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?
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.
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.
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.
I agree that 1h flights for a copter is unrealistic. The best performers that can lift a reasonable payload (DSLR camera) operate in the 15-20 minutes range.
The general route for long flight times is high voltage batteries (4-6S), very low kv motors and big lightweight CF propellers in the 14-15" range.
My university just invented a new lipo with tin battery which has about 3X the power of current lipos. So don't worry, batteries are going to have a quantum leap in 1-2 years.
Unfortunately, I can't share them with you yet. You'll have to wait for mass production.
"3x the capacity, better cycle life, higher charge rates and less expensive"
We do have the technology to power longer flights, gas is a good example, but I look forward to a time when these anode alternatives arrive on the market.
That is not to say we cannot optimize more with existing technologies. To really achieve efficiencies on the order you are talking, you need the right props, the right motors, a direct drive motor system is not likely to be the best choice, and there may be a good case for variable pitch as well. I've worked the raw stats of many motor/prop combinations, and 10g/W is rare in practice. It can be achieved on a chart, but seldom is achieved in flight. I hope to publish more information about this soon.
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)
40 seconds sustained flight on human output, but look how it is achieved. the massive blades hardly seem to turn.
There's always a lot of hype in the battery game since so much money is dumped into the research and the scientist's reputation rides on making advances. I can't imagine spending 5-10 years of my life as a battery researcher and failing to make any progress, but that's usually the case.
We'll have to see how the WSU battery thing plays out. I trust their claims more than the rest because this didn't come from a multi-million dollar battery research lab. It was essentially a spin-off from basic materials/engineering research.
Right now the prototypes I've seen are watch battery size, so hopefully it will scale. WSU is really aggressive in the patent area and I think they've already been licensing this out. There was a ton of local press on the issue when this came out. Too bad reporters aren't more educated and always keep turning out "Soon your cell phone will last longer" stories with no real technical meat.
Jake: ohh, Endless-sphere.com reference. Perhaps I am talking to a fellow e-bike enthusiast? :)
Mike, I think that the moment you start adding gearing and variable pitch you are better of with the more efficient design of a helicopter. One of the reasons why a multicopter makes sense even though you have the efficiency loss of multiple motors and small propellers, is that it is so mechanically simple.
John, I'm on record as saying the same thing. I've been crunching numbers from live tests and I'm not as confident any more. There is the theory, then there are the practical performance from available components. I need to verify some data on my own, and that may take a while, as I need to build a better testing environment. But the patterns in the data have me wondering. If I get that far, I'll publish my findings, flawed as they may be, in this community.
I don't want to come off as a crackpot. I'm just working with the numbers. But as a practical illustration of theory (sound, repeatable, even demonstrable) the same can be said for a mono-bladed prop, it has an edge in efficiency, but we just don't see them in use. There are reasons. And so there are scaling issues and torque issues as we scale the blades, and scaling issues with increasing to larger motors, etc. But there is more, and when I can describe it with numbers, from third-party tests on a large, empirical scale, I'll share. Until then, I'll just remain concerned about the delta between the theory I know and have studied, and the numbers I've been crunching...
Interesting. Was there a reason why the props were overlapping, or was it just to save space?