Question RE: appropriate RTH altitude

[FlyawaySOB]

So I've followed numerous crash threads (both here & other forums) & spent a lot of time thinking about the areas I intend to fly near. I do not have any experience with a DJI drone yet.

I totally understand the idea of taking the highest obstacle in the vicinity, adding some extra feet for good measure, and having that be RTH altitude. However common sense that may seem, I'm a little amazed at how many people that logic has eluded or in some way failed.

I won't pretend I'm any more virtuous or sharp so I'm wondering... To simply avoid RTH altitude issues, why not just set it to the highest allowable by law and not worry about it? Is it a battery life thing (eg. too much consumed unnecessarily ascending & descending)?

 

[halley]

Highest by law is usually "altitude above ground level (AGL)" and the Mavic sensors have no idea what altitude it has above the ground. The Mavic's altitude is measured "altitude above take-off point (ATO)" which could be wildly inappropriate in the case of canyons or mountains.

 

[F6Rider]

You answered your own question, exactly that. If you are out at distance and are so mesmerized by the awesome scenery that you get a low battery RTH, and have you RTH height set to 400' then you are going to have to go for a walk to retrieve your bird, because it is going to go into auto-land.

 

[Logger]

That doesnt make sense to me F6Rider. Low battery RTH is smart enough to occur when it has sufficient battery to return home. Will autoland back at home point sure, if you do not intervene, otherwise cancel and land it yourself. What am I missing, Why would you have to go for a walk?

 

[DebianDog]

A drone does not come straight down very fast due to flight characteristics of drones in general. I have never timed it I would guess that if you fly to 400 feet then hit RTH it would be a good minute... minute and a half... two minutes? Not sure what he means by you would have to walk either as the Drone gains altitude and flies forward relatively quickly in comparison. My fear would be that it would drop out of the sky for the last hundred feet.

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[Ender]

Okay, the two "dangerous" factors are battery as mentioned above and wind.
Wind can be harmless close to the ground and REAL stormy at 150m Altitude. And have a different direction.
So if you always go for a huge RTH Altitude its not only about the increased battery usage for goung uo and later going down all the way, it can also be drastically increased because it has to fight the wind AND it can be too much wind to counter. Of course this is only a common phenomenon in some areas, but where i live i have experienced it numerous times even to the point of having 70 km/h difference in wind speeds from ground to high Altitude.

So my "test ground" has Trees that are 35m high and i set my RTH ALt to 45m safe margin but not overdone.
But if unsure i would go for a very high Altitude indeed but i will go up there manually to have an idea about the wind situation and i will watch battery usage even closer.

Greetings,

Ender

 

[Kilrah]

I have never timed it I would guess that if you fly to 400 feet then hit Autoland it would be a good minute... minute and a half.

3m/s max descent speed for the Mavic, it shows up on your screen.

 

[The Editor]

Okay, the two "dangerous" factors are battery as mentioned above and wind.
Wind can be harmless close to the ground and REAL stormy at 150m Altitude. And have a different direction.
So if you always go for a huge RTH Altitude its not only about the increased battery usage for goung uo and later going down all the way, it can also be drastically increased because it has to fight the wind AND it can be too much wind to counter. Of course this is only a common phenomenon in some areas, but where i live i have experienced it numerous times even to the point of having 70 km/h difference in wind speeds from ground to high Altitude.

So my "test ground" has Trees that are 35m high and i set my RTH ALt to 45m safe margin but not overdone.
But if unsure i would go for a very high Altitude indeed but i will go up there manually to have an idea about the wind situation and i will watch battery usage even closer.

Greetings,

Ender

Yet another example of where switchable Atti is an advantage.
Simply fly up to your intended RTH height and then switch to Atti and watch the direction and speed of the drift. You then have a good idea of winds aloft.

 

[Logger]

Okay, So how is the battery a "dangerous" factor? RTH will allow for remaining battery life and act accordingly. Nothing dangerous there.
Wind strength at altitude can indeed be stronger (Not sure what you mean by Stormy) and is a reason I set mine lower at times. Depending on the wind direction vs flight path it can also be beneficial during RTH.

 

[DebianDog]

Yes I would love to have a manual Attitude mode.

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[DebianDog]

3m/s max descent speed for the Mavic, it shows up on your screen.

Wind gust and other weather conditions will dictate that

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[Kilrah]

Nope, that's what's programmed in. Stick fully down is a command to do a 3m/s descent.

 

[DebianDog]

Nope, that's what's programmed in. Stick fully down is a command to do a 3m/s descent.

Taking it out of return to home mode sure it will do its best to descend. But at four hundred feet or 121 meters that is still over 40 seconds

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[The Editor]

Okay, So how is the battery a "dangerous" factor? RTH will allow for remaining battery life and act accordingly. Nothing dangerous there.
Wind strength at altitude can indeed be stronger (Not sure what you mean by Stormy) and is a reason I set mine lower at times. Depending on the wind direction vs flight path it can also be beneficial during RTH.

Because these 'intelligent' batteries are anything but.
An already heavily depleted lipo may well behave unexpectedly under full load during a climb out procedure. If the pack hasn't been looked after particularly well then it may well let you down when called upon.
DJI have attempted to make their packs idiot proof but they are not. At the end of the day, they are common Lipo chemistry and will react no differently to any other pack (in fact worse since there is current limitations and LVC built in which means you cannot sacrifice a pack to get your aircraft back should that be necessary)

 

[Logger]

I see. Well I guess it may let you down during normal flight too then. I wont be setting my RTH heights based on the LIPO battery letting me down. I don't see it as a serious threat. It MAY work perfectly well too, or just let me down out of the blue mid flight. In any case if the RTH triggers with the AC below the programmed RTH Height it will be called upon for a climb at your full load regardless. Just for a shorter time for lower for a lower set RTH which is arguably less risky.

 

[Wacker2611]

You really don't need to procrastinate about it too much. For the vast majority of situations, 50 metres is more than enough. I just leave mine on that, although I very rarely use rth. Over time you'll get familiar with, for instance, roughly how high a tree/building is in the area your going to fly and you can change your rth up to accommodate it.


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[Glenn Goodlett]

I sometimes fly near hills, so, I've seen my altitude at 130 meters but my VPS sensor showing I'm only 5 meters above the hilltop. If I hit RTH at this point, I'm fine. If I go to the back side of the hill and drop down, I'm screwed. It does not matter what I've got my RTH altitude set at.

I don't think there is a best setting for every circumstance. When flying, think about what you are doing and the limitations of the hardware and software. In the future I think this limitation may be overcome with updated firmware like the P4P already has.

 

[Logger]

I sometimes fly near hills, so, I've seen my altitude at 130 meters but my VPS sensor showing I'm only 5 meters above the hilltop. If I hit RTH at this point, I'm fine. If I go to the back side of the hill and drop down, I'm screwed. It does not matter what I've got my RTH altitude set at...

You are only screwed in the sense that you cannot manually RTH, which apparently is your SOP every flight (as you mentioned elsewhere). It very much matters in your example what the RTH altitude is set at, because shortly after your signal loss from dropping below the hills, fail safe RTH will trigger. If you had set your RTH < 130 = possible crash. If you set it > 130 to say 150m you would be all good.

I fly on a 2000 acre farm with rolling hills. Have flown a survey with Mavic to establish the hilltop tree heights with regard to my home point. Now I know I am safe as long as RTH is set to 60m it will top them all.

 

[Glenn Goodlett]

You are only screwed in the sense that you cannot manually RTH, which apparently is your SOP every flight (as you mentioned elsewhere). It very much matters in your example what the RTH altitude is set at, because shortly after your signal loss from dropping below the hills, fail safe RTH will trigger. If you had set your RTH < 130 = possible crash. If you set it > 130 to say 150m you would be all good.

I fly on a 2000 acre farm with rolling hills. Have flown a survey with Mavic to establish the hilltop tree heights with regard to my home point. Now I know I am safe as long as RTH is set to 60m it will top them all.

Problem is that if you set your RTH at >130 meters you are no longer legal once you get out of the hills. But, you are correct, 150 meters would do the trick. I didn't think you could set the RTH to above 122 but you can set it up to 500 meters. Just checked.

 

[Logger]

Where I live you are not legal beyond line of site, so you would have already broken the rules by going behind the hill in the first place. I guess this is a shortcoming of routinely use RTH every-flight instead of as a backup for when things go wrong. My drone wont be shooting up to those flight levels unless things go wrong. Where as yours does every flight.