(1) Field of the Invention
The present invention relates to maneuvering an underwater vehicle, and more specifically to systems and methods for generating vehicle maneuvering forces from a post-swirl propulsor.
(2) Description of the Prior Art
Standard torpedoes and Unmanned Undersea Vehicles (UUVs) utilize a single propulsor at the stern in combination with control surfaces to provide the vehicle with necessary forces and moments to produce control of the vehicle. At higher speeds, this combination is generally satisfactory in terms of offering sufficient control. At low speeds, control surface effectiveness is significantly diminished, with the extreme condition being zero forward velocity (e.g., Bollard condition). There are several operations where low speed control is vitally important for UUV mission requirements. These include UUV recovery, station keeping and synthetic aperture sonar.
In the art, side forces have been generated using thrust vectoring. In thrust vectoring, the thrust is re-directed off axis to generate side forces for control. To meet low speed requirements, autonomous research vehicles have utilized tunnel thrusters to offer lateral and vertical control.
The difficulty is that thrust vectoring is most effective for zero speeds. As the flow velocity is increased, tunnel thruster effectiveness is significantly diminished. For example, it has been shown that tunnel thrusters are only 20% effective above five knots. The tunnel thrusters also increase drag such that maximum velocities are reduced. In addition, tunnel thrusters use considerable volume that could otherwise be used for energy or payload.
Another prior art design is referred to as the Haselton bow propulsor. In this concept, a pair of propellers, one at the bow and one at the stern, is used in tandem to provide vehicle control. Side forces are generated via cyclic pitch actuation similar to that used for helicopter rotors.
The Haselton design utilizes a swashplate so that angle of attack is varied over a single propeller rotation. For example, if maximum and minimum angles of attack are reached at 0° and 180°; the higher thrust force at 0° and lower thrust force at 180° will generate a moment couple. By adding rake and skew to the propeller, it is then possible to generate a substantial side force component.
A disadvantage is that the Haselton bow propulsor concept remains mechanically complex for implementation on undersea vehicles. In addition, placing a propulsor at the bow of the vehicle interferes with the forward looking sonar that is used on most UUVs and torpedoes.
In previous research, a pre-swirl propulsor capable of generating side forces of sufficient magnitude to provide vehicle control and maneuvering was investigated. It was found that varying the pitch angles of the upstream stator blades of a ducted, pre-swirl propulsor can both generate a mean stator side force and subsequently vary the axial velocity and swirl that is ingested into the inflow. The rotor can then generate a side force in response to the inflow.
The research also showed that the stator row generated significantly larger forces compared with the rotor. The rotor generates side forces in response to the stator modified inflow. For baseline rotor designs, the side forces are in a direction opposite to the stator forces (i.e., the rotor effectively attenuates the stator side forces).
Additionally, it was found that open propulsors (i.e., propulsors without a shroud) generated side forces in a manner as to be more efficient than standard vehicle control surfaces. By sinusoidally varying the stator blade pitch distribution about the circumference, it is possible to generate significant side forces.
However, ducted pre-swirl propulsor configurations (i.e., propulsors with a shroud) generated side forces with magnitudes approximately three times smaller than open configurations. The shroud produces opposing forces resulting in the diminishment of the overall side forces. For this reason, open pre-swirl propulsor configurations were proposed for further examination to offer an alternative for vehicle control.
There are several undersea vehicle applications where a ducted propulsor configuration is desirable. This includes pumpjet and rim-driven electric motor configurations. For these cases, a post-swirl propulsor is utilized with an upstream rotor blade row and a downstream stator blade row. For normal post-swirl designs, propulsive efficiency can be improved with effective stator blade row design. The rotor swirls the flow ingested into the stator. The stator can be designed to remove the swirl and at the same time generate roll moment to counter the rotor roll moment.
What are therefore needed are systems and methods for maneuvering an underwater vehicle having a ducted post-swirl propulsor. The systems and methods should be effective at reasonable operating speeds; should not significantly reduce maximum velocities; and should not take up considerable volume. Additionally, the systems and methods should be relatively simple to implement without interfering with forward looking sonar.