Long Endurance multirotors

This is a compressed little story of an endurance test today with a new hexa multirotor. Some time ago I noticed a post from Ecosynth made here, which attracted my attention. It outlined the use of an octocopter with 30 minutes of flight time for the purposes of ecological analysis and 3D modeling. On the same website I found a concept design that someone specified for a hexa which was cheaper, lighter and offered slightly more endurance. This hexa is almost a 1:1 realization of that design.

The result above shows it easily achieves 30m30s in hover with 168g payload, reducing the battery from full to nominal voltage. Running the battery flat to 3.4V per cell or so I guess I could have taken out some extra 5-9 minutes. The calculations show it should be possible slapping another battery on for 53 minutes total, but in practice I'm noticing the signal going to 3 motors is getting a bit high already, so I'm not sure if there really are power reserves to do that. With 500g payload the duration is reduced by some 6 minutes or so (theoretically).

Next thing to do is see how endurance keeps up when the vehicle is in actual flight. At slow speeds like 5 m/s I don't expect too much reduction in endurance, it may actually improve slightly.

I dove into the subject of multirotor endurance and found some really good information here: http://www.rcgroups.com/forums/showthread.php?t=1880665

There are three main design pillars for multirotors: agility, weight and endurance. Optimizing towards one or two will definitely remove capabilities in the other. The choices for propellers, motors, frame, battery should be made on the basis of this understanding. There's a tool called e-calc , which was instrumental in verifying the design.

The objective of this vehicle is to map areas up to 1km2 for 2D and 3D modeling and perhaps some video surveillance assistance. All the electronics are 3DR, drive system from rctimer.

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  • Yeah my only beef is with the landing skids/gear. The original kit i got off ebay was all glass fiber and came in over 200g, with all the "mounts" for fpv and stuff it was like 500g, think heavy rubber grommets. Although i love the rail design, i might have to print a couple replacements or find something lighter. I've already replaced all the glass fiber rods with carbon fiber and without sponges (only used when i'm landing on something like concrete), but i'm still at 90-100g for the gear.

    Any recommendations for landing gear that's pretty light? I've been printing a couple replacement options, but most of them are pretty brittle.

  • T3

    Nico, I suggest to try 16'' props. I tested these motors with 17'' (on a hexa) and there was no positive effect in flight time but handling was worse. For longer flight times you need to reduce weight. 

    Gerrard, on my current setup every 200g reduce flight time about 5min only - but its is a hexa... What I also found is that the calculations are often conservative and you can expect longer flight times especially for survey missions. 

  • Nice! I did the calculations for a quad back in the day and that showed it should be possible to draw 56 mins (on a quad) from the 11Ah+6Ah battery that maxamps sells. For a quad, every 200g really decreases the flying time by 12 mins or so. Looks like what you have is good enough and it looks like a solid machine, I like it. That landing gear looks a bit heavy perhaps, how much weight does that add?  On mine, I added 81g of weight with my gears.

    Sub 14V isn't necessarily that bad while the battery is under load. The current draw and internal resistance determine how much of a voltage drop there really is, but that's usually around 0.6V or so. When I did the flights for aerial mapping I took it down to 13.8 and 13.6 at landing time.

    When you go up from 12", the flight characteristics also change a bit. In forward flight it sort of starts to float around. I never checked it, but some people claim that in forward flight with these machines it's reducing current draw by a bit and you get better endurance vs. hover.

  • I'm getting some great flight times on my 4S setup.

    You can check out my video here https://www.youtube.com/watch?v=Xdo5brnvK1I

    Basically running weight without battery and gimbal/camera is 1200g, gimbal and camera is another 250g. Probably close to 2kg all loaded up. Depending on my flying style i'll get easy 15-20 minutes on a single 4S 5000mAh battery and pulling logs from the Pixhawk shows i've consumed around 3000mA. Without gopro camera/gimbal and with just a mobius and FPV i can easily hit 20+ minutes, bare bones stripped of all non-essentials i'm above 25, hit 30minutes once just to say i did but battery was dipping below 14volts.

    But i can't recommend this setup enough. The batteries are cheap, about $40 each (ebay, new) and i get a solid 15 minutes of fast flying. At hover loaded with FPV it's about 12-13amps of draw.

    Drone Build List:
    H4 Alien carbon fiber 680mm frame with custom 3d printed parts. (30 inches in diameter, folds into a backpack)
    RC Timer 5010-14 (360kv)
    RC Timer 15x5.5 CF blades (got some 17x5.5 ordered)
    Pixhawk Flight controller (dual GPS and compass, IMU etc)
    Tarot t2d gimbal
    Gopro Hero3+ Silver
    4S 5000mAh battery, (30 minutes flight, 20 with FPV)

    Here's the drone in flight


    Here's the drone folded


    I'll excited to see what the 17x5.5 props will do, i'm sure they'll make it fly alot slower (right now, pretty peppy for a 15").

  • Great job!I am very interest in map area and reconstruct the 3D map.

  • 3s and 4s are 3 and 4 cells, so the voltage is different. adding more cells means the weight goes up. Here are the equations:P=U*I    and P=U*Ah=x Wh

    The delivered power is voltage times current, so a higher voltage on your drive system reduces the necessary current that's flowing, which is very much subject to current losses.

    The other is voltage*Ah=Wh, the total power available over a timeframe. If Ah stays the same and voltage goes up, then a 4s has more Wh's available to drive the power system (since it has one more cell).

    A 6S setup is more efficient than a parallel 3S setup, because parallel 3S delivers more current to achieve the power, whereas the current for 6S is probably around half that and then slightly less because of current and efficiency losses. Some people report 10-15% better overall efficiency on 6S vs. 3S. For multirotors it's a bit more complicated to find suitable ESC's for 6S, which is the only reason I went up to 4S on my setup.

  • Gerard. Thanks for your feed back, this is going to be very helpful for my next build.

    I have just rebuilt my large quad into a X8 Octo quad. I'm going to do some tests to compare flight times versus batteries an weights. I realize there is a point as you add more weight with batteries the graph starts to declines in flight time so i am wondering where this point would be to know which is the most efficient battery powers v weight = best flight time.

    My X8 Octoquad build  ( later on I will be able to add lower KV motors and up to 16" props with this setup)


    T-motor 2216-12 KV800  (top mount)

    Jdrones Motors AC2836-358, 880Kv (bottom mount, So its close to 7-8% faster per Kv)

    12x45 CF props (top mount)

    12x45 EPP Style, Composite. (bottom mount)

    30amp ECS

    Quad weight including with BL gimbal/gopro (no battery) 2.830kg

    (strangely enough this X8 copter build with the two extra motors and ESCs  is 150g lighter than my hex with same gimbal/gopro setup)

    Using the NASA paper that suggest the most efficient coaxial ratio is that the top prop should run at 91-93% from the bottom props this top and bottom motor combination with using the same prop size comes pretty close to this optimum coaxial percentage ratio.

    I have two 6600mAh 4s Batteries with a weight of 0.655kg each so I will do a number of flights with using one battery to get an average best flight time and then Ill do the same using both hooked up  in parallel and see what flight time I get with that to compare.  

    What I am curious about is if I used 3S batteries with the same mAh rating and weights why wouldn't I get nearly the same flight times? Or would I? I understand that the 4S gives a more responsive flight, but if for example doing  just a hover to try and test flight time endurance, I don't get why a 3S  and a 4S with the same weight and mAh rating would have that much difference in flight times.

    Secondly. say if I used 2x3S batteries in series to give a 6S kv output on the same setup as I had when using them in parallel do you think that the flight times would be much different?

  • The weight you specify seems indeed to be a sort of maximum for endurance. Anything beyond that certainly won't make an endurance machine. You arrive at that weight because you also max up the craft with batteries, yielding about 3.5kg with around 1.5kg or more of batteries.

    1. Besides long flight times, part of this effort is also about how much manoeuverability and wind resistance you want or must maintain and this impacts the performance in difficult situations (wind gusts, yawing+wind, etc).

    2. I find with my current motors and props that thrust is overspecified on the sheets and I think indeed that 360kV is more a 6S application. I'm expecting 530kV's for 4S which I hope provide better performance.

    3. They work, but I think the voltage is a bit low for good performance. Expect a machine that has reasonable slow flying performance and is level and stable, but is like driving a bus. It feels as if you have a lot of  inertia like a super tanker and all manual movements must be meditated in advance. It takes time for every throttle operation to take effect, about half a second and this confuses some operators because they overcompensate.

    6. Lower kV's are always more efficient, but you are not likely to get the thrust performance that's calculated, limiting endurance anyway because you can't take all the batteries along. Recovery is also slower as you're already at the high end of the operation envelope.

    7. I still use the 15x5.5's, but others are using 17x5.5, however on a quad. Larger props eventually require you to increase the arm length, which increases weight so making them larger from there doesn't help. Longer arms also increase the distance a motor must travel to remain level, so the amps go up at some point. for these reasons I think 17" should really be the maximum you should be going for, but 15" probably gives a more snappy response.

    8. I'm now going for 14x8 on 530kV. So there's still something inbetween.

  • I am tying to get my head around which is the best prop,motor combination to get the longest flight time using 4S batteries. I see on some forums  they are using 330-360kv motors and getting great flight times, but most of these are using 6S batteries and I don't want to use more than 4S.

    I am building a Hexcopter to get the longest flight time possible, but I don't want to use any batteries over 4S in voltage. The maximum flight weight of my hex including two 6000mAh lipos is around 3.5-4kgs including camera and gimbal. 

    1. Can anyone recommend which motors and prop combination are the most efficient to get the longest flight time for this flight weight using 2x6000mAh 4S batteries?

    2.I know the 330-390kv motors are very efficient, but I see on their suppliers test sheet it only recommends using 6S 22V batteries. How will these motors perform using 4S?

    3.What would be the problem using 330-390kv size motors with a 4S? 

    6. For this weight using a 4S would it be more efficient to use say 600-690Kv motors or are the 330-390Kv still more efficient?

    7. Which prop size is the best match for efficiency and flight time using 4S batteries for 330-390kv motor?

    8. Which prop size is the best match for efficiency and flight time using 4S batteries for 690kv motor?

    Thanks in advance for any advise.  John

  • The rctimer web site seems to be down last few days , or is it only me?

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