Fastening into lightweight UAV sandwich structures, without crushing the core

Unmanned air vehicles (UAVs) are increasingly being designed with thin composite sandwich panels to maximise payload capacity and flight efficiency. These structures can be just millimetres thick, and every gram matters.

In this project, bigHead's customer needed reliable mounting points inside the payload module of a medevac drone. The challenge was simple to describe but difficult to solve: how do you fasten into a structure so light that even pushing on it can damage it?

The customer first approached bigHead for tightening-torque guidance on an internally threaded fastener bonded to the back of the sandwich panel. They recognised the risk of crushing the core during assembly, and wanted to understand how to avoid it.

Once the material properties had been reviewed, it became clear that the allowable tightening torque was less than 1 Nm, while most tightening methods have greater inaccuracy than that. Even a slight over-tightening could crush the sandwich core, but alternative fastener installations could cause skin disbanding.

In short, the fastening method the manufacturer was considering was too delicate to control, and too risky for a flight-critical component. Mounting points were needed, but the structure couldn't tolerate the forces required to create them.

The customer had experience with rivet nuts and other inserts, but bigHead fasteners were unfamiliar. Their team was already carrying a heavy cognitive load (most pressing of all: 'This thing can't fall out of the sky'). So instead of sending samples or datasheets, bigHead took the mental load off them. Engineers at bigHead evaluated their sandwich structure, calculated the compressive limits, and presented three viable installation options, with clear pros and cons. bigHead also recommended an appropriate adhesive, and matched thread length to their exact sandwich thickness.

The best option proved to be a bigHead blind socket fastener with a customised thread length to match the 5.5 mm sandwich panel. This configuration ensured that the fastener, not the sandwich structure, carried all compressive loads; there was no crushing, disbonding, or fatigue risk. It provided a lightweight solution aligned with UAV performance requirements, with clean internal surfaces suitable for medical payload equipment.

Even though the customer was only building ten units, the design needed to be right the first time, and fast. The customised bigHead allowed the company to move confidently from concept to prototype; within two weeks, the customer had moved from uncertainty to a clear, validated solution, and within six weeks, it had built the prototype and was ready for field testing.

Whether you need a standard bigHead, a customised fastener, or a brand-new creation, bigHead here to help you find a solution that protects your structure. And your payload.