What's up with the slow speed?

[Zandian]

Ok, so I got the crap scared out of me today. Flew my Mavic for the first time in a while and almost didn't make it home. I was 3100 ft out with 30% battery and started coming back, to discover I could only go about 5mph! I was getting wind warnings. I checked the weather ahead of time and it said 12mph. I tried everything, dropped to about 70ft, even tried flying sideways in case the front throttle on the controller was having issues. Nothing worked! I was looking for a place to ditch it and go get it and suddenly my speed increased. Got it back up to 20mph. Made it back with 5% battery!

I'm really not sure what's going on. Anyone have an idea what speed of wind would be high enough that the Mavic couldn't fly against it? I can't imagine the gusts were getting that high, but I guess it's possible. Certainly quite scary!

Anyone think it might be a defect, or simply high wind?

Z



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[erkme73]

I don't know if there is a way to tell what the actual head wind might have been. Was the flight heading away faster than normal? There were several reports early on (maybe 6 weeks ago) where people were barely making it back because of wind. Switching to S-mode increases the tilt of the air frame, and will allow you go up to 40MPH. Did you try that? It chews through batteries quicker, but you get home faster.

 

[Zandian]

No, flying away I was maxing about 21mph. Yes, I did put it in S mode with no change.

Strange that it almost seemed like my speed increased incrementally as I got closer to home...and almost the whole time I was flying over open fields.


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[erkme73]

That is certainly odd. If it was wind, you would expect the horizontal speed to increase by the difference between the modes. There must be something else going on. Since OA is turned off on S-mode, it isn't the forward vision sensors. Did you get any errors? I'd say @msinger or @BudWalker would be able to give you more detail, but they'll want you to upload your flight record (txt) from the phone, and/or the DAT from the MP.

 

[Zandian]

That is certainly odd. If it was wind, you would expect the horizontal speed to increase by the difference between the modes. There must be something else going on. Since OA is turned off on S-mode, it isn't the forward vision sensors. Did you get any errors? I'd say @msinger or @BudWalker would be able to give you more detail, but they'll want you to upload your flight record (txt) from the phone, and/or the DAT from the MP.

Thanks for the info. Is there another way to reach them besides twitter? I don't use it.

Z

 

[erkme73]

Not that I know of. But, for future reference, when you put @USERNAME in a post, it will send them an email notification (if they've enabled it) letting them know they've been mentioned. You can, of course try to start a conversation with them. On a computer browser, look just to the right of your username - there's an envelop. You can start a conversation there. Just add their usernames to the participant field...

 

[Zandian]

Ok cool, thanks for the education. Thought you were sending me to Twitter.

Also, when I export to csv via ios where does it go? I see a brief link to somewhere but it appears to be some sort of folder and I don't know how to get there


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[james_mav]

Ok cool, thanks for the education. Thought you were sending me to Twitter.

Also, when I export to csv via ios where does it go? I see a brief link to somewhere but it appears to be some sort of folder and I don't know how to get there


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What temp was it outside?

 

[erkme73]

Ok cool, thanks for the education. Thought you were sending me to Twitter.

Also, when I export to csv via ios where does it go? I see a brief link to somewhere but it appears to be some sort of folder and I don't know how to get there


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Sorry, someone else will have to chime in. I've never owned any iFruit products...

 

[Zandian]

What temp was it outside?

About 32F


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[james_mav]

About 32F


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32 degrees with a 20 plus MPH wind

About 32F


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I wouldn't be surprised if that was the problem. A 20 degree windchill on land & think what the temp was at 100 feet.

 

[Zandian]

It may have been a factor. Not sure the wind speed. Again, Dark Sky weather app said it should be about 12mph-at least at ground level. But I was able to go 20mph in the other direction and the speed actually increased as I got closer to home.

Z

 

[Zandian]

Hey @msinger and @BudWalker ,

Ok, I have the DAT file(s)-I'm not sure which is the actual flight. How can I get it to you?

Z

 

[BudWalker]

Hey @msinger and @BudWalker ,

Ok, I have the DAT file(s)-I'm not sure which is the actual flight. How can I get it to you?

Z

Most pilots use Dropbox and then post a link here. Google Drive is also an option.

 

[Zandian]

Ok, here they are. I have three because I'm not sure which was the actual flight.

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-45-01.DAT

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-47-37.DAT

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-48-51.DAT

Any advice would be much appreciated!

Z



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[BudWalker]

Ok, here they are. I have three because I'm not sure which was the actual flight.

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-45-01.DAT

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-47-37.DAT

Dropbox - DJI_ASSISTANT_EXPORT_FILE_2016-12-30_10-48-51.DAT

Any advice would be much appreciated!

Z



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I looked at the second .DAT as it was the only one that had a motorStart and actual flight. In summary the flight appears to be normal and without issues. However, there were some headwinds encountered that caused the Mavic to have a slower ground speed in some parts of the flight.

In response to full elevator being applied the AC will lower the pitch to achieve a faster ground speed. But, there is a limit to how much the pitch will lowered. In normal flight, that limit appears to be about -16 degrees. In an RTH the lower limit appears to be about -24 degrees. This can be seen here

At 316 full elevator is being applied resulting in a -16 pitch. At time 431 the RTH is initiated and the pitch is then lowered to -24. Both are at the lower pitch limit for their respective flight regime. At time 237 the negative pitch isn't being limited because the desired ground speed is being achieved.

Note the slightly different background color that indicates the switch from GPS_ATTI to GoHome.

The desired ground speed for full elevator in normal flight appears to be about 9 m/sec. This is being achieved at time 237 where the pitch isn't limited. At time 395 the ground has dropped to 3 m/sec because of a headwind and the downward pitch is being limited. At time 431 when the RTH is initiated the speed starts increasing because the pitch now has a lower limit.

The headwind is the difference between the ground speed where pitch isn't being limited and the ground speed where pitch is being limited. That's about 20 mph.

 

[Zandian]

I looked at the second .DAT as it was the only one that had a motorStart and actual flight. In summary the flight appears to be normal and without issues. However, there were some headwinds encountered that caused the Mavic to have a slower ground speed in some parts of the flight.

In response to full elevator being applied the AC will lower the pitch to achieve a faster ground speed. But, there is a limit to how much the pitch will lowered. In normal flight, that limit appears to be about -16 degrees. In an RTH the lower limit appears to be about -24 degrees. This can be seen here
View attachment 3321
At 316 full elevator is being applied resulting in a -16 pitch. At time 431 the RTH is initiated and the pitch is then lowered to -24. Both are at the lower pitch limit for their respective flight regime. At time 237 the negative pitch isn't being limited because the desired ground speed is being achieved.

Note the slightly different background color that indicates the switch from GPS_ATTI to GoHome.
View attachment 3323
The desired ground speed for full elevator in normal flight appears to be about 9 m/sec. This is being achieved at time 237 where the pitch isn't limited. At time 395 the ground has dropped to 3 m/sec because of a headwind and the downward pitch is being limited. At time 431 when the RTH is initiated the speed starts increasing because the pitch now has a lower limit.

The headwind is the difference between the ground speed where pitch isn't being limited and the ground speed where pitch is being limited. That's about 20 mph.

Thanks for the detailed reply. So it sounds like the slow speed is likely due to a headwind?

Any idea how much of a headwind would cause it to slow to 6mph?

Am I reading you correctly that if I encounter high headwinds and slow speeds, if I need to get it home I might be able to get faster speeds by putting it into RTH?

Z

 

[BudWalker]

Thanks for the detailed reply. So it sounds like the slow speed is likely due to a headwind?

Any idea how much of a headwind would cause it to slow to 6mph?

Am I reading you correctly that if I encounter high headwinds and slow speeds, if I need to get it home I might be able to get faster speeds by putting it into RTH?

Z

Yes, IMHO headwinds are the cause of the slow speed.

Lemme think some about your 6 MPH question. The problem may be under constrained and it may not be possible to answer that question without more data points. In fact, it may not be possible to calculate the actual headwind from this data and my estimate of a 20 mph headwind is questionable.

Instead of initiating an RTH maybe a better option would be to switch to Sport mode.

 

[james_mav]

Yes, IMHO headwinds are the cause of the slow speed.

Lemme think some about your 6 MPH question. The problem may be under constrained and it may not be possible to answer that question without more data points. In fact, it may not be possible to calculate the actual headwind from this data and my estimate of a 20 mph headwind is questionable.

Instead of initiating an RTH maybe a better option would be to switch to Sport mode.

What about the temperature? Does your data show the RPM's for the blades? I was wondering if the temp might have decreased the power output.

 

[Zandian]

Actually, I thought I had it in sport mode, but I think I may have turned it off on the way out. To be honest, I don't remember for sure if I did ever have it on. I'm sure the data says whether I did or not. I think I was freaking out so much knowing it wasn't going to get home at that speed and planning a field landing location that I didn't think about sport mode.

But to the issue of temp, it is still strange that the speeds increased when I got closer to home. Maybe the bird warmed up, maybe the headwinds died down. I guess it's hard to tell. I thought I'd be pretty safe as long as I was around 32 degrees. I've flown in colder temps with no apparent impact on performance-based the wind may have been lower too...