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Bell 200 / XV-3 / XH-33

 

bellxv-3
 
In August 1950, the Air Force and Army announced an official Tilt Rotor design competition. Bell's proposal for their Model 200 won the competition for the XV-3 (originally designated the XH-33), and Bell was awarded a contract in May 1951. The XV-3 was built and initially flight tested at Bell's Ft. Worth, Texas, facility. Two prototypes were built, carrying tail numbers 4147 and 4148.

The basic configuration was a metal fuselage with the metal wing mounted mid fuselage. A helicopter-like rotor was mounted on each wing tip. The rotor shafts were oriented in a vertical position for take-off, landing and hover like a helicopter, and moved to the horizontal position for forward flight like a conventional aircraft. A single Pratt & Whitney R-985-AN-1 Wasp Junior piston engine, producing 400hp for flight and 450hp for take-off, was located in the fuselage just behind the wing. A series of gearboxes and drive shafts transferred power to the rotors. Fuel was stored in a 380 litres tank located in the fuselage just under the wing. The fuselage length was 9.25m, the wing span was 9.53m, the height was 4.14m, and the wing area was 10.8 sq.m. The maximum speed was predicted to be 252km/h.

Three-bladed, fully articulated, 7.63m-diameter rotors were used initially. They were driven through a two speed transmission that could be shifted to a lower gear to allow the rotors to turn at a lower speed while the engine maintained a higher speed for greater cruise efficiency. The rotor tilt was controlled by an electric motor enclosed in a fairing at each wing tip. Each rotor could rotate through a 90-degree arc from vertical to horizontal in 10 to 15 seconds. The pilot could stop or reverse the rotor motion at any point. Steady, stable flight could be maintained with the rotors in any intermediate position.

The XV-3 was intended as a research vehicle to evaluate the Tilt Rotor concept and to provide design and test data, but the military contracting rules at the time required that it also be designed and demonstrated with a military mission capability. The XV-3's military mission was observation, reconnaissance, and medical evacuation. It had mounting points for two litters.

Tests confirmed that good propulsive efficiencies for cruise could be obtained using helicopter rotors as propellers by reducing the rotational speed to about half of that used for hover. Only minor design changes were recommended as a result of tests. What the wind tunnel tests did not look at was the impact of the large, slow turning rotors on the aeroelastic and longitudinal stability of the aircraft at cruise speeds. Aeroelastic effects, which are the coupling of the bending motions of various flexible components on the aircraft, plagued the XV-3 throughout its early life.

The first XV-3 (serial 54-147) was constructed between January 1952 and December 1954. The aircraft rolled out on February 10, 1955, and following roll out, airframe and control system proof load tests, vibration surveys, and system functional tests were performed. Then, with the XV-3 mounted on a tie down stand, full conversions and operation in helicopter and airplane modes were tested. Ground runs included a rotor stability survey and ten hours of full power operation. These ground tests were completed by August. No major difficulties were noted, and rotor stability checks showed the aircraft to be free of any resonance tendencies.
 

Bell-200

 

The first hover flight was made on August 11, 1955, by a Bell test pilot, but limited progress was made in flight testing during the next three years due to continuing problems with wing/pylon/rotor instabilities during flight. The first of these occurred after only one week and 1.2 hours of flight time, during an air taxi test. It resulted in a hard landing, and the XV-3 sustained rotor and airframe damage. While the damage was not extensive, the discovery of the instability raised major concerns. Repairs were made, and several modifications also were made which were felt would eliminate the elastic coupling problem. These included stiffening the rotor controls and adding external struts to make the wing more rigid. The trailing 25 percent of the wing was made to droop during hover to reduce disturbance to the rotor airflow during hover, and also to reduce stall speed to 85 knots, making transitions at a lower speed possible. 200 hours of ground runs and tie down tests were made to evaluate these modifications before clearing the XV-3 to fly again. The next hover flight finally occurred on March 24, 1956.

Bell finally began envelope expansion flights in June 1956 and performed a nacelle tilt of 5 degrees on July 11. By July 25, they reached 70 degrees tilt and 150km/h forward speed when another rotor instability was encountered. Flight testing resumed on September 26 after more modifications were made to the rotor system and ground run evaluations were completed. On October 25, during another test flight, 4147 encountered another very severe inflight rotor instability and crashed. The pilot was seriously injured, but survived. Bell took a serious look at the entire rotor system, and decided that the basic design and characteristics of the three-bladed articulating rotor system were unsatisfactory for the XV-3.

Prior to resuming flight testing with the second prototype, 4148, Bell made numerous changes. They kept the R-985 engine, despite its limited power, because of its very good reliability record. The flush engine cooling air inlet was replaced with a scoop that spanned across the top of the fuselage, from one wing root to the other. As had been done with 4147, the rotor controls were stiffened and the wing was braced with external struts on the bottom. The number 2 XV-3 was shipped to NASA's Ames Research Center, where it flew in their 12m x 24m wind tunnel in September and October 1957. Serious flutter problems with the original three blade rotors were confirmed, and Bell decided to replace them with two-bladed, semi-rigid type rotors of 7.3m diameter, mounted on shorter masts. The semi-rigid, versus the fully articulated design, in itself further increased rigidity, which would decrease the possibility of aeroelastic coupling. While in the wind tunnel, pilots were able to practice conversion procedures and gear changes. Further changes to the two bladed rotor/pylon design were still required to eliminate pylon oscillations, but these changes were tested while still in the tunnel, and again appeared to eliminate the problem in all conditions that were tested. The pilots found conversion to be quite easy, but engine gearbox shifting proved to be difficult, requiring considerable manipulation of the pitch controller and throttle throughout the twenty second process.

Flight testing of 4148 (serial 54-148) began at Bell on January 21, 1958. Conversions to 30 degree pylon angle and speeds up to 205km/h were accomplished by April 1, 1958. Pilots also demonstrated autorotation to a landing following a simulated engine failure. Helicopter characteristics were rated as good, especially in high speed forward flight, and vibration levels were lower than expected. By May 6, another rotor oscillation was encountered in flight at a 40 degree pylon angle, and the aircraft again was grounded. Bell, the Army, and NACA decided that another series of wind tunnel tests was in order. The 12m x 24m tunnel at Ames Research Center was heavily scheduled through the summer, and the XV-3 sat grounded until being shipped to Ames in September. During this time, Bell conducted analog computer simulations to further analyze the instability problem and recommend configuration changes prior to beginning the wind tunnel tests.

During October 1958, the XV-3 finally went into the 12m x 24m foot wind tunnel at NASA Ames, and more refinements resulted. Changes included increasing the control system stiffness to three times greater than original, adding counterweights in to the rotor collective control mechanism, and increasing the blade sweep angle.
Flying resumed on December 11, and on December 17, the XV-3 achieved 30 degrees tilt, and on December 18 achieved 70 degrees tilt. After making a few minor rigging corrections, a second flight was made and a full conversion to 90 degrees finally was achieved. The conversion was made in steps, starting at 167km/h in full helicopter mode and finishing at 213km/h in full airplane mode. A second full conversion was made the next day. This was three years after the date projected for full transition at the time of the XV-3's roll out. Over the next ten weeks, final aerodynamic and control system refinements were made, including adding a large plywood ventral fin to improve the poor directional stability.

With all the changes that were made to the rotors and pylons, Bell tried something new to see if the wing really was contributing to the instability. The strut attachments were modified with locking devices that allowed the XV-3 to fly with the supporting struts either locked in place to maintain wing rigidity or loose to provide no support to the wing. They were hydraulically actuated, and the pilot could switch between the two settings in flight. Bell assumed that if the instability resumed with the struts unlocked, then the wing indeed did contribute to the problem. The new device was tested on January 16, 1 959, at rotor angles up to 85 degrees from vertical with no instability problems. On the January 22, the struts were removed and the XV-3 flew up to 220km/h with the rotors 50 degrees from vertical, again with no instability noted.

A wheeled landing gear was added to the landing skids, and short take-offs were made on April 13, 1959, using less than 60m of runway, and using only two thirds power. Optimum rotor tilt angle was found to be ten degrees forward of vertical.

The first in-flight gear change was made on April 14, 1959, and a forward speed of 220km/h was achieved at a much lower engine speed with an accompanying lower vibration level. The gear change took about ten seconds to accomplish. The process was very similar to shifting a manual transmission in a car, requiring manipulation of the aircraft's collective control, throttle, and clutch.
 

Bell-XV3-03

 

By April 24, 1959, the XV-3 was ready for formal evaluation by the military. It was shipped to Edwards AFB for a two month flight evaluation that began on May 14, 1959. The joint Air Force and Army evaluation consisted of 38 flights and a total of 29.6 hours. Forty conversions were made, as were twenty gear shifts to lower the rotor speed while in airplane mode. Also demonstrated were power-off conversions from airplane mode to helicopter mode followed by autorotation to a safe landing. Flights up to 3660m were performed.

Air Force and Army evaluators concluded that conversions could be performed easily at all airspeed and fuselage attitudes that were tested. They considered the concept to be operationally practical because of low down wash velocity and temperature, low vibration, reasonable noise levels, and excellent reliability. The XV-3 demonstrated good behavior during stalls, good rolling take-off performance, and good basic controllability without electronic or mechanical stability augmentation.

On the negative side, items related to the prop rotor concept included an erratic lateral darting tendency and roll oscillations during hovering in ground effect. A large increase in power was needed as hovering flight was approached. Weak longitudinal and lateral-directional stability was also observed at low speed in helicopter mode, as was excessive blade flapping during longitudinal and directional maneuvering in airplane mode. There was high parasite drag in all configurations at high speeds. And last, the XV-3 displayed a fore/aft surging motion, especially severe in rough air, attributed partly to the use of large, lightly loaded rotors as propellers.

Performance was significantly less than predicted and weight had grown. The design empty weight was 1580kg and grew to 1895kg, before the Air Force added 72kg of instrumentation. At a gross weight of 2165kg, only the test pilot and about 190 litres of fuel (half the fuel tank capacity) could be carried. This gave a maximum endurance of one hour.

In airplane mode in high gear and full power at 1220m of altitude, cruise speed was 189km/h true airspeed and wing stall was 185km/h. In low gear and full throttle, cruise increased to 213km/h and wing stall was reduced to 174km/h. The XV-3 was dived to 287km/h, which was the top speed due to limits of collective pitch. Short period longitudinal dynamic stability began to deteriorate at 222km/h, and was considered unacceptable at 240km/h.

At the completion of Air Force testing, the XV-3 was shipped to NASA Ames where it remained on flight status through July 1962. By the time its flying career ended, the XV-3 had been flown 270 times by 11 pilots for a total of 125 hours. 110 full conversions were made by nine different pilots, six of whom performed the conversion on their first flight.

Flight testing of the XV-3 clearly demonstrated that technology had not yet developed an understanding of the aeroelastic coupling that could occur between the wing and a large, slow-turning rotor located at the wing tips. It wasn't until the mid-1960s that engineers finally had a relatively complete understanding of the coupling problem, following the development of sophisticated analytical tools and more capable computers. Although the XV-3 demonstrated the feasibility of the Tilt Rotor concept, it was limited because of low power, an unsophisiticated flight control system, and relatively low twist, helicopter-like rotor blades. Several times Bell proposed replacing the powerplant with a Lycoming T-53 turboshaft engine of 600hp and making rotor and control modifications to eliminate the concept-related deficiencies identified during the Air Force evaluation, but the XV-3 never flew again.

However, the XV-3's career was not over yet. Under a NASA contract, Bell continued studying the phenomena of aeroelastic coupling and ways to improve high speed flight. Two additional sets of wind tunnel tests using the XV-3 were performed. The first was done in July 1962, evaluating several changes aimed at increasing maximum speed, improving high-speed flight characteristics, and decreasing the rotor flapping that occurred during maneuvering. The tunnel was run at 240km/h before instabilities set in. Following modifications, the instabilities did not return until 295km/h.

Using these results, Bell felt that they finally had a good understanding of the problem. In May 1965, the XV-3 returned to Bell, where further study, modifications, and ground runs were performed between July 1965 and March 1966. It then returned to Ames and again entered the 12m x 24m tunnel for the fourth, and last, time in May 1966. This time, the XV-3 was tested to 365km/h, the limit of the wind tunnel, without encountering any of the oscillations that had plagued the aircraft throughout its career. However, on May 20, while running at maximum tunnel speed and taking the last planned data point, both rotors tore loose following a wingtip fatigue failure, damaging the aircraft and permanently ending the XV-3's career.

Following these events, the XV-3 was placed in storage for many years, spending time at Wright-Patterson AFB and Davis-Monthan AFB. Eventually the U.S. Army Aviation Museum at Fort Rucker, Alabama, acquired it for display.


Small size and lack of capital of Platt-LePage Aircraft, along with lack of orders for military aircraft caused the Platt-LePage Aircraft Co. to shut down in August of 1946. The McDonnell Aircraft Co. obtained most of the helicopter patents from Platt-LePage during the liquidation of the company, along with the personnel responsible for the twin engine project. Robert Lichten an ex-Platt-LePage engineer went to Bell Helicopter and developed the Tilt-Rotor idea into the XV-3, an examination of the original patent by Haviland Platt shows a remarkable similarity between the two ideas, and Bell Aircraft eventually paid Haviland Platt for the use of his patent.

Bell XV-3
Engine: 1 x Pratt & Whitney R-985, 330kW / 450hp
Type of rotors: 2 x 3-blade combined rotor-propellers
Propeller diameter: 7.32m
Wingspan: 9.15m / 30 ft
Length: 30 ft
Take-off weight: 2177kg
Empty weight: 1633kg
Max speed: 280km/h
Ceiling: 3600m

Seats: 4

 

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