I've been reading about Micro Drones claim of 1 hour flight and I have been pouring over some numbers trying to figure out if this is possible with LiPo batteries. My answer is simply no.

 

I've look at different batteries and different motors. Typically the motors are able to thrust 10g per W. This means that for every gram of thrust needed you are going to consume 0.1Wh (watts per hour). Even this number is slightly optimistic, some motors / props have half of that output others go as “high” as 11-12g per watt.

 

Let’s use 0.1Wh per gram of thrust.

 

Now, let’s look at batteries (I’ve compared thunder rc and hk batteries) and the best pack for the punch is about 650 grams for a 8400mah battery (3s). With LiPos you are supposed to drain about 80% which gives about 75Ws for this battery. If you divide the weight of the battery with the Whs you find that you get about 0.11Wh per gram of battery weight.

 

So, to simplify; just to carry the weight of batteries you will burn up all the charge in one hour. It doesn’t matter HOW many batteries you add. The equation is still the same. However, I have seen that typically LiPo batteries packs more punch the bigger they get, but I could not find any reasonably priced batteries larger than 8400mah.

 

My dream of building a 1 hour flight octo with 4lb carry capacity is just impossible.

The only possibility I see is to tweak and build a really large copter and gain a few % here and there or come up with another source of energy than LiPos.

 

Can someone correct my math if it’s off please, or direct me to more efficient motors and batteries?

 

Thanks,

Roger Larsen

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Replies

  • So, what if you created an octacopter where the you use 4 large props low kv engines and add 4 higher kv, smaller prop engines for stability?

  • An hour is possible right now, but not with a 4lb payload.

    Quadrocopter 65min 29sec flight time from Ferdinand Kickinger on Vimeo.

  • I found some interesting motor / prop configurations;

    http://www.rctigermotor.com/show.php?contentid=142

    13x4.5, 7.4v, 6.3a (47watts) with a thrust of 730g. This gives 15.66g/W. A configuration like this might give 30+ minute flight times.

  • I've got an idea.  Try to locate  three blade props that are about the same size of your current props.  You'll get more thrust so your rpm required should be less.  See if that improves your flight time.  If that works, maybe we can increase further with a 4 blade prop ?     My quadpod uses 5" 3 blade props.   I tried 5" 2 blade to see what effect it would have since i had the parts in my r/c parts box... it had less thrust and since I had to throttle up more the batteries did last a shorter period of time.

    Maybe we can experiment with that ?

  • My engineering group talked about this thread this morning. We determined that it is technically possible though the design would need to be changed. Much larger, slower turning blades would be the best way to slow the battery drain. Prop efficiency is one of the largest keys to sustained flight.

    I submit to this thread for consideration, the world record flight of a human powered heli. (multirotor quad)

    http://www.theatlantic.com/technology/archive/2012/06/human-powered...

    40 seconds sustained flight on human output, but look how it is achieved. the massive blades hardly seem to turn.

  • Motors generally are going to be about 75-85% efficient, and the LiPoly technology is what we're basically stuck with.

    The two variables which are under your control are propeller FM and disk loading.  In this worked example of a 4-pound quad, the ideal power (FM=100) is about 19 watts per gram.  If ultra efficiency is your goal, off-the-shelf props are going to disappoint you.

    DIYD Thread - The Case For Large-Scale Electric Multicopters

    As Jack said, if you made the disk loading light enough, it might work, but then rotational inertia would impact the controllability severely.  Ultimately, you're right - the energy density of batteries is still not there yet.

    DIYD Thread - Worked Example of Energy Density Solutions

  • If the propellers are big enough & the change in air velocity is reduced to near glider levels, it could probably do it.  It wouldn't be flyable in the wind.

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