Vehicles, such as fixed and rotary wing aircraft, have a variety of inlet and exhaust apertures formed on the outer skin of the fuselage structure for intaking and expelling fluids. These apertures are typically configured to maximize the amount of fluid flow that is admitted into or expelled from the interior of the vehicle, while minimizing head-loss or back-pressure. The shape, size, and location of the aperture all play an important role in determining the fluid flow efficiency.
For the purpose of simplification, the vehicle referred to herein is an rotorcraft, but it should be understood that the invention can be employed on any vehicle, e.g., automotive, other types of aircraft, etc.
The apertures are generally used in combination with a duct which operates to channel or funnel the fluid from or to a specific area within the aircraft. A frequent use for such apertures is to exhaust hot gases generated by one or more internal mechanisms, e.g., oil coolers.
One of the most efficient ways for channeling a fluid flow into or out of an aircraft is by locating the aperture in direct line of sight of the internal component and connecting the two utilizing a straight duct. For example, an oil cooler would be located directly forward of an exhaust aperture so as to permit the exhaust gases to flow directly out of the aircraft. Such a design requires the least amount of ducting for channeling the fluid. Furthermore, the straight shape of the ducting minimizes the likelihood of the flow becoming disrupted, i.e., turbulent.
It is oftentimes desirable, however, to configure the aperture to channel the fluid in a direction other than that of the initial flow. For example, the engine exhaust aperture in a helicopter with a ducted anti-torque system is designed so as to direct the exhausting gases to the side of the aircraft in order to provide a thrust for reacting the torque induced on the aircraft by the main rotor system. Handling qualities may also influence the design of the duct. For example, exhaust gases may be directed away from areas on the aircraft where ground personnel are likely to be performing maintenance. Furthermore, the aircraft aerodynamics may affect the desired direction of intake or exhaust, e.g., the exhausting of a gas may be directed away from the rotor downwash.
One common way for providing directional control of the intake and exhaust of fluids is to angularly orient the duct with respect to the outer skin such that the walls of the duct direct the fluid in the preferred direction. There are two primary deficiencies with this type of arrangement. Firstly, the area within the aircraft wherein the duct is to be mounted may be limited in size and, therefore, the size and shape of the duct may be so limited. Secondly, the additional duct wall structure needed to angle the duct in the preferred direction increases the overall weight of the duct.
Another method for directing fluid flow involves the mounting of guide vanes within the aperture as depicted in FIG. 1. The guide vanes are generally small airfoil-shaped structures which are disposed within the passing flow of fluid and oriented so as to direct the passing flow of fluid in the preferred direction. The vanes are typically affixed to the duct wall or to the structure surrounding the aperture. While guide vanes provide sufficient directional control over the fluid flow they require additional structural support in order to maintain their desired shape and orientation. This results in a relatively heavy duct structure.
Track and scan radar tracking systems utilize a transmitter to emit a radar signal, i.e., electromagnetic energy, toward an aircraft and a receiver to sense reflected electromagnetic energy. The electromagnetic energy returned to the radar source represents the aircrafts radar signature. The stronger the radar signature of the aircraft, the more likely it is that the aircraft can be detected and tracked by the radar source. Many of the internal components, e.g., compressors, transmissions, oil coolers, etc., of modern military aircraft are manufactured from metallic materials such as steel which tend to reflect electromagnetic energy.
To reduce the aircraft's radar signature, internal metal components are, where possible, located out of direct-line-of-sight of an aperture as shown in FIG. 2. Shaped-ducting is utilized to direct the fluid flow to or from the component. This type of design minimizes the likelihood of a returned signal inasmuch as the signal must reflect off the duct walls to reach the internal component, then reflect back to exit through the aperture. An example of such a shaped duct wall is described in U.S. Pat. No. 5,016,015, entitled AIRCRAFT CONSTRUCTION. One deficiency with a shaped duct configuration is that a substantial amount of weight is added to the aircraft.
Another method for reducing the radar signal return is to coat the duct wall surface with a radar absorbent material (RAM). The RAM coating acts to absorb the radar energy as it impinges on the duct wall. Thus, a shorter length duct wall can be used while maintaining a low radar signature. The RAM coating, however, increases the weight of the overall duct structure inasmuch as the entire duct must be coated.
In order to prevent foreign object debris (FOD) from entering an exhaust or intake aperture, wire screens are oftentimes affixed thereto. While the screens provide an effective means for preventing relatively large pieces of FOD from entering the aperture, the screens do not provide a means for directing the fluid flow. Furthermore, the screens may also be coated with RAM to reduce the radar return that would otherwise be generated.
A need, therefore, exists for an improved duct cover which provides directional control over a fluid flow passing therethrough, while minimizing a direct-line-of-sight of any structure contained within the vehicle.