This is a compressed little story of an endurance test today with a new hexa multirotor. Some time ago I noticed a post from Ecosynth made here, which attracted my attention. It outlined the use of an octocopter with 30 minutes of flight time for the purposes of ecological analysis and 3D modeling. On the same website I found a concept design that someone specified for a hexa which was cheaper, lighter and offered slightly more endurance. This hexa is almost a 1:1 realization of that design.
The result above shows it easily achieves 30m30s in hover with 168g payload, reducing the battery from full to nominal voltage. Running the battery flat to 3.4V per cell or so I guess I could have taken out some extra 5-9 minutes. The calculations show it should be possible slapping another battery on for 53 minutes total, but in practice I'm noticing the signal going to 3 motors is getting a bit high already, so I'm not sure if there really are power reserves to do that. With 500g payload the duration is reduced by some 6 minutes or so (theoretically).
Next thing to do is see how endurance keeps up when the vehicle is in actual flight. At slow speeds like 5 m/s I don't expect too much reduction in endurance, it may actually improve slightly.
I dove into the subject of multirotor endurance and found some really good information here: http://www.rcgroups.com/forums/showthread.php?t=1880665
There are three main design pillars for multirotors: agility, weight and endurance. Optimizing towards one or two will definitely remove capabilities in the other. The choices for propellers, motors, frame, battery should be made on the basis of this understanding. There's a tool called e-calc , which was instrumental in verifying the design.
The objective of this vehicle is to map areas up to 1km2 for 2D and 3D modeling and perhaps some video surveillance assistance. All the electronics are 3DR, drive system from rctimer.
Comments
I think the 360kv motor is sized for 16". I was just being a bit careful on torque issues in case the specs aren't exactly right. Too much torque could stall the motor and it's again bringing certain limits closer within the operational range. In light setups the amount of throttle for takeoff can be pretty low (according to calculations!). So if you have 30% throttle for takeoff, then if you descend and move sideways that particular motor could temporarily 'switch off' or switch on/off, causing the vehicle to dip.
So I took 15" as a good starter, also considering how even larger props make it more susceptible for control and handling issues. You can always slap a 17" on if you're happy with your results. But beware of the issue from the prop shape. Some props can only be mounted on the motor pointing up and you probably don't want to take apart the vehicle to rotate the arms.
Gerard,
Thanks for the info. I saw on some forums people using 17x5.5 prop with this motor.
Could you tell me why you didn't go for a bigger (16 or 17 in ) prop?
Regards,
Saad
@Gary McCray regarding handling: I was surprised to see how agile it still responds to stick input. With larger craft and props the drag and wind surface area also increases and I think that's the largest factor of how it handles differently. So the attitude handles rather similar to a disco, of course not exactly as responsive, but it moves quickly left and right. Here's a video of another test showing how it responds:
Throttle control is the difficult bit. I need to restrain myself because it takes only a couple of ms to see large changes. I'm used to the Discovery and Naza, so probably I should use some D/R on throttle. I'm also thinking that a slight lower response of the vehicle on throttle triggers the integrators, so I'm going to look at reducing those.
You can't make it out really well in the video of this article, but in wind gusts this vehicle gets pushed around a lot more. That's because the surface area is larger (big battery, arms, motors, props). I also have experience with APM on the 3DR hexa and that hexa moves around more aggressively, but seems to be subject to the same average deviations (up/down, sideways). It just moves quicker and is more aggressive in returning to its position. A slightly slower response can help to achieve more stability actually.
Although hover is usually taken as the performance and endurance test, eventually the vehicle is taken for a flight. Hover however isn't necessarily the most efficient of states. See articles on hover and translation lift here:
http://www.dynamicflight.com/aerodynamics/
In the ETL article there's a blanket statement which says that more horizontal wind is always better. This is true when you only consider efficiency increases from the perspective of efficient use of air to produce thrust. Of course from the perspective of spent energy there are many other effects like drag, higher rotational velocities to produce lift, etc. that are not considered in that statement. In the end a mission needs to be flown which has a fixed distance y and the question is which angle alpha has efficiency x, corresponding to some forward speed v, yielding a flight of duration d, which results in the least amount of energy spent e. My mission speed for now will start with 5 m/s. The main point is that we shouldn't assume that with little forward speed v, endurance is always reduced because the motors need to work harder. Depending on what that graph looks like, endurance could reduce a bit, it could stay the same or it could increase. What I certainly agree on is that there's some angle beta after which spent energy increases significantly in an exponential manner.
To finish off and to increase your interest in this area, here's a video from the guy who built that 95 mins quad in heavy wind:
12mm Frame Quad big FPV endurance test 57min from Ferdinand Kickinger on Vimeo.
Check out:
- 1:12: the low "purr" of the motors compared to a regular quad.
- 4:00: the high wind condition
A quad built like that (two carbon rods in a cross) with very basic FPV gear, no gopro and a maxamp battery should easily give you 75 minutes hover time. That should give you 40-45 minutes of fun, more than some fixed wings I know.
(His secret to 95 mins+60mins flight is his battery of 770g. They're Panasonic NCR 18650B soldered together by hand (dangerous)).
@Saad: Lots of the details are in the motor page itself: http://www.rctimer.com/product_575.html . For resistance I just took a value to be honest. Most important is idle no-load current @ 10V (0.2A assumed). 14 poles and 40mm diameter, 360 rpm/V.
@Thorsten: excellent! I'm planning the same on wednesday or somewhat later. First thing noticeable is the zig-zag deviation from the course line. I'd expect smaller props to have slightly better tracking. Of course you stated high wind, but in my hover I notice that tiny gusts already move the copter about a bit. Most important bit though is that none of the arms make even the slightest dip-and-recover movement, which in my case it didn't. I'm going to analyze your report in a second.
@Randy: The concept design is made by the people at Ecosynth. I just built it with a bit of theoretical verification.
@G McCray: I use the props that Thorsten pointed out. I also bought the other "TM-1555". Notice how those TM's have a different shape up top. Because of that shape, you can't use them with the motor when pointing downwards, they have to be mounted upwards. The other Carbon-1555 can be mounted downwards.
When I buy stuff for multirotors, I always purchase 2 more than needed. You'll see how with some vendors the axle is bent, an ESC is faulty, a prop drilled incorrectly, etc. It's far cheaper to buy some extra due to shipping than going through the process of buying more.
Encouraged by Gary's comment ("... I am sure that these deficits will be entirely acceptable because the extra endurance will be most important.") I took the hexa on a first waypoint mission today - under heavy wind: no crash!
Details can be found here: http://ecosynth.org/profiles/blogs/stress-test-of-stephen-s-long-en...
Gerard,
Can you tell me what data you used for the motor in ecalc? I don't see the motor listed under RCTimer motors on ecalc, so thought you might be using custom data.
Regards,
Saad
Gary, these are the props I ordered for my hexa: http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=...
Since my hexa has the same problems as Gerard's I assume he has the same. Hence, I suggest to take different ones...
Gerard,
Really interesting! I was actually hoping for a T3 competition based on endurance to try and focus the community on figuring out what worked well. Looks like you've gone ahead and figured out a lot of it without that. Really valuable information here!
Hi again Gerard,
Talked myself into it, just ordered 4 of the RCTimer 5010 360's on their special weekly deal.
($77.34 delivered for all 4)
They have a good selection of 16" props as well varying from $35.00 for 2 pair to $95.00 a pair.
I thought I'd research that a bit first could you supply the part number for the ones you are using?
I like the motor prop on the bottom method you are using because it is definitely more efficient to not have the motor support strut directly in the prop wash.
There is a little bit of loss with it on top, but in a hover the loss is very small.
The trick is getting decent landing gear under it and getting the CG low enough in relation to the prop centers of thrust for stability.
In any case I will be exploring both endurance and stability.
Looking through your article, and the other ones in DIYDrones and Ecosynth above are really starting to give us a feel for how these large pancake motor slow turning propeller multicopters behave.
Clearly they do support the concept that for a given power output and weight the biggest, slowest turning propeller will give the greatest overall efficiency and endurance.
But they also point out a few of the pitfalls, the tendency to yaw one direction or the other, the fact that if allowed to drop rapidly they recover very slowly if at all and the obvious (and apparently difficult) requirement to have the CW and CCW props be exact mirror images of each other and to be drilled properly.
Unstated, but probably also expected would be a more sluggish response to controls or external forces (like gusts) and possibly a more noticeable complex instability caused by relative prop tip position.
For some even many applications I am sure that these deficits will be entirely acceptable because the extra endurance will be most important.
But it is important that people understand that maximum endurance does come with significant compromises.
At this point these trade offs are not at all well understood and pretty much completely unquantified but, that will need to be done.
Your research into endurance (and its resulting problems) is laying the groundwork.
I also really like your low cost affordable approach, the RC Timer motors and props are many times cheaper than the T-Motor ones and are in a normally hobby affordable price range.
I will probably try to build a quad based on the motors and props you are using.
My expectation is that the Quad will amplify the negative stability and handling characteristics of the big slow propellers.
But that also serves as a good model for understanding them and for how to develop methods for dealing with the problems generated.
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