Hydrodynamic Propulsive Performance of a Hybrid Aquatic-Aerial Rotor-Wing System under Collective Pitch and Rotational Speed Matching

Imagine a vehicle that can fly through the air and then dive to swim beneath the waves, using the same set of spinning blades for both journeys. This is the promise of hybrid aquatic-aerial vehicles, machines designed to bridge the gap between sky and sea. While the idea of a single platform serving two such different worlds is appealing, the physics of moving through air and water are vastly different. Air is thin and light; water is dense and heavy. A rotor blade that slices efficiently through the atmosphere often struggles to push effectively through the ocean, and the presence of a fixed wing behind the spinning blades adds another layer of complexity. When a rotor spins in front of a wing underwater, the water it pushes backward hits the wing, creating a chaotic interaction that can either help or hinder the vehicle's forward motion. Understanding exactly how these two parts talk to each other is essential if engineers hope to build machines that can cruise efficiently underwater without needing a completely new set of propellers for the dive.

Researchers at Wuhan University of Technology set out to solve this puzzle by looking closely at how a spinning rotor and a stationary wing behave when working together underwater. They focused on two specific controls that pilots or computers can adjust: the angle of the blades, known as the collective pitch, and how fast the blades spin. The goal was to find the perfect pairing of these two settings to make the vehicle move forward with the least amount of wasted energy. Using powerful computer simulations that modeled the swirling water around the blades and the wing, the team tested a wide range of combinations. They did not just look at the rotor alone or the wing alone, but studied the entire system as a single unit, observing how the water flowing off the rotor changed the forces acting on the wing, and how the wing, in turn, changed the forces on the rotor.

The study revealed that simply spinning the blades faster or slower is not enough; the angle of the blades must be carefully matched to the speed to achieve the best results. When the researchers adjusted the blade angle and rotation speed together to meet specific thrust requirements, they found they could improve the efficiency of the rotor itself by as much as 16.2 percent compared to a standard setting. This improvement happened because changing the angle and speed together altered the shape of the water swirling behind the blades, known as the wake. However, the most surprising discovery was not just about the rotor, but about the relationship between the rotor and the wing. The team found that the spinning blades create a wake that actually pushes water in a way that reduces the drag, or resistance, on the wing behind it. In some cases, this interaction reduced the wing's drag by more than half while simultaneously increasing its lift.

This reduction in drag is a significant win for the overall system. Even though the interaction between the rotor and the wing did not drastically change how efficiently the rotor spun on its own, it made the entire vehicle much more efficient at moving forward. The researchers calculated that by taking advantage of this interaction, the total propulsive efficiency of the system could improve by up to 5.2 percent. This benefit grew larger as the blade angle increased, but there was a limit. If the blades were angled too steeply, the rotor itself became inefficient, and the gains from the wing interaction were not enough to make up for the loss. The study showed that the best overall performance was found at a specific middle ground where the blade angle was high enough to create a helpful wake for the wing, but not so high that the rotor struggled to turn.

The researchers also looked at how stable the system was under these different settings. They found that when the blades were set to a very high angle, the forces on the rotor became much more erratic, with the water flow becoming unstable and creating strong vibrations. This instability was linked to the water separating from the blade tips, creating a messy wake that shook the rotor. In contrast, at lower angles, the flow remained smooth and steady. The team concluded that while the interaction between the rotor and wing is a powerful tool for saving energy, it cannot fix a rotor that is poorly designed for the job. The best approach is to find a balance where the rotor spins efficiently on its own while simultaneously creating a wake that helps the wing slice through the water with less resistance.

These findings provide a clear roadmap for designing future hybrid vehicles. Instead of treating the rotor and the wing as separate parts that happen to be near each other, engineers can now view them as a team where one part actively helps the other. By carefully matching the speed of the spin with the angle of the blades, it is possible to create a flow of water that reduces the effort needed to push the vehicle forward. This does not require new materials or complex new shapes, but rather a smarter way of using the controls that are already there. The study confirms that the complex dance of water between a spinning rotor and a fixed wing is not just a source of trouble, but a hidden opportunity to make underwater travel more efficient, provided the settings are tuned with precision.