The F-35 Joint Strike Fighter has been flown in air-to-air combat maneuvers against F-16s for the first time and, based on the results of these and earlier flight-envelope evaluations, test pilots say the aircraft can be cleared for greater agility as a growth option.
The availability of additional maneuverability is a testament to the aircraft’s recently proven overall handling qualities and basic flying performance. “The door is open to provide a little more maneuverability,” says Lockheed Martin F-35 site lead test pilot David “Doc” Nelson.
“When we did the first dogfight in January, they said, ‘you have no limits. It was loads monitoring, so they could tell if we broke something. It was a confidence builder.” AF-2 was the first F-35 to be flown to 9g+ and -3g, and to roll at design-load factor. The aircraft, which was also the first Joint Strike Fighter to be intentionally flown in significant airframe buffet at all angles of attack, was calibrated for inflight loads measurements prior to ferrying to Edwards in 2010.
The operational maneuver tests were conducted to see “how it would look like against an F-16 in the airspace,” says Col. Rod “Trash” Cregier, “Pilots really like maneuverability, and the fact that the aircraft recovers so well from a departure allows us to say to the designers of the flight control system laws, ‘you don’t have to clamp down so tight,’”
Departure resistance was proven during high angle-of-attack (AOA) testing, which began in late 2012 with the aircraft pushing the nose to its production AOA limit of 50 deg. Subsequent AOA testing has pushed the aircraft beyond both the positive and negative maximum command limits, including intentionally putting the aircraft out of control in several configurations ranging from “clean” wings to tests with open weapons-bay doors. Testing eventually pushed the F-35 to a maximum of 110 deg. AOA.
High angle-of-attack testing included intentional departures with weapons bay doors open.
"An aggressive and unique approach has been taken to the high alpha testing", says Nelson. “Normally, test programs will inch up on max alpha, and on the F-22 it took us 3-4 months to get to max alpha. On this jet, we did it in four days. We put a spin chute on the back, which is normal for this sort of program, and then we put the airplane out of control and took our hands off the controls to see if it came back. We actually tweaked the flight control system with an onboard flight test aid to allow it to go out of control, because it wouldn’t by itself. Then we drove the center of gravity back and made it the worst-case configuration on the outside with weapons bay doors and put the aircraft in a spin with yaw rates up to 60 deg./sec., equal to a complete turn every 6 sec."
"That’s pretty good. But we paddled off the flight-test aid and it recovered instantly,” he says.
Pilots also tested the ability of the F-35 to recover from a deep-stall in which it was pushed beyond the maximum AoA command limit by activating a manual pitch limiter (MPL) override similar to the alpha limiter in the F-16.
“It’s not something an operational pilot would do, but the angle of attack went back and, with the center of gravity way back aft, it would not pitch over, but it would pitch up. So it got stuck at 60 or 70 deg. alpha, and it was as happy as could be. There was no pitching moment to worry about, and as soon as I let go of the MPL, it would come out,” Nelson says.
Following consistent recoveries, the test team opted to remove the spin chute for the rest of the test program.
“The airplane, with no spin chute, had demonstrated the ability to recover from the worst-case departure, so we felt very confident, and that has been proven over months of high alpha testing,” says Nelson.
“It also satisfied those at the Joint Program Office who said spin chute on the back is not production-representative and produces aerodynamic qualities that are not right.” Although there are additional test points ahead where the spin chute is scheduled to be reattached for departure resistance with various weapons loads, the test team is considering running through the points without it.
With the full flight envelope now opened to an altitude of 50,000 ft., speeds of Mach 1.6/700 KCAS and loads of 9g, test pilots say improvements to the flight control system have rendered the transonic roll-off (TRO) issue tactically irrelevant. Despuite earlier being highlighted as a “program concern” in the Defense Department’s Director of Operational Test and Evaluation (DOT&E) 2014 report, now TRO “has evolved into a non-factor,” says Nelson.
“You have to pull high-g to even find it.” The roll-off phenomena exhibits itself as “less than 10 deg./sec. for a fraction of a second. We have been looking for a task it affects and we can’t find one.”
