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Aviation History
1948
1948 - 1714.PDF
458 FLIGHT OCTOBER 14TH, 1948 Tie-Rod Transmission An Ingenious Drive System Used on the Landgraf Helicopter By MAURICE F. ALLWARD THE Landgraf Model H-2 Helicopter contains severalunusual features, not the least interesting of whichis the tension-rod drive ti the twin rotors. This drive incorporates one worm only on the crank- shaft, meshing with two worm wheels which are connected directly by the tension rods to discs under the rotors. Thus there are no toothed gears of the bevel or spur type, and no long rotating shafts in the transmission. Each end of the twelve high-tensile steel rods is connected to special end fittings incorporating a bearing which rotates on bolts fixed to the worm wheel and rotor discs. A third of the way along from each end, two Micarta spreader rings are positioned, which prevent the rods from bending outwards under centrifugal loads when in operation. These rings are very lightly loaded, the rods turning in simple oilite bushes, while the rings are retained by swaged flanges on the rods. In action the drive_is quite silent and almost 100 efficient as the linking i almost pure tension, rods, shown unprotected in the illustration, are nor- mally encased in the two ^wing stubs which carry the rotors. It is true to ray that necessity This sketch of the Landgraf rotor drive emphasizes its simplicity and lightness. was the mother of this drive. The construction of theprototype during the war was severely hampered by short- ages, and it proved impossible to get any new bevel gears THE Landgraf H-2 helicopter is an* experimental single-seater with two laterally disposed three-bladed rotors.Instead of changing the pitch of the blades, their camber is varied by hingedflaps on the trailing edges. cut for the normal type of drive which had been designed, nor was it possible to utilize any existing gears. And so, faced with the prospect of a complete stoppage of work, Fred Landgraf set himself the task of developing an alter- native drive which did not use any bevel gears; the ten- sion-rod drive was the result. It was originally intended to use this only in the prototype, but it has proved so outstandingly successful that it is to be retained in the production aircraft, unless some hitherto undeveloped snags appear. One advantage is that the complete installation is some 30 per cent lighter than the normal torque-tube bevel- gear drive would be, an important consideration on helicop- ters with their present limited pay loads. Another is that the inherent '' spring'' of the rods very effectively dampens the transmission of engine vibration to the rotor heads, with consequent decrease in wear. The same '' give '' also allows the rotor wing stubs to flex under gust loading with- out harming the drive system. The installation in the Landgraf Model H-2 has been run at 30 per cent above the normal speed of 485 r.p.m., and at twice the normal loadings on a test bench for many houn without any faults developing. To ascertain the effect of a r9d fracturing, two were loosened at one end for one test; when run up, the rods merely bent slightly without fouling, and the system still functioned fairly well, but with per- ceptible vibration, just enough to warn a pilot that some- thing was wrong. Although it is a dangerous thing to claim in the aero- nautical world, this drive appears to be the first of its kind to be used on an aircraft, with the possible exception of an accessory drive of an early type of aircraft piston engine. The system should lend itself well to larger rotors and lower speeds; in fact, the slower the speed, the more efficient the drive becomes. After the "bugs" that, to follow tradition, are sure to appear, there seems to be no reason why the system cannot be used elsewhere successfully and with advantage. POLYTETRAFLUOROETHYLENE '"PHE above name—and what a name—is that given to a-I- quite extraordinary new plastic material; extraordinary is used advisedly, for the electrical and mechanical propertiesof the material are unique. Known generally for convenience as P.T.F.E., this new polymer is manufactured in this countryin powder form by Imperial Chemical Industries. However, there are considerable difficulties in the forming techniqueand it is only after two years' of intensive development work that British Mechanical Productions, Ltd., of London andBristol, have successfully established a method of production. In this, they had the benefit of considerable primary researchwt>rk undertaken at the Royal Aircraft Establishment, Farn- borough. Its molecular structure gives P.T.F.E. a greater resistanceto temperature than any other plastic and combines first-class electrical properties with high chemical resistance. Thesequalities have particular applicability to electrical and inter- communication equipment in aircraft, where relatively hightemperatures may be reached. Transparent in thin sections, but waxy in appearance in thick sections, P.T.F.E. withstandsthe attack of all other materials with the exception of alkali metals. It can be boiled in alkali hydroxides, hydrofluoricacid, fuming nitric acid or aqua regia with no change in weight or properties. It is water-repellent, non-absorptive, non-flammable, insoluble and has no true melting point. When subjected to heat, P.T.F.E. undergoes a transitionfrom its solid phase at 327 deg. C, with a sharp drop in strength. Its maximum surface temperature for retention ofshape is 250 deg. C, but it will not decompose below 400 deg. C, whilst its strength and dielectric qualities are retained at tem-peratures between 38 and 260 deg. C. SECURITY IN SHIPMENT OF JET UNITS HE Edo Corporation of America, well known as manufac-turers of floats for marine aircraft, recently turned their attention to a different subject, the packing of turbojetengines to assure freedom from damage during transit. Then new "elastic" container unit was designed to a U.S. Navyspecification, and would appear to give full protection from the heavy impact shocks, capable of disturbing the balance ofsuch power units, and liable to be suffered through rough handling during transportation. Made almost entirely from aluminium, the complete unitcomprises three main components ; an engine stand on wheels ; an anti-moisture barrier in the form of barrels encasing theengine completely, pressure-balanced through a Silica-gel filled orifice; and the elastic outer cage made up of springingmembers arranged to absorb the greater part of any shock suffered by the complete cargo. Tests showed thaj the con-tainer would reduce an exterior load of 47g to one of 8g on the engine itself. The engine in its stand and moisture drum but without theprotective cage is in a convenient form for air transportation, and the weight is some i.ooolb less than when a wooden crateis used for the same purpose. BI 6
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