After much thought and calculations, I realized that my original design for the Santa Cruz Project was faulty.
I had built a very lightweight frame from 3D printed plastic and carbon fiber tubing. It was light and strong. The rather large motors had 16" (now 18") propellers sitting on top, and the 4 big battery packs hung vertically underneath the center platform. The 4 batteries (together) weighed more than the entire rest of the craft. That is just what I wanted.
But I soon realized one thing: In order to go fast horizontally, the craft has to tilt up to 45 degrees, and in order to tilt, the 'copter has to fight any weight that is below the center of gravity (CofG). With so much battery weight below the CofG, I would waste a lot of energy just tilting the craft. What I need is a center of gravity that is on the same plane as the center of lift (CofL) - the propellers!
I need to re-do my frame to put the batteries much higher in the craft. And with the CofG on the same plane as the CofL, when I drop a battery to the ground, the craft's CofG will not change much, making the control system more stable and easier to 'tune'.
So... back to the drawing board. I'm still going to use my "BOX - X design", where 4 thin arms connect the central core platform to each of the motors, and 4 very thin rods connect the ends of the arms - in other words, a box with an 'X' connecting the apexes of the box. Only this time, my battery 'box' will extend upwards such that a portion is above the plane of the propellers.
Now, I need to find a good place to put the flight controller - which should also be near the CofG. This is precisely the area used by the batteries. I may have to make a square tube opening in the middle of the batteries just to hold the flight controller.