The Revolutionary Evolution of the F-16XL

Nov. 1, 1983

When Lt. Gen. Lawrence A. Skantze spoke at the rollout of the first F-16XL on July 2, 1982, he was speaking in his then-role as Commander of Aeronautical Systems Division. He characterized ASD’s perspective as having “one foot in the present and one foot in the future.”

For the present, he noted on that July day that the General Dynamics F-16 program “has been one of the singular, outstanding successes that we have had in acquisition management during my tour in the Air Force.” It is being produced ahead of schedule, on cost, and meeting all its performance objectives, General Skantze said, and furthermore, the aircraft had acquitted itself well in combat. He cited General Dynamics’ receipt of an award for $6.8 million for proving that F-16 reliability and maintenance “far exceed the specifications that we laid down in the contract.”

Looking to the future, General Skantze said that “somewhere out there is a new and advanced technology fighter,” and that sometime soon, USAF’s present exploratory work would lead to the definition of that new aircraft. Meantime, he said, it’s “our responsibility to take the fighter craft we have today and evolve those into higher performers, better performers, and improve their margin and hone the edge of their cutting abilities as the future goes before us.”

That has been accomplished in the F-16XL. In a cooperative program, General Dynamics and the Air Force have demonstrated that, at rather modest cost, the F-16XL delivers double the range or payload of the current impressive F-16 performance.

That is revolutionary evolution indeed. The story of how it came to pass is an excellent illustration of industry initiative and risk-taking being applied to US Air force needs, with USAF taking a share of the costs in order to capitalize on the advances created. The result if the aircraft is chosen for production of up to 400 copies for USAF, will be a low-risk, high-payoff for the taxpayers.

D. Randall Kent is Vice President and Program Director for the General Dynamics F-16XL program that involved a team of more than 600 specialists. He summarizes the XL program this way:

“The F-16XL flight-test program has conclusively demonstrated that the XL performs as predicted. This performance level represents a significant increase in mission capability for USAF. Coupling this with the affordability and low risk of the F-16XL presents USAF with a viable way to increase mission capability while simultaneously growing to a forty-wing TAC force structure.”

In addition to its potential as USAF’s derivative fighter, the F-16XL is reportedly being considered by the Japanese Air Self-Defense Force as a replacement for its current ground-attack aircraft. Also, because of its extended range, payload, and suitability for both ground-attack and air-to-air roles, the F-16XL is a prime candidate for US maritime defense operations. That option is now being studied by defense officials and is yet another example of blending USAF and US Navy capabilities to enhance defense performance.

Genesis of the F-16XL

Above a hanger door at the Air Force Flight Text Center at Edwards AFB, Calif., is a white sign with faded blue lettering. It reads “Lights-weight Fighter Program.” The LWF program was a competition between Northrop’s YF-17 and the General Dynamics YF-16. GD won the USAF competition in 1974, and then in 1975 won the international competition to provide fighters for four NATO countries. The F-16 has since been sold to the air forces of six more foreign countries in addition to SAF and its original partners of Belgium, Denmark, the Netherlands, and Norway. The other foreign buyers of the F-16 are Egypt, Israel, Korea, Pakistan, Venezuela, and, most recently, Turkey. Turkey ordered 160 F-16C and D models in early September.

The LWF sign still hangs at Edwards, and, more than ten years later, another General Dynamics fighter is being evaluated from the same flight line and in the same air-space as the YF-16.

When General Dynamics won the LWF competition with the YF-16, David Lewis, the company’s Chairman and Chief Executive Officer, looked ahead. Among other decisions, Lewis set GD’s designers to work to develop derivatives of the F-16.

Harry J. Hillaker was chief project engineer for the advanced versions of the F-16. Harry has been involved in the advanced design of every major aircraft produced at Fort Worth since 1942. He served as YF-16 deputy chief engineer and director of F-16 deputy chief engineer and director of F-16 marketing before turning to leading the F-16XL design effort. The advanced designs that led to the F-16XL were undertaken with company funds and with the cooperation of the National Aeronautics and Space Administration (NASA) and USAF.

Hillaker said that the objective of the F-16XL program was to achieve a logical evolution from the basic F-16 that would provide significant improvements in all mission performance elements. At the same time, it would retain the fundamental F-16 advantage of low procurement and operating costs. Although the principal improvements were to be in range and payload capabilities, simultaneous improvements in all other mission elements were to be given equal emphasis. For example, survivability was to be a prerequisite to longer range. Higher military power (non-afterburning) penetration speed, lower observables, increased maneuver agility, and reduced vulnerable area increased the survival rate so as to be consistent with a longer-range/deeper-penetration capability. Many of the improvements resulted from the design team’s innovative approach to integrating the weapons and airframe rather than hanging weapons on in the conventional high-drag, destabilizing manner.

To say that Hillaker’s design team achieved its objectives is an understatement. Example: For an air-to-surface mission, the F-16XL can carry twice the payload of the F-16A up to forty-four percent farther, and do it without external fuel tanks while carrying four AMRAAM (Advanced Medium-Range Air-to-Air Missiles) and two Sidewinder AIM-9 infrared missiles. With equal payload/weapons and external fuel, the mission radius can be nearly doubled. When configured for a pure air-to-air mission, an F-6XL with four AMRAAMs and two AIM-9s can go forty-five percent farther than an F-16A and can do so while conducting a combat action that is equal to thirty percent of its internal fuel.

As for penetration and survivability, the F-16XL can dash supersonically with a load of bombs at either high or low altitude. It can climb at high rates with the bombs aboard. And it has a speed advantage of up to eighty-three knots over the F-16A at sea level at military power setting and 311 knots on afterburner at altitude while carrying a bomb load.

Two additional capabilities of the F-16XL contribute to survivability. First is improved instantaneous maneuver ability coupled with greatly expanded flight operating limits (with bombs), and second is reduced radar signature resulting from the configuration shaping.

Importance of High Turn Rate

For a decade and a half, many fighter tacticians have stressed the paramount importance of being able to sustain a high turn rate at high Gs. The rationale was that with such a capability, enemy aircraft that cannot equal or better the sustained turn rate at high Gs could not get off a killing shot with guns or missiles.

With developments in missiles that can engage at all aspects, and as a result of having evaluated Israeli successes in combat, the tacticians are now leaning toward the driving need for quick, high-G turns to get a “first-shot, quick-kill” capability before the adversary is able to launch his missiles. This the F-16XL can do. Harry Hillaker says it can attain five Gs in 0.8 seconds, on the way to nine Gs in just a bit more time. That’s half the time required for the F-16A, which in turn is less than half the time required for the F-4. The speed loss to achieve five Gs is likewise half that of the F-16A.

All of these apparent miracles seem to violate the laws of aerodynamics by achieving greater range, payload, maneuverability, and survivability. Instead, they are achieved by inspired design, much wind-tunnel testing of shapes, exploitation of advanced technologies, and freedom from the normal contract constraints.

The inspired design mates a “cranked-arrow” wing to a fifty-six inch longer fuselage. The cranked-arrow design retains the advantages of delta wings for high-speed flight, but overcomes all of the disadvantages by having its aft portion less highly swept than the forward section. It thus retains excellent low-speed characteristics and minimizes the trim drag penalties of a tailless delta.

Although the wing area is more than double that of the standard F-16 (633square feet vs. 300 square feet), the drag is actually reduced. The skin friction drag that is a function of the increased wetted (skin surface) area is increased, but the other components of drag (wave, interference, and trim) that are a function of the configuration shape and arrangement are lower so that the “clean airplane” drag is slightly lower during level flight, and forty percent lower when bombs and missiles are added. And although the thrust-to-weight (T/W) ratio is lower due to the increased weight, the excess thrust is greater because the drag is lower – and excess thrust is what counts.

The larger yet more efficient wing provides a larger area for external stores carriage. At the same time, the wing’s internal volume and the lengthened fuselage enable the XL to carry more than eighty percent more fuel internally. That permits an advantageous tradeoff between weapons carried and external fuel tanks.

Through cooperation with NASA, more than 3,600 hours of wind-tunnel testing refined the shapes that Harry Hillaker and his designers conceived. More than 150 shapes were tried, with the optimum design now flying on the two aircraft at Edwards.

As an additional technology, the XL’s wing skins are composed of an advanced graphite composite material that has a better strength-to-weight ratio than aluminum, is easier to form to the compound wing contours, and has higher stiffness to reduce undesirable flexibility effects.

Two features of the basic F-16 played an important part in readily accommodating what appears to be a drastic change in configuration. First, the modular construction of the airframe allows major component changes with local modification only. And second, the redundant quadriplex fly-by-wire flight control system has the inherent ability (one of its strongest features) to accommodate configuration changes readily.

The modular component construction permitted the addition of a twenty-six-inch “plug” between the center and aft fuselage components to carry the additional wing loads, and a thirty-inch “plug” between the cockpit and inlet component to accommodate the increased wing chord (length). Each “plug” is added at an existing manufacturing splice or mating point.

Finally, since the design and fabrication was entirely a company project, the design team was not constrained by irrelevant requirements and specifications. As Harry Hillaker puts it: “Every piece on this aircraft earned its way on.” That design freedom kept the team concentrating on achieving “performance objectives” in this derivative of the F-16.

Late in 1980, General Dynamics approached the Air Force’s Aeronautical Systems Division for cooperation and support in flight-testing the design. USAF supplied the two test aircraft to be modified to the F-16XL configuration, two turbofan engines, a new two-place cockpit, and funding for the flight-testing. A Pratt & Whitney F100 engine powers the single-seat F-16XL; its sister two-place aircraft is powered by a General Electric F110 derivative fighter engine.

Proof is in the Flying

At the Air Force Flight Test Center, I was privileged to fly in the F-16XL with Experimental Test Pilot Jim McKinney of General Dynamics. Jim flew the maiden flight of the F-16XL on July 3, 1982. That was accomplished twenty months after GD, having received Air Force assurance of support, decided to turn their design concepts into a flying aircraft. Also, I was able to discuss with Jim and Harry Hillaker, who is now GD’s Vice President and Deputy Program Director for the F-16XL, the derivative fighter evaluation program the aircraft has been undergoing for more than a year. For that purpose, we joined Lt. Col. Marty Bushnell, USAF, who commands the Combined Test Force (CTF) on the F-16XL evaluation, and Lt. Col. Joe Bill Dryden, USAF, the chief Tactical Air Command member on the CTF.

Under the derivative fighter evaluation program, 240 F-16XL flights were planned to be completed by May 15, 1983, by two aircraft: a single seater and a dual seater. In fact, within the time and funding provided, 369 test flights were accomplished. Colonel Bushnell said that the reliability and maintainability of the F-16XL appear to be the same as that of the operational F-16. These features should support XL sortie rates in service similar to those of the F-16. About thirty-six sorties per month were averaged in the basic test period through May 15. Among other results of the tests was validation of the predicted improved performance of the aircraft. An extended test plan called for an additional seventy-two flights, but more than that will be achieved by year’s end, the CTF people believe.

Our flight was in aircraft 75-0747. It was the third F-16 full-scale development aircraft. Its sister ship is single-seater 75-0749, which was the fifth full-scale development aircraft. First, we discussed characteristics of the aircraft and specific plans for this flight. Jim McKinney explained that we would explore the four corners of the F-16XL’s performance envelope: high altitude/low speed, high altitude/high speed, low altitude/low speed, and low altitude/high speed.

The aircraft was loaded with twelve Mk 82 50-pound general-purpose bombs, four dummy AMRAAM missiles, and two AIM-9 Sidewinder missiles. Internal fuel was 10,200 pounds (full fuel for the prototype is 10,600 pounds). Allowing for fuel consumption for engine start and taxi, gross takeoff weight was 43,500 pounds. Jim estimated the takeoff roll at a bit more than 3,000 feet.

The aft cockpit of the F-16XL test aircraft is configured with the current avionics and sensors that are in production standard F-16C and D aircraft. Should the derivative fighter evaluation result in the F-16XL’s becoming USAF’s dual-role fighter, the avionics suite will be the same as that being developed under the Multi-Stage Improvement Program (MSIP) for the F-16C/D, which will start being delivered, with initial core systems, in 1984.

When fully implemented, MSIP will provide the desired night/under-weather, navigation/weapon-delivery and beyond-visual-range (BVR) missile capabilities. The back seat in the Dual-Role Fighter version would have the controls and displays, including a color-moving map, added to provide the independent or interactive task coordination required to fulfill the dual-role missions. If additional, or future, avionics are needed, the MILSTD-1553 avionics multiplex bus will be able to accommodate virtually anything by a simple reprogramming of its software.

Jim McKinney re-familiarized me with the rear cockpit controls and emergency procedures. Then we put on personal equipment and walked to the aircraft for preflight.

The F-16 design has always impressed me. It looked functional yet appealing, a design already in the classic category. Approaching the F-16XL with an F-16 alongside reinforced the appeal. Just parked on the ramp, the airplane looked efficient, and you wanted to get in and fly to see what it will do. The walk-around inspection reinforced the feeling, and verified features of the XL design discussed earlier.

Of particular interest were the control surfaces on the aft edge of the cranked-arrow wing. The F-16XL does not have a horizontal tail. Thus, the control surfaces for both pitch and roll are on the rear edge of the wing. The inboard surfaces are mainly for pitch control, while the out board surfaces take care of roll control. However, thanks to the automatic flight control system, when performance requires it, all four surfaces can act in either pitch or roll.

The drag chute s another difference noted on the walk-around. Except for the Norwegian configuration, standard F-16s do not have a drag chute. It was installed on the F-16XL for operational advantages. It enables the aircraft to recover at airfields whose runways have been shortened through enemy action, as is the threat in Europe. With the drag chute, the F-16XL can recover on runways shorter than 2,000 feet, and it can attain higher-gross-weight takeoffs for the short, critical field lengths of NATO runways. The drag chute allows aborts on a wet runway under hot day conditions at the maximum gross takeoff weight of 48,000 pounds.

Also on the walk-around, we could see close up how the designers mated external payload to the new wing. The method is called “semi-conformal mounting.” The normal method uses a pylon protruding from the wing, with a bomb rack that contains multiple ejectors, and then the bombs. That approach imposes high drag and weight penalties.

With the F-16XL method, only the ejectors protrude from the wing and the bombs are thus snugged up close. Their arrangement conforms to the wing shape. Also, the wingspan is larger enough to permit staggered placement from centerline outboard, and n line from fore to aft. With one bomb behind the other (in line) the second bomb has half the drag of the first one and the third bomb has half the drag of the second one.

By staggering each row of bombs inboard to outboard, the interference drag is also reduced. Thus, the total drag of this innovative carriage concept is sixty percent lower than the conventional concept. The result is another performance bonus: supersonic flight with a full bomb load. While up to sixteen Mk 82 bombs can be hung from the F-16XL’s big wing, twelve were on 75-0747 for our flight.

Supersonic in Seconds

Takeoff from Edwards AFB’s Runway 22 with maximum power at gross weight of 43,500 pounds was achieved in les than 3,000 feet. Jim eased back the power to climb away from the Edwards traffic pattern and take up a northerly heading for the test airspace assigned to us.

Cleared to climb to 30,000 feet, Jim applied afterburner and back pressure. Our weight was diminished only by the fuel used for takeoff and the brief excursion out of the pattern. We climbed at more than 20,000 feet per minute, leaping from 4,000 to 27,000 feet in sixty-seven seconds. Jim eased the power back while turning into the supersonic corridor and getting cleared by Edwards Control to begin a supersonic run. Jim applied afterburner and the aircraft accelerated smoothly from Mach 0.95 through 1.0 and to 1.2 in seconds. Even with the heavy bomb load aboard, the aircraft went supersonic without a tremble. Handling characteristics at mach 1.2 with the heavy ordnance load were remarkably similar to those of the standard F-16 without bombs.

Jim pulled the throttle back to military power. The aircraft continued to coast supersonically for a long period before the mach meter showed that we were once again subsonic at 0.97.

Next, we maneuvered at slow flight speeds and high angles of attack, demonstrating the F-16XL’s agile handling in that corner of the performance envelope. With airspeed below 150 knots, Jim invited me to try a roll to the left. Pressure on the side-stick controller resulted in a fast roll, with no sensation of lagging because of the heavy payload. Release of pressure stopped the roll immediately. I tended to “ratchet,” and tried to end the roll with opposite pressure. That’s unnecessary with the F-16XL’s system, as Jim demonstrated. I tried it again, more smoothly this time.

We accelerated back to more than 400 knots and I tried more 360° rolls. Once I was accustomed to the correct control stick pressures, the roll rate was fast and the controls crisp. The same feelings were apparent at 500 knots – quick, sure response, with no feeling of carrying the heavy bomb load.

Next, Jim demonstrated the F110 engine’s ability to accelerate from idle to max afterburner by slamming the throttle forward. Engine response was smooth with no coughing or stalling, thanks to General Electric’s advanced electronic engine controls.

Then we descended to low level for penetration at high speed. Jim set up the aircraft at 600 knots indicated airspeed at 100 feet above ground level. The ride quality on a very hot day was smooth. The G-indicator on the head-up display (HUD) showed excursions of less than 0.2 above the below 1.0, but they were undetectable in the body. On similar flights with an F-4 as the chase aircraft, its G excursions were as high as 2.0, making for an uncomfortable ride and heavy concentration on flight controls.

In the loaded configuration, the F-16XL can penetrate at low level at airspeeds fifty-to-ninety knots faster than the basic F-6 when similarly configured. In fact, at every corner of the performance envelope, the aircraft has power in reserve, according to members of the Combined Test Force at Edwards.

Next, we conducted simulated weapons passes on a ground target, using the continuously computed impact point system (CCIP) displayed on the HUD. With this system, even this novice pilot, who has difficulty with a non-computing gun-sight, achieved on-target results. Attack maneuvers resulted in G forces ranging to +7.0. With the heavy bomb load aboard, the F-16XL is cleared for maneuvers up to +7.2 Gs, compared with 5.58 Gs in the F-16A. This demonstrates how the designers were able to increase the aircraft weight while maintaining structural integrity and mission performance.

We returned to Edwards to land on Runway 22. Touchdown speed was 170 knots. When Jim deployed the drag chute, its effect was instantaneous, slowing us to less than eighty knots in less than 1,000 feet.

With the F-16XL, the US Air Force has the option to gain markedly improved range, payload, and survivability performance over current fighters. According to its designers, the F-16XL in production would have a unit flyaway cost of about fifteen to twenty percent more than the F-16C and D.