F-16 Fighting Falcon
The air war experience in Vietnam, where the lack of maneuverability of US fighters at transonic speeds provided advantages to nimble enemy fighters, was the stimulus for the Lightweight Fighter program. The Air Force and designers of the Lightweight Fighter therefore placed great emphasis on achieving unprecedented transonic maneuver capability with excellent handling qualities.
In January 1972, the Lightweight Fighter Program solicited design specifications from several American manufacturers. Participants were told to tailor their specifications toward the goal of developing a true air superiority lightweight fighter. General Dynamics and Northrop were asked to build prototypes, which could be evaluated with no promise of a follow-on production contract. These were to be strictly technology demonstrators. The two contractors were given creative freedom to build their own vision of a lightweight air superiority fighter, with only a limited number of specified performance goals. Northrop produced the twin-engine YF-17, using breakthrough aerodynamic technologies and two high-thrust engines. General Dynamics countered with the compact YF-16, built around a single F100 engine.
The evolution of the YF-16 design at LMTAS included studies of configuration variables such as wing design, maneuvering devices, number and location of engines, control surfaces, number and location of tail surfaces, and structural concepts. As the configuration options matured, two candidate configurations competed for priority. The first configuration was a simple wing, body, and empennage design, while the second design was a twin-tailed, blended-wing body with vertical and horizontal tails on booms. The LMTAS team selected the best features of both configurations for the final YF-16 design. After considerations of performance, stability, and control were addressed, the YF-16 configuration incorporated a rather wide, blended forebody that produced strong vortices at moderate angles of attack. LMTAS had attempted to weaken the strength of the vortices by promoting attached flow, but these attempts were not successful.
In the early 1960's worldwide interest in the phenomenon known as "vortex lift" increased as a result of aerodynamic studies of highly swept configurations such as the Concorde supersonic transport. The favorable effects of vortex on lift were demonstrated during development of the Swedish Viggen canard configured aircraft. The favorable effects of the canard trailing vortex on the lifting capability of a close-coupled wing might also be extended to higher angles of attack by the strong leading-edge vortex flow of a slender lifting surface. The leading edge of the blended forebody be sharpened to increase (rather than decrease) the strength of the vortices, which could be exploited for additional lift. This modification allowed the forebody vortices to dominate and stabilize the flow field over the aircraft at high angles of attack, improve longitudinal and directional stability for the single-tail configuration, and stabilize the flow over the outer wing panels. In addition, the sharpened strake significantly reduced buffet intensity at transonic maneuvering conditions. The wing-body strake of the F-16 is regarded as a key contribution to its success as a maneuvering fighter.
When the YF-16 team analyzed the effects of deflected leading- and trailing-edge flaps and the sharp-edged wing-body strake on directional stability at high angles of attack, they found that the stability contributions of a single vertical tail were significantly enhanced. However, the contributions of twin vertical tails were markedly degraded. As a result of this analysis, the YF-16 was configured with a single vertical tail. Thus, the Langley recommendation for a sharpened wing-body strake favorably impacted other configuration features of the aircraft.
Increased maneuverability for the YF-16 necessitated extended flight at high angles of attack where aerodynamic deficiencies caused by separated airflow can result in sudden decreases in stability and controllability. Therefore, special emphasis was placed on tests to insure that the YF-16 could provide the pilot with "care-free" maneuverability. To provide superior handling characteristics at high angles of attack, any undesirable handling characteristics were pushed out of the operating envelope of the aircraft and the flight envelope was limited with an advanced fly-by-wire flight control system by LMTAS. This concept has proven to be highly successful and has been used in all variants of the F-16.
Reliance on the flight control system to insure satisfactory behavior at high angles of attack required research on the ability of fly-by-wire control systems to limit certain flight parameters during strenuous air combat maneuvers. The F-16 employs the concept of "relaxed static stability" in which the aircraft is intentionally designed to be aerodynamically unstable while the flight control system provides integrated stability by sensing critical flight variables and making the control inputs required to stabilize the aircraft. Of particular concern was the ability of the horizontal tails and longitudinal control system to limit the aircraft's angle of attack during maneuvers with high roll rates at low airspeeds. Such maneuvers are critical because rapid rolling maneuvers produce large nose-up trim changes due to inertial effects, whereas the aerodynamic effectiveness of the horizontal tails becomes significantly reduced at low airspeeds and high angles of attack.
Early on, tests of a YF-16 model indicated that if angle of attack was not limited by the flight control system, the aircraft could pitch up and attain an undesirable trimmed condition at very high angles of attack with insufficient nose-down aerodynamic control to recover normal flight. NASA Langley researchers viewed this "deep" stall as a serious problem that would require significant research for resolution. High-angle-of-attack test results obtained on models of the early production version of the F-16 configuration showed the same deep-stall trimmed condition that was noted in the YF-16 results. In subsequent high-angle-of-attack flight evaluations at Edwards Air Force Base, an F-16 that had been subjected to rapid rolls at diminishing airspeeds in vertical zoom climbs suddenly entered a stabilized deep-stall condition and the pilot was unable to recover the aircraft with normal aerodynamic controls. Fortunately, the test aircraft was equipped with an emergency spin recovery parachute that was deployed to recover the aircraft to normal flight conditions. This event brought all high-angle-of-attack flight tests of the F-16 to a stand-still while a solution to the deep stall could be found. The ultimate fix for the problem (which also improved takeoff performance) was increasing the size of the horizontal tail about 25 percent. This solution has been incorporated in all F-16 production aircraft.
When the Lightweight Fighter competition was completed early in 1975, both the YF-16 and the YF-17 showed great promise. The two prototypes performed so well, in fact, that both were selected for military service. On 13 January 1975 the Air Force announced that the YF-16's performance had made it the winner of its Air Combat Fighter (ACF) competition. This marked a shift from the original intention to use the two airplanes strictly as technology demonstrators. General Dynamics' YF-16 had generally shown superior performance over its rival from Northrop. At the same time, the shark-like fighter was judged to have production costs lower than expected, both for initial procurement and over the life cycle of the plane. At the same time, the YF-16 had proved the usefulness not only of fly-by-wire flight controls, but also such innovations as reclined seat backs and transparent head-up display (HUD) panels to facilitate high-G maneuvering, and the use of high profile, one-piece canopies to give pilots greater visibility. Thus, the Air Force had its lightweight fighter, the F-16.
|Join the GlobalSecurity.org mailing list|