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Aviation History
1937
1937 - 0791.PDF
March 25, 1937 Supplement to ©CCR? 302a cfSA** FLIGHT ENGINEERING SECTION No. 134 (Vo,^S?3XIV) 12th Year March 25, 1937 LONGITUDINAL STABILITY An Investigation with Particular Reference to Low Wing Monoplanes By W. R. ANDREWS, A.F.R.Ae.S. T~^VEN the casual student of aeronautics cannot have f~4 failed to notice that modern low-wing monoplanes •*—' appear to be fitted with " outsizes " in tailplanes, and he may have wondered why. Many designers have found to their cost that although a new type had satis factory stability in pitch at most speeds, it became unstable on climb, and cases are on record of such new machines being successively fitted with larger and larger tailplanes in an attempt to get longitudinal stability at all speeds. In the following article Mr. Andrews explains his theory concerning the reasons for a form of instability which has taken several designers by surprise. We have had this article in hand for some months but have not had space for it until now. However, the subject is still a serious design problem and deserves close study. Previous attempts have been made (Refs. 1 and 2) to obtain simple formulae for the design of tail surfaces to ensure satisfactory longitudinal stability. These earlier attempts were made before the vogue of low-wing mono planes. As a consequence, it was not considered necessary to allow for the vertical position of the Centre of Gravity which did not appear in any of the formulae. In view of the present popularity of the low-wing arrangement, it seems worth while to re-open the subject (making allowance for the vertical C.G. position), especially since certain low-wing designs have suffered from instability, although the " tail volume " (based on biplane data) has appeared satisfactory. Any formula now derived must be capable of application to biplanes as well as to all types of monoplanes, so that the wealth of data collected in the past can be applied to the low-wing monoplane problem. With this in view the present investigation has been carried out. Certain simplifying assumptions have been made, but it will be found that they are well within the accuracy to be expected from any simple formula adaptable to the requirements of the practical engineer For a theoretical aeroplane having no inertia, neutral stability is obtained when (at constant speeds) dKMw , dKMT -I — „ o da dKMK da dKMr da = Slope of wing moment curve (about C.G.) = Slope of tail moment curve (about C.G.) This approximates to the condition when an aeroplane flies in a gusty atmosphere, such that the whole aeroplane is subjected to the same change in incidence without appreciable loss of speed. For complete stability <*K, da + dK* must be negative. The pitching moment of inertia of the actual aeroplane materially affects the resulting stability. A simple investiga tion allowing for inertia is not possible and any other becomes so involved as to defeat the object of the present article, which is to produce a simple formula by means of which comparisons can be made. In general we may write aa ua It will be shown that (1) p.
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