Super Cobra Solution Will Be Applied To Entire Marine H-1 Fleet

A survivability upgrade to the current fleet of AH-1W Super Cobras has turned out to be a solution to tail boom structural problems encountered during developmental testing of AH-1Z and UH-1Y aircraft.

Turning the exhaust away from the helicopter's tail boom will not only further decrease the AH-1W's infrared signature, but engineers here expect it will help with decreasing engine exhaust heat on AH-1Z and UH-1Y tail booms.

H-1 program officials paused flight testing February 5 because of decreased structural strength in tail boom components caused by increased engine exhaust temperatures and higher loads and torque imparted on the tail boom by the upgraded T-700 engines.

"Tail boom heating has had our interest since flight-testing began in 2000, but the full impact has only recently become apparent," explained Col. Doug Isleib, H-1 program manager here.  "We'd prefer that these sorts of things didn't crop up, but we'd rather deal with them now than have them be problems for Marines out in the fleet.  This is why we do developmental flight test in the first place."

Advanced non-destructive inspection technology enabled NAVAIR engineers to understand the problems caused by the heating before they became safety of flight issues.  A pause in the flight test schedule to incorporate the upgrade before safety issues arise is a recognized part of the developmental flight test environment.

The AH-1Z and UH-1Y both use the T-700 series engines.  Higher aircraft gross weight for the Y/Z, coupled with an upgraded transmission, allows the engines to operate at optimal performance, creating more power and resulting in increased exhaust gas temperatures, loads and torque.  The higher temperatures of the exhaust gases hitting the tail boom structure caused greater-than-expected weakening of the metal, known as annealing.

Following an intensive review of data and engineering review, the aircraft have been returned to a flight status with a reduced gross weight envelope and are undergoing interim modification of the tail booms that will restore a full envelope.  This interim modification involves adding skin and structure "doublers" to the tail boom that will return it to original design specifications for heat tolerance.

"Interim fixes that allow the test team to resume flight test will involve strengthening the structure locally which will allow the resumption of testing with a full flight envelope while a long-term fix is developed," Isleib continued.  "The interim fix of the tail boom only addresses the symptoms of the problem so that flight testing can continue.  The optimal fix addresses the root cause of the problem and involves diverting the exhaust gases from the tail boom."

Turning the exhaust improves the life of the tail boom by reducing the amount of hot exhaust impacting it.  It also reduces the aircraft's overall infrared signature - a feature that will greatly increase survivability according to lessons learned in Operation Iraqi Freedom.

Diverting the exhaust is an accelerated engineering effort that has already been initiated for the currently fielded fleet of AH-1Ws to provide additional survivability.  Because the AH-1W shares the same engines and basic tail booms as the upgraded H-1s, this survivability upgrade was projected to forward fit to the AH-1Z/UH-1Y aircraft.  Increased concerns about tail boom heating accelerated the introduction of the turned exhaust for the upgrade aircraft.

"We were going to do this all along to gain additional survivability," Isleib explained, "but we found that what works for IR signature reduction also works to lower tail boom temperatures - making turned exhaust the optimal solution to both concerns."

While other solutions continue to be explored, none appear to be as promising as turning the exhaust nor do they address the root cause of the problems.

Program officials estimate the first prototype turned exhaust kit to arrive within two weeks.  Installing and testing the turned exhaust on the H-1 upgrade test aircraft may shift the operational evaluation of the platform by approximately four months.  Originally planned delivery dates and the Initial Operating Capability date are not expected to be affected.

The program recently passed a major milestone by gaining approval from the Defense Acquisition Board to begin low-rate initial production. With the resumption of flight testing, the program expects to enter its next operational assessment, OT-IIB, in mid April.

After remanufacture, the H-1 Upgrades aircraft will feature the latest technology in rotor and drive train design, avionics, sensors and weapons. They also share approximately 84 percent of their parts, making them far more maintainable, supportable, survivable and deployable than today's H-1 aircraft.

The H-1 Upgrades Integrated Test Team here currently has achieved approximately 1,800 flight test hours with five aircraft (three AH-1Z and two UH-1Y test aircraft, of which all but one AH-1Z are production representative).  The test aircraft have flown 222 knots, maneuvered from -0.4 to +3.5 g's and been well above the 10,000-foot altitude mark.

By 2014, the Marine Corps will have procured 100 UH-1Y Hueys and 180 AH-1Z Super Cobras.

FMI: http://pao.navair.navy.mil/