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TH Barometer Weirdness Continues...

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Exciting news for me today - haven't done any flying since New Year's Eve, but at last work chilled out a bit, so I had some time to get myself airborne again.
The good news is that after reasonable degree of battery care I still got 13 mins of flight per pack, and that all the major bits were still working. 3 very nice controlled flights with no flight or drift issues.

Initially I did a more thorough than normal visual inspection, and waited on the ground for 15 mins to refresh the GPS almanac etc, and then was good to go.
First flight; perfect everything - no worries there.

Second flight, and as soon as I started the motors my reported altitude went from 0 ft to 50 ft in the space of 2 seconds, then levelled out and stayed there, despite the craft having not yet left the ground. 'Uh oh' I thought, 'if I take off now, it'll do the refuse-to-come-down thing later if my theories about that are correct'. So I stopped motors, restarted them, and this time altitude remained hovering around 0. Took off - another perfect flight, no sign of the descent refusal issue.

3rd pack, and we had the same again, but this time twice in a row - exactly the same behaviour - motor start, instant 50 ft gain on reported altitude, and level off there.
So, stopped and restarted motors, same again, but on 3rd time everything remained normal, as did the full flight that followed it, and again, no sign of any problems descending.

So, in the new season, we completed our 'empty field safety test session' with a cautious 'pass' but with that ongoing mystery...still ongoing.
With hindsight, and replete with the knowledge as I am of at least 6 different ways to get it down after the refusal issue, I suppose the better thing to have done would have been to not restart the motors, and confirm that we did indeed get the descent refusal issue if we just left it - that would have added a lot more weight to my theory.

But anyway, reporting it here because I know I'm not the only person that gets this, and so we don't forget it's still a thing we have to watch :)
 
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PatR

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Has me wondering if starting the motors may reduce pressure inside the airframe through a form of venturi effect.
 

Steve Carr

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The barometer and the accelerometers both help determine altitude. So it might help to do an accelerometer cal next time you are out.
BTW, I often restart the motors to zero out the alt reading. The barometer seems to be affected by the internal temp of the aircraft.
 
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Cheers, @Steve Carr - I will do an accel calibration next time I'm out...

Venturi effect, hey ? Interesting thought. But, wouldn't the open vents found throughout the case negate that / make it impossible for pressure to accumulate or seriously change inside it ? And could that still be the case where one motor start initiates the problem, but another one 5 seconds later reports normal ? Could the pressure change so much so fast ?
 
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PatR

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IMO, the vents on the case would be the feature that might allow a venturi effect to happen. Just a wild a&$ed theory but we have 6 motors surrounding the middle body, when the motors start up it creates a vertical wind tunnel with the vented body suddenly experiencing a lot of air moving through or by the body vents. The body interior is a smaller space than the airflow zone outside of the body. Lateral flight would allow pressure equalization as it would displace the focused column of air around the vented body.

Just a thought and absolutely nothing to support it.
 
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Altitude in telemetry reflects what I saw at the time... fast but even rise, then hold at 50 ft until I stop motors...

Flight 198-V-A.png
 

PatR

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Interesting. Altitude change match increases in prop speed. Internal heat accumulation does not occur that quickly.
 

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