Advanced (active) flight control systems

Control systems, either fully manual, power-assisted, or power-operated, which rely upon push-rods, bell-cranks, pulleys and power augmentation for the human hand and foot at high speed, are well proven in effectiveness. However, they are heavy and vulnerable to changing force gradients with changes of flight phase, and configuration. They can also be jammed by nuts, bolts, split pins and tools left inside them.

Automatic flight control systems isolate the pilot from the aeroplane. This has the advantage of giving him enhanced handling qualities of stability and control. Stalling can be avoided, or its consequences avoided. Overstressing can be prevented. A uniform stick-force per ‘g’ can be provided throughout the flight envelope. Data processing, digital computation, system multiplexing, fault toleration can all be provided by computer(s) linking pilot and flying controls by conduits and cables carrying laser or electronic signals.

Disadvantages exist, primarily at the interface between pilot and aircraft. There are marked difficulties being experienced in the conversion of airline pilots to airliners with advanced flight control systems. Learning — perhaps one should say relearning — times are longer than expected. Type conversions of pilots who, knowing their routes backwards in conventional aircraft, reveal difficulty in adjustment to glass (video-screen) cockpits, redesigned control layouts and procedures, and the flexible scope inherent in the new systems. There is difficulty in relating to such aircraft. Not only is there a mass of material to absorb on the ground, but airlines are recommended to convert their pilots during a considerable number of different training legs on company routes. The need is proven, yet there are airlines which refuse (as this is written) to take such a prudent step, preferring, it is said, to let flight-deck crews loose on an aircraft without adequate experience of its capabilities. There is alleged circumstantial evidence that this has caused at least one fatal public transport aircraft accident in Europe.

Active controls enable an unstable aeroplane to appear stable and wholly controllable to the pilot, because they are under the direction of the fully automatic flight control system. This can be disconcerting for self-confidence in oneself. The author knows from experience the need for enough manual reversion to enable pilots to remain current with live (‘hands-on’) handling, especially during stages of the terminal approach and landing.

8.7.1 Fly-by-wire (FBW) and fiber-optic (fly-by-light (FBL))

High-order (active) electronic control systems relying upon pilot inputs to a computer, which then operates control actuators via electric cables, or via fiber conduits which conduct light, has attractive advantages. Both use the spectrum of electromagnetic radiation, while differing only in wavelength and frequency. Not least among their advantages is the ability to connect the avionic and other navigational equipment into the system, so that the pilot exercises command and control with less risk of error. Add to that the ability to program the power and flight controls for automatic take-off, climb, cruise, descent and landing, and automated all-weather operation, unhindered by pilot limits, becomes a reality.

A practical advantage of such advanced control systems is that there is potential for reorganizing space within the cockpit, replacing the central control-column, stick, or aileron wheel by sidestick controllers, on the basis of one for each pilot. This leaves space ahead of each pilot for unhindered sight of the

instruments. One digital FBW or FBL system in a large public transport aeroplane might feature 5 main computers, operating all of the primary and secondary flight controls by means of electromagnetic signals and hydraulic jacks. Pilot pitch and roll commands are transmitted through small sidestick controllers to two computers. These have what is called redundant architecture to provide safety levels which are as high, if not higher, than the mechanical systems they replace. The system provides flight envelope protection, to prevent the aeroplane being over-rotated into dangerous attitudes, so preventing structural and aerodynamic

limitations from being exceeded. For example, if the pilot moves the pitch control fully aft, the aircraft will not exceed its ‘alpha-floor’ angle of attack, a safe airspeed above the stall will be maintained, and the throttles will be opened automatically to establish a safe climb. The system is programmed to make it impossible to exceed g-limits when maneuvering. If the pitch control is moved to full nose-down, the aeroplane will not exceed the maximum operating speed, in knots or Mach number. An attempt to over-bank, beyond 300, simply results in the bank angle returning to 300 when the sidestick is released.

A further step is to multiplex inertial systems which provide different normal and alternative control laws, in the form of g-demand in pitch and rate command in roll. If there is a multiplex system failure which removes attitude information from the pilot display, the system can be arranged to revert to a direct mode, in which control surface angle is directly related to sidestick position. Control of the rudder and tailplane angle is by means of rudder pedals and a manual trim wheel.

The final step beyond such automation is to remove the pilot, which it is seriously argued could eliminate risk of pilot error. In fact that would leave an aeroplane and its payload of high-value freight and passengers in the hands of an electronic moron, lacking discrimination and subject to the law of

‘Garbage-IN = Garbage-OUT’. It could also leave them as victims of equally dangerous engineer error without the safeguard of a pilot to redeem the situation.

Note: on this last point, in a little under 50 years flying, 16 of which were military, the author had three engineer-related technical emergencies: an engine fire, an engine flame-out, an oxygen failure. In more than 20 years flying as a civil airworthiness test pilot he had 13 forced or emergency landings, all due to engines and fuel systems being improperly prepared for the tests by engineers. Some of the flights could have ended badly (but did not) in which case they might then have been regarded as being due to pilot error. The roles of machine and man are complementary. Engineer error is every bit as dangerous as pilot error, although the term does not have the same common usage). This is one of the most cogent reasons for live 'hands-on’ currency of a pilot, advocated earlier.

Vectored thrust

The use of thrust-vectoring to improve maneuverability is not new. There was a patent application by an engineer called von Wolff in 1944. Modern advances with computers, FBW and FBL, and engine configurations which enable thrust to be directed up, down and sideways, as well as fore and aft, have provided the aircraft designer with the tools to handle established degrees of freedom in new and flexible combinations. Tail surfaces can be eliminated, saving weight, wetted area and structural complexity. All that the pilot needs is for the engine(s) to keep running and the control elements, be they aerodynamic, or those which mechanically deflect the jet efflux for pitch, yaw and roll (when possible) to remain functional.

There are various techniques. For example, graphite fins or vanes working in the rocket exhaust were employed with the German V2s during World War II, and in subsequent postwar space programs. A

three-paddle variant of the four paddles arranged around the nozzle of the F-105, shown in Fig. 7.8(c), bestow control authority and agility in maneuver by deflecting the turbojet exhaust of the experimental Rockwell/DASA X-31A EFM (Enhanced Fighter Maneuverability) demonstrator. The paddles are manufactured from carbon compounds.

(picture)

Plate 8.4 Rockwell/DASA X-31A EFM (Enhanced Fighter Maneuverability) research aircraft. For control at extreme nose-up attitudes the foreplane. engine intake lip, leading-edge flaps, trailing-edge flaperons, and three thrust-vectoring paddles attached to the rear nozzles, bestow agility. The paddles deflect the exhaust through 100 for yaw-control. They can also act as air brakes (see also Appendix F).

A primary cause of conventionally controlled aircraft accidents is the unexpected and uncontrollable departure from the desired flight path, examples of which are: stall-spin, pitchup, and inertia coupling in a rolling maneuver. These can result from lack of authority of the aerodynamic controls in the unusual attitudes reached during departures. Confusion and disorientation can result when resort is made to a recovery procedure which is apparently illogical and interfered with by the effects of surprise, and maybe subsequent panic. This can happen, for example, when caught out by an unexpected spin, when the pilot is out of spinning practice.

The effectiveness of thrust-vectoring is, of course, dependent upon engine, fuel system and control reliability. Given that its advantages are plain, because its mechanical control authority in unusual attitudes is independent of the peculiarities encountered when airflows break down over conventional aerodynamic control surfaces. Exceptional attitudes, achieving angles of attack in excess of 900, are attainable. Tumbling

maneuvers, erect and inverted, are within reach, giving scope in air combat previously regarded as impossible. Pointing ability of the fore and aft axis of an aeroplane, without resort to roll and bank, is available again for deflection shots and setting angles-off in air-to-air and air-to-ground attack, as it was with much slower fighters and scouts long before jet aeroplanes came along, with their demand for vast volumes of sky within which to maneuver.

Aeroplanes designed to take full advantage of thrust-vectoring will be different in appearance from those now regarded as conventional. The area of greatest weakness as this is written lies in achieving vectored thrust authority in roll, which is the equal of that of vectoring in pitch and yaw.

Active wing

While control in roll by means of vectored thrust is not yet comparable with what has been achieved in pitch and yaw, resort to an active aero-elastic wing (AAW) appears to have potential. The technique is fraught with mechanical and structural complexity, and needs considerable research. The origin of the idea lies in the loss of lift/drag ratio and high lift-dependent drag which accompany the low aspect ratios of conventional fighter configurations.

Agility in maneuver involves high applied g. Load an aero-structure and, like any other, it deflects. Excessive flexibility normally spells trouble. Although conventional fighters with low aspect ratio wings are strong, they can suffer lack of stiffness. To build in enough stiffness introduces structural weight penalties.

During early test flying of the McDonnell Douglas F-18 the pilot is reported to have encountered control reversal, when roll control was applied at M = 0·6 at sea level. To roll to the right the pilot applied right aileron, putting the left aileron down. This twisted the left wing leading edge down, so that instead of rolling right the aircraft rolled left. The same happened when attempting to roll left.

Accepting but controlling the amount of twist occurring with aileron reversal, while ensuring that it is constrained by adequate stiffness, it appears possible to use small aileron deflections of up to about 50 to twist the wing through no more than half that amount, which is adequate enough at high dynamic pressure, q (see Eqn (1-5)). At lower airspeeds and dynamic pressures ailerons operate conventionally. Research must involve investigation at full-scale Reynolds numbers, high Mach numbers, angles of attack and in high-g maneuvers, if designers and pilots are to feel relaxed and confident in the concept. Added to that, fatigue-life investigation will be crucial.

Figure 8.23 summarizes schematically points covered in this chapter, in the form of an experimental project for a tailless, agile, stealth-technology research aircraft. Its features and flaws are discussed in Appendix E.2.2.

Fig. 8.23 Features of an experimental design study for agility and stealth. No conventional tail-surfaces. Thrust-vectoring for pitch and yaw control. Few reflecting surfaces and lines normal to other than transient lines of sight. Basic aircraft unstable. High-order multiplex aerodynamic and power control systems. Power-assisted manual ailerons for back-up if wing incidence-roll control by changing dihedral fails. Pitch-roll-yaw coupling tricky. Small and expensive!

Part 4 GROUND AND WATER OPERATIONS