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Aviation History
1950
1950 - 0260.PDF
172 FLIGHT, 2 February 1950 ELEVATOR way as it responds te action by the human pilot. Basic Principles.—The use of power-assisted flying controls has followed as a logical develop- ment upon earlier experiments with manually- energized control-assisters. These assisters were in the form of trim tabs, balance tabs, flying tabs, and mass balances, by means of which it was con- trived to supplement pilot effort. In their day, all these devices were reasonably satisfactory, though of limited application. In Fig. i is shown a diagrammatic arrangement of a system in which the trimming surface is placed forward of the actual.control surface but aft of the fixed surface and the hinge hue. The principle of this configuration is that the actual (i.e., rearmost) control surface is available for normal flight control but not for extreme conditions of operation, such as take-off and landing. The main control- surface is actuated by the pilot's usual controls, and the trimming surface by an irreversible control. In practice, it has been found that the combined aerodynamic loads of the two surfaces, both of which have to be resisted by the pilot, assume so great a magnitude that the necessary reduc- tion-gear precludes all possibility of fast operation should an emergency occur. Stick Forces.—Stick-force characteristics are of salutary sig- nificance in any evaluation of power-assisted flying control systems. Conventional stick-force gradients have been extra- polated on the basis of Ib/g, and are known to be critical with certain aircraft types. This embraces consideration of de- stabilizing influences created, in the case of long-range fighters, by such excrescences as external wing-tip fuel tanks. It is desirable that the stick-force gradient should lie somewhere between 3 lb and 9 lb per g, but it must be remembered that this function varies with altitude and with changes of e.g. location. Between sea level and 35,000ft the stick-force gradient per g unit varies by approximately 4.5 . C. E. Pappas (chief of aerodynam- ics, Republic Aviation Corporation), addressing the New York section of the Institute of the Aeronautical Sciences, recently gave a stimulating exposition of these characteristics as applied to the F-84 Thunderjet. He made the point that whereas the specification for this aircraft per- mitted only a 6 Ib/g variation be- •tween extreme flight conditions, the ; Thunderjet originally had, in fact, • ii.5*lb variation. '; • By way of reducing the stick-loads induced by the relatively large ailer- ons, a maximum power-boost control- ratio of 10 :1 was decided upon in conjunction with internal aero- dynamic balancing. The amount of aileron deflection necessary for the j very high rates of roll was not com- patible with the aerodynamic balance which could be accommodated within the wing envelope, and it was this' - fact which necessitated a 10 : 1 power-boost ratio. It is under- stood that this combination of aerodynamic balance and power-assisted aileron control has proved eminently successful in practice. • At high indicated air speeds, and with wing-tip fuel tanks: fitted, it was subsequently discovered that the stick force per g curve had a pronounced peak between 45 and $g, as shownin Fig. 2 for empty tanks and in Fig. 3 for full tanks. In general, these peaks occurred earlier with full tanks than withempty tanks. Exhaustive tests were carried out with the object of elimin-ating these peaks, and the results-proved that the moments and lifts were substantially constant with load factor, whilethe moment contribution of the tanks was sufficient to cause a 2.22 per cent forward movement of the Thunderjet's neutralpoint. Since measurements of stick-force gradients disclosed that the tanks caused a 2.5 per cent forward movement of theneutral point at Mach numbers below approximately 0.70, the tank moments were shown to account for the bulk of thechange. . The System Proper.—The Vickers power-boost system in-stalled in the Martin Mars provides an interesting example of an actual installation. Whereas a landplane can be manoeuvredby wheel brakes, the absence of such a facility jn a flying- boat demands effective directional control at slow speeds. Mostof the earlier tab controls failed to achieve this purpose, be- cause their functioning was generally contingent upon therebeing a certain minimum airload on the trimming surface. The 3 4 5 LOAD FACTOR (yunrls) Fig. 3. Comparison of stick force with g for Republic Thunderjet at high speeds (I.A.S.) with full and empty wing-tip fuel tanks. ig. 4. Schematic arrange- ment of Vickers power boost system as fitted to Martin Mars Flying Boat. The letters refer to the description in the text. application of power-assisted flying controls to large ffying- boats is, therefore, of particular merit. A simple diagrammatic illustration of the system fitted to the Martin Mars is shown in Fig. 4. The servo valve and the boost cylinder are the vital elements, and the ratio of their piston diameters is directly proportional to the boost ratio. Interposed between the servo valve and the control column is a scissors linkage designed to facilitate immediate straight- through manual operation of the system in the event of fluid- supply failure. This linkage also operates the servo valve in response to control-column movement. The operation cycle of the system is as follows: Assuming the elevator surface to be held in its neutral position by equal airloads from above and below, the pilot logins to move the control surface upward by moving the control column backward. The first slight motion of the control column rotates link A about point B. Since the elevator surface is held in a neutral position by airload, link C (which is pivoted on the aircraft structure) does not move. Rotation of link A pushes the servo-valve piston aft. Hydraulic fluid under pressure is ad- mitted to the booster cylinder, causing the elevator to move up against airload as the control column continues to move aft. As the down airload on the surface increases, pres- sure builds up in the booster cylinder. It also builds up in the servo valve cylinder, thus providing a propor- tional load on link A and on the con- trol column. As the elevator moves up, link C is rotated about point D, thus moving pivot point B. The control column moves aft at the same rate as the elevator moves up, so that the distance between points E and F remains constant. In so far as Fig. 4 is a purely diagrammatic representation of this mechanism, it is necessary to explain that points D. G, E and F are actually on a common centre-line. When the pilot stops moving the control column, the elevator continues to move momentarily, because fluid is still being supplied through the open servo valve to the booster cylinder. The additional movement of the surface rotates link C about its pivot point D. Point E is held motionless by the control column, however, and point B therefore moves forward, pulling the servo-valve piston closed. Under this stable condition, the down airload on the elevator surface is resisted by a pressure in the lower end of the booster cylinder, which is greater than that in the upper end of the cylinder. This same pressure is transmitted to the servo-valve piston which, through the linkage, acts on the control column, tending to move the column forward. The pilot must hold against this load. Should he release the control column, the servo-valve piston would then be allowed to move so as to relieve the excess pressure. This in turn would relieve the pressure exerted on the booster cylinder and allow the elevator surface to return to neutral. Should the control column be left free, it will in turn leave the servo-valve piston free to move in either direction. Obviously, the piston will not remain in any position unless the pressure in each end of the cylinder is the same. If this pressure is equal, the toad on either side of the surface must also be equal In order to obtain proportional leel, the pressure line con- necting the booster cylinder to the area between the centre and
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