Avionics Navigation Systems, 2nd Edition

8 Air-Data Systems

8.1 INTRODUCTION

An air-data system consists of aerodynamic and thermodynamic sensors and associated electronics. The sensors measure characteristics of the air surrounding the vehicle and convert this information into electrical signals (via transducers) that are subsequently processed to derive flight parameters. Typical flight parameters calculated by air vehicles include calibrated airspeed, true airspeed, Mach number, free-stream static pressure, pressure altitude, baro-corrected altitude, free-stream outside air temperature, air density, angle of attack, and angle of sideslip. This information is used for flight displays, for autopilots (flight-trajectory control and control-loop gain adjustment), for weapon-system fire-control computations, and for the control of cabin-air pressurization systems.

Air-data systems are an outgrowth of the airspeed indicator and altimeter used in early aviation. In those primitive, pneumatically driven instruments, the computation was performed by nonlinear spring mechanisms incorporated into specially designed bellows, which expanded or contracted in response to changes in sensed pressures, thereby moving the dials of the flight instruments. In the late 1950s, analog computers were interposed between the pressure sensors and flight instruments. The transducers and computation elements found in typical analog air-data computers of that era were documented in the first edition of this book [27]. In those designs, servo-driven ...