The Tri^2 Tricopter
I wasn’t happy about it but it was clear to me I needed to do a complete rebuild of the tricopter if, “I wanted to do it right”. I hate those words. But there were a number of things I needed to fix. I wasn’t happy with the Delrin body and the Castle ESCs needed to go back on to the arms. I was also now leery of the O ring arm retainers, as they were a potential source of vibration. They were great for smaller props, but were never considered for props the size I was using.
I had also come across some very interesting vibration isolating motor mounts from an outfit called AGL Hobbies. They have some nicely designed stuff, all made in small quantities. And I found a lot of good things said online about their motor mounts. They weren’t terribly expensive and it looked like I could make them work on my 10 mm square carbon fiber arms.
The first step was to replace the Delrin body, and for that I had been looking at G10 fiberglass sheet. G10 is cheap (Amazon sells it) and easy to cut/machine. It’s about 20% heavier than Delrin, but it’s four times as strong and about six times as stiff. So, theoretically, I could end up with something even lighter, yet stiffer and stronger. Sounds good.
I wish I could regale you with how I designed the replacement G10 body using finite element analysis and 3D CAD. In truth I just put the Fortis Delrin body plates into my flat bed scanner, scanned them into a graphics program, and edited the edges to make it a slightly more beefy shape. It worked. The G10, being only 1/16″ thick, was easy to then cut out with a small bandsaw.
I gave up on the Fortis O ring arm retention methodology and returned to the Delrin Pro tricopter scheme of using 4-40 screws as clamps and pins. It would still allow for folding, but a screw would have to be removed first. That’s not a deal breaker if the damn thing works.
I had a lot of trouble with the Delrin landing struts on the Fortis airframe poking things when the frame was folded and inserted into a backpack. Since I had plenty of cheap G10, and I now knew how strong it could be, I whipped up some ultra-lightweight landing struts. These are held in place with nylon screws and wingnuts, and fold down to nothing when removed. Again, some assembly required, but if it works….
Finally, the vibration isolating motor mounts arrived and they turned out to be excellent! I wasn’t sure if I could adapt them from their intended use on 21 mm carbon fiber tubes to my 10 mm square carbon fiber arms but they worked. Because the rear arm has the yaw servo mechanism on it, and it was already rather tall, I decided to use the motor isolators on only the two front arms.
I took to calling this one Tri^2 since it was far removed from the Fortis design and it seemed more advanced than just a “Mark 2”. As the design improvements to me seemed more exponential, I like the “squared” part. Geekiness abounds.
My first test flight was with the 14″ props. Visually it was very stable, but hovering at 67% throttle. It’s desirable to have your hover closer to 50%.
Changed the props to the 16″ props. Acccck! Up, down, up, down….It wouldn’t hold a solid altitude. Looking at the flight data logs showed the vibration levels were somewhat improved over my previous attempts using 16″ props, but still well out of bounds for decent flight. Adding insult to it all, I found with these larger props it was still hovering at 64% throttle. That was hardly an improvement from the 14″ props.
Well, that just really sucked. It was a whole lot of work for almost no gain in flight capabilities. I was pretty much out of options as I had pulled every trick I could think of with that frame. I wasn’t sure where the vibrations were coming from, but they were there.
While I was sulking about this I happened to place a Hobbyking order. For the blissfully uninitiated, Hobbyking is a Chinese Internet company selling RC planes, copters and equipment at ridiculously low Chinese prices. They are known for their great pricing and long shipping times. Anyway I noticed they were selling very light carbon fiber propellers very cheaply. I’ve never used CF props so I thought, “What do I have to lose?” I ordered some 14″ props, since I knew those worked OK on the tricopter. Then I said screw it (or a reasonable facsimile thereof) and ordered some friggin’ 17″ props. They were all of $9 each, about what I was paying for USA made plastic props. Yeah, I know I had enough problems with 16″ props, but maybe there’s an “other side” to it. It’s a theory, anyway.
After some time the props arrived and I was impressed. They looked like good quality and didn’t need much balancing. I had built a motor test stand for prop/motor testing (I concede I may have got too deep into all this) and put the Hobbyking CF props on it.
The 14″ props were about worthless. They provided less thrust and consumed more power than my plastic 14″ props. But the 17 inchers…..They were interesting. They weighed about half of my heavy plastic props (and it’s rotating mass!) but put out considerably more thrust.
I quickly put the 17″ carbon fiber on the Tri^2 and headed out for a test. A rock-solid hover! On top of that I was hovering at 47% throttle, another success. And this thing was quiet, real quiet. Review of the flight controller’s data logs showed the vibration levels were way down, into the “good” levels. Looks like I might just win this after all.
After all the changes the tricopter still seemed a little hefty at 1,672 grams, with the heavy Canon SX230 camera. However…..a 4S, 3,300 mAh battery gave me a flight time of just over 17 minutes! I did not see that coming. Swapping out the Canon for a much lighter Mobius HD video camera, in a brushless gimbal, and kicking the battery up to a 4,000 mAh size, I had a flight time of just over 20 minutes. That’s long enough to start getting bored. And the sucker handles really well.
So, it looks like I exceed even my optimistic design goals of a 15 minute flight with the Canon camera, and all is puppy dogs and rainbows, right? Um, not so much.
I’ve found that with a certain amount of forward motion (or holding position in a wind) some sort of vibration does surface. It manifests itself as “jello” in the video or as a larger number of blurred images from the Canon still camera. My feeling is it’s due to the fundamental flexibility or resonance of the fairly small 10 mm square carbon fiber arms. And there’s not much I can do about those arms. It’s frustrating as I have more than my desired flight time, but the flight quality really curtails what I can do with all that air time. I’d say it’s at 75% of where I want it to be.
But you know, AGL Hobbies has that neat 21 mm diameter carbon fiber tubing their vibration isolation mounts are made for. And 21 mm CF tubing is waaaaay stiff. And a variant of my current body plate can still be made to fold with round tubes. And I still have lots of G10 fiberglass.
I guess this next one will be Tri^3……
While this design does suffer from vibration when moving at speed (probably due to arm flex) those big props and slow motors give it quite a bit of endurance. Putting on a 5,000 mAh 4S LiPo battery (a big sucker) I was able to fly the Tri^2 around for just over 27 minutes. Well, that was sweet, what else could it do?
How about a fully autonomous mission covering 4.6 miles?
Using that same 5,000 mAh battery, I sent the Tri^2 off to do a big pre-programmed rectangular course. Looking at its log after the flight, I found it had flown beyond my RC control signal on 10 instances. Since I figured it might, I had programmed it to continue with its planned course if it lost my signal, instead of its usual return to launch point. It was dead on its planned course, again flying itself much better than I possibly could. Its total flight time to do the 4.6 miles was 15:20 and it returned with considerable battery remaining.
So, sorta great aircraft if I could only do something about those flexy arms….