DUPLEX AIRSCREWS Power for the Brabazon I ; Coaxial and Contra-Rotating Airscrew Arrangements

DECEMBER 26TH, 1946 FLIGHT DUPLEX AIRSCREWS Power for the Brabazon I ; Coaxial and Contra-rotating Airscrew Arrangements

Primary test rig for the twin-Cen- taurus installation. In this view the transmission shafts can be seen in addition to the engine mounting arrangements and the cardan shaft for driving the auxiliary gear box.

C. B. Bailey-Watson

EFORE we start to unravel the various skeins of the screw and the inner shaft carrying and driving the front Brabazon I power installation, we might with advan- airscrew. B tage first take time to define the difference between This arrangement, finally adopted after every other contra-rotating and coaxial airscrew designs. Although at alternative had be.n the subject of intensive design study first this would appear to be somewhat difficult since is, in fact, the simplest of all, involving as it does the use coaxial airscrews contra-rotate and, equally, contra-rotat- of only a single pair of gears (the reduction bevels) for each ing airscrews are mounted coaxially, in point of fact it is engine/airscrew drive, thus incurring the lowest possible reasonably simple. The accepted basic definition is that transmission less penalty. The target mechanical efficiency co-axial airscrews are entirely separate airscrews mounted was 99.3 per cent—an inordinately high one—but Bristols on co-axial shafts each driven by its own engine, usually have the great satisfaction of having proved that, to judge through reduction gears in a common box. A contra- by the considerable running tests so far done, the drop in rotating airscrew is a single airscrew unit of two blade efficiency actually encountered is satisfyingly less than that banks mounted on co-axial shafts driven by one. -or more estimated in assessing the target figure. engines—usually the former. So much for terminology. Viewing the power installation as an overall problem In passing, it might be noted that since a contra- one may appreciate the enormous amount of work entailed by considering a typical example. The size of the bevel rotater is a two-bank single unit airscrew, the singular case wheels in conjunction with certain unusual design .features is used, i.e., one refers to a contra-rotating " airscrew,'' but and the degree of accuracy required were quite outside the in the complementary application of coaxial airscrews, as capacity of any gear generating machines known to exist. each bank of blades is a separate airscrew driven by its own To overcome the difficulty Bristols made a close study of engine, the plural is used and the term is thus coaxial " air- the problem, found a solution, ar.d crews." put up the proposal to David VeW briefly, a.id in the most Brown and Sons, Ltd., who agreed general sense, the prototype Bristol THIS article forms the frst sect/on of a to undertake manufacture. Special 167 power installation can be said ' review dealing with coaxial and contra- equipment was designed and built, to comprise four pairs of Centaurus rotating airscrew drives. A coaxial arrange- and the gears produced. In c :3ence engines, the individual units of each ment is dealt with this week,, and a contra- one can sum up the main problem pair being angularly disposed one rotating design will be included in next week's issue. as being one involving the reorien- to the other so that their crank- As representative of the coaxial layout, we tation and scaling-up in si . of con- shaft axes centre at a vertex angle have selected the power installation of the ventional elements; most of the of 64 degrees. The crankshafts Bristol 167, and by so doing serve a double difficulties incurred being concerned drive torsion shaft; which, in turn, purpose, for not only is it an excellent example with refining and simplification of drive bevel pinions meshing with of the type of engine/aircrew drive we wish design, together with maintenance bevel wheels carried on the respec- to consider, but at the same time it is topical of efficiency in the increased sizes tive airscrew shafts. The latter are and of interest in itself as part of a great rather than difficulties of actual nested coaxially, the outer shaft project. planning design as such. carrying and driving the rear air- 694 FLIGHT

DUPLEX AIRSCREWS

FINE-PITCH STOP There is nothing exceptional about the MECHANISM Centaurus themselves. They are just FINE-PITCH standard Mk 57 units without individual STOP reduction gears, but with the exhaust MECHANISM stacks from each cylinder taken back to - '' half-horseshoe '' manifolds, air-muffed for heat insula- tion, which terminate in dis- charge elbows protruding be- neath the skin line of the lower wing surface. In this guise the engines are termed Centaurus XXs. Each engine is housed in a sealed cell divided by a diaphragm picking-up to a sealing sleeve at the rear extremi- ties of a simple wrapper cowl which shrouds the cylinder heads; thus the differential across llid diaphragm is the cooling pressure.' The intake is through ram heads in the leading edge, each aperture having horizontally hinged flaps; and outlet is via gill slots in the upper and lower engine cell doors. When one airscrew is feathered—or in the event of fire in an engine—the appropriate air intake flaps and exit gills can be closed to reduce drag and/or seal the cell to . preclude dissipation of the fire-smothering agent. This special Flight cut-away drawing Engine mounting is unique in that each Centaurus is sup- shows the drive arrangement between ported on one side only. The mounting structure consists the transmission shafts from a pair of INNER of two '' tripods '' the apices of which attach by spherical- Centaurus and the respective airscrews, AIRSCREW seat quick-release couplings to the wing structure, whilst giving the relative disposition of the SHAFT the base of each tripod leg anchors to attachment brackets various components and permitting the on the crankcase. In conjunction with the tripods, two operative motion of the mechanism to be stabilizing struts form a forward extending vee in the verti- grasped. cal plane. the front cover, (ii) the main casing, (iii) the auxiliary Gear box Details drive casing, and (iv) the rear cover. Mounting is through The power input into each gear box is made by trans- the main casing, which is circumferentially bolted to a mission shafts joined to the respective Centaurus crank- mounting ring built integrally with the monocoque trun- shaft flanges through the medium of Layrub flexible coup- cated-cone housing which, in turn, is anchored to the wing lings, and transmitting through Hooke-type universal joints structure. This monocoque carries not only the dead into the gear box. Inductor-rings for electric torque-meters weight of the airscrews, but also supports any dynamic out- are conveniently arranged around the universals. of-balance forces of airscrew rotation, in addition to being, To consider the gear box itself: the casing is formed in via the gear box, the main thrust transmission member be- four sections, all of cast magnesium ifiloy. They are (i) tween airscrews and airframe. Starting from the back and working forward, the gear box rear cover and auxiliary drive casing support between them a series of gear trains which drive the synchronizers, airscrew controllers, gear pumps, and auxiliary gear box drive. In the rear cover itself is housed the airscrew oil distributor-unit by means of which control oil delivered from the controller units is transferred/ to the appropriate tubes for suppiy"to the airscrew actuating cylinders. These tubes, two to each airscrew, are radially disposed about a common axis and are housed within the inner airscrew shaft.

This full-size test rig is a mock-up wing section of the 167 housing one of the twin-Centaurus power installations. The photograph shows the air-intake ram- heads and gives a good idea of the aerodynamic cleanliness. DECEMBER 26TH, 1946 «95

MAIN THRUST REAR AIRSCREW BEARING for REDUCTION DRIVE STARBOARD ENGINE REAR AIRSCREW TRANSMISSION SHAFT

SYNCHRONIZER ALTERNATORS

AIRSCREW OH. DISTRIBUTOR UNIT

CARDAN SHAFT DRIVE FOR AUXILIARIES

AIRSCREW OIL TUBES MONOCOOUE STEADY HOUSING BEARINGS FRONT AIRSCREW REDUCTION DRIVE TORQUE-METER PORT ENGINE INDUCTOR RINGS TRANSMISSION SHAFT FREE-WHEEL

The auxiliary drive casing carries the roller bearing in rear wheel, is splined to its airscrew shaft. Both bevel which the rear spiral bevel wheel is mounted on the inner pinions are splined to input shafts carried in roller and ball airscrew shaft. Immediately forward of the rear bevel is a bearings, the shafts taking the drive from the universal duplex roller and ball bearing for journal and thrust loads joints on the transmission shafts. of the inner shaft. These bearings are supported in the rearmost part of the main casing (which is in the form of Bevel Drive Cooling a double-walled cone), and housed in front of them is a Obviously the heat generated in the bevel meshing zones double-row ball bearing acting as the main thrust and tail- will be very great, considering trie power being transmitted, support bearing for the outer airscrew shaft. An interesting and to effect cooling clustered oil jets are set on the " down- design aspect of the gear box is that all major loads are stream " side of the meshing zones, discharging directly focused at a single point on the airscrew shafts' axis, the down the tooth flanks. Since the tooth-face temperature- shaft bearings and cone apices of the main casing dia- rise will be at its peak immediately after meshing, this is phragms centering as close to this point as possible. The the logical spot to apply coolant. It will be obvious that front bevel is of internal straight-tooth type and, like the most of the heat is at the surface whence it is most easily abstracted by the oil Cooling-oil flow-rate is approxi- mately 300 gall/hr and is delivered by the gear pump in the rear cover. Whilst on the subject we may note that oil is fed from an outside tank and cooler to the suction side of the pressure pump, passes through the pump and thence through a Purolater pressure filter, and so into the gear box. Total flow-rate is about 330 gall/hr, of which roughly 30 gall/hr is for lubrication. The oil is a low vis- cosity type with special additives for specific use with gears of high tooth pressures. Drive is taken from the gear box through a dual free- wheel system to the aircraft auxiliaries gear box mounted back in the wing and remotely driven through two cardan shafts with Layrub couplings. The auxiliaries are powered Diagrammatic plan of paired from the reduction gear box in such a way that, should one engine install- engine fail, the drive will be maintained. This is achieved ation showing by taking a drive from each airscrew drive via a twin free- drives and re- wheel device. The drive is so arranged that there is a lationship to slight r.p.m. difference as between the input drives to the wings free-wheels with engines synchronized, and thus the drive is

- r*. 6g6 FLIGHT DECEMBER 26TH, 194O

DUPLEX AIRSCREWS normally taken from one engine only. Should this engine be shut down, however, one half of the free-wheel would idle and the drive be taken up from the other engine by the previously idling section. The free-wheel is a conventional wedging-roller type. Although we have referred to the inner and outer airscrew shafts, only the former is a continuous member, the outer shaft being composed of three separate sections, viz. : (i) that to which the driving bevel is splined, (ii) the centre section extending from gear box to the forward end-wall of the monocoque structure; and (iii) the foremost section on which the rear airscrew is carried and driven. The front and rear portions are arranged with '' belled'' ends to which the central portion—a relatively thin-walled large- diameter cylinder—is flange-bolted. Roller journal bearings for the inner shaft are housed within the outer shaft, whilst the latter is in turn supported by a roller bearing in addition to those in the gear box main casing and front cover. The inner and outer shafts are respectively No. 6 and No. 7I S.B.A.C. standard sizes. Airserew Operation The airscrews are special three-blade Rotol hydraulic This view of one of the constant-speed, feathering and braking units of 16ft dia- paired-Centaurus engines, meter, the blades being of improved-wood and of Hordern- taken from the starboard Richmond manufacture. Since each airscrew is a separate quarter, clearly shows the entity, it is obvious that the rear airscrew's piston and two '' half-horseshoe '' cylinder assembly must be of annular form to permit pas- exhaust manifolds with sage of the shaft carrying the front airscrew. their insulating air-muffs. As previously stated, the actuating oil is delivered from the controller units to the distributor in the gear box rear are pivotally attached to offset pins on the blade-root cover, whence it is transferred to the oil delivery tubes bases, so that the linear motion of the piston is converted housed within the inner shaft. Each airscrew is fed with to rotary motion of the blades to effect pitch change. fine- and coarse-pitch oil by separate tubes, those serving In structure and operation the front airscrew is similar to the rear airscrew discharging into a distributor inside the the rear unit, excepting the cylinder which, of course, does inner shaft (the front airscrew tubes passing straight not need to be of annular form. However, the front air- through), from which the oil is transferred through the screw's piston is annular since the central part of the shaft wall. On emergence from the shaft the oil is directed cylinder is taken up by the fine-pitch stop mechanism, the to the appropriate side of the annular piston through a piston sliding on a sleeve bolted to the cylinder base. further distributor enclosed by a sleeve on which the piston Normally the fine-pitch stop is a positive mechanical slides. The enclosing cylinder is flange-bolted to the air- type of abutment, but in this case, where the blades have screw hub, and at its forward end it carries a roller bearing to turn through fine pitch in order to give negative thrust which supports the inner shaft. for braking, no form of positive fixed stop can be employed. Pressure oil feed to one side or other of .the piston causes Nevertheless, a fine-pitch stop of some sort has to be em- it to slide in the cylinder, and eyeboife, bolted to the piston, bodied, and the way in which this has been achieved is quite interesting. The piston travels forward for fine- pitch blade motion and, as it ap- proaches the fine-pitch limit, it butts against the end flange of an axial sleeve and carries it forward against the pressure of a large coil spring. Between this sleeve and that upon which the piston slides is a third sleeve, this latter being also carried forward by ball abutments between it and the end-flanged sleeve. The balled sleeve is attached to an axial sliding tube extending the whole length of the inner airscrew shaft back to, and through the centre of, the oil- distributor on the gear box rear cover. Thus, when the piston butts against the end-flanged sleeve, the third sleeve is also moved forward and, consequently, the sliding tube travels forward too. The movement of the tube is used to alter, by means of a mechanical linkage, the position of the piston-valve in the constant-speed unit of the airscrew controller, and so direct This illustration of the full-size test rig installation shows the arrangement of the oil to the front (coarse-pitch) side of engines in the wing leading-edge forward of the front spar. A considerable number the airscrew piston to prevent further of hours' running has been done. movement of the blades to finer pitch. DECEMBER 26TH, 1946 FLIGHT 697

DUPLEX AIRSCREWS

For the rear airscrew the same end is achieved by having tions. In view of this we might thus review the syn- an annular plate at the rear of the hub, actuated through chronization part of the system first. spring-loaded plungers moved by strikers on the blade On the rear cover of each gear box are mounted two syn- roots. As the blades turn to finer pitch the strikers depress chronizer alternators and two corrector motors (one each the plungers and the annular plate is moved rearward. An for each engine)—the alternators and corrector motors for Oilite rubbing pad bears on the plate, and a linkage from all engines being inter-wired. The system is based on the this is taken to the c.s.u. in the controller unit. selection of one engine as a "master," the remaining en- When airscrew braking action is required, the balls in gines being subject to automatic continuous correction to the front-cylinder sleeve are allowed to escape into recesses, the governing r.p.m. of the master. (The left-hand engines and so permit the unrestrjgted forward movement of the of each pair are arranged as masters—only one of which will, of course, be employed as such at any one time.) The rotors of the corrector motors are joined by worm gears and a mechanical linkage to the constant-speed units of the airscrew controllers, and one winding of each corrector is taken to its own engine's alternator, the other winding

Three-quarter rear view of a gear box, showing airscrew shafts, input drive universal joint, and mounting faces on rear cover for synchronizers, airscrew controllers, auxiliary carden shaft drive, etc. piston without concurrent movement of the sliding tube. Sjmilarly, on the rear airscrew the arrangement of spring loadings on the plungers permits the fine-pitch ^stopping to be appropriately overridden, and so allows the requisite negative-thrust pitch-change movement of the blades. Engine / airscrew control in this power installation is unique, and on this count alone is thus worthy of study, quite apart from any intrinsic merit of desigjfper se. The system embraces synchrmous control qf^the engines as a complement and an addition to the/Contrfel of airscrew Three-quarter front view of a right-hand engine, showing the unique double-tripod-and-vee mounting structure, and input pitch for constant-speeding, leathering and braking func- half of transmission shaft coupling. Note the doors in the cylinder head sealing shrouds for access to the sparking plugs.

TRANSMISSION-" being connected to the alternator of the master engine. /iNNULUS Each corrector is, in fact, an electric differential which, when all engines are in phase, will not turn. But when INPUT a speed difference does exist, it will rotate at a speed PINION proportional to the difference, and in a direction determined by whether its engine is overspeeding or underspeeding relative to the master. Airscrew Controlling The correctors are interposed in the linkage between cockpit and c.s. units, and in operation the system is very simple. When all engines have been started, the requisite r.p.rn. are roughly selected, the master engine selected, and the synchronizer then switched on. At once, all seven a. Centaurus will be synchronized with the r.p.m. of the AUXILIARIES' CARDAN SHAFT eighth, master, engine ; this action will continue providing DRIVE no one speed control lever is moved relative to the others more than an amount equivalent to a difference of 250 r.p.m. Should the selected master fail for any reason, one of the remaining three possible masters can be substituted by switching over, the faulty engine being stopped and feathered: all seven remaining engines will continue to run synchronously. Detail of free-wheel device illustrating wedging-roller and Since the Brabazon I airscrews have to do more than drive members. A drive diagram is shown on page 698. merely constant-speed, the conventional constant-speed E 13 FLIGHT DECEMBER 26TH, 1946

DUPLEX AIRSCREWS governor unit is not sufficient for overall control. Thus units which are termed "airscrew controllers" are used, and these incorporate the normal c.s. units in addition to other elements. As is well known, the constant-speed unit is a valving medium for the direction of actuating oil (sup- plied by a pump) to one side or the other of the airscrew piston to effect blade pitch-change. The valve is governed by the action of centrifugal bob-weights to uncover the appropriate valving ports as engine speed needs to be increased or decreased by fining-off or coarsening the blade pitch. Airscrew Braking However, for the braking action the c.s.u. must be isolated, or rendered ineffective, since in the negative-pitch range constant-speeding is not required, the airscrew being, in effect, a fixed-pitch type for this duty. To permit this, a change-over valve is embodied in the airscrew controller, this valve being actuated by solenoid-operated valves, themselves operated by the action of taking the efigine throttle Diagram of alternative drives through wedging-rollers, trans- levers back through a gate for reverse-thrust braking. A mission annulus and central wheel to cardan shaft for special high-delivery "reverse" pump, incorporated with auxiliaries drive.

TO AIRSCREW the c.s.u. pump, normally passes its FINE PITCH STOP DEVIC output through the change-over valve BRAKING UNBRAKIIMG SOLENOID in an idling circuit; but when the '' braking '' solenoid is energized the change-over valve is moved and the reverse-pump delivery is directed to the fine-pitch side of the airscrew piston, the high output resulting in the very rapid pitch change rate of about 50 deg/sec. Normal oilflow between c.s.u. and airscrew is taken through the change-over valve, and when this is actuated for braking, it closes the ports connecting to the c.s.u., so rendering it ineffective. The c.s.u. pump output is concur- rently fed back through a relief valve to the suction side of the pump. Feathering is conventionally carried ( uREVERSE \J PUMP out by means of manual selection of the requisite c.s.u. valve position, and Schematic layout of control system for one airscrew. The flow of control oil may energization of the separate high-out- be followed together with synchronizer and fine-pitch stop actuation. put feathering pump.

AERO GOLFING SOCIETY AVIATION CENTRE N Tuesday last week Flight had the pleasure and honour Further contributions to the Londonderry House and Aviation O of entertaining members of the Aero Golfing Society to Activities Fund dinner at the Royal Aero Club, 119, Piccadilly, W.i. Trophies Fairey Aviation Co., Ltd 1,000 0 0 were presented for last year's competitions, and the occasion The Bristol Aeroplane Co., Ltd 1,000 0 o served also as a meeting for the election of new officers. Rolls-Royce, Ltd ...1,000 00 Several informal and characteristically humorous speeches The De Havilland Aircraft Co., Ltd 1,000 o o Vickers-Armstrongs, Ltd. (& Associated Cos.) ....." 1,000 o o were made, notably by Lord Brabazon of Tara, who presided. Short Brothers (Rochester & Bedford), Ltd 525 o o Sir Frederick Handley Page, retiring captain, and the new Dunlop Rubber Co., Ltd 500 o o captain (proposed by Mr. Robert Blackburn) Major A. Imperial Chemical Industries, Ltd 500 o o Michell Clark*. Mr. Claude Wallis and Mr. Geoffrey Smith Titanine, Ltd, 26.5 5 o, welcomed the guests. Alvis, Ltd. 26,; 10-4 Prizes presented included the Instone trophy to Mr. D. P. Shell-Mex & B.P., Ltd 302 10 o Cameron (83 less 4 = 79), the Flight trophy to Group Capt. Intava, Ltd 262 10 o P. W. S. Bulman (100 less 18 = 82), Colonel W. A. Bristow's Tube Investments, Ltd 250 o o J. Stone & Co., Ltd . 250 o o . Jubilee trophy to Mi. H. G. Bentley (75, Stableford, two Southern Forge, Ltd. . 250 o 0 rounds), and the Cellon trophy also to Mr. H. G. Bentley (64 Sir W. G. Armstrong Whitworth Aircraft, Ltd 250 o o plus 2 = 66). The Captain's prize went to Mr. A. E. L. Skinner, Blackburn Aircraft, Ltd 250 o o who returned the best aggregate for the year. James Booth & Co., Ltd 250 o o Sir Frederick Handley Page proposed a vote of thanks and Armstrong Siddeley Motors, Ltd 250 o 0 presented a handsome watch to Commander Harold Perrin, Birmid Industries, Ltd 250 o 0 the society's indefatigable secretary. Airwork, Ltd ; ; : v 250 o o Almin, Ltd 250 00 In addition to the President (Lord Brabazon) and the Cap- Percival Aircraft, Ltd 250 00 tain, officers for the year 1947 were elected as follows: Vice- A. V. Roe & Co., Ltd 2.50 00 Captain, C. S. T. Thorn; Committee: P. W. S. Bulman, J. R. Irving Air Chute of Great Britain, Ltd 250 00 Bryans. II G Bentley, D. P. Cameron, L. G. Callingham, Miles Aircraft, Ltd 25° ° ° C. F. H. Gough, Maior R. H. Mayo, A. E. L. Skinner; Secre- Northern Aluminium Co., Ltd 250 00 tary, Commandei Harold E. Perrin British Aviation Insurance Co., Ltd 25° 00 JANUARY 2ND, 1947 FLIGHT DUPLEX AIRSCREWS Part II. An Analysis of the Rotol Hydraulic "Contra Prop" By C. B. BAILEY-WATSON the matter of definition in terminology referring to of three blades each, both hubs being of simliar basic airscrews seems to occasion a certain amount of design and construction, actuated by the conventional A misunderstanding over what is, essentially, a straight- Rotol hydraulic system employing a stationary piston and forward issue, we might with advantage amplify the defi- sliding cylinder. The pitch-change motion for the rear nitions given last week. bank of blades is transferred from the front bank through A contra-rotating airscrew is, in effect, a single airscrew the medium of a translational bearing unit. Contra-rota- the thrust area of which is divided into two banks of blades tion is achieved by the interposition "of an idler pinion in rranged to rotate in opposite directions. But since both one half of the reduction gear, the normal direction of I ade banks are simultaneously controlled for constant- rotation being left-hand for the front bank of blades and ' speeding, feathering and braking, and one blade bank right-hand for the rear. cannot be operated independently of the other, it is logical We have mentioned that the singular case is used in to assess the airscrew in the singular case. A contra- referring to a contra-rotating airscrew, as distinct from rotating airscrew is most often driven by one engine, but coaxial airscrews where the plural case is correct. This as may be seen in the layout is exemplified by the particular diagrams, it can be driven by airscrew we are considering ; the two engines through clutch two blade banks are driven by couplings. If one engine should one engine, constant-speeding is fail, it is de-clutched and the controlled by one c.s. governor remaining engine alone- then unit, and the pitch change drives the airscrew. motion is actuated by a single Conversely, coaxial airscrews piston and cylinder assembly. are entirely separate units which are mounted in tandem on The Hubs coaxial shafts. Each airscrew is Since the basic design and driven by its own engine and is structure of each hub is similar, independently controlled for all we can make the description its functions. That is to say, general. Each of the airscrew the speed of rotation can be blades, which are solid duralu- different—although in practice it min forgings, is screwed into a is not so, because of vibrational steel blade adaptor on which a complications—and one airscrew Timken taper bearing is an- may be stopped and feathered chored between two buttress- whilst the other continues to threaded ring nuts. Thia operate. The method of driving assembly is secured in the hub may vary; in the case of the barrel, with the centre race of Bristol 167 each engine of the the taper bearing held against four pairs of Centaurus which a shoulder-flange inside the bar- power the aircraft drives its air- rel by a buttress-threaded retain- screw through reduction gears ing nut screwed into the barrel housed in a box common to each mouth. The design has the pair of engines. However, on merit oi being smaller, lighter the Macchi-Castoldi racing sea- and neater than the original plane a different system was form of blade housing, the im- employed : the two Fiat engines provement being directly attri- were mounted in tandem with butable to the employment of a the transmission taken from The Rotol six-blade contra-rotating airscrew fitted to the Rolls-Royce Griffon 85 engine of a Spitfire 22. single duplex bearing in place of between them up to coaxial the multiple bearing stacks pre- shafts passing forward between viously used to accommodate the cylinder banks of the front engine and so to the indi- centrifugal-thrust and journal loads. vidual airscrews. Other possible drive layouts are illus- The three barrels which make up the blade-carrying part trated in the diagrams on the next page. of each hub are made in an integral unit; this is bolted We ought, too, perhaps, to deal with the subject of to an internally splined driving centre which mates with "handed" airscrews, since this is a form of contra-rota- the airscrew shaft for the transmission of torque. Static iiip-u. The case is exemplified by the de Havilland Hornet and thrust loads are taken upon cone seatings in the con- and Lockheed Lightning, both of these being: twin-engined ventional manner, the hub being secured by means of a aircraft with each engine carrying and driving its own retaining ring nut screwed on the end of the shaft and, airscrew. The airscrews are, however, in so doing, pinching-up hub and arranged to rotate in opposite direc- cones. tions in order to take advantage of Inside the inner airscrew shaft is various benefits made available by IN last week's issue we reviewed the a distributor unit to which control this system. As may be seen in the • power installation of the Bristol 167 oil from the constant-speed unit is illustrations, the Hornet airscrews Brabazon I, taking it as an excellent ducted. Into this distributor are example of coaxial airscrew application. screwed two coaxial tubes which, at rotate inwards at the top, whilst the To complement this, and to provide the Lightning airscrews rotate outwards. remainder of our survey of duplex air- their forward ends, carry the piston ; Rotation in this instance is deter- screws, we this week deal with the Rotol this latter component is enclosed by mined by aerodynamic considera- six-blade hydraulic contra-rotating air- a large-diam«ter shallow cylinder tions. screw as used with the Rolls-Royce Griffon supported by a hollow stalk boused The Rotol airscrew can briefly be 85 engine to power the Spitfire 22. inside th^sjjjrscrew shaft and sur- described as consisting of two banks rounding-uie oil tubes. Pressure oil FLIGHT JANUARY 2ND, 1947

DUPLEX AIRSCREWS CLUTCH IDLER from the c.s.u. is fed via the oil tubes to the forward side of the piston for coarsening blade pitch, and to the rear of the piston for fining pitch, the oil pressure reacting against the piston area and thus causing the cylinder to move forward or rearward to effect pitch change. Transmission between cylinder and blades is by means of an articulated linkage whereby the linear motion of the cylinder is converted to rotary motion of the blades. Pitch Change The articulated linkage for the front bank of blades is a simple link bolted to the cylinder and sliding in guide sleeves through the front and rear walls of the hub. The central part of the link is swollen and bored to accommo date a sliding Tufnol block, itself recessed to accommodate an operating pin anchored on the base of the blade; thus, as the piston moves the link fore and aft, the Tufnol block can slide laterally in the link to conform with the arcuate motion of the operating pin. The front blade links are tapped at their rear ends for receiving a connecting bolt which picks up lugs on a sleeve secured to the inner race of the translation bearing. The outer race of this bearing is correspondingly enclosed by Two typical yet diverse methcds of driving a contra-rotating a housing to which the rear blade operating links are airscrew. Top : By two parallel engines driving compour.d articulated. Also embodied in this outer housing is a reduction gears through clutch couplings. Bottom : By one driving joint in the form of a yoke with which registers a engine, as in Griffon and Merlin installations, where the crank- driving pin attached to the rear hub. In this manner, the shaft powers a double pinion and an idler is used to give rear blade links are relieved of the duty of rotating the contra-rotation.

outer race of the translator bearing, the transmission loads being conveyed solely through the pin ; however, transmission loads for the inner race are carried by the bearing links of the front hub, a duty which they can well afford since the cantilever is short and thus the bending moment small. The translation bearing is enclosed within a trun- cated cone housing attached to an annular plate bolted on the rear of the front hub. Reference has already been made to the supply of control oil from the constant-speed unit to the cylinder via the coaxial oil tubes to effect pitch change, but it might be

Examples of handed airscrews. Directions of rotation are determined by aerodynamic effects on the aircraft.

LIGHTNING

worth while in conclusion to review the constant-speed operation of the airscrew. First, one must bear clearly in mind that the pilot has two major engine controls, (i) the throttle, which con the degree of supercharge given to the engine and has only an indirect bearing on the engine r.p.m. ; and (ii) the speed control which, through the medium of the constant-speed unit, by regulating the pitch of the airscrew blades, deter- mines the speed at which the engine shall run, irrespective, within limits, of the load and likewise of the boost pressure. Let us consider an example : an aircraft is flying straight and level at normal cruise conditions; the boost, we will assume to be plus 6 lb/sq in and the engine r.p.m. steady at 2,100. The pitch angles of the blades at these values Three typical methods of driving coaxial airscrews. A : By we will take as being 40 deg. for the front blades and 38 parallel engines driving through spur reduction gears, one incorporating an idler pinion. B : By raked engines driving deg. for the rear blades, the latter having a relatively bev«l reduction gears, one an internal type. This scheme finer basic pitch setting to compensate for their having to is used for the Bristol 167. C : By opposed engines driving operate in the helical wash from the front blades. right-angle simple bevel reduction gears. The pilot then decides to climb a few thousand feet; JANUARY 2ND, 1947 FLIGHT

load on the engine is at once increased and thus the r.p.m. from the engine. Meshing with the driving g^ar is an idler tend to fall, and (although the boost value does not change) gear mounted on a short hollow shaft incorporating a non- immediately this happens the constant-speed unit regu- return valve through which engine oil enters the unit. lates the blade pitch to finer angles, say 30 and 28 deg., These gear wheels form the pump which feeds oil, via allowing the engine to speed-up to its original figure of ducts in the casing, into the interior of the hollow driving- 2,100 r.p.m. gear shaft, from which two outlets give access to the oil On the other hand, should the pilot start diving, the pipes connecting to the airscrew tubes. Inside the hollow load on the engine is relieved, the "revs" tend to soar gear shaft is the piston valve which has lands upon it and and the blade pitch is consequently coarsened until the is moved by the centrifugal governor, so determining to engine r.p.m. are brought back to their selected figure. As which oil tube the high-pressure oil shall he fed. When the time-lag between enghve r.p.m. change and corrective the piston valve is in a central position IIOIIJ -outlets are action is very small, engine speed and boost pressure blanked-off by the lands, and the high-pressure oil from remain virtually constant. The actual power output of the the pump is by-passed back through a relief valve to the engine is thus maintained at a determined value, irrespec- suction side of the pump. tive of the attitude of the machins, although naturally The centrifugal governor consists of two L-shaped fly-

BEARING DRIVING PIN ASSEMBLY OPERATING LINKAGE

he design and construction of the TRANSLATION TRoto/ 15 Geg. pitch range six-blade BEARING contra-rotating airscrew are clearly UNIT shewn m this special "Flight" cut- away drawing. How the movement of the sliding cylinder is transmitted through the operating links and translation bearing for pitch-change motion can easily be followed.

the speed of flight is dependent upon attitude. When climbing, L for instance, the engine would be \\ .turning over at the same speed for •s*8HfJ~same boost, thus producing the same power as for level flight, but due to the finer blade pitch which permits this condition, the aircraft will not be pullett weights, the lower arms of which bear against the outer through the air for so great a distance per revolution of race of a ball bearing mounted on the end of the, piston the airscrew, and thus the climbing speed will be lower. valve. On the other side of the ball bearing is a flange against which bears a, coil spring to compensate the centri- Constant-speed Unit fugal movement of the fly-weights. Above the spring is The constant-speed unit is mounted on and driven by a sleeve along which are cut racking teeth to mesh with the engine, and is composed of three main elements, viz., a toothed segment controlled from the pilot's cockpit. a gear-type oil pump, a centrifugal governor, and a piston Thus, according to the position to which the segment is valve, all of which are housed in a three-part casing. The turned, the tension of the coil spring is either lessened gear-type pump is also the drive for the centrifugal governor or increased in order to balanGe the centrifugal moment of and comprises a driving gear mounted on a long hollow the fly-weights, and so determine the position of the piston shaft, one end of which is splined to transmit the drive valve and the consequent distribution of the oil to the FLIGHT JANUARY 2ND, 1947

COARSENING PITCH

Rising r.p.m. cause the fly-weights to lift the piston-valve permitting oil to flow to the front of the piston so coarsening blade HIGH PRESSURE OIL pitch. TO FRONT OF PISTON

When the airscrew is "on speed" no oil flow takes place and the blades are held in the obtain- ing pitch

Falling r.p.m. allow fly-weights to lower piston - valve so feeding oil to rear of piston to fine-off HIGH PRESSURE OIL the blade pitch. TO REAR OF PISTON

type, since this quality is not necessary with a single- DUPLEX AIRSCREWS engined aircraft. Feathering types are, however, very similar, the main difference being in the provision of a longer cylinder to accommodate the increased pitch range. airscrew oil tubes. Inside the racked sleeve a further coil Extra movement of the pilot's control into the feathering spring is housed, the object of this being to ensure that position moves the c.s.u. piston valve to the positive coarse- the sleeve is kept in the position for normal cruising r.p.m. pitch position, and a separate, electrically-driven high- ' should the control from the cockpit be shot away or output pump feeds oil to the c.s.u. whence it is become inadvertently disconnected. delivered to the airscrew to effect feathering movement of The airscrew we have dealt with is not a feathering the blades.

D.H. "OVER-TWENTY" PARTY •/ "SAMOPOMOC LOTNIGZA" ^ INETEEN more people who have served the de Havilland INTRODUCING the Polish Air Force Association (Samopomoc N Enterprise lor twenty years were initiated into the Over- -»- Lotnicza) at a gathering in the headquarters at 14, Colling- Twenty Fellowship at its annual party at Hatfield on ham Gardens, S.W.5, Grp. Capt. W. Makowski, C.B.E., ex- December 20th. The parent company formed by Sir Geoffrey plained the aims and objects of the Association. de Havilland and the executive team of the old Aircraft Manu- In brief the P.A.F.A. has been formed—on the lines of the facturing Co., Ltd., after the first Great War, is now twenty Royal Air Force Association—to prevent its members (now years old. and 106 of its members have been admitted to this about 8,000) from becoming a liability to the community. A. 1 Fellowship since it was originated in 1940. registered war charity, the Association is entitled to accept f^ It was considered that Major Frank Halford and those ol donations, but understanding is a more urgent requirement. his staff who have been engaged lor twenty years on de Havilland work also qualified for membership as from 1926, FORTHCOMING EVENTS the year in which he began the design of the first Gipsy an. 3rd.—I.Mech.E. : " The Development of Aircraft Hydraulic Ma- engine. This year's gathering was made particularly cheerful, chinery," H. G. Conway, S. M. Parker, and D. A. L. Robson. therefore, by the initiation of Major Frank Halford and Mr. Jan. 4th.—R.Ae.S. Reading : Annual general meeting. John Brodie who has been at his right hand since an even Ian. 4th.—British Interplanetary Society : " The Energy and Stability of Atomic Nuclei," L. R. Shepherd. earlier date. Ian. 6th.—R.Ae.S. Medway : "Design o< a Two-seater Sailplane," F. M. Another director of the de Havilland Engine Company, who Reynolds. )an. 7th.—R.Ae.S. Belfast : " Gliding," Professor Hill. ior all his youth now completes twenty years of service, is Ian. 7th.—R.Ae.S. Bristol : Brains Trust, with Capt. 1. Laurence Pritchard flMi. Hugh Buckingham. The others initiated were: Mr. F. W. as Question-master, •\mos, Mr. L. T. Boreham, Mr. G. N. Cross, Miss E. D. Ian. 8th.—R.Ae.S. Southampton : " Elementary Metallurgy ol Aircraft Materials," A. Black. Cramer, Mr. M. H Eagle. Mr. A. W. Hunt, Mr. H. J. Heading, Ian. 9th.—" Aerotech " Flying Club, No. I : "Naval Aviation," R. G. Mi, H. R. Hooper, Mr. T. E. Johnson, Mr. W. F. King, Mr. Worcester. H. ^eale, Mr. R. J. Nixon, Mr. R. I. Tugsley, Wing Cdr. Ian. 10th.—I.Mech.E. : "F2 Development and Shock Wave Investigation C. ft. Pike. Mr. T E. Parnell, and Mr. T. Radford. at Metro-Viclc." Dr. Smith and K. Todd.