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Aviation History
1933
1933 - 0655.PDF
September 28, 1933 Supplement to FLIGHT Edited by C. M. POULSEN September 28, 1933 CONTENTS Page The Centre of Gravity Position of an Autogiro. By J. H. Crowe, M.Sc, A.F.R.Ae.S tjl Technical Literature— Summaries of Aeronautical Research Committee Reports .. 65 THE CENTRE OF GRAVITY POSITION OF AN AUTOGIRO By J. H. CBOWE, M.SC., A.F.R.Ae.S. THE problem of the Autogiro at large angles of incidence is mainly one of stability and control. In this respect it is similar to the Helicopter. It has been amply demonstrated that a screw can be designed which will give the necessary lifting power to take a machine up vertically. There remains the problem of producing equilibrium between the forces involved when there is no air stream, apart from the induced velocity of the rotating blades, in which to operate a control. The question of whether an Autogiro can land truly vertically can only be settled by a consideration of the stability and control of the machine involving as this does a discussion on C.G. position. The aircraft must be balanced at all possible angles of incidence when we can see what degree of control is required to hold the air craft at any particular gliding angle. Also from our tail setting curves or overall moment curves we shall see the degree of longitudinal static stability for dif ferent C.G. positions. This latter should give us possible C.G. positions, since an aircraft that has inherent static instability at steep angles of glide is obviously inadmis sible, and, knowing the possible C.G. positions, we should be able to arrive at possible gliding angles. That the C.G. positions affects the possible gliding angle will be apparent later on. There is always, of course, the psychological aspect of vertical descent. It is quite likely that descent at 45 deg. to the horizontal looks to be, at any rate if not vertical, nearer vertical than 45 deg., since we are not accustomed to aircraft descending at such steep angles. The small run on landing (the forward velocity at a gliding angle of 45 deg. is about 16 m.p.h.) con siderably aids the imagination in picturing vertical descent. This is an aspect of the problem that need not concern us. There are three possible positions for the C.G. of an Autogiro; it may be forward of the vertical shaft, directly beneath it or behind it. Take first of all a very elementary consideration of the balance of the machine descending vertically. Since the axial induced velocity will be something less than the vertical velocity, the resultant air velocity acts upward; there is therefore a positive lift on the tailplane. It is obvious, therefore, that the C.G. cannot be in front of the vertical shaft, since the only other forces acting are the weight acting through the C.G. and the drag of the blades acting vertically upwards through the shaft; the aircraft cannot be in equilibrium. At a vertical velocity of 30 f.p.s., which would appear to be about the minimum speed at which an Autogiro descends vertically, assuming that it can so descend, the drag of the tailplane, allowing an air speed equal to the speed of vertical descent minus the axial induced velocity, can only be of the order of a few pounds. Neglecting any moment due to the body therefore, the only possible C.G. position is a fraction of an inch behind the vertical shaft. It will be shown later on that the aircraft is definitely unstable with the C.G. under or behind the vertical shaft. The degree of instability rises as the incidence increases. There is also the fact that since the air speed over the tail Mould be of the order of 10 f.p.s., even with an all- moving tailplane with 90 deg. movement the amount of control available in this attitude is practically negligible. It would appear, therefore, that an Autogiro cannot descend truly vertically in still air. That the actual gliding angle possible depends on C.G. position is easily demonstrated. Consider the aircraft descending on an even keel with the C.G. a few inches in front of the shaft. The thrust of the Autogiro acts vertically upwards, and will produce an anti-clockwise moment about the C.G. requiring a negative lift on the tail- plane to balance. As the incidence increases so the elevator will have to be held more negative until all the control, on both tailplane and elevator, is used up. This determines the maximum gliding angle. If we let the C.G. move further back less negative lift will be required on the tailplane, hence for the same negative setting the incidence, and therefore the gliding angle, can be greater. We can therefore secure a steeper gliding angle by putting the C.G. further aft, but we are limited as to the backward travel of the C.G. from a point of 972 a D
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