AlexPeterson
Well Known Member
So, right on time after 1000 hours I decided to formally calibrate my airspeed indicator! Didn't see any reason to rush into it. It is a Van's version, knots only, no TAS calculator built in. Pitot is the heated version with the tip in the same location as Van's plans recommends for the simple tube. While there are various methods for doing this, I chose to fly closed laps, N-W-S-E on the heading, hold altitude, nail indicated airspeed, record ground speed. Of course I also recorded altitude (which was 3500'), OAT and baro. I flew laps at 110 through 170 indicated at each 10 knots. I'm sure the test settings (i.e., altitude) are not optimum for this work, but probably good enough.
With the four ground speeds, I then used Paul Beale's calculator
http://spot.colorado.edu/~beale/TrueAirspeed/TrueAirspeedCalc.html
to calculate TAS for each run. Next I used
http://www.paragonair.com/public/aircraft/calc_TAS.html
in a trial and error fashion to get CAS for each run. (I would enter different CAS until the TAS matched)
The results were well behaved, although something was apparently wrong with the data I took at 170 kias, so I discarded that point. The result is presented here graphically:
The best fit straight line fits quite nicely to the data, and the slope is right around .95. So, this means when I indicate 100 knots, CAS is 95. Indicate 150, CAS is 143. I printed a little table and stuck it next to the airspeed indicator.
Seem reasonable?
With the four ground speeds, I then used Paul Beale's calculator
http://spot.colorado.edu/~beale/TrueAirspeed/TrueAirspeedCalc.html
to calculate TAS for each run. Next I used
http://www.paragonair.com/public/aircraft/calc_TAS.html
in a trial and error fashion to get CAS for each run. (I would enter different CAS until the TAS matched)
The results were well behaved, although something was apparently wrong with the data I took at 170 kias, so I discarded that point. The result is presented here graphically:
The best fit straight line fits quite nicely to the data, and the slope is right around .95. So, this means when I indicate 100 knots, CAS is 95. Indicate 150, CAS is 143. I printed a little table and stuck it next to the airspeed indicator.
Seem reasonable?