XuGong v2 Pro and 3DR Pixhawk - build and thoughts

I had dream: to have foldable multirotor vehicle with gimbal and GoPro on it.
I've built Tarot 650 already and it's still 650mm when folded and not encourage to carry in backpack.
After poking around internet I found four options worth to look in details.

• Black Snapper, which slowly became Black Snapper Pro L from Globe Flight.
• AD1 from ActionDrone.
• QU4D from SteadiDrone, now they have variety foldable quads.
• XuGong v2 PRO from ImmersionRC.

In a beginning I bounced back and forth between Black Snapper and AD1 with leaning toward German creation. Snapper is quiet small when folded but heavy, total weight with battery and gimbal for ~15minutes flight time ends up close to 2.2-2.6 kg. This is pretty heavy for taking it for hiking, biking or any other non-motorized adventure. And I'm not counting remote and at least one spare battery to carry. Bad thing is that Snapper does not offer theirs nice front attached gimbal any more, they moved to DJI Zemuse hanging down option which made it less attractive for me.

AD1 is a little longer when folded, weight and capabilities are similar. California team offering combo with nice gimbal based on BaseCam SimpleBGC controller. When folded it's quiet long - 510mm and doesn't seem very portable. Just 140mm shorter than Tarot 650. And fully loaded weight is more than 2kg.

QU4D was nice and it was first foldable and really portable quad I saw on the market these days. Cannot say any more details. Its successors are good, but they are all in 2.2kg+ category.

Now I'm looking at XuGong v2 PRO. Light, small and advertising ~20 minutes flight time. Down sides: small compartment to put electronics. Frame is designed for DJI NAZA, power distribution board and OSD all tuned for this FC. And I'm APM person, especially because of guided flight modes. To imagine how small compartment is I can say that AMP 2.6 with headers facing to sides will not fit. There is option to use FC with connectors facing up and I thought about it.

How to fit Pixhawk?

Here I hit my first problem - I never was able to make APM to work with 3DR GPS/compass module. May be because my APM is a RCTimer clone, I don't know. I decided to move to Pixhawk FC. It's a little longer than APM, so need to do something with it.

First I removed existing headers and cleaned up solder.

Second I soldered new straight connectors. Don't look at gaps, since it's quadcopter, there is no need for all eight servo connectors. Also one of AUX pin was scarified.

After such vandalism act survived Pixhawk FC fits nicely in small compartment and all connectors are accessible.

There are round holes on side walls where safety switch goes as this place was make exactly for it :)

At this point we have main part assembled. In next step everything needed to be connected.
One thing: USB port is completely blocked. Thinking to get remote one from 3DR.

How to control gimbal from FC

Here is one more caveat with OSD/Camera control. Remember - everything here is for DJI NAZA. Gimbal pitch control and camera switch are implemented on PDB. It requires PPM input from receiver and uses CH7 for pitch and CH8 for camera switch. Chanel selection can be adjusted in GUI from ImmersionRC. Caveat is that PDB reads PPM from middle point before it hits FC.

First. Need to "improve" PDB for our needs. To do so, I removed two 100 Ohm resistors R11 and R12 to disconnected IC on PDB from gimbal connector.

And soldered single servo wire to three point: brown-GND, red and orange are PWMs for pits and roll from FC. I forgot to document which of them is what, but it's easy to figure out.
Note: on other end of wire you need two servo connectors, not one.

But this is not enough to make gimbal behave as wished. You need to change firmware to [Rocker Position Mode] G3 2-Axis Gimbal Firmware V2.13 version or later, where "Position Mode" is essential. Firmware can be found here under G3 2-Axis For Aircraft.

Results

Here is maiden:

Pictures above shows Multistar 4006 700KV motors on it with 10" props. Later they were replaced with Tmotor AirGear set. Power consumption in both cases is approximately the same.

Bad things

1. Because the main body is hanging down on 8 rubber dumpers system is susceptible to oscillations especially with high D-rates.
2. Gimbal is hard mounted to frame, as a result any oscillations coming to frame go directly to camera and video. This is visible even on 60 FPS footage when windy or flying fast.
3. Arm locks are the weakest point in whole design. Any impact will slice a little plastic notch and arm become freely rotating. In video below copter hit tree and then fell down from ~7 feet -- two locks were lost. It was third minor crush and I used all spare locks.
4. Gimbal itself is slow in response. In video below you can see that camera is tilting too slow to catch person movements.

There is a thread on FPVLabs where there is drawings for bigger frame plates.

Bottom line

 Nice, light and compact frame but fragile and not good for missions where you may not be able to revoke control back and save craft.

Tarot 650 quadcopter - Labding gear upgrade - field test results

Here is quick update.

First field test of custom made landing gear proved it didn't improve anything... So I moved back to stock option. Decided to spend more time on tuning electronics.

Update.
Because of fragility stock one broke again :(

So, need to do something.... 10 mm aluminium rod came very handy

After this mode I had pretty successful flights in NW Nevada:

Tarot 650 quadcopter - fragile skid bars

Tarot put woven carbon fiber 10 mm tubes with 0.7-0.8 mm wall as skid bars. They are connected to 16 mm vertical tube by T-join. To keep skid bar in place it suggests to use M3 inner bolt which actually make whole construction even more fragile. I broke them in very first land when copter was not perfectly straight and "dropped" from about 2-3 cm.

I had cheap landing gear from F-450 like quad decided to use.

Set contains four 8 mm carbon fiber rods, so wrapped with tape it perfectly fit inside.

As can be seen it still working well with broken outer tube.

Tarot 650 quadcopter - folding joints

What is good in Tarot 650 frame  - light (~450g), strong, foldable and relatively cheap.

Foldability is goodnes but it's a weakness at the same time. 

Joins

There are two M2.5 bolts screwed from both sides. Holes in CF plates are exact diameter to hold bolt tightly. BUT! Not forever and not all of them. When you fold/unfold it bolts start to loose. Vibrations helps in that also.

You can say, Okay, tighten them before each flight. Agree if they are accessible. Since main plate design allows to put electronics on top or bottom, most likely bolts will be covered.

I looked at Tarot's Hex and Octo frames and they had joints designed slightly different. Why not to make same thing here?

What you need is enlarge hole to 3-3.2 mm so M3 - 30 mm bolt will go through. 30 mm is perfectly fit with nylock nut. This will keep all pieces together and no need to tighten every time.

Landing gear

Copter with such frame will weigh at least 2kg, most likely 2.5kg and stock legs can hold it only if landing gently and straight down. Any deviation - need spares.

Each leg has three connection points and moderate landing may result with bent metal parts and broken CF.

After it happened with me I decided not to order replacement, next time same thing happened. Instead I ordered 16 mm CF tube to cut segments. Found some old glass fiber boards and cut parts from them.

I'm using two small pieces of 16 mm tube, it looks steady and reliable but in case of mistake, it can be replaced by one long one, there is space inside frame for it.

New legs folds only 2/3 way or so. May be later I'll make these green parts longer to fold legs in full.

Now I need to reorganize surfaces and put all electronics back.

Tarot 650 quadcopter - quick overview

Hi All!
I built Tarot 650 quadcopter. It's nice frame but still not as good as it can be. Even for it's price engineers had to spend some time flying it.
Weak points:

1. There is not too much space to put electronics.
2. Folding joints are bad, after some time bolts starting to fall off. I lost two from arms during flight and one form leg.
3. Legs are fragile, especially 10mm horizontal piece. I broke them very in first landing when started to tune flight controller.

Let's go over all these weak points and try to enhance them:
Fragile landing skid
Joints and Landing gear