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Aviation History
1928
1928 - 0297.PDF
APRIL 19, 1928 THE VARIABLE PITCH AIRSCREW With a Description of a New System of Hydraulic Control THE'paper under above title, by Dr. Hele-Shaw and MrBeacham, read before the R.Ae.S. & I.Ae.E. on April 12 proved not only a most interesting one, but also highlycontroversial. The first part of the paper was devoted to outlining theadvantages of variable pitch aircsrews, and curves were shown, taken from Dr. Watts' book and from THE AIRCRAFT ENGINEER (Technical Supplement to FLIGHT), the latterreproducing one of the set of curves from Mr. C. C. Walker's article on " Climbing Efficiency of Aircraft," which wepublished on January 27, 1927. On the subject of the mechanical design of variable pitchairscrews, the authors divided into three classes the mechan- isms which have been put forward of late years : Manually-operated gears ; power-operated gears ; and power-operated gears in which a governor adjusts the pitch. The authorsof the paper, having selected as the best solution of the problem the third class, very naturally devoted their atten-tion to this, and outlined the past history of the Hele-Shaw- Beacham variable pitch airscrew. An order was given to the authors of the paper by the AirMinistry for a hydraulic variable pitch airscrew to be fitted to a Rolls-Royce " Condor " engine. The hydraulic gearwas made by Harper, Sons and Bean, and the propeller successfully passed various spinning tests and was fitted on a" Condor " and flown at Farnborough. Following the first appreciation of their invention by the Technical Departmentof the Air Ministry, the authors of the paper secured the co- operation of the Gloster Aircraft Company in the furtherdevelopment of the hydraulically-operated variable pitch airscrew. A large number of designs for the chief types ofengine were prepared by Mr. H. L. Milner under the direc- tion of Mr. H. P. Folland, and the first design to be actuallyproduced was one for a " Jupiter " engine. The airscrew for this had Duralumin blades manufactured by the special" Gloster" process which that firm has been developing of late. The Gloster Company has completed spinning testsof this airscrew, and the results of tests in the air will be published shortly. The general problems of hydraulic control were dealt withnext, the authors pointing out that hydraulic machinery is usually associated.with operations requiring enormous force,but that nevertheless the forces available hydraulically are greater in proportion to bulk and size than any other formof steady action. A small working model was exhibited in which a variable stroke rotary pump gave pressures up to3,000 lb./sq. in., although merely driven by hand. It was pointed out that a much smaller pressure than that, less thanone-third in fact, was sufficient to operate the blades of a large airscrew fitted to the " Condor " engine. The variablepitch airscrew designed by Dr. Hele-Shaw and Mr. Beacham depends upon the action of such a pump, but operating aram instead of the pressure gauge, the ram in turn moving the airscrew blades. The control of the pump, instead ofbeing effected by hand, is effected by a centrifugal governor. Before proceeding to a description of their airscrew, theauthors discussed briefly the question of weight, pointing out that there is obviously a minimum weight for the econo-mical employment of the variable pitch airscrew. If it imposed serious additional weight, then it must give advant- increasing the engine power. The experiments made so far indicated that the weight of hydraulic gear and governor could be reduced to a comparatively small item, and that the bulk of the weight increase was due to the blades, the blade mountings and the hub. Metal blades were not m themselves heavier than wood blades, but the large centrifugal forces involved must be sustained on ball races of substantial size. The diameter of the hub must be increased to house these ball races, and its length increased to accommodate the journal bearings. , , In addition to the question of weight, there was that ofreliability, and the authors called attention to the fact that the gear used in their airscrew was substantially similar tothat used for the steering gear of ships, of which many hundreds had been fitted with Hele-Shaw gears without anyfailures. The third point of importance was the proviswn against possible failure of parts, and the Hele-Shaw-Beachamairscrew gear was so designed that in case of a fracture ot a pipe line, by a machine gun bullet, for instance, the pitch of the propeller wjuld automitically return to the " normal "angle, and the propeller be the exact equivalent of an ordinary fixed pitch airscrew. Without the use of illustrations, it is difficult for us to givean adequate idea of the details of the " Gloster " Hele-Shaw- Beacham variable pitch airscrew, but the following extractsand summary from the paper may serve to give at least a very general idea. " The pitch of the air-screw blades isvaried by means of a double-acting hydraulic piston, operated by oil pressure from a variable stroke pump driven by theengine. The stroke of the pump is in turn controlled by a governor, also driven by the engine, so that whatever the airconditions may be, the pitch of the airscrew sets itself so as to keep the engine running at a constant, predeterminedspeed. The speed at which the governor operates can be altered by the pilot within certain limits by means of a smallcontrol lever ; that is, the pilot has it in his power to speed up the engine and obtain extra power in an emergency, andalso, on the other hand, to reduce his engine speed so that he may cruise at full throttle—this, of course, being thecondition for minimum petrol consumption per brake horse- power developed by the engine. The two airscrew bladesare mounted in a special hub in which they are free to rotate independently about an axis at right angles to the shaft. Oneach blade is fastened a crank, both cranks being operated from a common crosshead, the crankpin being connected tothe crosshead in each case by a small slider, free to slide a short distance in order to compensate for the angular move-ment of the propeller blades. The hydraulic piston moves the crosshead forward to vary the pitch in one direction, andaft to vary it in the opposite direction." Each propeller blade has two journal bearings and a ballrace to sustain the centrifugal force on the blade without undue friction. The hydraulic piston, which is situated in its cylinderon the front of the hub, is connected to the crosshead by two bolts. The oil pressure provided by the variable stroke rotarypump is led to the two sides of the hydraulic piston via two separate pipes, one to the front and one to the rear. Thevariable throw crank of the pump is operated by a small governor. In the normal condition, the rod operating thechange of stroke is in mid position, and no oil circulates. As soon, however, as the engine increases or decreases its speed,the governor moves the rod which determines the variable stroke, and the pump forces oil through one or other pipe,to front or back of hydraulic piston, which in turn moves forward or back, taking with it the crosshead to which thecrank arms of the blades are connected. The pilot's control takes the form of an arrangementwhereby the force on the governor spring is varied, and consequently adjusts the speed at which the governoroperates. Finally, it should be pointed out that the crosshead hasa centering spring, so that in the event of the hydraulic pressure failing, this spring is strong enough to return the crossheadto its central position, i.e., that corresponding with normal pitch. The hydraulic pressure available is so powerful thatit can easily overcome both the force on the propeller blades and the spring pressure. Much of the success of the " Gloster " Hele-Shaw-Beachamvariable-pitch propeller depends on the design of the variable- be necessary, which for obvious reasons cannot be given here, although we may be in a position to give such a description at a later date. In order to keep the pipe system full under all conditions, a small non-return valve is fitted between each pips and a connection to the oil reservoir. In the case of the pipe under pressure, the non-return valve closes. If leakage takes place anywhere, a vacuum will tend to be created in the other pipe, and the corresponding non-return valve will automatically open, and oil will be drawn in from the reservoir to fill the void which would otherwise occur. This ensures that at all times the whole pipe system and hydraulic cylinder is absolutely full of oil, and the crosshead is thereby held rigidly, owing to the incompressibility of oil. There is consequently no backlash to start any fluttering action in the blades. The Discussion The Chairman recalled that as long ago as 1916 or so it was agreed thatit was desirable that something should be done to encourage the development of the variable pitch airscrew, and that as a result a variabie-pitch airscrew 269
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