V-4
February 2008
Legacy
V-5
February 2008
Legacy
Trailing Cone Calibrations
Another method utilizing a “trailing cone” can be utilized for “solo” data gathering
which will eliminate the hazards and transfer errors of a pacer calibration and the
time consumed by tower passes as well as increase the measurement accuracy. This
data can be gathered at several altitudes, across the full speed range, with far greater
accuracy, and require no external assistance. This requires the use of a drag cone
which is trailed behind the aircraft (typically from the top of the vertical fin) and
at such a length that the pressure field of the aircraft has decayed. (This is on the
order of 35 feet for the Lancair.) Static ports are located in the tubing forward of
the cone 10 to 12 cone diameters. Since the “Cone” provides true outside pressure
altitude, a differential pressure gauge between the Cone and the aircraft’s system will
display the error. This eliminates the error associated with the comparison of two
absolute measurements but requires the use of tables of pressures versus altitudes
in the range of inches of water for the altitudes flown. The result is however an
extremely accurate calibration of your static source such that you will know that
your corrected 8000 feet in IMC conditions is really 8000 feet, and you’ll know
the effects of gear and flaps on indicated altitude also. The Trailing Cone is used
world wide for static source location development and certification purposes.
Once these data are gathered by whichever means, they can be plotted on the fol-
lowing chart and then represent the calibration for your aircraft specifically. These
corrected altitudes should then be flown for all subsequent airspeed system tests.
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