Stalled after two accidents, the V-22 Osprey programme is regaining momentum. With other projects making progress, have tiltrotors come of age?
Three years ago the tiltrotor concept appeared to be dead. Bell Boeing's V-22 Osprey had suffered not one but two fatal accidents within 12 months. US marines as well as aircrew had died. Politicians were lining up to condemn the project as fatally flawed. The media unravelled evidence of cover-ups, accusing programme officials of concealing the Osprey's inadequacies.
The Osprey's first operational evaluation (Opeval) report in 2001, while assessing the tiltrotor transport as "operationally effective", said it was "unsuitable" to enter service with the US Marine Corps or Air Force as the V-22 had failed to meet reliability, availability and maintainability requirements. A later General Accounting Office report criticised programme officials for restructuring the development flight-test effort to relieve cost and schedule pressures.
"We went heads-down for a while after that," says Bell Boeing V-22 programme director Mike Tkach. "We were set a series of challenges by an independent 'Blue Ribbon Panel', and decided that the best way forward was for us to keep a low profile and work on them."
Today, programme officials have more of a spring in their step. They say the main problems are fixed and the Osprey will be in shape to both sail through its second Opeval, due to start in February next year, and to secure full-rate production clearance later in 2005. Significantly, they also know that if they subsequently fail to make that date, it is not the end of the world.
"Progress is now firmly event-driven," says programme manager USAF Col Craig Olson. "We were criticised by the panel for focusing on budget and schedule milestones so we switched emphasis completely. I am now under absolutely no pressure to start Opeval until the airframe has demonstrated through test that it's ready. That's a comfortable feeling for a manager to have."
Olson believes they will make it, nonetheless. "The VMX-22 operational test squadron is currently engaged in a pre-Opeval assessment, with three aircraft flying over 100h, looking at a cross-section of the missions it might encounter. Results will give me and the other managers a heading check, to see if there is anything we've missed. The assessment has been going on for three weeks now and I know they are achieving their goals. All the indications are that we will be ready for this truly robust test of the aircraft's capabilities."
Delivery
Later next year comes the Defense Acquisition Board decision on full-rate production. Bell and Boeing are still in low-rate initial production mode, but are ramping up the assembly process "at a sustainable rate" in anticipation of approval, Olson says. "We are delivering Block A aircraft to the Marine Corps Air Station New River, to VMX-22. Commanding officer Col Glenn Walters has seven MV-22s, with another on the way." Seven more are in developmental flight test at NAS Paxutent River, Maryland and two USAF CV-22s are at Edwards AFB in California. Block B aircraft, which improve aircrew access to the cabin through bigger doors, and maintenance access to the engines via extra inspection hatches in the nacelles, are under construction.
After another review last year by Pentagon acquisition chief Michael Wynne, the V-22 team was given further targets to work towards in three main areas: production rate, capabilities and affordability. Olson says they developed plans that address all those issues, "and I think they follow pretty closely Wynne's original guidelines".
The ultimate aim, Olson says, is to arrive by 2010 at an aircraft with a fly-away cost of $58 million - more than $15 million below the sticker price. A lot of this, he says, can be achieved by simple economies of scale. "But it looks like we will get the necessary adjustments to the programme that will help us further to achieve that goal. Our 2006 plans are under review at the moment and we are expecting a decision later in the summer."
The V-22 mishap in April 2000 was attributed to the aircraft entering vortex ring state (VRS) - also known as settling under power - an aerodynamic phenomenon that can be experienced by rotorcraft during descent under power, whereby air recirculates below the rotor discs and lift is lost. The V-22's reaction to this state was compounded by the fact that only one disc was affected, adding an uncommanded roll problem to the difficulties facing a crew already handicapped by a lack of training in either recognising or dealing with VRS. It later transpired the crew was operating some way outside the cleared flight envelope, and descending at a particularly high rate.
After the accident, Olson says: "We underwent a thorough investigation to look at the phenomenon and its effect on the V-22 airframe. We subsequently defined the area within which it could occur as a much, much narrower one than you would find within an ordinary helicopter's flight envelope. The 'optimum' low-speed descent rate that can initiate the first stage of vortex ring in the V-22 is nearer to 2,000ft/min [10.2m/s], compared with a fleet placard limit for all naval rotorcraft of 800ft/min. There is no tactical justification for descending at 2,000ft/min with a low airspeed."
Pilots also now know that if they ever enter VRS - up to 20s after hearing an accompanying audible "sink-rate, sink-rate" message in their helmets and seeing a visual warning on a multifunction display - all they have to do is "beep" the nacelles forward to put the proprotors into clean air, rather than lowering the nose and reducing power (the standard reaction in a helicopter). "As a result," says Olson, "the V-22 loses far less altitude and can recover to a climb much quicker. We have extensive flight-test data to demonstrate this. The features have been incorporated into the Paxutent River simulator software, so that pilots can learn to recognise the phenomenon."
The second V-22 accident, in December 2000, was attributed to a combination of a hydraulic leak inside one of the nacelles and a flight control system (FCS) software error that led to the wrong information being fed to the crew. The leak was caused by abrasion of a hydraulic line, which had been routed too close to another component.
The Osprey was redesigned to make sure there could be no contact between the various fuel, oil and hydraulics lines at any stage. "In practice, it meant stripping the engine back almost to its basic components and starting from scratch," says Olson. "Once we had found a layout that worked, we tested it both statically and dynamically to find out how it was behaving. After returning to flight, in April 2002, we made regular checks to ensure the clearances were being maintained. Software engineers also wrote new lines of code for the FCS and designed a clearer set of advisory messages for the crew."
Problem solving
The team still faces challenges with theV-22's digital fly-by-wire system. "We identified some uncommanded yaw oscillations. They were relatively minor and didn't read across to any control problems, but we didn't quite understand them. We found that there was a portion of the FCS envelope that was rather too narrow, so we increased the stability margins. It was in a part of the flight envelope that you don't often visit - both high-lateral and high-aft centre of gravity - but we fixed it," says Olson. "We have a little further to go before I can consider the aircraft ready for Opeval," he adds, "but we are at the right point on the curve. Some reliability and maintainability issues are still not where they should be, so we are making further hardware and software changes, but I believe we will get there."
The US Army examined the V-22, but dismissed it as too small to carry its combat vehicles including the Stryker light armoured vehicle and the Humvee. But, looking to the future, army chief of staff Gen Peter Schoomaker has said: "There's a dimension beyond that. It's like V-22 capability with a CH-47 - or C-130-size box - an 'advanced tactical transport' tiltrotor."
Schoomaker was referring to Bell's proposed quad tiltrotor (QTR), one of several ideas mooted to address the army's need for a heavylift intra-theatre transport capable of lifting 25,000kg (55,000lb) vertically. The QTR has larger internal dimensions than Lockheed Martin's C-130 Hercules and can accommodate the Stryker or Humvee with room to spare.
QTR model tests have been conducted at NASA Ames's windtunnel at Moffett Field, California under the auspices of Bell's new XworX research and development centre in Arlington, Texas. The new organisation, modelled on Lockheed Martin's Skunk Works and Boeing's Phantom Works, will be responsible for the rapid prototyping of new rotorcraft such as the QTR and Eagle Eye tiltrotor unmanned air vehicle.
Interaction
"The model will allow us to conduct aeroelastic tests to examine the aerodynamic interaction between the two sets of wings and proprotors in aeroplane and conversion mode," says Bell director of advanced concepts development Mark Gibson. "The tests are key to proving to ourselves that the QTR concept will work. With funding, we could start the tests by the end of this year and finish within 18 months: without it, it might take three or four years."
At the opposite end of the tiltrotor scale, a full-size pre-production prototype Eagle Eye is under construction, having been selected by the US Coast Guard for its Deepwater modernisation programme. AAI, Lockheed Martin and Textron Systems are working with Bell to prepare the Eagle Eye system, including air vehicle, ground station and payloads, for military and para-military applications - and for civil roles such as oil pipeline surveillance. Bell's first civil customer, Evergreen Aviation, has signed a letter of intent for three UAVs.
The HV-911D Eagle Eye for Deepwater recently passed its preliminary design review, showing that the system is within margins for risk, performance, supportability and cost. A 7/8th-scale model of the UAV flew for over 90h, achieving 200kt (370km/h) in level flight, while carrying a 95kg payload. The full-scale prototype is 12-15% larger and weighs just under 1,360kg and has a payload capability of up to 135kg, says programme manager Rob Dompka.
The full-scale prototype is set to fly at the end of this year or early next, 14-16 months before the first Deepwater prototype, says Dompka. The USCG's HV-911D will be powered by a 640shp (475kW) Pratt & Whitney Canada PW207D turboshaft and will carry a Telephonics 1700CG surveillance radar and FLIR Systems Star Safire electro-optical/infrared sensor. It will have a 5h endurance, operating from coastguard cutters after becoming operational in late 2007.
Progress on the V-22 has been firmly linked to that of its smaller civil cousin, the Bell/Agusta Aerospace (BAAC) BA609, for several years. Now that life is becoming relaxed on the Osprey programme, engineering resources are being used once more on the six- to nine-passenger civil tiltrotor.
Since the initial 14h flight-test programme ended in April last year, the US/Italian team has been evaluating refinements to the BA609 in the vehicle management system integration laboratory, which replicates the hardware and software in the actual aircraft. Technical director Ross Menger has also maintained the tempo of work on the aircraft, although not all of it is taking place on US soil.
"We have reorganised and restructured the entire programme with Agusta," Menger says. "They are now taking on a significantly increased proportion of the work; particularly the flight-test segment. Three out of the four prototypes will find their way to Italy." The second BA609 has been shipped to the Cameri air base west of Milan, where it has been undergoing safety-of-flight proof-load tests.
Test requirements
"That aircraft is scheduled for proof loads of flight-control surfaces including elevators, flaperons and rotor controls," says Menger. "These are certification test requirements to demonstrate that the surfaces will move freely under load throughout their ranges, without excessive deflections or binding. Aircraft two was sent to Italy as, essentially, two large primary structures - a wing and a fuselage - and so in parallel with the tests it will have the systems integrated and assembly completed.
"Aircraft one will return to flight first towards the end of this year, followed by number two in Italy a few months later," says Menger. "We will then progress through the conversion corridor to full aeroplane mode." In the meantime, aeroplane-mode FCS software development is continuing, together with that of the pressurisation and environmental control systems. "Aircraft three and four are still here, in various stages of completion of the primary structure, and will be shipped to Italy later this year," he says. "The wing and fuselage of aircraft three are taking shape and four is still in early stages of assembly."
On target
As a result of initial flight tests, brake pedal geometry is being reconfigured to ease operation; the main landing-gear shock strut is being modified to allow softer landings; the glareshield is being reduced in size to improve visibility; and, to add protection in the event of a birdstrike, two small overhead windows in the cockpit have been eliminated. The two Pratt & Whitney Canada PT6C-67A engines will have new mechanical fuel-control units, designed to improve fuel scheduling during start and help achieve faster acceleration during one-engine inoperative conditions.
"Once the aircraft starts flying again," says Menger, "we will concentrate on expanding its flight envelope and then look at certification issues. We are still on target for US Federal Aviation Administration certification in 2007 and first deliveries soon after." BAAC says it has orders for 65 BA609s from 43 customers - 30% corporate, 30% from offshore operators and the rest for emergency-medical, government and utility missions. The US Marine Corps is showing interest in the Osprey's smaller sibling as the basis of an armed escort for the MV-22. Clearly the concept is again gaining lift.
ANDREW HEALEY / LONDON
Source: Flight International
