Frame Physics: Geometry, Resonance, and Materials
CRITICAL The physical frame is not just a holder for components; it is a structural element that defines the control authority and vibration characteristics of the vehicle. A "floppy" frame cannot be tuned perfectly, no matter how good the software is.
1. Frame Geometry: The Thrust Map
The layout of the motors defines how the PID loop interacts with the physical world.
1.1 True X
- Layout: Motors are equidistant from the center and form a perfect square.
- Physics: Roll authority equals Pitch authority.
- Pros: Perfectly symmetrical handling. Easy to tune. The "Gold Standard" for freestyle and racing.
- Cons: Propellers are often visible in the FPV camera view.
1.2 Wide X (Squashed X)
- Layout: Motors are wider apart laterally than longitudinally.
- Physics: Roll inertia is higher than Pitch inertia. Roll authority is slightly lower.
- Pros: Moves props out of the camera view.
- Cons: Requires asymmetric PID tuning (Pitch P often needs to be higher than Roll P). Separation of "Clean" vs "Dirty" air is different for front/rear props.
1.3 Deadcat (DC)
- Layout: Front arms are swept back to clear the camera view. Rear arms are standard. The motors are not equidistant from the Center of Gravity (CG).
- The Physics Problem: The front motors are often closer to the CG (or further) than the rear.
- Torque Imbalance: In a fast yaw spin or snap roll, the lever arms are different. The FC must work harder to mix the motors correctly.
- Tuning: Deadcat frames often have "yaw washout" or strange dipping behavior in hard turns because the thrust vector center doesn't match the mass center.
2. Material Science: Stiffness is Speed
PID loops react to error. If the frame bends, the motor moves, but the Gyro (mounted in the center) doesn't feel it immediately. This delay allows the error to grow.
2.1 Carbon Fiber Grades
- Toray T300: Standard "Hobby Grade." Good cost/performance ratio.
- Toray T700: High Tensile Strength. Used in premium frames. It is stiffer and less prone to delamination on impact.
- Weave:
- 3K Twill: The standard "checkerboard" look. Good strength in both X and Y directions.
- Unidirectional: All fibers run parallel. Extremely stiff in one direction, very weak in the other. Used in arm cores ("sandwiched" carbon) to maximize arm stiffness against upward thrust.
2.2 Resonance and Harmonics
Every object has a natural frequency at which it vibrates.
- The Goal: Push the frame's resonant frequency above the operating frequency of the motors.
- Stiff Frame: Resonates at > 150Hz.
- Floppy Frame: Resonates at < 80Hz.
- The Problem: If the frame resonates at 100Hz, and your motors are spinning at 100Hz (mid-throttle), the vibration amplifies violently. The Gyro sees this noise and tries to correct it, causing the motors to twitch, creating more noise. This is a feedback loop (D-Term Oscillation).
3. Vibration Management: The Clean Gyro
3.1 Software Filtering: The Notch
ArduPilot uses a Dynamic Harmonic Notch Filter.
- Function: It tracks the throttle position (or ESC telemetry RPM) to predict the motor frequency.
- Action: It places a narrow "Notch" filter exactly at that frequency to erase the noise before it reaches the PID controller.
- Result: You can fly a noisy, bent, or beat-up frame smoothly because the computer ignores the specific frequency of the vibration.
3.2 Mechanical Isolation
- Stack Isolation: "Gummies" (silicone grommets) decouple the Flight Controller mass from the frame.
- Cable Management: A loose capacitor or battery wire tapping against the Gyro chip creates massive spikes that no filter can fix. Tape down all wires.
- The "Mass-Spring-Damper" Model:
- Mass: The Flight Controller.
- Spring: The Gummies.
- Damper: The internal friction of the rubber.
- Tuning: If the gummies are too soft, the FC "bounces" (low frequency oscillation). If too hard, they transmit motor noise (high frequency).