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Aviation History
1928
1928 - 0318.PDF
32 TO PLIGHT APRIL 26, 1328 THE AIRCRAFT ENGINEER TABLE 3.—(C): " Clear-View " Two-Bay Biplane : Parasitic Drag Synthesis (Ref. Figs. 9 and 11-C) Paras. DragComponent Total: Body Wheels : Internally sprung, 700 x 100 Vertical struts : (Wing) ... Diagonal struts : (Body) Axle : (divided, as above) Total: Undercarriage Frontal Area (ft-2) 10-0 2-0 0-5 0-7 Drag Coeffi- cient g(lb. at) (abs.) (lOOft.s.) 0-18 42-8 012 5-7 0-06 0-7 006 10 — Nil 7-4 26-0 0-0055 3-4Tail plane and~] -„ , elevators Based on , Fin and rudder J surface area | 120 0-0045 1-3 Bracing: (Cantilever) — — Nil Tail skid — (approx.) 10 Total: Tail "I 0-72Inner interplane struts :type Outer interplane struts : " I " 0-60 type Inner lift wires : ^--in. stream- 0-18 line Outer lift wires : -^-in. streamline 0-26 Inner anti-lift wires : J-in. stream- 0-10 0-20 line Outer anti-lift wires : J-in. stream- 0-19 0-20 line Wiring lugs (24 at 0-05 lb. each) 1-2 0-04 0 04 0-20 0-20 0-7 0-6 0-9 1-2 0-5 0-9 Total: Wing bracing ... ... ... 6-0 Total: Miscellaneous fittings ... (approx.) 2-0 Total: Free air drag 63-9 Interference drag factor ... + 20 per cent. 12-8 Total: Parasitic drag 76-7 Explanatory Notes Total machine weight: 2,600 lb. Engine : 400-500 h.p- radial. Body : Oval-section monocoque with extended (integral) wing roots. Frontal area based on maximum cross-section of body. Drag coefficient includes body complete with radial cylinders, windscreen, cockpit opening, etc. Undercarriage : Rigid type with internally-sprung wheels and divided chassis. Faired connections at bottom wings and wheel shields. Tail: Full-cantilever type, no external bracing. Wing bracing : Two-bay truss with " Clear-View " centre- section. " I "-type interplane struts. Miscellaneous Fittings : Includes various external acces- sories. Interference Factor : Assessed from empirical data. parasitic drag, between Figs. 7 and 9, though contrary to a first impression perhaps, the two-bay truss scores a little from the cleaner structure under body and slipstream influence : for this reason it is important that any comparative model tests should be complete with working airscrew. On the other hand, Fig. 7, could be cleaned up still further at the body connections, by eliminating the top inner planes and constructing each side as a true single-bay truss, without the " V" wing-body struts, as shown. It would mean the sacrifice of the pronounced dihedral of the inner sections, though sufficient wash-out and a limited amount of extra dihedral towards the body could be obtained by suitable tapering of the wing spars at their inner extremities. Thus, the front spar would be tapered down and the rear spar tapered up. This would be advantageous in the case of the TABLE 4.—Comparative Summary : Parasitic Drag Type (A): Conventional single-bay biplane. (Ref. : Table 1, Fig. 11-A.) Type (B): " Clear-View" single-bay biplane. (Ref. : Table 2, Figs. 7 and 11-B.) Type (C): " Clear-View " two-bay biplane. (Ref. : Table 3, Figs. 9 and 11-C.) Parasitic drag (lbs. at 100 ft.s.) Type: Group Body Undercarriage Tail Wing bracing Miscellaneous fittings Total: Free air Drag Interference drag Total: Parasitic drag (A) Slipst. Drag 40-5 3-0 6-7 2-8 2-0 55-0 13-7 68-7 Total Drag 50-0 12-3 6-7 8-0 30 80-0 20-0 100-0 (B) Slipst. Drag 34-2 2-0 5-7 1-9 10 44-8 9-0 53-8 Total Drag 42-8 9-9 5-7 6-7 2-0 67 1 13-4 80-5 (C Slipst. Drag 34-2 0-6 5-7 1-8 1-0 43-3 8-7 52 0 ) Total Drag 42-8 7-4 5-7 60 2 0 63-9 12-8 76-7 rear spar as the neutral axis would be concave against the lift loading in normal flight and so reduce the total bending moment in the bay. Since this argument would also operate against the corresponding convexity of the front spar, it would be advisable in that case to interpose a single small body strut to support the downward taper, as shown in Fig. 8. Further, in the case of wing fuel tanks for gravity feed, this extra strut has sometimes been found necessary, and could be made to take the fuel supply line from the tank. Naturally, for the reverse loading conditions of inverted flight, the rear spar would be under this same disadvantage, the curvature of the neutral axis adding to the bending moment, though in this case the loads are less and the spar would probably be up to the required factors. Purely from the interference standpoint. Fig. 8 is perhaps the best solution, and in the absence of model data, an Interference Drag Factor of 15-18 per cent, would be a fair assessment, in place of the more usual 25 per cent, for the conventional arrangement of Fig. 11 (A). Fig. 10 is a sketch suggesting the underlying idea of these proposals, (A) showing the conventional, and (B) the proposed " clear-view " scheme of wing connections. From this it will be apparent that if the pilot's cockpit is situated near the highest point of the body, there will be a general uptrend of the air flow in front of this region for most streamline shaped bodies, this flow being upwards relative to the normal air- stream over the outer portions of the wing, which are sensibly outside the influence of the body streamlines. It is clear then, that if the centre portion of the top plane is to attack this body layer of air at the appropriate incidence common to the whole wing, for any given lift condition, it must be brought gradually into alignment with the upward slope of the body lines where connected or faired into them, and so be inclined, at a negative angle or decreased incidence to the outer portions. Such a feature should tend to harmonise the air flow between the wing and body, and thus reduce the mutual interference losses at this region; the upward flow of the body streamlines being progressively changed or " twisted " across the inner portions of the span, to a positive or greater incidence at the outer portions, which are maintained at a uniform incidence as usual, with the possible exception of the overhang where a shaped wing tip may necessitate a certain amount of wash-out towards the extremities. Incidental to the advantages of smoother air flow, it will be seen that the geometric effect of the twisted wing root is to produce an increased degree of dihedral towards the body, which together with the negative slope of the root section and its grading down in thickness (it need be only about 2£-3 ins. for small fighters), will be of advantage to the pilot's forward vision : allowing an unrestricted fighting view directly ahead along the gun sights, and over the inner portions of the top plane and engine cowling when registering on a target; and
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