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?
Interesting. Was there a reason why the props were overlapping, or was it just to save space?
I would think it has to do with weight as much/more than space. Prop size, no doubt, is driven by lift requirements and wing loading, but prop placement and spacing must be a struggle with weight, rigidity, and extending power out to the prop point of axis within acceptable specifications
There are a couple of ways to look at this, and certainly the most obvious is getting a lighter disk loading in a more compact space. The research paper that sealed the deal for me, and the seminal work which validated the whole Redundant Array of Independent Rotors (RAIR) concept aerodynamically, was this one from Gordon Leishman at the University of Maryland:
The most significant prediction made by Dr. Leishman is that there might be a range of dual propeller overlap which is actually more efficient than two isolated systems. Maybe the Father of multiple rotors and the "Flying Banana" designer Frank Piasecki (who also spawned the CH-4X series) was crazy like a fox. Leave it to a nut like me to try it with 36 (long story, but 12 ought to be the right number in production).
The funny thing about aerodynamic physics is that there really isn't an underlying "master theory" used to extrapolate performance. Almost all the formulas used are empirically-derived. Certainly, classical mechanics can be used for basic disk actuator theory (F=MA) and there is that constant reminder that the laws of physics have to be observed. However, there are some counter-intuitive things which arise, like the Reynolds Number paradox (in small props, faster is usually more efficient).
Check out the seeop spin wing.
Check out the "watch video of conversion link" there's also a pdf in another thread that details the use of large airfoil shaped blades.
Thanks for that link. Brilliant work from them.
However, they have a heck of a long way to go to get to 3 meters. 60 seconds isn't out of reach - they've already done 50, but the craft is clearly in ground effect, and the energy to get it up out of ground effect will be huge, and likely impossible for a human unassisted for more than a few moments.
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 ?
I found some interesting motor / prop configurations;
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.
Yep and you can effectively make the propeller larger by staying with the current size and going 3 blade too.
More blades = less efficient, unless there is some other factor such as optimal RPM that comes into play. Unless you have space problems you should go with 2 blade designs.
I should measure to see what my max prop size can be. I'm pretty sure I'm at max for a 450mm
so the added lift should out way the efficiency loss or it's not worth it. The same applies to
Battery size. Is it the friction of the added blade that drops the efficiency ?
My quadpod actually flies better with 3 blades. I thought maybe the same would apply to my 450mm AMP 2.0 quadcopter. Jake do you have any links so I can read about this ? are efficiency formulas posted ?
Good call, Jake. Yes, there's no getting around the fact that the more blades you drag around trying to accelerate the same column of air, the less efficient you are. The smaller diameter equivalency of 3-blades versus two (~10%) has to do with power loading.
There are a whole batch of reasons why larger-scale ships go to 3, 4, or even more rotors, including lower pitch-change blade inertial moments, higher (easier to dampen) vibration harmonics, and (in the case of single-rotor helicopters) more manageable and softer on-set of lagging blade stall at higher forward speeds.
Solidity Ratio is a common term in helicopter aerodynamics; it's the ratio of the blade area to the total rotor disk area. It also happens to be in the denominator of the classic Prewitt figure-of-merit equation:
FM = (Ideal Power Based on Disk Loading) / (Induced Power) + (Profile Power),
Where Profile Power = ((Solidity Ratio * Aggregate Drag Coefficient) / 8)
I know how much you love math. :-)
From practical experience with R/C, a 3 blade requires more watts to spin at the same speed as a 2 blade of the same size. And the added lift is nowhere near 1/3, more like 1/4. But they look and sound cool, and so they are used a lot. :)