CASE
13
Care-Free Maneuverability At High Angle of Attack
Since the airplane’s earliest days, maintaining safe flight at low speeds and high angles of attack has been a stimulus for research. As well, ensuring that a military fighter aircraft has good high-angle-of-attack qualities can benefit its combat capabilities. NASA research has provided critical guidance on configuration effects and helped usher in the advent of powerful flight control concepts.
Case-13 Cover Image: A drop model of the F/A-18E is released for a poststall study high above the NASA Wallops Flight Center. NASA.
At the time that the National Aeronautics and Space Administration (NASA) absorbed the National Advisory Committee for Aeronautics (NACA), it also inherited one of the more challenging technical issues of the NACA mission: to “supervise and direct the scientific study of the problems of flight with a view to their practical solution.” Since the earliest days of heavier-than-air flight, intentional or inadvertent flight at high angles of attack (high alpha) results in the onset of flow separation on lifting surfaces, stabilizing fins, and aerodynamic controls. In such conditions, a poorly designed aircraft will exhibit a marked deterioration in stability, control, and flying qualities, which may abruptly cause loss of control, spin entry, and catastrophic impact with the ground.[1] Stalling and spinning have been—and will continue to be—major areas of research and development for civil and military aircraft. In the case of highly maneuverable military aircraft, high-angle-of-attack characteristics exert a tremendous influence on tactical effectiveness, maneuver options, and safety.
Some of the more notable contributions of NASA to the Nation’s military aircraft community have been directed at high-angle-of-attack technology, including the conception, development, and validation of advanced ground- and flight-test facilities; advances in related disciplinary fields, such as aerodynamics and flight dynamics; generation of high-alpha design criteria and methods; and active participation in aircraft development programs.[2] Applications of these NASA contributions by the industry and the Department of Defense (DOD) have led to a dramatic improvement in high-angle-of-attack behavior and associated maneuverability for the current U.S. military fleet. The scope of NASA activities in this area includes ground-based and flight research at all of its aeronautical field centers. The close association of NASA, industry, and DOD, and the significant advances in the state of the art that have resulted from common objectives, are notable achievements of the Agency’s value to the Nation’s aeronautical achievements.
The Early Days
Early NACA research on stalling and spinning in the 1920s quickly concluded that the primary factors that governed the physics of stall behavior, spin entry, and recovery from spins were very complicated and would require extensive commitments to new experimental facilities for studies of aerodynamics and flight motions. Over the following 85 years, efforts by the NACA and NASA introduced a broad spectrum of specialized tools and analysis techniques for high-angle-of-attack conditions, including vertical spin tunnels, pressurized wind tunnels to define the impact of Reynolds number on separated flow phenomena, special free-flight model test techniques, full-scale aircraft flight experiments, theoretical studies of aircraft motions, piloted simulator studies, and unique static and dynamic wind tunnel aerodynamic testing capability.[3]
By the 1930s, considerable progress had been made at the NACA Langley Memorial Aeronautical Laboratory on obtaining wind tunnel aerodynamic data on the effectiveness of lateral control concepts at the stall and understanding control effects on motions.[4] A basic understanding began to emerge on the effects of design variables for biplanes of the era, such as horizontal and vertical tail configurations, wing stagger, and center-of-gravity location on spinning. Flight-testing of stall characteristics became a routine element of handling quality studies. In the race to conquer stall/spin problems, however, simplistic and regrettable conclusions were frequently drawn.[5]
The sudden onset of World War II and its urgency for aeronautical research and development overwhelmed the laboratory’s plodding research environment and culture with high-priority requests from the military services for immediate wind tunnel and flight assessments, as well as problem-solving activities for emerging military aircraft. At that time, the military perspective was that operational usage of high-angle-of-attack capability was necessary in air combat, particularly in classic “dogfight” engagements wherein tighter turns and strenuous maneuvers meant the difference between victory and defeat. Tactical effectiveness and safety, however, demanded acceptable stalling and spinning behavior, and early NACA assessments for new designs prior to industry and military flight-testing and production were required for every new maneuverable aircraft.[6] Spin demonstrations of prototype aircraft by the manufacturer were mandatory, and satisfactory stall characteristics and recoveries from developed spins required extensive testing by the NACA in its conventional wind tunnels and vertical spin tunnel.
The exhausting demands of round-the-clock, 7-day workweeks left very little time for fundamental research, but researchers at Langley’s Spin Tunnel, Free-Flight Tunnel, Stability Tunnel, and 7- by 10-Foot Tunnels initiated a series of studies that resulted in advancements in high-angle-of-attack design procedures and analysis techniques.[7]
New Challenges
Arguably, no other technical discipline is as sensitive to configuration features as high-angle-of-attack technology. Throughout World War II, the effects of configuration details such as wing airfoil, wing twist, engine torque, propeller slipstream, and wing placement were critical and, if not properly designed, often resulted in deficient handing qualities accentuated by poor or even vicious stalling behavior. The NACA research staffs at Langley and Ames played key roles in advancing design methodology based on years of accumulated knowledge and lessons learned for straight winged, propeller-driven aircraft. Aberrations of design practice, such as flying wings, had posed new problems such as tumbling, which had also been addressed.[8] However, just as it appeared that the art and science of designing for high-alpha conditions was under control, a wave of unconventional configuration features emerged in the jet aircraft of the 1950s to challenge designers with new problems. Foremost among these radical features was the use of swept-back and delta wings, long pointed fuselages, and the distribution of mass primarily along the fuselage.
Suddenly, topics such as pitch-up, inertial coupling, and directional divergence became the focus of high-angle-of-attack technology. Responding to an almost complete lack of design experience in these areas, the NACA initiated numerous experimental and theoretical studies. One of the more significant contributions to design methods was the development of a predictive criterion that used readily obtained aerodynamic wind tunnel parameters to predict whether a configuration would exhibit a directional divergence (departure) at high angles of attack.[9] Typical of many NACA and NASA contributions, the criterion is still used today by designers of high-performance military aircraft.
As the 1950s progressed, it was becoming obvious that high-alpha maneuverability was becoming a serious challenge. Lateral-directional stability and control were difficult to achieve, and the spin and recovery characteristics of the new breed of fighter aircraft were proving to be extremely marginal. In addition to frequent encounters with unsatisfactory spin recovery, dangerous new poststall motions such as disorienting oscillatory spins and fast flat spins were encountered, which challenged the ability of human pilots to effect recovery.[10]
Group photo of X-planes at Dryden in 1953 exhibit configuration features that had changed dramatically from the straight winged X-1A and D-558-1, at left, to the delta wing XF-92A, top left, the variable-sweep X-5, the swept wing D-558-2, the tailless X-4, and the slender X-3. The changes had significant effects on high-alpha and spin characteristics. NASA.
Automatic flight control systems were designed to limit the maximum obtainable angle of attack to avoid these high-angle-of-attack deficiencies, but severe degradations in maneuver capability were imposed by this approach for some designs. Researchers considered automatic spin recovery concepts, but such systems required special sensors and control components not used in day-to-day operations at that time. Concerns over the cost, maintenance, and the impact of inadvertent actuation of such systems on safety discouraged interest in the development of automatic spin prevention systems.
As the 1950s came to a close, the difficulty of designing for high-angle-of-attack conditions, coupled with the anticipated dominance of emerging air-to-air missile concepts, resulted in a new military perspective on the need for maneuverability. Under this doctrine, maneuverability required for air-to-air engagements would be built into the missile system, and fighter or interceptor aircraft would be designed as standoff missile launchers with no need for maneuverability or high-alpha capability. Not only did this scenario result in a minimal analysis of high-angle-of-attack behavior for emerging designs, it resulted in a significant decrease in the advocacy and support for NASA research on stall/spin problems. In the late 1950s, Langley was even threatened with a closure of its spin tunnel.[11]
Revelation and Call to Action
During the Vietnam conflict, U.S. pilots flying F-4 and F-105 aircraft faced highly maneuverable MiG-17 and MiG-19 aircraft, and the unanticipated return of the close-in dogfight demanded maneuverability that had not been required during design and initial entry of the U.S. aircraft into operational service. Unfortunately, aircraft such as the F-4 exhibited a marked deterioration in lateral-directional stability and control characteristics at high angles of attack. Inadvertent loss of control became a major issue, with an alarming number of losses in training accidents. A request for support to the NASA Langley Research Center by representatives of the Air Force Aeronautical Systems Division in 1967 resulted in an extensive analysis of the high-angle-of-attack deficiencies of the aircraft and wind tunnel, free-flight model, and piloted simulator studies.[12]
The F-4 experience is especially noteworthy in NASA’s contributions to high-angle-of-attack technology. Based on the successful demonstrations of analysis and design tools by NASA, management within the Air Force, Navy, and NASA strongly supported an active participation by the Agency in high-angle-of-attack technology, resulting in requests for similar NASA involvement in virtually all subsequent DOD high-performance aircraft development programs, which continue to the current day. After the F-4 program, NASA activities at Langley were no longer limited to spin tunnel tests but included conventional and special dynamic wind tunnel tests, analytical studies, and piloted simulator studies.
The shocking number of losses of F-4 aircraft and aircrews did not, however, escape the attention of senior Air Force leadership. As F-4 stall/spin/out-of-control accidents began to escalate, other aircraft types were also experiencing losses, including the A-7, F-100, and F-111. The situation reached a new level of concern when, on April 26, 1971, Air Force Assistant Secretary for Research and Development (R&D) Grant L. Hansen sent a memorandum to R&D planners within the Air Force noting that during a 5-year period from 1966 through 1970, the service had lost over $200 million in assets in stall/spin/out-of-control accidents while it had spent only $200,000 in R&D.[13] Hansen’s memo called for a broad integrated research program to advance the state of the art with an emphasis on “preventing the loss of, rather than recovering, aircraft control.” The response of Air Force planners was swift, and in December 1971, a major symposium on stall/poststall/spin technology was held at Wright-Patterson Air Force Base.[14] Presentations at the symposium by Air Force, Navy, and Army participants disclosed that the number of aircraft lost by the combined services to stall/spin/out-of-control accidents during the subject 5-year period was sobering: over 225 aircraft valued at more than $367 million. Some of the aircraft types stood out as especially susceptible to this type of accident—for example, the Air Force, Navy, and Marines had lost over 100 F-4 aircraft in that period.
An additional concern was that valuable test and evaluation (T&E) aircraft and aircrews were being lost in flight accidents during high-angle-of-attack and spin assessments. At the time of the symposium, the Navy had lost two F-4 spin-test aircraft and an EA-6B spin-test vehicle, and the Air Force had lost an F-4 and F-111 during spin-test programs because of unrecoverable spins, malfunctions of emergency spin parachute systems, pilot disorientation, and other spin-related causes. The T&E losses were especially distressing because they were experienced under controlled conditions with a briefed pilot entering carefully planned maneuvers with active emergency recovery systems.
The 1971 symposium marked a new waypoint for national R&D efforts in high-angle-of-attack technology. Spin prevention became a major focus of research, the military services acknowledged the need for controlled flight at high-angle-of-attack conditions, and DOD formally stated high-angle-of-attack and maneuverability requirements for new high-performance aircraft programs. Collaborative planning between industry, DOD, and NASA intensified for research efforts, including ground-based and flight activities.[15] The joint programs clearly acknowledged the NASA role as a source of corporate knowledge and provider of national facilities for the tasks. With NASA having such responsibilities in a national program, its research efforts received significantly increased funding and advocacy from NASA Headquarters and DOD, thereby reversing the relative disinterest and fiscal doldrums of the late 1950s and 1960s.
One of the key factors in the resurgence of NASA–DOD coupling for high-angle-of-attack research from the late 1960s to the early 1990s was the close working relationships that existed between senior leaders in DOD (especially the Navy) and at NASA Headquarters. With these individuals working on a first-name basis, their mutual interests and priorities assured that NASA could respond in a timely manner with high-priority research for critical military programs.[16]
From a technology perspective, new concepts and challenges were ready for NASA’s research and development efforts. For example, at the symposium, Langley presented a paper summarizing recent experimental free-flight model studies of automatic spin prevention concepts along with a perspective that unprecedented opportunities for implementation of such concepts had arrived.[17] Although the paper was highly controversial at the time, within a few months, virtually all high-performance aircraft design teams were assessing candidate systems.
Accelerated Progress
NASA’s role in high-angle-of-attack technology rapidly accelerated beginning in 1971. Extensive research was conducted with generic models, simulator techniques for assessing high-alpha behavior were developed, and test techniques were upgraded. Active participation in the F-14, F-15, and B-1 development programs was quickly followed by similar research for the YF-16 and YF-17 Lightweight Fighter prototypes, as well as later efforts for the F-16, F-16XL, F/A-18, X-29, EA-6B, and X-31 programs. Summaries of Langley’s contributions in those programs have been documented, and equally valuable contributions from Dryden and Ames will be described herein.[18] Brief highlights of a few NASA contributions and their technical impacts follow.
Spin Prevention: The F-14 Program
Early spin tunnel tests of the F-14 at Langley during the airplane’s early development program indicated the configuration would exhibit a potentially dangerous fast, flat spin and that conventional spin recovery techniques would not be effective for recovery from that spin mode—even with the additional deployment of the maximum-size emergency spin recovery parachute considered feasible by Grumman and the Navy. Outdoor radio-controlled models were quickly readied by NASA for drop-testing from a helicopter at a test site near Langley to evaluate the susceptibility of the F-14 to enter the dangerous spin, and when the drop model results indicated marginal spin resistance, Langley researchers conceived an automatic aileron-to-rudder interconnect (ARI) control system that greatly enhanced the spin resistance of the design.[19] The value of NASA participation in the early high-angle-of-attack assessments of the F-14 benefited from the fact that the same Langley personnel had participated earlier in the development of the flight control system for the F-15, which used a similar approach for enhanced spin resistance. Extensive evaluations of the effectiveness of the ARI concept by NASA and Grumman pilots in the Langley Differential Maneuvering Simulator (DMS) air combat simulator reported a dramatic improvement in high-alpha characteristics.
An F-14 used in Dryden’s high-alpha flight program extends its foldout nose canards. Spin tunnel tests predicted that the airplane’s flat spin would require this modification. NASA.
However, after the ARI system was conceived by Langley and approved for implementation to the F-14 fleet, a new wing leading-edge maneuver flap concept designed by Grumman was also adopted for retrofit production. Initial flight-testing showed that, when combined, the ARI and maneuver-flap concepts resulted in unsatisfactory pilot-induced oscillations and lateral-directional deficiencies in handling qualities at high angles of attack. Meanwhile, NASA had withdrawn from the program, and Grumman’s modifications to the ARI to fix the deficiencies actually made the F-14 more susceptible to spins. Made aware of the problem, Langley then revisited the ARI concept and, together with Grumman and Navy participation, corrected the problems. Development and refinement of the ARI system for the F-14 continued for several years.
In the mid-1970s, senior Navy leaders were invited to NASA Headquarters for briefings on the latest NASA technologies that might be of benefit to the F-14. When briefed on the effectiveness of the ARI system, a decision was made to conduct flight evaluations of a new updated NASA version of the system. Joint NASA–Grumman–Navy flight-test assessments of the refined concept took place with a modified F-14 at the NASA Dryden Flight Research Center[20] in 1980. Flight tests of the ARI-equipped aircraft included over 100 flights by 9 pilots over a 2-year period during severe high-angle-of-attack maneuvers at speeds up to low supersonic Mach numbers. Results of the activity were very impressive; however, funding constraints and priorities within the Navy delayed the implementation of the system until an advanced digital flight control system (DFCS) was finally incorporated into fleet airplanes in 1999. The system, designed by a joint GEC-Marconi–Northrop Grumman–Navy team, was essentially a refined version of the concept advanced by Langley over 25 years earlier.[21]
In retrospect, the F-14 experience is a classic example of inadequate followthrough on the technology maturation process for new research concepts. No doubt, if NASA had continued its involvement in the development of the ARI and been tasked to resolve the ARI/maneuver flap issues, the fleet would have benefitted from the concept much earlier.[22]
New Levels of Departure Resistance: The F-15 Program
After its traumatic experiences with the F-4 stability and control deficiencies at high angles of attack, the Air Force encouraged competitors in the F-15 selection process to stress good high-angle-of-attack characteristics for the candidate configurations of their proposed aircraft. As part of the source selection process, an analysis of departure resistance was required based on high Reynolds number aerodynamic data obtained for each design in the NASA Ames 12-Foot Pressure Tunnel. In addition, spin and recovery characteristics were determined during the competitive phase using models in the Langley Spin Tunnel. The source selection team evaluated data from these and other high-angle-of-attack tests and analysis.
In its role as an air superiority fighter, the winning McDonnell-Douglas F-15 design was carefully crafted to exhibit superior stability and departure resistance at high angles of attack. In addition to providing a high level of inherent aerodynamic stability, the McDonnell-Douglas design team devised an automatic control concept to avoid control-induced departures at high angles of attack because of adverse yaw from lateral control (ailerons and differential horizontal tail deflections). By using an automatic aileron washout scheme that reduced the amount of aileron/tail deflections obtainable at high angles of attack and an interconnect system that deflected the rudder for roll control as a function of angle of attack within its Command Augmentation System (CAS), the F-15 was expected to exhibit exceptional stability and departure resistance at high angles of attack.
Radio-controlled drop model of the F-15 undergoing checkout prior to a flight to assess spin susceptibility at a test site near Langley Research Center. The F-15’s reluctance to spin was accurately predicted in model tests. NASA.
NASA’s free-flight model tests of the F-15 in the Langley Full-Scale Tunnel during 1971 verified that the F-15 would be very stable at high-angle-of-attack conditions, in dramatic contrast to its immediate predecessors.[23] During the F-15 development process, spin tunnel testing at Langley provided predictions for spin modes for the basic airplane as well as an extensive number of external stores, and an emergency spin recovery parachute size was determined.
Langley was also requested to evaluate the spin resistance of the F-15 with the outdoor helicopter drop-model technique used at Langley for many previous assessments of spin resistance. During spin entry attempts of the drop model with the CAS operative, it was once again obvious that the configuration was very spin resistant. In fact, an exceptional effort was required by the Langley team to develop a longitudinal and lateral-directional control input technique to spin the model. Ultimately, such a technique was identified and demonstrated, although it was successful for a very constrained range of flight variables. This spin entry technique was later used in the full-scale aircraft flight program to promote spins. In 1972, Dryden constructed a larger drop model with a more complete representation of the aircraft flight control system and a larger-scale prediction of the airplanes spin recovery characteristics. Launched from a B-52 and known as the F-15 spin research vehicle (SRV), the remotely piloted vehicle verified the predictions of the smaller model and added confidence to the subsequent flight tests.[24]
Meanwhile, testing in the Spin Tunnel concentrated on one of the more critical spin conditions for the F-15 aircraft—unsymmetrical mass loadings. Model tests showed that the configuration’s spin and recovery characteristics deteriorated when lateral unbalance was simulated, as would be the situation for asymmetric weapon store loadings on the right and left wing panels or fuel imbalance between wing tanks. Fuel imbalance can occur during banked turns in strenuous air combat maneuvers when tanks feed at different rates. The results of the spin tunnel tests showed that the spins would be faster and flatter in one direction, and that recovery would not be possible when the mass imbalance exceeded a certain critical value. As frequently happens in the field of spinning and spin recovery, a configuration that was extremely spin resistant in the “clean” configuration suddenly became an unmanageable tiger with mass imbalance.
During its operational service, the F-15 has experienced several accidents caused by unrecoverable spins with asymmetric loadings. At one time, this type of accident was the second greatest cause of F-15 losses, after midair collisions.[25]
Comparison of theoretical predictions, spin tunnel results, drop-model results, and flight results indicated that correlation of a model and airplane results were very good and that risk in the full-scale program had been reduced considerably by the NASA model tests.
Relaxed Stability Meets High Alpha: The F-16 Program
Initially envisioned as a nimble lightweight fighter with “carefree” maneuverability, the F-16 was designed from the onset with reliance on the flight control system to ensure satisfactory behavior at high-angle-of-attack conditions.[26] By using the concept of relaxed longitudinal stability, the configuration places stringent demands on the flight control system. In addition to extensive static and dynamic wind tunnel testing in Langley’s tunnels from subsonic to supersonic speeds and free-flight model studies for high-angle-of-attack conditions and spinning, Langley and its partners from General Dynamics and the Air Force conducted in-depth piloted studies in a Langley simulator. The primary objective of the studies was to assess the ability of the F-16 control system to prevent loss of control and departures for critical dynamic maneuvers involving rapid roll rates at high angles of attack and low airspeeds.[27] General Dynamics used the results of the study to modify gains in the F-16 flight control system and introduce new elements for enhanced departure prevention in production aircraft.
One of the more significant events in NASA’s support of the F-16 was the timely identification and solution to a potentially unrecoverable “deep-stall” condition. Analysis of Langley wind tunnel data at extreme angles of att