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Aviation History
1948
1948 - 2197.PDF
DECEMBER 23RD, 1948 FLIGHT 759- Powered Controls Exposition of Current Ideas on the Use of Powered Flying Controls of Various Types the satisfaction of pilots with conventionally proportionedcraft having spans above 150ft to 200ft. Thus, as seerIN our last week's issue, we included a brief resume ofthe paper Present Thoughts on the Use of Powered Flying Controls in Aircraft, read before the Royal Aeronautical Society by the author, Mr. D. J. Lyons, B.Sc, A.F.R.Ae.S., on December 16th. The author is the Principal Scientific Officer, Aerodynamic Flight Section, Royal Aircraft Establishment, and, by virtue of the specialized study he has made of the subject is, per- haps, the most highly qualified person in this country to deal with powered controls as a lecture subject. Great ingenuity has been expended in developing various^f'lunds of fixed aerodynamic balance to lighten the increased I control hinge moments which have arisen from growing sizesof aircraft and controls, increased flight speeds and increased demands for manoeuvrability. In practice, it is essential toallow for certain variations in the value of b, (where b, is the rate of change of control hinge moment coefficient with controlangle), the most important of which are those due to (i) dif- ferent flight conditions and control deflections, (ii) manufac-turing tolerances, and (iii) ice accretion. These variations tend to cause over-balance of control either in prototype or production testing, and experience hasshown that the value of -Kb, for any I such control (where K is a motor repre-| senting aircraft response effects) should I be greater than 0.035 to avoid over-balance due to manufacturing tolerances only, greater than 0.08 to avoid over-balance due to the combination of ice accretion and manufacturing tolerances,and greater than 0.12 to avoid difficulty in design and to give reasonable repeat-ability on the production line without individual adjustment. The curve shown in Fig. r relates tothe top manoeuvring speed of a typical fighter to the design value of Kb2 neces-sary to obtain a certain aileron perform- ance. The dimensions (based on theMeteor) are assumed constant, and the actual conditions assumed are 15 deg ofaileron applied at top speed for 50 lb stick force. On these assumptions de-sign would become difficult with a top speed of 230 m.p.h. and serious risk ofover-balance would occur with a top speed above 420 m.p.h. After surveying the main parametersof aerodynamic servo controls, Mr. Lyons went on to observe that the mainproblem with fixed, aerodynamically balanced controls is whether the operatin; air-seen at present, if powered controls are used on aircraft above theselimits, a fully duplicated system should be used. There is no fundamental reason why duplicated poweredcontrols should not be used in cases where manual reversion is practicable; but it would appear wise to rely on manual rever-sion in the emergency case as often as possible. The pilot requires some forces on the cockpit control, or"feel," to give him some idea of the position of his controls and to return a control to the trimmed position on release,but the layout of the complete powered control system has to be altered according to which method is employed. When proportional feed-back of hinge moment is used togive "feel," the control forces are trimmed out by the pilot with the normal trimmers. If reversion action takes placewhen the pilot has a load on the cockpit controls, and the resultant load on the control, i.e., out of trim load with power on proportion of feed-back is too much for the pilot to hold, theensuing motion of the aircraft is the same as if he released his controls before thepower was switched off with the same trimmed conditions. Unfortunately,the friction in the control circuit between pilot and powered control unit may beholding a load on the control surface uji to the limiting friction proportion of feed-backwhich the pilot will have to hold him self on reversion.It is thought at present that the best compromise is to base a requirement onfriction grounds alone by stating that the proportion of feed-back must begreater than the control circuit friction the maximum sudden load the pilot can be expected to hold.Examples of this value for an aircraft with wheel control are 80 Ib for theaileron, no lb for the elevator and 200 lb for the rudder.When completely artificial '•' feel " is used, there is no direct and simple rela-' tion between the hinge moment held by the powered control unit and the cockpitcontrol loads. This is immaterial when the power is on, so long as the artificialfeel is adjusted to suit the aircraft. It is believed that the best solutionis to provide continuous automatic 5OLB. STICK FORCE 15°AILERON ANGLE AT TOP S AIRCRAFT BAS ON METEOR DIMENSIONS OVERBALANCF_ LIKELY DURING PRODUCTION TESTING BELOW ,.THIS VALUE 2OO 4OO AIR SPEED (M.RH. EAS) 6OO Fig. I. Curve showing variation with top speed of the required control hinge moment coefficient on the aileron control of a typical fighter. loads to the pilot can be reduced to a sufficient degree, whereas with a servotab system the question is whether the tab, upon winch re- liance is placed, can be guaranteed to produce power to movethe main controls in certain critical conditions. With powered horr certai l cond pflying controls neither of thes* problems causes as much1 worrv but, instead, there exist the more mechanical problms ofproducing a stable, non-oscillating power system which; requires very small parasitic forces to operate it and, in addition, thtensurance of good reliability with sound reversionary scheme.s to cope with the emergency resulting from any likely form offailure in the power system. . • If manual reversion is accepted as the recognized escapethe event of any failure in the main powered flying c,>ntro system, it must be ensured that the loads which the pilot canexert are enough, when manual operation is being used to enable the aircraft to return to its base and lane! In addition. at the moment of change-over from the failed powe> system to the manual operation, the pilot must be able tocontrol the aircraft satisfactorily. Even at the lower speeds, manualdifficulty, as shown in Fig. 3. It fa for these emergency conditions ^ould become-Kb,<o.i2) for aircraft with spans above about 150ft _ it is not considered likely that manual reversion can be used to ons that become difficult f trimming-out of the control surface hinge moments bymeans of tabs whether the power is on or off. The hinge moment on the control is measured mechanically by the de-flection of a spring: when the load and, therefore, the deflec- tion exceeds a given amount, a switch is made which allowscurrent to pass to an electric motor which, in turn, winds tbs tab in the direction to cancel the hinge moment load.To help in preventing instability, a compromise can be made between the maximum error allowed in the trim and the rateof operation of the trimmer. Added sensitivity can be obtained by slowing down the motor gradually as zero erroris approached. In the ideal system, the reversion to manual control in theevent of power failure should be automatic whatever the kind of failure that has occurred, although indication should bogiven to the pilot or engineer that the change-over has hap- pened. At present, the tendency is to let matters slid.;? a littleby taking advantage of the fact that, if any form of failure occurs, the pilot's one obvious remedy is to cut the poweredcontrol out and return to manual control. It is wrong to do this, however, for even an alert pilot will not be able to cutout a control which is lapidly running away (possibly up to 30 to Go deg/second) before damage has occurred. If the pilotis not expecting this rare trouble, or the automatic pilot is in control at the time, even a slow run-away may be catas-
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