The True Airspeed applet will allow a pilot to accurately determine the true airspeed of an aircraft using a GPS or loran unit. The airborne procedure requires approximately 10-15 minutes of stabilized flight to collect groundspeeds in three separate directions, and the ground procedure requires only a few minutes to type those groundspeeds into the applet.
instructions
In the
aircraft at the desired altitude, set the aircraft power to the desired setting
(e.g., 75 percent). Configure the GPS or loran unit to display groundspeed.
Turn to one
of the cardinal headings (north, east, south, or west) and allow the aircraft
to stabilize at its trimmed airspeed. Record the cardinal heading and the
groundspeed readout from the GPS or loran.
Turn to
another cardinal heading and allow the aircraft speed to stabilize. The
indicated airspeed should be very close to the indicated airspeed in the
previous step. If it is not, you may be in an updraft or downdraft and may need
to allow the aircraft more time to stabilize at its trimmed airspeed. Record
the cardinal heading and the groundspeed readout.
Turn to a
third cardinal heading and allow the aircraft speed to stabilize. Again, the
indicated airspeed should be very close to the previous airspeeds. Record the
cardinal heading and the groundspeed readout.
On the ground, enter the recorded groundspeeds into the input boxes in the left half of the True Airspeed applet. Enter the groundspeed recorded while heading north in the top box, the groundspeed recorded while heading west in the left box, etc. As you enter the groundspeeds in the input boxes on the left, the groundtrack display on the right will show lines representing the legs of the airborne test. When you have finished entering the three groundspeeds, the groundtrack display will contain a no-wind representation of your groundtrack. For example, if you first flew north, then east, then south, the line displayed would go up first, then right, then back down. (If any of the directions are opposite to the previous direction, its line will draw on top of the line for the previous direction, so it will look as if only two lines have been drawn.)
If the
groundspeeds are not entered in the same order as the legs were flown in the
air, the groundtrack representation will not be accurate. To fix this problem,
click the mouse in the input box where the first-flown groundspeed is entered
and then press and release the control key (Ctrl) on the keyboard. Repeat the
same procedure for the input box where the second-flow groundspeed is entered,
then the same for the last-flown groundspeed. At this point the groundtrack
display should contain an accurate no-wind representation of your groundtrack.
(On some browsers, pressing the control key may not be necessary; clicking the
mouse in the input box may be enough to change the order in which the legs are
displayed.)
The applet
determines the wind and the aircraft's true airspeed by making an initial guess
and then continuing to make better guesses until the solution is found. If you
want the applet to display only the final solution, uncheck the "Show
Iterations" checkbox. Leaving the checkbox checked will cause the applet to
display each guess as it is made so that the groundtrack display becomes an
animation starting with the applet's initial guess and ending with the final
solution. (Since the applet makes a new guess every tenth of a second, there is
not a significant performance penalty to leaving the checkbox checked so the
applet's progress can be observed.)
Click the "Compute" button with the mouse. This will cause the applet to begin the guessing process and eventually determine the true wind and aircraft airspeed, usually within seconds. The groundtrack display will now contain a wind-corrected representation of your groundtrack, and the values for true airspeed and the wind will be displayed below the groundtrack display.