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Force Balance Mechanism

Technical Information

Catalogue No: C0105
Category: Air Data
Object Type: Module/Sub-Assembly/Component
Object Name: Force Balance Mechanism
Part No: None
Serial No: None
Manufacturer: Elliott Bros
Division: Unknown
Platform(s): Buccaneer  Lightning  VC10 
Year of Manufacture: circa 1965
Dimensions:
Width (mm):
108 
Height (mm):
45 
Depth (mm):
120 
Weight (g):
610 
Location: Main Object Store
Inscription(s):

None

Notes:

Carrier force balance (electrical) mechanism with (possibly) a barometric connection. Rectangular electrical connection. The metal base plate 12mm thick.

An experimental prototype that probably led to the related item. It was originally in the Marconi Collection Ref: 2021

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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