When German military air power was unleashed over Europe, a semi-secret Junkers military development made air force officials of all nations more conscious of the potentialities of dive-bombing. This development was the Junkers Ju-87 Stuka dive bomber, which proved quite formidable against poor defenses. Another Junkers military aircraft which has gained considerable attention is the high performance Ju-88 twin-engined general purpose bomber. Both of these aircraft have been equipped with Junkers Jumo 211 engines and, with some exceptions, Junkers hydraulically-operated constant speed propellers.
Although Junkers airplane and engine design and production had been extensive before 1938, it was not until then that the first experimental production models of a Junkers designed controllable propeller appeared. Production of this propeller indicated a desire of the Junkers firm to avoid the foreign patents under which they had been previously licensed to produce American designed controllable propellers in Germany. A recent announcement that a Junkers Jumo 004 gas turbine was being used in the Messerschmitt jet-propelled fighter indicates that advances in design development have not been hindered by the war.
Standard non-controllable Junkers propellers had featured dural or magnesium blades held in a one-piece hub by screw threads on the blade root and in the hub socket. These propellers were adjustable on the ground, and in addition to dural and magnesium blades they could be fitted with hollow metal and wooden blades. To obtain better engine cooling with the ground-adjustable propeller, blade cuffs were used to determine the blade shape most desirable for satisfactory cooling, then blades with the selected shank form were made and put into use.
As originally designed the Junkers controllable propeller consisted of a single-piece hub having magnesium or compressed wood blades. Blade actuation for pitch change was accomplished by an oil-operated gear motor attached to the front of the hub and geared to the blades by a spur and worm reduction gearing system.
By 1940 British authorities had made a thorough investigation of a production model Junkers which had been on a Stuka shot down in England. Their study disclosed that the Germans had already made several design changes, possibly due to operational demands or ailments. Further gearing had been added to raise the gear reduction to 2:1, and counterweights had been put on the blades to relieve the twisting moment to low pitch.
Recent examination of a 1943 model Junkers disclosed that major changes had been made which completely altered the pitch change gearing. This change had been made earlier but had not been so completely embellished with detail redesign. Several intermediate models having minor modifications indicated that the Germans either were really striving for improvement or just liked to tinker with various ideas. The fact remains that Junkers propellers are still largely confined to Ju-87, Ju-88, Ju-188, and Dornier 217 bombers while the VDM electrically-operated propeller continues to be installed on the rest of the Luftwaffe's first-line aircraft.
Several conditions contribute to this disparity in total installations. All the above mentioned aircraft except the Dornier use Junkers Jumo 211 engines, to which the Junkers propeller is particularly adapted, while the VDM is adaptable to any engine. Also the VDM has had a much longer service record, having been originally developed before 1935.
Fig 3 shows operation and construction of the 1940 model; a later description will cover the features of the 1943 model to point out the design changes made in three years.
Power for pitch change is obtained from an engine-driven governor incorporating a spur-geared oil pump. Oil lines from the governor, which is mounted on the rear of the engine, lead to the propeller through a rotating joint in the propeller shaft. Cored passages, through the pitch lock assembly in the hub, direct oil to the oil motor in the pitch change power unit attached to the front of the hub. Oil pressure at approximately 170 psi is directed through one side or the other of the oil motor, depending upon which direction pitch change is required. The oil under pressure passing through the two helically-toothed gears comprising the oil motor causes them to rotate and thus establish a moving gear train to the propeller blades.
A drive shaft splined through one of the motor gears and extending out of the front of the motor housing is fitted with a small spur gear which meshes with a large gear to provide a 2:1 reduction at this point. The large gear is fixed to a shaft which extends through the other motor gear, emerging at the rear end of the motor housing with a small gear which meshes with the internal spur gear teeth of the coordinating ring gear. The outside of the ring gear has helical teeth which engage one of the gears on each of three intermediate transverse shafts.
A second helical gear on each of these shafts meshes with the final drive to the blade, which consists of three more transverse shafts equipped with worm gears meshing directly with worm wheel teeth cut in the main blade adapter. The motor-to-blade reduction ratio thus obtained is 1,548:1. A compound worm gear reduction system such as this is mechanically very inefficient, especially for a propeller. The Germans apparently realized this, as will be shown by subsequent description of a later, improved model.
In the hollow center of the propeller hub, a pitch lock, (K) in Fig 3 is fitted. This pitch lock consists of a cored magnesium casting incorporating a transverse rotatable sleeve inclosing a relief valve. The sleeve is positioned so that a spline shaft connected to one blade adapter causes the sleeve to be rotated when the blade angles are changed. Slots in the sleeve come into registry with the high and low pitch oil passages when the pitch lock comes into action.
This action is required because the hydraulic governor is connected to the throttle in such a manner that sudden opening of the throttle when the blades are in low pitch would cause over-speeding. Thus, in operation, when the throttle is closed and the blades rotate to a predetermined degree of low pitch, the sleeve uncovers passages so that low pitch oil pressure is bypassed into the high pitch line, neutralizing the gear-motor and stopping blade rotation.
In this position, if the engine is given a sudden burst of power there will be sufficient blade angle to absorb the speed increase. When the throttle is closed and the engine speed decrease causes operating oil pressure to drop below 7 psi, the bypass valve closes and the blades move to rest against the low pitch positive stops integral with the base of the blade socket.
The Junkers hydraulic governor shown in Fig 4 provides the oil pressure and constant speed operation of the propeller. It consists essentially of an oil pump, distributor valve with a spindle and ball flyweights, and a spindle loading spring. The spindle spring is loaded directly by a selector control for flight positions such as takeoff and cruising, and it is loaded automatically by linkage from the engine throttle through a differential gear.
Body of the governor is made up of several magnesium castings bolted together. The base, which is bolted to the engine, contains the low and high pitch oil passages leading to the propeller The oil pump section just above the base houses the two oil pump pinions, one of which is driven by an idler gear in a housing extension for the engine drive to the governor. One pinion of the gear pump carries a steel cruciform mounted on a flat plate and separating four free flyweight balls. Over this assembly is fitted the main housing which contains the selector valve, relief valve, and oil passages for direction distribution of pitch change oil. The lower end of the selector valve spindle fitted in its sleeve is made with a cone to fit over the flyweight balls. The upper end of the spindle fits the loading spring housed in the top casting.
Contained in the top casting is the differential gearing that links the engine throttle and over-ride selector control to the rack which impinges on the selector valve spindle loading spring. Quite elaborate for a governor loading device, the gearing is comprised of epicyclic gears, a dashpot, cam, and coil spring. The two levers for throttle and selector control act through the gearing to the spring rack via different gear trains so that the selector control lever may be operated to override the throttle setting. When the throttle lever is moved, its shaft rotates a cam linked to the shaft by a torsion spring. Attached to the cam is a piston fitting a dashpot integral with the housing. Thus, no matter how fast the throttle lever is moved, the damping action of the dashpot delays the final transmission of movement to the spring rack.
In operation, oil enters the inlet side of the oil pump from the propeller return and engine supply, passes through the pump and past a relief valve to annular passages around the selector valve spindle sleeve. Under this condition, oil pressure awaits valve direction to flow to the propeller, or if the governor is in neutral, or "on speed" position, the relief valve by-passes the oil to the inlet side of the pump.
The ball bearing type flyweights fitted inside the cone-shaped end of the selector valve spindle move outward under the influence of centrifugal force. Therefore, if engine speed increases to the point that the balls push against the sides of the cone and cause the spindle valve to rise against the pressure of the loading spring, oil from the common delivery annulus will be directed to the high pitch side of the propeller to effect an engine load and bring the governor back to the constant speed condition which is desired.
Conversely, if engine speed drops, the spring pressure will be greater than the effect of the flyweights and the valve will drop to uncover the low pitch passage to the propeller. The oil pressure generated by the pump is approximately 170 psi, and there is a rate of flow of about 11 gal/min.
Machined from a one-piece molybdenum steel forging, the Junkers hub is cadmium-plated. Bell-mouthed blade sockets are provided at their outer internal lip, with buttress threads to receive the blade retaining nut. Machined in the base of each blade socket is a bearing recess for the inner journal bearing of the main blade adapter Also machined integrally with the base of each blade socket are the projections which engage blocks fixed to the blade butt, thereby limiting the low pitch blade angle. A hole bored through the base of one blade socket allows a small splined shaft to engage the butt of one blade and the rotatable valve sleeve of the pitch lock which is housed in the hollow center of the hub.
The hollow hub center is made possible by dispensing with the usual splined propeller shaft. Hub attachment is therefore accomplished by Hirth-type serrations on the rear face of the hub, these serrations mating with similar teeth on the flange face of the engine shaft. Eight studs in the hub rear face project through the engine flange and retain the hub. A large synthetic rubber sealing ring is fitted between the rear hub face and engine flange to prevent leakage from the hub, which is bathed internally with oil at a slight positive pressure. The front face of the hub has a flange to receive the gearing apertures cut into each of the hub sockets so that the drive worms will contact the blade adapters.
Fabricated from Heine-process compressed beechwood, the blade is screwed into a tapered steel adapter. Butt of the blade is sealed by an aluminum disk sprung into the adapter. Inner end of the blade adapter has V-type threads which screw into the main blade adapter, which is a steel shell incorporating worm teeth on its inner rim, an integral bearing ledge for the thrust bearings, and a bearing surface for the outer journal bearing. Inner journal bearing consists of a double row of ball bearings held in races which fit into the bearing recess in the base of the blade socket and over an extension of the inner end of the main blade adapter.
The thrust bearing is composed of bronze caged roller bearings seating against the adapter bearing ledge. The centrifugal loads carried by these bearings are transmitted to the hub by the retaining nut which screws into the blade socket, forming the outer thrust bearing race. Just above the thrust bearing, a bearing surface around the main blade adapter, and loose roller bearings fitting into a bearing groove in the retaining nut, form the outer journal bearing. Both bearing surfaces formed from the main blade adapter are case hardened and ground.
Inner lip of the main blade adapter is beveled to receive a bronze cone which is wedged between the main adapter and blade adapter to prevent the latter from unscrewing and the blade becoming detached. This cone secures the top of the blade adapter so that the inner adapter threads are relieved of bending loads. Threads on the outside of the main adapter receive a counterweight ring which is threaded internally to contain the locking ring nut which impinges on the bronze adapter cone. The counterweight, a block of steel, is held to the ring nut by bolts. Tapered oil seals in the rim of the retaining nut prevent oil leakage.
Fitting on the front of the hub, the power unit consists of an oil motor with reduction gearing and a worm gear train to the blades. The oil motor consists essentially of two hollow-steel helically-toothed gears enclosed in an aluminum alloy housing. Apparently the helical teeth are intended to provide a smoother drive than spur teeth and to allow a slight oil seepage which would prevent oil sluggishness at low temperatures. Roller bearings support each end of the oil gears in their housing. One of the motor gears with internal spline teeth drives a shaft fitted with a small spur gear which meshes with a larger gear. This large gear is keyed to a shaft extending through the other (hollow) motor gear and emerging at the rear end of the motor equipped with a small spur gear. An extension of the rear of the motor housing carries the coordinating ring gear which has internal spur teeth to mate with the motor drive spur gear plus external helical teeth to drive the first stage of worm gear reduction. This coordinating ring gear, constructed of two pieces riveted together, is mounted on ball bearings.
The worm gear reduction system is fitted into a solid magnesium alloy casing finished all over. First stage of three worm shafts is equipped with a bronze worm and mounted in the casing with ball bearings, while the second series has bronze worms keyed in place and equipped with roller bearings for radial loads and ball thrust bearings for the thrust loads imposed when rotating the blades. The entire reduction assembly is fitted into the recess in the front face of the hub so that the final drive worms engage the worm teeth on the rim of the main blade adapter. The motor assembly fits over the gearing and is held in place by retaining bolts. A magnesium dome shell fits over the entire power unit and is sealed to prevent oil leakage.
The propeller which we are concerned with here was apparently constructed in 1943 by Junkers and a subcontractor whose markings were not identified. Four principal design changes were noted: The entire oil motor and the gear reduction assembly were changed, the hub components were altered, and the blade assembly and governor had undergone modification.
This unit was fitted to a 1,200-hp Junkers Jumo 211 engine in a Ju-88 twin-engine bomber. It is a three-blade right-hand tractor propeller 11' dia, equipped with wooden blades. In external appearance this model appears to be almost identical with the previous design. Only the shape of the oil-motor dome has been changed. Whereas it was formerly semi-spherical it now somewhat resembles the outline of a German soldier's helmet. Close examination indicates that the blade counterweight size has been increased and a new type finish has been applied to the hub.
Oil motor and attendant gear train comprising the power unit has undergone the most complete redesign of any component. The oil motor itself, consisting of two pinion gears, has been retained in principle, although straight spur teeth are now used instead of the helical type. Spur gears on the front of the motor are still used to provide an initial reduction, and they transmit their drive by a shaft extending through one of the oil motor gears to the rear of the motor housing. From this point to the blades, an epicyclic gear train has been interposed to supplant the former worm gear system. Drive gear at rear of the motor housing meshes with the involute teeth of a ring gear bolted to a cage carrying three planet pinions. These planets fit over and rotate around two sun gears, one of which is fixed to the hub and the other a sleeve gear free to rotate relatively to the fixed gear and the hub. This rotatable sleeve gear has, at its inner end, bevel teeth which mesh with the teeth on the main blade adapters, thus connecting the gear train to the blades.
A heavy cast dural dome featuring an oil drain plug fits over the entire assembly and is held to the hub by bolts. The motor housing is bolted to the inside of this dome, while the main reduction gearing is fixed to the hub. The oil motor housing is fabricated in two pieces of gray cast iron in contrast to the previous model's aluminum alloy housing which was approximately half as heavy. The oil supply tubes screw directly into the rear of the housing and are led to the motor gears by cast-in passages.
The housing cover, which also acts as the front support plate for the motor gears, is made of cast iron and located in position by two dowels and secured by through bolts. The motor gears are supported on each end by roller bearings and are equipped with roller thrust bearings contained in a race similar to a washer. Both gears are bored through their centers.
One gear, though, is hollow, which is rather incongruous, considering the extreme weight increases over the previous model. This hollow gear has been featured in all the Junkers models for reasons not clear except, possibly, to the Germans. The effort to reduce weight by this small item might be worthwhile if the theme of design was lightness. As it is, a 1-lb solid steel shaft has been used to fill an alternate gear drive passage, although the plug carries no stress and hence such practice does not seem reasonable.
Exactly how the one motor gear is made hollow has long been a controversial matter. The inside diameter of the hollow gear is so much greater than the opening in the center of the integral shaft that a complicated expanding tool would have to be used if the interior was machined. However, a certain roughness of the interior indicates that the gear could have been centrifugally cast by a highly uniform process. Even if this was done the value of the result is questionable.
A series of spur gears on front of the motor provides initial speed reduction and drives a shaft carried in the body of the motor housing on a double row of loose roller bearings. The spur gear which is driven by the motor meshes with a case-hardened ring gear drilled around its rim with lightening holes. This ring gear is attached to the planet cage by six bolts. The planet cage is a solid aluminum alloy machining of thick section. Planet gears are mounted in apertures milled in the periphery of the cage on hollow pins pressed into the cage walls.
Three rows of loose roller bearings run on the shaft and in the planet, and they are separated by small spacers. Two steel bearing races are pressed into the front and rear of the planet cage so the cage may rotate on loose roller bearings having for their inner races the front of the fixed sun gear and the outside of the blade drive gear. The fixed sun gear is doweled to the hub and has two ground, hardened surfaces providing inner bearing races for the blade drive gear which fits over the fixed sun before it is secured to the hub. Loose roller bearings are included in this final drive as well as a flat thrust bearing washer containing small rollers.
All steel parts in this power unit assembly have been treated with a black chemical stain having good penetrating quality. Dural parts have been left untreated. Finish of all parts is not highly refined, and the fits of gears and bearing surfaces are not very close.
Fig 6 is a view of the power unit assembled on the hub with the enclosing dome removed. One blade main adapter is installed in the hub socket and a complete retention assembly and counterweight is fitted with a stub blade.
The oil motor shown at the front of the gearing in the schematic drawing of Fig 7 has a 21:4 speed reduction in its transfer of rotational motion from the oil motor gears to driving pinion A, having twelve teeth. Pinion A drives ring gear B which has 62 involute teeth. Mounted on a pinion shaft at the periphery of B is planet gear C of eleven teeth. Planet C meshes with the fixed sun gear D of 39 teeth and movable sun gear E having 42 teeth. Final drive to the blade is made through bevel gear F, integral with gear E and equipped with 36 teeth which mesh directly with the blade gear G of 40 teeth.
As may be observed in the drawing and noted in the following calculation, the greatest speed reduction occurs at the point where planet C rotates around the sun gears D and E which have a differential of three teeth, causing gear E to rotate 3/42 of a revolution for every revolution of gear B
Thus, 21/4 × 62/12 × 42/3 × 40/36 = 422:1, the reduction ratio from the oil motor to the blade.
The hub remains essentially the same as in the original model, except that the pitch lock in the hollow center has been eliminated and the drilled hole in the base of one socket is not present. Also, the front face extension has been modified to receive the fixed sun gear and blade drive gear.
Lightening holes have been drilled in both the front and rear face of the hub, and it is noted that some of these holes have been filled to achieve balance. In machining the base of the blade socket, positive stops have been left integral with the hub and bronze inserts pressed into the recess in the base to provide a liner for the outside race of the inner journal bearing.
Although no provision for spinner or de-icing fluid distribution had been made, such equipment may readily be attached. Spinner diaphragm security is achieved by ring straps bolted around the rim of each hub socket. The straps incorporate a thick metal section containing holes to receive diaphragm retaining bolts. The diaphragm has a sliding ring which typifies the German quick-detachable spinner as introduced by VDM. In contrast to the VDM propeller which has holes drilled directly into the hub socket walls to retain the spinner, no holes of any sort are drilled in the Junkers hub. If de-icing is desired, a pickup flange is installed on the rotating blade at the cone locking nut. The interior and exterior of the hub has been treated with a black chemical stain and the exterior has been coated with dark green paint.
Redesign is also evident in the blade, too; for construction of the later unit differs from that of the previous model. Compressed wood is only used in the shank portion which screws into the blade adapter. The compressed wood shank is spliced to the blade proper by tapered laminations. The mass of the blade is composed of approximately 14 laminations of natural wood boards consisting of all sorts of odds and ends of wood glued together by a pink cement. There is no uniformity of grain or maintenance of quality. Occasional knots are noted.
Possibly this use of all grades of wood is due to a conservation policy stemming from a lack of materials a lack also indicated by the use of poor quality wood laminated and glued together to form the stock of the standard German Mauser rifle.
The whole surface of the blade is covered with a coarse mesh fabric next to the wood. Tacked to the leading edge of the blade is a strip of fine mesh copper gauze to which a brass leading edge sheath is sweated. Except for the leading edge sheath, the blade is totally covered with a cellulose acetate sheet about 1/16" thick applied to the blade under pressure. An enamel and the usual dark green paint completes the covering. This type of wooden blade protection, originated by the Schwarz Propeller Co before the war, has proven to be a fairly satisfactory treatment for the wooden blades. Except for a few specimens most blades have shown that the covering has fair abrasion resistance and good adhering and moisture-excluding qualities.
The blade retention system has undergone a few modifications but has remained basically the same in design. As has been described, a bevel gear blade drive supplants the former worm drive. This change has necessitated bevel gearing on the main blade adapter which extends around half the inner edge of the adapter. Bottom of the adapter, channeled to fit over the positive stop in the socket, is fitted with adjustable plugs for pitch adjustment at the feathered and low pitch positions. The inner journal bearing which had been fitted with balls is now fitted with roller bearings in a cage which may be disassembled when the dural preload nut and steel washer securing it are removed.
A dural plug is screwed into the bottom of the adapter and locked. The main thrust bearing still consists of roller bearings in a bronze cage riding against an integral bearing ledge on the adapter and against the adapter, or blade, retaining nut. A dural ring nut screws into the retaining nut to compress the oil seals.
The bronze blade adapter locking cone is tightened against the blade adapter and main adapter by a nut which screws into the counterweight ring. The counterweight ring fits the threads on the outer lip of the main adapter. The counterweight itself has been increased in weight to the point that it has been calculated that the twisting moment created by the weight against the centrifugal twisting moment of the blade amounts to 40 percent of that of the blade. A mark on the rim of the counterweight ring coincides with degree markings stamped on the hub so that blade angle position may be easily determined.
The constant speed governor has been modified by a simplification of the control head and the addition of a feathering pump, motor, and valves. The method of loading the oil distributing spindle valve spring by a series of epicyclic gears from the control levers has been supplanted by a simple rack bearing on the spindle spring, and operation is by a pinion rotated by a pulley wheel. A cable from the governor control is attached to the pulley and causes it to rotate when the control is moved. A positive stop and adjustment are provided in the control head to secure limits of operation. The oil pump, consisting of two spur gears driven by an intermediate gear direct from the engine mounting pad, has not been changed, nor have the ball-type flyweights and cone-bottomed distributing valve.
However, an additional housing has been added to enclose two helically-toothed feathering-oil-pump gears. This feathering oil pump is independently operated by a reversible Bosch electric motor controlled by the pilot to feather and unfeather the propeller when such action is desired. Automatic valves in the housing direct the feathering oil to the propeller and render the constant speed system inoperative when they come into play. Inclusion of a feathering system in the constant speed governor has resulted in a complicated, bulky governor unit, but this has consolidated the two functions nicely.
The Junkers propeller is heavy, weighing approximately 425 lb, and it does not show the generally thorough German design practice. Intermediate models not covered in this examination of two propellers, with distinctly different features, have exhibited various and sundry experiments apparently directed at improvement. While the spur gear reduction of the latter model is more efficient than the worm system, size of the blade counterweights has been increased to make up for lack of blade angle changing power. In its function of constant speed engine control, however, the propeller probably renders reasonably good rugged, but not sensitive, service. No especially fine tolerances are observed in the mechanism, but the loose bearing feature throughout is most likely a maintenance headache. No part in the whole unit, with the possible exception of the main adapter, is such a specialized piece of work as to require other than routine equipment and facilities. And, since no part appears difficult to manufacture, production could be high if the propeller was desirable.
This article was originally printed in the April, 1945, issue of Aviation magazine, vol 44, no 4, pp 117-123.
The original article includes 8 figures: 3 photos, 4 drawings (three of which are shown above), and one graph.
Photos are not credited; graph credited to Junkers; 2 drawings redrawn from Aircraft Engineering.