engine designs to meet this requirement. These aircraft became known in the popular press as the "flying battleships" and the "flying dreadnoughts."
In 1936 contracts were awarded to Consolidated and Sikorsky to build prototypes: the XPB2Y-1 and the XPBS-1, respectively. Both the XPB2Y-1 and the XPBS-1 were flown in late 1937. The XPB2Y-1 was the one selected for production and became known in service as the PB2Y Coronado. After the competition, the XPBS-1 was used by the navy as a transport and made several high-priority trips to the South Pacific in the early days of World War II. It was wrecked in a landing accident at Alameda, California, on 30 June 1942 with Adm. Chester W. Nimitz, commander in chief of the Pacific Fleet, on board.
Even though the Glenn L. Martin Company's design model 160 was not awarded a development contract in the four-engine battleplane competition, Martin continued to explore the design with company funds. As part of its research, Martin constructed a one-quarter scale, man-carrying flying model of the model 160. Even though the model 160 project was eventually canceled, this man-carrying model is an important element of Martin's PBM story.
Four-engine patrol bombers were expensive. A single PB2Y cost as much as three PBY Catalinas. In the austere economy of 1937, cost was of great significance to navy procurement officials. Therefore the navy invited proposals for a two-engine version of the battleplane.
The Glenn L. Martin Company proposed its design model 162, a new flying boat that used two Wright R-2600 Cyclone engines. The model 162 offered speed, load, and armament advantages over the PBY, and it cost much less than a four-engine aircraft.
On 30 June 1937 Martin received a development contract for a single prototype, designated XPBM-1. To speed development and improve its position for a production contract, Martin converted the one-quarter scale model of the model 160 into a three-eighths scale model of the model 162. Framed in wood and covered with aluminum sheeting, the model 162A "Tadpole Clipper" was powered by a Martin-modified Louis Chevrolet light-plane engine driving two propellers by means of belts. Ready in mid-November 1937, extensive testing of the model 162A confirmed Martin's performance projections for the XPBM-1.
A possible competitor was Consolidated's model 31, developed with company funds. It was an innovative and promising design, using the powerful Wright R-3350 engines, a low-drag Davis wing, and self-contained tricycle beaching gear. Designed as a commercial transport to carry up to fifty-two passengers, Consolidated's model 31 had the potential to be a fine navy patrol bomber. The model 31 flew on 5 May 1939 but demonstrated poor hydrodynamics characteristics at high gross weight. After much modification, it was acquired by the navy in April 1942 as the XP4Y-1 "Corregidor."
Martin's Tadpole Clipper testing was very timely. In spring 1937 Maj. Reuben Fleet, the colorful president of Consolidated, went to Washington, D.C. He stormed into the U.S. Navy's procurement offices, disputing Martin's performance claims for the model 162 and threatening political action if Consolidated lost the next navy contract.
Supported by the Tadpole Clipper test data, the navy awarded Martin a production contract on 30 December 1937. The navy covered its bets by the politically astute move of splitting the 1937 procurement: twenty-one PBM-1s were ordered from Martin for $5.1 million and thirty-three PBYs were ordered from Consolidated for $4.5 million.
The XPBM-1, Bureau Number (BuNo) 0796, first flew on 13 February 1939. The XPBM-1 had a 118 foot gull wing and twin tails. It was powered by two 1600 horsepower Wright R-2600-6 engines, it was double-decked forward. Aft of the cockpit, the flight deck had standup room for a navigator, a radio operator (and later a radar operator), and a flight engineer. A complete galley was located beneath the flight deck. It had powered bow and dorsal turrets with single 50-caliber guns and 50-caliber waist guns in unpowered gimbal mounts. Enclosed bomb bays were located in extended engine nacelles, each with a capacity for six 500-pound bombs. The wing floats were located well inboard from the wingtips and retracted inboard for flight. The XPBM-1 was a sturdy and businesslike design that served the navy well for many years.
Flight tests of the XPBM-1 went well. Major faults uncovered were poor directional stability and the tendency to "porpoise" (develop hydrodynamically induced oscillations) at high water speeds. The porpoising tendency was reduced by moving the main step of the hull slightly aft and directional stability was improved by giving the horizontal stabilizer dihedral. The vertical stabilizers remained fixed at 90 degrees to the horizontal stabilizers, giving the PBM a unique cranked tail appearance.
The XPBM-1 had a remarkably long career for an X-model aircraft. It served in a training role with patrol squadrons VP-55 and VP-56, received a major overhaul in Norfolk in 1941, and was assigned to the aircraft armament units at Norfolk, Virginia, and Patuxent River, Maryland, until stricken in May 1944.
Even as the XPBM-1 prototype flight test continued during 1939, the Martin Company began series production of the service model PBM-1. Thanks to the performance data verification by the Tadpole Clipper, which had allowed the early production decision, deliveries of the production PBM-1s (Bureau Numbers 1246 through 1266) to fleet squadrons began in October 1940, only nineteen months after the first flight of the prototype. Deliveries were complete by May 1941. The PBM-1s were almost immediately deployed to the North Atlantic for neutrality patrol missions.
The second production PBM-1, BuNo 1247, was redesignated XPBM-2 and modified to test the concept of achieving extra-long range by catapult launching of an overloaded flying boat. Fuel capacity was increased from 2,700 to 4,815 gallons. The structure was reinforced and fittings were added for catapult launching. The Baldwin Locomotive Works built the XH Mark 3 catapult that was installed on a catapult lighter (AVC-1) intended as a mobile base. Specifications for AVC-1 have not yet been located, but extrapolation of its size from two photographs indicate that it was approximately three hundred feet long with a seventy foot beam. Successful tests of the catapult with dead loads of up to sixty thousand pounds were completed in 1942. However, the requirement for the extra long-range flying boat was canceled and the XPBM-2 remained at the Naval Air Material Center in Philadelphia as a static test article until it was stricken from the navy inventory in June 1944.
As part of President Franklin D. Roosevelt's massive rearmament program, an order for 379 PBM-3s was placed in November 1940. The PBM-3 displayed significant differences from the PBM-1. The retractable wing floats were replaced with fixed, strut-mounted floats, the nacelles were lengthened to increase the bomb-bay capacity, and the size of the aft hatches was increased. The gimbaled waist guns were replaced with 50-caliber guns that were arranged to fire front the aft hatches. Armor was added for crew protection and Wright R-2600-12 engines of seventeen hundred horsepower were installed. The basic airframe configuration of the PBM-3 remained unchanged for the rest of PBM production.
The pilot's handbook for the PBM-3D provided an excellent description of that aircraft (and the later PBM-5):
Maximum speed was about 215 miles per hour, with a service ceiling of approximately 20,000 feet; fuel capacity, with droppable bomb bay tanks was 3,496 gallons, and range was 2,300 to 2,400 miles. Maximum weight was about 58,000 pounds.
Armament consisted of eight 50-caliber type M-2 machine guns: two each in the bow, deck, and tail turrets, and one mounted at each of the two waist hatches. The turrets were all electro-hydraulic, controlled with Mark VIII electric gun sights. The waist guns, also 50-caliber M-2s, had hydraulic controls or could be used manually. A bomb load of 4,000 pounds (bombs, depth charges, or mines) was carried in bomb bays located at the aft end of the engine nacelles. Also, two Mark XIII torpedoes could be carried, one under each wing, between the hull and engine nacelle.
The fuselage of the plane was divided into five watertight compartments by bulkheads provided with watertight doors. (1) The bow compartment contained the bow turret and the bombardier's station. Aft of the bow compartment to about three feet forward of the leading edge of the wing, the fuselage divided into two floor levels. The upper floor was the flight deck, which contained the raised pilot's control station at the forward end, with a seat for the pilot on the left and for the copilot on the right; a navigator's station on the left behind the pilot; a seat between the pilot and navigator for the radar operator; a radio operator's station on the right behind the copilot and the flight engineer's station at the aft end of the flight deck. The lower level was (2) the galley compartment, with refrigerator and electric range. Directly aft was (3) the fuel compartment and the lavatory. Beneath the center wing structure were two small compartments. The forward one was the bunk room and the aft held a fuel tank (the saddle tank). Above that was the auxiliary power unit, on what was called the putt-putt deck. This unit supplied power for the plane when the engines weren't running. The navigator's hatch was also on that deck. Aft of that was (4) the after hull compartment, containing the waist gunner's stations and the deck turret. A tunnel from the aft hull compartment provided access to (5) the tail compartment, which contained the tail turret.
A passageway extended from the bulkhead under the front spar, down through the center of the plane, and back to the tail turret. Forward of the front spar, the passageway extended down the left side of the plane to the galley compartment. A flight of stairs led to a door on the right, which opened on the flight deck just forward of the flight engineer's station. Two manholes in the bottom wing skin, in the fuel compartment, allowed access to the center wing section and the bomb bays. Personnel protection against gunfire consisted of two pieces of armor plate behind the pilot's and copilot's seats, six pieces of armor plate for protection of crew members on the flight deck, one piece of armor plate on the aft section of each waist hatch door for protection of the waist gunners, and face and body armor plate for the projection of each turret gunner.
One unique feature of the PBM-3 design was the addition of four "vortex airfoils" to the horizontal stabilizers. The PBM-1 had suffered an undesirable tail flutter or "buzz" caused by the airflow pattern at the intersection of the vertical and horizontal stabilizers. To cure this problem on the PBM-3, Ellis (Sam) Shannon, a Martin test pilot, conceived the idea to install small airfoils on the horizontal stabilizer to smooth the flow. This installation solved the problem and the "Shannon vortex airfoils" became another unique PBM feature.
The PBM-3 was produced in a numbers of variants: PBM-3C patrol bombers, PBM-3S antisubmarine versions for the South Atlantic campaign, PBM-3R transports, one PBM-3E radar testbed, and the PBM-3D patrol bomber that had self-sealing tanks, increased armor, and nineteen hundred horsepower R-2600-22 engines. A total of 677 PBM-3s were built.
By 1941 the U.S. Navy recognized that the PBM needed more power and ordered 230 PBM-4s to be powered by the twenty-two horsepower Wright R-3350 engine. Correspondence detailed concern as to whether or not the existing Mariner empennage design was a satisfactory match for these power plants, but the question became moot when national priorities directed all R-3350 engine production to the B-29 Superfortress. The PBM-4 project was canceled before a prototype could be built.
With the cancellation of the PBM-4, another approach to increasing Mariner power was necessary. Two PBM-3s, Bureau Numbers 45275 and 45276, were designated XPBM-5 and used as testbeds for the installation of Pratt and Whitney R-2800-22 engines of twenty-one hundred horsepower. The design was satisfactory and delivery of the PBM-5 began in mid-1944. In addition to more powerful engines, the PBM-5 had provisions for four of the newly developed rocket assisted takeoff (RATO)/jet assisted takeoff (JATO) bottles of one thousand pound thrust each. These rockets would prove invaluable in rough water operations in the Pacific. A total of 628 PBM-5s were delivered. Delivery continued after the end of World War II although many of the postwar aircraft went directly from the factory to storage. This postwar production was justified because of the way contract termination costs were structured. It was cheaper for the navy to take delivery than it was to cancel the contracts. The postwar production decision was a sound one. Most of the stored aircraft were later activated to serve in the Korean War and during the cold war era.
The versatility and military usefulness of an amphibious aircraft was well demonstrated during World War II by the PBY-5A amphibian version of the Catalina. The navy decided to expand the operational utility of the Mariner by building an amphibious version. A standard Mariner flying boat, PBM-5 BuNo 59349, was converted into the prototype XPBM-5A amphibian and delivered in April 1946 to the Naval Air Test Center in Patuxent River, Maryland. Tests of the prototype were successful and thirty-six production PBM-5A amphibians were ordered. The final Mariner delivery was a PBM-5A on 9 March 1949.
Martin Company archives refer to "a proposed PBM-6 design proposed in 1945," but apparently the navy took no action on this proposal; no navy record of a PBM-6 designation was found.
Excerpted from The Fighting FLYING BOAT by RICHARD ALDEN HOFFMAN Copyright © 2004 by Richard Alden Hoffman . Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.