AH-1Z tops 20k fatigue test hours
NAVAIR News Release
Release Date: 11/26/2002
By John C. Milliman, PMA-276 Public Affairs Officer
NAVAIR PATUXENT RIVER, MD - Two years and three months may not seem a long time.
For some Marines, it's series of crossed off days on a calendar marking time until the end of a tour -- a tour of late nights and early mornings (sometimes even blending seamlessly into each other) spent on flight lines around the Globe keeping aging and fatigued helicopters mission ready.
For the Marines' newest "recruit," though, it's a double lifetime.
It's actually 20,000 hours and twice the required fatigue life of the AH-1Z SuperCobra and UH-1Y Huey. And the AH-1Z Structural Test Article recently achieved this milestone at the Bell Helicopter test facility at Fort Worth, Texas - joining an elite group of aircraft to have done it.
"10,000-hour fatigue life is a new requirement for us," explained Bill Evans, Bell Helicopter's lead SuperCobra airframe stress engineer. "The AH-1W requirement was for 6,000 hours. And previous aircraft were only required to meet 4,000 hours."
The milestone, reached at 11 a.m., Nov. 22, marked only the end of the "low-cycle fatigue test," according to Evans. Following the low-cycle tests will be high- cycle fatigue testing and static loads testing.
The low-cycle, or Ground-Air-Ground, portion tests load extremes by placing the structural test article in a fixture and using actuators to place stresses on it in various places to reflect actual loads the airframe experiences in flight.
"We utilize 36 hydraulic actuators," explained Mark Woods, a Bell structural test engineering specialist. "Six to hold the ship in place and 30 to apply the flight loads. We currently are monitoring 245 strain gage channels. The strain gages measure the strain in the individual components, which can then be converted to stress. During the course of two lifetimes, we applied 1.2 million load conditions."
"In the GAG testing, we go to the extremes of the load cycle," Evans explained. "From sitting on the ground, to full 3G maneuvers and then back to a hard landing. We essentially ran the most extreme normal flight maneuvers back-to-back for 20,000 (accelerated) hours."
The purpose of all this testing is determining the life and durability of the aircraft, explained Evans, as well as determining inspection intervals. And proving all of it to the Fleet.
"We want to find out where the problems are, and when they occur," he said. "That way, we'll fix them in production so they won't be problems out in the Fleet. To prove to the Fleet that the aircraft is good for 10,000 hours, we go to 20,000."
Such extensive fatigue testing hasn't ever really been done before, according to Evans.
"This is normal for fixed wing platforms, but I'm not aware of any rotary wing program with this level of low-cycle hours," he said. "Some other programs primarily did high-cycle testing and didn't include GAG."
In the past, platforms became functionally obsolete before they wore out, according to NAVAIR's Aging Aircraft Integrated Product Team. From the '50's through the '80's, new platforms came along every few years and the older platforms went to the bone yard long before they reached fatigue life limits. Since the Clinton-era "procurement holiday," however, platforms have been asked to soldier on. And for the first time, platforms are reaching the upper limits of their fatigue life (and in some cases, exceeding them) with no replacements hitting the fleet anytime soon.
"What we have seen in other platforms," said Bob Ernst, head of the Aging Aircraft IPT, "is that we are charting new waters -- we're going where we've never been before in structures and fatigue life and Fleet maintainers are the explorers. What they're finding isn't always exciting or pretty, and it's making for a lot of work just to keep these aircraft mission ready."
With more fatigue testing done up front, though, future maintainers shouldn't have as many nasty surprises waiting for them when the AH-1Z becomes geriatric.
"Traditionally, we wait until problems pop up and then we do the 'forensic' autopsy to see what went wrong," said Ernst. "By investing in proactive approaches like fatigue testing, we can remove some of the wizardry and fix potential problems at a fraction of the cost. This platform will age better and a heck of a lot more gracefully than any of its predecessors because we've been to the back side of the age curve, fatigue-wise, before it's even in production."
And that's due in large part to what the testers at Bell have found -- much of which will be incorporated back into the design to "inoculate" it against the effects of age.
While perhaps unsung, the success of the accelerated fatigue tests were no accident, according to the Bell testers.
"Actual flight test gets all the press while we do all the work!" Evans joked.
"Everybody worked really hard," echoed Jim Lappin, Bell technical specialist for Airframes Structures. "A lot of people worked a lot of weekends to keep this program running. Not only that, but we got excellent response from NAVAIR Structures."
Even though it's achieved significant testing results, the AH-1Z STA won't be allowed to rest on its laurels.
"Now that we're done with GAG," explained Ben Settle, Bell Helicopter Structural Testing IPT lead, "we're going to totally refurbish the aircraft, including reskinning, and start over with high-cycle tests. That's where we set the actuators (that input stresses to the STA) to flight loads and then add in the oscillatory loads from the main and tail rotors. After that, we'll refurbish it again for the static loads test."
But even after all that, no easy life in a museum awaits this test veteran.
"We'll take it out and shoot it," Settle added.
H-1 Upgrades Program Update
Meanwhile, the three AH-1Z and two UH-1Y flight test aircraft here continue to battle adverse weather to make progress in the schedule.
To date, the three SuperCobras have amassed nearly 400 flight hours, achieved 220 kts, maneuvered to -0.3 to +3.5 G's and reached a 10,000-foot altitude since December, 2000. The two Hueys have flown more than 140 hours, reached 190 kts and maneuvered from -0.4 to +2.8 G's since December, 2001.
Both platforms continue with envelope expansion as well as having recently supported cockpit mapping for the Thales Top Owl helmet-mounted display.
AH-1Z STA.jpg Shake, rattle and roll -- The AH-1Z SuperCobra recently rocked around the clock to complete 20,000 hours of accelerated low-cycle fatigue testing and prove a 10,000-hour airframe life. Such testing helps identify fatigue-related problems, and enables design tweaks, before the aircraft presents headaches to future maintainers. The milestone is thought to be a first for rotary wing aircraft. Photo courtesy of Bell Helicopter - a Textron Company.
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