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Aviation History
1953
1953 - 0061.PDF
16 January 1953 waters, where they will be put to a possibly severe routine of photography and air-sea rescue. A Sycamore is to be on hand, with Leonides running, at every fly-off of aircraft, and also during landmg-on. Rescue of ditched personnel is now usually accomplished by two destroyers—on the face of things the Sycamore appears a more economical proposition By virtue of the new landing gear, this Sycamore has much- improved ground-stability, and the rotor clearance is naturally somewhat greater than previously. Furthermore, the increased ground-incidence makes it possible to fly-off the Mk 50 with the stick central, instead of aft—a feature which has earned universal approbation, and reports have it that Boscombe Down consider this the best of all the light rotary-wing aircraft from the handling viewpoint. The type has hydraulic, self-priming brakes (which do not require an engine-driven pump); five seats; and a fully sprung panel carrying all the cockpit instruments. The first aircraft carries the British serial XA 221, and the all-up-weight is now at 5,400 lb. As a result of the company's diverse experience in the operation of helicopters under so many types of conditions for a variety of purposes, a form of "universal Sycamore" is under development. This should be a most useful vehicle, for it will not only embody all the lessons of the Sycamore's development, but provision will be made for the incorpora tion of any desired feature in the basic airframe—winch, stretchers, cameras and like equipment—without any struc tural alteration. This general-purpose variant, which appears worthy of big production, will seat the pilot on the right. America has adopted this layout, which appears to be generally approved; it enables the pilot to fly with his right hand, with a central collective-pitch lever. Twin-engined Variants Turning to the Bristol 173, which represents so notable a contribution to helicopter engineering and operation, it may be recalled that the first prototype of this twin-engine, twin- rotor aircraft originally experienced some trouble in the mat ter of ground resonance. This was cured by Unking the left- and right-hand oleo legs of the undercarriage by small-bore hydraulic piping, which was carried round from the tops of the legs in prominent external arcs. The 173 Mk 2, G-AMJI, is fitted with a cleaned-up hydraulic linkage, the piping now being internal. Some lateral damping is afforded by hydraulic resistance within the thin linking-pipes, but strong tension springs are now located beside each leg both to provide damping and decrease fuselage-torsion on the ground. The latter has now been reduced to a fraction of. its original value. The Mk 2 air craft, which is now in the final assembly stage, is unique in having non-castoring rear wheels and car-type front wheels complete with king-pins and track rods. These front wheels are not steerable, but are castoring and centred by springs. (In the first 173, all wheels were fully castoring.) Another development seen in the second 173 is the incor poration of a very large main door capable of passing bulky loads and stretchers; it divides and folds along hinges at top CAUSES OF A DETAILED study of the formation of condensation trails -**- behind aircraft is given in the second edition of a Meteoro logical Office pamphlet* on the subject. Trails are formed when water vapour is condensed into droplets of water (i.e., cloud) after the passage of an aircraft. There are several causes of condensation in such circumstances, but by far the commonest is that petrol or kerosine, when burnt in an engine, produce water vapour. Further, the colder a mass of air, the less water vapour it can contain; consequently, if the excess moisture produced by the engine more than offsets the heat generated at the same time, condensation is liable to take place. In the British Isles, this normally occurs only within an altitude-band ranging from about 21,000ft (winter) or 28,000ft (summer) to a few thousand feet into the stratosphere. The "immunity temperature" is that above which trails will not form even in an already-saturated atmosphere, *M.O. 479: "Condensation Trails from Aircraft." Her Majesty's Stationery Office, York House, Kingsway, London, W.C.2 (or from booksellers Price <jd. Two sets of wings help to carry the weight of the Bristol 173'Mk 3. Two Alvis Leonides Majors will provide the power for the later'versions. and bottom, the top forming a large canopy and the lower section a very neat step. The two parts are interlinked and are approximately balanced; a somewhat similar arrangement is used in the B.E.A. Elizabethan. Further development at Bristol is, of course, known to include the 173 Mk 3 and the 181. The former is a "stretched" development for some 18 passengers, and it is shown in the small G.A. drawing on this page. Two sets of stub wings are fitted; these help to unload the rotors at speed, the resulting reduction in angle of blade incidence permitting considerably increased flight-speeds. The Leonides Major is scheduled as the power-plant, and it will be a happy arrangement if both the airframe and engine can be developed concurrently. There would appear to be a very considerable future for a machine of this type, with its generous load- carrying capability and good cruising speed. Looking even farther ahead, it may be recalled that British European Airways formulated a specification for a "Bealine Bus" helicopter, with a seating capacity for about 45, which the Corporation consider to be the optimum tar get at which builders of commercial helicopters should aim. The 181 is known to be a larger aircraft than the 173, and Bristols have been at work on this project for some months. It would indeed be strange if they did not make a bold attempt to capture some, if not all, of the great market open to a "Bealine Bus" builder. The present position is one in which fully developed air craft are available for sale for a number of purposes. On the other hand, many potential operators are not inclined to buy until they are sure of what lies "just around the corner," and yet few can pay development costs. No helicopter builder can work off development costs on civil production, and it may be that more rapid military ordering is the real solution. Orders are the key to prosperity in helicopter development, as in all other things; it is in the national interest to ensure that development of new projects is not retarded by the lack of bold decisions on the part of future purchasers. CONTRAILS and it can now be predicted with reasonable accuracy. This tem perature therefore prescribes the lower limit of the altitude-band: th» upper limit, in the stratosphere, has its origin in the fact that the atmosphere there grows drier, but not colder, with increasing altitude. The trails may be of short duration, or long and per sistent: the latter type are frequendy, but not always, associated with cirrus or cirro-stratus cloud. In direct contrast to those caused by addition of water to the atmosphere, trails may also be formed behind airscrews or wing- tips by the pressure-reduction that occurs in these locations. If pressure is reduced, expansion of air follows, causing a drop in temperature. If this drop is sufficient, a "cloud trail" may form. Other, less familiar, causes of condensation are the mixing of layers of air by airscrew or jet efflux, the effects of electrically charged panicles, and the ability of exhaust-supplied particles to act as nuclei of condensation. The subject is clearly of more than academic interest, as the position of an aircraft is immediately given away if it sails through the air with a dense white plume streaming away behind it.
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