Lockheed Martin is claiming that all three versions of the F-35 Joint Strike Fighter (JSF) will have kinematic performance better than or equal to any combat-configured fourth-generation fighter. The comparison includes transonic acceleration performance versus an air-to-air configured Eurofighter Typhoon and high angle-of-attack flight performance vis-à-vis the Boeing F/A-18E/F Super Hornet.
"The F-35 is comparable or better in every one of those metrics, sometimes by a significant margin, in both air-to-air, and when we hog-up those fourth-generation fighters, for the air-to-ground mission," says Billy Flynn, a Lockheed test pilot who is responsible for flight envelope expansion activities for all three variants.
But the Lockheed claims are strongly disputed by other sources, including one veteran Super Hornet test pilot with thousands of hours in that aircraft. "These claims are technically inaccurate from my point of view as a professional test pilot," he says. "An aircraft with small control surfaces intended for stealth cannot produce such fantastical results in maneuverability; a little wing cannot produce a lot of lift period."
Flynn says "that the F-35 can go out on any given day, and we have, gone to the red line of the airplane" with a full internal weapons load. Going to the limits of the aircraft's envelope with a full load of weapons is "inconceivable in any of the other fourth-generation airplanes, including Typhoon, which most would say has the best performance of those four fourth-gen jets," says Flynn, who is a former test pilot for the Eurofighter and Lockheed F-16. All variants of the F-35 are capable of flying at Mach 1.6 and 50° angle-of-attack, he says. The A and C models have a maximum speed of 700 knots calibrated airspeed (KCAS-1296 km/h) while the F-35B can fly at 630 KCAS (1167 Km/h). The A, B and C variant are rated at 9g, 7g and 7.5g's respectively.
But at issue is exactly what constitutes a combat load out. An F-35 loaded up with two 2000lbs bombs and two air-to-air missiles internally is not carrying an equivalent payload to a Eurofighter Typhoon with four 2000lbs bombs and five air-to-air missiles or a Super Hornet armed with a mix of bombs and air-to-air missiles. "What was the combat load out?" the Super Hornet pilot asks. "If you compare apples and oranges --you can make claims like that." One highly experienced pilot flying the Lockheed F-22 Raptor adds, "They need to compare the performances based on similar amounts of ordnance carriage."
Another point that must be considered, however, is that the F-35 will only be relying on its internal weapons payload during operations against a very robust threat environment. "Internal carriage is only required if you need the stealth," another F-22 pilot says. "At which point a fourth generation jet may not even be able to deliver on the target."
Stealth is a point that Lockheed emphasizes. "The game-changer is stealth," Flynn says. "No one is going to see us coming or going." But exactly how many targets an F-35 could attack with its internal payload versus a non-stealth platform during a campaign is debatable. "There is a whole other story on how many targets the F-35 could hit with the limited internal carry versus the fourth-gen plus jets," the second Raptor pilot says.
Asked to address the issue of transonic acceleration compared to the best performing fourth-generation machines, in this case an air-to-air configured Typhoon, Flynn reiterated that the F-35 was better than or equal to that aircraft. Even with the reduced transonic acceleration times mentioned in the Pentagon's director of operational test and evaluation 2012 report, the F-35, including the C-model which had its specifications reduced by 43 seconds, still out accelerates competing aircraft in a combat configuration, he says.
But others are skeptical. "Forty-three seconds tells me there is a massive decrease in the expected performance because of some serious shortcomings," the Super Hornet pilot says. "How that's parlayed into 'we're better than the rest of the world combined' on every measure, I don't know, and I don't believe it." More important is the question of how the reduction in performance impacts aircraft survivability. "So what if you can accelerate better than a [F-16] Viper or Typhoon, can you live against an SA-20?" asks the second F-22 pilot.
If one were to overlay the energy-maneuverability (E-M) diagrams for the F/A-18, F-16 or Typhoon over the F-35's, "It is better. Comparable or better than every Western fourth-generation fighter out there," Flynn says. That applies even to the F-35 B and C models with their respective 7g and 7.5g limits. "You're not going to see any measurable difference between the aircraft," Flynn says. In terms of instantaneous and sustained turn rates and just about every other performance metric, the F-35 variants match or considerably exceed the capabilities of every fourth-generation fighter, he says.
The first F-22 pilot says he is surprised to hear that there are already E-M diagrams available. "The reality is that I would be floored if they had accurate E-M diagrams right now," he says. "They are probably computer generated, and very inaccurate. Also, 'real' E-M diagrams come from OT/DT [operational test/developmental test], not the contractor."
In terms of high angle of attack (AOA) performance, Flynn says the F-35 is better than the Boeing F/A-18E/F, even though the Super Hornet is capable of reaching higher angles than the JSF's limit of 50°. "We are better than any airplane out there," says Flynn, a veteran Canadian Forces CF-18 Hornet pilot who has also flown thrust-vectored prototype variants of the F-16 and F/A-18 Hornet at NASA. "You can go to higher degrees of angle-of-attack in the F/A-18, the flight control system will not limit you, but that's not necessarily controlled flight." In the F/A-18, Flynn says that past 50° there is a lot of very violent buffeting.
"You maneuver the airplane much like an F-22 or a lot like I maneuvered the prototype F-16 20 years ago with thrust vectoring," Flynn says. "You maneuver the airplane back and forth with amazing controllability at the highest degree of angle-of-attack, and that is not the case with the only other Western airplane that can go to high AOA, the F/A-18." The one other exception is the Raptor, which Flynn does acknowledge as having better high AOA performance than the F-35 due to its thrust vectoring capability. The Typhoon, by comparison, has a 25° AOA limit. In the F-35, Lockheed made the decision to limit the AOA to 50°, but test pilots have flown the aircraft well past that.
The high AOA limit gives the F-35 "great" instantaneous turn performance. "We knew that 50°, from our years of research, is about as far as you need to go to take advantage of the aerodynamic performance" of the jet, Flynn says. "There is no reason to be there [at extreme AOA]; you're not going to get much more capability at 75° than you would at 50°." The limiter will allow an F-35 pilot to fly with "reckless abandon", which Flynn says is not possible in a Hornet because an F/A-18 can depart from controlled flight.
Both Raptor pilots take strong exception to the phrase "reckless abandon" that Flynn uses. The same terminology was used in the F-22 Dash-1 manual until one particular incident where a Raptor pilot experienced an "inverted spiral". Both say using the phrase is a serious mistake.
The Super Hornet test pilot, who also has thousands of hours in the older A to D model Hornets, refutes Flynn's statement as dated. He clarifies that early model F/A-18 Hornets could depart from controlled flight if maneuvered very aggressively at high AOA with a heavy external weapons load back during the 1980s. Subsequent updates to the flight control system, particularly the 10.7 software load, "has made all the older Hornets extremely robust and very maneuverable and with a great deal of departure resistance," he says.
The Super Hornet has always been extremely capable at high angles of attack right from the outset. "We have no angle of attack limits in any symmetric configuration, and we can maneuver without any lateral stick or pedal input limitations at any angle of attack. The Super Hornet still has complete roll control at 50° AOA and has demonstrated this many times while flying at low altitude with a full combat load because there is no departure issue," the Super Hornet pilot says.
Lockheed declined to compare F-35 performance to clean configuration fourth-generation fighters saying such comparisons are irrelevant. "This comparison doesn't mean much, because a clean fourth-gen isn't carrying weapons," the second F-22 pilot says. The assumption for such a comparison would mean that a fourth generation fighter was forced to jettison its weapons and, if taken literally, its weapons pylons too. "Losing pylons is not a good thing during a protracted air campaign," he says.
Source: Flight International