Calibration of one's ASI using a GPS is often done by finding the direction
of the wind and then averaging the GPS ground speed on courses directly
toward and away from the wind. Some error is introduced if the wind
direction is not accurately known when using this method.
There is another interesting approach to calibrating the ASI using a GPS.
One flies 3 GPS ground tracks which are 90 degrees apart (e.g. 0, 90, 180) at
the same indicated airspeed on each track, noting the GPS speed on each. The
wind need not be known although it is assumed to be constant for the duration
of the measurements. The density altitude is assumed constant since TAS
(rather than CAS or IAS) is found by this method. Error is introduced if the
density altitude or indicated airspeed differs between readings.
Assuming that the wind, density altitude, and the ASI reading are constant
while the GPS speed readings are made, the true airspeed is calculated as:
TAS = Sqrt ((V1 **2 + V2 **2 + V3 **2 + (V1 **2 * V3 **2) / (V2 **2)
) / 4)
Where: V1 = GPS measured velocity on first track
V2 = second course, 90 degrees different ground track
V3 = third course, ground track is 180 degrees from V1
(Also record the altitude and temperature to allow calculating density
altitude or read it directly if a uMonitor or equivalent is available.)
The above equation yields the TAS, which should be converted to CAS (using
an E6B) for comparison with the indicated airspeed flown while the
measurements were made to find the ASI error.
If needed, the wind can be found from:
W1 = (V1 - V3)/2 [ directed along V1,V3 ]
W2 = 0.5 * (V2 - (V1 * V3) / V2) [ directed along V2 ]
Wind velocity is the vector sum of W1 and W2:
Wind Velocity = Sqrt ( W1 **2 + W2 **2)
The wind direction can be found from W1 and W2 using trigonometry or using an
E6B's wind triangle side. Alternatively, the magnitudes and directions of W1
and W2 plus common sense will yield a fair estimate of wind direction.
This method was published in "Kitplanes" several years ago. Unfortunately,
the article contained a typo which made it difficult to verify the method
without re-deriving it. (Most of the work in deriving this method is in
constructing the drawing of all the vectors.) This method is less intuitive
than flying directly into and out of the wind so one may want to compare
methods before becoming a believer in an approach derived via simultaneous
equations.
John N44EU
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