This is a project to create quad-X (true-X and left-right symmetric) frames by applying the tetrahedral structure, which has been shown useful for building a quad-plus frame, called the Tetra frame, having very good vibrational resistance as in my previous posts.

Design Concept

Regarding the Tetra frames, four motors are placed at four vertices of a tetrahedron. In the QT(=Quadruple Tetrahedron) frames, four sets of two adjacent motors are placed at the two vertices of each of four tetrahedrons, with each motor supported by the two adjacent tetrahedrons.
This QT frame structure is a type of so-called Z-frame, mounting two front motors and two rear motors at different heights. In a conventional Z-frame structure, the front and rear motors are aligned in the same direction. However, to orient the rear motors downward and align the front and rear propellers horizontally, the frame design must account for the motor heights. I adopted the latter approach.
In the Tetra frame, each arm serves a dual role as both motor mount and electronics (or battery) mount, whereas the QT frame requires an additional electronics mount plate. In other words, there is no upper limit to the height of electronics stack, which is a highly convenient feature of the QT frames.
To achieve the above structure with carbon plates, numerous braces are required. Therefore, I have tried to reduce aerodynamic resistance as much as possible and improve efficiency of the propeller airflow. Consequently, all plates except those for mounting the motors, camera, electronics, and battery are arranged vertically.
For compatibility with horizontal mounting of electronic devices, the true-X QT frame for double-stack layouts is not suitable for builds smaller than 3-inch prop size. Therefore, I created a single-stack specification as a 2.5-inch build. In fact, it became slightly tight even for 3-inch builds, and the frame size became very close to the 3.5-inch specification. As for the 3.5-inch build, it is the ideal size requiring absolutely no compromise regarding electronic component mounting.

As can be easily confirmed from the CAD images above, in each design, I added braces not present in the design concept image and connected the side braces to the battery mount plate. These braces play a crucial role in suppressing frame resonance caused by side braces that are excessively long despite insufficient thickness. Before adopting these braces, I spent several months attempting to improve frame resonance characteristics without using additional carbon plates, but all those efforts proved futile. Furthermore, each diagonal brace connecting the front (rear) motor mount plate to the battery mount plate is secured to the battery mount plate with two screws, enhancing the overall frame strength. This is just one tiny example of the anonymous golden rule of frame design: "Each arm of a modular drone should be secured with at least two screws."

Builds

QT25

• Frame : QT25 KIT
  Wheelbase : 124.4mm
  Thickness : 1mm
  Motor mount : 9x2mm, 4 holes, M2
  FC mount : 25.5x25.5mm, 20x20mm, 16x16mm, M2
  Stack height : 5+mm (requires four M2 standoffs of length at least actual stack height + 5mm )
  Compatible with whoop style USB connector
  Compatible camera width : 14mm (including O4 Lite)
  Built-in battery strap holder (can use up to 15mm width strap)
  Weight : Approx. 13.5g (Carbon only)
• AIO FC : DarwinFPV F435 45A AIO
• Motor : Amax 1504 6000KV
• Props : HQProps T63mmx8
• Receiver + VTX : NeutronRC VTX+ELRS AIO
• Camera : Caddx Ant
• Weight : Approx. 114g

QT3

• Frame : QT3 FOR 19-20MM WIDE CAMERA KIT
  Wheelbase : 152.7mm
  Thickness : 1.5mm
  Motor mount : 9-12x2mm, 4 holes, M2
  FC mount : 25.5x25.5mm, 20x20mm, 16x16mm, M2
  VTX mount : 25.5x25.5mm, 20x20mm, M2
  Stack height : 11+mm (requires six M2 standoffs of length at least actual stack height + 5mm)
  Compatible with whoop style USB connector
  Compatible camera width : 19-20mm (including hard mounted O3 and O4 Pro)
  Built-in battery strap holder (can use up to 20mm width strap)
  Weight : Approx. 33.7g (Carbon only)
• AIO FC : CCRC AT32F435 2-6S 40A AIO
• Motor : FPV Cycle 23mm Short Motor 3450KV
• Props : HQProps Duct-76mmx8
• Weight : Approx. 207g

QT35

• Frame : QT35 FOR O4 PRO CAMERA SOFT MOUNT KIT
  Wheelbase : 175.4mm
  Thickness : 2mm except 1.5mm bottom plate
  Motor mount : 9-12x2mm M2, 16x3mm M3, 4 holes
  FC mount : 25.5x25.5mm, 20x20mm, 16x16mm, M2
  VTX mount : 25.5x25.5mm, 20x20mm, M2
  Stack height : 11+mm (requires six M2 standoffs of length at least actual stack height + 5.5mm)
  Compatible with whoop style USB connector
  Compatible camera width : 23mm including damper (compatible with ordinary M3 FC dampers, My custom damper file for 3D print service)
  Built-in battery strap holder (can use up to 20mm width strap)
  Weight : Approx. 49g (Carbon only)
• FC : JHEMCU F722 NOXE
• ESC : SEQURE Blueson A1 2-6S 65A
• Motor : Amax 2207 3250KV
• Props : HQProps Duct-89mmx8
• Weight : Approx. 320g