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Aviation History
1939
1939 - 0847.PDF
MARCH 23, 1939. Supplement to U 304* 1 AIRCRAFT ^ ENGINEER No. 158. /Volume XVII\ I No. 3 ) 14th Year March 23, 1939 CONTROL AXES An Overlooked Factor : The Influence of Fuselage Inertia on Lateral Control By W. E. GRAY, D.F.C. THE importance of arriving at a sound basis on which to compare lateral control test results arises, of course, from the fact that at large angles of incidence it has always been rather easy to lose control of one's aero plane temporarily, with very serious results if near the ground; and even after thirty-four years of flying, good men and valuable machines continue to go to destruction through loss of control. This loss may be due to misuse of the controls by the pilot or to inadequacy of the controls for their purpose—and lateral control has nearly always suffered from this inadequacy, as witnessed by the long standing custom of judging one's near ness to the stall by the weakness of this control. In fact the inability of the lateral control to give the pilot power in all circumstances to roll his machine, even in a spin, has probably been responsible for more loss of life than any other single factor. Past Methods Ever since data from control experi ments began to be collected there has been a need for some basis on which to compare the results, so that sound conclusions could be drawn from the various sets of experiments. Hitherto it has been customary to measure the power of an aeroplane control by the turning moment it can exert on the machine about its centre of gravity with reference to rather arbitrarily chosen "body axes." This basis of i/omparison seems to have proved satisfactory when considering rudder action, and also aileron action at the smaller incidences, but when applied to ailerons at the higher incidences the method appears to be unsound and misleading. It is with special reference to lateral control at these higher inci dences that it is proposed to review the question of axes. Attention will be drawn to an important factor that appears to have escaped notice so far, namely, the influence of the fuselage inertia on the motion caused by the ailerons. That lateral control is still a problem after years of research perhaps lends weight to the suggestion that something has been overlooked, and it may be that the oversight is due to the fact that the mathematicians who are in the habit of studying these problems are not usually themselves pilots. Since "chord" axes and "body" axes are almost the same, and as chord axes have no uniform meaning but yary with wing section, they will be treated as synonymous in what follows, and will be regarded as coinciding with the '' wind axes " at an unspecified small value of the lift co efficient only. In other words, it will be assumed for clarity that the wing chord from which incidence is measured is parallel to the principal X-axis of inertia of the aeroplane. Now, the authorities on control and stability, at any rate in this country, hold the view (and have for years) that a lateral control should exert a rolling moment about the longitudinal '' body'' axis without any accompanying yawing moment about the yawing "body" axis. Yet it is difficult to understand how this body-axis belief arose as a proper basis on which to compare lateral controls, for it is surely obvious that if an aeroplane is flying near the stall, with its fuselage at a large angle to the flight path, and it is- rolled about its body axis one wing-tip will immediately be ahead of the other and the machine will be fly ing with sideslip. Now it is not the slightest good having a supposed lateral control that acts with changing incidence as a wildly-varying com bination of aileron and rudder; it is a treacherous arrangement. Authoritative Nonsense For the above reason it is funda mental that the lateral control ought to roll the aeroplane about the "wind axis " or direction of motion, for it is well known that if sideslip is intro duced at high incidence, a powerful rolling moment is set up. It follows that the plotting of rolling and yawing moments on a body-axis basis in a vector diagram for the purpose of assessing the merits of a lateral control is just plain non sense—with all due respect to the eminent men who com pose the Stability and Control Sub-Committee of the Aeronautical Research Committee. It is probably due to this Sub-Committee or Panel, in issuing their general .report in 1926 (R. & M. 1000) on the lateral control of stalled aeroplanes, that the body axes have been regarded for so long as sound. In their conclusions they said that they had, in a measure, reached "a thorough understanding of the principles governing the control of stalled aircraft," and one of the chief conclusions was that a lateral control should not exert any yawing moment about body axes; thus was an apparent seal of authenticity set upon these axes. It would seem that their rightful anxiety to condemn ailerons that dragged as they lifted, with reference to body axes, had blinded them to the fact that ailerons giving pure rolling about the body axis are still terribly bad at high incidence.
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