I'm finally getting back on track with the Santa Cruz project. The frame needs to be very strong, yet very light. I have settled on the "Box - X" design, that is, the frame is a normal "X", but the arms of the "X" are fastened together with small diameter carbon fiber tubing.
By connecting the ends of the arms, the diagonals (the "X" part) can be much thinner while still maintaining overall strength and rigidity.
The craft will need a "landing gear", so I plan on extending short legs down from the end of each "X", and to add strength to those - as well as further preventing twisting of the arms, I'm adding a very thin carbon fiber "X" between all the legs. Since these "arms"are - in effect - trusses, they can be thin while being strong.
I'm planning on the main "Box X" part (3 pieces to make the "X", and 4 to make the "Box") from 6mm round carbon-fiber tubing. The legs (approx 6" long) will also be made 6mm carbon fiber tubing, while the "X's" that connect the legs will be made from 3mm carbon fiber rod (solid). 4 more pieces of 3mm carbon fiber rod will connect the bottom of each leg back to the center platform of the copter. These additional supports are needed to handle the 5-6 lb weight of the batteries, which will hang under the center of the craft. Carbon-fiber disks glued and screwed to the carbon fiber tubing will act as motor mounts and a few 3-D printed plastic pieces will aid in holding things in place. An acquaintance who is very familiar with (real) aircraft frames and carbon fiber fabrication and has a machine shop has tentatively agreed to provide help. I'll need it!
At least three of the 4 batteries that power the craft are mounted vertically in "chutes" made from carbon fiber angle stock. A nylon string (fishing line?) connected across the chute under each battery holds them in place. A 26GA nichrome wire is twisted around each nylon string and the ends of the wires brought to the battery controller. The fact that the batteries are mounted vertically explains the quite long length of the legs.
The battery controller consists of a Microchip PIC 18F2321 uC. 4 outputs are routed to 4 N-FETs acting as a level shifters. The drain of each of the N-FETs is connect to a 60A,40V P-FET held in the "OFF" state with a 10K resistor. Each battery is connected to the Source of a separate FET. The drains of all FETs are connected in parallel and feed power to the craft's main power rail.
4 channels of 10-bit A/D (in the 18F2321 uC) are each connected to a separate battery. When initiated by a trigger signal, the uC brings the first output high which turns on FET #1 and battery #1 powers the craft. When battery #1 voltage drops below a set potential, the 18F2321 brings output #2 high. This has two effects: It turns on the FET connecting battery #2 to the main power rail, and it also feeds power to the nichrome wire wrapped around the nylon string. Within a second or two, the heat generated by current flowing through the nichrome wire melts the nylon string and battery #1 is no longer held in place. It drops down the (short) chute by means of gravity and a short loop of wire allows the battery to gain a bit of momentum before it "yanks" on the 3.5mm bullet connectors that connect it (electrically) to the craft. The disturbance from the "yank" is small enough, and of short duration, so the craft simply shudders and continues running on battery #2. This process repeats for battery #2 and battery #3. Battery #4, of course does not need to be disconnected!
I'm laying out the PCB for the above controller right now.
I have the basic airframe designed in AutoCad, and a friend is converting the design to SolidWorks. SolidWorks has a FEA 'add on' that will show strengths and stresses. I will use that to refine the airframe.