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Air Data System

Airspeed Subsystem Calibration

This calibration procedure is primarily applied to a total or static system when the system volume is not negligible.

The ground calibration procedure for static systems is to attach a chamber and an altimeter to the static source The chamber is then evacuated to an indicated altitude well above the ceiling of the air vehicle. An adjustable valve is then opened to the atmosphere setting up an indicated rate of sink ( - dh/dt) This rate of sink or the rate of climb established during the evacuation is never larger than rates of climb or sink anticipated in flight. The instantaneous value of applied pressure (measured as pressure altitude is recorded on a photo-panel, time correlated with the on-board recording system which is recording altimeter gage pressure altitude (Hic).

A plot is constructed as in Figure 1 where it can be seen from this plot that:

where:

s = Pressure lag error in the static system - sec

d Hic/dt = Slope of line through Hic data - ft/sec

Note: For this case (Hic1- Hic) and Hidt will both be negative values.

Equation (19) above can be seen to be the same as Equation (1). The static pressure lag error is corrected to sea level standard day conditions by the following form of Equation (4):

Where:

s = Sea level - standard day' static pressure lag error - sec

Ta= Ambient temperature - deg K

Pressure corresponding to Hic - in Hg

The ground calibration procedure for total systems is similar In this case, a pressure tank with an airspeed indicator is attached to the total head to be calibrated and the static Bide of the system and the airspeed indicator is left open to ambient pressure. The tank is pressurized and then the pressure is bled off from the tank to the atmosphere. This process does cause a rate of change of tot”tal pressure to be sensed by the airspeed indicator on the calibrator and the indicator on the air vehicle. These two indicator values are recorded simultaneously.

A plot is then constructed as in Figure 2 where the value can be obtained.

With the assumption that,

and dHic/dt = 0, it is proven that:

Where:

t = Total pressure source pressure lag - sec

t is corrected to Sea Level standard conditions in the same manner ass in Equation (20).

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