Going Backwards?

With thanks and acknowledgement to Aviation Week Network

A Lockheed Martin F-35A Joint Strike Fighter (JSF) was outperformed in the type’s first basic fighter maneuvering exercise by a 20-plus-year-old F-16 fighter, according to a leaked Lockheed Martin report prepared by the pilot who flew the mission.

Inferior energy maneuverability (EM), a limited pitch rate and flying qualities that were “not intuitive or favorable” in a major part of the air-combat regime gave the F-16 the tactical advantage and allowed its pilot to get into both missile-launch and gun parameters over the F-35. Another drawback was that the large helmet and F-35 canopy design restricted the pilot’s rearward view.

Lockheed Martin and the JSF Program Office confirm that the document, originally leaked by the War is Boring website, is genuine but says “the interpretation of the scenario results could be misleading.” Maj. Gen. Jeffrey Harrigian, director of the U.S. Air Force F-35 integration office, says that it is “too soon” to judge the F-35’s maneuverability.

JSF risk reduction and development have been under way since late 1996 and more than $80 billion has been spent.

F-35 in high-angle-of-attack testing. The fighter has a combat advantage in that area, but it takes too long to get in and out of it, a pilot reports. Credit: Lockheed Martin/Tom Reynolds

The test mission in the report was carried out on Jan. 14. It could have been one of those flown from Edwards AFB in California by Lockheed Martin site lead test pilot David “Doc” Nelson and reported by AW&ST in April. At that time, however, the question of which aircraft had the advantage in the engagement was not discussed. The author’s name has been removed from the copy of the report obtained by Aviation Week.

The key performance parameters set for the JSF do not call for the aircraft to be superior to the F-16 and F/A-18C/D in air-to-air combat, but they demand comparable performance in terms of sustained and instantaneous g and acceleration. However, Lockheed Martin executives and pilots have stated that when including sensor fusion, stealth and other attributes, the F-35 will be superior in air combat, by margins of 400-600%, to so-called “fourth-generation” fighters (a term coined by Lockheed Martin to denote both older U.S. fighters and current non-U.S designs).

The F-16 that outmaneuvered the F-35 in the January test was an F-16D Block 40, one of the lower-performing versions of the family, delivered between 1987 and 1994. The Block 40 was beefed up to carry more weapons than the preceding Block 30 and incorporated Have Glass radar-cross-section reduction measures, boosting its empty weight, but lacked the higher-performance engines introduced on the Block 50. The F-16 retained its two 370-gal. external fuel tanks during the engagement and was limited to 7g until they were empty.

Lockheed Martin notes that AF-2 is an early development aircraft without stealth coatings, although those are not relevant in within-visual-range (WVR) combat and their absence would make the airplane lighter. It was also not equipped with simulated air training missiles, but neither was the F-16. Lockheed Martin says AF-2 lacked “software that allows the F-35 pilot to turn, aim a weapon with the helmet, and fire at an enemy without having to point the airplane at its target,” but this capability is available in clear daylight on any modern fighter, and was not used by the F-16 in the test. Also, the F-35 cannot exploit this capability in stealth configuration because there is no suitable high-off-boresight air-to-air missile planned for internal carriage on the aircraft.

The report identifies changes to F-35 flight control software that could mitigate some of the problems found in the test. However, none of them can correct the deficit in energy maneuverability, which is defined at any given airspeed by the aircraft’s available thrust, drag and weight and indicates its ability to accelerate, climb or change maneuver state.

The F-35 has higher angle-of-attack (AoA) limits than the F-16, which should normally be an advantage, but a combination of factors, including a limited pitch rate and the inferior EM, made it less useful. It took too long to reach high AoA, and the lack of energy maneuverability meant the F-35 could not quickly re-accelerate into high-speed maneuver states.

The sortie included 17 engagements starting between 18,000 and 22,000 ft. with a 10,000-ft. floor, with starting speeds between 380 and 440 kt. indicated airspeed, the report says. The test was primarily designed to “stress the high AoA control laws during operationally responsive maneuvers utilizing elevated AoAs and aggressive stick/pedal inputs,” the report says. The pilot judged the test as being “extremely effective at providing data that are not achievable with scripted test cards,” it adds.

The first observation in the report is that “the most noticeable characteristic of the F-35A in a visual engagement was its lack of energy maneuverability,” and the pilot notes that the F-35A has a smaller wing than the F-15E, similar weight and 15,000 lb. less thrust.

The “insufficient pitch rate exacerbated the lack of EM” the pilot reported, because energy would bleed off continuously as the pilot tried to get the nose up. The F-35 would have had more shot opportunities in some cases if the pitch rate limit (imposed by control laws rather than aerodynamics) had been more relaxed, the report says. Offensive and defensive gun moves were slow and easily detected and countered, the report says, and the highest g loading was around 6.5g in breaks or on entering a turn, decreasing as the aircraft slowed down. The F-35 airframe is designed for 9g.

High AoA flight allowed “a few offensive opportunities” against the F-16 (which is AoA-limited and roll- and yaw-limited towards its maximum angle of attack). For instance, a long full-rudder input could yaw the F-35 sharply enough to cause the F-16 to cross its nose and get a missile shot, the report says. But this maneuver “required a commitment to lose energy” with the F-35 headed for the floor, and “meant losing the fight unless the bandit made an error.”

The F-35 has a sophisticated flight control system (FCS) that changes the response of the control surfaces to stick and rudder commands as the airplane moves through its flight envelope, with low- and high-AoA regions and a “blended” region between them, at 20-26 deg. This was the area where the aircraft “fought best,” the pilot reported. But it was not easy, because flying qualities “were neither intuitive nor favorable” and “the lateral/directional response was often unpredictable.”

The problem, in the pilot’s view, is the way that the FCS adjusts response according to AoA. In flight test, the pilot aims at a specific AoA and expects a certain response, but in a dynamic flight “attention is focused on the bandit rather than the specific AoA [and] the response was often confusing,” the report says. The pilot was left waiting for a roll rate that did not happen, or expected yaw and got sideslip.

In one case the pilot applied full rudder to no immediate effect, then tried a stick input – just as the rudder kicked in. The pilot added more rudder and got a “fantastic yaw rate” that was promptly quashed (“immediate, abrupt and forceful”) by the anti-spin logic in the FCS.

Both the anti-spin logic and the slow pitch rate meant that the F-35 could not escape a gun attack by the F-16. “No successful guns defense was found,” the report says. For instance, a standard escape maneuver – unload, roll and pull to change the plane of the aircraft’s movement – was bogged down by pitch rate, so “the result was an out-of-plane maneuver that was easy to track.”

The size of the helmet-mounted display system presented a problem. “There were several occasions where the bandit would have been visible, but the helmet prevented getting in a position to see him,” the report says. The “eyebrow” formed by the visor assembly also blocked the tally at times.

Some mitigating measures are advocated in the report, such as relaxing restrictions on AoA onset and pitch rate. Both would let the pilot move more quickly into and out of the higher-AoA regime, where the F-35 has a controllability advantage over the F-16. The pilot also advises expanding the “blended” control regime, avoiding shifts in control laws in a key part of the combat envelope and giving the pilot more yaw authority versus the anti-spin logic.

In the pilot’s view, the F-35’s departure resistance is good enough to allow more latitude. Other aircraft, including the F/A-18 Super Hornet, have benefited from flight control software changes over time, which have allowed pilots to better exploit the aircraft’s strong points and mitigated early problems.

However, the basic energy maneuverability deficiency is more difficult to address, and most contemporary fighters are much less restricted at high AoA than the F-16. (“Drag him into the phone booth” has long been the adversary’s best way to fight the F-16.) “The F-35 goes out against something like a Sukhoi or a Typhoon and they’re going to eat his lunch,” remarks an experienced military pilot. “They have the advantage in turn rate, [specific excess power] and energy bleed rate.”

Pilots who talked to Aviation Week about the leaked report – some but not all of whom work or have worked for other fighter manufacturers – were surprised by the magnitude of the shortfall in energy maneuverability but not by its existence. The limitations of the Block 40 F-16 and the fact that it retained its tanks “should have been a plus for the F-35,” says one, and the engagement altitude – “above mid-range” in terms of within visual range combat – should have favored the F-35 because of its modern, powerful engine.

“People all need to look at what F-35 really is,” another aviator comments. “A stealth A-7 bomb truck, capable of first-night suppression of enemy air defenses, with limited self-escort.  It is, as software-configured right now, not a light air-combat-maneuvering-capable platform. This is what we saw with the early F/A-18E/F blocks: millions of lines of code, and in need of constant update. But, in this case, it’s becoming increasingly easier to rewrite the code laws to allow for those changes.”

“Anyone can see that it is not a very agile aircraft,” a third pilot comments, pointing out that changing flight test laws may be more of a problem if – as some operators expect – pilot flying hours are reduced and training is transferred to lower-cost aircraft and simulators.

Part of Lockheed Martin’s response to the release of the report is to downplay the importance of maneuvering combat. “The F-35’s technology is designed to engage, shoot, and kill its enemy from long distances, not necessarily in visual ‘dogfighting’ situations,” the company says. But as one of Aviation Week’s sources says, reliance on deciding the fight beyond visual range may not always be possible in the early stages of a conflict (power projection, show-of-force) or where rules of engagement limit BVR shots. “My belief is that the tactics against the F-35 will be something which we are not used to saying: If you see one — get close.”

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