FLIGHTTESTING of the Boeing/ Sikorsky RAH-66 Comanche scout/ attack helicopter is "making progress and staying safe", the team says. With only one prototype available, and an annual fight for funds to keep that aircraft flying, safe expansion of the flight envelope is paramount, says flight-test director Clarence Hutchinson.
A second prototype is to join the flight-test programme in 1998, but will be grounded soon after for installation of the mission avionics. As a result, the first prototype is being used for more than just envelope expansion. The test team, which combines Boeing/Sikorsky, the US Army and engine supplier LHTEC, is collecting data on aircraft aspects ranging from systems to signatures, while pursuing "a safe, building-block approach" to expanding the flight envelope, Hutchinson says.
"With only one prototype, we cannot dedicate different aircraft to test different aspects," he says. Instead, the prototype is instrumented heavily, allowing "several thousand" parameters to be recorded on each flight of just over 1h. Under its integrated test plan, the team has already taken preliminary looks at the Comanche's acoustic and infra-red signatures, and evaluated key aircraft systems.
EXTENDED VALIDATION
The Comanche is being flight-tested under a $1.7 billion "Phase III" contract signed in January. This is an extension of the earlier $1.9 billion demonstration/validation effort, and runs until late 2001,when engineering and manufacturing development is scheduled to begin. The US Army's current plan calls for initial operational capability in late 2006.
Phase III includes the flight-testing of two prototypes and the construction of six "early operational-capability" Comanches. These will be delivered to the US Army for a one-year user evaluation in 2003-4. While this is under way, Boeing/Sikorsky is scheduled to build ten low-rate initial production RAH-66s with which to complete development and equip the first operational Army unit. Some 500h of flight-testing is expected to be accumulated on the two prototypes during Phase III. Testing began in January 1996 and, so far, more than 30 flights have been completed at Sikorsky's West Palm Beach, Florida, test centre, for a total test time exceeding 35h.
While this would seem a small return for more than a year of testing, Hutchinson says that more has been achieved in the first 35h than on any previous programme with which he has been associated.
About 70% of the initial flight envelope has been cleared. The aircraft has been flown to a 149kt (275km/h ) true airspeed and 1.75g load-factor. "We had hoped to have completed initial envelope-expansion by now, but the aircraft has already shown itself to be better than any previous helicopter," Hutchinson claims.
A gear failure on the propulsion-systems testbed (PSTB) resulted in the transmission on the prototype being limited to 84% power. Failure of the input bevel-gear was traced to resonance, which has been overcome by attaching an elastomeric-damper plate to the gear web. As the original transmission had undergone around 100h tie-down testing on the PSTB before the failure, the aircraft has been cleared for 45h flight testing before the transmission must be replaced, Hutchinson says.
The hours remaining before the prototype is grounded in April, for installation of the modified transmission, will be spent evaluating fairings on the main-rotor torque tubes. These carbonfibre-composite "cuffs" transmit pitch-change commands to the blades, which are attached to a "flexbeam" bearingless hub. Hutchinson says windtunnel testing indicates that the fairings may alleviate vortex shedding, thought to be the cause of tail vibration.
The fairings, which are planned for the production Comanche's low-observable hub, were not previously fitted to the prototype. They are one of a series of fixes developed to overcome excessive loads and vibration observed on the tail of the prototype. Others already tested successfully include increasing the incidence of the horizontal stabiliser by 3í, to 7í, which has reduced symmetric loads on the tail. This change reduces the aircraft's pitch attitude by 2í, and "-gets the tail out of the flow", Hutchinson explains.
STABILISERCHANGE
Asymmetric loads have been eliminated by altering the relative incidence of the left and right horizontal stabilisers. This has been achieved on the prototype by attaching "Gurney" flaps to the trailing edges of the stabiliser. These are simple metal strips at 90í to the stabiliser (right up, left down), and equal in height to about 1% of the stabiliser chord. "They are a cheap way of looking at changing incidence angle, and will not stay on the aircraft," Hutchinson says.
After installation of the modified transmission and a planned software update, flight testing is scheduled to resume on 30 June and continue "full bore" for the next year, he says. Test goals for 1997 include: flight at the design 170kt horizontal speed; 2g load factor; 45kt sideward and rearward flight; and 90í snap turns at 80kt. The latter will use the full 450kW (600shp) capacity of the Comanche's "fan-in-fin" tail-rotor to turn the aircraft rapidly through 90í to point the gun in forward flight.
Based on testing so far, the team expects to meet its goals. The modified transmission will enable the helicopter to be flown to 107% rotor RPM in high-g manoeuvres. The rotor has been tested at this speed on the PSTB, which is used to ground-test the Comanche's dynamic system and which Hutchinson describes as "-everything above the chine line - transmission, shafts, hydraulics, electrics and flight controls". Some 280h of rotor testing has already been completed on the PSTB, including some time at 107% and 95% RPM. The latter will be used in "quiet mode" to reduce rotor noise.
Initial acoustic testing shows that the prototype, although not fully configured, is already within the noise specification, Hutchinson says. A test flight witnessed by Flight International, during which the helicopter was hovered within gun range of the observers, demonstrated the Comanche to be extremely quiet. Infra-red signature also meets the specification, he says. The helicopter has an ejector which entrains outside air to cool the engine exhaust, which enters the ejector at 600íC and exits at 260íC through slots running the length of the tailboom.
Maintainability is also good for a prototype, says Hutchinson. The helicopter has "unique" keel-beam structure which carries all the loads. None of the skin is structural, and there are many large panels which can be removed for access. The weapons-bay doors can be used as maintenance platforms when the aircraft is on the ground. "There have been no major maintenance issues," he says.
The first prototype has a limited mission-equipment package (MEP), which essentially provides subsystems control and the caution/ advisory/warning system. Most of the initial flight testing, therefore, is focused on the fly-by-wire flight-control system, which is supplied by Lear Astronics. The system has "dual-triplex" redundancy and is two-fail operational, explains chief project pilot Rus Stiles.
The "core" primary flight-control system (PFCS) provides a degraded, backup, capability and is not selectable by the pilot, says Stiles. In this mode, rotor pitch-change is proportional to sidestick displacement and there is automatic trim follow-up. Core PFCS mode is designed to provide Level 2 handing-qualities "up-and-away", meaning that pilot workload is higher than desired, but acceptable for normal flight.
The "mission" PFCS mode introduces yaw-rate feedback, which basically keeps the helicopter's nose facing forward, Stiles says. This requires four yaw-rate sensors to provide two-fail operational capability. Mission PFCS mode is designed to provide Level 2 handling-qualities in nap-of-the-earth (NOE) flight.
The mode to which the helicopter automatically defaults on start-up is the automatic flight-control system (AFCS). In its basic form, AFCS mode provides rate command and attitude hold - move the sidestick to the left and the aircraft will bank to the left, release the stick and it will hold the bank angle, Stiles explains. AFCS mode is designed to provide low-workload, Level 1, handling qualities for day, visual flight.
Agrowth AFCS mode will provide Level 1 handling qualities for NOE flight in degraded visibility,Stiles says. This will introduce attitude command and velocity hold. Basically, the helicopter will fly itself, he explains: move the sidestick forward and the aircraft with pitch down and accelerate, release the stick and the aircraft will level off automatically and maintain the new speed. A mode in which the helicopter will return automatically to level flight after any manoeuvre is required when flying NOE without any external horizon reference, Stiles says.
SIDESTICKCLIMB
The sidestick operates in four axes, although the Comanche also has a conventional collective level. When flying the helicopter through the AFCS, Stiles says, pulling up on the sidestick commands a climb rate of up to 800ft/min (4m/s). This is useful in the scout/attack helicopter's normal mode of operations of popping up to unmask its sensors and weapons, then dropping back behind the cover of foliage or terrain, he says.
Stiles says that the prototype has exhibited low vibration levels, even though it has no vibration-reduction system installed. Hutchinson says that there are provisions in the aircraft for such a system, and for which components have been made, but that flight-test results indicate that vibration absorption may not be required.
The next major milestone in the Comanche programme will be the first flight of the second prototype in 1998, leading to flight testing of the reconnaissance MEP in 1999. The next major challenge will be to avoid a six-month grounding of the first prototype, beginning in April 1998, when that year's funding runs out. The US Army is seeking an additional $40 million to keep the Comanche flying throughout 1998, having successfully secured additional funds to avoid a similar interruption this year.
Hutchinson wants to avoid a grounding not only because of its potentially adverse impact on pilot training and aircraft configuration-control, but also because a hiatus will increase pressure on the rest of the flight-test programme. "We have not removed any test objectives. If the Comanche is grounded, we will just have to get cleverer," he says.
Source: Flight International
