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Pitot-Static Unit

Technical Information

Catalogue No: C0092
Category: Air Data
Object Type: Sensor/Transducer
Object Name: Pitot-Static Unit
Part No: 81 D 14-A-1
Serial No: 224/68
Manufacturer: Elliott Bros
Division: Unknown
Platform(s):
Year of Manufacture: 1968
Dimensions:
Width (mm):
120 
Height (mm):
130 
Depth (mm):
135 
Weight (g):
940 
Location: Main Object Store
Inscription(s):

P-S Capsule Unit
Type No. 81D14-A-1
Ser. No. 224/68
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WRITTEN NOTE:
"Please handle with extreme care
"Vibrator fitted - very fragile"

Notes:

In September 1957 Elliott Brothers (London), Ltd., showed for the first time their Air Data Computer Mk 1, which constitutes a complete centralized source of aerodynamic data. It is assembled on a unit-construction basis and can be fitted with just the units required for a particular application.

Pitot-static and static pressure are measured in a two-capsule system by an A.C. pick-off whose signal is amplified and used to operate a motor tacho. Potentiometers provide outputs which are used in the electrical computations. Temperature readings are obtained from a resistance probe temperature unit, suitably corrected and computed. All this data is fed to the Air Data Computer. The Mk1 ADC operated from a 115 V, three-phase, 400 c/s power supply and had a consumption and weight, dependent on the particular arrangement, of 160-180 W and about 30 lb. The Mk 2 version weighed only half as much.

A wide choice of outputs was available in either linear or logarithmic form, including height, I.A.S., Mach number, T.A.S., vertical speed, static pressure, pitot-static pressure, ambient temperature, air density and acceleration.

The outputs were available either as voltage or synchro signals or as shaft rotations; and up to four separate signals could be provided simultaneously from each output.

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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