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Height Lock Transducer

Technical Information

Catalogue No: C0107
Category: Air Data
Object Type: Sensor/Transducer
Object Name: Height Lock Transducer
Part No: HL/10A/270/31
Serial No: 53174
Manufacturer: RW Munro
Division: Unknown
Year of Manufacture: Unknown
Dimensions: Width (mm): 95
Height (mm): 95
Depth (mm): 70
Weight (g): 450
Location: Rack RAA10 [Main Store]

Height Lock
R.W.Munro Ltd London.
[internal marks]
Inst. 53174
[other marks]
Inwards Goods number: IG733479


The object is an aircraft-type panel instrument in black Bakelite with a transparent front. Inside the aneroid capsule, potentiometer and linkage are visible. There are three electrical connections at the side (marked A,B,C) and three barometric connections at rear (marked P, S and blank). These inputs are from the Pitot Tube and the Static Port and they drive an aneroid capsule. The difference between these two pressure inputs gives the dynamic pressure which is a measure of Indicated Airspeed (IAS). The capsule drives a mechanical system that moves a wiper along a wire wound resistor such that an electrical output is given via the three external tags marked ‘A’, ‘B’ and ‘C’. The electrical output may have been used to drive an Altimeter or Airspeed indicator but its title (Height Lock Transducer) would suggest that it was part of the Autopilot system that can be used to maintain a preset altitude.
The instrument was made by R.W.Munro but formed part of a system marketed by Elliott Bros and was in the original Elliott Collection Ref: 2023.

The Air Data Transducer contains an aneroid capsule, or diaphragms, which expands and contracts with the pressure input from the Pitot tube. The case around the diaphragm is airtight and is vented to the static port. The difference between the pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed.
The units have a circular alloy base 6.7cm diam. with four fixing feet protruding. Unit surmounted by Perspex/plastic cylindrical lid. A solenoid within a circular armature and pitot & static barometric connections are visible. The solenoid coil's motion is sensed by an electromagnetic rotary sensor (around the 6-toothed disc).
The current in the solenoid produces a counter-force against that produced by the pitot-static air pressure difference across the diaphragm attached to the coil. External electronics continuously controls the coil current to keep the rotary sensor signal in the middle of its range (probably its null point). With this balance achieved, the coil current is exactly balancing the air-pressure difference and therefore proportional to it. Suitably manipulating the coil current value yields airspeed.

These little units were used in the Autostabiliser on such aircraft as the Lightning and later the VC10.

Air data systems provide accurate information on quantities such as pressure altitude, vertical speed, calibrated airspeed, true airspeed, Mach number, static air temperature and air density ratio. This information is essential for the pilot to fly the aircraft safely and is also required by a number of key avionic subsystems which enable the pilot to carry out the mission. It is thus one of the key avionic systems in its own right and forms part of the essential core of avionic sub systems required in all modern aircraft, civil or military.

The air data quantities; pressure, altitude, vertical speed, calibrated airspeed, true airspeed, Mach number etc. are derived from three basic measurements by sensors connected to probes which measure:

Total (or Pitot) pressure
Static pressure
Total (or indicated) air temperature

The total pressure, PT, is measured by means of an absolute pressure sensor (or transducer) connected to a Pitot tube facing the moving airstream. The Pitot pressure is a measure of ram air pressure (the air pressure created by vehicle motion or the air ramming into the tube). When airspeed increases, the ram air pressure is increased, which can be translated by the airspeed indicator.

The static pressure of the free airstream, PS, is measured by an absolute pressure transducer connected to a suitable orifice located where the surface pressure is nearly the same as the pressure of the surrounding atmosphere. The static pressure is obtained through a static port which most often is a flush-mounted hole on the fuselage of an aircraft located where it can access the air flow in a relatively undisturbed area. Some aircraft may have a single static port, while others may have more than one. When the aircraft climbs, static pressure will decrease.

High performance military aircraft generally have a combined Pitot/static probe which extends out in front of the aircraft so as to be as far away as practicable from aerodynamic interference effects and shock waves generated by the aircraft structure. A Pitot-static tube effectively integrates the static ports into the Pitot probe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of the probe, outside the direct airflow, to measure the static pressure. Some civil transport aircraft have Pitot probes with separate static pressure orifices located in the fuselage generally somewhere between the nose and the wing.

From the measurements of static pressure PT and total pressure PS it is possible to derive the Pressure Altitude, Vertical Speed, Calibrated Airspeed and Mach number. Measurement of the air temperature is made by means of a temperature sensor installed in a probe in the airstream and from this a function called Total Air Temperature can be calculated.

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