I wanted to build a lightweight quadcopter frame for some time, and I've eventually found time to do it. Hereby I summarize some aspects of the build that allowed me to save weight.
Note that this is a summary of the complete post in my blog.
I’ve chosen to build a very simple X frame for this build. The frame is made out of two square unidirectional carbon fiber tubes of 8mm x 8mm outer dimensions and 40cm (15.7 inches) length, crossing over each other and glued with epoxy glue at the center. A few bits of CF tube are added around the crossing point for rigidity. The overlapping tubes create an asymmetry that allows for overlapping props, thus enabling larger prop sizes than the frame would normally allow.
Power and motor leads often account for a significant factor of the All-up weight, especially if heavy gauge wiring and high current connectors are used. In this build I tried to shorten the lead length as much as possible, and get rid of all the connections, instead choosing to solder leads directly to the PCB. Removing the bullet connectors from the ESC and motor side alone gave a weight gain of 30 grams (1.05 oz). Removal of long leads from the PDB to the ESC gave an additional gain of 40 grams (1.4 oz).
I’ve drilled holes directly on the CF tube 19 mm (0.74 inch) apart, and used them to fix the motor. In addition, I’ve used a mix of steel and plastic screws to secure the motor. This combination shaves off a few grams from the build, without negatively affecting the frame vibrations.
With this reduction in weight a small battery should be enough to keep the aircraft on the air for quite some time. I’m using a 1550 mAh 2S battery, with a weight around 150 grams (5.3 oz). Further reduction could be achieved using a pair of 18850s in series.
I’m using plastic 10-inch props, which are overlapping a bit at the edges. These are really lightweight props, at the expense of being a bit flimsy. Still, I prefer them to a pair of heavy CF, as they have a much softer sound profile. I cannot measure the actual RPM, but from the sound of the quadcopter they are spinning at around 2000rpm when floating.
The frame weighs 483 grams (around 1.06 pounds) when measured at my kitchen scale. I don't expect this figure to be very precise, but it should be in the ballpark. I still have to remove the leads from two of the ESCs, so it should drop a bit more. A preliminary estimation by eCalc gives the hover time at around 15 min, which is quite satisfactory given the small battery. It's also interesting to note that around 80% of the weight of this quadcopter is drive weight. So with the current hardware (motors, ESCs, battery), I can't find any more ways to reduce weight on this one. Swapping the 2S cell for a lower capacity 3S should give 1-2 extra minutes of flight, according to eCalc.