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Is it ok to cancel drone initiated low battery RTH

sar104

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As a mathematician & aerodynamicist I love the discussion but I’d caution about being too theoretical. You can reduce drag by flying close to the ground, the way Charles Lindbergh conserved fuel on his epic transatlantic flight but for a small drone that is quite risky. But more importantly, local conditions can vary significantly so knowing the approximate wind speed from local met. services could be hazardous (local downdrafts & circulation from irregular terrain, hills, trees buildings etc. could change wind speed and direction significantly. There’s a saying amongst pilots “a good pilot never needs to use his skills”, good planning, contingency plans & caution will avoid tight situations.
I wonder if spurious RTH signals might be generated by the drone measuring a prolonged wind gust, and deciding RTH is necessay.
The smart RTH calculation is based purely on distance from the home point. Wind speed is not factored in.
 
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Cymruflyer

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depending on the wind direction but just for the sake of this thread, then if you have VLOS and are trying to get back directly into the wind
apart from reducing height you could use the same method as a sail boat uses and tack across the wind to make better progress
this would lessen the forces being applied to the drone to some extent, and of course would be dependant on terrain and amount of battery remaining
A sail boat does not make better progress by tacking. What tacking does is, it allows the sail boat to make forward progress when sailing into a head wind. The closer to the wind you sail, the less time it will take to get from point A to B. However too close to the wind and you chance the occasional luff, due to being too close to the head wind. Also tacking is not going to make it less time because you are covering a greater distance by constantly tacking. If you have to sail into a wind, the ONLY way you will be able to do this IS by tacking because a sail boat by design, can not sail directly into a head wind, it MUST sail at an angle towards the wind, deflecting it off its sail and thereby propelling itself forward, moving left and right into the direction of the wind, over a period of time.

As for flying back to home, if you think of the drone letting out a long string behind it as it goes about flying home. A straight A to B will be the shortest route and the shortest time. A zig-zagging A to B to C to D to E to F for example, will let out a lot of string behind the drone and when you lay those two piece out straight, you will see the length you flew in the zig-zag or tacking route is far, far greater than the straight line A to B route and obviously the time to travel that distance is longer than the shortest, straight A to B route.

Therefore, tacking (zig-zagging) with a drone is not something you want to do when trying to get home the quickest and tacking in a sail boat is not the quickest way either to get from A to B, it is simply done out of necessity. If you have a headwind to fly against, then simply drop to as low an altitude as safely possible to diminish that head wind component. In general, the wind is always at a lower speed, the closer to the ground you get.
 
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old man mavic

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A sail boat does not make better time by tacking. What tacking does is it allows the sail boat to make it easier to sail close to the wind and not chance the occasional luff, due to being too close to the wind. Also tacking is not going to make it less time because you are covering a greater distance by constantly tacking.

Same for flying back to home. If you think of the drone letting out a long string behind it as it goes about flying home. A straight A to B will be the shortest route and the shortest time. A zig-zagging A to B to C to D to E to F for example, will let out a lot of string behind the drone and when you lay those two piece out straight, you will see the length you flew in the zig-zag or tacking route is far, far greater than the straight line A to B route and obviously the time to travel that distance is longer than the shortest, straight A to B route.

Therefore, tacking (zig-zagging) with a drone is not something you want to do when trying to get home the quickest and tacking in a sail boat is not the quickest way either to get from A to B, it is simply done out of necessity. If you have a headwind to fly against, then simply drop to as low an altitude as safely possible to diminish that head wind component. In general, the wind is always at a lower speed, the closer to the ground you get.
@Cymruflyer i agree with you about it being a longer route i used the analogy in respect of the wind being to strong for the drone to make any head way into it and i did say it would be dependant on remaining battery power
 

Cymruflyer

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@Cymruflyer i agree with you about it being a longer route i used the analogy in respect of the wind being to strong for the drone to make any head way into it and i did say it would be dependant on remaining battery power
understood Old Man Mavic, but if the wind is that strong that no head way could be made flying straight into it, then no headway is going to be made flying at an angle to it either, only distance over the ground side ways could be achieved, which will not be beneficial to get to your home point, even if you had 100% battery left. Therefore any amount or type of tacking will never be of help to drone pilots trying to get home with a low battery.
 
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Thomas B

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understood Old Man Mavic, but if the wind is that strong that no head way could be made flying straight into it, then no headway is going to be made flying at an angle to it either, only distance over the ground side ways could be achieved, which will not be beneficial to get to your home point, even if you had 100% battery left. Therefore any amount or type of tacking will never be of help to drone pilots trying to get home with a low battery.
Not so... what Old Man Mavic meant was not movement only to the side, but at an angle to desired flight path like tacking a sailboat into the wind.
 
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Cymruflyer

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Not so... what Old Man Mavic meant was not movement only to the side, but at an angle to desired flight path like tacking a sailboat into the wind.
I understand him perfectly, however an aircraft in the air against the wind behaves nothing like a sail boat in water against the wind. You can not draw any similarities in how you control these two different craft. No amount of attempted tracking will allow the drone to move forward if a straight on approach will not allow forward movement, it is simple physics.

I am both a long time pilot and a sailor of yachts and I can assure you that trying to do a tacking maneuver has no useful purpose in any aircraft when attempting to penetrate a headwind that is so strong that the aircraft is standing still. That means your forward airspeed is the same as your headwind component, which would mean your ground speed would be zero MPH.

The only thing that might help in such a situation is to change altitude in the hope that the wind speed or direction changes at a different altitude, either, higher or lower (altitude that is). As I previously stated, in general, the speed of the winds aloft generally diminishes the closer you get to the ground and that is your only hope in such a situation, if you can get closer to the ground.

The wind direction and speed varies up in the sky and you can find a speed change as well as a direction change as you progress through flight levels. But with a drone your best bet is to get as low as possible, but under no circumstances will any attempt at zig-zagging or tacking or what ever you wish to call it, will help you progress forward, if the wind was, as suggest by OMM to be so strong a headwind component, that you were not able to penetrate it (move forward) at full throttle.

Your best option, in such a situation, would be to look left and right below you to find a suitable place to land and be able to be located once on the ground, and then walk there and collect your drone.
 

Cymruflyer

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Not so... what Old Man Mavic meant was not movement only to the side, but at an angle to desired flight path like tacking a sailboat into the wind.
The only reason that you tack in a sail boat, is to achieve a forward movement, because sailing too close to the wind or in a straight line head on into a headwind would not be possible. Therefore you tack left and right and slowly move forward.

Now, if you can image that the water was also moving against that sail boat, equaling, any forward gain that you may have made by tacking, or in other words, cancelling out any forward gain made by tacking, then that sail boat would not move an inch forward, it would simply sail left and right and never move forward from that position, as long as the wind stayed the same speed, so we could do these calculations of course. You can not compare a sail boat to a flying machine in the same way you can not compare a car driving into a headwind, to a sail boat.
 

Thomas B

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The only reason that you tack in a sail boat, is to achieve a forward movement, because sailing too close to the wind or in a straight line head on into a headwind would not be possible. Therefore you tack left and right and slowly move forward.

Now, if you can image that the water was also moving against that sail boat, equaling, any forward gain that you may have made by tacking, or in other words, cancelling out any forward gain made by tacking, then that sail boat would not move an inch forward, it would simply sail left and right and never move forward from that position, as long as the wind stayed the same speed, so we could do these calculations of course. You can not compare a sail boat to a flying machine in the same way you can not compare a car driving into a headwind, to a sail boat.
See your point.
I rarely encounter the situation as I try to fly so that my go home leg is downwind.
 

Cymruflyer

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See your point.
I rarely encounter the situation as I try to fly so that my go home leg is downwind.
That is always the wisest thing to do, the same in general aviation, if just going out for a local flight from your airfield if there is a fairly good wind blowing, especially if closer to sunset. The time it takes to get back home can be miscalculated and as the light fades you could find you may be landing in the dark. Setting a higher percentage for low battery warning is also another good idea, so you never find you are very low on power and far away at the same time.
 

old man mavic

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thank you all for the input to my post #12 i understand what you are all saying, and just for fun i am going to try out my thoughts the next time i fly in windy conditions,i still believe that moving forwards across the wind will allow greater progress than straight on into the wind time will tell
 
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thank you all for the input to my post #12 i understand what you are all saying, and just for fun i am going to try out my thoughts the next time i fly in windy conditions,i still believe that moving forwards across the wind will allow greater progress than straight on into the wind time will tell
You can tack across a light to moderate wind and travel a lot further to get home, but that won't demonstrate how things work in a wind that's too strong to make headway.
But if the wind is too strong to fly straight into it, tacking won 't help you at all.
It would expose your bird to the wind for longer trying to travel much further but still being exposed to a wind it can't make progress against.

Sailboats tack because they cannot sail directly into the wind .. even light winds.
They have to get a suitable sail/wind angle to be able to make headway in any wind.

If you find yourself in a troubling headwind situation, your only option is to get out of the wind as much as you can by getting lower down.
If that isn't enough to help, consider the experience an educational one so you don't go flying off downwind in a strong wind in future.
Prevention is the best way to solve strong wind problems.
 

old man mavic

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no worries i always try to fly into the wind when possible on the outwards leg but when you think about it if we are flying along a large arc to get a particular view then at some point the wind direction is going to change in relation to the forwards movement of the drone, you can tell when this happens if you keep an eye on your speed relative to direction of the drone, for me i always make sure i am coming home with plenty of battery left ,but i can understand that it is possible to get caught out if the weather changes and a sudden shift in wind direction takes place, personally i would try anything to get my drone home undamaged
 
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AMann

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I don't think you have to worry about how drag changes with airspeed - this is just an issue that the aircraft has a maximum airspeed



That's correct. Using some simplifying assumptions one can explore that relationship further. Assuming that the power required to stay airborne is fixed (~ 130 W for a Mavic 2), that drag goes with the square of the airspeed, and that the drag coefficient for the M2 is 0.025, the energy usage per unit distance, as a function of windspeed (headwind) and airspeed can be calculated analytically:

View attachment 79935

The curves show minima, as expected, that increase with increasing windspeed. Differentiating with respect to airspeed to get the location of those minima:

View attachment 79936

So for zero headwind, that indicates an optimum airspeed of 13.8 m/s (31 mph) to maximize flight distance as a function of energy usage. As the headwind increases the optimum airspeed also increases. 21 m/s (46 mph) is around the maximum airspeed of the M2, and so that suggests that it will struggle into a headwind of greater than 10 m/s (22 mph). We know it can go into greater headwinds than that however, indicating that the assumptions are not completely correct. However, the basic trend is correct.
Looks good! OK, how fast should one run to be the driest possible while getting out of a rainstorm? ;)
 

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... Is it OK to cancel drone-initiated low battery RTH and bring your aircraft back to land it manually?

Please explain your reasoning either way.
Yes, it is OK to cancel low-battery RTH, but only to find a more efficient path home. I do it so I can return home while lowering, thus returning at a descending angle as opposed to a horizontal then vertical rectangle. It is not OK to cancel RTH to linger or go further away. If you are correct about a tail wind on the return you might have no problems but you need to be aware that the next step is the Critical Low Battery warning that will force it to land, whether at a retrievable location or not.
 

sar104

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thank you all for the input to my post #12 i understand what you are all saying, and just for fun i am going to try out my thoughts the next time i fly in windy conditions,i still believe that moving forwards across the wind will allow greater progress than straight on into the wind time will tell
A very simple thought experiment shows that's not the the case.

Consider the motion of the aircraft in the frame of reference of a constant wind field, rather than the earth frame of reference. Power use is a monotonic function of aircraft tilt which means that, while the form of the function relating power to tilt is not fully specified, an increase in tilt always results in an increase in power. Decomposing the motion parallel and normal to the wind vector it is clear that the lowest power solution to holding against a wind field is a simple hover. That requires less tilt than flying back and forth normal to the wind vector which would require the same tilt to prevent moving downwind (parallel to the wind vector) and then additional orthogonal tilt for the motion normal to the wind vector. It therefore follows that flying upwind relative to the ground (which can now be considered an arbitrary frame of reference) uses the least power by flying directly into the wind, since it also uses the least possible tilt.

And at the extreme end of the flight envelope, when the maximum available tilt is required to hold against the wind, any attempt to move normal to the wind vector, by rotating the tilt (thrust vector) away from directly opposed to the wind vector, would result in drifting downwind.
 
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sar104

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Looks good! OK, how fast should one run to be the driest possible while getting out of a rainstorm? ;)
Good one. That's easy. For vertical rain and a constant spatial density of raindrops you intersect sideways the same volume, and thus same number of drops, however fast you move. However, the number of drops that hit you from above is directly proportional to the length of time you are in the rain, and so you want to get out as fast as possible.