Starting a new thread to show what I'm working on. I'm building a new Octocopter of my own design. The goal is to use it for Aerial Photography, lifting a mid-weight camera such as a Sony Nex5. It will of course be using Arducopter for control. ;)
I started off researching the various kits on the market, and starting getting analysis paralysis. I looked at all the options such as Droidworx AD8, Cinestar, CarbonCore, and SteadiDrone. But I just wasn't happy with the design on them, particularly for the price they ask. Combined with the fact that I can be pretty frugal, and have a strong DIY ethic, I decided to just design my own.
The basis of the design relies on many of the Hobby King quadcopter parts. I liked the design of the motor mounts, they are very professional looking, even compared to some of the high end machines. And I liked that the boom mounts are blocks of aluminum instead of plastic. Very rigid. I also liked the look of the dome on their H.A.L. quad, but since they don't sell it seperately, I bought the whole kit, it's only $34, amazing! I might end up using the rest of it someday, who knows? So, I basically emptied out their stock of Talon parts, so if you need to fix your Talon quad and the parts are backordered... sorry!
Once that was settled, I set about designing the center frame. My design required a few things, namely I want all the wiring hidden. I don't want any spaghetti showing. Particularly with the ESC's, while still allowing cooling airflow. This required the center hub to be larger than is typical. I then needed a smaller subframe to house the avionics.
You can see these plates below. The larger one houses the HAL dome. You will notice the 3 blue anti-vibration grommets in the middle. This is the APM1 pattern. I did that just because. Maybe I'll use an APM1 as a stand along gimbal controller. The second plate had grommet mounting for an APM2. The final plate is the top plate, and has bolt patterns for the Ublox GPS and the magnetometer.
This next photos shows the avionics frame built up with an APM2 mounted. 25mm aluminum standoffs are used. I don't like the plastic standoffs typically used. They get loose, and lead to vibration. They also break easily. This structure ends up being quite rigid.
Here is the avionics frame mounted on top of the main frame. Yes, the main frame is HUGE. I actually don't see the point in having a tiny center frame, and then long arms. This seems like it just leads to flex, and doesn't leave you with any real estate to mount your avionics.
This photo shows one of my design features. I put some cutouts on the bottom frame for weight savings, and then made matching cutouts on the top frames. This creates some nice conduits to run wires neatly.
Here it is with the HAL dome mounted and one of the Talon arm mounts bolted in. I'm really happy with how stiff the assembly is already.
Here it is with one of the arms mounted. These are the short 220mm arms, I also got some of the 320mm arms. One may wonder how I'm going to get away with such short arms and 13" props.... Astute readers will know what I'm planning already. ;)
This last photo shows one of the ESC's in position. This is a Hobby King F-40A, it's somewhat larger than typical 20A units which is partially why the frame is so big.
That's it for now. I had the plates cut out of aluminum to start with because it's cost effective. If it all works out, I'll probably have it redone in carbon fiber plate for the weight savings. I was a little uncertain about the weight, but it looks like the frame will come in at 1070g all in aluminum. That's about 300g heavier than a Droidworx AD8, but not too bad. It will drop about 1-200g if redone in CF.
I have a Super decathlon on my flight simulator it flies as fast as the jets Beautiful plane .
Just to be clear, the whole idea behind overlapping rotor disks was to increase the effective disk area in a more compact footprint. Dr. Leishman's efficiency "bucket" prediction is only ~5% of unity, so I would expect to find no difference at 20% overlap unless the lab conditions were very tightly controlled (which is what he said in the paper). It may very well be that a grid layout is best for this approach versus a classic multicopter "star" pattern.
However, the efficiency cost of a coaxial configuration is easily discernible in testing, at a range of 20-40%. To be fair, the downstream rotor should have a higher pitch (which you can calculate via the induced velocity of the air stream of the top prop) for it to work the best. That said, the whole disk-loading vs. thrust-spacing vs. control-response vs. center-of-mass conversation is a whole other kettle of fish.
The efficiency of motors peaks at about 1/3rd of rated power and falls off rapidly after that. A unit rated at 85% peak efficiency might only be about 60% at full power (remembering too that our friends in Asia tend to play fast and loose with rating numbers). In other words, a 300 watt motor is trying to scrub off 120 watts of waste heat...so, yeah, it will get hot.
Anyway, nice work Robert. I'm sure the finished product will exceed expectations.
Yeah, I think the variability in my testing could easily be 5%, unless I used some kind of computerized equipment that captured PWM/Watts/Thrust constantly throughout the run, and either averaged or presented the data at each point as a bar.
Just for completeness , here's my second overlapping setup:
Brad, regarding lower propeller with higher pitch in a coaxial setup. From a more practical hand's on standpoint. If you tried swapping bottom propellers (same size, different pitch), would the one that resulted in the same current draw from both top and bottom motors at the same throttle setting be the correct one?
Yes, actually, we're trying to achieve the same AoA for each prop. Since the upper prop is developing its own inflow, and the lower prop is being "fed" air from the upper, then the lower prop needs a higher pitch for the same AoA. Considering that Cd (and Cl, of course) for an airfoil varies according to the AoA, and given that total drag is proportional to the total power, and since the current is proportional to the latter...you're on the right track.
As an aside, I mentioned before that you can calculate the induced velocity of the air stream using momentum theory (actuator disk), but that would only be an approximation. The actual velocity varies considerably, depending on the distance the disks are separated. In a somewhat counter-intuitive sense, the far-field wake is twice as fast as the velocity at the disk due to flow contraction effects. This flow contraction also means that the tips of the lower rotor "see" less velocity than the rest of the radius, which also means that the precise optimal geometry of a lower prop is rather...complicated. This is why the coaxially-induced power factor Kov is 1.41 for near field and 1.28 for far field, using rotor blades of essentially the same geometry. If you've ever noticed that some small-scale toy coaxial copters have the upper and lower rotors separated by what seems to be a ridiculous amount (half the disk diameter), now you know why.
Robert, you are way ahead of my next build and have noticed the same issues with wiring congestion. Why the small center section when just slightly larger will help several issues? I also have a strong DIY slant (1st quad frame was all scratch built). To that end here is a teaser of where I am going...
The larger base/top plates will allow:
The material will still be 1/8 aircraft plywood. The stuff is about 10% lighter than the FR4 material. Another goal is to allow someone to purchase most of the raw frame material at a DIY store, hobby shop, or craft shop. A dual battery box under the base plate ads rigidity and protection for the $$ LiPos.
By adding a new work bed to my home brew CNC, I think I can just cut these plates. If not, that is what other woodworking tools are for.
Oh, before anyone comments, I have three CAD programs and still keep going back to paper and pencil. I know, Caveman methods..but it works for me. G-code for the CNC machine...I still do that manually too. I'd rather spend the $$ on flying stuff than some software that will go obsolete in a few years.
I still sketch things out by hand too. I do my brainstorming on paper, but soon as it gets to the point I would need to break out a ruler, then I just go to CAD.
Here's where it's at now. I painted the main plates with a nice gunmetal paint and clear coat I had lying around. These are the long arms. They sure are long! The diameter is now 960mm.
Another point of this design, besides just real-estate for the ESC's, is cooling. The ESC's need to have some air for cooling. But the avionics do not. Most designs either strap the ESC's on the arms, good for cooling but ugly. Or the high end machines cover it all up with a dome that turns it into a oven. Seems sort of counter-intuitive. This designs leaves the big cutouts for ESC airflow, while covering the avionics.
Here is the landing gear from the H.A.L. I really like the machined mounts, but the orange plastic joints have to go. Easy enough to replace with some aluminum blocks.
Looking good. I'm in the planing phase for a 100% scratch built octo myself also. Only way to get it just right. :)
The main reason being that I want to have base plates with dedicated room for all electronics to keep things tidy and easy to replace. Other than the price of materials (big 3K CF sheets are worth their weight in gold), there is no point in trying to keep the base as small as possible. On a 1000mm octo it just looks weird anyways.
Ordered a bunch of 25mm aluminum tubes (.90 size helicopter tail booms) at HobbyKing. Almost as rigid as CF, and close to the same weight also. And the best thing, less then $3 a piece. :)
I will probably also look into using overlapping propellers to keep the size down. 1000mm octo's are a drag to transport.
Yeah, I know, I'm looking at this thing now and thinking man... it's huge! I hope it fits in the trunk of my little car!
I've got a van and truck too but, even then it will consume a huge amount of space.
So far, I'm not super happy with the boom mountings. They can flex up and down a fair amount, and actually side to side as well if the screws aren't really tight. The issues is these clamps really on mount to the bottom plate. The standoffs that go up to the upper plate don't do much to stiffen the arms in the vertical direction, the base plate can just deflect. I would like to drill one hole in the topside of each block so that a screw can rigidly pin it to the top plate.
Either that, or use to blocks per arm, but that takes up more room. I'll think about what to do.
The booms clamp very tightly into the bore of the blocks, that's not a problem. It's just the block itself, and the block-frame interface is too loose. I don't know yet if this will cause a problem in flight, we'll see.
Ideally, I think I could design new blocks. Maybe them 1" deep, with 4 mounting bolts top and bottom. The problem is allowing for the clamping, the top holes in the frame would need to be slotted or something. I guess you could simply change to a horizontal clamping scheme. Similar to all the other machines out there, but make they out of an aluminum block instead of plastic.
So I've been thinking about the results of my testing, what they mean, and what I'm going to do with the design.
I sent back the 13" props to the vendor, and will be ordering some 12" props. That's sort of good, because it means I can get APC's which I think are superior. But I'm not really happy with the size of the thing.
On Saturday, when I realized that the motors could not handle the 13" props, I was disappointed in the Turnigy motors, as well as eCalc since it had showed they could. But I only realized this morning there is a massive flaw in my test methodology. Simply put, the number of batteries you have per motor is a critical aspect of the design. I had noticed this funny business going on with eCalc, but didn't understand why, or what it means exactly. I thought the eCalc was showing me the effect from the extra weight of the batteries, but that's not it at all!
Simply put... the higher the load on the battery, the greater the internal voltage drop. Therefore, if you have more batteries per motor, the voltage getting to the motor will be higher. The higher the voltage to the motor, the more power it will put out. When I did the single motor test, it was the equivalent of having 8 4S 5000 packs on my Octo! And if you plug that into eCalc, the voltage getting to the motors is 13.93V, and the result is 650W per motor! This is almost exactly what I experienced, and why I throttled back. However, if you drop to 2 4S 5000 packs, the voltage to the motors drops to 12.1V, and peak power to 445W. Within limits of the motors. So it turns out I *could* use the 13" props! Oh well, too late, I already shipped them. And as I say, I like APC props better anyway and I can get them in 12". eCalc shows the loss in flight time to be only about 1/2 minute, and with the added bonus, I could fly with 4x 4S 5000 packs for a 19 minute flight time if I wanted. But 4x packs on the 13" props might overload the motors again.
I actually did run a test where I had two motors running with full seperation, and it showed almost the exact same thrust/power numbers as the 18% overlap test. Certainly within the margin of error of my test method.
Anyway, keep this all in mind when doing your design guys.
Now, the secondary thing to all this is that it somewhat invalidates my thrust testing. For the non-overlapping test, I ran a single motor on that single battery, making the voltage drop even less. Thus, voltage to the motor was even higher, and I think this might have made them appear more efficient than they really are? I really need to re-run the test with 2 non-overlapping motors running simultaneously.
So now, back to the actual octo. I'm sort of unhappy with how big it has become, and the long, thin arms a bit flimsy for my liking. I also don't like the fact that my octo is becoming more and more "me too". It's nothing new, no advancement to the science of these things. However, now that I'm going to go with 12" props, it opens up an interesting possibility. I have 220mm tubes, and 320mm tubes. If I cut the 320mm in half to 160mm, I would end up with a 20% overlap again! And the diameter of the Octo will shrink to 623mm! That's just a hair bigger than my 3DR Quad! Much easier to pack! And it will actually fit on a shelf!
Yes, I'm excited!
It will also benefit from the added stiffness due to the arms being half as long. To get around the "upside-down motor getting in the picture" problem, I'll just do what I did on my test stand. I'll flip them up and use stand-offs. I'm not entirely sure how big the seperation has to be. But I used 30mm spacers on the bench, and so no problem at all. The disks don't deflect at all, but that's not to say they wouldn't in flight. We'll see. On the test stand, I used 30mm male-female threaded aluminum spacers. For the airframe, I'm thinking of using 35mm unthreaded aluminum spacers with 40mm screws. I don't at all like the idea of using aluminum threads long-term. Though I guess it's true that the female threads in the motor are into aluminum.... Still, I'd rather use steel screws.
If I go this way, it will result in the tips of the blades being within 20mm of the main frame of the quad. I don't forsee any issue with this. Anybody? If anything, it will increase cooling airflow the ESC's.
So what you are proposing is long - short - long... arms with a motor offset?