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RD / C-21 / C-26 / C-29 / OA-3 / OA-4 Dolphin

During the 1920s and 1930s, the Army's air transport organization had acquired only a few dozen airplanes under the cargo designation. In the mid-1920s, the service purchased eleven Fokker trimotor transports, designated C-2, along with thirteen Ford trimotors, designated C-3, C-4, and C-9. The service also operated about two dozen Douglas Dolphin twin-engine amphibians, useful in miscellaneous duties in the Philippines and the Panama Canal Zone and for occasional coastal patrols.

The Air Corps was responsible for coastal patrol of the United States and its possessions and territories (Hawaii, Philippines, Puerto Rico, Canal Zone, etc.) and needed aircraft able to operate from both land and water. Several variants in the Douglas Dolphin series of amphibious aircraft were purchased in small numbers for use by the Air Corps.

During the summer of 1931, the Air Corps ordered eight Douglas Dolphin amphibian aircraft and designated them Y1C-21. These planes were intended for use in the coastal defense mission in secondary roles like observation, scouting, pathfinder navigation and "at sea" rescue. It was originally envisioned that the Y1C-21 would fly along with a coastal defense bomber formation, but faster monoplane bombers soon made it impossible for the slow C-21 to keep up. In 1933, the C-21s were re-designated OA-3 (Observation Amphibian) and used primarily as staff transports and search and rescue planes. The C-21 (OA-3) was in some respects the first effective amphibian operated by the Air Corps. Many search and rescue techniques were pioneered with the C-21 and similar Navy aircraft during the early 1930s. The OA-3s remained in service until the late 1930s assigned primarily to bases in the U.S. territories of Hawaii, Panama Canal Zone, and the Philippines.

The Douglas Y1C-26 was an improved version of the Y1C-21 intended for use in coastal patrol, observation and at-sea rescue missions. The Y1C-26 was very similar to the Y1C-21 in appearance, but had many improvements incorporated into the design. The most obvious external difference was the removal of the two auxiliary vertical fins from the tail. The area of the vertical stabilizer and rudder was increased and the drag-producing fins were removed. One of the less obvious, but major design changes was the lengthening of the fuselage by about 18 inches to make room for an extra passenger seat. The Y1C-26 also had a 25 percent increase in fuel capacity, extending its range to 650 miles.

The Air Corps bought two Y1C-26s and followed-up the order by buying 14 more improved aircraft and assigning them as the -A and -B models. The Y1C-26 was re-designated as C-26 in 1933 and then reclassified as OA-4 (Observation Amphibian) in 1934. The C-26 (OA-4) was updated several times during the 1930s with major modifications to the engines -- more powerful versions; wings -- plywood to metal skinned; and cabin -- six place transport to two place observation interior.

About a month after placing the order for the two Y1C-26s, the Air Corps ordered eight additional aircraft and assigned them the Y1C-26A designation. The -A model was very similar to the basic C-26 and differed only in minor details. While the planes were being built, a more powerful version of the Pratt & Whitney Wasp Junior became available and these were installed in the new planes. The engines allowed the plane to lift a larger payload, but did not increase the speed.

Like the Y1C-26s, the Y1C-26A was re-designated as C-26A, then reassigned in the Observation Amphibian category as OA-4A and finally four of eight aircraft built were upgraded and re-designated as OA-4C. The C-26A/OA-4A were modified and upgraded several times during the 1930s as more powerful engines were installed. Wings were upgraded and interior layouts were changed to suite the mission changed from passenger/staff transport to coastal observation and at-sea rescue.

In the spring of 1933, the Air Corps ordered six additional Douglas Dolphin amphibian aircraft. The first two were extensively modified and re-designated as C-29s before delivery. The other four were also updated, but only in minor details and became C-26Bs. The C-26B had a different model of the Wasp Junior radial engine, but was otherwise similar to the C-26A. The C-26Bs were in service for about one year when they were designated as OA-4B observation amphibian aircraft.

The Douglas RD started as the Sinbad, going through some iterations before the final configuration was established. It appeared to be smaller and lighter than the later Dolphins. The U.S. Coast Guard aircraft register for 1933 shows the RD named Procyon CG-27 based at Cape May, New Jersey, and allocated the international radio call-sign 'NUMRG' and Coast Guard call-sign "24 G." Apparently this first production aircraft was delivered to the USCG in New York direct from the Douglas factory in February 1931. This was a flying-boat, not an amphibian.

The RD-2 Adhara was delivered in July 1932, and in 1933 it was based at Gloucester, Massachusetts, with the international radio call-sign 'NUMRJ' and Coast Guard call-sign '24-J.' The RD-1 Sirius followed on 5 August 1932, being based at Miami, Florida in 1933 with international call-sign 'NUMRH' and Coast Guard call-sign '24 H.' The first RD-4 was not delivered until nearly three years later, on 20 February 1935. All four types were externally different in fuselage, engine and tail configuration. . .The U.S. Coast Guard aircraft were used extensively in search and rescue (SAR) missions and as flying lifeboats, often flying far out to sea from several air stations to rescue stricken mariners or seamen in need of urgent medical care to hospitals ashore. Upon U.S. entry into World War II in December 1941 the U.S. Coast Guard became part of the U.S. Navy and the surviving Douglas RDD-4's were assigned to security patrols along the United States seaboard.

The Coast Guard acquired 13 total of four different variants of the famous Douglas Dolphin. They began acquiring Dolphins soon after the prototype model, named Sinbad, was introduced in 1930. It had an all-metal hull with room for 8 passengers and two flight crewmen. It featured a plywood-covered cantilever wing similar to the Fokker model acquired by the Coast Guard during this time. The first example was powered by two tractor-fitted J-5C Wright engines with two Hamilton Standard fixed pitch propellers. It was purchased for $31,500 in 1931 and was designated as RD for multi-engine transport, Douglas. It was delivered on 19 March 1931. It was not an amphibian but utilized beaching gear.

The Coast Guard christened it Procyon and assigned it to Air Station Cape May, New Jersey. LCDR C. C. von Paulsen flew Procyon to its new duty station. The Coast Guard soon after acquired two other Douglas RDs and were somewhat different than Procyon in that they were true amphibians and were powered by more powerful Wright engines. They were christened Adhara (designated as RD-2 and delivered in July, 1932) and Sirius (delivered 5 August 1932). Procyon was later converted to an amphibian configuration. The service acquired upgraded Dolphins in November, 1935, and these were officially designated as RD-4s.

The Douglas RD started as the Sinbad, going through some iterations before the final configuration was established. It appeared to be smaller and lighter than the later Dolphins. The first RD-4 was not delivered until nearly three years later, on 20 February 1935. All four types were externally different in fuselage, engine and tail configuration. . .The U.S. Coast Guard aircraft were used extensively in search and rescue (SAR) missions and as flying lifeboats, often flying far out to sea from several air stations to rescue stricken mariners or seamen in need of urgent medical care to hospitals ashore. Upon U.S. entry into World War II in December 1941 the U.S. Coast Guard became part of the U.S. Navy and the surviving Douglas RDD-4's were assigned to security patrols along the United States seaboard.

Manufacturer Douglas
Designation RD-4
Other Designations, if any: SD-178-4
Aircraft Type Multi-engine transport, amphibian
Unit cost: RD = $31,500.00; RD-1 = $36,500.00; RD-2 = $43,250.00;
RD-4 = $60,000.00;
Wing Span 60'
Height 14' 7"
Length 45' 3"
Fuel Capacity 240 gallons
Top Speed 147 mph
Cruising Speed 110 mph
Stall Speed 63 mph
Range 660 miles
Empty Weight 6,467 lbs.
Gross Weight 9,737 lbs.
Crew 3
Service Ceiling 14,500 feet
Engine(s) 2 x Pratt & Whitney R-1340-Cl "Wasp"; 454 hp



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