FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1929
1929 - 1339.PDF
JON* 27, 1929 Substituting for k :— 54 6Lq •* = * " THE AIRCRAFT ENGINEER SUPPLBMKNT TO FLIGHT (7) To find the shear stress :— from (2) fa = — Putting in the values of T and Io from (1) and (4) and making r = \ ckxh, we find :— This is a maximum when x = I and is/8 „„, = ^ ' Taking the second example of the previous article, in which the central load was 4,000 lb., the other particulars being : — =12 lb. per square foot, I = 23-59 ft., suppose the O.P. position at top speed to be 0-5 chord. q will then be 0-5 — 0-3 = 0-2. If we stipulate that the angular distortion under normal load is not to exceed 1 deg. at the wing tip, i.e., 6 = 57-3 radians, then the weight of the torque member necessary to fulfil these conditions will be :— 54 500 X 12 X 0-2 X 57-3 ., W< = 25X 1,728 x 10" x 0-125* X *•» = 72-5 lb. The maximum shear stress is :— _ 1 1,728 X 106 x 0125 = 1,258,000 lbs./sq. ft. = 3-9 tons per square inch. Thus we see that if the torque member is designed to give reasonable torsional stiffness to the wing, the actual torsional stress is comparatively low. This does not necessarily mean that excessive structural weight is involved, as, owing to the inherent efficiency of the thin circular-section conical tube as a torque member, the weight is comparatively small; in the example chosen, for instance, this is about half the weight of that part of the structure required to resist pure bending. Each particular design involves its own load factors and requirements regarding torsional stiffness, so that it is impossible to give a definite relationship between the weight of the spar and the weight of the torque member, but once the requirements are known, the suggested methods enable a rapid approximation to be made of the main struc- tural members of a cantilever wing. AIR-COOLED HIGH-SPEED SEAPLANES By W. G. CARTEK, M.B.E. In a discursive examination of the relative merits of air v. water-cooled racing machines, it may be considered that the ultimate justification for developing either type is con- tingent upon the degree in which the most successful practice may be applied to the requirements of progressive design. Not inconsistent with this principle is the necessity to segregate those characteristics providing an advantage for racing purposes, while subject to depreciation or entirely discounted when dissociated from that special and particular concept. • The most significant item for consideration in this respect ifijthe procedure adopted for dissipating surplus heat from Water-cooled engines by using a comparatively large expanse of superficial radiating surface, this arrangement conferring an advantage amounting to six or seven per cent., but unsuit- able in many respects for service and general-purpose machines, a tube block radiator with its accompanying high resistance being a standard fitment under these condi- tions. In all other respects, alternative types compete on terms of equality, the disparity between frontal areas of respective engines being the principal contributing factor in determining relative speed values. By rearrangement of engine accessories it has been possible to reduce the frontal area of water-cooled units within normal requirements of sufficient fuselage accommodation for pilot, fuel tanks and equipment. Similar treatment and modification is not so readily applicable to radial engines, the frontal area of which, in addition to greatly exceeding " V," or similar units, is of such configuration or shape precluding the possibility of a body system comparable in outline with competitors. The effect of these matters in relation to speed values has formed the subject of comparative experiment and investigation on different occasions ; the resulting evidence, however, has frequently been insufficiently convincing, or in other respects inconclusive, and there accordingly exists a wide divergence of opinion not unusually somewhat exaggerated in favour of water-cooled installations. One of the more recent designs constructed for this purpose was fitted with an air-cooled Bristol " Mercury" engine, each cylinder projecting through the fuselage and individually faired with suitable covers having apertures at each extremity for cooling purposes. The governing factors prescribing a layout according to this conception were (1) the necessity to provide a good degree of visibility ; and (2) the restricted diameter of the airscrew blades due to high R.P.M. of an ungeared engine. An officially-timed speed test returned an average of 232 m.p.h., with a maximum brake horse-power of 650, and 2,175 r.p.m., instead of 2,500, for which the airscrew was designed, the thrust horse-power under these conditions not exceeding 520, and implying a resistance of 72-5 lbs. for the complete machine. After certain modifications had been made in regard to position, area and shape of the air intake, a considerable improvement in available brake horse-power was obtained. By that time, however, the special metal airscrews were unserviceable, and those constructed in timber and in use as a temporary expedient gave, as was expected, results of no comparative value. It is interesting to reflect that a quarter-scale model of this machine, subjected to experiment and research at the N.P.L. had, under conditions of close similarity, predicted a resistance of round about 70 lbs.—a somewhat remarkable verification, in view of possible scale effect in reproducing a body system of this arrangement. A comprehensive survey of this work has been published, and, having regard to authen- ticity of agreement between model and full scale, will serve as an authoritative index in considering alternative methods of installing an engine to the best advantage. The minimum drag recorded by successive observation with different shapes and types of engine cowling proved to be 65-5 lbs., occurring in association with extremely elongated covers fitted to the projecting cylinders. Considering this problem from an opposite standpoint, evidence is not unavail- able in support of an opinion that entirely to enclose the engine within a fuselage diameter may prove a solution for reducing drag to a minimum value, this arrangement being particularly suitable when regarding the fuselage as an engine nacelle, and accommodation for the pilot, with the necessary range of visibility, is provided elsewhere. A possible method of installation under these conditions is to superimpose a fairing of adequate size about the normal fuselage surface, providing, in effect, an annular space in which repose the cylinder extremities. To ensure a steady and continuous supply of air at relatively low velocity passing through this concentric area, an orifice is formed by means of a duplex spinner attached to the airscrew, or alternatively by a small enlargement of the external cowling immediately 522c
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
