Test pilots at Edwards Air Force Base, Calif., have successfully performed “airstarts” on the F-35 Joint Strike Fighter over the last few weeks, an important milestone in opening up the aircraft’s test syllabus, according to a Lockheed Martin test pilot.
During an in-flight “airstart” the pilot secures power to the engine at altitudes between 15,000 and 30,000 feet and then restarts it in a carefully prescribed sequence to ensure the safety of the aircraft and surrounding airspace.
David Kidman, the Air Force Flight Test Center technical expert for the propulsion integration flight test at Edwards AFB, said data derived from ground testing played a significant role in the recent flight testing.
Kidman credits the success of the propulsion flight testing program in part to data and technical expertise derived from past F135 engine altitude development testing and the partnership with AEDC.
“AEDC’s contribution to our recent in-flight engine airstarts is that the engine has just gone through development at AEDC,” Kidman said. “As a result, AEDC engineers had firsthand knowledge of how the system should operate. Additionally, AEDC engineers had firsthand knowledge on how the system should be tested, including where to expect the best performance and where to expect problems and what test procedures might work best. AEDC engineers are sharing this system and test knowledge with Edwards’ engineers through regular communication and the AEDC QLR’s (Quick-Look Reports).” Dr. Charles Vining, AEDC’s Turbine Engine Ground Test Complex technical director, said he is one of Kidman’s counterparts in this collaborative effort.
“My role is providing advice and coordination with Dave Kidman on developing common analysis approaches,” Vining said. “The over-arching objectives include improving the use of statistical methods in test and evaluation and to develop an understanding of the similarities and differences between ground test simulations of altitude starting characteristics compared to flight test data.”
Vining said the program office and both test centers benefit from this collaboration through improved test and evaluation and communications. This is why he and Kidman re-energized the Propulsion Integration Ground/Flight Test Interchange in August of 2010.
Kidman added, “The Air Force benefits from this collaboration by the sharing of system and test expertise learned from AEDC testing. As a result, we have a pretty good idea what to expect from the engine. The new piece is that now the system is fully integrated with the airframe (e.g. inlet and power extraction) which changes engine operation.”
Capt. John Dayton, a Turbine Engine Ground Test Complex project engineer, and Mike Wrenn, an ATA lead analysis engineer, traveled out to Edwards to provide support.
“The reason that we were out there for airstart testing specifically, is because that’s one of the easiest direct links you can make between what we test on the ground here at AEDC and what they test in the air,” Wrenn said. “We’re making a direct comparison and a correlation between the test points that they’re running [and what] we’re looking at here. “There are lessons learned and guidance that we can provide to them, including the timelines that we developed on how long it would take the engine to restart. This corresponds directly to what they’re using out at Edwards to help put the safety mechanism in place to ensure their pilots understand what they’re getting into, that they fully understand the altitude loss during the maneuvers and how to accomplish that flight test safely.”
Dayton added, “We were out there in an observational and a relationship-building role with the flight test engineers in the program. One of our goals coming back is [to determine] where can we get more involved in the future, to improve our products and analysis. And then also, where do we best inject our experience into the test plan or test procedures.”
John Kelly, AEDC’s Turbine Engine Ground Test Complex F135 test manager, and Melanie Link, Propulsion Integration Lead for the F-35 Integrated Test Force at Edwards, worked with the rest of the team with one goal.
“We were trying to figure out how we, as a test enterprise, can seamlessly go from one test to another, so ultimately our [ground test data] output would be the input to flight test; our reports would be what guides the plans for a flight test,” said Kelly. “During the flight test the pilot literally turns the engine off and then he can either use ram air effect from the motion of the airframe or he can also do a starter-assisted start and basically use both the air and the starter to get it started. And we’ve done those tests here before they ever did them out there.”
Kelly said the pilot’s feedback during the flight and airstarts literally becomes another “data point” to support the tests.
Link put the recent airstart testing into perspective.
“We’re still in the early stages of air-start testing of the aircraft,” she said. “Airstart testing for the STOVL (Short Take-Off and Vertical Landing) jets is still in planning stages, only ground start testing has been done at Naval Air Station Patuxent River, Md., thus far. We’ve been flight testing since 2007; airstart testing for the production engine though has just begun. Air-start testing for STOVL jets will also be done at Edwards AFB.”