Flight height minus

[Jimmy777]

Today during takeoff everything was normal takeoff from 0 mtr. During flight I already see something was wrong: the height not shown correctly. I returned home and landed in the same spot as where was takeoff. I land at a height of minus -7.4 mtr. Anybody know what can cause this problem?

 

[Krieger’s Drone]

Today during takeoff everything was normal takeoff from 0 mtr. During flight I already see something was wrong: the height not shown correctly. I returned home and landed in the same spot as where was takeoff. I land at a height of minus -7.4 mtr. Anybody know what can cause this problem?

Earth rotation? [emoji12] jast kidding

 

[FoxhallGH]

Did you get the message from the controller to confirm that GPS location was updated, before you lifted off?

 

[Jimmy777]

Did you get the message from the controller to confirm that GPS location was updated, before you lifted off?

Homepoint was recorded

 

[sar104]

Today during takeoff everything was normal takeoff from 0 mtr. During flight I already see something was wrong: the height not shown correctly. I returned home and landed in the same spot as where was takeoff. I land at a height of minus -7.4 mtr. Anybody know what can cause this problem?

How long did the flight last? If the local atmospheric pressure is changing fast then the barometrically measured elevation will change. Atmospheric pressure can change by 200 Pa/hr, which would correspond to a change in displayed elevation of around 17 meters per hour.

 

[Jimmy777]

How long did the flight last? If the local atmospheric pressure is changing fast then the barometrically measured elevation will change. Atmospheric pressure can change by 200 Pa/hr, which would correspond to a change in displayed elevation of around 17 meters per hour.

Then this could be the problem the flight was 21 minutes.
But this could be dangerous when for example you set RTH attitude to 20mtr the bird only flies 12 mtr. and could crash into something on the way home.

 

[Kilrah]

Things are not perfect, you always need margin.

 

[sar104]

Then this could be the problem the flight was 21 minutes.
But this could be dangerous when for example you set RTH attitude to 20mtr the bird only flies 12 mtr. and could crash into something on the way home.

That's one of the risks of not maintaining visual line of sight to the aircraft.

 

[Ken K]

You can be somewhat prepared for a rapid air pressure drop / rise by, before your flights, referencing a weather app that forecasts pressure hourly over a period of hours.
Also, though not as accurate, you can check your hourly weather forecast for any rapid change in temperature as this would indicate a corresponding change in pressure.

 

[Ken K]

How long did the flight last? If the local atmospheric pressure is changing fast then the barometrically measured elevation will change. Atmospheric pressure can change by 200 Pa/hr, which would correspond to a change in displayed elevation of around 17 meters per hour.

Question ...... Could the updraft / downdraft created by a change in terrain be sufficient to be a problem with RTH. I'm thinking of a large paved area, a large sandy beach, large body of water, etc?

 

[FoxhallGH]

Question ...... Could the updraft / downdraft created by a change in terrain be sufficient to be a problem with RTH. I'm thinking of a large paved area, a large sandy beach, large body of water, etc?

RTH is not usually affected by those things. RTH works by the drone registering its position with GPS, and altitude with a barometric sensor after you do the start sequence. The drone should show at altitude = zero when you get the GPS ready message, and from there, that's the height reference used for RTH. However, when the drone does its landing in a RTH sequence, it will actually use downward sensors as well - to determine height above the ground, and if the surface is suitable for the landing.

 

[sar104]

Question ...... Could the updraft / downdraft created by a change in terrain be sufficient to be a problem with RTH. I'm thinking of a large paved area, a large sandy beach, large body of water, etc?

Are you thinking about the direct effect of strong wind drafts, or about the effect that they might have on the barometric readings? Both are potential issues if the induced pressure gradients are large enough.

 

[FoxhallGH]

You can be somewhat prepared for a rapid air pressure drop / rise by, before your flights, referencing a weather app that forecasts pressure hourly over a period of hours.
Also, though not as accurate, you can check your hourly weather forecast for any rapid change in temperature as this would indicate a corresponding change in pressure.

Usually there is a clue with a huge great wall of cloud - as a cold front (or worse, a squall) comes through ...

 

[sar104]

Usually there is a clue with a huge great wall of cloud - as a cold front (or worse, a squall) comes through ...

It's also usually rather windy with such a large spatial pressure rate of change.

 

[kilomikebravo]

Yep, the old isobars can get pretty close together.

 

[Ken K]

RTH is not usually affected by those things. RTH works by the drone registering its position with GPS, and altitude with a barometric sensor after you do the start sequence. The drone should show at altitude = zero when you get the GPS ready message, and from there, that's the height reference used for RTH. However, when the drone does its landing in a RTH sequence, it will actually use downward sensors as well - to determine height above the ground, and if the surface is suitable for the landing.

I was thinking more in terms of clearing obstacles above or below rather than the actual landing. I also was relating the draft to pressure and I'm not sure if that's valid.

I'm trying to get my head around the pressure / height relationship. In the OP's example the height (elevation) registered negative so his AC thought it was lower than it actually was. This means, I think, that if his AC had been in RTH mode it would have been flying "higher" than his intended RTH set height. Yes?
In this situation, would the pressure have increased or decreased? Ie. if the pressure rapidly INcreases will the AC be flying higher or lower than the preset RTH height?

I hope I'm making some kind of sense here.

 

[Ken K]

Usually there is a clue with a huge great wall of cloud - as a cold front (or worse, a squall) comes through ...

Or it could be the other way around, from stormy to clear and calm. But, of course, nobody would have their drone in the air then anyway.

 

[sar104]

I was thinking more in terms of clearing obstacles above or below rather than the actual landing. I also was relating the draft to pressure and I'm not sure if that's valid.

I'm trying to get my head around the pressure / height relationship. In the OP's example the height (elevation) registered negative so his AC thought it was lower than it actually was. This means, I think, that if his AC had been in RTH mode it would have been flying "higher" than his intended RTH set height. Yes?
In this situation, would the pressure have increased or decreased? Ie. if the pressure rapidly INcreases will the AC be flying higher or lower than the preset RTH height?

I hope I'm making some kind of sense here.

Correct - the actual RTH altitude would have been higher than set in that case.

When it returned to ground level it showed the elevation as negative. Since pressure decreases with height, that means (if the issue really was due to a change in atmospheric pressure) that means that the local atmospheric pressure had increased.

 

[Ken K]

........(if the issue really was due to a change in atmospheric pressure) ......

Of course. I'm in "what if" mode here trying to understand the pressure effect on the actual height of the AC.

So if, while in flight, the air pressure drops, the AC will be actually lower than any preset heights. I'm thinking here of a long waypoint mission, although the probability of this ever becoming a problem is pretty slim.

 

[sar104]

Of course. I'm in "what if" mode here trying to understand the pressure effect on the actual height of the AC.

So if, while in flight, the air pressure drops, the AC will be actually lower than any preset heights. I'm thinking here of a long waypoint mission, although the probability of this ever becoming a problem is pretty slim.

I've looked at maybe a couple of events where it appeared to have been a factor. But typically you would not be flying any distance in those kinds of weather conditions due to wind speeds.