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D307-26 · Miscellaneous
c.1955
Object Overview
Before us is a , manufactured approximately around . The instrument belongs to the () type. Its designation stands for () with a measuring range of . VD-20 altimeters were serially produced by , later renamed the . In 1954, a branch of the Ramenskoye OKB-149 design bureau was established at the plant; it was later transformed into the independent . In 1961, Plant No. 280 officially became the Ulyanovsk Instrument-Making Plant, and in 1990 the production association was renamed .
611143
self-elastic stacked capsule movement
aluminium, beryllium, brass, copper, glass, steel
The instrument was designed to determine the relative barometric altitude of an aircraft — that is, its altitude relative to a selected reference level: the departure airfield, the destination airfield, or another point for which the barometric pressure is known. Its operating principle is based on the predictable decrease in absolute atmospheric pressure with increasing altitude. In effect, the altimeter is a highly accurate aneroid barometer whose scale is graduated not directly in pressure units, but in units of altitude. According to the official technical description of the VD-20, its measuring range extends from 0 to 20,000 m.
The instrument is housed in a substantial cylindrical metal case designed for mounting behind an aircraft instrument panel. Judging from the casting, relatively low weight, and appearance of the metal surface, the case is made from a light aluminium alloy. The outer cylindrical section has a grey protective paint finish with a pronounced fine wrinkle-textured surface. The front mounting section and the bezel surrounding the glass are finished in matt black.
The cylindrical form is determined by the internal arrangement of the instrument: almost the entire mechanism is positioned along its longitudinal axis directly behind the dial, while the front is formed as a square mounting flange with rounded corners. This construction allowed the altimeter to be installed in a standard opening in the instrument panel, leaving only the dial, glass, and setting knob visible to the pilot.
At the front, the case is closed by a flat protective glass. In accordance with the factory construction of the VD-20, the glass rests on a rubber gasket and is pressed against the case by an outer flange. On this example, the flange is secured by eight screws arranged around its perimeter.
A pressure connection fitting is located on the rear wall of the case. Through this fitting, the interior of the altimeter is connected to the aircraft’s static pressure line. Static atmospheric pressure is therefore admitted into the sealed case and acts upon the sensitive aneroid capsule assembly.
The rear cover of this example bears an unusual raised factory emblem. At its centre is a stylised representation of a circular measuring instrument with a scale, flanked by wings; below it is a small oval element containing a coded sign.
Emblems of this type belong to a still poorly studied post-war system of markings used by Soviet aircraft-instrument manufacturers. Their symbolism is entirely appropriate to the system of the MAP — Ministry of Aviation Industry of the USSR: the circular scale represents instrument manufacture, while the wings represent the aviation industry. The lower mark was apparently a code identifying a particular factory. A consolidated official key to these codes has, to date, not been published in open sources.
The emblem is therefore best regarded as a coded factory mark of an aircraft-instrument enterprise within the Soviet MAP system, characteristic of the mid-20th century. The VD-20 itself is, in any case, documented as part of the production range of Plant No. 280 in Ulyanovsk.
The dial is made from a thin stamped aluminium plate. Its front surface is coated with a deeply matt black instrument enamel or lacquer. This finish was primarily functional: it suppressed unwanted reflections from the glass and cockpit lighting and provided maximum contrast between the background and the scale markings. In production, the aluminium base may have been chemically oxidised or anodised beforehand to improve corrosion resistance and the adhesion of the subsequent paint layer; however, the precise coating process used specifically for the VD-20 dial is not described in the available sources.
A substantial proportion of the numerals, principal graduations, and both pointers are coated with a pale yellow-green photoluminescent compound of temporary action. Such a dial, using a light-storing luminous material, was intended to be used in conjunction with continuous ultraviolet illumination of the aircraft instrument panel, using lamps whose principal emission was invisible to the human eye.
Accordingly, normal night-time operation of this dial did not require a radium-based radioactive luminous compound. Soviet aircraft employed special ultraviolet instrument-panel lamps whose radiation excited the photoluminescent material applied to scales, pointers, and inscriptions. As a result, only the necessary markings glowed, while the surrounding cockpit remained dark.
The dial carries two concentric altitude scales, each served by its own pointer.
The outer scale indicates metres. One complete revolution of the large pointer corresponds to a climb of 1,000 m. The numerals indicate hundreds of metres — from 0 through 1, 2, 3 and so on to 9 — after which the next complete revolution begins again at zero. There are ten subdivisions between each adjacent hundred-metre mark, so each small division represents 10 m.
The inner scale indicates altitude in kilometres and covers the range from 0 to 20 km. The small pointer makes one complete revolution over the entire measuring range of the instrument — 20,000 m. The transmission ratio between the two pointer mechanisms is 1:20: during one full revolution of the small pointer, the large pointer completes twenty revolutions. The simultaneous position of both pointers therefore allows the pilot to read both the total altitude in kilometres and the remaining hundreds and tens of metres.
A distinctive feature of the altimeter is the presence of two additional movable triangular indexes pointing downward. One is positioned against the outer metre scale, the other against the inner kilometre scale.
These indexes are linked to the barometric setting mechanism. When the setting knob is rotated, the barometric scale turns and both indexes move simultaneously. They indicate, on the corresponding altitude scales, the altitude equivalent of the change in barometric pressure relative to the adopted standard pressure of 760 mmHg. One index expresses this correction in metres, the other in kilometres.
A large knurled setting knob is located at the lower left of the front panel. It allows the altimeter indication to be matched to the actual barometric pressure at a selected reference point.
On conventional Western aircraft altimeters, the same function is performed by the so-called Kollsman window: the pilot turns a knob to set the required pressure in a small window, while the mechanism simultaneously shifts the relationship between the scale and pointers. The VD-20 uses a different visual implementation of the same principle. Instead of a small rectangular window, it has a rotating barometric scale in the lower part of the dial. The setting knob is mechanically linked to this scale, to the pointer system, and to the two indexes described above. Turning it therefore changes the barometric reference level from which altitude is measured.
If altitude is to be measured relative to the departure airfield, the pointers are set to zero using the known local pressure. To determine altitude relative to the destination airfield, either the barometric pressure at that airfield or its elevation above sea level together with the corresponding barometric correction must be known. During long-distance flight, pressure information for the destination or an intermediate point could be transmitted to the crew by radio; the pilot would then set the required value using the setting knob, and the altimeter would indicate altitude relative to the selected point.
The factory manual specifically describes the flight-level mode: to maintain an assigned flight level, the barometric scale was to be set to 760 mmHg, or the indexes aligned with the zero-altitude mark. In other words, the instrument was then referenced to a common standard pressure used by all aircraft, allowing safe vertical separation to be maintained.
At the heart of the VD-20 is an aneroid capsule assembly consisting of two elastic capsules. Each capsule is formed from two thin corrugated metal diaphragms hermetically joined around their circumference. Static atmospheric pressure, admitted into the case through the pressure fitting, acts on the diaphragms from the outside.
Near ground level, pressure is at its greatest and the capsules are therefore in their most compressed state. As the aircraft climbs, atmospheric pressure decreases, the external force compressing the diaphragms becomes smaller, and the elasticity of the corrugated membranes causes the capsules to expand. Their deformation therefore depends directly on changes in atmospheric pressure and, consequently, on altitude.
The expansion and contraction of the aneroid capsule assembly is microscopic in comparison with the movement required at the pointers and must therefore be amplified considerably. This is achieved by a complex system of links, levers, a toothed sector, pinions, and gears.
Movement of the movable centre of the aneroid capsule assembly is transmitted through a link and fork to the sector shaft. The toothed sector rotates together with the shaft and remains in constant engagement with a pinion. A gear wheel is rigidly mounted on this pinion and engages another pinion, on whose shaft the large pointer is mounted. In this way, the very small movement of the diaphragms is greatly amplified and converted into rotation of the large pointer over the outer metre scale.
The small kilometre pointer is mounted on its own hollow concentric shaft. It is connected to the main mechanism through an additional gear train with a ratio of 1:20, consisting of several gears and pinions. As a result, the large pointer makes twenty revolutions during one revolution of the small pointer. This arrangement allows both pointers to be mounted concentrically while simultaneously displaying two different orders of magnitude — hundreds of metres and tens of kilometres — on a single dial.
Additional elastic elements are incorporated into the transmission mechanism to eliminate backlash. Radial clearances are taken up by a hairspring fitted to one of the pinions, while axial clearances are adjusted by the position of the centring screw and associated mounts. This ensures the most precise and unambiguous pointer position possible despite the extremely small movements of the aneroid diaphragms.
The indication of an aneroid altimeter depends not only on pressure but also on temperature: as temperature changes, the modulus of elasticity of the diaphragm material changes, and therefore so does its deformation under the same pressure. To reduce this error, the VD-20 employs two separate temperature compensators.
The first-order compensator is located at the movable centre of the sensitive element and is constructed as a bimetallic component. In VD-20 documentation it is described as a bimetallic roller. Its temperature-dependent movement compensates for changes in the position of the diaphragm centre caused by changes in membrane elasticity. In effect, this compensator corrects the initial shift in indication that would otherwise occur approximately uniformly across the scale as temperature changes.
The second-order compensator is installed on the sector shaft and affects not merely the initial position of the system but its transmission characteristic. As the temperature changes, it alters the effective geometry of the crank-and-link mechanism and thereby compensates for the temperature error that increases with altitude. In general descriptions of aircraft altimeters, such compensators consisted of bimetallic elements made from materials with different coefficients of thermal expansion — in particular, steel and Invar.
This dual compensation clearly demonstrates the mechanical sophistication of the VD-20. It is not merely a barometer with an altitude scale, but a specialised aircraft measuring instrument designed to maintain acceptable accuracy across a very wide range of both pressure and temperature. The factory specification provided for operation of the VD-20 at ambient temperatures from +50 to −60 °C, placing particularly high demands on both the sensitive element and the transmission mechanism.
The VD-20 barometric altimeter is a characteristic and technically expressive example of Soviet aircraft instrumentation from the mid-20th century. Behind its comparatively restrained black dial lies a complex, entirely mechanical measuring system: two sealed aneroid capsules, a lever-and-gear transmission, a two-pointer reduction mechanism, an adjustable barometric setting system, two altitude indexes, and dual bimetallic temperature compensation.
The instrument was designed not simply to display atmospheric pressure in arbitrary units, but for direct operational use by the pilot in determining altitude relative to the place of departure or landing, relative to a selected barometric datum, and during flight at a standard pressure level. The rotating barometric scale performed a function analogous to the Kollsman window of Western altimeters, while the photoluminescent markings allowed the instrument to be read confidently in a darkened cockpit under ultraviolet illumination.
The combination of its substantial sealed case, exceptionally clear two-pointer indication, characteristic green luminous compound, complex mechanical construction, and coded emblem of a Soviet aircraft-instrument manufacturer makes this example not only a functional measuring instrument, but also a vivid surviving witness to an era in which demanding flight-instrument tasks were solved entirely through precision mechanics.