In This Issue
Centennial of Aviation
March 1, 2004 Volume 34 Issue 1

Technology and the F-16 Fighting Falcon Jet Fighter

Wednesday, December 3, 2008

Author: Harry J. Hillaker

The judicious application of advanced technologies and design innovations gave the F-16 unprecedented performance capabilities at an affordable cost.

The Lockheed F-16 Fighting Falcon was initially ridiculed and rejected by both the company and the Air Force for being too small and too light to go anywhere or carry anything significant. Nevertheless, the F-16 has proven to be an extraordinary fighter. The F-16 has been ordered by 24 countries and is operating in the air forces of more than 20 countries. The judicious application of advanced technologies combined with design innovations gives the airplane its unprecedented combat performance at an affordable cost.

A successful fighter must be orchestrated, not played with a single instrument like a violin or a clarinet, but, rather like an organ, with a vast capacity of sounds, from the highest to the deepest notes and from soft, sweet tones to loud, crashing crescendos. The design of the F-16 was orchestrated in such a fashion. The airplane can also be considered a unity of contradictions: "small" but with long range; "light" but of high strength; "low drag" but with high controllability; and high "g" but pilot tolerant. In the past, interactions between the pilot and the aircraft tended to require that specific, conscious choices be made between various disciplines, but on the F-16 the interactions are largely mutual - one discipline enhances another. Each discipline, or element, is part of an interwoven pattern, with dynamic relationships between the elements.

That the F-16 turned out to be a huge success is not accidental. We dissected the problem, studied it until we understood its interworkings, and adopted a rigorously rational approach. We were very careful not to let even a hint of emotion or bias influence our actions, fully recognizing that a decision based on either would not necessarily be rational. We accepted only facts - facts based on experimental data.

We did not use technology "for technology’s sake." We did not consider a technology or a feature unless it reduced weight or increased operational capability with no overall weight penalty. If we added something involving an incremental weight increase, it had to lower the overall weight.

To realize our size objective, we sized the airplane at the start of combat, the condition at which it was to perform its primary mission, instead of at its takeoff gross weight, the point at which its wing loading and thrust loading are defined. The wing loading (wing area), thrust loading, and structural criteria were defined at this point rather than at the takeoff point.
We knew that, with rare exceptions, external fuel tanks are added to fighters after the airplane is defined. Therefore, we decided to include external fuel tanks as part of our configuration definition. The external tanks would be used for takeoff, ascent, and outboard cruising to the start of combat. Under battle conditions, they would be jettisoned, and combat would start with full internal fuel tanks.

By sizing the airplane at the start of combat with full internal fuel tanks - combat, return cruise, and reserve fuel - and using external fuel tanks for the outbound leg of the mission, the empty weight was reduced by 1,470 pounds, and gross weight was reduced by 3,300 pounds. The weight and drag saving produced a generous 30-percent improvement in acceleration time and increased the sustained turn rate by 5 percent. At $289.70 per pound of weight empty this represented about a 10 percent saving in unit flyaway cost.

Achieving our stated design weight objective of less than 20,000 pounds was indeed a challenge, but we were careful not to skimp on anything related to the operational mission objectives. It would have been easy to skimp on structural strength and fuel load - the principal elements that could be varied - but we chose not to. We decided the configuration, or form, would have enough structural strength to match the plane’s maneuvering capacity and enough fuel to sustain its combat capability at effective, in fact higher than normal, structural criteria.

The F-16 may be light and small, but it can "bench press" with the best. It is not generally appreciated that the aircraft’s "light weight" was not achieved by resorting to high strength-to-weight materials, like titanium or composites, or other exotic materials. Nor was structural integrity sacrificed for the sake of lighter weight. The structural criteria were fully compatible with the aerodynamic capacity of the airplane and the pilot’s physical tolerance. The structural design load factor was set at 9.0 g’s at start of combat with full internal fuel, as contrasted to the normal military specification of 7.33 g’s with only 60 percent internal fuel.

Our approach to the structural design ensured that we could demonstrate the maneuvering capability of the airplane. Air Force ground rules dictate (appropriately) that an airplane be limited to 80 percent of its design takeoff weight until a structural static test has been completed. Because a structural test was not possible within our $37 million contract, we decided to design the two prototypes to 125 percent (a 25-percent margin) of the design loads (80 percent of 125 percent is 100 percent). The weight penalty was small (418 pounds or 3.5 percent). This was an unappreciated decision that contributed indirectly to the success of the prototype program; it was the means by which we were able to demonstrate the sensational, eye-opening maneuverability of the airplane that would otherwise have been restricted by the 80 percent load limit.

We also recognized that U.S. military aircraft have a history of remaining in the active inventory for a very long time. Therefore, we doubled the design service life from 4,000 hours to 8,000 hours. To my knowledge, no other fighter has ever been designed to such stringent requirements.

The most visible of the selected technologies and design innovations was the blended wing-body configuration that maximizes the total vehicle lift with a minimum increase in drag. The wing-body blending and forebody strakes are significant beneficial features of the airplane that represent radical departures from other airplane configurations and give the aircraft its unique look. The F-16 has a smooth fairing, or blending, of the body (fuselage) into the wing, rather than the usual sharp intersection; the fuselage blends into the wing cross-sectionally and longitudinally, or lengthwise. Although more of the wing is covered up, the lift lost in that area is more than regained from body lift at high angles of attack when the lift generated by the wing begins to diminish because of flow separation. The forebody strakes generate a strong vortex flow that improves directional stability, delays flow separation over the wing (thus extending life), and gives a more favorable center of lift.

One of the distinguishing features of the F-16 is the configuration of the engine air inlet, a considerable departure from more conventional configurations. We concluded early on that it would take radical departures from the norm to achieve our lightweight fabrication goals. Thus, the engine air induction system (inlet and air duct) was given major consideration in the definition and integration of the overall airplane configuration.

A nose inlet similar to the F-86 Sabre, the F-100 Super Sabre, and Vought’s F-8 Crusader or A-7 Corsair II, in which the air is undisturbed, would seem to be a logical choice for high angle-of-attack maneuvering. However, there were some drawbacks; the air duct from the inlet to the face of the engine is quite long, extending roughly one-half the length of the fuselage. Long ducts can be beneficial in straightening, or diffusing, flow or pressure distortions that result from high angle-of-attack flight. However, there are also penalties. The high internal duct pressure that occurs during supersonic flight adds considerable weight, and excessive duct length means more friction losses that result in lower pressure recovery. In addition, unless the air duct goes around the cockpit, as it does in the F-100 Super Sabre, instead of under the cockpit, as it does on the Vought F-8 Crusader and A-7 Corsair II, the added side area would have a destabilizing effect on directional stability and would require a bigger vertical tail, which would increase drag and weight (400 pounds). With our emphasis on low weight and maximum thrust, that nose inlet configuration was not acceptable. Besides, the requirement that advanced search-track radars be mounted in the nose precluded the use of such an inlet.

We concluded that we needed an innovative approach to locating the inlet that would be better suited to the desired maneuvering characteristics of the airplane. We finally settled on a position on the lower surface of the forward fuselage, set back some distance from the nose of the airplane - far enough forward to allow a gradual bend in the air duct up to the engine face to minimize flow losses and far enough aft so it wouldn’t weigh too much or be too draggy or destabilizing.
We incorporated automatic leading-edge flaps on the wing that increase lift by as much as 18 percent and reduce drag by as much as 22 percent when performing a sustained turn at M = 0.9 at 30,000 feet and reduce the drag at maximum lift by nearly 70 percent. During an instantaneous turn, the energy loss is only a fraction of the loss you get with a fixed camber wing.

One of the revolutionary features of the F-16 is its flight control system, a notable product of advanced technology. The heart of the airplane that gives it life- the brain that determines how it performs and the muscle to make it work - the flight control system is the link that integrates the pilot and the airframe into a highly responsive and effective combat fighter. This advanced system is a radical departure from previous systems. "Fly-by-wire" is a totally electronic system that uses computer-generated electrical impulses, or signals, to transmit the pilot’s commands to the flight control surfaces instead of a combination of the push rods, bell cranks, linkages, and cables used with more conventional hydromechanical systems. Without question "fly-by-wire" was the star technology of the F-16. It’s the only way to go! Any modern airplane, whether military or commercial, that doesn’t incorporate fly-by-wire is archaic, and any company that fails to see its benefits must have its head in the sand.

"Fly-by-wire" is not really an adequate descriptor. After all, early airplanes were flown by stranded wire cables through a series of pulleys that ran from the control stick directly to the control surfaces. The control surfaces did only what the pilot could force them to do, and the pilot got direct feedback from the control surfaces’ actions. The pilot literally controlled the airplane; he provided the sensing, the signals, and the power - the muscle. With a fly-by-wire system, the pilot commands roll, pitch, or yaw rate- not surface deflection, as in a conventional system. When the pilot makes a roll input, for instance, he commands a roll rate that varies with the force of his input. If he releases his input, the airplane maintains the resultant bank angle until he makes another control input; he doesn’t have to center the "control stick" to maintain the bank angle as he does with a conventional system.

Beyond the necessary control of flight, the pilot has precise response control. The reduced lags and overshoots afforded by the better kinematics of the electronic circuitry results in greatly improved and expanded flying qualities, which, in turn, significantly improve the response and tracking accuracy of the pilot-airframe system. A much higher level of precise, nonvarying control response is possible throughout the flight envelope with the fly-by-wire system than with conventional flight control systems.

The resulting system is a quad-redundant (fail-operative, fail-operative, fail-safe), high-authority, command-and-stability augmentation system. The system consists of a series of sensors (accelerometers, rate gyros, air data converter), computers, selectors, transducers, and inverters that collectively generate the pitch, roll, and yaw rates that are transmitted as electronic signals to the five triplex electrohydraulic, servoactuators that control the flaperons (roll and flaps), elevons (pitch and roll), and rudder.

The weight saving resulting from the absence of cables, linkages, bell cranks, and the ratio changer was translated into redundancy. The redundancy level and the freedom of routing afforded by wire harnesses improved the reliability and increased the operational survivability of the airplane and contributed to its compactness and small size. With a conventional flight control system, we would have had to route the cables externally, as on the old Ford Tri-motor, to maintain the size or else make the airplane bigger.

Because of the fly-by-wire, all-electronic, computer-based flight control system, some have called the F-16 the "electric jet" or "an airplane wrapped around a computer." I don’t feel comfortable with that; I want the "smarts" to remain in the cockpit, to be vested in the pilot. The pilot uses his natural, inherent intellect to govern the airplane; he (or she) makes the airplane "obedient." It responds with minimum commands to the pilot’s needs.

In conjunction with the fly-by-wire flight control system, we changed the dynamics of the airplane, which dramatically enhanced its capabilities. We adopted what we called relaxed static stability, which meant that, with the center of lift forward of the center of gravity instead of aft of it, the airplane would be statically unstable. No matter, though. With artificial stability, the airplane would be much more dynamic - as much as two and one-half times as dynamic as the F-4C Phantom. Relaxed static stability allowed the airplane to achieve an initial pitch rate of 5 g’s per second with "deadbeat" damping-no overshoot. Maneuvers could be instantaneously initiated and precisely controlled - a very important factor.

Another key feature that greatly enhanced the airplane’s combat effectiveness was the cockpit, the pilot’s office. I know of no other fighter as attentive to the pilot’s needs. The canopy, which is one piece - no separate windshield - provides unsurpassed visibility. Some pilots say it makes them feel as though they are riding on the airplane rather than in it. The seat is tilted back 30 degrees, instead of the normal 17 degrees, which increases the pilot’s tolerance to high g maneuvers. The control stick is located on the pilot’s righthand-side console and has no movement - the force exerted on the stick, or controller, provides signal inputs to the control surfaces.

Speaking metaphorically, the F-16 initially had a torturous mountain to climb, a vast turbulent ocean to cross, a vicious dragon to slay on its way to reality, and it has proved to be one of the premier fighters in the world. Nothing can detract from that. The F-16 shows that the application of advanced, and unproven, technologies need not result in an expensive aircraft. When applied properly, it can do just the opposite. It should be noted that 30 years after its first flight the F-16 is still being procured.

About the Author:Harry J. Hillaker is retired vice president and deputy program director for the F-16, General Dynamics Corporation, and an NAE member.