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Object Overview
Before us is a Polish Aviation airspeed indicator, the PZL PR-250S, belonging to the group of diaphragm-type flight and navigation instruments and manufactured in 1978. The instrument was intended primarily for gliders and light aircraft and serves to determine indicated airspeed from the difference between total and static air pressure. Its operating range is 50–250 km/h, with major graduations at 10 km/h intervals. The instrument was housed in a standard 80 mm diameter case, weighed no more than 0.4 kg, and had an error of approximately ±3 to ±10 km/h depending on the part of the scale and the temperature.
The instrument was manufactured by the Warsaw enterprise . The history of this specialised factory began on 16 February 1952, when, by order of the Polish Minister of Heavy Industry, WSK "Warszawa II" was established on the basis of several enterprises located on Podskarbińska Street in Warsaw. One of its earliest fields of production was precisely diaphragm-type aviation instruments and their sensitive elastic elements — diaphragms and bellows. The enterprise subsequently became one of Poland's principal manufacturers of flight and navigation equipment: altimeters, airspeed indicators, variometers, gyroscopic, electrical, and magnetic instruments.
B7806037
self-elastic capsule movement
aluminium, brass, copper bronze, enamel, glass, plastic, steel
The enterprise received its modern designation PZL-Warszawa II in the mid-1970s. In 1970 the trademark Delta was introduced into its name, and on 14 January 1975 it was replaced by the PZL mark. The State Archives in Warsaw even preserve a separate 1975 file concerning the replacement of the trademark "Delta" by "PZL" in the name of the enterprise. The PZL emblem on the data plate of this instrument therefore does not denote the name of a specific manufacturer: in socialist Poland it was used as a common industry-wide mark of the unified aviation industry and was incorporated into the names of various PZL enterprises.
The designation PR-250S reflects the purpose of the instrument. The letters PR derive from the Polish prędkościomierz — "airspeed indicator"; the number 250 corresponds to the upper limit of the scale — 250 km/h. The letter S denotes the glider version (szybowcowy), in accordance with the PZL system of designating specialised glider instruments. In factory and operating documentation this model regularly appears as PR-250S seria B — "PR-250S, Series B".
In operating principle and purpose, the PR-250S is the Polish functional equivalent of the Soviet US-250 airspeed indicator. Both instruments are mechanical differential pressure gauges that convert dynamic air pressure into an airspeed indication and have an upper limit of 250 km/h. It would, however, be inaccurate to regard the PR-250S as a literal copy of the US-250: for example, the documented range of the PR-250S begins at 50 km/h, whereas the scale of the Soviet US-250 could begin at 10 km/h. Soviet technical literature describes the US-250 according to the same basic principle: its sensitive element is a pressure capsule located inside a sealed case and responding to the difference between total and static pressure.
This relationship between the instruments should be viewed within the broader context of the post-war Polish aviation industry. During the first post-war decades, a substantial part of that industry did indeed develop on the basis of Soviet licences and technical cooperation. Polish factories produced, for example, the MiG-15 and MiG-17 under the Lim designations, the An-2, Soviet-type engines, and a large quantity of aviation equipment. At the same time, within the Council for Mutual Economic Assistance, individual countries gradually developed specialisations in particular types of production.
In the aviation industry, some of Poland's most important specialisations became light aircraft, gliders, agricultural aviation, helicopters, and aviation instruments. The Polish Aviation Museum explicitly states that within the Comecon Poland was assigned the role of a producer of agricultural aircraft and helicopters. Between 1960 and 1990, Polish enterprises manufactured about 15,000 aircraft and helicopters in agricultural versions, most of which were exported. At the turn of the 1970s and 1980s, PZL-Mielec became the world's largest manufacturer of general-purpose and specialised aircraft of this class.
One of the most characteristic examples of this division of production was the Mi-2 helicopter. The aircraft was designed in the USSR by the Mil Design Bureau, but series production was transferred entirely to the Polish WSK Świdnik factory. The first Mi-2 manufactured entirely in Poland flew in November 1965; more than 5,400 aircraft were ultimately built at Świdnik, most of them supplied to the Soviet Union. The Mi-2 thus became a unique Soviet helicopter that was not serially produced in the USSR itself, but exclusively in Poland.
Within this production system, aviation instruments also became an important Polish specialisation. WSK PZL-Warszawa II manufactured practically the complete range of mechanical instruments for Polish gliders: airspeed indicators, altimeters, variometers, compasses, and gyroscopic instruments. As early as 1975, the Polish professional press noted that Polish glider instruments had gained an international reputation owing to their good quality combined with a relatively low price.
The PR-250S became an almost standard airspeed indicator on mass-produced Polish SZD gliders. Its use is documented, among others, on the SZD-9bis Bocian, where a 1978 publication explicitly lists the PR-250S among the standard PZL instrument equipment; on the SZD-41A Jantar Standard, whose 1975 manual refers to the prędkościomierz PR-250S seria B; on the SZD-51-1 Junior, where the same model is shown in the instrument installation diagram; on the PZL KR-03A Puchatek, for which the PR-250S is a mandatory instrument; and on the SZD-50-3 Puchacz, where it was also included in the standard equipment.
The entire instrument is enclosed in a substantial black cylindrical case with a standard mounting diameter of approximately 80 mm. A Polish technical publication from 1975 explicitly describes the PR-250S housing as a standard Bakelite case.
In this context, the historical term Bakelite should not necessarily be understood as referring specifically to material sold under the Bakelite trademark, but rather to a thermosetting phenol-formaldehyde moulding compound — phenolic resin moulding compound. The material exhibits features clearly visible on this specimen: a thick-walled black moulded shell, a hard slightly matt surface, a fine moulding texture, and the absence of a metallic substrate beneath scratches. Unlike thermoplastics, phenol-formaldehyde moulding compound cures irreversibly after hot moulding and possesses high rigidity, good dimensional stability, and resistance to heat, making it a suitable material for aviation instrument cases.
At the front, the case is closed by a substantial flange secured with four large mounting screws. Behind it is a circular protective glass separating the dial and mechanism from the cockpit environment. The instrument itself is sealed: static pressure enters the interior of the case only through the dedicated connection.
The rear wall carries two pneumatic connections marked with the letters C and S:
C — ciśnienie całkowite, total pressure;
S — ciśnienie statyczne, static pressure.
The C connection was connected to the total-pressure receiver — a Pitot tube or combined air-pressure probe. The S connection was connected to static ports on the surface of the glider, positioned in an area where disturbance of the airflow was minimal. Fine protective screens are fitted inside the connections to prevent contamination from entering the narrow pneumatic passages of the instrument.
The dial consists of a thin aluminium plate coated on its front surface with a black, fine-textured, matt anti-glare paint finish. The textured surface reduces specular reflections from the glass and improves the legibility of the scale in bright sunlight.
Against the black background appears the inscription PRĘDKOŚĆ — "speed" — with the unit marking km/h ×10 below. The scale figures are read with a multiplier of ten: 5 corresponds to 50 km/h, 10 to 100 km/h, 20 to 200 km/h, and 25 to 250 km/h. Beyond the 200 km/h mark, the final values of 220 and 250 km/h are transferred to an additional inner arc, allowing the full range to be accommodated within the standard 80 mm case while retaining large and easily legible numerals.
The large numerals, principal graduations, and working portion of the pointer are covered with a thick cream-yellow coating with a characteristic slightly granular surface. This is not ordinary paint. The technical description of the PR-250S family specifies for the PR-250S-B version a coating of the marks and pointer described as self-lighting paint of permanent operation. This is a radioluminescent compound intended to make the indications continuously visible in darkness without prior "charging" by an external light source.
For a late example dating from 1978, the combination of evidence allows this compound to be identified with a high degree of probability as a composition based on promethium-147 (^147Pm) and a zinc sulphide, ZnS, phosphor. In radioluminescent coatings of the second half of the twentieth century, Pm-147 gradually replaced the considerably more hazardous Ra-226, while ZnS was one of the most widely used phosphors. Sources dealing with radioluminescent coatings specifically record the use of the combination of ^147Pm and zinc sulphide, while a 1976 standard for radioactive luminous pigments used in Eastern Bloc countries likewise describes materials containing ^147Pm.
Promethium-147 is a relatively soft β-emitter and, for a museum object, has a very short half-life of approximately 2.62 years. If the luminous compound was applied in 1978, then over the almost 48 years that have since elapsed approximately 48 ÷ 2.62 ≈ 18.3 half-lives have passed.
After 18.3 half-lives, only about 3 × 10^-6 of the original Pm-147 remains, corresponding to approximately 0.0003% of the original activity.
The original Pm-147 has therefore now almost completely decayed, which is consistent with the fact that an ordinary Geiger counter shows almost no increase above natural background when measuring this specimen.
The ZnS phosphor, however, physically remains in the paint. When the dial is illuminated with an ultraviolet torch, the numerals, major graduations, and pointer once again glow brightly with a characteristic green light. When the instrument was new, this same green luminescence was excited continuously by the β-particles emitted by Pm-147, without any external light source.
Originally, the luminous compound was probably considerably paler, with a cream or yellowish-green appearance. Over the decades, the organic binder and the phosphor itself have aged, causing the surface to acquire its present warm yellowish-cream tone and a slight local unevenness.
Inside the PR-250S is a completely mechanical pneumatic measuring system requiring no electrical power. Its principal sensitive element is a large differential pressure capsule, which occupies a substantial part of the internal volume of the instrument.
The capsule consists of two thin, circular, elastic diaphragms joined around their outer circumference to form a sealed internal cavity. Each diaphragm has a large number of concentric annular corrugations. These corrugations perform several functions at once: they increase the compliance of the diaphragm, provide a sufficiently large axial displacement in response to a very small pressure difference, and make its deformation more predictable and repeatable.
In this instrument, the pressure capsule has a characteristic copper-bronze colour. Thin corrosion-resistant copper alloys — bronzes or related spring alloys — were traditionally used for highly elastic measuring diaphragms of this type. From the very beginning, WSK Warszawa II specialised specifically in the manufacture of measuring diaphragms and bellows; the modern successor to this production likewise lists copper alloys, bronzes, and steels among the traditional materials used for elastic elements.
The internal cavity of the pressure capsule is connected through a thin copper tube to the C connection, and therefore to total pressure. The space inside the sealed case surrounding the capsule is connected to the S connection and is maintained at static pressure.
Thus, the inner surfaces of the diaphragms are subjected to total pressure, pt, while externally they are subjected to static pressure, ps.
When the aircraft or glider is stationary, pt ≈ ps, and there is practically no pressure differential.
When the aircraft moves relative to the air, the total-pressure probe facing the airflow brings the incoming air to rest. As a result, total pressure becomes greater than static pressure: pt = ps + q, where q is the dynamic, or velocity, pressure.
The pressure capsule therefore responds only to the difference between these pressures: Δp = pt - ps = q.
At sufficiently low speeds, where the compressibility of air may be neglected, Bernoulli's equation gives q = ρV² / 2, where q is dynamic pressure, ρ is air density, and V is true velocity relative to the surrounding air mass.
The corresponding relationship for velocity is V = √(2q / ρ).
For this reason, an airspeed indicator is, in its physical essence, a differential pressure gauge, rather than an instrument that directly "measures kilometres per hour". It measures the difference between two air pressures, while its mechanical transmission and the form of the scale convert this pressure differential into a speed indication.
As pt - ps increases, the pressure capsule expands axially. The displacement of its central portion amounts to only fractions of a millimetre and therefore has to be greatly amplified. This is achieved by means of a fine linkage connected to a system of levers and a toothed transmission.
Photographs of the dismantled mechanism clearly show the vertical link running from the centre of the pressure capsule to the transmission assembly. It moves a small lever, which in turn transmits the motion to a toothed sector engaging with a small pinion on the pointer spindle. This combination of lever and toothed sector greatly magnifies the slight linear motion of the diaphragm and converts it into a substantial angular movement of the pointer.
The transmission incorporates a spiral hairspring, clearly visible in the upper part of the mechanism. It applies a small constant torque that takes up backlash between the teeth of the sector and pinion. As a result, the pointer does not flutter within the clearances of the gearing and reproduces readings consistently both as speed increases and decreases. A similar principle of eliminating backlash by means of a flat spiral spring was used in other diaphragm instruments manufactured by WSK PZL-Warszawa II and is explicitly described, for example, in the technical manual for the W-12S altimeter.
The mechanism contains several adjustment points. During factory calibration, adjusting screws allowed the zero position, effective lengths of individual levers, and transmission ratio to be altered. After final adjustment, some of these screws were secured with bright red inspection lacquer. Such red seals are well preserved on this specimen and indicate that the mechanism was not intended to change its calibration inadvertently during service.
An additional small spiral spring is associated with temperature compensation and the kinematic characteristic of the mechanism. It is necessary because the elasticity of the diaphragm material and the properties of the transmission components vary slightly with temperature. In addition, the relationship between speed and dynamic pressure is not linear: V is proportional to the square root of Δp.
If the movement of the capsule were transmitted to the pointer in a perfectly linear manner, equal increments of pressure would produce equal angular movements, whereas the airspeed scale must follow the quadratic relationship. The geometry of the levers and sector is therefore selected so as to produce the required non-linear characteristic.
It is also important to understand the meaning of the term indicated airspeed. The PR-250S is not calibrated to determine directly the true velocity of the aircraft relative to the surrounding air mass, but rather according to the standard relationship between dynamic pressure and airspeed. Its indication corresponds to the speed the aircraft would have at the measured dynamic pressure under standard atmospheric conditions. As altitude increases, actual air density decreases, so the true airspeed corresponding to the same indicated reading becomes higher. For navigational calculations, corrections must therefore be introduced for density, temperature, altitude, as well as instrumental and aerodynamic errors.
The PZL PR-250S is a characteristic example of the mature Polish aviation instrument industry of the 1970s. Despite its outward simplicity, it is a high-precision, entirely mechanical measuring instrument combining several specialised technologies: a sealed case made of phenol-formaldehyde moulding compound, a thin aluminium anti-glare dial, radioluminescent night markings, a precision corrugated pressure capsule, and a miniature lever-and-gear transmission.
At the same time, the instrument reflects the broader industrial context of its period. It emerged within the system of close technical cooperation between the countries of the Comecon, but already belongs to an era in which Poland possessed an independent and highly developed tradition of glider manufacture and onboard equipment production. The PR-250S became one of the standard instruments used by several generations of Polish gliders and was exported with them far beyond Poland.
This example, manufactured by WSK PZL-Warszawa II in 1978, retains not only its original mechanism and Bakelite case but also its characteristic luminous scale. The Pm-147, which has almost completely decayed over the intervening decades, can no longer sustain the original continuous glow, yet the surviving phosphor still reacts brightly to ultraviolet light. The instrument therefore remains both a technical monument to classic pneumatic aviation instrumentation and a vivid record of the materials and technologies employed in European aviation during the second half of the twentieth century.