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Aviation History
1947
1947 - 0003.PDF
JANUARY 2ND, 1947 FLIGHT DUPLEX AIRSCREWS Part II. An Analysis of the Rotol Hydraulic "Contra Prop" By C. B. BAILEY-WATSON A the matter of definition in terminology referring toairscrews seems to occasion a certain amount ofmisunderstanding over what is, essentially, a straight- forward issue, we might with advantage amplify the defi- nitions given last week. A contra-rotating airscrew is, in effect, a single airscrew the thrust area of which is divided into two banks of blades rranged to rotate in opposite directions. But since both I ade banks are simultaneously controlled for constant- ' speeding, feathering and braking, and one blade bank cannot be operated independently of the other, it is logical to assess the airscrew in the singular case. A contra- rotating airscrew is most often driven by one engine, but as may be seen in the layout diagrams, it can be driven by two engines through clutch couplings. If one engine should fail, it is de-clutched and the remaining engine alone- then drives the airscrew. Conversely, coaxial airscrews are entirely separate units which are mounted in tandem on coaxial shafts. Each airscrew is driven by its own engine and is independently controlled for all its functions. That is to say, the speed of rotation can be different—although in practice it is not so, because of vibrational complications—and one airscrew may be stopped and feathered whilst the other continues to operate. The method of driving may vary; in the case of the Bristol 167 each engine of the four pairs of Centaurus which power the aircraft drives its air- screw through reduction gears housed in a box common to each pair of engines. However, on the Macchi-Castoldi racing sea- plane a different system was employed : the two Fiat engines were mounted in tandem with the transmission taken from between them up to coaxial shafts passing forward between the cylinder banks of the front engine and so to the indi- vidual airscrews. Other possible drive layouts are illus- trated in the diagrams on the next page. We ought, too, perhaps, to deal with the subject of "handed" airscrews, since this is a form of contra-rota- iiip-u. The case is exemplified by the de Havilland Hornet and Lockheed Lightning, both of these being: twin-engined aircraft with each engine carrying and driving its own airscrew. The airscrews are, however, arranged to rotate in opposite direc- tions in order to take advantage of various benefits made available by this system. As may be seen in the illustrations, the Hornet airscrews rotate inwards at the top, whilst the Lightning airscrews rotate outwards. Rotation in this instance is deter- mined by aerodynamic considera- tions. The Rotol airscrew can briefly be described as consisting of two banks The Rotol six-blade contra-rotating airscrew fitted to the Rolls-Royce Griffon 85 engine of a Spitfire 22. of three blades each, both hubs being of simliar basic design and construction, actuated by the conventional Rotol hydraulic system employing a stationary piston and sliding cylinder. The pitch-change motion for the rear bank of blades is transferred from the front bank through the medium of a translational bearing unit. Contra-rota- tion is achieved by the interposition "of an idler pinion in one half of the reduction gear, the normal direction of rotation being left-hand for the front bank of blades and right-hand for the rear. We have mentioned that the singular case is used in referring to a contra-rotating airscrew, as distinct from coaxial airscrews where the plural case is correct. This is exemplified by the particular airscrew we are considering ; the two blade banks are driven by one engine, constant-speeding is controlled by one c.s. governor unit, and the pitch change motion is actuated by a single piston and cylinder assembly. The Hubs Since the basic design and structure of each hub is similar, we can make the description general. Each of the airscrew blades, which are solid duralu- min forgings, is screwed into a steel blade adaptor on which a Timken taper bearing is an- chored between two buttress- threaded ring nuts. Thia assembly is secured in the hub barrel, with the centre race of the taper bearing held against a shoulder-flange inside the bar- rel by a buttress-threaded retain- ing nut screwed into the barrel mouth. The design has the merit oi being smaller, lighter and neater than the original form of blade housing, the im- provement being directly attri- butable to the employment of a single duplex bearing in place of the multiple bearing stacks pre- IN last week's issue we reviewed the • power installation of the Bristol 167 Brabazon I, taking it as an excellent example of coaxial airscrew application. To complement this, and to provide the remainder of our survey of duplex air- screws, we this week deal with the Rotol six-blade hydraulic contra-rotating air- screw as used with the Rolls-Royce Griffon 85 engine to power the Spitfire 22. viously used to accommodate centrifugal-thrust and journal loads. The three barrels which make up the blade-carrying part of each hub are made in an integral unit; this is bolted to an internally splined driving centre which mates with the airscrew shaft for the transmission of torque. Static and thrust loads are taken upon cone seatings in the con- ventional manner, the hub being secured by means of a retaining ring nut screwed on the end of the shaft and, in so doing, pinching-up hub and cones. Inside the inner airscrew shaft is a distributor unit to which control oil from the constant-speed unit is ducted. Into this distributor are screwed two coaxial tubes which, at their forward ends, carry the piston ; this latter component is enclosed by a large-diam«ter shallow cylinder supported by a hollow stalk boused inside th^sjjjrscrew shaft and sur- rounding-uie oil tubes. Pressure oil
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