Zonal stiffness
Summary¶
Zonal stiffness is a structural design strategy that deliberately assigns different stiffness values to different regions of a structure, routing crash energy to a controlled sacrificial zone rather than distributing it uniformly. In libdrone, the centre body is stiff (PCCF sandwich, high infill) and the arms are compliant (PETG or TPU, lower infill, slimmer cross-section). A crash load enters at the motor, travels up the arm, and ideally breaks the arm at the arm-body interface before the energy reaches the electronics stack. Zonal stiffness is the engineering basis of the failure hierarchy.
Concept¶
Stiffness gradient as energy routing¶
When a structure is loaded to failure, it fails at the point where stress exceeds the local material strength. If every region has the same strength, failure location is unpredictable — it may be a prop, an arm, the body, or the flight controller mount. If the structure is designed so one zone (the arm) is the weakest link by a controlled margin, failure always occurs there. The electronics survive; only the arm needs replacing.
The stiffness gradient works as follows. A stiff core resists deformation and redistributes the crash impulse across the joint area rather than concentrating it at one point. The compliant arm absorbs and dissipates the remaining energy by deforming plastically before the core reaches its yield point. The transition between the two zones — the arm-body joint — is designed as a stress concentrator: the arm cross-section reduces at the joint, ensuring that if the arm is going to fracture, it fractures at the joint base and separates cleanly rather than mid-arm with a jagged break.
The three zones¶
Zone 1 — Motors and propellers: Designed to survive all normal loads but release under crash loads. Propellers shatter; motor mounts flex. Energy absorption is primarily plastic deformation of propellers and the TPU bumper sleeve on CF rod ends.
Zone 2 — Arms: The primary sacrificial zone. PETG arms on Core and Bandit are designed to fracture at the arm base under crash loads exceeding a defined threshold. Replacement is the intended repair action, not prevention of failure. TPU arms on Bandit deform without fracture — they absorb crash energy and recover shape. Ghost's CF plate arms are stiffer and rely more on the motor-end deformation.
Zone 3 — Body sandwich: The protected zone. PC-CF or PETG sandwich with high infill and 6-bolt clamping pattern. Should survive all crashes that Zone 2 handles correctly. Electronics, FC, and ESC live here.
Consequence of violating the gradient¶
Over-stiffening the arms (printing PC-CF arms with 80% infill) transfers crash energy directly to the body. The arm does not yield; the body does. FC mounting holes crack; the electronics stack loosens. This is not a catastrophic crash failure — it is a subtle structural degradation that produces intermittent electrical contacts and unexplained flight instability.
Under-stiffening the body (printing the sandwich at 10% infill to save weight) allows the body to deform under normal flight vibration, not just crashes. The battery mount loosens; the GX12 connector alignment shifts.
Reference¶
| Zone | Component | Material | Infill | Role |
|---|---|---|---|---|
| 1 | Props, motor mount | ABS/nylon props; TPU bumper | — | First absorber |
| 2 | Arms | PETG (Core/Pro), TPU 95A (Bandit) | 25–40% | Sacrificial zone |
| 2 | Arms | PC-CF (Pro option) | 40% | Semi-sacrificial |
| 3 | Body sandwich | PCCF layers | 40% | Protected zone |
| 3 | Body sandwich | PETG top/bottom | 25% | Protected zone |
Procedure¶
Verify zone stiffness at print time¶
- Print a coupon section of each arm and body material at the specified infill. See → coupon-validation for the coupon workflow.
- Bend the arm coupon by hand at the arm-body joint geometry. It should deflect noticeably before any body coupon deflects — the arm must be measurably more compliant.
- Drop test: drop the assembled frame (without electronics) from 1 m onto a hard surface. The arm should fail before the body shows any damage. If the body cracks first, reduce arm infill or increase body infill.
Rationale¶
The zonal stiffness approach was adopted because it converts an unpredictable event (crash) into a predictable outcome (arm breaks, body survives, replace arm and fly again). The alternative — maximising overall stiffness — produces a frame that either survives the crash entirely (if under-loaded) or fails catastrophically (if over-loaded), with no controlled intermediate. For a platform used in workshop teaching contexts where crashes are expected, a predictable, cheap repair path is more valuable than maximum crash resistance. Arms cost €0.50 to print. Flight controllers cost €55. Protecting the FC at the cost of the arm is the correct trade.
Connections¶
yaml requires: - [frame-structure-overview](<./frame-structure-overview.md>) - [failure-hierarchy](<./failure-hierarchy.md>) related: - [sandwich-structure](<./sandwich-structure.md>) - [pre-tensioning](<./pre-tensioning.md>) - [exact-constraint-design](<./exact-constraint-design.md>) - [monocoque-structure](<./monocoque-structure.md>) - [petg](<./petg.md>) - [pccf](<./pccf.md>) leads_to: - [failure-hierarchy](<./failure-hierarchy.md>) - [coupon-validation](<./coupon-validation.md>)