How is airspeed measured?

[khorton]

I've got a D-10A EFIS, and will be using a speed course type method to determine the aircraft's static system position error.  This method relies on comparing the indicated airspeed against the speed determined via another means.  You correct the IAS for instrument error, and assume that any remaining error is due to static system position error.  More info here: http://www.kilohotel.com/rv8/phplinks/out.php?&ID=31

I need to understand how the airspeed is determined, so I can be sure to do an instrument error calibration that will be valid at altitude.  

Is there a differential pressure transducer, or do you subtract the output from a static pressure transducer from the output from a pitot pressure transducer?

How is temperature sensitivity accounted for?  There is quite a difference between cockpit temperature in winter vs summer.  Once the unit temperature has stabilized, should I expect there to be any variation of instrument error with cockpit temperature?

Thanks,

 

[dynonsupport]

Airspeed is indeed measured via a differential pressure transducer, though this technical point should not affect your testing method. No matter what's going on inside the unit, you still need to hook up pitot and static to it.

Differences in the unit temperature are taken into account with a data map that is generated during the calibration of the unit here. Once the unit temperature has stabilized over the course of the first 15 minutes or so, you should see reliable, accurate performance.

 

[khorton]

Thanks for the info.  I was hoping the unit would have a differential pressure transducer, as it does simplify my testing to determine instrument error.  With a differential pressure transducer, just plumbing one end of a water manometer into the pitot port will do.  The other end of the manometer and the static port will both see the local ambient pressure, which will be the same at both places, assuming still air.  If you used pitot and static transducers, then such a test would only be testing the pitot transducer, as the static transducer would always see the same pressure during the test.  I would need to come up with some other method to check the calibration of the static transducer.  But this isn't the case, so no issue.

As far as the temperature sensitivity goes, does your calibration testing cover the full range of expected stabilized unit temperatures that would be seen during hot day operations?  The greenhouse effect of my bubble canopy means that cockpit temperatures may be very high for quite some time in the summer.

 

[dynonsupport]

As long as the unit is not displaying a "temperature out of range" error, then the unit is within the temperature range that we calibrate it at.

If it is displaying a temp out of range error, we suggest you get some cooling into your avionics stack since the EFIS and all of your other equipment is running at somewhere north of 150 degrees F.