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About WIG Craft...

WIG's are advanced hybrid air cushion craft offering the highest possible combination ofspeed, fuel efficiency, and ride smoothness. Unlike it's close cousin the hovercraft, a WIGrides above the surface like an airplane on a dynamic air cushion that is produced by thecraft's forward motion. In contrast, a hovercraft rides on a captured air bubble (CAB) that is

produced by a mechanical blower---usually in the form of a fan or cyntrifugal compressor.The Hovercraft's air cushion is held captive by an inflated skirt that surrounds the barge-likehull of the craft. The speed and efficiency of a WIG's can be considerably higher than thoseof hovercraft because they have no power lost to a blower and no bulging skirt to forcethrough the air or drag over the waves.

WIG's share technologies with several other types of air and marine vehicles. Thehydrodynamics of the main hull are similar to those of high performance powerboats,seaplane floats, or flying boat hulls. The hydrodynamics of the sponsons or wing endplatesare similar to catamaran hulls or hydroplane sponsons. The flow of ram air under the wing issimilar to that under tunnel hull racing boats. In the areas of propulsion, and operation onthe water, WIG's are similar to hovercraft and airboats. The flight performance, stability, andcontrol of WIG's are similar to aircraft. The design of a safe and practical WIG craft dependsdirectly on the understanding and successful orchestration of all of these technical elements.

Our History and Technology...

Wing-in-ground-effect R&D at Seair Craftbegan in 1986 with a requirement for a

surface-skimming small craft that would provide maximum ride comfort and efficiency athigh speeds. Seair founder Peter Longwood commissioned a technical survey by The X-AeroCompany to study the feasibility of such craft. This study compared the pros and cons, andpotential efficiency of various high speed marine vehicles. Catamarans, tunnel hulls,

hydroplanes, deep V's, hydrofoils, hovercraft, airboats, SES craft and various hybrids wereconsidered. The study concluded that an air cushion vehicle based on wing-in-ground-effecttechnology would have the best chance of satisfying the given requirements. WIGs wereshown to share many of the unique characteristics of hovercraft, hydrofoils, and airboats, buttheoretically had the potential to excel in some areas---chiefly, the ability to maintain asmoother ride over rougher waves at higher speeds. WIGs also had the potential for muchgreater fuel economy than other types.

Alexander Lippisch* built one of the world's first WIG's in the U.S. in the early 1960's, butthe majority of serious WIG development has historically taken place in Germany* and

Russia*. In other parts of the world the handful of WIG research programs that existedthrough the late 1970's were generally of a more academic nature without full-scale testvehicles. By the mid-1980's, most interest in WIG technology fell dormant outside of theSoviet Union. The X-Aero study pointed out that throughout the 60s and 70s WIGtechnology had failed to find its niche. Of the several dozen WIG craft built up to that time,few had reached even limited production status. In fact, until the early 1990s all WIGs builtoutside of the Soviet Union were prototypes or proof-of-concept research craft. Several of

these prototypes were direct copies or minor variations of the 1960s Lippisch concept. Somewere simply misguided "design-by-guess" attempts resulting in expensive failures.

The X-Aero study concluded that to make WIG's a viable transportation alternative one mustbe able to reliably design practical, mission-oriented craft without relying on expensive trialand error testing of multiple full-scale prototypes. When Longwood commissioned the study

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in the mid-1980s unclassified technical information applicable to WIG design was very hardto come by. Published reports consisted mostly of general ground effect theory and academicstudies of a few generic WIG concepts. Frustrated by the apparent lack of practical WIGtechnology in the U.S., Seair embarked on a long-term R&D program to build useful civilianWIG design capability from scratch.

Seair's "Building Block Approach"to acquiring WIG know-how is illustrated symbolicallybelow. Research began with reviews of all existing WIG craft and published technical data tohelp formulate practicaldesign requirementsfor viable WIG's. This was followed byexperimental and computational work, building capability for full-scale craft designs.

The experimental phase beganwith studying fundamentalWIGaerodynamics andhydrodynamics.

Lessons learned with wind

tunnel and hull models werelater extended to flight testing

with large-scale radio-controlled models.

Test data were used to deriveand calibrate computationalmethods and simulations.

Such prediction capabilityallows a systematic design

approach and more accurateWIG conceptual design studiesfor specific applications.After spending several years doing our "homework", the engineering tools, methods, anddata have been used to do more detailed design work on smallWIG craft for personaltransportation and recreation.

Wind Tunnel Model Test Dynamically Scaled R/C Test

WIG Design Requirements

Based on a study of potential uses of small WIG craft (less than 10 tons gross), Seaircompiled a list of specific characteristics which should be met if they are to be successful assafe, practical marine transportation. Seair has used these proposed characterictics since1986 to guide the development of key technologies for their WIG designs. Importantrequirements and objectives included these

"Ten Commandments" of WIG Design:

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Remain "boat-like" in most respects: Maintain good "boating" characteristics (running

on plane at below lift-off speeds---critical for operating in high traffic areas, narrowrivers, or any area with restricted maneuvering room). Also good cockpit visibility forseeing and avoiding other marine traffic. Have practical (reasonably "boat-like")docking / mooring capability.

Avoid IMO "Type C" which is unable to capture the cost savings of licensing as a

"boat". Design for reasonable operating speed margin between initial lift-off and maximum

cruise speed.

Moderate power required for takeoff (horsepower-to-weight ratio of < 7 % ) withreasonably slow lift-off speed and short takeoff runs (< 30 seconds or so). Also have

ability to takeoff in other than "nearly calm" wind and waves.

Good resistance to blow-over if "bounced" by wind or waves above normal height,and no dangerous loss of stability or control at maximum speeds. Be able to safelyrecover from significant pitch or roll upsets.

Have good maneuverability (i.e. "boat-like" turn radius --or better-- at cruise or

harbor-taxi speeds) and the ability to quickly execute emergency stopping or othercollision avoidance maneuvers.

PAR takeoff assistance (if required) must not be too mechanically complex, costly ornoisy for commercial use.

Props or fans need to be located in areas not prone to debris damage, or sand and

spray erosion, and should not create a potential hazard to dock attendants, nearbyboaters, or other personnel.

1 to 6-seat craft should be conveniently folded or dismantled for transport ortrailering and should have walk-on "deck" areas for boarding, engine access (or for

swim platform, sun pad, etc.).

Avoid ultra-light aircraft type construction that is not robust enough for seriousmarine use and avoid "low-cost" FRP boat construction that is too heavy to fly well(use aircraft structure where necessary).

Every one of of the historical WIG examples investigated during Seair's survey exhibited atleast four deficiencies relative to the proposed list. During the last half of the 1990s several

WIG developers emerged with "next generation" designs that have addressed some of theprevious shortfalls, promoting the formal recognition of WIGs by the IMO and various CoastGuards. It is good to keep the above factors in mind when making your own evaluation ofany current or future WIG craft.

Experimental Aerodynamics & Hydrodynamics

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Wind Tunnel Testing: During 1987 -1991, Seair conducted exploratory wind tunnel tests on1/20 scale models of several wing, body, tail, and endplate combinations. This testing was

accomplished in a small research tunnel atAeronautical Testing Servicesof Arlington,Washington. Data from these tests were used to develop practical aerodynamic predictionmethods, identify viable configuration options, and to help understand various ground effectphenomena. Wind tunnel tests helped determine several viable tail and endplate shapes frommore than a dozen options. Wing pressure measurements were used to assess the influenceof several airfoil section shapes and flaps on takeoff lift and cruise height stability. The windtunnel program employed several measurement methods to gather a range of data:

Longitudinal force and moment data

Off-surface and downstream wake survey Flow visualization (tufts)

Wing and ground plane pressures

Free-to-trim pitch-heave dynamics

Some the data obtained over eight tunnel entries was the first of its kind collected on WIG'sanywhere (and some may still be unique in 2001). Pressures measured on the ground-planeunder a WIG wing with endplates graphically showed why the ride of some WIG's has beendescribed as feeling like that of a large luxury car with soft suspension. A hovercraft mustsupport itself on a short cushion of air bounded by the skirt around its hull, and a hydrofoil issupported only at its ends on stiff struts. In contrast, the experimental data showed that aWIG's supporting cushion extends well forward and aft of the wing. Its total pressure"footprint" on the water (or in this case on the tunnel ground-plane) is three to four times

the length of the wing chord. The length of the WIG's air cushion acts as the aerodynamicequivalent of a long wheelbase that smoothes out the bumps when the craft is "platforming"over a choppy surface. Just as the craft exerts a force on the water surface well out in front

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of the wing, oncoming waves feed a pressure force back to the wing of the craft allowing it'smotion a slight "lead" on the shape of the surface when "contouring" at an angle to longswells.

Model Scale Flight Testing:

From 1987-1996 Seair Craft Inc. conducted extensive outdoor testing with 1/5 and 1/4 scalepowered models, and two small hand-launched gliders. Hull planing characteristics werestudied with towed and radio controlled models.

Just as the wind tunnel models provided insight into the aerodynamic lift and pitch stability,the first generations of towed-hull and R/C model tests provided an understanding of thehydrodynamics. We found that traditional seaplane float and boat hull design guidelinescould be used to build WIG endplates and hulls that had good "boating" characteristics, butthese were not necessarily compatible with the aerodynamic requirements. Conversely, thelowest drag and highest lift aerodynamic shapes had very poor hydrodynamics. These were

difficult to get over the "hump" and onto plane, to unstuck from the water, and could havebad "slamming" characteristics in choppy water.

When properly executed, the reverse-delta planform of Lippisch type WIG's has been shownto offer good height and pitch stability, but the arrangement is inherently larger in width xlength for a given square footage of lifting surface. Besides being difficult to build compactly,the wing geometry is potentially more difficult and costly to build than a rectangularplanform. The simple tandem rectangular wing arrangement favored by Jorg is perhaps themost straight-forward and results in very fast cruise speeds over smooth waters. We wereconcerned that the tandem wing craft did not seem to be very robust in stability or takeoffperformance when in choppy waters or gusty winds.

We initially investigated single-piece rectangular wings with simple trailing edge flaps and

flat-bottomed or S-shaped airfoils. First results were disappointing. After performingcomputational fluid dynamics (CFD) on nearly 50 airfoil modifications, we arrived at a familyof custom hybrid airfoils and wing planforms that provided reasonable pitch stability, lift,

unstick from the water, when combined with a specially designed center hull andsponson/endplates. We settled ona preferred general arrangement formula consisting of a mid-mount low aspect ratio mainwing, coupled with a high-mounted, moderately tapered horizontal tail surface of high aspect

ratio. The horizontal tail is supported and stiffened by twin vertical fins (in the case of singleengined craft) or a single centerline tail behind side-mounted twin engines. Although mosttesting was conducted with open props, the preferred design employs shrouded props orducted fans. The resulting combination enables takeoffs without the use of auxiliary enginesor PAR devices at static thrust-to-weight ratios or less the 1:4. The horsepower to weightratios are comparable to efficient PAR WIGS. This makes the complexity of powered lift orhovercraft type air cushions unnecessary (unless amphibious capability is desired).

Material on this website is copyrighted 2000 by C.P.Nelson, Seair Craft Inc.

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Computational Prediction and Simulation Tools

CAD Layout of 2-Seat Cockpit

CFD Analysis (Panel Methods)

Tail-boom Finite Element Model

Operational Simulation (SC-180T)

Computational Tools Used By Seair:From the beginning, Seair has used PC-based tools to help study and understand

WIG craft design in ways unavailable toexperimenters in the 1960's and '70's.

Early in our studies, commercial PCsoftware was not widely available for thekinds of tasks at hand, so much of theinitial simulation, analysis, performanceestimates, and lofting tools were customwritten in-house. As time progressed wehave been able to employ a range ofcommercially available software tools.

Computational tools employed in Seair

studies have included the followingexamples...

Specialized 3D surface loftingsoftware

3D Computer Aided Drafting

(CAD) Custom Airfoil Design & 2D

Analysis 3D Computational Fluid

Dynamics (CFD)

Performance and Vehicle SizingCodes Structural Finite Element

Models Batch-processed 3 and 5-D.O.F.

vehicle dynamics simulations Piloted operational simulations Spreadsheet-based vehicle cost

and weight estimation Project planning tools 3D solids rendering and

animation

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Simulation Time-History Plot

WIG Conceptual Design Studies

SC-122T 150-Ton PAR-WIG Concept

SC-38T Ferry Concept 3-View

SC-38T In A Banked Turn(click picture for larger image)

A number of concepts for commercial,governmental, and private WIGSwereinvestigated by Seair during 1996-98. Bothconventional and powered lift (PAR or SES takeoff)

variants were investigated. The SC-38T WIG ferry/utility transport shown on this page was one ofthe non-PAR concepts investigated for IMO Type Aand B WIGS from 4 to 30 seats in size.

The SC-38T is a multi-use craft.It was sized to

carry a pilot, and 9 passengers in a "commuter"version or 6 passengers + up to 750 lbs ofbaggage / cargo as a "6-Pack" configuration. Atrest, the wing serves as a walk-on deck forpassenger loading. With an alternative "RV-style"interior the -38T could be outfitted by privateowners for high-tech cruising ---imagine Seattle to

Glacier Bay, Alaska in two days! Twin-enginedreliability, 6-hour endurance, and the ability tooperate in 20 kt winds and 4 ft waves make itsuitable for SAR, customs patrol, or islandambulance duty.

Type B WIG craftcan skim well above the waves,so high cruise speeds can be maintained withbetter comfort than any other type of high speedmarine vehicle. They can also takeoff and land in

larger waves than Type A WIG's and can skim oversandbars, floating ice, logs and debris. The -38T'sfloating draft is less than 2ft fully loaded, so it can

operate from shallow bays or rivers. A"ruggedized" version would also be capable ofoperating from flat expanses of ice or snow.

WIG craft are very "environmentallyfriendly". The high energy efficiency of WIG'sallows the 4.25-ton SC-32T to achieve five times

the gas mileage of an equivalent sized deep-V hulleven while cruising at 60 knots! Low noise, lowemissions, no submerged prop, and zero wake(while skimming) make the -38T well suited forecotourism or scientific access to sensitive remoteareas. The high fuel efficiency gives the -38T anunrefueled radius of action of 160 NMi (in still air).

Dollarwise, WIG craft are generally more

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SC-38T USCG Configuration(click picture for larger image)

SC-38T At Anchor

Simulation: Captain's View

expensive to build than a conventional boat (butconsiderably cheaper than a seaplane or helicopterof similar capability). The high fuel efficiency, highseat-miles-per-hour productivity, and lowmaintenance actually offset the higher initial price.USCG and other regulatory bodies have agreed to

license Type B WIG's as boats rather than aircraft ---the WIG captain, taxi operator, mechanic, etc. donot need FAA certification. The engines are

marineized GM V-8's so parts and maintenance areeasily obtained in any part of the world. Theconstruction is of corrosion-resistant aluminum andcomposite materials.

Simulation: SC-38T Fast Cruise

Simulation: Tail Camera

* Digital animations of the operation of the SC-38Twere created in-house using data from piloted and

batch processed engineering simulations, 3D CADsolid models, and detailed performance

calculations.

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Personal and Recreational WIG Craft

"SC-14" Single-Seat WIG

"SC-19" 2 or 3-Seat WIG

SC-19 Dynamically Scaled Model

One, Two, and Three Seat Recreational WIG's :After spending several years doing our "homework",Seair used the engineering tools, methods, and data

we have accumulated to do more detailed designwork on small WIG craft for personal transportationand recreation.

Although these craft have much of the utility of asmall seaplane, they are actually IMO Type B WIGwater-craft that do not require a pilot license tooperate and do not have to be maintained by acertified aircraft mechanic. These factors, plus thefact that an emergency "landing strip" is alwaysavailable a few feet below, allows significantsimplifications over aircraft and large cost savingsrelative to a conventional seaplane of similarpayload. Cost to build is on the same order as thatof the smallest homebuilt kit aircraft, and onlyslightly more than conventional speedboats orrecreational hovercraft.

While operating on water, these craft can be usedmuch like an Airboat. At higher speeds (30-45 mph)the craft will lift free of the water surface and skimalong in ground effect.

The pilot regulates the cruise height and speed byusing a combination of tail trim and throttle setting.Full throttle is normally used only for takeoffs so the

engine duty cycle is well suited to automotive typepowerplants. The loss of lift and increase in dragexperienced out of ground effect, plus the ductedfan's natural decrease of thrust with forward speedserves to curtail the craft's ability to sustain flight ataltitudes greater than about one wingspan above thewater.

A combination of shallow banking and yawing is usedto provide minimum radius turn capability in groundeffect.

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"SC-24" 4-Seat WIG

"SC-24" Folded For Transport

"SC-24" Stowed On A Large Yacht

Four, Five, and Six Seat PersonalTransportation WIG's: Engineering dataaccumulated by Seair in studying small recreationalWIG's can be easily "scaled" to larger sizes for moreserious transportation use. Seair has doneconceptual studies of single-engined WIG's of up to

eight seats, based on the same general arrangementas the smaller SC-19. More detailed preliminarydesign work has also been done on a four seat

personal WIG called "SC-24". A preliminary CADdefinition of the SC-24 design has been used tocreate the 3D solids used in the digital renderingsshown on this page.

As with the smaller Seair designs, the wings and tailassembly of the SC-24 fold to allow roadtransportation or storage. The overall dimensions ofthe SC-24 make it compact enough to stow on largeyachts. With proper lift equipment the 1700lb craft

can be unfolded and lowered into the water for rapidlocal transportation or sightseeing. Expectedperformance of the SC-24 includes a range capabilityof 140 nautical miles (with 20 minute reserve fuel),and a payload of nearly 1000 pounds. The maximumstill air speed is near 100 miles per hour (75-80knots) and the normal cruise speed is estimated to

be about 55 knots. At cruising speed the fueleconomy is 18-20 statute miles per gallon. Evenhigher fuel economies should be achievable underoptimal conditions at slightly slower flying speeds.The normal cruise height is 2 to 6 feet above themean wave surface. Based on model test data, theSC-24 should be capable of operating over three to

four foot waves and in winds of up to 15-18 knots(when flown by an properly trained experiencedskipper).