Method of and apparatus for using an alternate pressure to measure mach number at high probe angles of attack

Abstract

A method of and apparatus for obtaining angle of attack, local static pressure and local pitot pressure from measured pressure values at ports of a probe having angle measuring ports with angle measuring axes lying in a plane and a pitot port having an axis lying along an axis in the plane bisecting the angle between the measuring ports. A table of known values of angle of attack relative to measured pressure ratio at measured Mach numbers determined by P,/P,is stored in memory and used when P,is greater than the pressure sensed at the pitot port (P,), P,is the pressure measured at the one of the angle measuring ports that is higher than the pressure measured at the other angle measuring port, and P,is an average of the pressures at the angle measuring ports. The correlation of the ratio of measured pressures on the probe relative to angle of attack is determined using the known values P,/P,at different angles of attack either in wind tunnel tests or by computer simulation. The known values are stored in a look-up table in an onboard air data computer. The values that have been stored then can be retrieved to solve for the correct angle of attack, using the measured pressures P,, P,and P,. The angle of attack values are used with an algorithm to determine local pitot and static pressure.