My Cuties [QT35, QT3, and QT25; QT=Quadruple Tetrahedron]

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.

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. In the QT image shown in the right (or above) figure, the central green line and the two blue lines must not be parallel. Otherwise the two adjacent diagrams containing both the green line and the blue line would become two-dimensional planar diagrams rather than tetrahedrons. The green line belongs to all four tetrahedrons simultaneously, corresponding to the battery mount. Therefore, the battery mount plate is critically important for the overall frame strength. However, sufficient strength is easily achieved by combining it with the bottom plate for mounting electronic components. Because the battery mount is supposed to be horizontal just like an ordinary frame, the blue lines are inclined.
　
Mathematically, QT is an intermediated state of the minimal triangulation of an octahedron. An octahedron is a polyhedron obtained by identifying bases of two square pyramids, with the four vertices of the base corresponding to the four motor mounts. Here, the minimal triangulation means adding an edge connecting top vertices of the two square pyramids. This edge corresponds to the battery mount.
　
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, 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."

QT25

• Frame : QT25 KIT (prototype still in progress)
  Wheelbase : 121.7mm (nearly true-X, slightly wide; horizontal distance between motors is 88mm left-right, 84mm front-rear)
  Thickness : 1mm (not suitable for bando bashing)
  Motor mount : 9x2mm, 4 holes, M2; front-rear gap is 32mm
  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. 14.3g (Carbon only), 17.3g (w/ M1.4 hardware, w/o M2 hardware)
• AIO FC : DarwinFPV F435 45A AIO (worth a try if cheep)
• Motor : Amax 1504 6000KV (11.7g, powerful and silent, suitable for 8-blade props)
• Props : HQProps T63mmx8 (my favorite)
• Receiver + VTX : NeutronRC VTX+ELRS AIO
• Camera : Caddx Ant
• Weight : Approx. 114g

QT3

• Frame : QT3 FOR 19-20MM WIDE CAMERA KIT (CARBON ONLY)
  Wheelbase : 152.7mm (true-X)
  Thickness : 1.5mm (not suitable for bando bashing)
  Motor mount : 9-12x2mm, 4 holes, M2; front-rear gap is 38mm
  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), 39.3g (w/ M1.4 hardware, w/o M2 hardware)
• AIO FC : CCRC AT32F435 2-6S 40A AIO (add to cart during sales)
• Motor : FPV Cycle 23mm Short Motor 3450KV (20g, always out of stock)
• Props : HQProps Duct-76mmx8 (my favorite)
• Weight : Approx. 207g

QT35

• Frame : QT35 FOR O4 PRO CAMERA SOFT MOUNT KIT (CARBON ONLY)
  Wheelbase : 175.4mm (true-X)
  Thickness : 2mm except 1.5mm bottom plate (not suitable for bando bashing)
  Motor mount : 9-12x2mm M2, 16x3mm M3, 4 holes; front-rear gap is 38.5mm
  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 (My custom damper CAD file for 3D printing service, Shore hardness 60A recommended)
  Built-in battery strap holder (can use up to 20mm width strap)
  Weight : Approx. 49g (Carbon only), 57g (w/ M2 hardware except standoff related)
• FC : JHEMCU F722 NOXE (cheep and good)
• ESC : SEQURE Blueson A1 2-6S 65A
• Motor : Amax 2207 3250KV (35g)
• Props : HQProps Duct-89mmx8 (my favorite)
• Weight : Approx. 320g

For size comparison purposes, the three builds above are combined into a single image as shown below:

Hereafter, I show the results for QT35.

As usual, I ran the PIDtoolbox Basement tuning in angle mode before flying it outdoors.

On the test flight using a CNHL 4S 1300mAh 130C battery, I performed 0-100% slow throttle punches, some quick flips and rolls, and obtained a blackbox log file. I think the results are good enough to enjoy HD FPV for casual flight.

Amax 2207 3250KV motor (35g) is quite heavy for a 3.5-inch quad, but it provides ample power for free flight enjoyment. With a 4S battery, the KV value is slightly low, and my QT35's top speed isn't particularly fast. However, with a 6S battery, the top speed easily exceeds 200km/h. Under light wind conditions using CNHL 4S/6S 1300mAh 130C batteries, I obtained the following results :

• Maximum speed with 4S 1300mAh (157g) : 43.90m/s (158km/h)
• Maximum speed with 6S 1300mAh (225g) : 62.01m/s (223.2km/h)

Since the maximum current of a 6S battery is only about 110A (27A per motor), it is reasonable to expect that the lighter 6S battery achieves a higher maximum speed.