The F-35 Lightning II may not meet acceleration goals, a Lockheed Martin official said. (Lockheed Martin)
The F-35 Lightning II’s transonic acceleration may not meet the requirements originally set forth for the program, a top Lockheed Martin official said.
“Based on the original spec, all three of the airplanes are challenged by that spec,” said Tom Burbage, Lockheed’s program manager for the F-35. “The cross-sectional area of the airplane with the internal weapons bays is quite a bit bigger than the airplanes we’re replacing.”
The sharp rise in wave drag at speeds between Mach 0.8 and Mach 1.2 is one of the most challenging areas for engineers to conquer. And the F-35’s relatively large cross-sectional area means, that as a simple matter of physics, the jet can’t quite match its predecessors.
“We’re dealing with the laws of physics. You have an airplane that’s a certain size, you have a wing that’s a certain size, you have an engine that’s a certain size, and that basically determines your acceleration characteristics,” Burbage said. “I think the biggest question is: are the acceleration characteristics of the airplane operationally suitable?”
A recent report by the Defense Department’s top tester, J. Michael Gilmore, says that the Navy’s F-35C model aircraft, which has larger wing and tail surfaces, is not meeting requirements for acceleration.
The report doesn’t say whether the F-35A and F-35B have hit similar snags.
Richard Aboulafia, an analyst with the Teal Group, Fairfax, Va., said that the revelation was not particularly surprising.
“It’s a strike fighter,” Aboulafia said. “It’s not an interceptor; it’s not an F-22.”
Aboulafia said it was unclear whether additional engine power could boost acceleration in the difficult transonic regime. So far, doubts about the aircraft’s aerodynamic performance haven’t diminished Lockheed’s sales prospects, he said.
The F-35 transonic acceleration specifications were written based on clean-configuration F-16 Fighting Falcon and F/A-18 Hornet fighter, Burbage said.
But unlike the Hornet or the F-16, the F-35 has the same configuration unloaded as it does loaded with weapons and fuel, Burbage said. When an F/A-18 or F-16 is encumbered with weapons, pylons and fuel tanks, those jets are robbed of much of their performance.
“What is different is that this airplane has accelerational characteristics with a combat load that no other airplane has, because we carry a combat load internally,” Burbage said, the F-22 Raptor notwithstanding.
Even fully loaded, the F-35’s performance doesn’t change from its unencumbered configuration, he said.
In the high subsonic range between Mach 0.6 to Mach 0.9 where the majority of air combat occurs, the F-35’s acceleration is better than almost anything flying.
Thus far, Lockheed has not had issues with the plane’s acceleration, Burbage said. There are top level Key Performance Parameters from which lower level detailed engineering specification are derived and Lockheed’s job is to meet as many of those specifications as possible within the laws of physics, he said. Discussions are underway about if those original specifications are relevant given the jet’s acceleration in a combat configuration, Burbage added.
Air Force Lt. Col. Eric Smith, director of operations at the 58th Fighter Squadron at Eglin Air Force Base, Fla., and F-35 test pilot, said that flying the aircraft is a thrilling experience.
“I can’t even explain the adrenaline rush you get when you light the afterburner on that thing,” Smith said. “The acceleration is much better than an F-16.”
But the F-35’s aerodynamic performance is not what makes the jet special, Smith said. The F-35 powerful sensors and data-links and how that information is fused into a single coherent and easy to use display are what will make the jet an effective warplane.
Burbage added that while the F-35 is designed as a supersonic fighter, it’s not optimized for the extremely high supersonic speeds that the Raptor was designed to operate at.
“This is not a supercruising airplane like the F-22,” Burbage said.