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Frame

🟢 Start — zero knowledge, plain words. 🟡 Hands-on — building or buying, specifics and tradeoffs. 🔴 Specialist — the physics and math behind it.

🟢 Start. The frame is the skeleton — everything else bolts to it: motors, electronics, battery. Frame size is named by the propeller it fits: a "five-inch" (5″) is the classic freestyle machine, 7″ means long range, and 65–85 mm tinies are indoor whoops. The material is almost always carbon fiber: at the same weight it's several times stiffer than aluminium, and a drone that regularly meets trees needs exactly that.

🟡 Hands-on. Frame geometries differ in how the arms are laid out. "True X" (motors on the corners of a square) flies the most neutrally. "Stretched X" (longer spacing along the flight axis) stabilizes fast straight-line flight — a racing favourite. "Deadcat" pushes the front arms outward so the props stay out of the camera's view — the choice for cinematic builds. Arm thickness for a 5″ is typically 4–6 mm; thicker means stiffer and tougher but heavier. Also check mounting real estate: the standard electronics hole patterns are 30.5×30.5 mm and 20×20 mm.

   true X          stretched X         deadcat
  M4     M2        M4       M2       M4       M2
    \   /            \     /          |       |
     [X]              [ X ]           [  X  ]
    /   \            /     \            \   /
  M3     M1        M3       M1        M3     M1
  neutral          stable at         props out of
  handling         speed             the camera view

🔴 Specialist. Carbon is a composite: stiffness depends on the weave (3K plain/twill), fiber orientation and pressing quality; cheap plates carry micro-cracks from cutting. Frame stiffness directly sets the arms' resonant frequency — in a blackbox spectrogram, resonance shows as a horizontal line at a fixed frequency, independent of throttle. Compute the center of gravity instead of guessing:

xCG=∑imixi∑imix_{CG} = \frac{\sum_i m_i x_i}{\sum_i m_i}

Mount the battery so the CG lands in the geometric center of thrust. Two electrical traps: carbon conducts electricity (chafed insulation on an arm = short circuit) and it shields radio waves — an antenna lying in the shadow of a carbon plate loses real decibels.

🖼️ Photos: your own top-down shots of the three geometries; Wikimedia Commons "carbon fiber weave" (CC0/PD filter).