YF-23 Designer Weighs In On How Lockheed Martin Could Evolve The F-35

Losing the competition to develop the successor to its own F-22 presents Lockheed Martin with a challenge and an opportunity: retaining its dominant position in the combat aircraft industry and evolving the F-35 to fill the gap.

Lockheed Martin CEO James Taiclet has made clear his plan is to “create a best-value bridge” from today’s fifth-generation fighters to the sixth-generation Boeing F-47, winner of the U.S. Air Force’s Next-Generation Air Dominance (NGAD) program.

It will take at least five years—perhaps as long as a decade—for the F-47 to enter service, Taiclet told analysts on July 22. He sees a need and an opportunity to provide the capability to bridge that gap.

“How do we bridge capability there?” he said. “We’re going to port a lot of our own NGAD R&D over the F-35, and potentially the F-22 as well, striving to get 80% of the effectiveness of sixth-generation in stealth and other aspects at 50% of the cost per unit.”

Darold Cummings, who was chief configurator for Northrop’s YF-23 fighter, has some ideas about how the F-35 could be evolved. Still an active aircraft designer, he this year helped Aviation Week analyze both the F-47 (AW&ST April 21-May 4, p. 13) and China’s latest combat aircraft (AW&ST Jan. 13-26, p. 14).

At Lockheed Martin Aeronautics in Fort Worth, engineers are looking back at the F-16 program for insights into how best to evolve the F-35, which is expected to remain in production for more than a decade, based on the current backlog.

In particular, they are looking at how the arrow-wing F-16XL was developed as a company-funded initiative to evolve the F-16 from its origins as a lightweight combat aircraft into a long-range strike fighter. Cummings similarly takes the F-16XL as a starting point.

“I believe one way Lockheed Martin can meet its goals is by employing two strategies,” he says. One is stretching the airframe to add fuel and improve fineness ratio, as well as adding lifting area with a canard or more wing area. The second is to use thrust vectoring to eliminate the tail.

Starting in April, Cummings developed three conceptual designs for evolved F-35s based on past Lockheed experimental projects; the new designs offer short-, medium- and long-term improvements in performance and increased stealth.

In the near-term F-35EX concept, the fuselage is stretched 60 in., similar to the F-16XL, and vertical and horizontal tails are replaced by a canard foreplane and multi-axis thrust vectoring. The latter is based on the Low-Observable Axisymmetric Nozzle first flown on an F-16C in 1992 and the Axisymmetric Vectoring Exhaust Nozzle now fitted to the X-62A VISTA (Variable In-flight Stability Test Aircraft), a highly modified two-seat F-16D.

“The short length of the F-35, combined with the fuselage cross-sections necessary to contain weapon bays and landing gear, contribute to a poor fineness ratio,” Cummings says. “This is the main reason the F-35 cannot supercruise. The F-16XL solved this problem with a 56-in. fuselage stretch.” Stretching the fuselage increases fineness ratio for higher lift-to-drag ratio in supersonic flight while adding volume for fuel and equipment.

Cummings’ F-35EX concept employs the maximum amount of F-35A and F-35C components, including using the larger F-35C wing with a canard for pitch control and maximum lift. The fuselage stretch adds 4,000 lb. of fuel volume, he calculates. Another 1,500 lb. of fuel is housed in an external tank with the same outer mold line as the F-35C external gun pod. Together they increase fuel capacity by 30% over the baseline F-35C. Cummings sees the F-35EX as a near-term alternative to the U.S. Navy’s F/A-XX.

The midterm F-35FX concept uses the core F-35EX fuselage but employs technologies from another past Lockheed experimental program, the conceptual X-44 MANTA (Multi-Axis No-Tail Aircraft). Derived from the F-22, the X-44 was intended to test the feasibility of full-authority pitch, roll and yaw control without a conventional empennage, relying purely on three-dimensional thrust vectoring.

The tailless F-35FX has a clipped-delta planform similar to the MANTA for lower radar cross-section and better supersonic area distribution, as well as an advanced, stealthy 3D vectoring nozzle. With the stretched fuselage and larger wing, total internal fuel volume is about 30,000 lb., Cummings calculates, 50% more than the F-35A.

The F-35GX is a long-term concept that maximizes low observability. This configuration starts with the F-35FX layout and modifies the forward fuselage shape to a straight 70-deg. chine. The inlets are moved under the chine. The wing tips are clipped to match the 70-deg. chine angle, and all aft edges—wing, fairing and nozzles—are aligned to reduce radar signature.

The spirit of the F-16XL is evident in Cummings’ concepts, but evolving the F-35 may not be as easy as it was for the F-16, for technical and financial reasons.

In December 1980, General Dynamics CEO David Lewis gave the Fort Worth division $53 million in company funds (more than $200 million today) to build two F-16XL flight-test prototypes. “We aren’t about to sit on our laurels and risk the F-16 becoming obsolescent,” Lewis said in an article in The Wall Street Journal cited by Albert Piccirillo in his book on the F-16XL, Elegance in Flight.

The Air Force provided two F-16A development aircraft for modification into F-16XLs, one of which had been damaged when its nose landing gear failed. Component manufacturing began in January 1981, and the first aircraft flew on July 3, 1982—19 months after Lewis greenlit the program.

That speed of execution was due in part to the F-16’s design, as the wing attached to the side of the body, making it relatively straightforward structurally to change the wing. This was done with the F-16XL as well as with Japan’s F-16-based Mitsubishi F-2, which has a bigger wing and longer fuselage.

In the F-35, the wing and fuselage structure are integrated to reduce weight. This could make it more complicated to stretch the fuselage and replace the wing. For this reason, Cummings located the rear wing spar on all three concepts with the rear wing-spar fuselage carry-through structure.

“When we did the YF-23, three of the most labor-intensive, painful things were the crew station and canopy, the weapon bay and the landing gear—and especially the landing gear relationship to the weapon bay,” Cummings says. “These things are all done on the F-35. Compared to those three things, implementation of new wings and other components are much less complex. So the F-35 has a running start.”

Ultimately, the F-16XL lost to Boeing’s F-15E in the Air Force’s Dual-Role Fighter competition, but the F-16 continued to evolve, culminating in the Block 70 variant still in production today. Whether Lockheed decides that evolving the F-35 justifies a similarly substantial investment of company R&D funding will become clear in the months ahead.