I wanted to build a Toothpick-class park flyer as light as possible, preferably under 30g (dry). Something that could use spare 0803 motors, and many of the Whoop-class batteries that I already had. I might eventually check out 1102 or 1103 motors, but I don't have those right now. I used a 1S-2S F4 flight controller, but I could be convinced to try something heavier or lighter if the respective trade-off is worth it later.

What would you have done to reduce the weight further, or optimize the power train?

This build summary is broken down into pre-build estimation, initial build, final build, and batteries.

Pre-Build Weight Estimates

Base

• Frame: 4.8g (no hardware)
• Flight Controller/ESC/Receiver: 4.2g (no pigtail, no hardware)
• Motors: 9.6g (no screws)
• Propellers: 1.8g
• Camera/Video Transmitter/Video Antenna: 3.4g

Hardware

• Pigtail: 1.4g
• Frame bolts (4), nuts (8), standoffs (4), spacers (0), and rubber dampeners (4): 2.0g (estimate)
• Motor screws (12): 0.8g (estimate)

Miscellaneous

• Camera Mount: 1.3g
• Rubber band: 0.2g (estimate)
• Solder: 0.5g (estimate)
• Glue and tape: 0.0g (avoiding use)
  Estimated TOTAL (DRY): 30.0g

My initial estimate seemed to be on point at the time. Without yet adding the flight controller and dampeners, camera, canopy, and solder, I was hitting the expected ~20g weight.

Initial Build and Crash

While I was waiting for some parts to arrive, I transplanted parts of my modified Eachine Trashcan to the frame. I use a Larva X canopy in that build to hold the nano camera. The heavier camera and VTX added some weight. Brining it to 31.2g. But at the same time I did not use the frame's metal screws because they were too short. Instead I used a stack of nylon screws and standoffs. It may have saved a little weight to offset the heavier FPV system, but it was not smart, as many reviewers have noted.

After several test flights over grass, on my next to last pack as I was bringing it back to the parking lost from where I was flying it crashed mild to moderately and broke apart easier than i have ever seen a quad before. Lesson learned, so I found some longer metal screws for the final build below.

Final Build Measured Weights

Base

• Frame: 5.0g (no hardware)
• Flight Controller/ESC/Receiver: 4.1g (no pigtail, no hardware)
• Motors: 9.7g (no screws)
• Propellers: 1.8g
• Camera/Video Transmitter/Video Antenna: 3.0g

Hardware

• Pigtail: 1.0g
• Frame bolts (4), nuts (1), standoffs (4), spacers (0), and rubber dampeners (4): 2.0g
• Motor screws (12): 0.5g

Miscellaneous

• Camera Mount: 1.9g
• Rubber band: 0.2g
• Solder: 0.0g (estimate; negligible)
• Glue and tape: 0.0g (did not use)
  Final TOTAL (DRY): 29.2g

A couple things were slightly heavier than advertised: frame, motors. But my original estimates were slightly too high in others: flight controller, motor screws, pigtail. The motor screws at 3mm are too short. They'll likely have to be replaced with 4mm which will add some weight being metal. That is, unless i get a big batch of RENY screws to save a little across all my brushless builds. I could have trimmed the pigtail a little shorter, but i didn't want to re-solder it. I am not accounting for the jumper when flying 1S, which is an additional 0.2g. I may also try out some alternative props which could vary weight +/- 0.2g. The custom 3d printed camera mount was the biggest addition over the original estimate, but I wanted a 35 degree mount. I could have used the lighter BeeBrain mount and tilted it, but I'm using that mount on a whoop. Instead, I drilled out a couple pieces of unnecessary plastic on the 3D print to try to save a little weight. The VTX antenna originally had some rubber and metal sheathing, but removing them, shortening the wires, and direct soldering the VTX decreased weight by 0.4g.

The frame kit originally came with 10mm screws, but this was not sufficient unless you wanted to invert the board, but then the motor plugs hit the camera mount. To keep the intended board orientation I used 15mm scresws and 5mm standoffs from an old build. It added weight over the frame kit, but was still in line with my initial estimate by getting rid of the metal nuts. I ended up only needing 1 nylon nut for the rear screw, the 3D print canopy holds itself snuggly in place.

Battery Options and Testing (all PH2.0 unless noted)

• 250 mAh 1S: 6.3g (not likely to use, for reference)
• 260 mAh 1S: 7.0g (not likely to use, for reference)
• 300 mAh 1S: 7.6g
• 350 mAh 1S: 8.6g
• 450 mAh 1S: 13.0g
• 525 mAh 1S: 13.5g
• 650 mAh 1S: 14.5g
• 300 mAh 1S x 2: 15.2g
• 300 mAh 2S: 16.0g
• 350 mAh 1S x 2: 17.2g
• 350 mAh 2S (converted to PH2.0): 21.5g
• 800 mAh 1S (recovered and converted from a failed 2S): 25.0g
• 450 mAh 1S x 2: 26.0g
• 525 mAh 1S x 2: 27.0g
• 450 mAh 2S (XT30): 28.9g (not likely to use, for reference)
• 525 mAh 2S (XT30): 29.1g (not likely to use, for reference)
  Battery Weight Range: 7.6g-27.0g
  

I have a fair amount of 450 mAh 1S batteries. I also have a variety of other batteries lying around, and I picked up some 650 mAh and 350 mAh 1S batteries to try out on thiis quad. They seem to have a relatively high Milliwatt-Hour:Weight ratio. And the 350's could be doubled up for an efficient 2S option.

On the first couple of flying sessions it's flying good on just about everything for basic cruising, 250-650mAh 1S. Punch is sufficient for some acro, but time will tell as I get more agressive. Motors are still getting hot on 2S, so I may lean towards 1S for this ultralight, but it's nice to have the option.

All-Up Weight (AUW) with a 450mAh 1S battery: 42.2g

What Else Did I Consider
How did Kabab build something similar that only weighed 38.7g (AUW)?

His motors were 0703 for a weight savings of 0.8g (and he could have saved more with 0802s). And I think he used only a single pigtaiI (~0.4g savings). I can't see if he has a camera mount under the canopy, but if it doesn't that's 0.3g. Maybe fewer nuts and only nylon (0.4g). Perhaps less standoff-type hardware (0.4g). And slightly lighter propellers (0.2g) That's a total savings of 2.5g. (And the camera mount, mentioned above). But at the same time, working against him is that I believe my frame is lighter. I was never able to figure out how he achieved the rest of the savings. But it's flying good at the time, so I'm happy.