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Aviation History
1942
1942 - 1252.PDF
594 FLIGHT JUNE IITH, 1942 t»ic AIRCRAFT ENGINEER FLYING BOAT PROJECT Initial Stages in the Planning of a Design to Fulfil a Given Set of Specification Requirements By JOHN A. SIZER, A.R.Ae.S. ET us suppose that we are handed, in our design office, the specification of a required flying-boat design, and that we have to rough-out its general proportions and qualities quickly. This article outlines the procedure adopted before a design is finalised. 1. Specification.—We will say to be worked to is as here written. (a) All-up weight not more than 10,000 lb. (b) To avoid complication no slats or flaps to be fitted. (c) Alighting speed not to exceed 60 m.p.h. \d) Crew to consist of pilot, observer, wireless operator, engineer. (e) Top speed not less than 150 m.p.h. at sea level. (J) Rate-of-climb at sea level not less than 8oo-ft./min. (g) Endurance not less than 6 hr. at full load and at 75 per cent, full-throttle. (h) Any engine or engines may be used. No specification would, of course, read so wide and handsome as the above, but it will serve our purposes here. 2. The Wing.—Wing Loading.—Since no flaps are to be fitted, it will be necessary to restrict the wing loading to a somewhat conservative figure, in order to have a reason ably low alighting speed and take-off. Therefore we will say , all-up weight that the wing loading i THE following article, by a writer who has been actively [ ; engaged upon flying boat and landplane design since 1929, \ \ will, we believe, serve as,o very useful introduction to a \ i fascinating subject, partigHarly for the younger generation in ; j the drawing offices, wjiose normal work is concerned with j ! details rather than wM the general design. The author is noma technical officer at the R.A.E., but \ \ the views expressed are his own. . j that the specification shall be 15 lb./sq. ft. wing area With a wing loading of 15 lb./sq. ft., the area of wing surface required to support the loaded weight of 10,000 lb. can easily be estimated, thus : 10,000 Wing Area required = = 666.3 S<1- "•, approx. 3. Aspect Ratio.—We now have to choose a wing plan form having a span and chord which will, multiplied together, ^give us the 666.3 sq. ft. of required wing area. Aspect ratio comes into this because it is a measure of the span divided by the chord ; it can also be expressed as squaring the span (in feet) and dividing by the wing area. Here be it noted that a high aspect ratio is aerodynamically a good thing, but structurally uneconomic. In theory, a wing of infinite span is the best. In practice an aspect ratio of 9 is considered daring, and 5 or 6 is the common rule. In some cases even lower figures have been used, but the induced drag has been high unless the wing had a very good plan form, i.e. elliptical. We will use the span2 method for aspect ratio estimation, choosing a span of 60ft. which should add to aerodynamic refinement. Span8 60 x 60 =—rr- = 5.4 approx., which seems to be in area 666.30 ->*»*'*' line with convention. Knowing our aspect ratio, then, we obtain the mean chord by dividing span by A.R. Chord (mean) —= 11.1ft. approx. So now we have the plan form of the wing in units and are able to visualise its size. It is 60ft. in span and n.ift. in mean chord. We could build a wing to that shape, but it would not be very easy on the air nor clean to the eye. Therefore we will consider making it prettier by rounding off the tips nicely, and tapering the wing [i.e. sweeping the leading and trailing edges towards one another at the wing tip). A point to be borne in mind here is that a tapered wing has advantages, apart from its lower induced drag (the resistance caused by a wing in producing lift). The principal gain is a marked saving in structural weight on account of air loads tapering off in-magnitude towards the wing-tips. The wing having a tapered plan form and thickness can be looked upon as a beam with a graded load, whereas wings of rectangular plan form are structurally heavier on ac count of being, in effect, bekifef with more nearly evenly distributed loads upon them. How ever tapered wings " stall" or lose their lift at the tips first, causing wings to " drop " on the stalled side and leading sometimes to vicious tendencies such as spinning. The rectangular plan form wing, however, stalls at the centre- section first, the stall gradually creeping outwards to the wing-tips. This is reasonably safe, as any stall can be overcome, generally without having to fight out a spin. 4. Taper Ratio.—A taper ratio (root chord divided by tip chord) of 2 to 1 seems to be in general use nowadays and has been chosen in this case. So, having a mean chord of, say, nft., our wing will have a root chord of 14.67ft. (at the 1 of the aircraft) and a tip chord, of 7.335ft. By rounding off the tips slightly we shall lose a small amount of wing area, and shall probably end up with something like 650 sq. ft., which is used throughout the text. 5. Wing Section.—At one time there were so few wing sections from which to choose that reliable choice was fairly easy. Data have since been published of literally hundreds of wing sections and consequently the choice is complicated by the variety. However, for our projected design we will consolidate Anglo-American unity by choosing one of the N.A.C.A. 23000 patterns. A thickness/chord ratio of 15 per cent, will be used at the root and 10 per cent, at the tip ribs. Our wing sections will therefore be : Root—N.A.C.A., 23015. Tip—N.A.C.A., 23010. To digress for a moment, it might be helpful to explain that the figures 23015 in the N.A.C.A. nomen clature each have some significance in identifying the w;ng section. In this manner— The " 2 "—indicates the camber of the chord line, as a percentage of the chord. The " 30 "—defines, as a percentage of the chord, tlfce position, back from the leading edge of the section, of the maximum thick ness ordi nates. The "15 "—defines the thickness/chord ratio as a percentage. From the above definition, therefore, it is possible for us to visualise the shape and size of things at both root and tip of our wing. Since the root chord is 14.67ft., the maximum depth of the section will be -ft. •= 2.2ft., 1 100 approx. And this maximum thicknes# will be 30 per cent., i.e. 4.4ft., back frorn the leading edge of the wing root rib. For the tip we have 7-335 X 10 100 = 0.74ft. for the maximum thickness, and the position of this depth is 1.48ft. from the tip rib leading edge. The contour of the wing ribs in between these two sections will be obtained at a later and more finalised stage in the design, from a wing ordinograph specially prepared, or by the more length ' and questionably accurate method of layout to as large a scale as paper sizes and office space will allow. For the moment, having an idea of the wing size and
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