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Air Data System

Airspeed Subsystem Calibration

A typical airspeed calibration is processed using a trailing cone example and the following method. In-flight calibration data was normalized in the form:

DP/qcic versus CLic

Where:

DP = Static pressure error

qcic = Pt - Ps = Differential pressure (PSF)

Pt = Total pressure (PSF)

NOTE: Pt = Pt' for subsonic flight less than .6 Mach

Ps = Static pressure (PSF)

CLic = W/ (qcic x Sw) = Coefficient of lift

W = Aircraft gross weight (lbs)

Sw = 1234.5 = Wing area (ft2) (example)

Data processing will involve the following steps:

DP = Ps - Ps(cone) = Ship to cone static differential pressure corrected for instrument error and converted to PSF from inches of H20 (PSF)

Hic = HXTS = Cone pressure altitude (coarse)

Ps(cone) = PaSL[(1-6.87535 x 10-6) x Hic]5.2561 = Cone static pressure (PSF)

PaSL = 2116.216 PSFDP/qcic(cone) = +.01 = Cone

( The +.01 is used for illustration purposes.)

calibration position error coeff.

qcic = Pt - Ps(cone) = DPPS or DPCS = Differential pressure - system total to cone

static DP(cone) = P/qcic(cone) x qcic = Cone calibration converted to differential pressure (PSF)

Pa = Ps(cone) - DP(cone) = Ambient pressure (PSF)

Ps = DP + Ps(cone) = Static pressure (PSF)

qcic = Pt - Ps = DPTS or DCTS = Differential pressure - system total to system static

DP/qcic = DP C qcic

CLic = W/ (qcic x Sw) = Coefficient of lift

W = Aircraft gross weight (lbs)

Sw = 1234.5 = Wing area (ft2)

The above procedure was utilized to produce DP/qcic versus CLic curves. The procedure is applicable to pilot's and copilot's pitot- static systems.

The DP/qcic versus CLic curves were utilized during data expansion to provide airspeed and altitude position error corrections. Expansion of the DP/qcic curves into airspeed and altitude position error correction involve the following steps:

Vic = Assumed indicated airspeed (KIAS)

GW = Assumed gross weight (lbs)

Hic = Assumed pressure altitude (ft)

PaSL = 2116.216 = Ambient pressure at sea level (PSF)

Sw = 1234.5 = Wing area (ft2)

Ps = PaSL{1-[(6.87535 x 10-6) x Hic]} = Static pressure (PSF)

qcic = PaSL{[1+.2(Vic/661.48)2]3.5-1} = Differential pressure (PSF)

CLic = GW / (qic x Sw) = Coefficient of Lift

DP/qcic = From flight test curves at CLic

DP = qcic x DP/qcic

qc = qcic + DP = Compressible dynamic pressure (PSF)

Pa = Ps - DP = Ambient pressure (PSF)

Vc = 661.48 5[(((qc/PaSL)+1)1/3.5)-1] = Calibrated airspeed (KCAS)

DVpc = Vc - Vic = Airspeed position error correction (KCAS)

Hc = [1-(Pa/PaSL)1/5.2561]/(6.87535 x 10-6) = Calibrated pressure a ltitude (ft)

DHpc = Hc - Hic = Altitude position error correction (ft)

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