I always loved my Gecko but while trying to squeeze out every bit of performance during PID tuning, I always ran into the fact that the single-baseplate Armattan frame suffers from quite a lot of resonance. So I decided to move everything over to a new frame; the awesome ImpulseRC Micro Apex 4" frame!

Building the frame itself is fairly straightforward, except:

1. The edges of the carbon are not chamfered, they expect you to do this yourself with the two provided files. You'll waste half an hour of your life doing this, and much more if you don't take precautions against inhaling the carbon dust! I recommend to simply wet the frame and the file so the carbon dust will stick in place.
2. The center stack screws must be secured before mounting the arms, and cannot be reached afterwards. You can use either M3 or M2 screws, but the holes are not countersunk far enough for the M3 screw heads, so you'd have to drill it out half a millimeter further. For the M2 screws, which I used in my build, they provide some tiny rubber rings that help to prevent the screw from damaging the hole. It worked great for 3 of the screws but on the 4th the little ring squeezed out all the time. After countless re-alignments and trying other rings I managed to find the right balance between having a loose screw and squeezing out that ring.

And while ImpulseRC provides some amazingly detailed build instructions for the original Apex (http://apex-docs.impulserc.com/), there's no such guidance for its little brother...

To make sure the frame would have the least amount of resonance as possible, I followed Chris Rosser's advice and applied a thin layer of Nyogel 767A damping grease between the arms and bottom plates. I also used that grease instead of the usual blue threadlocker, and it works really well to keep the screws in place.

After this I bumped into one challenge after the other:

1. The Caddx Tarsier's flat cable I had was not long enough to put the boards in the back. I considered putting it in the center but that would have made the build weird - I think. Right? So anyway, I had to order the 10 cm cable from Banggood and wait almost a month before it arrived...
2. The camera plates in front are spaced 20mm apart, so I had to find two 0.5mm spacers to make the Tarsier's camera fit. It's a pity they didn't provide these spacers. Perhaps they expect you to use the tiny rubber rings but those wouldn't hold the camera angle very well I'm afraid, so I had to find some metal washers myself.
3. The 25V 470 µF capacitor I had would not fit nicely into the frame. I solved it by putting a 330 µF capacitor between the two stacks, and putting another 330 µF capacitor in the front. This turned out to provide excellent filtering!
4. There was no room to fit the battery powered beeper either, but I found a nice solution by taking it apart and putting the tiny battery behind the camera, and sticking the beeper board with VHB tape to the side of the camera cage. This way the beeper's button is perfectly reachable, better than it was on the Gecko. I just hope the longer wires between the battery and the board won't get damaged in a bad crash.
5. The most frustrating problem appeared when I powered everything up for the first time; the analog feed was showing a lot of interference and diagonal lines!?! I wasted a lot of time trying different cabling and different components, but it turned out that there was a hidden electrical connection between the frame and one of the motors.

The maiden flight didn't go without a hitch either. The ExpressLRS EP1 receiver, which should give me a range of many kilometers, failsafed when I only flew 100 meters away!

For some reason the receiver didn't like to be in the front camera cage at all. Perhaps it was because the antenna lead was squeezed under the ESC together with the Tarsier's cable which gave it some weird interference? After I put the EP1 in the back next to the VTX and made sure the antenna lead would not be parallel to other cables, the reception seems totally fine now.

Luckily, it was all worth it! This kwad flies amazingly well even though it's 20 grams heavier than before. The frame shows nearly zero resonance so I could disable all gyro lowpass filters and just keep the D-term filter slider at 1.5. The PID master slider can also go up to 1.5 or even higher, which results in excellent propwash handling. I'm sure that it'll allow me to learn some of those sweet Sbang Sbang tricks - this kwad is just as capable as its bigger 5" brothers!

More about the PID tune and other experiences at https://intofpv.com/t-implulserc-micro-apex-4