Effects of frame design on barometer performance ( and all other non 3.3 specific AC questions)

According to Randy the know issue of altitude loss after a high speed run is at its heart a frame issue.

This seems to me a very worthwhile discussion to have.

Once the majority of us understand this, we will be less likely to muck up other threads.

Take it away Randy...

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Zubax gps have baro on itself....

Randy, I've been flying 3.4 today on my sawn-off (330mm) Turnigy Talon v2 framed quad (with AUAV-X2) and I must say it flies beautifully. With the external Baro, I get a much smoother lift off in AltHold than with 3.3.3 (with internal baro), the lift off is very decisive on raising above 50% throttle, whereas with 3.3.3 doing the same, the revs actually drop, then raise (like its recovering from some pressure related confusion), then it launches, but its definitely not as steady as with 3.4. Great work! As always with much respect and thanks, Paul

I have an update on the Gill static pressure port 3D model. I've continued to work on the design and manufacture. At this point I believe that I have a feasible, cost-effective solution. The 3D print house, Shapeways offers relatively high resolution prints at a very competitive price.

Here is a link to an updated pressure port model that can be purchased from Shapeways.


The material is a scintered nylon, and several color options are available.

Just ordered 2 of these Zubax GNSS 2 modules with the barometer built in to see if this does the trick. May be easier than incorporating the barometer sensor separately.

Emin, The Zubax unit looks awesome, but then so is the price - dearer than the most expensive pixhawk clone I ever bought (and that was with all the accessories!). As for the GLB link you posted here, I am not certain that this unit is i2c compatible. It certainly doesn't mention it. I have bought two of the i2c 5v baro's so far - one connected to my Pixhawk running the Plane f/w - just hooked into the i2c splitter, and the other one chained to the mag connections on my m8n GPS unit, and both work very well, and at a very cheap cost. The ones I used were off eBay here: http://www.ebay.co.uk/itm/141799881200

Its very easy to connect - just 4 connections to chain onto the i2c bus: VCC(+5v), Gnd, SCL, SDA 

You also would need to be running the 3.4dev Copter firmware (or Plane for fixed wing).


I was looking for the explanation of altitude loss when moving copter horizontally forward (and other directions) - but I never found a simplest explanation. When the craft is moving forward  it is leaning towards the front (pitch) i.e. at the constant throttle level it must lose altitude due to a decreased upright thrust. It applies to all movement directions and actually I observed the same behaviour - losing altitude when moving in all directions. So, it suggests that it has nothing to do with

frame, pressure bubble etc but rather with losing a vertical component of thrust due to a pitch/roll change.


I think it's all of the above. I added a plexi glass cover over my pixhawk and it instantly solved the problem so it's definitely not just aerodynamics but air pressure. After all it uses a barometer that can detect the difference in pressure in under a meter!! So it's a sensitive little bugger... 

it also has a lot to do with your thr_min and thr_mid parameters, set thr_min to the lowest possible setting which will spin your motors, than adjust thr_mid accordingly.  I think what it does it increases the resolution of the PWM signal, so more fine throttle control is possible. Also, oneshot125 improves throttle control in stab mode noticeably even on relatively low KV motors - 900kv, this translates in better alt hold performance. 

Any news about altitude loss with Zubax GNSS 2

Still waiting for them to arrive.

After a few false starts, I can verify that the Zubax GNSS 2 modules work great with the latest firmware for the PixHawk (3.4rc1). I had been using the Pixhack, but it does not support UAVCAN. I switched to a PixHawk, but received a clone which did not use the usual DF13 connectors. I sent it back and bought a genuine 3DR PixHawk. After following the tutorials on Zubax's website and setting the barometer to 20Hz directly on the GNSS 2 module (not through Mission Planner), and setting GND_PRIMARY to '1' in Mission Planner, I got everything up and running. Now with the barometer readings up on the mast along with compass and GPS, the altitude holds MUCH better. Even at high speeds I am seeing a good altitude hold now.

Thanks for the  feedback

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