I made the following post to the Pupman Tesla Coil mailing list on March 8, 2018:

If you are looking for a truly fascinating coil design with ground breaking potential, try building a magnifier with three tertiary coils on a common grounded frame. From my decades of studying Tesla literature, this is how Tesla built the flying triangle, which has since been perfected by US black projects. Back in the 1990s, there was more Tesla information on the Internet and BBS forums, which has since disappeared. One such report was of Tesla having built a model flying triangle configuration in his lab.

There is one primary and secondary coil, which drives three tertiary coils. All are grounded to a single aluminum frame, which is not in contact with Earth ground. The tertiary coils are each 1/3 out of phase with each other, and have variable inductance or variable capacitance to finely adjust the resonance.

The three tertiary coils develop a rotating magnetic field around the entire coil setup, which creates what I call a space bubble. The space in this macro rotating magnetic field becomes gravitationally separated from the space around it. By changing the characteristics of the of the rotating magnetic field space bubble, you can build a free moving vehicle that can navigate through space with reduced gravitational influence from the Earth. The orientation of the tertiary coils with respect to the Earth ground plane is what keeps the system from tumbling. With the tertiary coils perpendicular to the Earth ground plane, the rotating magnetic field orientation will also have an axis perpendicular to the Earth ground plane.

I believe Tesla's first model was an open frame. The rotating magnetic field space bubble will exist whether the frame is open, or whether it is completely enclosed around the Tesla coil system. However, a closed frame system should be more stable, as the conductive frame will give the rotating magnetic field space bubble a more reliable structure.

This is something I have been planning on for years, but have not yet put together. The key component of this system will be the design of the tertiary coils to be in 1/3 phase with each other and also to have a controllable variable inductance or controllable variable capacitance with just the right range of adjustment.

I followed up with the following comment:

Thanks, Jim, for the link to Antonio's work. This just clarified for me how the three tertiary coils should be tuned. A tertiary coil would be way out of tune with the resonating primary and secondary power supply if it was tuned to 1/3 wavelength. Instead, there should be three, quarter-wave steps and a missing step, such that the tertiary coils would fire X-X-X-O, or 1/4, 1/2, 3/4, 0. This makes sense since a magnetic field, whether rotating or not, is asymmetrical.
And it is more than just the magnetic field that is at play, here. Tertiary coils are acting strongly on the electrostatic field, in addition to the magnetic field. They are pushing electrons and ions. When three tertiary coils are mounted to a common ground, and tuned to produce maximum potential in succession, they will push the ions circularly, and perpendicular to the axis of the frame. This is what creates a rotating magnetic field plasma. The plasma will likely be double layered, with negative ions moving in one direction, and positive ions moving in the opposite direction.
The resulting plasma is the space bubble that surrounds the frame and coils. Naturally, the aim for tuning is to minimize streamer production and keep as much of the energy as possible within the system.