I am trying to understand the purpose of setting airspeed ratio to 2 (approximately) for the airspeed sensor I am working with, the MS4525DO. Currently I am using the raw data from the sensor to get the differential pressure (psi), then using the relevant formula to output speed (KIAS). When I compare this to ArduPilot parameters and code, I see the airspeed ratio parameter. What is it for?
I’ve moved this from the “Blog” categor to “Plane 3.6” because it was appearing on the front page of ardupilot.org.
I’m afraid I don’t know the answer to this question but hopefully others will.
You are right that there is a simple equation, based on Bernoulli’s principle, to convert ram pressure to airspeed.
I believe the reason for an adjustable ratio is to account for all the possible errors which are not captured by the equation. Having a tunable ratio is a simple and effective way to mitigate their effect on the airspeed measurement.
Yes, I thought about that as well. The resultant airspeed, Calibrated Airspeed (CAS) in the aviation world, is Indicated Airspeed (IAS), corrected for position error etc. But if the airspeed ratio in ArduPlane were specifically designed for that purpose, I think it would be somewhat less than a factor of two. Something more like 1.01 or 1.02 to start.
I am just getting up to speed with the C and C++ languages and might be able to discern a purpose for the ratio by deciphering the math behind the coding.
You’re right that the correction is a small amount, meaning the corrective multiplier would be near 1. I think the equation’s factor of 2 (or .5) is combined with the corrective factor, giving a ratio value closer to 2.