TRUE AIRSPEED CALCULATOR
I found a spreadsheet on the web that makes checking TAS rather simple and what appears to be accurate. Instead of flying just one course and then it's reciprocal, three courses are flown. The heading of these three courses is not really important but the farther spread apart the three are, the more accurate the calculation. I fly at 0, 120 and 240. I use a micro cassette recorder plugged into my passenger headphone jack so everything I say on the intercom gets recorded. This turns out to be much easier than trying to write everything down during my flight testing.
Here is how the testing goes. While maintaining a constant altitude (as much as possible) fly 0 degrees. Record the GPS track and the Groundspeed. ( I usually record the IAS as well). I take five sets of readings at each heading as quickly as possible ( the longer you take the more likely you are to drift off from the current track and altitude. Then turn to 120 degrees and take five more readings, then to 240 degrees and take five more readings.
Back at the ranch, average the five readings at each of the three headings. Enter the average GPS Groundspeed and GPS track in the green boxes on the JavaScript TAS calculator below that my son coded and added to this web page. Press calculate and the TAS will be displayed in the blue box. The IAS yellow box is just there for a reference and does not play into the calculation (it does not have to be filled in). Also as you can see, the wind speed, wind direction and true headings will also be calculated.
If you want to save the TAS as an Excel spreadsheet, click the link below then pull down the file menu and do a save as to save it on your local computer.
The default numbers you see below are actually my last TAS test at 7000 feet with an outside air temperature of 58 deg F. If you press calculatae with these default numbers, you will see that my TAS was 118 mph.