Bell 533 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 during the 1960s . The helicopter was a YH - 40 — a pre - production version of the UH - 1 Iroquois — modified and tested in several helicopter and compound helicopter configurations . The Bell 533 was referred to as the " High Performance Helicopter " ( HPH ) by the Army , and reached a top speed of 274 . 6 knots ( 316 . 0 mph , 508 . 6 km / h ) in 1969 , before being retired . Design and development . The U . S . Army began a program to study improvements to current helicopters that could be demonstrated by testing . Analyses by three helicopter companies showed that performance could be greatly improved . Bell Helicopter was one of the participants and investigated improvements to the UH - 1B Iroquois . After the study , Bell submitted a proposal for the " High Performance Helicopter " . 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 . In response , Bell built the Model 533 from a YH - 40 - BF , one of the six pre - production UH - 1 . 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 . The Bell 533 was flight tested in three main configurations . The first configuration was the basic YH - 40 helicopter with drag reduction changes . The second configuration added a pair of jet engines for additional thrust . The third configuration added swept wings for extra lift . Operational history . 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 sandwich 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 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 CAE J69 - T - 9 turbojet engines in nacelles mounted directly to the aft portion of the fuselage . The thrust from the two 920 - pound ( 4 . 09 kN ) 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 , 357 km / h ) 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 ( 7 . 56 kN ) thrust J69 - T - 29 engines . The additional thrust helped make 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 ( 13 . 3 kN ) 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 . Survivors . The Model 533 is on display ( in highest speed configuration ) at the US Army Aviation Applied Technology Directorate , Fort Eustis , Virginia .