We used the sprayer tank system on our T18 octocopter for almost a year, in various locations and field conditions. We also received great feedback from the customers and potential end users. Several key points that we walked away with:
- The vehicle needs greatly improved flight time (we are getting approx 8-10 minutes with a full tank)
- exposed wiring/autopilot/ESCs are not acceptable, they are fragile and can be easily damaged by dirt/dust/pesticides during spraying and rough handling on the ground
- The vehicle needs to be sealed from dust and water
- The clamping system for mounting arms and motor mounts is unacceptable. The integrity of the mount is dependent on tightening lots of small screws, which strip easily and loosen each flight
- The vehicle should be able to be broken down for shipping, and fit in the back of a pickup truck at the worksite.
- The vehicle needs to carry a variety of payloads. Ideally the same unit should be able to be configured to carry the sprayer system, a camera system, and have a standard interface that allows other future payloads
- Be able to be maintained with simple hand tools (ie field replaceable parts, no complex mechanical assemblies)
Armed with this information, we looked around to try to find a vehicle that met our needs. After months of searching, we came up empty handed. We realized that the performance of our tank system was being held back by the performance of our vehicle, so we decided to build a multicopter that could meet our needs.
We settled on the X8 layout because it allowed us to use the largest propellers for a given frame weight. We intended to keep the AUW of this machine under 55lbs, and the X8 layout allowed us to fit 30" props vs 18" props on a flat octo.
Once we had decided on the layout, we set about designing the center electronics housing. Most frames on the market utilize a 'tube and plate' construction, which is fine for personal use, but is not the most efficient structure.
In our case, because we are giving up some efficiency with the coaxial prop setup, we needed to optimize the weight of our system as much as possible. To do that we settled on a central body that was machined out of a single piece of 6061-T6. We performed extensive FEA to determine how thin we could make certain critical sections, and the result is a central body that measures 12"x12"x3" that weighs just 1.4lbs (~630g), and can easily carry a 40lb payload.
The aluminum monocoque also has the interesting feature of doubling as a heatsink for the ESCs. All of the avionics are mounted inside of the enclosure, and the ESCs are mounted directly to the aluminum with 3M 8810 thermal tape. This allows the heat generated by the ESCs to be conducted to the main structure itself, and then radiated/convected away. The system works very well, and you can feel that the body of the copter is warm after a long flight.
Another area where we were able to gain some efficiency was by using an airfoil shaped motor boom. While most companies use round arms, from an aerodynamics point of view round arms have approx 10 times the drag of a streamlined shape of the same projected area. By reducing the drag on the boom we get a direct increase in efficiency. Second, because most frames use a clamping system to grab onto the booms, there is a lot of extra weight in the design.
We designed a streamlined airfoil shape, and had unidirectional carbon fiber tubes manufactured. We then designed machined aluminum hardpoints that were then bonded into the carbon tubes with Loctite E120-HP (a very common structural adhesive in aerospace). There are precision holes on the hardpoints, which mate up with precision dowel pins on the central structure. In this way, we can easily remove the arms from the vehicle for storage or transport, and reinstall them back into perfect alignment with just two bolts on each arm. In the unlikely event of a crash, there are aluminum fracture pins that allow the arm to break before damaging the center section (the most expensive part).
Another advantage of the airfoil arms has to do with vibration reduction. A major cause of vibration is caused by the low pressure bubble that forms downstream of the arm collapsing. This phenomenon is called vortex shedding. Depending on the speed of the flow and the shape of the body, it can have quite dramatic effects. The streamlined airfoil arms help to mitigate this effect, which can be especially dramatic with the large props that we are running. I also suspect that the airfoil shape acts like a stator in a jet engine, and helps to remove some of the swirl from the upper prop wash, thus helping increase the efficiency of the lower prop as well (although I have no proof of this).
When all is said and done, we ended up with a copter with the following specs:
KDE 7215-135 motors
KDE 95A HV ESCs
Tmotor 29x9.5" props
Pixhawk (of course!)
Custom designed frame (detailed above)
48Ah 8S batteries
In the end, we ended up with a vehicle that meets all of our requirements, and is something that we are proud to put our name on. We are seeing flight times of around 45 min with a Tiny2 Gimbal, Git2 camera, and a Connex HD video link, and flight times around 35 minutes with our sprayer tank system.
If you made it this far, thanks for reading. I want to thank the Arducopter development community for compiling top notch software, and documenting it all. I just wanted to give back to the community and share my project, I hope you liked it. I am happy to answer any questions you may have.
Just wanted to post some short videos in case anyone was interested.
The first is a short clip on the vehicle takeoff and hovering
And another of the sprayer system in action at a commercial vineyard
@Brian awesome build! Overall system looks very professional, especially the work done on spraying mechanism.
Some time ago here at DIY drones, one of the members did some real world tests with square versus round tubing of the same dimension on a standard quadcopter, they found, as I recall, approximately a 4 percent improvement in efficiency using relatively small arms of the round versus square.
Since square counts as flat plate area and round counts very nearly as 1/2 flat plate area, you might expect as much as a total of 7 percent or so efficiency improvement of flat (square) versus fully aerodynamic.
He also did some tests using wider flat plates as are common on the the small carbon fiber or fiberglass plate motor spar copters and got a reduction in efficiency of over 15 percent.
So clearly it does make a difference.
On a coaxial copter I would expect the benefit to be less, but probably better than half as much because the air straightening of the airfoils is probably more beneficial than the completely turbulent result from a round or square tube.
Brians copter's spars have a thin vertical cross section which is already better than a symmetrical square or round one, but the airfoil will improve on the efficiency as much as possible.
As for the vertical cross section causing problems in wind absolutely right, the more it has the more it is affected.
However, Brian's copter is a big heavy copter and wind effect is in relation to mass.
It could easily blow a light copter all over the place, but on a semi-truck like Brians, you are going to get a lot more effect by the wind passing over the prop blades changing the local lift than you are from the copter body/frame itself.
I do think that helis will eventually fill a lot of the needs in te heavier lift realm, because that big rotor makes them much more efficient and they handle unfavorable conditions better and can be made at least as reliable as any multi.
Excellent low maintenance rotor hubs already exist and as UAS use becomes more common, they will proliferate and become more widely accepted.
Regardless, for various reasons, heavy lift multis will have a place as well.
The ventilated arms are a work-in-progress only :)
(Along with a few dozen other things).
My current working X8 just uses square aluminum tube booms. Cheap, versatile, easily-modified during 'R&D'. But not as wind-efficient as they could be.
If your applications don't require precise position-hold, or even close, then no doubt it's less important. But if they do, wind proves to be a very big deal in everyday use. Not just for the endurance penalty of constantly fighting the wind to restore position, but even more importantly, the outright inability to carry out the payload function safely or effectively at all.
(In fairness, crop spraying probably doesn't require precise position hold, so in fact, given the intended use of your machine, my original criticism was probably not so relevant)
P.S. - if taking the approach of removing material from the sides of the boom arms, aluminum arms might be a better bet than anything fibre-based, especially pultruded or uni-directional fibres.
Even if there is no aero benefit (which is unlikely) the structural benefits are real as you pointed out. I too have thought about the increased profile causing issues in the wind, but so far we have not seen anything to that effect. I think that the (relatively) large mass on the central body coupled with the (relatively) small cross section of the arms helps to mitigate the effects of any wind.
I would like to see any pictures of your X8 if you have them, your arm design sounds interesting.
I'll second Giovanni's skepticism about the benefits of the airfoils-as-booms (while commending the rest!).
My guess is the very minor portion of the props' total angle of revolution which actually passes over the boom, compounded by the fact that only 4 out of 8 props are affected, makes the gains very limited.
What won't be limited is the much more important, and major, increase in wind resistance caused by such a high vertical profile on the boom arms.
In practical use, wind, not minor differences in endurance, is the bane of these multi-rotors, compared to variable pitch heli alternatives.
Roughly speaking, it's the tops and bottoms of the booms taking most of the stresses (because most of the stresses are vertical), and the sides effectively performing the role of the webbing in an I-beam (as well as providing torsional rigidity).
So, I wonder if significant gains in wind stability might be had by (carefully, gradually) taking material out of the sides, to allow the wind to pass through.
I'm currently playing with this by using over-sized aluminum tubes (to gain the major stiffness advantages of a wide separation between top and bottom of boom arms, with more and more material drilled out of the sides (to 'ventilate' them, so to speak).
Good luck! (I'm a heavy-lift X8 guy, too).
We have not done any bench tests of the arms with the coaxial arrangement. However, when we measure our current consumption during flight, we are seeing approx. 10% more current than the manufacturer data would suggest. This leads me to believe that the boom is doing something, or we just got lucky with the vertical motor spacing.
The airfoil shape has other benefits when compared to a round tube though. Because the shape is elongated, with more material away from the neutral axis, we see a significant stiffness improvement vs a round tube of similar mass. Also, the bonded hardpoints are much lighter than an equivalent round tube/clamp system, and greatly increase the stiffness of the arm.
What are the vibrations like on your large machine? We are seeing very low vibrations in hover, on the order of .1G on the x and y axes and .2G on the Z axis.
Brian, do you have any pictures of the electronics layout? Very clean looking outside!