[TheOtherCliff]

Airspeed Sensors I Just Bought

MS4525D Pixhawk PX4 Airspeed Sensor with big fancy pitot and tubing, on eBay for $47 shipped, took 15 days shipping arriving Dec 17. Here is a direct link to the PX4 (MS4525D) version.  Beware that this page also sells the cheaper MPXV7002, apparently also with the big heavy pitot for $25.
https://www.ebay.com/itm/JMT-PT60-APM-PX4-Airspeed-Tube-Pipe-Digital-Airspeed-Meter-For-DIY-Fixed-Wing/283185821007?var=583596360248

White sensor chip/module says 4525D.  Black board says PX4AIRSPEEDV1.1 and 5V SCL SDA GND, so that at least votes for 5V.  Reading several threads on various sites, it looks like everyone should be using a 3.3V part, but the initial design was wrong in that it used a 5V part and that is what we are stuck with now.  The datasheet says 2.7V to 5.5V so I take that to mean the 3.3V part can run at 0.6V below that.  If I have I2C errors, I may try to run this 5V part at say about 4.4V rather than messing with a level shifter.

That is the cheapest MS4525D with big pitot I found while taking shipping cost into account.

The packet containing just the metal pitot tube and silicone tubing weighs 30 grams.  Pitot alone is 14.1 grams.     The cheaper pitot that often comes with the MPXV7002 feels a good bit lighter.  Lighter tubing is surely possible too...


Xt-xinte has the MPXV7002 (usually black sensor chip/module?) with lighter weight pitot for $14.  I got free shipping and listing says free shipping, but it is xt-xinte...  Shipping time about 3 weeks (due here in less than a week).
https://www.xt-xinte.com/h-product-detail.html?goods_id=138778

[jdl]

Report:

I've recently got this one MS4525D Pixhawk PX4 on sale from AliExpress, $35.34, Free shipping via AliExpress Standard Shipping (16 days transit). Lighter, all-metal pitot tube, silicone tubing and sensor board with cable.

Option available: Sensor only for $29.49

https://www.aliexpress.com/item/32560997823.html?spm=a2g0o.cart.0.0.21f93c00C3V7HQ&mp=1

Meanwhile the price raised but now I see it again on sale (same lower price) till 10 Jan 2020.

This evening I found some time to test it, works really fine. Connected it directly to Flexi Port (5V). No I2C errors, no interrupts. Data coming seems plausible and doesn't drift with temperature, unlike MPVX7002.

[karla]

Testing the MS4525DO unit in air, looking very promising 

Mounted it in the pre cut-out compartment in the wing and had to cut down the length of the plastic tubes a lot, to fit it in.



Its mounted in the left wing and I wanted to calibrate it.
Just using the default settings for the air speed sensor as posted before here by f5soh and me.



This maiden flight is 5 min at the beach. The wind is coming from the sea at just below 6 m/s, very stable not gusty. I am flying close figure eights turning in to the wind, then land. I don't trust myself to fly in manual mode so this is in attitude, attitude, manual, manual in Basic AttEstAlgo.



This is the log from the 5 min flight showing airspeed and gps reported groundspeed.
I think the average airspeed and the average groundspeed align just fine at around 15.5 m/s.



Over at ArduPilot its suggested for the zero calibration of wind speed:
( https://ardupilot.org/plane/docs/calibrating-an-airspeed-sensor.html#manual-calibration )
The airspeed varies with the square root of the pressure, so for differential pressures near zero it varies quite a bit with very small pressure changes, while at flying speeds it takes much greater pressure changes to produce a similar change in speed. If you see mostly 0, 1, 2, with an occasional bounce to 3 or 4, consider it normal. You will not see that sort of variability at flying speeds.

I think that's exactly what can be seen in the picture after landing with no wind - yellow circle.

Think the windspeed sensor is okay and ready to go.

[TheOtherCliff]

After you have tweaked PIDs beware that you may / will find that PID I term windup before flight is a problem.  If tilted left in your hand, you will get more and more right aileron.  That left tilt will cause it to go hard right at takeoff.  Elevator affected as well.  Higher PIDs makes this worse.

I have seen this when switching from Manual mode to Atti mode in flight.  I presume that you get the same problem from just holding it in your hand, in Atti mode, with the motor running.

Of course you really need Stabilization->ZeroTheIntegral enabled (you probably already have this) so that as long as throttle is zero it will not wind up.  Then it is just between throttle up and launch that it can wind up.  JDL does bungee launch at zero throttle, then adds throttle when in the air.

One other factor about this is that you can reduce the I term windup by reducing the ILimit in StabilizationSettingsBank1(or which ever bank) ->
  RollRatePID
  PitchRatePID
  RollPI (not needed for default settings where I term is zero)
  PitchPI (not needed for default settings where I term is zero)

(Other readers should note that we don't usually stabilize fixed wing yaw so it isn't mentioned here.)

[karla]

I thought that windup problem could be solved with the "jdl-method"?

https://forum.librepilot.org/index.php?topic=3558.msg29816#msg29816

We talking about the same thing right?
(PIDs windup on takeoff in Attitude, after arming, switching from manual to attitude and applying throttle).

[TheOtherCliff]

If I recall correctly, the @JDL method allows you to fly in the air something that is effectively Manual mode, and in air switch to Attitude mode without it going crazy; where if you use real Manual mode and switch to Attitude, the windup is a problem.

What I am talking about is like an Attitude mode quad taking off tilted from the side of a hill.  The longer you wait, the more it winds up.  When it finally gets in the air the downhill side has much more thrust and it tends to flip "up the hill" like rolling a ball up the hill.