Multi-rotor endurance factors?

I'm still trying to get the basic concepts into my head. What I can't really wrap my head around is the endurance, or better, which factors influence the endurance how?

Of course, it's clear that weight is a factor and battery capacity. Then probably, the motors have some range in which they operate more efficiently.

If I look e.g. at commercial drones like the Microdrones things. They boast endurance of up to 70 mins in a quadrotor.. Here my problems start because as I understand it...:

If I want to have more endurance, I need more mAh. More mAh = bigger batts = more weight = less endurance. Much more weight = overload = need for bigger props + bigger motors, amybe bigger airframe alltogether = more power consumption = less endurance.

But that is only my view of the things, based on basic understanding and some applied logic.

So the question remains...:

Which factors influence the endurance of a multi-rotor in which way? How do I build a multi-rotor which can carry 2 cams (FPV + recording) with, let's say, an hour flight time?

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  • hahaha..

    1.  that is a typical nightmare of any designer... let me try and make it simple for u... for a multicopter..... u will never hit one hour of flying time, unless the scientific community some up with a solution of super conductivity in ur life time.

    2.  However.. please understand... if u have a particular machine... which ever one it may be... and that machine is the constant, then best flying wt is when ur throttle is around 50 to 60 % . so u can just increase batt capacity till u can hover between 50 and 60% throttle..

    3.  Now , u mention, one FPV cam and one recording cam.. what is the wt of the recording cam.. if its a DSLR like canon 7D with along with the lens weighs 1.5kg, and add another 1.5 kg for 10,000 mAh lipos.. that it self is a wt of 3kg, for this ur Hexcopter would itself weigh 3.5kg.. .so that is a total wt of 6.5kg.... and u will hover between 50 and 60% throttle... u cannot get one hour flying time... u will get a total time of 15min.. which u cannot use cause if u drain ur lipos to 0 Volt u will loose the batt.. so keeping a safe margin u will get 12 min... if u want more... go for a gasser heli.. that is why it was invented... 

    4. u can get only that much and no more from multicopter... 

    5. or buy lots of lipos, so that if say a lipo give u 6 min of safe flying.. make sure u have 10 lipos at hand.. land.. change lipo.. lake off.. only that way u can get 60 min of fly... 

    6. All the best


    Ramesh Tahlan

  • Developer

    I don't think microdrones has much on it when it's flying over 1 hour so that's a very high hurdle to reach.  Not impossible of course, the pros are only human as well!

  • longer props have a higher aspect ratio (better lift:drag ratio, more lifting force for a given ammount of work. i.e. better prop efficiency). But longer props require a bigger frame, which is more weight.

    Higher voltage batteries/motors means less amps (thinner wire, smaller mosfets/ESC, less weight) for the same ammount of work. So, A High voltage motor with long props would be best - e.g. low KV motor. But low KV motors are bigger and heavier (more copper). Relativelty small/light low KV motors are the most expensive kind (strongest magnets, etc).

    So there's a ballancing act between prop efficiency (long props, low kv motors) and motor efficiency (thin wires, light motors). If you bring costs into it, there's a sweet spot between 800-1400kv motors, 8"-13" props, and 2-4 Cell lipos. outside those bounds things start to get a bit exotic (expensive).

    There's another thing. Imagine a multicopter that hovers at 100 percent throttle; it's brushless motor is being used most efficiently, and there is minimal dead weight (all of the capacity of the motor, wire, esc, etc is being fully utilised). But if it's hovering at 100% throttle, it can never climb. What's more, it needs a certain "stability margin" for stabilisation. For sedate flying, I think about 10% throttle is a usefull stability margin (as in 10% in reserve at "full throttle", not hover). For acrobatic flying style or gusty conditions, the autopilot probably need more like 15% "unused" capacity for stabilisation. If this imaginary hovvering quad is very highly tuned and flying in a windless stadium maybe it could get away with only a few % stability margin - but how is that contrived situation better than simple pole? :)

    It's the same for strength - to maximise hover time you could make a frame that's exactly strong enough to hover, using expensive super light materials. But if you want to manouver, you would need to make it slightly stornger (heavier). If you wanted it to survive the rigours of practical use, it would need to be stronger still. And of you wanted it to be affordable, you have to pay even more weight penalty.

    Similar deal with batteries. If you drain every last drop of power out of your LiPos you ruin them. If you use Lithium Phosphate batteries (which can be safely discharged more fully), you pay a 20% reduction in energy density. Personally I plan to limit the drain on my LoPos to ~60% capacity, to prolong their life and leave some capacity for emergency. (e.g. 4Ah 3Cell Lipo theoretically has about 48 Watt-hour capacity, I'd plan to land by the time it's done 30 Watt-hours work).

    In practice, with the energy density we have with todays batteries, 5-10 minute flights with a utility airframe of 1-2 KG is practical. You can get away with slightly mismatched components and 20% AUW of payload (cameras etc) if you are happy with 5 minute flights. If you want 10 minute flights, you need reasonably well matched, good quality components, and not to much payload relative to the size of the quad. If you want to push past 10 minutes, you need to make carefull compromises.

  • I had this discussion non  AeroQuad, and to summarize, the following item help to increase endurance:

    1. Low RPM, high torque motors allows the use of larger props.  The larger props mean higher larger surface area, and thus more efficient lift production.
    2. The more efficient motors use higher voltage batteries.

    I'm sure there are more factors, motors and props are the major factors.

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