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?
I don't wish to get dragged back into this discussion. It's theoretically feasible, but probably not commercially viable in the hobby market.
At a larger scale (manned aircraft) where my particular focus is, it is absolutely doable; there's a lot more mechanically to a conventional helicopter than just the engine. The reasons for the large UAVs currently looking like single-rotor helicopters are many, but most have to do with parochial thinking from both the designers and customers. Low cost and simplicity are not high on most military buyers' priority lists.
"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.
I'm not trying to revisit that discussion. I just wanted to represent to the OP that there are differing opinions on the issue and mine is not even the most authoritative here on the site.
You were being very gracious by paying me a compliment even whilst disagreeing. Thank you.
There is always something to be said for "running the numbers". For example, I hope most of the flight efficiency mystique has been stripped away by my incessant pontificating about disk loading and FM. "Yah canna violate the laws of physics, Jim!"
It is clear that a great many people are spending billions of dollars on the electrical energy storage density thing. Something useful is bound to come out of it.
I don't understand ppl claiming "It is clear that a great many people are spending billions of dollars on the electrical energy storage density thing. Something useful is bound to come out of it."
The fact the somebody is spending more and more money on a technology, usually means it has reached steady limit, nothing quarantees breakthrough. For example, we are spending most money on taxes and banks. They have consultants, research departments. Yet, there is nothing ultra spectacular achieved by new taxes nor banking in the past century beyond idea of bourse money rental, money transfer etc.
Amount of money spent on energy density in batteries would rather mean it is easier to get funding for this sector beause it is easiest to draw imaginative stories around it. What you will get in shops in ten years from now, is leaving research labs right now. And what do you have in the labs? Energy densities raising by 50%, at best.
Yep, but my point is that we would only need two of those to change the world. Double density + 1/2 charge time would do that.
And that graph stops at 2002. The research in battery tech exploded when LiPos came to market. Look at all the grants and buzz around this. Great things are going to come of it.
Nissan Leaf would be 400 miles in range and charge in 15 minutes. Now, that's when the world changes. The point is; these kind of jumps are really plausible, and we only need 4 times the density of current tech for this to happen. If I were oil companies I would sell all the oil I had.. now.. Because it will be worthless for travel in 2 generations.
7% increase in density every year is a doubling ever 7 years. However, we are already looking at a doubling this year.
+160%. Real deal.
LiS is 600+ wh/kg. And that is real, with some minor tweaks (more recharge cycles).
The point is that the implications are giantic if we say 4x current density.
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.
@Krzysztof - I agree with you.
If I am correct, the lipo batteries I am working with are 11ah 4s at 820g, that would make the energy density 198wh/kg?
11ah X 14.8v = 163wh 163wh X 1.18 = 192wh/kg
If this is correct, that would make the Evia Systems about double the density at 400wh/kg.
Anyone know of a standard heli design that uses an electric tail rotor motor? To me that seems like that might be a decent way to go.
Is there an easy way to calculate how much tail thrust you need to counteract your reaction torque?
Anyways; here are the motors / props I'm ordering. Skipping the T-motors due to a lot of bad reviews.
I've also seen some strange looking thrust benches. I'll try to build one which does not use a L to apply force to a scale, and place the motor on a long stick attached to a 4kg weight at the end. Place the weight on my scale and read how much it can lift. If you have to much stuff right behind the prop it will give you incorrect readings (since the force of the prop will push the plate down at the scale and counter the thrust of the prop). Kinda like what happens if you put a big plate underneath a helicopter and try to take off with the plate attached (yep, I saw this on Myhtbusters :) ).
2-6 cell batteries (where it makes sense to use them).
(already have this one)
(Graupner 14x8 e-prop)
I feel I need some more props to test; but these seems to be what people recommend. I did find some 16" graupners but no CCW :/
I'll keep you posted!