On the Vertical Horizon:
Bell Designs Are Accelerating at Full Tilt
By Mike Hirschberg

Bell Helicopter Textron has nearly 50 years experience with tiltrotor aircraft designs. After building the XV-3 in the early 1950s and then the XV-15 in the mid 1970s, Bell studied scores of different designs for various commercial and military applications. With the initiation of the JVX program in 1981, Bell finally established a production line for a tiltrotor, the V-22. With the V-22 Osprey now in Initial Low Rate Production, Bell has unleashed a barrage of exciting new concepts using tiltrotor technology.

QTR

At the Pentagon on 8 September, Bell participated in a special Tiltrotor Technology Presentation at the Pentagon. The first production MV-22 landed on the Parade Grounds along with the XV-15, dressed in Coast Guard colors. Bell also unveiled its Quad Tiltrotor (QTR) design to the public (see Vertiflite, Summer 1999).

With the Quad Tiltrotor concept, Bell seeks to capitalize on V-22 investments to develop a large payload, high speed, Vertical and/or Short Take Off and Landing (V/STOL) capability for the military within 10 years. The QTR would use V-22 propulsion and support systems: engines, rotor systems, drive train, transmission, hydraulics, electronics, and generators, except that QTR would have four engines, instead of two, mounted on fore and aft wings. The wing structure outboard of the flaperons would also be common; however the front wing would have a slightly longer span than the V-22 to accommodate the wider fuselage The rear wing would be longer than the front wing, putting the rear rotors outboard of the front rotors for higher performance and fuel economy in cruise. The front and rear propulsion systems could be interconnected for additional reliability. The glass cockpit, avionics, instruments, and threat warning systems could also be adapted directly from the V-22.

The QTR fuselage would be the size of a Lockheed Martin C-130-30 Hercules transport, and could transport a wide assortment of loads: eight 463L pallets, 90 passengers, 70 stretchers, a helicopter as large as an AH-64 Apache, a 155 mm howitzer, or three HMMWVs. A rear ramp, rollers and rails would facilitate common logistics equipment used for the C-5, C-17, C-130 and C-141 loading. According to Bell, an advanced concept technology demonstrator (ACTD) could fly by 2005, with production deliveries beginning in 2010. Although it would be possible to use a modified C-130 fuselage for a demonstrator, there are very different structural requirements since the QTR has two wings versus the single wing of the Hercules. For production, lower weight and a better match for the expected payloads would be possible with a new fuselage. Although Boeing builds the V-22 fuselage, Dick Spivey, Bell's Director of Advanced Concepts, said Boeing would not necessarily be a partner on the QTR; either Bell or a subcontractor could build the fuselage. Bell has recently determined that it can eliminate the vertical tail entirely, and provide directional stability via differential rotor thrust.

Figure 1. The QTR would be able carry up to eight times the cargo of the V-22.

The QTR would be able to deliver cargo from airfields and port facilities directly to ground maneuver units and to ships at sea, needing as little as 1/2 acre to land. The QTR would allow a practical means to transport up to 30,000 lb externally or 40,000 lb internally far from shore bases (due to its size, however, it would obviously not be able to be stowed below deck). With twice the propulsion system of the V-22, the QTR could hover at 100,000 lb and have a maximum weight of 140,000 lb; internal volume would be 6-8 times that of the V-22. Maximum unrefueled range would be 2,000 nautical miles and it could cruise at 280 kt. Shown below is the payload the QTR could deliver to a given range (to dry tanks) for both sea level and 95 °F/4,000 ft elevation conditions after taking off from a hover out-of-ground effect, hover in-ground effect (10 ft wheel height), STOL take-off, and at maximum weight. A maximum payload weight of 42,000 lb has been assumed as a design constraint. A 10% fuel reserve would reduce the ranges by 10%.

Figure 2. Payload-Range capability for the QTR (nm).

According to Spivey, Bell has flown two V-22s in close proximity to each other, approaching the distance between the fore and aft rotor system, with no difficulties. Water tunnel tests indicate that the rotor wake from the front rotors in forward flight flow down and inboard, below and inboard the rear rotors. It should also be noted that from 1966 to 1980, some 200 flight test hours were conducted on two Bell X-22 quad tilt duct demonstrators.

Using common parts with the V-22 would not only reduce the cost of the QTR, but also that of the V-22. It would allow existing support equipment, test equipment and spares pipeline to be used for both aircraft, reducing the logistics footprint. Efficiencies in maintenance and training could also be realized.

Bell is discussing the Quad Tiltrotor with the Services to define potential requirements (e.g., the Joint Transport Rotorcraft/Joint Common Lift mission), and is pursuing possible risk reduction activities with DARPA. With the QTR, the Marine Corps could deliver thousands of tons of supplies per day to forward troops without having to depend on land supply routes. The Navy could deliver of tons of supplies to carriers and even non-aviation ships while underway, without the need for a catapult or arresting gear. The Army is seeking to replace the heavy lift CH-47 Chinook helicopters it uses today with much greater capability, and is currently funding advanced rotors, transmissions and structures science and technology programs. The Air Force could supply its aerospace expeditionary forces directly from forward operating bases. Humanitarian relief, such as was needed for Central America in October 1998 after hurricane Mitch, could be greatly improved with a large heavy lift V/STOL transport. And non-combatant evacuations could be conducted from the continental US to anywhere in the world using the QTR's aerial refueling capability. For example, in a hypothetical rescue mission, a QTR could take off from Quantico Marine Corps Base in Virginia and fly to the American Embassy in Moscow with two refuelings over the North Atlantic. The QTR could then take up to 80 passengers out to a vertical recovery on an American ship in the Baltic.

Furthermore, since the Army has shown a desire for the capability to hover with a 20 ton payload and have a 500 ft/min vertical rate of climb, Bell has investigated Pre-Planned Product Improvements (P3I) that would allow the QTR to lift a 40,000 lb payload on a 95 °F hot day at 4,000 ft elevation, and still have a 1,000 km (540 nm) range (see graph). With advanced engine technologies, wing download reductions, higher transmission torque, and 30% additional rotor area, a QTR capable of lifting 22 tons on a hot day could be available for production by 2015.

Figure 3. QTR growth for 22 ton hot day requirement (km).

BA609

At the World Aviation Congress, held in San Francisco in October, David Snyder, Bell Vice President of Research and Technology, presented the latest concepts from the Bell design teams. Snyder stated that Bell/Agusta are holding steady at 77 orders for the BA609 tiltrotor, and that Bell has needed to "slow down to meet the commitments" of their military customers. The advance orders are from 41 different customers in 18 different countries: 40 from North America, 22 from Europe, 12 from Asia, and 3 from South America.

Bell's market forecast is for 1054 aircraft over the next 20 years. The design was initiated in September 1996, sized for nine 200 lb oil field workers. The test program includes four flight vehicles, with a first flight date of December 2000. Type certification is expected in May 2002 with the first production delivery in June 2002. Currently, 80% of the drawings (2,283) have been released, and 63% of the tooling (963) is ready. The BA 609 tiltrotor is expected to cruise at 275 kt with a range of 750 nm. The wing and fuselage for the first prototype are now in tooling, with first flight expected by 2001.

A number of variants of the BA609 have also been proposed. The HV-609 Search and Rescue Tiltrotor is targeted for United States Coast Guard missions such as search and rescue, drug interdiction, surveillance, and other law enforcement roles. The T-609 Tiltrotor Trainer is aimed at reducing the fleet training costs, student time in the training pipeline and the risks associated with conducting initial tiltrotor training in the V-22 Osprey. It would reduce the time to train at all levels of instruction, with comparable systems and performance to the V-22. It would allow development of skills in all three modes of flight - helicopter, transition, and airplane - prior to training on an actual V-22. The UC-609 Utility Tiltrotor is also offered for other missions where the vertical take-off and landing capability of the helicopter needs to be combined with the high speed, forward flight ability of a fixed-wing aircraft.

The projected cost of the BA609 family of tiltrotors would be significantly less than the anticipated $40M for each Osprey. It would also be equipped with a cyclic and thrust control lever, like the V-22, which began its Operational Evaluation (OPEVAL) test period in November and is slated to reach initial operational capability (IOC) with the Marine Corps in 2001.

Figure 4. XV-15 on the US Coast Guard Cutter Mohawk.

Bell has been studying the HV-609 concept since early 1997. Bell is part of the Lockheed Martin team on the United States Coast Guard Deepwater Project to study the Coast Guard's current and predicted 21st Century missions and to make recommendations for future ship and aircraft acquisitions. A typical rescue mission might begin with a crew of four and auxiliary fuel making a Short Take-Off or an In Ground Effect (IGE) vertical take-off, a 3 hour, 250 nm flyout to the scene, 45 minutes on the scene to pick up six people, and return to base. Mission equipment would include a hoist, basket, litter, raft, pump and other gear. On 17-18 May 1999, a Bell XV-15 tiltrotor demonstrator aircraft performed a series of common Coast Guard maneuvers. The XV-15, simulating an HV-609, made two landings on the flight deck of the Coast Guard Cutter Mohawk while the ship was underway off the coast of Key West, Florida. Later maneuvers tested the XV-15's ability to hover over a Coast Guard 41 ft rescue boat, simulating a rescue operation, and over a rescue swimmer who demonstrated recovery techniques in the water.

Figure 5. The Bell 619 (left) and 626 (right) concepts.

Snyder also unveiled two new Bell concepts for larger civil tiltrotor transports, the Bell 619 and 626. Snyder envisions that the Bell 619 will weigh 28,000 lb with two 3,000 shp engines. It could carry 19 passengers with four-abreast seating. The Bell 626, which would weigh 34,000 lb, is a 26 passenger quad tiltrotor, using two BA609 propulsion systems. Current plans do not include eliminating the vertical tail like the QTR. Although advances in flight controls and modeling and simulation allowed the BA 609 to be configured with a single, rudderless vertical tail, rather than the H-tails found on the XV-15 and V-22, Bell's Snyder remarks that it would be easier to convince government customers than commercial customers that the vertical surfaces are completely unnecessary.

Eagle Eye

On November 1,1999, Bell (with teammates Raytheon and Lockheed Martin) submitted an unmanned tiltrotor concept as an entrant in the joint US Navy and Marine Corps Vertical Take Off and Landing Tactical Unmanned Aerial Vehicle (VTUAV) initiative. Bell's VTUAV tiltrotor design is a derivative of their highly successful Eagle Eye demonstrator, designated TR911X. The TR911X demonstrated that it is capable of cruising at speeds of 185 knots and climbing to altitudes of up to 20,000 feet. The existing TR911X design has been upgraded to satisfy all the VTUAV performance specification requirements and half of the objective requirements, as well as to enable future growth.

Figure 6. The TR911X VTUAV demonstrator.

The Eagle Eye demonstrator used 70% off-the-shelf manned-rated helicopter and common hardware parts. It first hovered in early 1992 and conducted extensive tests for the US Navy at Yuma Proving Grounds, Arizona, in late 1993. Bell continued development of the Eagle Eye and early last year was awarded a contract under the Navy's "VTOL UAV Demonstration Program" to demonstrate the performance and maturity of the prototype and its control system.

According to the US Navy, the VTUAV system is intended to be a replacement for the Pioneer UAV system "to collect and pass information utilizing an airborne sensor platform that will provide the battlefield commander an extended and enhanced battlespace situational awareness." The VTUAV contract decision for the Engineering and Manufacturing Development (EMD) phase is expected in February 2000.

The Future

Once the Services begin crystallizing requirements for the Joint Common Lift/Joint Transport Rotorcraft, the Quad Tiltrotor could be a major contender. The final requirements, however, will determine the amount of commonality with the V-22 and therefore the degree of affordability the design will be able to offer.

The BA609 family of aircraft seem poised to bring tiltrotor aircraft into the mainstream. With the large number of advanced orders lined up, it seems certain that the civil tiltrotor will be successful. It also looks very promising that military variants - search and rescue, training, and other missions - will be developed. Once the BA609 has established itself, it is likely that larger civil tiltrotors could make commercial tiltrotor operations commonplace. Tiltrotors could certainly help relieve some of the tremendous air traffic congestion projected for the next century.

Bell has demonstrated four different tiltrotor concepts, operating from 2,250 lb to 52,780 lb. Three of the aircraft, the TR911X, the XV-15, and the V-22, are currently flying. With the experience from these programs, as well as with the development of the BA609, Bell has the expertise and the vision to lead the tiltrotor into the next century. In the coming months and years, we will see if the world is ready to accelerate at full tilt.

About the Author: Mike Hirschberg is an aerospace engineer at ANSER, Inc. He currently supports the Joint Strike Fighter propulsion system development and serves as the managing editor of Vertiflite.