I'm still trying to get the basic concepts into my head. What I can't really wrap my head around is the endurance, or better, which factors influence the endurance how?
Of course, it's clear that weight is a factor and battery capacity. Then probably, the motors have some range in which they operate more efficiently.
If I look e.g. at commercial drones like the Microdrones things. They boast endurance of up to 70 mins in a quadrotor.. Here my problems start because as I understand it...:
If I want to have more endurance, I need more mAh. More mAh = bigger batts = more weight = less endurance. Much more weight = overload = need for bigger props + bigger motors, amybe bigger airframe alltogether = more power consumption = less endurance.
But that is only my view of the things, based on basic understanding and some applied logic.
So the question remains...:
Which factors influence the endurance of a multi-rotor in which way? How do I build a multi-rotor which can carry 2 cams (FPV + recording) with, let's say, an hour flight time?
Replies
Hi i'm technical manager of Remote Aerial Surveys. In my experience most people want to design to one of 2 factors:
1) To fly in high winds - You need small props as they have a low inertia, and thus their speed can be changed very rapidly. This results in a highly stable platform. The Asctec Falcon 8 is a good example of a highly stable platform
2) For endurance/ efficiency - You need large props as they dont need to be spun as fast to generate lots of lift and are therefore more efficient. The best airframe for this is the X8 since you can maximise the props disk area for a smaller airframe weight penalty. A good measure of airframe efficiency can be calculated using "Disk Loading"as follows:
Airframe Efficiency = 1/ Disk Loading = Combined Prop Area / Airframe Mass
Examples:
Standard Ocirotor (8 13x4 inch props), Airframe Mass 2Kg
Airframe Efficiency = 531 inch^2/Kg
X8 Octirotor (8 17x6 inch props), Airframe mass 1.5Kg
Airframe Efficiency = 1211 inch^2/Kg
As disk loading reduces, efficiency increases. See: http://en.wikipedia.org/wiki/Disk_loading
The key to efficiency is to reduce the platform weight as much as possible and spin your blades as slowly as possible for max efficiency. The X8 configuration does this perfectly :) Might explain why 3DR have designed their X8+ in this manner!
If you keep increasing the size of your rotors, then you will end up with a highly unstable platform as the props will have a high inertia. Its all a trade-off between efficiency and stability
Hope that helps some of you
James
air or water, ERGO, a fluid
BTW, if anybody wants to see an interesting discussion about this topic from a bunch of guys who have less accademic training, but a whole lot more real world experience, check this out:
http://www.multirotorforums.com/showthread.php?5992-S800-alternativ...
This comment in particular is interesting, as it talks about noise and vibration with the DJI props (which seem like they are designed according to the aerodynamic principles being proposed here) which is eliminated when moving to the APC props which do not appear designed properly (at these small scales).
The discussion about stiffness is interesting. Should the blades be stiff, or soft? Traditional helis employ "flapping" blade hubs for a good reason. Multicopters have rigid blades. Maybe blades shouldn't be stiff? Or maybe the DJI blades are too stiff?
It also cannot be overlooked that much of this noise/vibration problem might not be due to the blades at all, but the simple fact that the DJI arm design/construction is incredibly poor.
Again for a good reference, the MD-1000 drone uses a specially shaped prop. The main surface area seems to be closer to the shaft of the motor, and tapered at the tips. This probably helps them to maximize performance. I suspect this shape is the optimal for multi-rotors.
In fact, the slow fly props are almost the opposite with the higher surface area being on the outside, and tapering towards the shaft.
Like just about everyone here, I started out just tinkering around. However, I quickly realized that aircraft design is not trivial, and if you wish to pioneer uncharted territory or just make your own unique craft, it certainly helps to understand what you're doing. Even things that fly have to obey the laws of physics. Far from being abstract and detached, if you don't heed these laws you won't be successful, or at the very least, your trial-and-error efforts won't teach you anything.
Making thrust from propellers has been analyzed for about 90 years, so it is fairly well established. The original post from Stefan was related to "basic concepts". If you read any technical literature on helicopters, one of the first concepts to be discussed is disk loading. If you don't know what that is or how it affects efficiency, then what is the point of having a discussion at all?
I didn't jump into this thread because I love controversy - far from it. But to have such a lengthy discourse about rotor craft endurance without this fundamental topic at least being mentioned, it seems to me, does the entire community a disservice.
It is true that many engineers tend to over-analyze the hell out of things only to be foiled by some "real world" variable for which they didn't account (for electronics engineers, parasitic reactance comes to mind). However, if you don't at least understand the theoretical basics for the science, then you're just poking around randomly in the dark. It's perfectly ok if you want to just tinker. Do yourself a favor and buy a kit or use a preselected parts list with known performance attributes. Stefan indicated that he wanted more - he wanted to understand.
So here we are, the two basic concepts for rotorcraft endurance which directly respond to the "which factors" question:
#1 Disk Loading
http://en.wikipedia.org/wiki/Disk_loading
#2 Rotor (propeller) Figure-of-Merit or FM:
The ratio of ideal power to total induced power, normally expressed as a 2-digit percentage (I couldn't find a good link for this straight away, but The Case for Large-Scale Electric Multicopters covers it fairly well)
As a final note, many of the formulas and values used in aircraft design are, in fact, empirically-derived from actual testing. You can find real-world wind tunnel data for many of the propellers used in small multicopters. What you'll find in the very-real-world is that the "turn it slower" advice doesn't hold water - most 13" or smaller props actually become more efficient at higher RPMs due to Reynolds Number (Re) scaling.
Santayana said, "Those who cannot remember the past are condemned to repeat it". Knowing what science has come before you is a bit like studying history - it will save you a lot of work and frustration.
I didn't exactly ignore aerodynamics of lift, I said 'magic 100% efficient propellers'. The purpose wasn't to design a realistic craft, it was to analyse the absolute limit of hovering endurance based on the specific energy of LiPos.
I just don't understand the Microdrones thing at all. I can't seem to build anything that gets much over 20 minutes flight time with eCalc. How do you make the leap to 60+ minutes? It's a huge leap! Because I can easily build something that gets 12-14 minutes flight time, but double the battery capacity, and it only goes to 20 minutes. Throw a third battery on, and now you get 22 minutes. Huge amount of diminishing returns. How in the world can you get over an hour?
Even if I build a "flying battery" with 380kV T-motors, Zero frame weight, and huge props, it's hard to get over 30 minutes!
Edit: Ok:
4x MT3515-15 T-motor 400kV, zero frame weight, 2x 6S5000 batteries, 18x4.7 props, 39 minutes. That's it. Zero frame weight!
Stefan
Your thinking is 100% correct. exactly this is what ALL multicopter designers are having a problem(challenge) with.
Take a look at the multicopter design simulator, http://www.ecalc.ch/xcoptercalc_e.htm?ecalc
Most of the factors can be simulated there, and you will see which other factors also have an effect on performance and duration, and it is nicely graphed for you too..
Unfortunately Gravity is still the only constant, and it ALWAYS wins!
I don't think microdrones has much on it when it's flying over 1 hour so that's a very high hurdle to reach. Not impossible of course, the pros are only human as well!
longer props have a higher aspect ratio (better lift:drag ratio, more lifting force for a given ammount of work. i.e. better prop efficiency). But longer props require a bigger frame, which is more weight.
Higher voltage batteries/motors means less amps (thinner wire, smaller mosfets/ESC, less weight) for the same ammount of work. So, A High voltage motor with long props would be best - e.g. low KV motor. But low KV motors are bigger and heavier (more copper). Relativelty small/light low KV motors are the most expensive kind (strongest magnets, etc).
So there's a ballancing act between prop efficiency (long props, low kv motors) and motor efficiency (thin wires, light motors). If you bring costs into it, there's a sweet spot between 800-1400kv motors, 8"-13" props, and 2-4 Cell lipos. outside those bounds things start to get a bit exotic (expensive).
There's another thing. Imagine a multicopter that hovers at 100 percent throttle; it's brushless motor is being used most efficiently, and there is minimal dead weight (all of the capacity of the motor, wire, esc, etc is being fully utilised). But if it's hovering at 100% throttle, it can never climb. What's more, it needs a certain "stability margin" for stabilisation. For sedate flying, I think about 10% throttle is a usefull stability margin (as in 10% in reserve at "full throttle", not hover). For acrobatic flying style or gusty conditions, the autopilot probably need more like 15% "unused" capacity for stabilisation. If this imaginary hovvering quad is very highly tuned and flying in a windless stadium maybe it could get away with only a few % stability margin - but how is that contrived situation better than simple pole? :)
It's the same for strength - to maximise hover time you could make a frame that's exactly strong enough to hover, using expensive super light materials. But if you want to manouver, you would need to make it slightly stornger (heavier). If you wanted it to survive the rigours of practical use, it would need to be stronger still. And of you wanted it to be affordable, you have to pay even more weight penalty.
Similar deal with batteries. If you drain every last drop of power out of your LiPos you ruin them. If you use Lithium Phosphate batteries (which can be safely discharged more fully), you pay a 20% reduction in energy density. Personally I plan to limit the drain on my LoPos to ~60% capacity, to prolong their life and leave some capacity for emergency. (e.g. 4Ah 3Cell Lipo theoretically has about 48 Watt-hour capacity, I'd plan to land by the time it's done 30 Watt-hours work).
In practice, with the energy density we have with todays batteries, 5-10 minute flights with a utility airframe of 1-2 KG is practical. You can get away with slightly mismatched components and 20% AUW of payload (cameras etc) if you are happy with 5 minute flights. If you want 10 minute flights, you need reasonably well matched, good quality components, and not to much payload relative to the size of the quad. If you want to push past 10 minutes, you need to make carefull compromises.
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