Bell 533

:"For an overview of the whole Huey family of aircraft see Bell Huey"

infobox Aircraft
name = Bell 533
type = Research helicopter
manufacturer = Bell Helicopter

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caption = Bell 533 with engines and wings added
designer =
first flight = 10 August 1962
introduced =
retired =
status = On display, Fort Eustis, Virginia
primary user = United States Army
more users =
produced =
number built = One
unit cost =
developed from = YH-40
variants with their own articles =

The Bell 533 was a research helicopter built by Bell Helicopter to explore the limits and conditions experienced by helicopter rotors at high airspeeds under contract with the United States Army. The Bell 533 was designated the High Performance Helicopter (HPH) by the Army and was tested in several helicopter and compound helicopter configurations, reaching a top speed of 274.6 knots (316.0 mph, 508.6 km/h) before being retired.

Operational history

On 7 August 1961, The Army's Transportation Research and Engineering Command (TRECOM) awarded a contract to Bell Helicopter for a high-performance helicopter to conduct research.Robb, Raymond L. [http://www.vtol.org/pdf/summer06robb.pdf "Hybrid helicopters: Compounding the quest for speed"] , "Vertiflite". American Helicopter Society, Summer 2006.] In response, Bell built the Model 533 from a YH-40-BF, one of the six pre-production UH-1 aircraft.Pelletier, Alain J. "Bell aircraft since 1935". Annapolis, Md: Naval Institute Press. 1992. ISBN 9781557500564] The dynamic components of the YH-40 were updated with components designed for the UH-1B. The research for the contract was split into two phases; phase one would be to determine the benefits of an overall reduction in drag, phase two would determine the benefits of auxiliary thrust.

Initial testing

During the first phase, a general clean-up of the airframe was performed to reduce drag. New aerodynamic fairings were developed using fiberglass honeycomb for the rear fuselage, a cambered vertical stabilizer was developed which, in cruise flight, aerodynamically unloaded the tail rotor. The skid landing gear also had streamlined fairings applied to it and the rotor mast was replaced by a mast that could be tilted in-flight.

The reconfigured helicopter was first tested in the NASA Ames Research Center wind tunnel, which confirmed the modifications had significantly reduced the aircraft's drag. The 533 made its maiden flight on 10 August 1962 at Bell's Fort Worth, Texas headquarters utilizing the two-bladed UH-1B rotor. Before beginning the program flight testing, the helicopter was fitted with a gimbal-mounted three-bladed rotor. Modifications to the flight controls allowed either rotor to easily be fitted to the aircraft in a short time, and the three-bladed rotor could be mounted on the gimbal or rigidly to the mast. In this configuration, the 533 achieved a true airspeed of 150 knots (173 mph, 278 km/h) in straight-and-level flight.

The Army funded the second phase to study the effects of auxiliary propulsion on the helicopter. Bell configured the 533 with two convert|26.8|ft|m|2 swept-back wings. The sweep of the wings was adjusted by maintenance personnel on the ground. Later in the program, the tilt of the wings was linked to the collective control to limit lift and to control rotor speed during autorotational profiles. After flight tests to determine the characteristics of the aircraft with the wings installed, the wings were removed.

Starting on 21 October 1963, Bell began test flights with two Continental J69-T-9 turbojet engines in nacelles mounted directly to the aft portion of the fuselage. The thrust from the two 920-pound thrust engines caused turbulent airflow over the standard elevators, so an additional elevator was mounted on the vertical fin on the opposite side of the tail rotor. Following the modification, flight tests were resumed with both wings and engines mounted on 2 March 1964. In this configuration, the Bell 533 achieved 186 knots (214 mph, 344 km/h) using maximum auxiliary thrust.

Independent testing

In April 1964, contracted testing was completed with the Army. Bell immediately fitted the two-bladed rotor with tapered blade tip caps for the company's own independent testing. The blade tip modification allowed the helicopter to reach 193 knots (222 mph) using the maximum auxiliary thrust from the Continental turbojets. In an effort to achieve even higher speeds, Bell removed the J69-T-9 engines and replaced them with 1,700-pound thrust J69-T-29 engines. The additional thrust had the effect of making the 533 the first rotorcraft in history to exceed 200 knots (230 mph, 370 km/h), reaching 205 knots (236 mph, 380 km/h) on 15 October 1964.

Six months later, on 6 April 1965, the Bell 533 became the first rotorcraft to reach 217 knots (250 mph, 402 km/h). In addition to the higher speeds, Bell test pilots were able to demonstrate that the additional thrust from the auxiliary engines allowed the compound helicopter to maintain sustained 2 g turns at 60-degree bank angles.

Follow-on research

By early 1968, the Army and Bell were working together again to expand the envelope of the Model 533. Bell removed the wings previously used and replaced them with a stub wing mounted higher and further back on the airframe. Bell also replaced the J69 turbojets with Pratt & Whitney JT12A-3 (J60) engines, capable of producing 3,300 pounds of thrust each, mounting the new engines to the ends of the stub wings. Bell also modified the helicopter flight controls to change pitch control from inputs to the rotor during lower flight speeds to airplane-style elevator control during high-speed flight. On 15 April 1969, the Model 533 achieved its highest speed of 274.6 knots (316.0 mph, 508.6 km/h).

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. Today, the Model 533 is on display at Fort Eustis, Virginia.

ee also

aircontent
related=
* UH-1 Iroquois

similar aircraft=
* Lockheed XH-51
* Sikorsky S-69
* Sikorsky S-72

lists=
* List of VTOL aircraft

see also=
* Gyrodyne

References

* Bell Helicopter, TRECOM Technical Report 63-42, "Summary Report, High-Performance-Helicopter Program, Phase I", U.S. Army Transportation Research Command, September 1963.
* Bell Helicopter, TRECOM Technical Report 64-61, "Summary Report, High-Performance-Helicopter Program, Phase II", U.S. Army Transportation Research Command, October 1964.

External links

* [http://www.vectorsite.net/avhuey.html#m9 Huey modification information on Vectorsite.net]
* [http://www.jefflewis.net/rotorcraft_limitations.html Rotorcraft Speed Limitations]