What would you say is the most efficient and economical forward MP speed for conserving battery during a flight? I'm guessing around 20 mph. I'm trying to figure out at what speed to set my Litchi missions .
most efficient MP speed for conserving battery?
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What really going to make a difference is if you calculate your wind directions with your speed, that is the key to getting the most out of your battery.
Phantomrain.org
Gear to fly you Mavic in the rain.
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It's in the Mavic Pro specifications:
Max Flight Time: 27 minutes (no wind at a consistent 15.5 mph (25 kph))
Max Flight Time: 27 minutes (no wind at a consistent 15.5 mph (25 kph))
This has bothered me from day 1. If you fly faster the battery doesn't last as long. But it's using 3 , 4, 5 times the power to stay up as it is to go forward. If my battery has 1200 MaH, and hovering uses 3 amps. I've got .4 of an hour; if going forward 3MPH uses 4 Amps I've got .3 of an hour so I can cover 0.9miles. If going forward at 6MPH uses 5Amps I get .24 of an hour so I can cover 1.44miles. And if going forward at 9MPH uses 6Amps I get .2 of an hour but cover 1.8 miles.
What I don't know is the currents are for different speeds. Do you get to maximum pitch before you stop increasing the distance ?
What I don't know is the currents are for different speeds. Do you get to maximum pitch before you stop increasing the distance ?
No - the optimal speed for maximizing distance is less than top speed. For example, for the Mavic 2:
Max Speed (near sea level, no wind) 72 kph (S-mode)
Max Flight Time (no wind) 31 minutes (at a consistent 25 kph) (= 13 km)
Max Hovering Time (no wind) 29 minutes
Max Flight Distance (no wind) 18 km (at a consistent 50 kph)
Note, however, that this is the optimal airspeed, while you are setting the target ground speed.
sorry, don't know the difference between air speed and ground speed, to me 15.5 mph is 15.5 mph on land, air, or water 
i understand wind conditions will affect actual speed in the air, but aside from that, let's say in calm weather, no wind, setting Litchi mission cruise speed at 15.5 mi will get me the best battery performance, right?
flew 3 miles in a straight line at 15.5 mph today, almost no wind. got there with about 60% battery left. gonna be looking into battery mods to go on 7-10 mile missions....or maybe not.
i understand wind conditions will affect actual speed in the air, but aside from that, let's say in calm weather, no wind, setting Litchi mission cruise speed at 15.5 mi will get me the best battery performance, right?flew 3 miles in a straight line at 15.5 mph today, almost no wind. got there with about 60% battery left. gonna be looking into battery mods to go on 7-10 mile missions....or maybe not.
sorry, don't know the difference between air speed and ground speed, to me 15.5 mph is 15.5 mph on land, air, or water
flew 3 miles in a straight line at 15.5 mph today, almost no wind. got there with about 60% battery left. gonna be looking into battery mods to go on 7-10 mile missions....or maybe not.
Correct - in calm conditions. My point was that into a headwind, 15.5 mph ground speed will be a higher airspeed (by the value of the wind speed) and downwind it will be a lower airspeed, neither of which will be optimal. But it may be too much complexity to try to take that into account when setting the mission speeds, so just using 15.5 mph ground speed may be the best you can do.
Are you sure about those times ? Staying in the air longer flying forwards than hovering ?
18 km @ 50kph would be 21.6 minutes. ~ 3/4 of the hovering time above, so 4/3 of the consumption.
So simplistically using 1/3 of hovering power for thrust gets you 50 kph,
IIRC, drag doubles if you increase the speed by square-root-of 2. So double the the thrust, would mean consuming battery at 5/3 of hover rate and be expected to give about 72Kph, so I'd expect it to last 3/5 of the hover time time ... ~17 minutes it will get 18 KM in 15 mins ... so my model obvious isn't quite right.
That's what I'm trying to get to in my head... what multiplier of battery use while hovering comes with any given speed. Or at least the maximum distance at each speed. Really I need to go out and do some long runs with my drone at constant speed and look at the telemetry but I was hoping someone had numbers. I had a quick try at it but I didn't fly for long enough to get good data..
They should be correct .. they are DJI's numbers from the published specs
When the drone tilts more to go faster, the motors have to spin faster to compensate for the reduced vertical component in the motor's thrust.
Have you figured that in your model?
Unless it all somehow works differently for drones, those numbers shouldn’t be correct. I’m a retired helicopter pilot and rotors are subject to something called translational lift. As you start to tilt the rotors to move forward, you change the vertical component of airflow and, in simple terms, you use more power to hover in still air than you do to move forward. There’s a point at which the increasing forward speed starts to use more power again but certainly at lower airspeeds you should need less power than when hovering in still air.
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Well it's counter intuitive that hovering uses
edit I wrote the above back to front. You'd expect Up + Forward to use more power than just up.
Yes I was allowing for the tilt. "Up" part is output * cos(pitch-angle) and the forward part is output * sin(pitch-angle). Since the up part must be equal to the weight, the total output must always be weight/cos(pitchAngle). And at constant speed drag will be equal to sin(pitch-angle).
So the faster (greater pitch)the more power you needed. But the angles we fly at always have the majority of the power going upwards. So we haven't doubled the output of the props.
Try it the other way round. Let's say you told to drone to consume a given current, and adopt whatever pitch kept it at a constant height with the motors using that current, that would set your airspeed. What would the graph of airspeed against input power look like ?
If it were a simple 10 watts + 1 watt per m/sec, then 5m/sec would use15 watts, and 2m/sec would use 12 watts - with more invented-for-simplicity numbers, if the battery can supply 15 Watts for 20 minutes, it can can supply 12 Watts for 25. So I could do 120m/min * 25 = 3KM or 300m/min * 20 = 6KM.
And that's the question I'm trying to understand . Obviously it is not as simple as that (drag increases with square of speed for a start, so it won't be the same increase in speed for each added watt. )
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Alex Baxter
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There was an excellent scientific test done on a Phantom 3 pro to find out exactly that, I can't find any link to the tests at present, but it turned out to be close to the maximum speed. I'll delve further to see what I can find, I know they are aerodynamically different, but gravity being gravity, it'll have the largest effect.
Alex Baxter
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Here it is, proper scientific document..
long range drone DJI Phantom 3 professional
The DJI Phantom 3 Professional can be used for good long range flights. But how far it flies? This is calculated in this professional article.
seat108
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Thanks for those numbers, they are consistent with what I'm seeing, and it takes the stress out of Litchi mission planning somewhat. I've been tempted to turn off obstacle avoidance on fully vetted missions to see how it increases battery life.
Drag scales with ?² provided that the geometric drag coefficient doesn't change, but that's not the case as you change the pitch of a Mavic - it goes up too. You would have to factor that it to get a useful empirical model of power vs. airspeed.
Thats not what I said! To quote myself, “you use more power to hover in still air than you do to move forward”. Although hovering near the ground changes things slightly (because of something called ground effect), as a general principle in a helicopter (drone?) as you move from the hover into forward flight, the power needed to maintain height reduces. The effect may not be quite as marked in a drone as you are changing the speed of the rotors rather than the pitch of the blades and the blades tend not to cone up significantly, but I would think the basic principle still applies.
Sadly, it’s not just a matter of angles. There are aerodynamic forces involved (like translational lift) which change the power requirements significantly. If you look at a typical helicopter performance graph of power required to maintain level flight, the power you needed to hover reduces markedly as the airspeed starts to increase before gradually increasing again as drag starts to become a more significant factor. I’m making the assumption that the same basic principles apply to drones. In most of the helicopters I’ve flown, you only start to approach hover power again when you get over about 90 knots (significantly over, in some cases).
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Alex Baxter
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If one were to build a drone strictly for distance with no other considerations, would the body would be at an angle of c. -45° when hovering, and level at the most efficient speed? I would guess this to be the case. It would be an odd looking beast..
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