Payload

Abstract

The Atılım UAV payload subteam designed an airdrop system that releases a strobing beacon and a water bottle independently, without free-fall and without either release interfering with the other. A dedicated servo lock holds each payload fixed until the aircraft is on station, at which point the lock disengages and an 800 RPM DC motor lowers the payload at a controlled, constant descent speed on a solid cylindrical spool. Both winches are driven from a single HW-231 dual-channel H-bridge, and the entire module mounts to the airframe on rail-guided sliders so it can be serviced or reloaded without tools. This document covers the release mechanism, the detachable mounting scheme, and the spool geometry.

1 Design Choices

The payload subteam was tasked with designing an airdrop system capable of independently deploying two distinct payloads — a strobing beacon and a water bottle — onto separate targets during a single autonomous mission, without free-fall and without cross-interference between the two release events. Given the weight and reliability constraints of a heavy-lift platform already carrying a full navigation and imaging stack, the system needed to control descent precisely while adding minimal mass and mechanical complexity. The greatest challenge was ensuring that each payload could be locked securely in place throughout flight, then released independently and at a controlled speed, using a single shared actuation scheme rather than duplicating hardware for each payload.

Our solution combines a servo-actuated spool lock with a DC-motor-driven winch for each payload. Each payload is held in place by a dedicated servo-actuated lock that keeps it fixed at the bottom of the UAV regardless of motor state, preventing any accidental release during flight. When the aircraft is positioned directly above the designated drop location, the release sequence begins: the servo first disengages the spool lock, and only then does a dedicated 800 RPM DC motor begin lowering the payload at a controlled, constant descent speed, eliminating free-fall entirely. Both delivery mechanisms are driven by a single HW-231 dual-channel H-bridge motor driver, giving independent bidirectional control over each winch so the beacon and bottle can be released separately, at different points in the mission, without one system affecting the other. The winch mechanism itself is built as a detachable module on a rail-guided sliding attachment, allowing it to be installed or removed from the airframe without tools while a locking feature keeps it secured during operation.

2 Detachable Module Mounting

Because the airdrop system needs to be serviced, reloaded, or swapped quickly between flights, the entire winch module is built as a detachable unit rather than a permanent fixture on the airframe. The mount features a pair of machined grooved channels running along the top of the frame, into which two matching locking sliders key in from above. Sliding the module into place engages both channels simultaneously, and the sliders lock the module firmly against the mount to prevent any shifting or vibration-induced play during flight. To remove the module — for reloading a payload, servicing the winch, or swapping in a spare — the sliders simply disengage and the whole assembly lifts free of the rails, with no tools and no fasteners to undo. This keeps payload turnaround fast between competition runs and means a damaged winch module can be replaced independently without touching the rest of the airframe.

3 Spool Design

The spool geometry is intentionally simple: a solid cylindrical drum flanked by two circular plates, sized to hold the winch line and maintain constant torque throughout descent. The top flange carries a ring of twelve mounting holes matched to the servo horn's bolt pattern, allowing the spool to bolt directly onto the servo's output shaft rather than relying on a friction or spline fit — giving a secure, positive connection that won't slip under the load of the payload during release. A small center pin on the top face helps register the spool concentrically to the horn before the bolts are tightened. The bottom flange is a plain retaining disc that keeps the wound line contained on the drum as it pays out.

4 Testing, Implementation, and Integration

Before flight testing began, the release mechanism was validated at the component level: the servo-actuated locks and the DC-motor-driven winches were exercised independently to verify lock engagement, lock release, controlled descent, motor stop, and independent bidirectional control of each channel through the HW-231 driver. Repeated trials produced consistent, repeatable deployment for both payload channels.

With the release mechanism validated, the team then flew the complete Search-Detect-Deliver chain end-to-end, fully autonomously. The first test was flown at approximately 20 m AGL, keeping the release within visual range while the guidance chain was flown for the first time: both targets were detected, their positions were handed off to the guidance system, and the aircraft navigated on its own to the stored release coordinates. The strobing beacon landed on the tent target, and the water bottle landed approximately 5 m from the mannequin. The same sequence was then repeated at the 50 m mission altitude — detection confidence and delivery offset held at the same order of magnitude, and both payloads landed inside the 50 ft (15.2 m) scoring radius. Release in these tests was commanded from the stored geolocation; closing the terminal-guidance loop on the live image remains a software refinement rather than a change to the mechanical release system.