Technology Notebook ContentsEffective Date: 15 June 98
Position Error Calibration Methods
The following discussion presents the various methods that are available for ca1ibration of the test or production airspeed system. The trailing static cone is used as the primary reference standard for all calibra tions with the remaining methods used as backup or check of the trailing static cone
Trailing Static Cone
Basically, this system is the same as a trailing bomb. A light weight cone is mounted on a nylon hose to keep the hose stable throughout the air vehicle speed range Static holes are located radially around the hose and far enough behind the horizontal tail of the air vehicle to assure that there are no effects from the air vehic1e on the static source The distance required to minimize these effets is determined during airspeed calibrations in the development phase. The static pick-up from the trailing cone is compared to the various other static systems by use of sensitive differen tial pressure gages. These tests determine the static error in the other systems.
Stadiametric
This method is used to determine the error in the static system. In the stadiametric method, one camera which is stationed on the ground at a known elevation point photographs the air vehicle as it flies directly overhead. The pressure height above the ground is determined from the camera film by triangulation and. by recording the temperature and pressure at the ground station. It is also necessary to have a calibrated free air temperature system on board the test air vehicle.
Pacer Method
In this method a check calibration of the test air vehicle is made against another previously calibrated air vehicle (pacer). The results from this method can only be as accurate as the previously calibrated pacer It is desired that the pacer be calibrated immediately prior to or after use as a pace air vehicle.
NOTE: It is very important to obtain a "ground block" of the test and pacer air vehicles statically prior to becoming airborne.
In the basic form of this method the test air vehicle and pacer are flown side by side approximately one wing span apart to prevent air vehicle pressure field intersection When the air vehicles are stabilized at the desired speed. and altitude the following pacer information should be recorded
Time
Altitude
Gear and Flap Positions
Gross Weight
Airspeed
Free Air Temperature
A static pressure calibration can be obtained directly from
a comparison of the altitudeB or indirectly from a comparison
of airspeed.
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