X-15 Research Results: Chapter 7

X-15 Research Results

Chapter 7

The Dynamics of Flight

ONE OF THE MOST VEXING PROBLEMS for the aerodynamicist is the dynamics of motion of an airplane as it moves through the atmosphere. Dynamics of motion relates aerodynamic forces to gravity forces, and since an airplane is free to rotate around any one of its three axes, the mass and inertia characteristics are also of major influence. The airplane must have correct and stable orientation along the desired flight path and also be maneuverable. It is significant that these were the last of the problems to be surmounted before man achieved sustained flight. This was the field of the most notable of the many contributions of the Wright Brothers.

Although the same fundamental problems had to be solved for the X-15 as for the Wright Flyer, the scope and magnitude of present-day problems are vastly greater. A wide range of nonlinear airflow conditions is encountered by the X-15 from subsonic to hypersonic speeds, and for angles of flow to 30 degrees. In addition, a wide range of air pressures is encountered for flight within the corridor, as well as a fall-off to zero pressure for the space-equivalent region above it. All these varied effects present formidable control tasks to the pilot. They require careful balancing of aerodynamic configuration, control system, and pilot capability to achieve satisfactory airplane maneuverability and dynamic response.

While dynamics of flight are important for flight to high speed, they are a critical factor for flight to high altitude and reentry. The X-15's maximum altitude was extended to 354 200 feet, but not until after much trial and error. The high angle of attack required for reentry from such heights was found to be a difficult control region. In fact, under certain conditions the X-15 would be dynamically uncontrollable there. Aerodynamicists had to break with traditional stability-and-control concepts when they found that old criteria did not apply to this new aerodynamic region. Ultimately, a change in the X-15's vertical-tail configuration and a new control system were required to explore the craft's maximum potential. This work concentrated attention on dynamic-analysis techniques and the necessary, even critical, part that fail-safe electronic aids could play.

The dynamics of piloted flight is a field in which engineers and pilots have long had to discard familiar methods and assumptions and venture in new directions. This has resulted from continued study of the complex equations that describe the behavior of an airplane in flight. Such analysis provides a basis for understanding the motion of an aircraft along a flight path within the corridor, its navigation over the Earth's surface, and, more significantly, its angular rotation around its own center of mass. All of these factors are inextricably coupled and must be kept in proper balance.

The most important is a compromise between maneuvering control and inherent stability to maintain proper alignment along the flight path. This is not peculiar to aircraft flight. The maneuverability of a unicycle, for example, is much greater than that of a bicycle. But the lower stability of a unicycle is all too evident to the rider. Without the proper compromise, an airplane may be too stable and have limited maneuverability or be highly maneuverable but unstable, like a unicycle. The pilot can compensate for certain instabilities, and quite often he has to control an unstable aircraft condition. However, the history of aviation contains many tragic accidents that attest to the inherent danger involved, especially in regions of high air-pressure forces.

The fact that yawing and rolling motions are coupled severely complicates the stability-and-control problem. And though the tail surface provides stability in pitch and yaw, no purely aerodynamic means has been found to achieve roll stability, since the airflow remains symmetrical about the axis of rotation. The coupling between roll and yaw becomes more severe as vertical-tail size increases, and it has presented a multitude of problems to designers of high-speed aircraft.

The solution to the stability-and-control analysis is the development of an adequate mathematical model. But such an analysis also requires a mathematical model for the pilot. While the static displacements and force capabilities of a pilot actuating controls are well understood, the dynamic-response characteristics are not at all precisely defined. Some progress has been made for simplified tasks, but no one has yet been able to develop a handbook model that accounts for differences between humans, or for the effects of environment, G-loads, fatigue, incentive, or intuition.

This seeming vacuum in stability-and-control analysis has been filled from study of the response of the pilot-airplane combination. Engineers have learned to utilize a pilot's natural attributes and to augment them, so that he can operate a highly complex machine. From this, engineers developed criteria for flying qualities that relate airplane maneuverability and response to aerodynamic-design parameters. These parameters are, perhaps, modern mathematical forms for the Wrights' "seat of the pants". They are based on empirical methods, though, and the X-15 would take stability and control far beyond previous knowledge. In addition, no criteria had been developed for flight at angles of attack above 10 degrees, or for the space-equivalent region. Even definition of an acceptable stability level was not always clear.

The X-15's Powerful Roll Damper

As speed and altitude increase, one pronounced effect on airplane control is a drastic decrease in the aerodynamic restoring forces that retard the oscillatory motions about the center of gravity. These restoring forces, which damp the motion, are effectively nonexistent over much of the X-15's flight regime, except at low speed and low altitude. Therefore, for precise control, it was necessary to provide artificial means for damping motions, through the control system. Damping about the pitch, roll and yaw axis had previously been something of a luxury for high-speed aircraft, but it became essential for the X-15. Furthermore, it had to be much more powerful than before. Previous automatic-damper systems bolstered pilot control only slightly, but the X-15 roll damper has twice the roll-control capability of the pilot. This strong stability-augmentation became a predominant part of the control system.

A far more significant evolution was taking place, Modern design practice had previously achieved a configuration that was stable and controllable without automatic controls, though it had become increasingly difficult at higher speeds and angles of attack. The advent of powered controls was an avenue for improving aerodynamic-control characteristics by incorporating electronic networks, in addition to the pilot, in the actuating of controls. This increases system complexity, though, and the simplest pilot-control system that can accomplish the task is usually the best assurance of mission success. Experienced research pilots provide a degree of reliability unmatched by electronics. However, when the altitude above 250 000 feet came under assault, simplicity gave way to complexity. Quite a lot of electronic equipment was needed to perform automatic function essential to precise control for the reentry maneuver from the maximum altitude of 354 200 feet.

Operations have changed extensively from the original system in the course of the extensive flight-development program. Much has been learned about the use of a powerful damper system. Free play in control linkages and other effects of structural coupling with the control system have been troublesome. The critical dependence of proper control on the damper placed extra stress on system reliability, yet the consequences of a failure had to be anticipated. Originally, a fail-safe design, similar to that of the rocket engine, was considered mandatory. Any component failure would shut the system down. Modifications have improved fail-safe provisions and reliability. The system has evolved into one of duality and redundancy rather than simplicity.

The combination of stability augmentation and rolling tail has been eminently satisfactory for control from launch through reentry and landing. The new concept of combined roll and pitch control from horizontal-tail surfaces has proved to be trouble-free. Control during the powered phase of flight must be very precise, because the entire path of a 10-12-minute flight is established in the brief time of 85 seconds. Each flight consists of a climb along a predetermined flight path and either a pushover to level flight for a speed run or a fixed climb angle to reach high altitude. Techniques for trajectory control were developed on the flight simulator, with particular emphasis on backup or emergency modes for completing a mission in the event of component failures.

One flight-control area of early concern was the space-equivalent region, where jet reaction controls were to be used. Since the X-15 was the first aircraft to enter this region, the use of jet controls was an important research matter. An early objective was to determine criteria for the design and development of a system. Although new pilot-control techniques for space flight were acceptable, there could be no radical differences from aerodynamic control, for the pilot would always be faced with the low-aerodynamic-pressure region of mixed aerodynamic and jet reaction controls. Experience warned that transition regions are usually the most troublesome. Since the primary factors depend upon a dynamic-control situation, the flight simulator was used as the primary tool for control-system design and development. One goal was to develop a system and techniques that would reduce the control rockets' consumption of propellants to a minimum.

Despite early fears, control in the space-equivalent region quickly proved to contain few problems. Initial evaluations were made with a simple ground test rig that simulated X-15 characteristics. Later, limited flight tests were made in the X-1B rocket airplane and in an F-104. This work encouraged confidence that there were no inherent problems for aircraft control with small rocket motors, though a number of difficulties with H₂O₂ systems were uncovered. Pilots found they could easily learn space control, and the idiosyncrasies of jet controls were minor compared to those of coupling aerodynamic controls. The early emphasis on the consumption of jet reaction fuel as a criterion has been less important to the flight program. Since the X-15's motions in the space-equivalent region are undamped, the original control system was modified to provide automatic damping through electronic control.

Problems of Reentry From Near-Space

Reentry from flight above the corridor presents the most serious flight-dynamics problems. At suborbital speeds, the X-15's reentry differs in many respects from the reentry, at near-orbital speeds, of a ballistic capsule. With the latter, the reentry problem is to dissipate kinetic energy in near-horizontal flight at high altitude, and to convert to a vertical descent path through the low-altitude region. The X-15's reentry, in contrast, starts from a steep descent path, which must be converted to a horizontal flight path. The serious problem for a ballistic capsule is the dissipation of energy in the form of heating. The X-15's reentry is made at speeds at which aerodynamic heating is not an important factor. Had this not been the case, its reentry would have involved much more serious problems.

Even so, many difficulties had to be overcome to push to altitudes above 200 000 and 300 000 feet. These very high altitudes require steeper angles for the reentry flight path, and more rapid flight into the layers of atmospere within the corridor. They also produce more rapid change in the pilot's control sensitivity and the plane's dynamic response, while superimposing oscillations on the already high pullout forces required to keep from dipping too far into the corridor and exceeding the air-pressure limits. Another difficulty in returning from the higher altitudes is that the airplane approaches the structural design limits during pullout. Whereas considerable margin is allowable for reentries from 200 000 feet, the margin slims markedly as altitudes rise above that figure. It becomes a limiting factor. Thus, the reentry wasn't so important as just another new flight condition, or as an end in itself - the aftermath of every flight into space. It was important as a means of exploring the most severe flight-dynamics problems ever encountered in piloted aircraft.

The most serious problem that developed during the X-15's exploration of high altitude and reentry was that it could not have satisfactory control without automatic stability-augmentation during some of the most critical flight conditions. In the basic airplane, the pilot could, in fact, produce uncontrollable motions by trying to control either pitch or roll oscillations during reentry.

The pitch-control problem was not new. Neither was it serious, as long as the pilot did not attempt to control the oscillation. He could not gain precise control, but neither would the motions become divergent. However, the coupling between roll-yaw motions was such that he must use some control to keep the wings level, and without stability-augmentation at angles of attack above 8 to 10 degrees, any pilot control induced roll-yaw oscillations that diverged until the airplane was out of control.

From routine spadework during flight preparations, this serious control problem began to emerge as a critical flight region. While the original design criteria showed it to be an area for concern, they did not predict it to be an uncontrollable region. But dynamic instabilities are complicated phenomena, and previous experience had shown that it is often the severity of the problem, rather than the problem itself, that is unexpected.

The large vertical-tail surfaces maintain good directional stability at low and high angles of attack, and have a favorable effect on roll-yaw coupling at low angles of attack. But their effect on coupling at high angle of attack was known to be adverse. It was not clear at the time of the design which of these interacting forces would turn out to be the more critical. Not until flight-simulator studies began extensively probing this reagion was the magnitude of the problem revealed. It illuminated the critical importance of the roll-damper for reentry flight from altitudes above about 250 000 feet.

A three-pronged attack on this problem was undertaken. Its goals were: (1) to develop analytical techiques to understand the dynamics of the problem, (2) to reduce the magnitude of the problem through aerodynamic means, (3) to reduce likelihood of roll-damper failure. As is often the case, all three approaches contributed to solving the problem. The lessening of adverse roll by removing part of the lower vertical tail has been discussed. Significantly, this change reduces stability by about half at high angle of attack, yet it improves pilot control. The speed brakes were used to provide an added increment of stability where necessary during other phases of flight.

Noteworthy was the development of an analytical technique that predicted the roll-yaw control problem and related its severity to familiar aerodynanic parameters. The dynamics of the critical roll-yaw coupling are now understood, and the analytical technique shows designers how to avoid similar problems in future hypersonic aircraft. Reliabilty of the stability-augmentation system was improved and the system modified to provide redundant components and operation after component failure.

This work was carried out while the X-15 flight tests were going on. Extensive use was made of an F-100C airplane, which was modified to duplicate the X-15's characteristics. One of the prime aids for dynamic analysis in developing satisfactory pilot-control-system configuration was the flight simulator.

Not every approach was satisfactory. Since the basic control problem comes from the use of normal pilot-control techniques, extensive simulator studies and limited flight tests were made for nonconventional control techniques, wherein roll control is used to control yaw rather than roll. A technique was developed on the simulator that permitted flight in the fringes of the uncontrolled region. Exploratory flight tests showed the technique to be very difficult to use in flight, though, and of doubtful use in an emergency. Thus, an area of caution developed in the application of flight-simulator results. Although unorthodox control techniques for the X-15 have not been investigated further, they have been applied more promisingly to other flight programs. These new concepts may someday be accepted as suitable for control.

Development of Self-Adaptive Control

One very significant advance came from the development of a new control system for one of the three airplanes. The X-15 served to focus attention on the problem of obtaining satisfactory flying characteristics over the entire flight envelope. The increased performance of aircraft had stimulated research on a new concept for a control system during the mid-1950's, one that would adapt constantly to varying flight conditions. Under the stimulus of the Flight Control Laboratory at the Air Force Aeronautical Systems Division, this concept evolved into what is now known as the self-adaptive control system.

cockpit view of the X-15's control stick and panel.

This is the centrally placed control panel of the X-15's
remarkable and highly successful adaptive-control system.

By 1958, its feasibility had been demonstrated in flight tests of jet aircraft, and engineers were curious to find out if it could cope with the demanding flight conditions of the X-15. In early 1959, the Minneapolis Honeywell Corp. started the design of an adaptive-control system for the X-15. Although the primary intent was to test the technique in a true aerospace environment, it was decided to include in the system certain features that had evolved as important by-products of the self-adaptive concept. These were: dual redundancy for reliability; integration of aerodynamic and reaction controls; automatic stabilization for angle of attack, roll angle, and yaw angle.

The basic feature that distinguishes the adaptive system from other control systems is a gain-changer, which automatically adjusts the control-system gain so as to maintain the desired dynamic response. This response is governed by an electronic network that compares actual aircraft response with an ideal response, represented as a rate of roll, pitch, or yaw. Stability augmentation is provided by rate-gyro feedback for each axis.

Although adaptive control results in a number of unconventional flying characteristics, pilots are enthusiastic in their acceptance of it. An important feature is the integration of reaction controls and aerodynamic controls into a single, blended system. In combination with damping and automatic attitude control, this results in more precise command than was possible when a pilot worked the jet reaction controls himself.

The fail-safe provision of the adaptive-control system is a big improvement over that of the basic flight-control system. No single malfunction causes complete disengagement. Rigorous preflight and postflight check-out procedures are required, however, for the pilot cannot detect some malfunctions in flight.

Confidence in the system has grown so that it is now the preferred control system for high-altitude flights. It has enabled the X-15 to fly through more severe reentry conditions than it could have weathered without it. Not only does the adaptive system provide constant airplane response but it has excellent reliability and affords additional control modes for critical control tasks. This has increased pilots' confidence in automatic controls so much that consideration is being given to replacing mechanical linkages with electric wires. The adaptive concept may eventually enable a pilot to control all stages of a multi-stage booster as well as the glide-reentry spacecraft that the booster hurls into orbit.

As the roll-yaw coupling problem came to be understood, flights progressed to higher and higher altitudes and more severe reentry conditions. Reentries from 250 000 feet were explored with the original vertical tail and original control system; from 300 000 feet with the original vertical tail and adaptive-control system; from 354 200 feet with the revised vertical tail and adaptive-control system. Fifteen reentries altogether have shown that piloted flight reentry is both possible and practical. To be sure, each reentry explored progressively more severe conditions.

There are still minor regions at high angle of attack in which the X-15 is uncontrollable, yet flight at high angle of attack has been increased three-fold, from 10� to 30�. This is one of the accomplishments that will lead to the day when space ferries shuttle back and forth through the corridor between Earth and orbiting space laboratories.

The approach-and-landing maneuver following reentry has also been a fruitful area for research. It might seem that the navigation, approach, and landing of an X-15 would demand extraordinary piloting skill, since the pilot guides the airplane with power off from a position 100 miles away to landings that now average only 1000 feet from the intended touchdown location. Yet most X-15 pilots would point out that hitting the desired point is not a demanding task, for the craft's aerodynamic characteristics are conducive to spot landings. The critical nature of the landing task is to keep from hitting the spot at too high a vertical velocity, because of the steep approach angles.

These steep approach angles result from one of the penalties of a hypersonic configuration, the high drag at subsonic speeds, which in turn produces high rates of descent. In addition, relatively high approach speeds are required, which greatly reduce the time available for the flare maneuver. The combination of high rate of descent near the ground and short flare time leaves little margin for error in piloting judgment. For the X-15, "dive" angle might be a more appropriate term than glide angle, for it has encountered rates of descent as high as 30 000 feet per minute. Previous rocket airplanes seldom descended faster than 6000 feet per minute during approach.

In spite of anticipated difficulties, no landing problems caused by piloting errors have been encountered in some 120 flights. Confidence accordingly has developed in the belief that landings can be made with configurations that produce even steeper descent paths.

This confidence was achieved through extensive study to develop suitable techniques. The techniques were arrived at through analysis and through flight in airplanes that were altered to simulate the X-15's aerodynamic characteristics. This work started with an F-104 and F-100C, about one year before the first X-15 flight. The F-104, in particular, because of its close resemblance to the X-15, was used to define approach and optimum flare techniques. It continues to be an important training aid. Significantly, what appeared at first to be a severe landing problem was overcome not by altering any aerodynamic characteristics but by coming to appreciate the fact that they are not the limiting factors. Operational techniques were developed that significantly increased the time available for final corrections after the completion of the flare, and thus have given the pilot a margin for error commensurate with that in more conventional aircraft. This flexibility has reduced what at first appeared to be a critical maneuver to a routine one.

Thus, from launch to landing, unique dynamic flight conditions that place new demands on aircraft, controls, and pilot have been investigated. The reentry maneuver, more than any other, highlighted problems of hypersonic stability and control, and showed the need for the vital blending and augmenting of pilot control. Pilots are now willing to accept the fact that a direct link to the control system is not always possible, and electrical signals may have to be substituted. Both pilots and engineers plan with confidence piloted flight exploration of new aerodynamic conditions to be encountered farther up the manned, maneuverable flight corridor.

Significantly, the acceptance of electronic aids has not lessened the importance of the pilot or forecast his impending replacement. While exploratory flight research is very exacting, perhaps more important factors are versatility and flexibility. And for these functions, experienced research pilots are as yet unmatched by "black boxes". Thus, maximum use of the pilots' capabilities enables them to fill many demands in addition to those of flight control,