Bell 301 / XV-15
In 1965, the U.S. Army issued a Request for Proposal for what it called the Composite Aircraft Program. Composite, in this case, was for a vehicle that would have both helicopter and airplane characteristics, specifically, looking for a single aircraft to replace both the CH-47 helicopter and the C-7 "Caribou". Three contractors were selected to perform design studies in 1966 and Lockheed and Bell were chosen to perform further exploratory definition studies, which were completed in September 1967. However, the Army dropped the development due to limited funds.
Bell decided in 1968 to continue development for a proposed civil tilt-rotor aircraft, designated the Model 300. Initial work led to the design for a 4285kg aircraft powered by two Pratt & Whitney PT-6 engines powering 7.6m diameter rotors. One-fifth scale aerodynamic and aeroelastic models were built and tested extensively from 1969 through 1973. Full size rotor and rotating mechanisms were whirl tested to determine their hover performance and then tested in 1970 at the NASA Ames 12m x 24m wind tunnel at various rotation speeds, angles, and airspeeds up to the maximum tunnel speed of 370km/h. The rotor met or exceeded all performance and stability predictions.
Then, in 1972, NASA and the U.S.Army Air Mobility Research and Development Laboratory jointly started the Tiltrotor Research Aircraft Program. Since the Tiltrotor Research Aircraft Program was to be strictly a research program and would not lead to the production of an operational aircraft, costs were to be kept under control by not making weight minimization a major factor and encouraging the use of off the shelf components. Advanced technologies like fly-by-wire and composite structures were to be avoided. Weight growth and performance shortfalls would be tolerated in order to minimize cost and schedule impacts. Even the number of aircraft to be built was a factor. The two aircraft option was selected because of the high accident rate experienced by most other VTOL research programs.
Bell's proposal started with the Model 300's design and evolved it into Model 301. Bell kept the rotor and transmission, but replaced the engines with the more powerful Lycoming T-53 because of the requirement to hover with only one engine and the greater empty weight and useful loads required. Another benefit of the engine switch was that the T-53 already had an oil system that could operate with the engine pointed vertically, which had been developed for the CL-84 program. Bell's proposal was submitted on January 22, 1973, and comprised 300 volumes weighing 350kg. Bell's proposal was selected in April 1973. NASA awarded contract NAS2-7800 for $28 million for the final design, fabrication, and preliminary testing of two XV-15s on July 31. The total estimated cost of the six-year program was $45 million.
The 12.8m long fuselage design was of a conventional aircraft, semi-monocoque, and using light alloy material. There was no fuselage pressurization, and the structure was stressed from +3 to -0.5 G. The airframes were designed for minimum service lives of 1000 flight hours over five years.
The tricycle landing gear came from the Canadair CL-84. It utilized Goodyear magnesium main and nose wheels, and Goodyear hydraulically operated magnesium/steel disc brakes. The full-swiveling nose wheel incorporated shimmy dampers and a centering device. It retracted into a bay forward of the cockpit. The main wheels retracted into external pods on each side of the fuselage. A switch on the main gear strut prevented inadvertent gear retraction and tilting of the pylons more than 30 degrees from vertical when the aircraft was on the ground. The landing gear was structurally designed to withstand a touchdown sink rate of 3m per second at full gross weight. A 14500kg/sq.m nitrogen gas system provided for emergency extension in the event of a hydraulic failure.
The H-tail consists of a horizontal stabilizer with a vertical stabilizer on each tip. This configuration was selected to provide improved directional stability at and near zero yaw angles. Rockwell International's Tulsa Division built the fuselage and tail units under subcontract.
Two pilots sat side-by-side in Rockwell-Columbus LW-3B ejection seats, entering through a door on the right side of the cargo compartment. The flight deck was heated, ventilated, and air conditioned, but not the cargo compartment. The cabin could accommodate nine personnel if not filled with test equipment.
The wing measures 9.75m across, has a constant chord measuring 1.6m, and a area of 15.7sq.m (one of the design requirements was that the XV-15 be able to fit in NASA Ames' 12m x 24m wind tunnel, which influenced the wingspan and rotor size). It is swept forward 6.5 degrees, not for any futuristic aerodynamic reasons, but to insure there would be adequate clearance when the rotor blades flex in airplane mode. Wing dihedral is 2 degrees. Along the trailing edge, a flap measuring 1sq.m occupies the inboard third, and a flaperon measuring 1.85sq.m occupies the outer two thirds. The flaps can be deflected down to 75 degrees to help provide additional lift at low speeds. In hover, the flaps and flaperons deflect downward to reduce slipstream interference by the wing. The problems with the wing/rotor/pylon stability that plagued the XV-3 were eliminated by designing a very stiff wing and nacelle/wing attachment, and by placing the rotor hub as close to the wing as possible.
Each wing holds two fuel bladders that form a single crashworthy fuel tank in each wing. Together they hold a total of 830 litres. The pump in each wing tank is powered from a different electrical system. In the event of a pump failure, both engines can feed from the same tank, or in the case of an engine failure, one engine can feed from both tanks. Cross feeds activate automatically in the event of a pump failure to assure uninterrupted fuel flow to both engines. In the event of a complete loss of electrical power to both pumps, the engine driven pumps still can maintain adequate fuel flow.
An Avco Lycoming LTC1K-4K engine, a modified version of the standard T53-L-13B engine, is mounted at each wing tip. They are rated at 1250shp for continuous operation, 1401shp for 30 minutes, 1550shp for 10 minutes for take-off, and 1802shp for two minutes for emergency power. Power is transmitted from the engines to the rotors using a coupling gearbox and transmission, which reduce the engine speed of approximately 20000 revolutions per minute down to a rotor speed of about 565 revolutions per minute in hover. The three-bladed, semi-rigid rotors measure 7.6m in diameter and have a 36cm chord. They were made of stainless steel and have a large amount of twist. (In July 1979, Bell received a contract from Ames for preliminary design of a composite rotor blade that would offer improved performance and increased life expectancy, compared to the existing metal blades. A set eventually was tested, but did not work well.) There are no flapping hinges, but the rotors can flap forward or aft as much as 6 degrees. To assure power to both rotors in the event of an engine failure, a shaft that runs through the wing interconnects the two transmissions. As a result of the interconnect, both rotors turn when the first engine starts. In the event of a double engine failure, both rotors will autorotate at the same speed. The nacelle tilt can be varied from horizontal to 5 degrees aft of vertical. Interconnected double ballscrew actuators operate the tilt mechanism in each nacelle. This assures that both nacelles always will be at the same position. The interconnected drive shafts and redundant tilting mechanisms permit single engine operation and fail-operate tilt capability.
The cockpit has dual controls and resembles a helicopter cockpit, including a collective stick. The flight controls are designed to permit single pilot operation from either seat. In airplane mode, the control columns and rudder pedals work conventionally. In hover mode, the stick functions as a cyclic pitch controller. The mechanical mixing unit does everything needed to convert the controls from the helicopter mode to the fixed wing mode. Control authority between helicopter and airplane mode is phased in as a function of the nacelle tilt angle. This includes changing the rotors from cyclic pitch control in vertical flight to constant speed control for fixed wing flight. In airplane mode, the collective lever can still be used as a power lever. Moving the collective lever causes the throttles on the center console to move.
Two switches, mounted on the collective lever and operated by the pilot's thumb, control the nacelle tilt angle. One pivots the nacelles from end to end in about 12 seconds and allows them to be stopped at any position. The other switch moves the nacelles between pre-selected angles of 0, 60, 75, and 90 degrees (relative to horizontal). To rotate the nacelles, electrical valves activate hydraulic motors. In the event of a complete electrical system failure, the pilot can manually open the valves using T-handles in the cockpit. This will drive the nacelles to the helicopter position.
Sperry Rand built the original navigation/guidance system. A digital computer provides navigation and control information to the pilot using advanced mechanical and electronic displays. The Calspan Corporation of Buffalo NY designed the Stabilization Control Augmentation System to improve its flight characteristics. The XV-15 does not incorporate fly-by-wire. Ailerons, elevator, and rudder are hydraulically boosted with a triple hydraulic system. They remain active in all flight modes.
The XV-15's empty weight is 4315kg with a vertical take off weight of 5865kg. This allows 495kg for instrumentation, 180kg for pilots, and 630kg of fuel, while leaving a few left over for growth. Original estimated performance included a maximum level speed of 610km/h, service ceiling of 8845m, and a range of 800km. None of these goals ever were achieved, but the XV-15 did achieve its primary objective of proving the practicality of the tilt rotor concept.
In April 1974, Rockwell International (Tulsa Division) received a contract for the construction of the fuselage and the tail assembly of the two XV-15 airframes.
On 2 October, 1975, components of the first prototype were delivered to Bell Helicopter in Fort Worth where final assembly would be undertaken. Aircraft No.1 (c/n 00001, N702NA) was rolled out, at Arlington, on 22 October, 1976. Before tiedown dynamic tests simulating all flight modes, the XV-15 No.1 underwent an extensive integration checkout.
Ground runs began in January 1977, and included 100 hours of system qualification tests on an elevated test stand in both the helicopter and airplane modes to demonstrate that the aircraft met final flight qualification requirements. The first hovering flight was performed on May 3, 1977, followed by hover and low speed evaluations. This short test effort consisted of only three hours of hovering during May. No problems that warranted corrections were uncovered. Following these flight tests, the transmissions and rotors were torn down, inspected and reassembled.
Because of NASA's insistence on full-scale wind tunnel tests before attempting a conversion, aircraft #1 was shipped by C-5A to NASA's Ames Research Center in March 1978 for wind tunnel tests. These tests were conducted in the Ames 12m x 24m foot wind tunnel in May and June 1978. Twenty hours of tunnel tests were performed at airspeeds between 110 and 330km/h. Configurations consisted of the rotors in helicopter and airplane positions, and numerous intermediate positions that would be encountered during transition. No unusual characteristics were noted in any of the tests conducted.
Following the wind tunnel tests, the #1 aircraft was torn down and refurbished at NASA Ames.
The second XV-15, #N703NA, was nearing completion. Since the program lacked funds to keep two aircraft on flight status, testing resumed with the #2 aircraft, beginning ground tests in August 1978 at Arlington. Numerous minor problems plagued the aircraft during these tests, including a stress corrosion crack in the left engine gearbox, a clutch misengagement, and foreign object damage within the transmission. It made its first hovering flight on April 23, 1979, with Ron Erhart and Dorman Cannon, Bell's XV-15 project pilot, at the controls. Conversion tests soon began, starting by rotating the nacelles 5 degrees forward on 5 May 1979. Successive tests gradually rotated the nacelles closer to horizontal, until the first complete conversion was made on July 24, 1979. The XV-15 also achieved a forward speed of 295km/h on this 40 minute flight. On 21 April, 1980, No.2 prototype reached 485km/h at 2530m. In one year of testing, aircraft No.2 logged 40 hours flying.
The Navy became interested in the XV-15 and because of continuous funding shortfalls, the Naval Air Systems Command began providing funding in 1979 and 1980 to insure the XV-15 flight testing would proceed up through the completion of envelope expansion flights. In exchange, the Navy would be allowed to perform flight evaluations.
During the contractor test program, all potential failures were simulated in actual flight or on the ground. On December 5, 1979. an actual engine failure occurred when the turbine seized. The transmission interconnect system worked properly, and both rotors continued to turn as designed. The predicted speed of 555km/h true airspeed was demonstrated with maximum rated power at a 4880m density altitude in June 1980.
The contractor flight test phase was completed in August 1980. The basic conversion corridor and airspeed/altitude envelope up to 16,000 feet was demonstrated. About 100 full conversions were made. Some resonance problems were uncovered, as is normal in any helicopter development, but they were fixed quickly.
Upon completion of the contractor flights, XV-15 #2 was shipped to NASA's Dryden Flight Research Center for continued testing, where it was joined by aircraft #1. Both XV-15s then operated at Dryden for a short period.
XV-15 #1 returned to Bell in September 1981. Flight testing by both NASA and Bell continued into the 1980s, and the two XV-15s proved to be virtually free of any significant problems. Additional accomplishments that were demonstrated included:
- 1.7 hour cruise endurance in airplane mode.
- Cruise speed of 425km/h.
- Take off as helicopter, fly twice as fast as a helicopter, and deliver payloads on half the amount of fuel when traveling distances of greater than 185km.
- Autorotation descents in helicopter mode, but never to a full touchdown.
- STOL take-offs with the nacelles tilted between 60 and 70 degrees, at the maximum gross weight of 6765kg.
For taxiing on wheels, it was found that tilting the nacelles forward of vertical only 1 degree was enough to start the XV-15 moving forward. Tilting the nacelles aft of vertical brings the aircraft to a quick stop. The XV-15 tends to rock a bit more than other aircraft because of the weight of the engines and props all the way out at the wing tips.
In hover, roll control is provided by differential rotor collective pitch, pitch control by cyclic pitch, and yaw by differential cyclic pitch. For maneuvering in the hover mode, many of the maneuvers normally performed by moving the cyclic control are done by tilting the nacelles. A combination of rotor angle and cyclic pitch also is used to vary the pitch attitude without moving forward. By tilting the rotors forward and simultaneously putting in aft cyclic control, the nose will pitch down, giving improved visibility over the nose.
Vertical liftoff is very easy, as the XV-15 holds attitude on liftoff. Lateral movement is accomplished by banking slightly so that the thrust has a small side component. The XV-15 can travel sideways at 65km/h with no tendency to turn into the wind. It can hover backwards up to 65km/h.
Touchdown surfaces can have an uphill or downhill slant of up to 15 degrees. Single engine performance is relatively poor, with single engine hover possible under only a very few conditions.
During conversion from hover to conventional flight, there is a tendency to lose lift and sink, requiring the pilot to add power, but this is normal on all VTOL aircraft.
With the nose up and full aft stick, level stalls in the clean configuration give a slight vibration at 205km/h. The aircraft will begin to sink, but there is no wing drop or other bad effects. In helicopter autorotation mode, the best descent rate of 11m/s is achieved at 140km/h. At 165km/h, the descent rate increases to 20m/s. For final approaches, pilots use nacelle tilt angle instead of pitch inputs to control airspeed.
The 630kg of fuel contained in the wings proved to allow for only about a 280km range. An auxiliary tank holding an additional 405kg eventually was added to the fuselage, which increased the range to about 520km.
In March 1982, aircraft #1 made a demonstration tour of East Coast facilities, which included seven flight demonstrations at six different locations in eight days. One flight included a stop at the helipad at the Pentagon. While on the tour, the XV-15 flew 4815km and needed only routine daily preflight maintenance.
Following this East Coast tour, #1 was modified at Bell's Arlington facility to perform an electronics mission evaluation. Items added included an APR-39 radar warning system and chaff dispenser system. The aircraft departed for NAS China Lake in California in May, then on to Ft. Huachuca in Arizona in June, and finally on to San Diego for sea trials. Shipboard evaluations were performed aboard the amphibious assault ship USS "Tripoli" off the San Diego coast in July 1982. Fifty-four vertical landings and take-offs (of which five were STOL take-offs) were performed.
Other mission related evaluations included over-water rescue and simulated cargo lifting, which were demonstrated in May 1983, and simulated air-to-air refueling, which was performed in September 1984. By 1986, both aircraft had accumulated a total of 530 flight hours, made 1500 transitions, and reached an altitude of 6860m (while still maintaining an 4m/s climb capability). In March 1990, #1 set numerous time to climb and sustained altitude records for this class of aircraft. These included a climb to 3000m in 4.4 minutes and to 6000m in 8.46 minutes, without even performing extensive climb tests to develop an optimal climb profile. It also sustained an altitude of 6860m with a dummy payload of 990kg in addition to more than 450kg of test instrumentation.
Under the new JVX programme Joint Services Advanced Vertical Lift Aircraft Program) the XV-15 served as test-bed. In direct relation to the JVX programme, XV-15 No.1 was tested in Fort Huachuca to evaluate its ability to accomplish SEMA missions (Special Electronics Mission Aircraft); the aircraft was sent to China Lake to measure its radar signature and, on 2-5 August, 1982, off San Diego, Lieut-Cdr John Ball and Dorman Cannon conducted the initial shipboard evaluation on board the amphibious assault ship USS Tripoli (LPH-10). This evaluation included vertical and short rolling take-offs, hovering flights and vertical landings. On this occasion, one of the 54 XV-15 landings was the ship's 60.000th.
The aircraft was then sent back to Fort Worth to undergo a complete overhaul and to receive several modifications. By the end of August 1982, the two prototypes had logged 289 hours of flight testing. The two XV-15s were then used in a research programme to explore the limits of the operational flight envelope and assess its application to military and civil transport needs. Late in 1987, the XV-15, piloted by Dorman Cannon and Don Borge, demonstrated its capabilities in the civil transport role at Washington and Chicago. The Chicago demonstration was conducted from Miegs Field in the very heart of the city.
As of June 1990, XV-15 #1 was based at Bell Helicopter's Flight Research Center in Arlington, TX, for continuing engineering development. XV-15 #2 was based at Ames for continuing tilt-rotor research. The two aircraft had accumulated 825 hours.
By 1990 the XV-15 was flown by over 185 pilots with widely varying experience and capability levels, including several low-time private pilots. Numerous admirals, generals, and at least one U.S. senator and one service secretary flew as guest pilots. Each flight consisted of a brief demonstration of helicopter, conversion, and airplane modes by a Bell test pilot. The guest pilot then took over the controls. After a few minutes of familiarization, he was talked through an airplane stall, single engine operation, and conversion/ reconversion at altitude. They then return to the airport for several take-offs and landings, usually converting to airplane mode and back to helicopter mode each trip around the pattern. Guest pilots rated the XV-15 as easy or easier than a helicopter to hover. Conversion was unanimously said to be straightforward, and with a low workload. Handling qualities in airplane mode were excellent. Most also noted the low interior noise and smooth ride.
FAA test pilots also flew the XV-15 in order to evaluate its potential for certification of a civil tilt rotor aircraft. While they saw no technical reasons for not being effective in the civil role, they determined that a review of Part 25, which sets standards for large transport aircraft, and Part 29, which sets standards for helicopters, would be needed in order to establish appropriate certification criteria.
XV-15 #1 remained in service at Bell's flight research center, where it was used as a concept demonstrator and marketing tool for the V-22 Osprey that was by then being developed. It was flown regularly until August 1992, when it was damaged beyond economical repair. A mechanical failure in the control system caused the aircraft to roll over while it was hovering. The crew was not injured, but the wing and one nacelle sustained extensive damage. At the time of the incident, #1 had flown nearly 841 hours. The forward fuselage was salvaged and put to use as a simulator to help develop Bell's upcoming civil tilt rotor aircraft.
XV-15 #2 remained at Ames through the 1980s. In 1986, it was fitted with composite rotor blades built by Boeing Helicopter. Sporadic testing was accomplished through 1991, when it was stopped due to a problem with the blade cuff that resulted in an emergency landing. While the blade cuff was being re-designed, NASA decided to put the airframe down for a major airframe inspection that would be due soon, anyway. Program funds again ran out before the inspection could be completed. #2 would remain partially disassembled until mid 1994. It had accumulated just over 281 hours.
With Bell anxious to resume tilt rotor development, they established a Memorandum of Agreement with NASA and the Army in 1994 which transferred XV-15 #2 to Bell and allowed them to return it to service at no cost to the government. The disassembled aircraft was shipped to Arlington, Texas, and the refurbishment and inspection began in mid 1994. The original metal rotor blades were put back on, and the aircraft resumed flight testing in March 1995.
Much of Bell's recent research has focused on reducing noise in order to make civil tilt rotor more acceptable for operating in crowded urban areas. Tests were being conducted to determine the major sources of noise.
As of the end of 1998, the remaining XV-15 had accumulated a total of 530 flight hours and remained in service at Bell's Arlington facility to continue developing and refining Tilt Rotor technologies.
Engines: 2 x Avco Lycoming LTC1K-4K turboshafts. 1,800-shp (1342-kW)
Propeller-rotors: 25ft / 7.62m diameter three-blade
Rotor speed forward flight: 458rpm
Rotor speed hovering flight: 565rpm
Transition: 12 seconds
Take-off weight: 5897kg
Empty weight: 4341kg
Max speed: 550km/h