Hi Larry,

 

I'd recommend everyone calibrate their airspeed indicator.  It's easy and fun and a great excuse to get some test pilot flying time.  Once you've done it, you won't need to guess how accurate your IAS is. 

 

I used the "four course GPS" method from the following web site:

 

 

You need GPS and a somewhat accurate outside air temperature sensor.  (Using the Chelton procedure for measuring the compressibility effect on temperature, my OAT at cruise speed is 4°F higher than ambient due to "friction" heating.  But the conversion from TAS to IAS isn't terribly sensitive to changes in OAT readings.)

 

I picked a day with relatively calm winds and smooth air and used the autopilot to fly the four courses.  I logged the IAS, TAS, ground speed, altitude, altimeter setting, and OAT for each course.  Back on the ground, I input the info into the web site's calculator.  I ran the calculator four times, once each with the following courses:

 

1.  north, east, south

2.  east, south, west

3.  south, west, north

4.  west, north, east

 

That gives TAS.  To convert to IAS, I used the following web site:

 

 

Enter the data and iterate with different IAS until the solved TAS equals the amount from the previous step.  That is your actual IAS (well, I suppose it's actually calibrated, or maybe even equivalent, airspeed, but for this purpose, it's okay to call it IAS).

 

It turns out my IAS was reading low, probably because I body-worked my Legacy's static ports flush with the fuselage.  Legacies with accurate IAS have static ports that are proud of the surface by 1/32" or so.  Modifying my static ports is on my to do list.  Until then, I enjoy a tail wind almost everywhere I fly!

 

Dennis Johnson

Legacy, 140 hours