# Impossible to get 1 hour flight with current technology?

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

Views: 7922

### Replies to This Discussion

I've seen a setup where a guy was able to get about 200w. There is a video of his setup on youtube. The hard part is getting these motors to start automatically.

If you were able to extract >1Kw out of 1 Liter of fuel then you might have something.

I'll test on my workbench first.    The idea is to start the engine before launch and control the generator by engaging and disengaging the clutch. Everything needs to be light weight.  I know my .049 engine is light, but there are smaller engines than that. heck I got excited when I found a company that makes micro sized clutches.   I'm going to look for a cool burning fuel.   There are high rpm engines that run on denatured alcohol too. The main issue is to see how much power can we actually get from a spinning brushless motor and how much power would it require to spin it fast enough to tap the three phases to charge the lipos.  Imagine using each phase to charge an individual cell.  We can use a 3s battery. We would need to create a charging circuit.  I wonder if it's possible to get the motor, generator, and fuel to weigh no more than 600g.  Weight might be offset by no more need for a large battery ? Go with a light 2200ma battery connected to the generator ?  It's just an idea.  First I need to sharpen my piloting abilities, add full telemetry and FPV.  I think the microturbine generator will be a great winter project.

maybe we want something easier to spin than a brushless motor ?  Maybe a homegrown 3 phase brushless generator.

I am far from becoming an expert on this.  You are the experts.  I've just got my feet wet and built my first quad two weeks ago.  I'm am still learning and there's lots to learn.  I did manage to use my skills to get an amazingly clean sonar signal! Have you seen my post regarding my maiden flight ?  No noise!

I soldered the leads as close to the pcb as possible. The insulation is against the pcb no wire exposted.

I used double sided tape between the #3 motor arm and the sonar.  the sonar is located 3" away from the quads central platform and the sonar is fastened with "permanent" double sided tape and the smallest zip ties I've ever used.

Regarding the sonar sensors; I have used these a lot in different settings. Adding a cap and a resistor cleans the signal up a lot. In addition maxbotix just released an new sensor which is much better.

http://www.maxbotix.com/products/HRLV.htm

The have much better resolution and are less subject to noise.

I'm suprised that people haven't picked up these yet. I ordered quite a few of them for a different project.

My goal with a generator would be in a complete automatic way where the generator can start and stop on demand. Light weight and produce a lot of power.

Envia systems are set to manufacture 400wh/kg cells this year, so perhaps all of this is not needed.

The problem is that these engines don't put out that much power, thus you can't generate much power from them.

It's highly unlikely you can get anything flying in this manner, let alone gain anything by doing it.

I have a gas fixedwing in the works which will use an alternator or tap the magneto to provide some power for the electronics.

Basically you just need a 3-phase rectifier, which you can find in any automobile alternator.  From there I plan to run the voltage right into a small lipo charger, which will handle the charging and balancing of the cells.

If I'm lucky, I'll be able to pull enough power to keep the battery charged, or at least greatly extend the run time.  But it's certainly not going to be enough to run an electric motor.

That's exactly what I plan on doing.  The quad will take off fully charged.  Yes I know a brushless motor is an alternator not generator.  This will be used to extend the flight,  not run the motor directly.

@ Jerry - You do realize how inefficient glow motors are and the published RPM limitations of the clutches you linked to.   I think this whole matter is a dead end.  Why not concentrate on gas powered conventional helicopters?  The beauty of a multi-rotor is simplicity.

Glow engines can be efficient.  I've got an 6lb 80" WS Piper Cub model with a Saito 56 that will fly for 30 minutes on just 11oz of fuel.

This is pretty much my own conclusion too. Even fuel cells is not going to work.

I'm packing over 1/2kw of energy. Recharging that is going to be really difficult. In addtion more weight = less efficency for props = even more W needed.

I'm not saying this is impossible; it's just a stop gap for something which will happen pretty soon: higher density batteries.

If we get 2 moore laws in batteries (double density + 1/2 charge time) the whole world will change. By my studing on this topic that will happen in less than a decade.

I was recently touting the efficiencies I built into my quad copter and its long flight duration.  "People in the know" weren't very impressed.  I was told, dead pan, that by 2020, there is to be expected, a 20 fold increase in battery energy density over today's typical Lipos.

As completely outrageous as the claim sounds, I have every reason to believe them.

I agree with Roger that this would change the world.

"I was told, dead pan by 2020, there is to be expected, a 20 fold increase in battery energy density"

Yes it they drop hydrogen bombs on major cities you can expect just that. In other scenarios... no chance.

I was told the same about processing power of a processor 10 years ago, when all thermal and density issues were already clearly visible. What we got now is a market of growing number of transistors and parallel cores that can accelerate some 10% of existing algorithms (the database of those numerical algorithms is a result of 100 years of research in most basic form of computational science).

What it means to you is that you have wasted important part of your life trying to impress a subset of fools from declining western civilisation.

I would like to add that replacing car travel by drug abuse might also reduce travel costs.

At 10-15 fold we can fly commercial airlines on batteries. Think about that.

(Possible through LiAir batteries or similar)

Regarding CPU power. People thought we would never break the 100nm barrier for transistors. Then the next goal was 50. Now you can buy 22nm stuff at walmart. Intel got plans for 10nm CPU/memory architectures.

Again, things which people thought was impossible.

A 4 fold increase in density renders gas engines obsolete, and that goal seems "easy" in comparison to producing transistors/nand cells at 10nm scale.

Sorry, no Moore's law for batteries...

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