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144 AIRCRAFT ENGINEERING June, 1931 The Manufacture and Wind-Tunnel Testing of a Centrifugally Operated Variable-Pitch Type Th e complete weight, which is extremely low for H E usual method of driving aero-generators th e capacity and compares very favourably with is by means of a small windmill mounted an y other on the market of a windmill suitable for T on th e shaft of the generator, the contour of a generator of 500 watt s output, is onl y 1 lb. 15 ozs. the latter being streamlined. By this means a separate prime mover is dispensed with, while, if Th e windmills and generators are designed for a the windmill is placed in the slip-stream of th e main normal speed of 3,500 r.p.m., which experience airscrew, some of the lost energy is re-generated. shows to be the most satisfactory speed for this The inherent drawback to this form of drive is, class of machine. however, that the speed of a normal windmill— Th e latest types of windmills, which are made in and hence of the generator armature—varies with several sizes suitable for generators of variou s output the speed of the aircraft, and many methods have capacities, are designed for a wind speed range of been devised to overcome this trouble. 70 to 240 miles per hour, and are thus suitable for One of the earliest methods was to twist the type s of aircraft with speeds of 200 m.p.h., and generator and windmill relative to the airstream over. by means of a lever from the cockpit, but the in Plan t and machine tools capable of a very high convenience of this arrangement to an already degree of accuracy have been installed for quantity production , and every effort is maintained to secure overtaxed pilot is very apparent. absolut e uniformity of quality and performance in An alternative method of allowing the windmill ever y windmill. Every precaution is taken with and generator to fluctuate over very wide ranges of th e selection and heat treatment of the various speed and controlling the generator voltage by materials, while all parts are made to jigs and some form of voltage regulator is also not ideal. Company, and is designed to meet the conditions gauges to a fine degree of accuracy. The windmills I t is obvious, owing to the wide range of speeds to arising from the ever increasing speeds of modern ar e subjected to severe overspeed tests and after be covered, that if the regulator is arranged to aircraft. ward s are carefully adjusted in a wind-tunnel to operat e at low speeds, th e generator must be racing Th e windmill (illustrated in position on the operat e a t the correct speed, and should require no a t a dangerous extent a t high wind speeds, while if generato r in Fig. 1) is extremely compact and further adjustment or attention throughout their designed to operate at high speeds, the instrument consists of a small elliptical hub containing the life other than periodic lubrication. is inoperative during a considerable range of low governor and operating mechanism, on which the wind speeds, so tha t a compromise has to be struck Some idea of the special equipment necessary for tw o blades are mounted on a special form of ball th e satisfactory production and testing of automatic thrus t bearing in order to reduce frictional effects to a minimum. The blades and blade holders are ver y light, being one piece forgings in a strong aluminiu m alloy, and are machined to shape and afterwards given the correct spiral twist. Th e movement of each blade is governed by a small cylindrical weight having a scries of splines parallel to its axis on its outer surface, and a series of splines of spiral formation on its inner surface. Th e inner splines mesh with a correspondingly cut membe r which is integral with the hub, and the oute r splines mesh with the blade holders. Centri fugal force arising from rotation of the windmill causes the weights to move outwards against the action of th e controlling springs and a rotar y motion is imparted to them by the spiral splines. This motion is, in turn, transmitted through the straight splines to the blades, the effect being tha t th e blades ar e moved in relation to the wind stream in such a manner that a balance is set up and a constant speed of rotation maintained over a wide range of wind speed. Experiments have shown the most useful values for spring an d weights, and the correct amoun t of damping, so tha t an almost isochronous which is not conducive to the highest efficiency or condition is maintained with complete absence of greatest safety for a given weight. huntin g or vibration. Development on the lines of a windmill driven constant-spee d windmills is shown by the fact that Th e windmill is entirely weatherproof, and is throug h a centrifugally governed clutch has also a special wind-tunnel for testing and adjusting the constructe d of strong aluminium alloy throughout, been carried out. This arrangement has the dis windmills under a reproduction of actual working wit h the exception of the following internal parts : advantag e tha t a large and fairly complicated piece conditions had to bo designed and manufactured. th e ball-bearings, governing weights, springs and of mechanism is mounted on a generator shaft Figs. 3 and 4 show the wind-tunnel and testing centra l pinions. The net result is an extremely revolving a t high speed, th e windmill itself revolving equipmen t used at Messrs. Haslam and Newton's light windmill of very neat appearance offering the independently of the driven shaft and being free Work s at Derby. The air is drawn through the least possible air resistance and giving a very high to attain, under some conditions, an excessive and tunne l by the large exhaust fan at the rear end, efficiency. dangerous speed. These frequent high speeds result which is driven b y a variable-speed motor of 170 h.p. Fig. 2 shows the windmill complete; and dis in rapid wear, while the constant slipping of the capacity . The windmills are fitted for the purpose mantle d into its component parts. clutch is likely to result in heating of the part s and of test on to an aircraft-pattern generator mounted th e necessity of constant adjustment of the friction in the wind-tunnel on a swinging gravity-con surfaces. trolled cradle, this cradle bein furnished with a Undoubtedl y the most satisfactory method of pointer moving over a tangen scale so that an obtaining a constant speed for the generator arma actua l measurement of the drag resulting from the tur e is b y means of a windmill furnished with blades windmill and generator can be obtained. th e pitch of which can be varied by means of centri Th e operation of the windmill is tested over the fugal weights; and a number of types of windmill range of wind speeds with the generator running hav e a t various times been placed on the market light and on load, and no windmill is passed out operatin g on this principle. In several of these from the Testing Department until its regulation types, however, constructional difficulties have ha s been proved to be within the limits laid down. mad e it necessary to use only one blade, with a Fo r routine testing, the generator is separately serious loss of efficiency and unbalanced aero excited a t a definite value from a secondary battery, dynami c forces. and the speed is read as volts generated, while wind An excellent type of variable pitch windmill is speed is measured by means of a standard Air th e "Marconi-Newton" patent constant speed Ministry-typ e wind gauge. For check testing and windmill, which is manufactured by Haslam and for experimental investigation, the speed of the Newton, Ltd., Derby, and sold by Marconi's Wire windmill in the wind tunnel is measured by means less Telegraph Co., Ltd. This two-bladed windmill of a stroboscope, the windmill being illuminated by is the latest of a series of designs produced by this reflected light from a suitably placed lamp.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jun 1, 1931
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