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Aviation History
1936
1936 - 0534.PDF
FEBRUARY 27, 1936 H THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 228^ cates the angle at which the aeroplane will drop its wing, and one is inclined to believe, at least as far as tapered wings are concerned, that this angle is identical with, or at least not far removed from, the one where stalling begins. The greater, therefore, the difference between the local K, of any section at the outer portion of the wing compared with the local KL max., the greater will be the margin between rolling stability and rolling instability. A rational criterion for determining where stalling is going to start first, is to determine the ratio of KL local over KL max. local or the difference between KL max. local and KL local. Although Mr. Andrews expresses the opinion that a comparison on the KL basis is misleading, I consider his diagram, Fig. 5, a more rational representation than his other figures, which are obscured by his bringing in the chord. (The misleading impression of diagram 5 results from comparing a tapered wing (A = .75) of varying thickness/chord ratio along the span with a rectangular wing of constant thickness. When comparing tapered aerofoils with a rectangular wing on an equal basis in regard to the aerofoils used along the span, it is quite obvious that the rolling stability of a rectangular wing is always higher (see Fig. 1).) If one divides the difference between KL max. local and KL local by dK^da one obtains a margin in degrees against stalling. In diagram 2, curves are plotted for various taper ratios indicating the local angular margin against burbling. The cunes were obtained by using calculated KL distributions for the local KL and Diagram 1 of Mr. Andrews' article in regard to local KL max. Needless to say that negative rallies of KL max. — KL have no direct physical meaning dKJda except that they indicate that burbling has set in. The straight line indicates the increase of local effective angles due to rate of roll psjv = 0.05. The taper ratios investigated are 1, 0.75, 0.5, 0.33, 0.238. One could have predicted without any further investiga tion that the aerodynamic characteristics of a wing of such a mild taper ratio as A = 0.75 would obviously be not far removed from those of the rectangular wing. For a wing having a taper ratio of A = 0.5 and using a section of high <am er (Km, = — 0.04), stalling will begin at the centre for ,.ero rate of roll. However, for rate of roll of psjv =0. 05 (a magnitude which is usually introduced when inter preting measurements on the rolling balance in regard to roll in stability) the major part of the wing is stalled. When using for the same wing a section of lower camber KmQ = — 0.02, the wing will stall at zero rate of roll between centre and 0.65 of the semi-span. For a wing having a taper ratio of 0.33 and Km0 = — 0.04, the wing stalls between the centre and 0.83 at zero rate of roll. For a wing with a taper ratio of A = 0.238 and Km0 = — 0.04, stalling will start at about 0.83 and will spread between the centre and 0.93s. The diagram illustrates how the point where burbling starts travels from the centre of the wing towards the tips rs the taper ratio increases. One is therefore inclined to p-edict that instability in roll will occur earlier as the taper becomes more pronounced. 859^—23321- -166*- *U&£"^ -"* FIG.3. I DOB A-.— Orifice ribs ,-25 percent stations B^ 45 tapered aerofoil—plan form and orifice location. This illustration forms Fig. 1 of American N.A.C.A Technical Note No. 521.
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