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Re: [Paparazzi-devel] Airspeed Sensor

From: alonso acuña
Subject: Re: [Paparazzi-devel] Airspeed Sensor
Date: Tue, 27 Aug 2013 21:36:27 -0600

I read elsewhere that in order to get true airspeed with a sensor like the Eagle Tree one needs the pressure diferential (reading from sensor), ambient pressure (one could get from barometer on IMU hopefully) and ambient temperature. I have no way to get temperature with my Aspirin so that won't work. A sensor like the one you recommended seems much better. 

On the other hand the only reason I am interested in airspeed at the moment is to prevent stalling at landing. I was wondering if someone knows if stall speed at a normal landing angle of attack is related to true airspeed or just having CAS would be fine? In other words does stall speed at landing vary much with temperature and or ambient pressure?

I found this page useful to understand what was meant by CAS and IAS and this one about stalling

On Tue, Aug 27, 2013 at 7:31 AM, Tobias <address@hidden> wrote:
Hey guys,

I spend some time with the eagle tree to get it working....

so I can confirm that there is a big problem with the offset at startup, thats why we have the calculation and no fixed values - however if you wane check your sensor at an pressure difference (maybe the equivalent for 15m/s for example) you will notice that the drift and noise are well in an acceptable range, just seems to be an issue with offset at startup...

just an idea for the calibration - some years ago I also started with calibrating my sensors on a car, but there are two things you should have in mind
1 you will probably stay inside the boundary layer of the car which will effect you measurement (hard to overcome when your not at the very front of it...)
2. even if you are able to calibrate it perfectly you will get IAS when mounted to your plane...

sounds good, but its not since you will have to correct this for the mounting error to get CAS (which is actually named as IAS in PPZ)

so what I do, is just checking the GS compared to IAS(ppz) in a constant circle at constant altitude.
IAS should be exactly in the middle between GSmax and GSmin
if it is, you probably calibrated it perfectly and therefor already corrected for any mounting error (you got CAS!)
if not adjust the scaling factor
thats an quick and easy way and is working just fine

just a tip, I changed to this sensor which has a temperature sensor and i2c, the code is already in v5 - looks very promising

hope that helps ;-)

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