Air–Water Hexarotor AUV
Six-DOF hexarotor that flies, dives, and changes shape in between. The arms retract and the props fold under water drag, so one airframe runs efficiently in both mediums.
- Role
- Mechanical design lead
- Organisation
- UCLA Mechatronics & Controls Lab
- Dates
- May 2026 – present
- Status
- Built, in testing
- Tags
- SolidWorks · Mechanism design · Waterproofing · CFD
Arms extended, props at full span. Maximum disc area for lift.
CAD states of the same airframe. One lead screw drives all six arms; the props need no actuator at all.
- Configurations
- Air (extended) / water (retracted)
- Control
- Full six-DOF in both mediums
- Hull
- Three bays, five printed sections, o-ring sealed
- Actuation
- Single lead screw, six arms
- Status
- Prototype built, CFD in progress
Why it exists
Underwater infrastructure inspection — pilings, hulls, dam walls, intake structures — is still largely a diving job, and diving is the dangerous part of it. The vehicles that do exist pick a side: an underwater hull cannot fly, and a multirotor is a poor submarine.
A single vehicle that flies to the site, enters the water, inspects, and flies back out removes the boat, the tether, and the diver from a routine job. The same capability applies to survey, sampling, and filming across a shoreline.
Requirements
Full six-DOF control in air and in water, with a transition between the two that does not require the vehicle to be recovered and reconfigured. Survive extended submersion. Stay manoeuvrable enough to inspect at close quarters, where a drifting vehicle is a liability.
My role
I own the chassis architecture and the full CAD. The mechanical subteam lead directs me and two other members who own waterproofing and simulation; because I designed the geometry, I have been involved in both and run my own simulations. I work directly with the electrical subteam to package their custom boards into the sealed bay.
How it works
The hull is three bays stacked in line: a sealed electronics bay on top, a flood bay in the middle, and a battery bay below. The flood bay is exactly what it sounds like — it fills with water on submersion, which means it does not have to be sealed, and it is where the arm-retraction drive lives.
A lead screw in the flood bay drives a plunger fixed to the screw nut. The plunger face is profiled to match the arc the arms sweep, so it stays in contact through the whole stroke and pushes all six arms down and inboard at once from one motor. Retracting the arms shrinks the frontal area and pulls the motor pods in toward the hull, which is where most of the underwater drag was coming from.
When the vehicle leaves the water, the flood bay drains by gravity through its own ports. The props fold passively: water drag folds the blades inboard, and in air centripetal force throws them back out to full span.
Key decisions
Fixed geometry vs. a hull that changes shape
- A purpose-built underwater hull — efficient submerged, will not fly
- A conventional multirotor frame — flies well, high drag and poor handling submerged
- A radially morphing frame — one airframe, two geometries
Radially morphing frame. Pressure sensors detect entry into water and trigger the transition.
The arms are parallelogram linkages, so the motor pods hold their angle through the full stroke and the thrust vectors stay valid in both configurations — retraction changes the footprint, not the control model. The lower link extends into the hull, where the arc-profiled plunger on the lead-screw nut drives all six arms simultaneously from a single actuator instead of six.
Two thrust systems vs. one that folds
- Separate air props and dedicated underwater thrusters
- A single compromise prop sized between the two
- A passively folding prop
Passively folding props on the existing six motors.
Long props make excellent thrust in air and stall the motors in water. Two thrust systems solves that with mass, cost, and a second set of motors — and six-DOF already requires six. The folding prop lets the medium do the switching: water drag folds the blades inboard so the motor sees a smaller disc, and centripetal force extends them in air. No actuator, no sensor, no control logic.
How to actually build the hull
- Machined aluminium — impractical at this size, heavy, bad for the buoyancy budget
- Carbon fibre layup — mould cost and lead time
- One large 3D print — exceeds the bed, and seals the bays permanently shut
A five-part printed hull, joined with o-rings and screws.
Every section fits the printer, and each bay opens independently for electronics swaps, maintenance, and re-sealing. It also concentrates the waterproofing problem into a small number of defined joints rather than spreading it across one large uncertain surface.
Analysis and validation
CFD is running now in SolidWorks Flow Simulation and ANSYS, characterising hull drag in air and in water. The sequence is the bare hull, then the motor pods and props alone, then the full assembly, each in both mediums and in both the extended and retracted configurations. The number that matters is retracted-versus-extended drag in water; that is the claim the whole morphing mechanism rests on. The same studies size the propulsion system and set mass distribution for neutral buoyancy.
The cheap analysis comes first: flood-bay displacement and a neutral-buoyancy budget straight out of CAD mass properties, which is a spreadsheet rather than a solver run.
This section is empty. Two or three specifics — a part that did not fit, an assumption that broke, a print that failed — will do more for this page than another paragraph of design description.
Overall dimensions extended and retracted, dry mass, flood-bay displacement volume, arm stroke length, target neutral buoyancy. All of these are already in the CAD.
Where it stands
CAD and design are complete, the vehicle has been prototyped and built, and electronics are integrated. CFD is running alongside to characterise drag and settle the buoyancy budget. Next is bench actuation testing, then air, then water, then the transition between them.
Gallery
The two prop-blade renders you sent in chat (folded and extended) never landed on disk — I could see them in the message but had no file to save. Drop them into assets/img/auv/ as auv-prop-folded.jpg and auv-prop-extended.jpg and tell me; they belong in the folding-prop decision above, which is currently text-only.