TinyStar, the 2-Day Dual-Flight-Controller Scale-Model Octorotor

That might be the most specific descriptors I’ve ever had to heap onto one build. It’s also kind of hard to explain how it got started… which I guess is true for some other stuff I do too. But as a reminder, here’s that mysterious picture that I posted a while back when I was working on Tinycopter:

Why does it have eight propellers!? That’s like, two quadrotors at the same time! And why is it so pointy?

First off, it’s a reasonably accurate scale model of a Cinestar 8. This was 100% intentional – not only am I out to one-up the quadrotor…uhhh, arms race? at MIT, but my buddy Shane Colton is way better at getting free stuff than me and he has a Cinestar 6 frame. But it is used for custom controller testing, so it has a legitimate purpose despite being free (an exercise of the classic industry strategy of “give academia free stuff so they can do weird things to it”). The Cinestars are probably some of the most badass-looking multirotor frames around – totally murdered-out and made of carbon fiber and black GFR nylon, and they kick ass. So much. Also, nice cameras make everything look more epic.

Second, the cool thing about Tinystar is that it uses two independent multirotor controllers (Hobbkying i86, which may be based on KKMulticopter’s firmware, but it’s just different enough that I’m not confident on it) feeding from the same receiver. It is quite literally a Siamese quadrotor twin – one controller is run in “X” mode and controls the 45 degree arms, and the other controls the 90-degree arms in “+” mode. This was a ‘dumb but potentially awesome hack’ idea spurred by discussion at MITERS about how you would go about making an octorotor quickly without having to build a full custom controller, and I’m glad to report that it turned out to be awesome – more on that shortly.

But Tinystar won’t be hauling any FS-100s or RED EPICs. Why?

because it’s tiny and adorable!!!

(Tinycopter is on the left, by the way).

It’s a little over 13″ across and weighs about 280 grams with battery. Now that I’ve shown a picture of the final product, this is how it went down. It literally happened this past weekend.

Strictly speaking, it took more than just 2 days, but sporadically so. The design work was completed in a day, and I intermittently printed the frame components on the Lab Replicator (I can actually say that now – are we living in the future?) over the past week or so. This frame uses the same tactic that I settled upon for Tinycopter v3 – 3D printed joists and clamping components stuck to carbon fiber tubing. Many multirotor kits are made this same way (though with injection-moldings instead of 3D printed parts of course) and the arrangement is sufficiently versatile and adjustable to be able to mess with the design quickly.

The two big round pieces form the main center “hub” of the frame, and the small H-looking things are motor mounts. The landing legs are made of 2mm laser-cut plywood, and will be painted to look more badass.

The arms are made of DragonPlate’s 1/4″ pultruded CF tubing, which I bought a fair amount of for Deathcopter a long time ago, cut to length using one of the small bandsaws.

Here’s the two multirotor control boaards. The mounting plates have mirror-offset holes which can be arranged in a regular octagon using standoffs. The two flight controllers are mounted right above each other, with the height chosen such that one is slightly above the height center of the props and the other is slightly below, for symmetry.

The controllers do not interface with each other at all – they are totally independent; if I had enough thrust, I could fly it as either quadrotor. The “copter mode” is selected by a few DIP switches and the control gains by trim potentiometer. It’s important that these boards are very simple gyroscope-only controllers. They do not help the frame achieve self-levelling due to the lack of accelerometers, so it’s a very different experience flying. I’m used to Tinycopter and its (vaguely) angle-controlled flight, so I’m still not very proficient at flying Tinystar.

Incidentally, a week after the “double quadrotor” discussion and of course right after I had ordered two of the i86 boards, Hobbyking unveiled the KK2 controller which has 8 outputs and performs self-levelling (though not quite the same as angle-control). And a LCD with menu system. AND a little piezo buzzer.

Hmph. Oh well – these things were already in the mail by then, so let’s just press on.

The motors for this thing are the HXT 5 gram outrunner motors – in fact the same ones that 4PCB uses. Tinycopter uses the slightly larger 10 gram motors, but I decided that they were too overkill for this project and could not use the 4 x 2.5″ propellers effectively. The 4 x 2.5 props are even a little small for the 5 gram motors in this application, and in retrospect I should have made the arms slightly longer to use 5 x 3″ props (the same type that Tinycopter uses) for more thrust and payload.

#2-56 screws are used to clamp the blocks to the tubing. I want to go back now and replace every one of these with real steel screws. The few grams I gain in weight will be more than made up by increased stiffness, because these screws can’t apply enough clamping pressure before they strip. Tinystar’s arms need a little adjustment every time I pull a rough landing (which is like, every time).

The frame mid-construction on Saturday…

I assembled the frame as the landing legs’ paint dried. The effect is definitely not carbon fibery, but it’s not bare wood. I am a fan of this style of landing gear and may make some (not so high-heeled ones) for Tinycopter. It’ll definitely fare better than my little 3d printed landing claws on it right now…

Starting to look like something…

The weird arrangement of legs is due to the real Cinestar 8’s need to carry a large camera on a gymbal underneath, with the rearward bias of the landing gear keeping them out of the camera frame further. It also looks more badass. That was all for Saturday – Sunday was reserved for wiring everything up!

oh god

Clearly as the number of rotors increases, wiring messiness increases nonlinearly. This is going to be difficult to arrange elegantly and in a vaguely easy to service fashion…

I guess it didn’t end up that bad looking. I pretty much daisychained each ESC together, so it’s actually quite suboptimal from a power distribution perspective – the last ESC in the chain gets the most screwed in terms of voltage drops and resistance. I would also mount these on the bottom side of the frame if I did it again, for easier access. A tree-style (8-to-4-to-2, for instance) distribution method would have been much better, though a bit more weighty.

Tinystar has noticeable trouble starting all 8 motors at once, something which I attribute largely to the daisychain wiring. Several motors pulsing at once mean the voltage on the positive connection can sag alot while the ground (0v) rises erratically, throwing off the ESCs’ starting routines.

The flight controllers have their own ‘decks’, onto which they are secured with fluffy double-sided tape. This is dramatically less isolation than Tinycopter’s block of memory foam, but it seems to work very well for one reason or another. Perhaps accelerometers really are more trouble than they’re worth…

Powered on! It took a few coin flips to get the motors spinning the correct direction. The controller assumes you have certain motors spinning one way because differential thrust is needed to turn (yaw) and if the props are wrongly-handed the direction of spin for a certain thrust differential will be opposite, leading to hilarity.

I tested one “subrotor” at a time – first, the four 45-degree motors in the “X” configuration, then the 90 degree motors. This was just to make sure I didn’t put everything together only to have something die or not work – I had to go back in after the “X” stage and replace one of the ESCs (Side note: the 6A ESCs come in a old and a new version, the latter of which does not work with 3S lithium batteries but the new one does)


It took me a while to get used to flying in rate mode….

…okay, it actually took me a while to dare fly it at all since it’s too pretty and my past first-flights have all ended in sadness. I still have issues with “station keeping”, or holding it steady in one place, but I also have that issue with Tinycopter and more flight hours can only help it. Rate mode is literally 1 integral away from angle mode, so the joystick movements required are much different. One tactic that has come in handy is briefly flicking the stick in the direction I want to move in- this step-changes the attitude since you command a strong rotation and then none. It’s not the smoothest of movements but it gets the job done.

One thing we noticed immediately is that this thing is so stable. I’m thinking it’s due to two main reasons. First, it has more ‘unit vectors’, so to speak. More directions it can move in while only changing one motor’s speed. A quadrotor moving in a direction that is not parallel to its arms needs to spin up or down at least 2 motors, whereas an octorotor can additionally move in the 45 degree directions without having to do the same. Second, there’s six gyros in total because of the 2 flight controllers, so not only do you get parallel readings, but the flight dynamics make for some mean ‘mechanical averaging’ too – the motors and propellers are on the end of long arms which have some springiness, so noisy command impulses can be absorbed. The propellers themselves are viscous couplings to the air (Thrust is a function of velocity), so it will also tend to damp erratic differnces between the motors.

Basically it was found out that I could set Tinystar flying very level and slow, and actually “dribble” it up and down, pushing down on the battery. It sinks a little, but then recovers.

Then I try flying it again and all hell breaks loose. Hmph.

Anyways, because Tinystar was made in mimicry of something used in cinematography, the test videos are a little more artsy than usual. There are, somehow, already two test videos. The first one was filmed indoors the day of:

And the second one filmed outdoors on the Third Ever MITERS Flight Day with even more arts:

I really should think about making a tiny 3-axis gymbal for this thing… once I switch over to 5 x 3 props so it can lift more than itself.

2 thoughts on “TinyStar, the 2-Day Dual-Flight-Controller Scale-Model Octorotor”

  1. You now need to make a 16 rotor, sci-fi style, flying aircraft carrier, with rows of tiny quadcopters and a few octocopters docked on it.

  2. It’s clearly missing a UFO-like peripheral row of LEDs one can run in a circular sequence (two, counter-rotating?) in order to properly mess with people’s heads in the dark. Just sayin’.

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