It took me 3 months to finally make it, so I would like to share all my experience and hope that would be helpful. My words are not very efficient but I'll try to write down all the important things and issues/solutions I have.
A little background. This is my first heli build but I'm pretty familiar with RC and Arducopter. I have no experience with heli before but thanks internet, DIY Drones and Rob, I've got a lot of info and help since day one.
My setup is an Align T-Rex 450 Pro, with 450L dominator tail boom and torque tube, and a Tarot DFC head running 350mm blades. Ceramic bearings are used on torque tube and gears.
Talk about the tail drive first. I like belt drive personally as it's very easy to set up and much less vibration, but it has static charge problem. 450 Pro is a torque tube heli, a special tail boom mount is needed to use belt drive. The gear and pulley that came with the boom mount was very bad and chewing my belt. After 1 min of bench testing my heli was full of black rubber dust, so I finally gave up, I don't believe belt will work well with any of such conversion. Torque tube is not bad at all, but vibration is indeed higher, and maintainance is trickier so not highly recommended. My tube is working great with two ceramic bearing though, no serious vibe/shaking at all.
Mounting the PX4 and wiring was one of the hardest task for me. 450 is a very small heli and apparently there is no room for such a large controller. People conventionally build a sub plate at the bottom, but I want things simple. I ended with a special laser cut plate bolted on the anti-rotation bracket, then foam tape the PX4 upside down on the "extended gyro plate". It's very simple but I think it's the best way to mount PX4 on a 450 heli. Besides I mount my tail servo like 450 SE just because I can use my carbon rod and can adjust the linkage easier.
Wiring the PX4 is a bit tricky, as one can't just plug the ESC/BEC on the servo output because PX4 has it's own power rail which can't power the servos. My first attempt was connecting all the servos and ESC on a separate pin rail then connect only the signal pin to PX4 with extra cables. It was a painful idea and it turned out to be too complex, you can find my previous wiring in the post about servo twitching. The final solution is actually very simple. I made an extra pin row on top of PX4 and convert all my servo connectors to 4-pin connectors. This row is interconnected, power cable is connected to this row instead of the second row. The servo pin out was Gnd-5v-Sigal and now becomes Gnd-empty-Signal-5v. I skipped the power rail of PX4 and I don't need extra cables. The less the cables there are, the less the inteference, the better signals I have. The only problem here is that the cables connected to PX4 is a bit loose due to the extra loose pin row. I'll check this very often to prevent any connector coming off.
Talk about inteference. You may already have heard servo twitching. Yes there it is. I have very serious twitching when my telemetry is on with a good antenna. As long as I grab the antenna, twitching is gone (it's not the 3DR telemetry, shame for me, my only non-3DR component). However I think the twitching is not due to the bad quality of the clone, but the wave that generates some weird signal inside PX4. I changed back to the stock antenna and put the it on the tail, twitching is gone as well. There is still twitching potential though but at the moment there is nothing I can do.
Long cable to the tail is not what I want but I'll temporarily live with this configuration. As this project goes further I'll make it beautiful, especially when I apply the direct drive tail.
Setting up the Heli firmware in MP is pretty simple and straight forward, thank you developers. Wiki has quite a lot of info to follow, and I would like to point out that the Frame type in MP is the one that has an helicopter icon in it, and that's the only thing which may confuse people. As I have flipped my PX4 for better mounting, I have set AHRS_Orientation to roll 180, also don't forget to set roll 180 in the compass as well.
One issue I had but was solved is the ESC signal. I have a Turnigy Plush 40A connected to PX4, and it always gives a beep telling me there is no signal or signal out of range. I don't know why it was like that exactly because it worked fine when connected on my Rx directly. So I tried to recalibrate it again but this time trimmed my throttle down 3 clicks in the calibrating process and whoa!!! Now it works. Guess it's a bug in the ESC or something.
I did a rough tuning with the tail first without main blade but failed to get a good working PID gain. I thought I got a good starting parameter for the tail and mounted main blades for real tuning, then it turned out I was wrong. The heli started to yaw left as soon as it left the ground. I got it stable by increasing the gain, then slowly adjusting everything. The heli was already flying pretty good after my first battery was close to empty. I was planning to fly the second battery but the wind here is too strong. So I just called it a day and came back quite happily.
My PID is not optimal yet but it works fine.
The rate roll/pitch KP can only be set at the same time when using CH6, and as is already pointed out in wiki they should be tuned separately. I really hope I can have the option of rate roll KP and rate pitch KP separately.
In the tuning phase, I happened to have an occational failsafe for unknown reason. I haven't set any failsafe though. The heli suddenly drops to the ground and the buzzer starts to beep. Luckly I only hover less than 1m height with training gears and no damages was taken. I guess it was the battery that came empty but I'm not sure.
I guess that's all. I hope my experience is helpful and I'll add more content if I remember more important things. Logs of today are also attatched, there are many empty time in it because I was messing up with blade tracking, parameters, and talking to curious people around etc so I'm sorry for that.
I've changed to a new frame a month ago, this VWINRC 450 frame is pretty awesome, it has quite a lot of space to fit a PX4 inside.
Now my heli project is getting close to finish, mounted two CF booms for aerial photography equipment, and of course, SpinBlades.
Regarding to the software, I've found that logging on PX4 seems not working with 3.1.2, this issues is already addressed so it will be fixed in the next release as far as I know.
Pre-arm safe checking will not pass if PX4 is mounted upside down even with AHRS_Orientation set correctly, Mission Planner still gives a warning of offset too high (I forgot whether it's the mag or IMU offset, but both orientation are set correctly in MP), so I have to turned pre-arm checking off.
S.Bus2 doesn't work with PX4 at the moment, the receiver I'm using is Futaba 7008SB, the S.Bus 1 (first gen. sbus) output on pin 8 is still no go, so something is wrong there. S6208SB with S.Bus 1 does work perfectly, so at the moment only FASST or S-FHSS receivers that uses S.Bus 1 will work.
GPS is still out of my plan for a short time, but will try to do that later after everything else is well tuned.
Finally have time to finish the DDVP tail for the project. I'm planning to use it on the Trex 450 Pro, let's see whether it will last, I'm not too confident with the laser cut POM material but it seems sturdy. There are already some great guys sharing their experience of DDVP tail, like this one. Mine is very similar, but I use an extra layer to save the process of countersink for the M2 screws and keep the original side plate for strengh. One thing I'm worried about is the long wire going through the boom, as it provides power to the motor, I guess the resistance is somewhat important. Thinking about long 18AWG wires but perhaps 16AWG is better and heavier, no idea. Any advice is much appreciated.