Patent Application
20090127384
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Not Applicable
1. Field of the Invention
This invention relates to manned and unmanned airships and other buoyant and semi-buoyant vehicles, and more particularly relates to the use of a wake ingesting embedded ducted fan to provide efficient propulsion thereof.
2. Description of the Related Art
In the past few decades the vast majority of aircraft designers have focused on high performance, high speed jet aircraft, where efficiency is highly dependent upon compression of the ingested air. As such, most aircraft designers go to great lengths to avoid ingesting either the boundary layer or aircraft wake, which requires more work to compress than the higher velocity free stream air. Yet for the purposes of slower propeller driven Proprietary Information of Michael T. Voorhees aircraft, such concerns are reversed. It is understood that in cold air propulsion, propulsive efficiency is proportional to mass flow and thus sensitive to the velocity of air ingested into the propulsor. All other things being equal, the slower the velocity of ingested air, the higher the propulsive efficiency.
In aircraft design, minimization of drag is also of extreme importance. The three major categories of drag being form drag (also know as pressure drag), skin friction drag (also known as viscous drag), and induced drag. For pure aerostatic vehicles operating at level pitch, induced drag is not a factor. For most heavier-than-air designs, form drag is more dominant than skin friction, whereas in buoyant aircraft, given the large surface area, skin friction becomes dominant. Chapman, whose patent is generically directed toward fluid-borne vehicles as diverse as submarines and airships, states in the abstract “A method and arrangement for propelling fluidborne vehicles is disclosed that results in reduction in the overall form drag of certain classes of vehicles”. He also states that an object of his invention is to “reduce form drag created by the vehicle while avoiding flow separation of the afterbody of the vehicle during under-thrusted conditions.” For submarines, which travel through the much denser medium of water, form drag is of greater concern. With airships, this is less so. The inlet arrangement specified by Chapman is not surprisingly sub-optimized for airships. As Chapman claims “A propulsion system arrangement for a fluidborne vehicle comprising: a vehicle including a forebody section, an afterbody section and a transition region joining the forebody and afterbody sections, the forebody section having an outer surface with a shape diverging from the forward most point of the vehicle to and including the widest portion of the vehicle, the afterbody section having an outer surface with a shape converging inwardly in the rearward direction of the vehicle to a shape with cross-sectional area smaller than that at the transition region and having a central outlet nozzle facing rearwardly therefrom, at least a portion of the surface of the vehicle contacting a fluid medium, a single inlet for inducting fluid from the fluid medium, said inlet located solely in the transition region . . . ” In computational fluid dynamics modeling of airships with a symmetric body of revolution shape, one finds that air moving past the hull at the above described “transition region” between forebody and afterbody sections is at a higher relative velocity than even the free stream. Thus it is clear that to maximize propulsive efficiency, this is the least desirable location for an inlet. Such a location also requires unnecessarily long ducts and incurs associated efficiency losses due to boundary layer effects. Furthermore, Chapman specifies, “the afterbody section is tapered at an angle of no more than 15° with respect to the direction of motion”. Such a structure will require more hull mass per enclosed volume, and thus less disposable lift.
The current invention is a propulsion system for the controlled flight of buoyant and semi-buoyant aircraft. It may be used to propel a vehicle with a predominately symmetrical body of revolution hull form, optimally with a fineness ratio (length to diameter) of between 3 and 4.5. Near the far aft section of the aircraft hull, ideally where the hull taper is at an angle of 18° or greater with respect to the direction of motion in level flight, an annular inlet circumscribing the aircraft is located so as to ingest air from both the boundary layer and aircraft wake. It is by accelerating a large volume of slow moving air through the propulsor that high propulsive efficiency is achieved. This overcomes previous inventions that inefficiently ingested free stream air via outboard propellers, or ingested air at the widest portion of the airship where air even faster than freestreem is prevalent in a misguided emphasis on reducing form drag. It also overcomes deficiencies of other sub-optimal designs for stern propulsion, including less efficient and noisy unshrouded propellers, and those with poorly designed ducts. The annular inlet opens to an annular duct providing a gradual deflection into and then rearwardly through a propulsive means. Avoidance of abrupt angular surfaces is desired and the cross-sectional area of the flow should gradually constrict to a minimum as air passes through the propulsor and then gradually expand toward the outlet. The propulsive means could be any of a number of prior art technologies, such as a fixed or variable pitch fan powered by electric motor, combustion engine, or gas turbine. When used as the main propulsive component in conjunction with steering means such as aerodynamic control surfaces, vectored thrust, or differential thrust, or some combination thereof, efficient controlled aerostatic flight may be achieved.
a shows the external view of the buoyant aircraft 1 as seen from the starboard side with the bow to the right and stem to the left. The annular inlet 2 and annular outlet 3 partially define the location and form of the annular duct 7. The forwardmost wall of the duct 4 is partially visible in this view.
b shows the system from the same viewpoint, but in cross section, whereby the buoyant aircraft 1 and components have been sectioned lengthwise in half, revealing the relative position and form of the annular duct 7 as defined by the forwardmost wall of the duct 4 and the outer wall of the duct 5, as well as the location of the propulsive means 6 as it operates within the duct.
Referencing
In the above manner, air of the slowest available velocity, located adjacent to the aft portion of the hull, is accelerated, maximizing the propulsive efficiency of the buoyant aircraft, while higher velocity air is allowed to bypass the propulsor. Furthermore, since the propulsive means 6, is located within the annular duct 7, blade tip flow reversal during static conditions is eliminated and a minimal acoustic signature is achieved.
| Number | Date | Country | |
|---|---|---|---|
| 60917933 | May 2007 | US |
