Drones That Fly and Dive: Why Ducted Fans May Matter More Than Turbofans

Most drones are built for either the sky or the water, not both. That sounds obvious, but it creates a real engineering problem. Air and water behave so differently that a propeller or motor designed for one usually performs poorly in the other. Still, researchers are trying to build drones that can fly, dive, and move between both environments without needing a separate vehicle for each job. One recent example is Nezha-D, a hybrid aerial-underwater drone that uses ducted fans. Instead of leaving the propeller fully exposed, the blades sit inside a circular housing that helps control the flow around them. This can be useful underwater, but the difficult part comes when the drone tries to break through the surface. Researchers found that thrust can drop during this transition, so Nezha-D uses underwater speed and momentum to help push itself into the air before normal flight takes over. Newer designs are trying to solve this problem in more flexible ways. Nezha-SeaDart III uses a dual-medium thruster that can switch between an aerial propeller and an underwater propeller depending on the direction of the motor. This makes more sense than forcing one propeller shape to work perfectly in two completely different environments. Other researchers are also experimenting with variable-pitch blades that can change their angle depending on whether the drone is flying or submerged. This is where turbofans come into the discussion, although the term can be misleading. A true turbofan, like the engines used on commercial aircraft, is a gas-turbine engine. It compresses air, mixes it with fuel, burns that mixture, and uses the turbine to drive a large fan. That system is built for air and would not work normally if it suddenly started taking in water. For hybrid drones, electric ducted fans are much more realistic. They look somewhat similar to the front section of a turbofan because the fan is surrounded by a duct, but the internal system is much simpler. There is no combustion core. The motor directly powers the fan. Even so, the comparison is useful because engineers can still borrow ideas from aircraft propulsion, especially when it comes to controlling airflow and improving efficiency. The biggest issue is still compromise. A propulsion system that works well in air may be inefficient underwater, while an underwater system can be too heavy or create too much drag during flight. Solving this may require drones that physically change how they produce thrust depending on where they are. That could be the real future of air-water drones. Instead of trying to make one fixed propeller do everything, engineers are building systems that adapt. If these designs continue improving, the same drone could eventually inspect ships, monitor oceans, assist search and rescue crews, or perform coastal surveillance without stopping at the waterline.