Wing In Ground-effect (WIG) Wingship
When a conventional airfoil is operated in a region close to the ground, its normal pressure distribution is distorted. Pressure tends to develop to a higher level under the wing and adds to the normal dynamic lift of the airfoil. This enhanced lift is well known as the ground effect. Wing In Ground-effect (WIG) aircraft, sometimes called a wingship, are "flying boats" intended to cruise just above wave crests so as to avoid all but very occasional water contact during flight. WIG aircraft possess one or more wings which are generally three orders of magnitude larger than the foils of hydrofoil craft. When a WIG aircraft has accelerated to a sufficient velocity through the water, the aerodynamic lift created by these wings lifts the aircraft entirely out of the water. By remaining close to the water's surface, WIG aircraft encounter significantly less resistance than they would encounter at higher altitudes because their aerodynamic lift is much greater closer to the water's surface than it would be at higher altitudes.
However, there are problems with such winged vehicles. Firstly, wings and water do not mix. If the tip of a wing catches the water, the results can be disastrous--and the problem is exacerbated because there is a relatively long moment arm represented by the length of the wing between the center of gravity of the vehicle and the point where the wing tip engages the water. Further, wings do not facilitate docking, loading and unloading of the vehicle.
There have been several versions of WIG vehicles developed over the years since the first one developed and patented by Finn Toivio Kaario in 1935. Since the first WIG device was designed in 1935, various attempts have been made to develop a feasible WIG machine. Some have had conventional high aspect ratio (AR.gtoreq.6) wings flying close to the surface, but their large wing span made them impractical machines to operate in geographically restrictive waterways such as harbors. Other versions incorporated low aspect ratio (AR.ltoreq.2) wing designs to keep the span within practical limits.
The aeronautical engineering arts have also advanced ground effect vehicles beginning with the Russian Ekranoplan KM, also known as the Caspian Sea Monster, which was developed in the 1960s for cargo transport and missile delivery applications. The KM uses extended wings with negative dihedral winglets on each end in order to promote the ground effect. The negative dihedral winglets are generally allowed to touch water if the KM is unintentionally flown too low. However, allowing the winglets to touch the water substantially increases drag, and may damage the wing or winglets. As such, the structural weight of the wing must be increased to account for water loads. If too much of the winglets contact water, the airplane may also experience stability problems.
The Wingship can actually transition from a waterborne to an airborne mode of operation. Wingships normally have relatively thin wings like aircraft for low drag coefficients when airborne. Ideally speaking however, a wing with the highest coefficient of lift would have a large radius leading edge and a very thick shape. It would also have a high camber in its most desired embodiment. Its shape requires that the air passing over the upper surface of the wing travel significantly further and hence faster than the air traveling over the upper surface of a thinner wing as used on aircraft. By Bernoulli's equations this results in a lower static head or pressure on the top of the instant invention's thick section wing and hence a much higher coefficient of lift compared to a thin section aircraft style wing. However, in the process of doing this there is, as speed increases, a separation of airflow from the top of the thick section wing.
Traditionally, the air trapped under the wing is the result of the aircraft moving forward utilizing a ram effect to produce enough pressure to form an air cushion. This ram effect was utilized to create the cushion of air necessary to lift the aircraft. This device also used the efflux from the propulsion system to create a "curtain" of air to contain the cushion in place of a rigid end plate. When the device is hovering, there is no ram force and most of the propulsion efflux is used to create the cushion. In the hovering mode, a great deal of efflux is required to offset the weight of the aircraft, in the same manner as a hovercraft.
Because small span wings have low aerodynamic efficiency, end plates were added to the low aspect ratio designs. End plates become hazardous in that they increase the danger of the wing "digging-in" as the end plates contact the water which becomes increasingly dangerous as speed operating ranges increase to those at which the WIG is most efficient (150 to 200 knots). So the WIG has a dilemma. If it flies high to avoid contacting waves, the "ground effect lift" diminishes rapidly and the WIG loses its advantage and becomes a poor airplane. If it flies low to maintain its aerodynamic advantage the end plates contact the water and high structural loads are imparted to the vehicle. A structure designed to withstand such loads must have increased weight and its consequent reduced payload capacity. In addition, the high structural loads seriously compromise the handling and maneuverability of the craft, thereby increasing the likelihood of pitch-in and capsizing.
Another difficulty experienced by WIG vehicles heretofore has been their inability to turn at high speed, a problem common to all high speed marine vehicles. Air cushion vehicles and surface effect ships have characterisitically large tactical diameters, hampering their military worth. The hydrofoil, in common with the airplane, enjoys smaller tactical diameters because of its ability to bank into a turn thereby producing large turning forces. The conventional WIG cannot bank into a turn because of its low surface clearance. Any attempt to bank at large angles to generate the turning force would cause the edge (tip) of the vehicle to dig in and overturn. This would especially be true of WIG vehicles with hard endplates.
Another disadvantage of most WIG designs is that since they gain their lift aerodynamically, they have no inherent hover capability. Such an added mode of operation designed into the vehicle would make it more useful. In some designs a pseudo hover capability is built in by the addition of floats for buoyancy lift (in water). These floats which also serve as the endplates cause the danger of water contact at high speeds referred to above.
A common problem of all ground-effect vehicles is that the increase in lift created by the ground effect with simultaneous reduction in resistances is only usable when there is a relatively small space between the support surface and the ground or water. This spacing which can be considered the flying altitude can only be maintained when there are no obstructions to fly over. This creates the risk that the ground-effect vehicle is stalled as a result of actuating the elevators, which makes recovery like an aircraft impossible due to the limited flying altitude. In addition when the elevators are lowered there is the danger that the wings of greater span while turning actually touch the water or ground. For this reason one must not use the maximum wing span that is most useful for starting, which is also a problem as a result of the larger space and the increased weight, the limited maneuverability when docking, entering a port, and the limited usability in rivers, canals, docks, and the like. In addition the necessary power for starting cannot be used as in an airplane to increase the travel speed. Since the angle is smaller with increasing air speed, the leading surface relative to the trailing surface is increasingly smaller at the rear edge so that the increased lift is lost as well as the automatic altitude stability. This is needed for the safe use of ground-effect vehicles in order to avoid touching the water when flying.
The WIG configuration that has reached the highest level of technical maturity is the Russian "ekranoplan." This is described further in the ARPA Report. A typical example of the "ekranoplan" configuration is embodied in the Russian Orlyonok. In this prior art WIG, turbofan engines are located on either side of the fuselage. These engines are used for underwing blowing PAR to increase the lift of the wing during take-off and landing thereby reducing take-off and landing speeds. The turbo prop engine provides efficient thrust for cruise. The horizontal stabiliser controls the pitching moment. A hydro ski can be lowered to reduce hull impact pressures on landing. The endplates help contain the pressure under the wing to provide increased PAR lift during take-off and landing. Because the endplates do not extend below the lowest part of the fuselage the effective air gap between the endplates and the water is no less than the gap between the lowest part of the fuselage and the water. The ability of the endplates to reduce the induced drag is therefore limited.
The Power-Augmented Ram Wing-in-Ground Effect Vehicle (PAR-WIG) utilizes the efflux from the aircraft engines to form the air cushion during the entire flight of the aircraft. The engines are placed near the front of the vehicle and tilted so that the efflux blows under the front edge of each wing. The efflux is partially trapped under the wing by end plates and movable trailing edge. A static pressure rise results under the wing, providing lift for the vehicle. The amount of lift is greater than the prior art devices due to the design of the propulsion system which entrains ambient air, hence filling the volume under the wing with high energy air. Tests have shown that the PAR-WIG cushion can be used with high wing loadings and relatively rough sea conditions, but requires relatively low thrust. Thus, this design is a large improvement over previous models. This design presents new problems, however, by the mounting of the engines near the nose of the aircraft. The engines are very vulnerable to the ingestion of salt spray and debris, both of which may damage the engines. Also, the structure of the aircraft must be more substantial in the nost section, causing an undesirable weight increase.
The U.S. Navy used thinner endplates with their PAR WIG model experiments disclosed at page 411 of the Mantle Report ["Air Cushion Craft Development (First Revision)" (DTNSRDC Report 80/012 (4727 revised) January 1980) by Peter J. Mantle]. These endplates were designed to pierce the waves but were unstable at cruise speed with a moderate angle of yaw. Even if these endplates did not fail, their relatively thick leading edge and forebody would make the drag of these endplates intolerably high when piercing waves at high speed.
Several large WIG designs have been proposed but never built. These are summarised in "Wingship Investigation" (Advanced Research Projects Agency, Sept. 30, 1994). Because the height of land varies so much it is normal to fly WIGs over water. All existing WIGs fly entirely above the water at the height of the highest wave expected to be encountered plus a margin of safety. This is because of the extremely high wave impact forces that would be incurred at cruise speed. The problem with the application of traditional aeronautical engineering to marine based wing-in ground effect vehicles is that there is currently no widely available civilian technology to produce a vehicle structure light enough to operate in complete free flight, yet sturdy enough to withstand the potential forty -G impact of hitting an eight foot wave at one hundred miles per hour. The ARPA Report concluded that designing basic structure and mission loads to tolerate impact with large waves is probably impracticable.
The ARPA Report also concludes that the induced drag increases and the Power Augmented Ram (PAR) lift decreases with the height of the endplates above the water. PAR directs the jet from engines located forward of the wing under the wing to provide added lift at slower speeds. Because of this there is an advantage for WIG endplates to penetrate the waves so that there is no gap at the wave trough between the bottom of the endplate and the water. The existing prior art has not taken advantage of this as it has been assumed to be impossible to design wave piercing endplates that would (i) have a low enough drag in the water and (ii) be stable at expected angles of yaw at design cruise speed.
As a result, the endplates of existing WIGs usually resemble slender hull shapes similar to high speed racing catamarans, some of which include steps to reduce water friction on take-off. Because these designs are still relatively thick they would incur severe wave impact pressures at cruise speed as well as high drag. Consequently, these endplates are designed to be no lower than the lowest part of the fuselage of the WIG. As a result there is always an air gap greater than the wave height between the wing tip or endplate and the trough of each wave. This restricts their ability to reduce the induced drag. Typical lift/drag ratios of Russian craft are around 18:1 and the ARPA Report study was unable to significantly improve on this figure even for a very large craft of 5,000 tonnes (after making changes required to achieve the longer range set by the study). As these lift/drag ratios are no better than those achieved by aircraft it is understandable why WIGs have never been commercialised.
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