Le Trifecta

In the beginning of 2015 HobbyKing announced their new tri-copter frame called the Trifecta (under their brand Quanum). The frame is a 330mm tri-copter that is foldable and in its folded state occupies less space than half a 250 quad. It is mostly a copy/clone/improvement of the pocketdrone but it does not come with foldable propellers and suitable such are hard to find.

Parts


Alternative components

The ESCs (RCX 12A Sky) i first picked are not optimal. I chose them out of curiosity while I was still ordering the motors from MyRcMart. But they are too big to fit in the tail or in the slot under the arms. In terms of performance they are good and they can be mounted in top of the arms instead. It could be preferable to use a smaller ESC like the 12A Afro  ESC or the KISS 12A. However, it should be noted that the Afro has only a 0.5A BEC and the KISS has none.

HobbyKing recommends their Multistar 2206 motor for this frame. Now, experiences from the Multistar lineup hasn't been fantastic. An alternative from HobbyKing is their Quanum MT 2208. It will deliver better when the power is needed but you'll be running closer to the limit of the ESCs. Using a 2208 size motor was in my considerations. Otherwise there doesn't seem to be that many options available for this size/kv/prop mount configuration except for this EMAX MT2206

I use a tail servo that I had in the drawer (I have too many things in the drawer). This particular servo is actually a spare cyclic servo for a 450 helicopter but it beats the specs of the HobbyKing recommended TGY-9018. When ordering the motors from MyRcMart their MG9025 would be an excellent alternative.

Pre-build reflections

The first impression of the frame and its design is very positive. You can see that a lot of thought has gone into developing this. I hope not all is stolen from pocketdrone without them getting any creds for it or that HobbyKing or the OEM itself has spent some serious effort on the design. You notice details like the placement of the holes where the power leads attaches to the distribution boards is located right where they must be for the cables to behave properly when you fold the arms. And the shelf tail servo slides right in between the potential battery and the body of the frame, all to allow maximum articulation of the retractable tail.

But should the tail really be retractable? With folding props the craft would be very compact with the retraction of the tail, but we haven't seen those yet. The mechanism and structure to support this consumes all the space inside the big chunk of plastic which is the body. Not much can go in there. But if you would have a fixed tail you could put a mini sized flight controller and receiver in there without any problem.

They've taken some short cuts in the work with the top part. They added some kind of shelf that should fit the mini APM perfectly apparently. But frankly, they should have included a more generic shelf instead to mount, say, all your FPV gear in one place. And the height of the shelf? There is no mounting point on the body for the flight controller, instead you should use a mounting board together with double sided tape/foam. This makes it very tight to connect servo connectors to the board while the shelf is on.

This is a build in progress. I'll update it continuously, but expect it to take a month or so because I haven't gotten all the parts yet and I will take time to consider and reconsider how I put things together. The build is tight, it should be, I hope I get it right the first time.

Build


Arm motors and ESCs

Please don't use the RCX Sky 12A, see below.

One of the things that surprises you with this frame is that you get screws to mount the motors with it. This is not in any way something that has been expect with previous cheap frames from HobbyKing but seems to have changed now. In the end, it saves a lot of time in the search for the appropriate screw.

My pick of ESCs is not a good choice in any way. It is too big to fit in the slot under the arm and I thought I'll put it on top of the arm as I have plans for other stuff (headlights) to hang under the arm later on. But this won't work. Initially I installed the ESC to close to the center and the arms wouldn't fold properly. After additional inspection it turns out that almost anything put on the top of the arms will prevent them from folding completely. There is space in the frame body for an ESC below the arm, but not above.

Front arms not folding completely due to the ESCs
I only know of two ESCs that will fit those arms properly. It's the KISS 12A and the Afro ESC Ultra Lite. I guess that was the HobbyKings plan all along.

The motors had long motor leads and I removed the ones on the ESC and soldered the motor leads directly to the ESC. I also replaced the power leads as they were just of the right length. From experience I know that soldering under such conditions often means they are 2mm too short.

Tail assembly


Tilt mechanism binding issues

There have been reports on the forums of people having problems with the tilt mechanism binding. My frame had this problem as well. When the mechanism was pushed together the tail became very hard to rotate at a level at which it would probably never be able to fly. To fix this you need to sand the center part of the hub. Unless using very fine sandpaper or polishing the parts afterwards the smooth surface will be lost. But with appropriate amount of sanding and some lubrication the tail will get a usable state. We can only hope the HobbyKing will come up with a v2 of the frame that has a ball bearing mechanism instead.

ESC and motor

Since the RCX 12A Sky does not fit in the tail, not even close, I was considering mounting the ESC outside of the tail. There is actually a nice spot under the tail servo. This location is suggested by manual to be used for a "telemetry device". It's a rather bad location for any thing, a hard landing will probably give whatever is located here a bit of a beating.

Instead I ordered a KISS 12A ESC. It came in the mail the day after. Fantastic little device if it keeps its promises.

Tail, motor and ESC
Tail assembly, motor and ESC

The KISS comes with nothing soldered. Which is nice, I re-solder all my ESCs anyway nowadays. I soldered power leads and the signal wire first. Soldering the motors leads is a bit complicated as you'll need to pull the wires through the hole in the tail first. I also added a small lead from one of the motors leads that will connect with the telemetry device later on. This way I will get motor rpm count in the telemetry logs, will be very interesting.


Tail ESC
KISS ESC 12A soldered

Tail servo

I have quite a few servos in my drawer doing nothing and therefore I didn't order any tail servo with the frame. I figured there had to be one in the drawer that could do the job. And yes, I found a ALZRC MG90S which in terms of specs beats the HobbyKing recommended one by a few decimals. It's also a perfect fit, even the splines goes well with the custom pinion gear that goes directly onto the servo splines.

Tail tilt mechanism
Tail servo
There is however some play in the servo attached gear. This has nothing to do with the servo and has been reported by users on rcgroups as well. This issue is that the washer that comes with the assembly isn't thick enough and the screw does not tighten properly. I had to use no less than 4 m2.5 washers in addition to counter this. Once in place the play in the gear is gone.

Telemetry sensor

There's a few electronic devices that needs to be installed on this limited body and I figured something will have to go on the inside. I decided to try to squeeze the telemetry sensor inside, I couldn't figure out how to put the receiver there without removing the casing.

I'm using a Unisense-E sensor with voltage, current, capacity, variometer and rpm readings. Unlike the e-fuelgauge it comes prefitted either with connectors or a massive copper wire sized for 100A installations. Naturally I needed to remove it and replace it with a more appropriate 16AWG wire.

Unisense-E with cabling redone

To make it fit I also needed to solder the signal wires directly to the pins instead of using the connector that came with it. And as if that wasn't enough I needed to cut away some plastic in the top part of the body assembly.

Slot for the the battery leads

I drilled and cut a slot in the bottom part of the body to make a better exit to the power leads that goes to the battery.

Inside of the body with all the stuff in place

And then, finally but still barely, the sensor would fit inside the body. As you can see on the foto above I also installed the filter for the camera and video transmitter inside of the body. This was where I first intended to put the receiver.


Power distribution

I used the PDB that came with the assembly. There really aren't that many options. I soldered the battery lead, the leads for the tail motor and the FPV filter from the inside. While doing this you realize that repairing this machine will not be easy. After this point, the whole assembly starts getting unpractical to work with.

The leads from the two arms come in from the sides like they're intended to. This works really well and the slots for it are well placed in relation to the point the they bend around.

Power distribution board soldered.

FPV Camera

There's really no place clearly intended for mounting an FPV-only camera on this frame. I usually use a wide angle board camera on my quads but it's going to be ugly if I make a mount for that on this frame. Fortunately I have a broken Fatshark pan-tilt camera in my drawer. This is not a favorite. It's not nearly as good a camera as the board cams on ebay for a third the price and one of the servos has given up. But I only need one servo and the camera is to expensive to go to waste. It needed some initial fixing of course since I rebuilt it to fit a FPV250. And one servo had to take the place of the broken one and I'm leaving the pan-servo out.

Fatshark pan-tilt soon to be tilt-only (more like pitch-only).

When you drop the pan-servo the bottom plate of the setup is quite flat which makes it perfect to mount on any other flat surface. But there is not space left over on the Trifecta for this. Instead I'm mounting the camera on top of the mobius mount. It is flat-ish and it makes the camera protrude enough to get propeller clearance.

Mobius mount with holes drilled for the zip-ties

 I drilled two holes to be able to attach the camera with zip-ties. I intended to use resin first but I usually tries to avoid due to toxicity and the fact that it's a pain to remove stuff if/when you need to.

Camera attached to the mobius mount

Two layers of double sided foam/tape was needed to make the mount sit flat on the mobius mount. I then secured it well in place with a zip-tie.

Trifecta with mobius mount + FPV camera

As you can see the camera protrudes well beyond the front motors and maybe a bit too much in front of the mobius itself. I tested with the mobius and the camera does show up on the recording, not even if you tilt it down.

Flight controller and receiver

Once I got to the top deck I stopped being impressed by the detailed design of the frame and start getting annoyed. Why wouldn't they make the top side of the frame flat? And maybe with a few holes drilled for easy mounting of a CC3D/Naze, KK or APM. And then, there's a top shelf for more electronics which is also not flat. Very annoying.

Why not a completely flat area up here?

HobbyKing recommends to mount the receiver on the narrow section right where tail enters the body. There actually a slot that seems to be made for that but I won't be placing my receiver here for two reasons. Firstly, the R7003SB 4 port receiver isn't small enough to fit in the slot. Secondly, the tail propeller would hit it, which might become somewhat of an issue.

Instead I'm placing the receiver and flight controller together under the to shelf. It's a tight fit but it works. I use the simple flight controller mount from HobbyKing and attach it with double sided tape to the body. This mount has a minor flaw. The holes are quite big, probably to allow a M3 nylon thread to pass through it, but when you can't reach to place a nut on the other side and need to screw in a self-threading screw, well... I filled it up with hot glue to be able to screw in self-threading screws.

20150217_222934
Flight controller and receiver

The ESC and servo cables are all soldered to the flight controller directly. My CC3D had straight pins soldered and I saw no possibilities to fit the under the included top shelf. Instead I removed them and soldered the cables directly.

I'm using a Fatshark FPV camera. It would be really simple to connect if I would also be using one of the large Fatshark video transmitters with built in 5V regulator. I'm instead using the small Skyzone TS5823 which does not have a 5V regulator which could make things a bit more complicated. This would normally pose no problem since all board-cams run directly from 3S LiPo. "Luckily" I'm not using a AfroESC but one with a more powerful BEC. I sourced the camera power directly from the BEC via the CC3D which makes for a neat installation in this case.

FPV Camera power source
Camera power from the CC3D/BEC

The video transmitter is sitting alone on top of the top shelf and I'm connecting it with a custom wire to the power distribution board and the video signal cable from the camera.

IMGP7535
Video transmitter with power and video signal cables

I intended to use the HobbyKing antenna mount for the antennas by placing the mount right in front of the top shelf. But the mount was way to clumsy for this frame and I used the classic zip-tie and heat-shrink technique. To my surprise the top shelf has a few extra holes drilled to allow quick and easy attachment of the zip ties. To ensure that the zip ties will not fall down during flight and vibrations I added a dab of hot glue to the them. They looked symmetrical and all until I heated the heat-shrink which also melted the hot glue and now they're not that symmetrical any more.

Antennas
Antenna installation

Result

The build has a few flaws, most notably the clumsy ESCs on the arms, but so does the frame. I could only wish that a v2 would change a few things like skipping the collapsible tail, making the upper surfaces flat and easy to build upon and extending the landing gears. I actually noticed that even with a small 1300mAh battery the landing gears a too short. Longer gears can be printed for the ones with a printer in reach (arms, tail).


Frame weight without mobius and battery

The final weight is about 20g more than calculated. It's more or less spot on since I didn't include cables in the calculation and ESC and servo specs doesn't either. Switching out the arms ESCs, loosing the telemetry sensor and avoid using the upper shelf could probably shave off 30-40 grams. 5 more grams could be won on using a non metal-gear tail servo. Other than that it would be hard to shave off more, maybe the tilting camera mount could be replaced.

CC3D Settings

I did an initial setup with the RCX ESCs. It was set a little high with them and I had some oscillations. Later on, having replaced the ESCs with Afro ESCs I tuned it using the TxPID module. While doing this I noticed I could set it much higher than with the old ESCs. Really nice!


CC3D Rate stabilization settings

I won't publish the attitude settings. Honestly I never really tune those settings but run with something that is good enough. I'm trying to learn to fly more in rate mode anyway.


Flight results and telemetry measurements coming soon!

Corrigendum

My choice of ESC was very bad. It is clumsy, heavy, ugly, badly built and one started failing during the second flight. I replaced both with Afro 12A Ultra Lite that I had coming in from a previous order, had I not had them ordered already I would probably have used KISS ESC instead since they are even smaller.

Addendum

For some reason I didn't install LEDs during the build. I considered it but skipped it. Anyway, while replacing the ESCs I also added a couple of front facing white LEDs and read/downward facing red and green ones. They're cheap, easy to install and gives a nice touch while only adding a couple of grams to the frame.

LEDs on the arms

I have equipped the frame with the longer legs that HobbyKing is selling. Now I don't have to belly land on the battery any more.







3 comments :

  1. Replies
    1. I'm sorry but I haven't had time to adjust the PID yet, only an initial configuration. I will post the PID as soon as I have a well flying setup.

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    2. I've added my latest PIDs for you now :)

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