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Air Data System

Airspeed Subsystem Calibration

This subsection presents the theory of a typical airspeed subsystem calibration

Generally, the pressures registered by the pitot-static system differ from free stream pressures as a result of

1) The existence of other than free stream pressures at the pressure source.

2) Error in the local source caused by the pressure sensors.

The resulting error is called position error. Position error may result from error at both the static and total pressure sources. For this discussion, it is assumed that all the error originates at the static pressure source.

The relation of static pressure, assuming an inelastic air vehicle, at any point within the pressure field of the air vehicle, to free stream static pressure depends on Mach number (M), angle of attack (), sideslip angle( ) , Reynolds number (NR) and Prandtl number (Npr).

(The symbol f denotes a functional relationship which is usually different each time it appears.)

Ps= Pressure at the static pressure source

Pa = Free stream static pressure

Defining the position error, as:

(6)

Equation (5) can also be written as:

Sideslip angles are kept small, Npr is approximately constant, and NR effects are negligible as long as the static pressure source is not located in a thick boundary layer. Hence, Equation (7) can be simplified to:

With no loss in generality, this equation can be changed to read:

Where

qcic = Indicated differential pressure,Ps)- lb/ft2

W = Air Vehicle Gross Weight - lb

= Ratio of specific heats, (1.4 for air)

S Wing area - Sqft

The term is termed "Position Error Pressure Coefficient"and is very useful in the reduction of pressure error data. The Static pressure position error causes error in the altimeter and airspeed indicator readings and in the Mach number derived from these quantities. The resulting errors, designated respectively are:

Where:

Hc = Calibrated pressure altitude-ft

Hic = Indicated pressure altitude corrected for instrument error - ft

Where

Vc = Calibrated airspeed - kt

Vic = Indicated airspeed corrected for instrutment error - kt

Mp = Mic - Mc

Where:

Mc = Calibrated or true Mach number

Mic = Indicated Mach number corrected for instrument error

In general, it is more convenient to work with position error correction rather than with the error itself. or:

These corrections are added to the indicated quantities to obtain the calibrated quantities.

Results from the calibrations are presented initially in the conventional form of total and static pressure coefficient versus indicated lift coefficient(CLic) for the low speed region and in the form of pressure coefficient versus indicated Mach number (Mic) for the high speed region These curves are constructed for both the ship and test systems From these pressure coefficient curves, airspeed, altitude and Mach number correction curves are constructed in a convenient form for each air vehicle configuration as described in Section.

Ground Effect

The ship system position error is obtained in ground effect just prior to rotation by the altimeter droop method during continuous takeoff and landing tests. The typical Takeoff and Landing (TOL) camera will be mounted on the air vehicle in a position whereby the air vehicle position along the runway can be correlated with airspeed and altitude The position error is obtained from a time history of air vehicle indicated pressure altitude runway elevationambient temperature and airspeed Runway elevation is obtained from the TOL camera data by deter. mining the air vehicle position along the runway. The position error is the difference between the change in runway elevation and the change in air vehicle pressure altitude. Calibration data for the test and ship systems are presented in the form of Calibrated Airspeed versus Indicated Airspeed Lag effects are not removed from these data since it is more useful to have it included during takeoffs.

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