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?
Roger, I am in the process of building a "professional" octocopter right now, aiming for 20+ minutes.
Here is my choice for low kw motor and 15" CF propeller
Preliminary tests show that I should get stable hover with a 8000mah 4S lipo battery and payload, using ~5A on each motor (~600W total). Theoretically this translates to 30min flight. But with the lipo 80% rule and so on, I will be happy with 20 minutes. And much more importantly, the motor/prop combination just looks sexy.. :)
Very purddy john. Very purddy indeed.
looks awesome! when you get this going, will you post specs and pics? I need to get my order going for a good octocopter and am more of a mechE than an EE. If you get good results, I would love to replicate an order.
Nice motor! Too steep in price for me. I'm learning to fly I'm sure there's going to be times when I bury the props into the ground. I'm sure the motor bearings don'y lime that. So I went with the NTM motors. $15.00 each. I was looking to do something similar, but the motors alone would cost the price of my current build minus the APM 2.0 board :)
I am running them at 11.1v with 10.47" props. So far the build as of today weighs 996g It flies well. I'm going to time the loiter duration as soon as I have my attopilot board installed and calibrated(waiting for mu deans ->XT60 connectors and 1" heat shrink tubing). When the cells hit 9.6v I'll go into LAND flight mode. With a 2200ma Lipo I'm getting @ 7 minutes. I ordered a 6000ma Lipo/ My current goal is around @ 20 minutes as well.
I'll be willing to throw 3 blade 10.47" props on my APM 2.0 450mm quad to get some real world measurements. I understand why it shouldn't be more effecient, but my little Snellflight Quadpod with 5" props has longer flights with 3 blade props than 2 blade props.
BTW.. are those carbon fiber props ?
My math has to be off. Looking at these batteries, motors and props this is what I came too:
It says right there that the same type prop (not graupner ones) gives about 6g/w thrust. The batteries weigh in at 1640g. The copter itself, 1000g (just a guess, but it seems reasonable). Each motor has to lift 660g, which is 110Wh per motor. 4 motors = 440Wh. Batteries come in at 260Wh, which is about 35 minute flight time. I'm confused. Just lifting the batteries alone is 293Wh.
Unless these props are just amazing :)
I have the same motors and batteries. Please correct me if I am wrong, but if the average voltage of a 4s lipo is 14.8v, then 14.8v x 22ah = 325.6Wh ?
With the same motors, (1) of the same batteries, and APC 14x4.7 props, I can keep my 2kg quad in the air over 35 minutes no problem.
Keep in mind that a hover is more like 1.15x mass, not 1x mass. And you want to have some headroom over and above that (usually a lot of headroom, if you are going to align your prop and motor into the most efficient portion of their respective curves.)
That is if you can drain the battery 100%, which is probably not a good idea :)
I run my batteries down to 3.2v per cell. They take a full rated charge or better and I've had no problems.
Interesting; so you're getting close to 100% of the 11,000mah? Btw the batteries linked here has about 40% more energy per weight than other brands I have sampled. These might make my 1hr 4lb setup possible.
@Roger - Those batteries are that good. Super expensive, but come in lighter, and take more charge than rated. If you figure they are guaranteed for 300 charges or 3 years, it takes some of the sting out.
At least that's how I rationalized it for the uavforge competition...
They actually sent me a hand written note to not keep them a secret... I should be getting a commission...
I just need to find a way to charge them at 5c. That's 55amps for one and I need to charge two in parallel at 110amps.