Bell 333
It is the Bell UH-1 Iroquois, commonly known as the Huey, that has emerged as the most widely produced light utility helicopter in history, with military variants operating across approximately 60 countries. What makes the UH-1 distinctive is its single-engine turboshaft design, which can be adapted for various customer needs.
According to Aviation Classics Issue 27—Bell UH-1 Iroquois, it was almost immediately after the design’s finalization that experimentation with the Huey began. It was in 1959 that the Transport Research Command of the US Army launched investigations into potential helicopter performance improvements.
To support this initiative, it was Bell that took the initial YH-40, 56-6723 and transformed it extensively into a research testbed, creating the unique Bell Model 533. The emphasis of the first phase of the project was streamlining, to which end a large fairing was added around the main rotor shaft, and the engine intakes were redesigned flush with this fairing.
The basic cross-tube mounts supporting the landing skids were converted into faired airfoil section legs, while the fin underwent cambering to reduce tail rotor load during forward flight. It was this latter modification that would be incorporated into production UH-1s, having demonstrated significant benefits for both performance and maneuverability. Even the door hinges underwent a redesign to eliminate as much drag as possible, resulting in a truly ‘slick’ airframe.
The first helicopter to fly faster than 200 kts
Powered by a 1400hp Lycoming T53-L-13 turboshaft, the 533 lacked the Bell trademark stabilizer bar on the main rotor; instead, it was fitted with a variable tilt main rotor mast that maintained the fuselage in the lowest drag attitude regardless of the position of the rotor disk in flight. The 533 made its first flight on Aug. 10, 1962, achieving 188mph (302.56 kph) in a shallow dive in March the following year.
The second phase focused on power, which was increased with the addition of two podded 920lb (417.3 kg) thrust Continental J69-T-9 turbojets on either side of the rear of the cabin, with which the 533 eventually achieved 214 mph (344 kph) in March 1964. The engine pods caused turbulence over the elevator, so an additional elevator was mounted on the fin opposite the tail rotor. The engines were later upgraded to the J69T-29 version of the turbojet, which produced 1700lb (771 kg) thrust.
During the trials, the original two-bladed rotor was fitted with broader chord blades before being changed for a three-bladed then a four-bladed rigid design, providing valuable test data for later developments of the Huey. The 533 was also fitted with two small swept wings to produce a compound helicopter, becoming the first helicopter to fly faster than 200 kts in October 1964 and achieving 250mph (402.34 kph) in level flight on Apr. 6, 1965.
Tremendous manoeuvrability
The wings bestowed tremendous maneuverability as well as speed, allowing the 533 to sustain 2G turns and 60 of the bank. The final phase of the program began in 1968. The wings were removed and new stub wings were fitted higher up on the cabin sides, each with a 3300lb thrust Pratt and Whitney JT12A 3 turbojet mounted on the end. In this configuration, the 533 reached 316mph (508.5 kph) in May 1969.
According to Wiki, the final configuration of the Model 533 for the research contract was with a proprietary, four-bladed, flex-beam rigid rotor developed by Bell. Afterwards, the Model 533 was returned to its original two-bladed configuration and retired.
The Model 533 is on display (in the highest speed configuration) at the US Army Aviation Applied Technology Directorate, Fort Eustis, Virginia.
Aviation Classics Issue 27: Bell UH-1 Iroquois is published by Mortons Books and is available to order here.
Photo by Own Work The Founders Intent via Wikipedia and U.S. Army Aviation Museum
