Fine mechanics for PGS support, trimmers


  PGS-state management requires the separation of each component of this state from the others. The presence of this feature using only mechanics gives a great advantage, since it allows to have the separate components under manual and visual control for high reliability, especially in emergency cases. Also, the control of each of these components must be sufficiently precise. For example, pitch control requires a precision of about 0.1° or better. This task is performed in the liftoplane design in a number of stages.


  The main stage of this task is reflected in the design of each rotor. In addition to utilizing the four-gears pitch steering scheme, it has special mechanics for steering the radial displacement and the angular position of the central gear of this scheme. An overall view of this mechanics can be observed from the inner side of the rotor, as pictured in the chart below.

Rotor, inner side

  This mechanics is located in the central area of the rotor. But the chart also exposes other elements in the peripheral area. Here are ten wing’s sockets of the light greenish tint, fixed on the faceplate of the light coffee tint. Ten bridges of the light magenta tint are mounted on these wing’s sockets for additional stiffness. In the windows of these wing’s sockets the bevel pinions are observed, as well as the bevel gears, but partially. Ten linking shafts connect these bevel pinions with the respective elements of the four-gears pitch steering scheme, which peripheral elements are retained between the faceplate and a back ring of the beige color. These linking shafts also have supports inside ten ribs that are placed around and under the back ring and are shown in dashed lines. A steady base in the sky color locates over the inner half of the back ring and has five holes along its perimeter for mounting the entire rotor on the body of the engine. Also, the steady base is rotatably mounted on the central powering shaft of the rotor pictured in the magenta color. Elements of the displacement mechanics are mounted on the steady base.

Rotor, inner side center area

  The chart above pictures the magnified main central part of the previous chart. The dashed lines under the steady base depict elements of supporting the cluster of the central gear of the four-gears pitch steering scheme. There is a shifting base, which is depicted as the ring area with circular clearance spaces inside and outside, and which is located inside the back ring. This base can be partially observed in the light teal color inside of the hole in the steady base. The shifting base can be moved in any plane direction, but it is fixed against rotation by a system of radial and tangential rods in an orthogonal relation. Three orthogonal radial rods are spanning between a steady flange, mounted on the inner area of the steady base, and respective clamps on its peripheral area. Three orthogonal tangential rods are mounted on respective sides of the peripheral area of the shifting base. Three cross-holes bearings are dressed on the respective pairs of the radial and tangential rods in the areas of respective slots having a sufficient clearance for their movement. The central cross-hole bearing is extended apart its sides by two crampons that can move in respective saddles mounted on the shifting base for additional reducing a remained rotation backlash.


  In respect of the pitch management of the PGS-state, the following elements perform it. A pitch worm gear of the dark red color rotates the pitch pinion that changes the angular position of the central gear, having a common shaft of the red color. A respective pitch worm in the red color is meshed with the pitch worm gear and is supported together with the common shaft in a flanged pitch bracket of the pink color, which is mounted on the shifting base. A telescopic universal joint assembly of the red and dark red colors can rotate the pitch worm. This assembly uses a spline-pair of the red and dark red colors that permits to transmit the rotation over the variable length distance. The outer shaft of this assembly is rotatably mounted on a pitch bracket of the light blue color, which is attached to the steady base. The last shaft is used as the pitch interface of the entire rotor (P-component).


  In respect of the gain and skew management of the PGS-state, the following elements perform it. A flange of a GS-variator of the cyan color is mounted on the steady base over a respective hole. This variator has inside of the flange a large skew worm gear depicted by the dashed line, which can be rotated by a respective worm placed under a skew bracket of the violet color, which is depicted by such dashed lines also. This skew worm gear has in the inside a steering lead, which is not shown, and which can be moved in the radial direction, and which has a round steering pin inserted in a corresponding hole in the shifting base. The movement of the steering lead is performed by a pair of a toothed rack and a gear, where the latter is mounted on a gain steering shaft of the teal color. A gain worm gear of the light green color is fixed on this shaft and can be rotated by a respective worm of the teal color, which is rotatably mounted on the flange of the GS-variator.


  Just connecting rotation to these two worms of gain and skew isn’t enough to have the independent gain and skew management. Indeed, let's consider that the gain has fixed and the skew is changing. So, in order for the gain to remain unchanged, the gain worm gear must be rotated at the same angular speed as the skew worm gear. But, it is fixed by the gain worm and induces an adverse movement of the toothed rack and undesirable changing of the gain. To avoid this undesirable effect, a Skew-to-Gain compensator or simply an SG-compensator is used.


  The elements of the SG-compensator are distributed on a gain bracket of the dark yellow color, which has a complex shape and is mounted on the flange of the GS-variator. The SG-compensator has at its core a differential, and simultaneously performs an additional function of reducing the rate of change of the gain to align it with the respective rate of change of the pitch when performing a typical flight operation with the biangular handling. So, there is also a reducer of rotation of an outer gain shaft of the green color, combined with the SG-compensator. The reducer consists of two equal pairs of a pinion and a gear, where one pair is the outer part, depicted in the green and the light green colors, the other is the inner part, depicted in the dark green and the teal colors, and the differential is inserted between them, where its outer and inner miter gears are depicted in matching colors. The differential has a spider of the dark magenta color, which can be rotated by a big compensating gear of the same color. Two intermediate miter gears of the differential, depicted in the light violet color are placed on a transverse shaft of this spider and can rotate freely. A compensating shaft of this spider passes freely through the outer reducing gear and the outer and inner miter gears of the differential. Rotation of a skew outer shaft of the blue color is transmitted to the skew worm by a pair of equal gears, where the second gear of the dark blue color is placed under the first of the blue color and has only a part of its hub visible. Also, this rotation through a shaft of the second gear is transmitted to the compensating gear through a compensating pinion of the dark blue color and an intermediate gear of the light blue color. The gain and skew outer shafts are used as the respective gain and skew interfaces of the entire rotor (G-component and S-component).


  The SG-compensator permits to compose the gain and skew values from the respective outer shafts to the gain and skew state of the rotor. However, the force balance on its differential may induct undesirable mutual rotation of the outer shafts. This problem does not matter at all when the rotations of both G and S outer shafts are managed by servos backed with encoders, for example upon a computer management. But in the emergency cases, the remained management is just manual, and the problem can be severe dependently on friction forces of the entire mechanics. A mechanism for for mitigation this problem exists in the cockpit instrumentation and will be discussed later in the current topic.


  The output P-G-S shafts are used for coupling with a PGS gearbox, which is used to redirect the rotation of these shafts toward the cockpit direction. This coupling ability allows to detach the entire rotor from the aircraft. The image below presents a view of the respective side of the liftoplane after detaching its rotor.


After detaching the rotor


  This view exposes a socket for the rotor. An electrical engine of the light teal color is observed in the center of this socket, and it can also be removed from the outside. Five mounting supports with nuts are mounted on the body of the engine and are used for mounting the steady base of the rotor. A central hole in a shaft of the engine is used to insert the central powering shaft of the rotor with subsequent fixation by clamping. Three couplers of the PGS gearbox are protruded from respective holes in the socket. The image below shows the PGS gearbox, looking at it from the inside.


PGS gearbox


  Three long primary P, G and S shafts are originated from the PGS gearbox and are directed toward the cockpit.


  The last stage of the fine mechanics for the PGS supporting is represented by two sets of special trimmers for serving each PGS component of both rotors, which are located on the control panel of the cockpit. These trimmers allow to control any component with a precision of about 0.1°. Similar trimmers are also used to control the pitch of the stabilators, the locking mechanics of the rotors and the target winding speed of rotation of these rotors. The placements of the scales of all these trimmers are shown in the image below.

Scales of trimmers

  Each trimmer has a designation by one or more capital letters on its scale. P, G and S for the PGS components, WST for the Winding Speed Target, SP for the Stabilator Pitch and L for the Lock control of the rotors. The small shield shown over the G-scale is placed under the big shield of the G-trimmer and has two auxiliary scales for the opposite and main pitch deviations in degrees, originated from the gain, which are shown in the respective windows. All other scales of the G-trimmer indicate values in percent of the Normalized Linear Gain (NLG). The trimmers for P, G, S and WST have three levels of bidirectional scales: general, intermediate and fine. The middle size arrows of the fine and intermediate rotating scales with the zero-letter inside are used for reading positive values. The small arrows of the fine and intermediate stationary scales are used for reading negative values. The large arrows in the centers of rotating shields are used for reading the values from the steady general scales. The trimmers for SP and L don’t have the intermediate scales, but the SP-trimmer additionally has the innermost rotating shield, which has a pictogram of the stabilator to indicate its pitch naturally. The small light gray circular shapes on the right are retractable handles that can directly rotate the fine scales, which are integral parts of respective primary rotating cans. The outer dark gray rings are used to retain respective glass lids of these trimmers and are attached to these primary rotating cans.


  The chart above shows the design of the P-trimmer. It has a case of the greenish tint, which is mounted under a corresponding hole in the control panel of the cockpit depicted in the light magenta tint. Actually, this case is a part of a common case of the P-G-S-trimmers, which has a triangle shape to be more compact than three separate cases. So actually, for each side there is a PGS trimmers block instead of three separate trimmers. A servo of the dark magenta color is mounted under the case. A primary shaft of this trimmer, depicted in the red color also services as its exit shaft in respect of interfacing, and has a universal joint on its end. This universal joint accepts rotating connection from a linking shaft in the floor direction and protrudes from a corresponding hole in a bottom and side cover of the control panel, depicted in the coffee color. This linking shaft participates in linking this trimmer with the primary P-shaft of the PGS gearbox. The primary shaft has a big exit gear of the red color that can be rotated by a servo gear of the magenta color using an intermediate gear of the dark red color that is meshed with both of these gears. Inside of the case, the primary shaft has a primary pinion of the red color.


  The primary rotating can that I mentioned upon describing the scales, is depicted in the orange color and is mounted on a central axle of the case by using two bearings, which are separated by a small ring of the dark violet color. A two-stages central axle of the sky color is screwed into a central hole of the central axle of the case and provides axial support for the both bearings and for the primary rotating can. A large primary central gear of the orange color is mounted on the tail of the primary rotating can and is meshed with the primary pinion. The glass lid of the light violet color lies inside a corresponding socket of the primary rotating can above a rubber ring of the dark yellow color and is secured by the retaining ring of the pink color. The handle of the dark brown color is inserted through holes in the retaining ring and the primary rotating can into a non-obstructed space between the walls of the case and the primary rotating can. A tube of the dark red color is dressed on a tail of the handle, can freely rotate on it and is secured with a washer and a screw from falling out. When the handle is pulled out, the tube retains it by some friction forces inside of the corresponding hole of the primary rotating can, which also allows the handle not to rotate in the pilot's fingers. Also, the washer prevents detaching the handle from the trimmer.


  All remaining gears of the trimmer aren’t exposed to significant loads. So, they are made of plastic to be cheap. A flanged primary central pinion of the orange color is dressed on the two-stages central axle and is fixed on the bottom of the primary rotating can, utilizing its wide flange for this fixation. The outer rim of its flange also provides its centering on the bottom of the primary rotating can, by entering to a corresponding circular recess. This centering permits to have some clearance between the central hole of the flanged primary central pinion and this central axle, so they aren't in touch. The central axle allows to mount two non-rotating cans in two stages. For this, the axle has two threaded areas for the respective nuts, which are used to clamp the respective cans between them. By such a way, there is a primary steady can of the light violet tint, fixed between the nuts of the gray-blue color, and a secondary steady can of the gray-green color, fixed between the nuts of the light blue color. The first of them bears the steady fine scale, and the second – the steady shield. Each of these cans also has a hole at the bottom for transmitting rotation. A secondary rotating can of the green color is placed inside the primary steady can, having a cluster from a secondary central gear outside and a secondary central pinion inside mounted on it. A secondary rotating can of the green color is placed inside the primary steady can and has a cluster of the same color, attached to it that includes a secondary central gear outside and a secondary central pinion inside. The secondary rotating can bears the intermediate rotating scale. A secondary gear of the yellow color is meshed with the primary central pinion and provides rotation to a secondary pinion by a common shaft inserted into the hole of the primary steady can. The secondary pinion is meshed with the secondary central gear. Similarly, rotation is transmitted inside the secondary steady can: from the secondary central pinion to a tertiary gear of the cyan color through a common shaft with a tertiary pinion meshed with a tertiary central gear of the blue color. The last gear is used to rotate a flange, which bears the rotating shield. Two plastic washers under the respective central gears are used for vertical aligning and slipping. A screw with washer fixes the tertiary central gear in the upper direction on the two-stage central axle.


  The chart above shows design of the S-trimmer, which is almost same as for the P-trimmer and differed by the presence of additional elements for locking in respect of resolving the problem of possible adverse changes of the skew upon changing the gain due to the forces induced on the differential of the SG-compensator. The S-trimmer uses a lockable variant of the primary rotating can, which has at the bottom a skirt for locking. Locking needles of the gold color are screwed into the skirt from outside, using their threaded ends, and are equidistantly distributed outwardly with their sharp ends. The number of these needles is equal to the number of ticks on the rotating fine scale of this trimmer, which corresponds to 0.1° of skew for the locking precision. A locking wedge of the magenta color performs the actual locking when it enters into a respective window in the case upon the force of a spring of the black color, which is dressed on the tail of this wedge and has a back support on the flange of a solenoid of the violet color. This solenoid is mounted inside the trimmer locking bracket of the light blue color and can pull the ferromagnetic tail of the locking wedge into the interior upon powering. A rigid wire of the magenta color is mechanically connected to the tail inside the solenoid and passes outward through some hole in the mounting wall of the locking bracket for the solenoid. This rigid wire has a hooked opposite end on which a soft string is tied. A pulley, pictured by the dashed lines, freely rotates on its axle, fixed in the locking bracket, and guides the soft string toward a skew locking knob.


  A bracket of the skew locking knob, depicted in the light teal color, is mounted under the control panel. A locking knob's shaft of the yellow color enters into an opened to top corresponding hole of the bracket, nesting in the lower part of this bracket and axially fixed by a screw of the light green color. A small spacer ring of the red color is dressed on this screw, allowing the shaft to rotate freely. The upper part of said hole has an increased diameter, so a clearance exists between the shaft and this hole, enough to place here the soft string, which is guided to this direction through a transverse hole by another pulley mounted on its axle in the bracket. The transverse hole encounters the shaft tangentially, and the soft string is wound around the shaft and has a knot inside that fixes it. A large spacer ring of the violet color is dressed on the shaft and enters inside the bracket, consuming the clearance above the soft string and lies on a thin circular step inside the clearance hole. A snapping ball of the pink color enters into the bracket from a corresponding hole and is continuously pushed in by a spring the other end of which is supported by a screw of the orange color into the bracket. Some hole exists in this spacer ring, in which the snapping ball can enter, providing an initial fixation of the spacer ring against rotation. Also, the spacer ring has a final fixation upon mounting the entire skew locking knob under the control panel, which clamps it along its flange. The shaft has two coned nests in which the snapping ball enters for the normal and locking position of the skew locking knob. A handle of the dark yellow color is fixed on the shaft after mounting the entire skew locking knob.


  When the skew locking knob is in the normal position, the soft string is maximally wound onto its shaft, and the locking wedge is out of the trimmer space, permitting the free rotation of the primary rotating can. The shaft is retained in this position by the snapping ball against the increased force of the wedge spring. When the skew locking knob is in the locking position, the soft string is maximally unwound from the shaft, so the remained rotation moment on the shaft is zeroed. If the solenoid isn't powered for this case, the locking wedge enters the trimmer space between a nearest pair of the needles and disables the rotation of the primary rotating can. But, if the solenoid will be powered for the latter case, the locking wedge will be out of the trimmer space, permitting the free rotation of the primary rotating can, so a loose-hanging loop of the soft string will be created. Having this loose-hanging loop isn't a well-secured solution. More than, the unwinding force of the soft string will be missed at all in the case of actuating the knob toward the locking position when the solenoid is powered, which will lead to jamming of the soft string. Thus, there is some intermediate pulley that is pulled by a spring with a low force and which is placed between the two main pulleys to solve this problem. This additional pulley isn't shown for simplicity. The solenoid is powered each time when the servo of its trimmer is powered. Sometimes this can be performed with high frequency. So, it is better to use the knob in the normal position for the non-manual handling to prevent the soft string from wearing out.


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