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Static Air Pressure Transducer

Technical Information

Catalogue No: C0289
Category: Air Data
Object Type: Sensor/Transducer
Object Name: Static Air Pressure Transducer
Part No: 3D753
Serial No: 1014/67
Manufacturer: Elliott Bros
Division: Unknown
Platform(s): Buccaneer  Lightning 
Year of Manufacture: 1967
Dimensions:
Width (mm):
165 
Height (mm):
125 
Depth (mm):
250 
Weight (g):
3,790 
Location: Main Object Store
Inscription(s):

Elliott
S. Transducer Mk.2. Type A
Type No. 3D753
Ref No. 6A/6826
Ser. No. 1014/67

Notes:

From ZF581, the former Gate Guardian at Rochester.

This Static Transducer unit was designed for and fitted to either the English Electric Lightning or the Blackburn Buccaneer.
The unit contains the pressure transducer which converts the Static pressure, from the aircraft sensor, to an electrical analogue. The static pressure system is open to the exterior of the aircraft to sense the pressure of the atmosphere at the altitude at which the aircraft is flying.
The difference between the Pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed. So the Static Transducer working with the Pitot unit has the capability to provide an output of indicated airspeed, limits on the altitude and also to manage the maintenance of a specific altitude; the height lock function. The Pitot and Static transducers will drive a cockpit instrument for indicated airspeed but will also feed into the Autopilot and automatically control the flying surfaces to maintain a pre-set speed or altitude.
The Differential Pressure Transducer within the box contains two aneroid capsules, 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 use of multiple aneroid capsules is to multiply the movement. The movement of the diaphragm is mechanically coupled to a non-linear cam assembly with two rows of fifteen adjustable grub screws to set the desired profile. Within the casing is a wire wound resistor which is driven by the cam mechanism such that an electrical output of indicated airspeed is provided corrected by the cam profile for the non-linearity of the diaphragm movement. A small servo motor was used to amplify the movement of the capsules to operate the internal gear train.

This is an early version of the Static Transducer probably for the first production run of Lightnings. The relationship between ‘Type’ and ‘Mk’ numbers is not known but it would seem that ‘Type’ (in this case ‘Type A’) can have incremental ‘Mk’ numbers; this is a ‘Mk2’.

This item also has a mod label:
"Mod No.
T.No. 3D753
S.No. 1014/67
A.D.S. Nos 72-88-109
118-124-132 136-148-169
170"

And has a white circular stamp: "EBL627"

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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