Measuring and testing – Navigation – Take-off and landing monitors
Reexamination Certificate
1999-02-22
2001-10-23
Oen, William (Department: 2855)
Measuring and testing
Navigation
Take-off and landing monitors
Reexamination Certificate
active
06305218
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of utilizing measured pressures from pressure sensing ports on an air vehicle for obtaining accurate indications of static pressure, pitot pressure, and angle of attack at very high angles of attack encountered in high performance air vehicles.
In the past, the problems of obtaining accurate measurements from air data sensors or probes during very high angles of attack have been recognized, and different approaches have been used for obtaining correction factors. For example, U.S. Pat. No. 5,205,169 provides for inverting the ratio of pressure differentials from sensing ports at high angles of attack, generally above 450°. In U.S. Pat. No. 5,205,169 the normal angle of attack ratio (P
1
-P
2
)/q
cm
) is inverted to be q
cm
/(P
1
-P
2
) when the denominator q
cm
approaches zero.
Other methods have been used, such as an iterative calculation of measured pressures which requires substantial computer capacity for carrying out accurate analysis of the measured pressures.
SUMMARY OF THE INVENTION
The present invention relates to a method for correcting measured pressures at high angles of attack, by determining Mach number using an alternate pressure signal and providing in the computation, derived correction factors from a calibrated “look-up” table in an air data computer. The calibration data or factors are either directly obtained from wind tunnel tests of the sensor probe configuration in actual operational use, or from computer simulations of such sensors at various Mach numbers and angles of attack.
The calibration values are used in a conventional manner for determining accurate angle of attack, static pressure, and pitot pressure from the measured pressures. The pressures that are measured at the ports are correctable up to angles of attack of in the range of 650°.
The alternate pressure signal is obtained from an angle of attack measuring port located on the sensing probe at a location where pressure increases as angle of attack increases, and the measured alternate pressure is used when the pressure from one angle sensing port of an angle of attack sensor is larger than the measured pitot pressure. The conventional local flow calibration approach is used when the pressure of the angle measuring ports is smaller than the measured pitot pressure.
The method of the present invention is applicable at angles of attack of between about 30° and 65°. Different shaping of the probe could extend the use of the present invention at angles up to about 90°.
The calibration values are provided for measured pressures indicating Mach numbers at angles of attack above about 35°. The calibration values can be easily placed in memory of an onboard computer to derive accurate readings of angle of attack, static pressure, and pitot pressure from the measure pressures.
The method is used in general with a cylindrical probe having angle sensing ports on the top and bottom of the probe, but it also is useful for flush mounted probes having angle of attack sensing ports that are sensitive to a change in angle of attack, in conjunction with a forward facing (pitot) pressure sensing port.
REFERENCES:
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patent: 3318146 (1967-05-01), De Leo et al.
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patent: 4718273 (1988-01-01), McCormack
patent: 4836019 (1989-06-01), Hagen et al.
patent: 5025661 (1991-06-01), McCormack
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Brochure 1044: “Angle of Attack Systems”, BFGoodrich Aerospace, Rev. Apr. 1995.
Brochure: “Upgrade and Save; Reduce DC-9-30/-40/-50 Angle of Attack Sensor Costs By Up to 75% ”, BFGoodrich Aerospace, published Sep. 1995.
Brochure: “BFGoodrich Aircraft Sensors Model 0861CAL Angle of Attack Sensor”, BFGoodrich Aerospace, published 1998, no month.
Brochure: “BFGoodrich Aircraft Sensors Model 0861HD Angle of Attack Sensor”, BFGoodrich Aerospace, published Dec. 1998.
Oen William
Rosemount Aerospace Inc.
Westman Champlin & Kelly P.A.
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