Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Sensors / Air Data System
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3.13 Air Data System
3.13.1 Review
Sensors measuring the local atmospheric conditions, post-processed to
obtain pressure height, air speed and Mach number are a constant feature on
aircraft. They are particularly important for generating an estimate of body
incidence required to stabilising fly-by-wire airframes. This function apart,
the plethora of alternative sensors has relegated air data to a reversion role
for dead-reckoning navigation and height keeping in the event of a major
systems failure. This work ignores magnetometers often found in air data
suites included to provide direction relative to the Earth. Air data also
provides an alternative height reference to barometers and NAVSTAR GPS
for stabilising a SDINS. One of its drawbacks in small vehicles is that the
sensors must be positioned in the free air stream and its recovery ratio in
bringing the free stream to rest calibrated. No fundamental change is
expected in these sensors, or their electronics and post processing.
3.13.2 Description
Collinson [C.8] provides a comprehensive description of air data sensors and
associated systems. The sensors measure three atmospheric properties, the
total and static pressures using a Pitot static tube, and the static air
temperature. These reference data are derived in the model shown in Figure
3-62 from the true geodetic height and the Mach Number (a Dryden wind
model is not included).
1
PZ_GDM
2
MACH
NUMBER
REFERENCE
INPUT
PRESSURE HEIGHT
GEODETIC HEIGHT
3.13-1
M_PZ_GFM I_PZ_GFM
PRESSURE HEIGHT
1553 I/F
GUI AIR SPEED M_V_AIR I_V_AIR
MACH NUMBER
MACH N0
AIR DATA REFERENCE, ERROR
AND PROCESSING MODEL
Figure 3-62 : Air Data Model
AIR SPEED
1553 I/F
M_MACH I_MACH
MACH NUMBER
1553 I/F
1
I_PZ_GDM
2
I_V_AIR
3
I_MACH
A transducer measures the total pressure exerted on the Pitot tube. This
comprises free stream static pressure and the pressure required to bring it to
rest. A temperature probe measures the total air temperature: the sum of the
free stream air temperature and the temperature rise in bringing it to rest.
These measurements are processed to obtain geodetic (pressure) height, true
airspeed and Mach number as shown in the decomposition in Figure 3-63.