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Aviation History
1951
1951 - 2204.PDF
November 1951 Fig. 2. Simplified arrangement of motor shown in Fig. 1. -', control. Friction force to be overcome before the system is set in motion is approximately equivalent to manual-control- circuit friction and is no more harmful. Normally, however, a restraining force of similar magnitude will be necessary to bring the system to rest again, and this restraining force can produce overshooting and oscillation which is additive to any that may be caused by phase-lag. Sources of Power.—Power may be transmitted from the engine to the powered control unit either hydraulically or electrically. There is no unanimity of opinion as to the best method, although there does seem to be some tendency to favour electrics for the large high-altitude aircraft and hydraulics for the smaller types. The electrically powered system incorporates its own pump driven by an electric motor and provided with a self- contained fluid reservoir. One of the merits of this method is that, since the electric motor and pump are running con- tinuously, the powered control unit is self-heating and is suitable for use at the lowest ambient temperatures without difficulty. On the other hand, the weight of the electric motor is considerable and it is only on the larger aircraft that rhe reduced weight of electric cables, as compared with hydraulic pipes, begins to mitigate the weight-penalty of additional electric motors. A further advantage is that a com- pletely self-contained unit can be initially tested or serviced away from the aircraft, while all refilling with fluid, replace- ment of worn parts, or other maintenance, can be carried out under carefully controlled conditions. Positioning the pumps on the engine, and piping the hydraulic fluid through the aircraft to the powered control, results in a comparatively simple system which, however, in some instances may involve difficulties caused by the low temperatures encountered in the wings. Transmitting sub- stantial powers over large distances calls for the use of large- diameter pipes, owing to the viscous nature of the fluid at low temperatures. A further disadvantage is that the output of the engine-driven pump is at a minimum when the require- ments for rate of control-movement are at their maximum, i.e., on landing. This may be considered a temporary difficulty in that it results only from the lack of large-capacity pumps at the present time. If a pump were designed to produce the required flow at landing (engine) r.p.m. it would produce excessive power under other flight conditions, but the penalty for this in terms of increased fuel consumption is negligible. If required, hydraulic accumulators may be installed in order to provide a reserve store of energy to meet the large flow requirements during landing. This, however, may be undesirable owing to the danger of the accumulator(s) becoming exhausted following an unusual amount of control movement. In the case of military aircraft the vulnerability of electric cables is generally considered to be less than that of hydraulic pipes. Irreversibility and Rigidity.—A manual control-circuit system is reversible and is to some extent elastic. Due to these characteristics it is normally necessary for control surfaces to be mass-balanced about their hinges in order to prevent flutter. If the operating member for the control Surface could be considered as a rigid structural member anchored to the wing, the control surface would, in effect, become part of the wing and the necessity for mass balance would be eliminated. The use of powered controls without manual reversion 571 immediately opens up the prospect of taking this step and saving a very substantial amount of weight—exceeding per- haps that of the powered control itself, which is in any case essential from the control-load viewpoint. In actual fact, of course, the powered control cannot be considered as a rigid structural member, and great care is necessary in the selection of powered control characteristics if the mass balance is to be eliminated. The first and obvious step is to avoid the use of any form of linkage which feeds back a proportion of the control-surface load to the pilot. Such feed-back is a useful method of providing stick "feel" which is frequently adopted in the case of power-assisted flying-controls, but it will readily be seen that despite a servo which is, in itself, irreversible, any feed-back of force to the pilot can cause the overall system to be reversible via forces transmitted back to the stick and fed into the servo input. In Fig. 1 is shown a simple form of servo motor suitable for development into a power-operated flying control. This unit is basically irreversible in that, even if the input connec- tion is constrained elastically, changes in reaction load applied to the output do not produce any deflection. In actual fact, this statement is only partly true; oil, despite the common belief in its incompressibility, is actually elastic to a surprising degree even when free of aeration, whilst the presence of air bubbles (which are difficult to eliminate) considerably aggra- vates this condition. Whereas, owing to the resetting action of the valve, steadily applied loads can be supported without appreciable deflection, shock loads applied to the jack will cause it to behave as a rather stiff spring, with a rate of deflec- tion-under-load determined by the compressibility of the fluid and the elasticity of the cylinder walls. The combination of an elastic control-surface anchorage and the mass of the surface itself determines the natural frequency of oscillation of the complete system, and the basic problem is to ensure that this frequency is so far removed from that of the wing as to ensure that the two do not com- bine to produce flutter. In general, this implies that the control-surface frequency must be substantially higher than the wing frequency since the latter is, in any case, fairly low. The considerations outlined above have led to the develo- ment of systems employing screw jacks driven by hydraulic motors, and to various refinements in hydraulic jack design aimed at increasing rigidity. Operating Life.—Until the advent of powered flying- controls, hydraulic equipment on aircraft had been designed for duties involving, during each flight, only a few relatively infrequent movements. It is very doubtful, for instance, whether the average undercarriage jack is ever called upon to operate more than ten thousand times during the life of an aircraft. Powered flying-controls may, in rough weather, be in movement during the whole of a flight and a very large number of reversals of direction can occur during quite a short period. A considerably improved standard of mechani- cal design—particularly of fluid seals—is therefore required, and metallic high-pressure seals which are permitted to kak slightly but are followed by low-pressure rubber seals, the intermediate space being vented to return, are frequently employed. Operating-temperature Range.—Most types of aircraft are now required to be capable of globe-wide operation without modification. Ambient temperatures can range from —85 deg C to +45 deg C in flight and from —40 deg C to + 90 deg C on the ground, giving a total temperature range of 175 deg C. No present-day hydraulic system is capable of operation at temperatures as low as —85 deg C, owing to the increase in fluid viscosity and to the hardening of synthetic- rubber seals. Heating of the powered control by fluid circula- tion, or some such means, is therefore essential on high- altitude aircraft. Types of Powered Flying Control Hydraulic Jacks.—One form of powered control has already been shown diagramatically in Fig. 1. A somewhat simplified version is depicted in Fig. 2, where the follow-up linkage is dispensed with by anchoring the piston rod of the jack to the aircraft structure and the jack body to the control surface. As the valve guide is attached to the jack body, an automatic
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