YR-13 / H-13 / OH-13 Sioux / HTL
The Bell (model 47) H-13 Sioux, with a crew of three, was one of the most popular light utility helicopters ever built. The Bell model 47 was produced continuously from 1946 to 1973, and in other countries through 1976. Produced in 20 different configurations, with model numbers ranging from A to T, the Bell model 47 was used in 40 countries. The combined total of commercial and military versions of this series was 5,000. The U.S. Army Air Force procured it's first YR-13 (model 47B) in December 1946. The OH-13 had a cruising speed of 70 mph (60 knots).
An important Bell innovation during the early development of the helicopter was the use of short weighted gyro-stabilizer bar at 90° to, and beneath, the main rotor. The gyro-stabilizer bar, with streamlined counterweights at both tips, was linked to the rotor in such a way that it determined the plane of the rotor, and maintained it generally in the horizontal, regardless of the angle of the mast. The stabilizer bar, connected to the cyclic pitch control, acted as a hinged flywheel utilizing gyroscopic inertia to keep the teetering rotor blades in-plane and independent of fuselage movement due to gusts of wind, providing stability during flight. Arthur Young, Bell's designer, ensured the system had high enough inertia so sufficient energy would be stored in the rotor to permit safe autorotation in event of engine failure, an important safety consideration.
With Piasecki's XHRP-1 in production and the XHJP-1 in the design stage, the Navy took further steps to acquire a suitable trainer and settled upon the Bell Model 47. Late in 1946 Bell Aircraft Corporation was awarded a contract for the first of a long series of Navy HTLs, a slightly modified version of the Model 47.
Until 1948 there was not a helicopter specifically designed for military observation in actual production. In existence, however, was the Bell model 47E in flight test at the Bell factory and the one Sikorsky S-52, which had already completed flight test; both of which, it was believed, could easily be converted for military observation use. At the time though, the Navy's HTL-2, an improved version of the HTL-1 which incorporated a larger engine and bubble canopy, was being produced in quantity for the Navy as a trainer and represented a close approximation to the final configuration of what could be expected in future observation helicopters. But until such time as either the experimental Bell or Sikorsky observation helicopters became operational and were available in quantity, considerable operational experience could be gained through operating a small number of the Navy's HTL-2s.
With this thought in mind, and with a desire to comply with the squadron's second mission of evaluating a helicopter as a replacement for the OY fixed-wing aircraft, Colonel Dyer recommended, on 28 April 1948, that the Marine Corps procure three HTL-2 trainers. The new Commandant, General Clifton B. Cates, who had relieved General Vandegrift on 1 January 1948, requested on 13 May that the CNO provide HMX-1 with the three HTL-2s.
On 9 August 1948, the squadron received the first of two HTL-2 from NAS Lakehurst, New Jersey. The Bell helicopter was two-place, dual controlled, and powered by a 178-horsepower engine driving a two-bladed main rotor. The cruising speed, similar to the H03S, was 80 knots but unlike the H03Ss, the gross weight was only 2,200 pounds. Since most of the pilots had received a minimum of 15 hours in the HTL-1 while undergoing training at HU-2, a familiarization syllabus was not necessary. Tests were immediately begun to compare the HTL-2 with the OY aircraft in artillery spotting, liaison, and aerial photography work. The results of the preliminary evaluations indicated that the HTL was superior in all respects to the OY, except that the OY's cruising speed was higher.
By November 1948 the evaluation had been completed and the results sent to the Commandant by Colonel Dyer. Based upon Colonel Dyer's letter, on 24 November 1948 General Cates asked permission of the CNO to change the complement of a Marine observation squadron from its previously authorized eight OY aircraft to four OY and four helicopters. The Commandant stated that all of the helicopters observed and tested as replacements for the OY aircraft, the latest model Bell HTL-3 and the Sikorsky S-52 closely met the Marine Corps' requirements of size, configuration, and gross weight, with the S-52 rated as the most desirable of all models. As a result of the favorable flight evaluation of the Sikorsky S-52, BuAer initiated a contract with Sikorsky for the S-52-2, a version of the original S-52 (Navy designation H05S-1).
HMX-1 took part in Operation PACKARD III which was the MCS amphibious command post exercise of 1949. The squadron was to "evaluate the operations of a small observation helicopter from an LST (Landing Ship, Tank) for artillery and infantry observation and liaison mission." During the exercise, the HTL-2 proved to be totally successful during its evaluation . The small helicopter operated for naval gunfire spotting and observation from the LST while the ship was both underway and at anchor. Although at times the LST pitched and rolled in the choppy seas, the HTL proved that small helicopters could work successfully from that type of vessel.
The HTL-3 trainer was similar in general configuration to the HTL-2 except that a 200-horsepower engine had been installed in place of the HTL-2s 178-horsepower engine which increased the useful load to approximately 706 pounds. As an interim measure, the Commandant recommended that 12 HTL-3s or S-52s be procured for the Marine Corps to implement the change in the VMO's aircraft complement. As a long-range recommendation, he requested that the design and procurement of a light helicopter be initiated to meet specifically the requirements for a military observation helicopter.
Aviation Plan Number 21-49, dated 7 April 1949, outlined the plans for outfitting the VMO squadrons. The plan specified that the HTLs were considered satisfactory for Marine observation requirements and that as the HTL helicopters became available they would replace half the observation aircraft in existing VMO squadrons.
The HTL-4 was similar to the previous Bell models except for a few added refinements. Two features affecting its appearance were the removal of the tail boom covering aft of the cabin, which made the helicopter 156 pounds lighter, and the substitution of a skid type landing gear in lieu of its wheels. The cabin could accommodate two passengers besides the pilot, whereas, all previous HTL models could carry only one passenger. The aircraft came equipped with provisions for carrying two external litters, each mounted parallel with the cabin across the top of the skid. The empty weight was 1,546 pounds with a maximum takeoff weight of 2,350 pounds. Sea level air speed was restricted to 80 knots, almost identical to that of the HTL-3s.
The H-13 was used for observation, reconnaissance and in the MedEvac role as a litter carrier in Korea, following initial fielding in 1951. In the MedEvac role a cocoon-like stretcher pod could be mounted on each skid. A distinctive feature of the Bell (model 47D) H-13D/H13E was the now familiar "Goldfish bowl" Plexiglas canopy, featured in the TV-series MASH (Mobile Army Surgical Hospital). The H-13 earned the nickname "Angel of Mercy" for evacuating some 18,000 United Nation's casualties during the war. The OH-13 Sioux also saw service during the early days of the Vietnam war before the fielding of the OH-6A Cayuse in early 1968. The Sioux had a single two-bladed main rotor and a metal two-bladed tail rotor. The H-13 had a speed of 106 mph (92 knots). The Sioux could be armed with twin M37C .30 Cal. machine guns on the XM1 armament subsystem or twin M60C 7.62mm machine guns on the M2 armament subsystem.
The Bell (model 201) XH-13F, powered by a Continental-Turbomeca XT51-T-3 Artouste I 220 hp turboshaft engine, was Bell Helicopter's first turbine powered aircraft.
An experimental armed Bell (model 207) Sioux Scout (1963), based on the a Bell (model 47G-3B1) body and engine, with a newly developed gun ship front end, aided in the development of the AH-1G Huey Cobra attack helicopter. The Sioux Scout featured revised seating for two in tandem, small stub wings containing additional fuel, and a remotely-controlled chin barbette with two M60C 7.62mm machine guns. The Bell (207) was powered by a Lycoming 260hp TVO-435-A1A turbocharged piston engine.
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