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DESIGN AND DEVELOPMENT OF THE SWIFT: A FOOT-LAUNCHED SAILPLANE

Ilan Kroo*

Stanford UniversityStanford, CA 94305

* Professor, Dept. of Aeronautics and Astronautics, Associate Fellow AIAACopyright ”2000 by Ilan Kroo. Published by the American Institute of Aeronautics and Astronautics, Inc., withPermission.

AbstractThis paper describes the development of what might beconsidered the first successful ultralight sailplane. TheSWIFT is a high performance foot-launched glider,designed to combine some of the convenience of hanggliders with the soaring performance of sailplanes. Ittakes off and lands like a hang glider, yet maintainsexceptional performance at high speeds, achieving alift-to-drag ratio of about 25:1. Although it is a fully-cantilevered rigid wing with aerodynamic controls andflaps, it is light enough to launch by running from ahillside and is easily transported on the top of a car.This paper describes the design, development, andflying of this unique aircraft.

Introduction and BackgroundPioneers of heavier-than-air flight were inspired withthe idea of being able to fly like birds – not for thepurpose of efficient, high-speed transportation, but forthe shear freedom that such a capability would permit.This was the motivation for, and is the appeal of,modern soaring aircraft such as paragliders, hanggliders, and sailplanes. Although the performance ofsailplanes has increased dramatically over many decadesso that lift-to-drag ratios of 60:1 have been achieved and1000 km flights are possible, certain aspects of highperformance sailplanes seem counter to the visionespoused by Lilienthal and others [1]. Especially for agroup of graduate students in the San Francisco BayArea, the cost of sailplane flying, along with the longdrive to an airport that supported such activities, meantthat achieving the goal of bird-like flight was onlysomewhat more realizable than it was 100 years ago.That one was often restricted to flights near this airportalso made the reality of soaring somewhat lessinspiring than the vision. This, of course, was one ofthe reasons that the sport of hang gliding becamepopular. But while hang gliding avoided many of theproblematic aspects of sailplane flying, it introducednew difficulties. Hang gliders were inexpensive andcould be flown from many local

sites, but their performance was such that long distanceflights were uncommon. Flying was often restricted toa very small corridor on a ridge, and more often thannot, consisted of an unimpressively short glide to thebottom of the hill. Furthermore, the simple, yetsubtle, techniques for hang glider control using pilotweight-shift seemed to limit further performanceimprovements and led to less than ideal handlingqualities under some flight conditions.

In 1985, I recruited a group of outstanding graduatestudents at Stanford, many of whom were hang glideror sailplane pilots, to investigate what was possible atthe boundary between these two aircraft types. The ideawas to consider the possibility of an airplane thatwould fly at the speed of birds, permitting launchingand landing like a hang glider, yet with the performanceand control that would permit extended soaring flightson good days. With affordable computationalaerodynamic analysis capabilities improving,composite structures evolving, and with some specificconcepts for efficient tailless aircraft configurations, webegan the design of a foot-launched sailplane that wouldeventually become the SWIFT.

Of course, we were not the only ones working on suchideas. Designs such as the Mitchell Wing, the Canard2FL, and lightweight sailplanes inspired by the SSA’shomebuilders’ workshops suggested that such anairplane might be feasible. Of particular relevance wasthe work of Brian Robbins, Erik Beckman, and BrianPorter of BrightStar Gliders, just two hours North ofStanford. BrightStar had been developing a rigid winghang glider, called the Odyssey, which Brian Porterpiloted to first place in the 1989 U.S. National HangGlider Championships. Brian Robbins suggested thatthe Stanford group might improve the Odyssey'sairfoils somewhat; but after several evenings ofdiscussions, we agreed to pursue a radically newdesign. Four months later, in December of 1989, theSWIFT took to the air over a small hill in MarinCounty.

This paper describes the technical development of theSWIFT, with a focus on the aerodynamics design

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concepts unique to this configuration and aircraft class.Additional descriptions of the evolution of this designmay be found in [2,3,4].

Figure 1. SWIFT on approach.

Design

ObjectivesThe design of the SWIFT began with a study of therequirements for cross-country soaring. Based onsurveys of thermal distribution and strength and inter-thermal downdrafts from [5], we developed a cross-country soaring simulation that would permit changingglider parameters and evaluating the effect on the likelyachievable soaring distance. Even without thissimulation on could see the direction required forextended soaring (see [6]). From data on 76 thermalsencountered in Dick Johnson’s flights over easternTexas and from our own experience in California andNevada, we created a statistical distribution of thermalsthat were spaced 1.2 to 12 miles apart with heights of1600 to 7000 ft. The interthermal sink varied from 1.4to –0.3 kts with an average of 0.3 kts and with 80% ofthe cases less than 0.5 kts. Based on this model onecould compute the probability of reaching the nextthermal – or of flying 100 miles. This is shown in thetable below as a function of the effective interthermalglide slope.

Effective P(next thermal) P(100 mi)Glide Slope10 .37 10-7

15 .81 .03418 .91 .2120 .95 .4440 nearly 1 nearly 1

It is clear that inter-thermal glide ratios of at least 15 to18 in the presence of the assumed 0.5 kts of sink isneeded to make this kind of soaring easily attainable.At the time that this study was first made time, only adozen 100 mile flights had been made by hang gliders.Today, although flights over 300 miles have been

made, most pilots (even most advanced pilots) havenot flown 100 miles. One of the factors limiting theflight distances of hang gliders is their speed. Theeffective inter-thermal glide slope in the presence ofsink is much lower for slow-flying aircraft. The tablebelow shows the aircraft lift-to-drag ratio required toachieve and inter-thermal glide slope of 18:1 in thepresence of a 0.5kt downdraft (or 9 kt headwind).

Cruise Speed (kts) L/D Required10 180.020 32.730 25.740 23.250 22.060 21.280 20.3

Furthermore, thermals are commonly encountered for arather limited time during daylight hours and withaverage cross-country cruising speeds of less than 20kts, one needs to fly for five hours to go 100 miles.Thus, extended cross-country soaring requires not onlya good enough glide to make it to the next thermal,but a fast enough glide to get there quickly and in thepresence of headwinds or sink. This is easily done bymaking large span sailplanes with high wing loading.But if the glider is to be foot-launched, it must belight (span not too large) and have a low wing loading.More refined studies of Johnson's data and barographrecords from George Worthington's Mitchell wingflights in the Reno area suggested that a foot-launchedsailplane with the required performance was just barelypossible. The following target performance figures wereestablished and work began to define the aircraftgeometry.

Target Performance for Foot-Launched Sailplane1. Minimum Sink Rate in 100' radius turn: 200 fpm2. Maximum L/D: 20:13. L/D at 60kts: 15:14. Stalling speed: no higher than existing hang

gliders for safe foot-launching and landing5. Weight: less than 90 lbs6. Exceptional controllability for safe flight at low

speeds

The fourth constraint meant that even with large flaps,the wing area would be 120 to 140 sq ft. With thisconstraint, the third goal would be very difficult,requiring an unprecedented level of aerodynamicstreamlining. To achieve the desired performance, lowdrag airfoils and an extremely clean pilot fairing wouldbe required. The sink rate polars in figure 2 illustrate

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the importance of streamlining, especially for lightweight gliders at high speed. The figure also showshow the predicted sink rate of the SWIFT compareswith other gliders; it is clearly in a class above hanggliders and compares very favorably with the Schweizer1-26 sailplane at speeds up to about 60 kts.

Figure 2. Sink rate polar comparison for some soaringaircraft.

Conceptual Design / Configuration ConceptUnless one does something very wrong, theperformance of a glider is determined primarily by itsweight, span, area, and streamlining. The selection ofthe configuration, whether conventional, canard,tailless, or something else, is based more on issuess u c h a s packaging, handling qualities,manufacturability, transportability, etc.. In thedevelopment of the SWIFT, several possibleconfigurations were studied. The results indicated verysmall performance differences between tailless,conventional and canard designs; however, theconventional design suffered some from the short taillength required for landing flair and take-off groundclearance. The directional stability of a slightly-sweptcanard was poor, and performance was alsocompromised by the short coupling. The taillessdesign was statically-balanced (empty c.g. near flightc.g.), compact, and did not pay the weight penalty thatwould be associated with a tail boom. (Note that evena 5 lb boom represents more than 5% of the emptyweight and a very large fraction of the wing bendingweight.) For these reasons, and to study severalaspects of tailless aircraft design, an aft-swept “flying-wing” design was selected.

One of the first steps in the configuration developmentwas a sizing study that started with a look at thesensitivities of performance to several designparameters (see figure 3). This was followed by a morecomprehensive optimization of span and area based onthe cross-country soaring simulation.

Figure 3. Effect of design parameters on lift-to-dragratio.

As the design evolved, several mock-ups wereconstructed to evaluate visibility and ground-handling.A high wing arrangement was adopted for good groundclearance and pilot visibility, with an arrangementsimilar to that used by the Odyssey. On the ground,the glider is supported by shoulder straps (Fig. 4).

Figure 4. Glider is supported by shoulder straps priorto launch.

After takeoff, the pilot rotates his or her legs forwardinto the aluminum cage structure. The pilot issupported by a retractable sling that provides acomfortable reclined orientation. (Fig. 5). The glidermay be flown with a fairing that covers the cagestructure, or with the pilot exposed to the air.Subsequent SWIFTs have included a small wheel andskid that permits feet-up landing when conditionspermit and even towed launches.

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Figure 5. The pilot rotates into flying positionsupported by a retractable sling.

Aerodynamic Design

The S.W.I.F.T. ConceptAlthough the tailless configuration is common amonghang gliders, for which portability is paramount,tailless designs are often considered an aerodynamiccompromise. Textbooks have noted the poormaximum lift capability of flying wings and somepenalty is generally assumed to be associated withtrim, whether through less-than-optimal reflexedairfoils or twist-related drag. Handling qualities havealso been undesirable for many aircraft of thisconfiguration. It was our goal then to see if we couldminimize or eliminate these difficulties by careful 3-Daerodynamic design of the wing. The combination ofsweep, taper, and twist was arranged so that ratherconventional airfoil sections with negative pitchingmoments (not reflexed airfoils) could be used (seefollowing section).

Figure 6. Horten VI planform and lift distributions

The use of sweep and twist, rather than sectionproperties, to achieve pitch trim on a stable tailless

aircraft is well known. It is used by most hang glidersand on the well-known tailless sailplanes developed bythe Hortens[7]. The penalties associated with suchwing twist can be large as evidenced by the Horten IVwhose spanwise lift distribution is shown in figure 6.When trimmed at a lift coefficient of 1.0, considerableupward deflection of the elevons is required leading tolower section Clmax and an induced drag penalty. TheSWIFT was designed to maintain very close to theideal lift distribution when trimmed, effectivelyeliminating such penalties.

The basic idea is that a swept wing can be stable andtrimmed, even with elliptic loading if the planformshape is properly chosen. What is required is that thecenter of lift associated with changes in angle of attack(the aerodynamic center) must be located outboard(hence aft) of the actual center of lift. If the wing iselliptically loaded, the center of lift is located at 42.4%of the semi-span. This is fixed. But the aerodynamiccenter can be moved farther outboard by increasing thesweep and tip chords. With typical airfoil sections, itis possible to move the center of additional lift (a.c.)sufficiently far from the center of total lift to achievereasonable static margins with moderate sweep andtaper, as long as the aspect ratio is sufficiently large.Another way to think about this effect is to imagineadding the lifts of an untwisted wing at angle of attackwith that of a twisted wing at zero lift. Although theinduced drag penalty of each of these components islarge, the sum provides an ideal lift distributionwithout trim penalty (Fig. 7). The planform of manytailless aircraft incorporate sweeps and tapers that arenot consistent with this concept.

Figure 7. The combination of washout and a tip-weighted additional load distribution produces adesirable net loading.

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Like any stable, swept tailless airplane, the SWIFTrequires more effective twist to trim at higher angles ofattack. This is not necessarily a penalty as theplanform is designed to require more twist at higherangles of attack to achieve the ideal span loading.However, the use of elevons to achieve this effectivetwist change is problematic for several reasons. One ofthe fundamental ideas behind the SWIFT controlstrategy is to deflect an inboard surface downward,rather than an outboard surface upward to achieve trimat higher CL. Such approaches were discussed byLippisch and others many decades ago [8,9], but havebeen used to great effect on the SWIFT. By deflectingthe inner flap downward one increases the effectivewing washout as desired, but also increases camber andmaximum lift. (Fig. 8). Because of the moderatelytapered planform, the SWIFT can exploit a trim-changing flap that covers about 45% of the wing span.When deflected down for higher lift, the glider nosesup slightly and trims at a lower speed. It may bedeflected downward as much as 55° for landing andapproach, reducing the L/D to a manageable value andslowing the glider down for stand-up landings. Thisuse of the inboard flap for pitch trim gives the aircraftits name. At the risk of confusion with the long line ofSwift aircraft, this SWIFT stands for Swept Wing withInboard Flap for Trim.

Figure 8. The inboard flap produces nose-up trimdespite the large negative section moments.

Wing OptimizationThe next step in the SWIFT's aerodynamic designinvolved complex trade-offs between wing taper, twist,flap size, flap deflections, elevon deflections, and wingarea. Changes that might benefit high-speedperformance might hurt thermaling ability or increasestall speed above acceptable limits. The final trade-offswere made by simulating a long cross-country flight onthe computer and using a numerical optimizer to selectthe design with the best overall soaring performance.The simulation included thermal models, inter-thermalsink, and a 3D inviscid aerodynamic analysis (panelmodel) of the design. The resulting configuration isshown in figure 9.

Figure 9. 3-View Layout of SWIFT

Airfoil DevelopmentAirfoils were designed especially for the SWIFT. Thesections have a small negative pitching moment andwere designed to operate in the Reynolds number rangeof 700,000 to 2,000,000. They make use of laminarflow over the first 25% of the chord, if they can get it,but are explicitly designed to experience littleperformance degradation if the flow is made turbulentby rain or surface irregularities. This amount of laminarflow was selected based on the idea that the first 25%of the airfoil could be quite smooth and accuratelyconstructed. The airfoil thickness at the flap and elevonhingelines was originally quite large to providestrength in this area. Tests on the first prototypesuggested that the strength in this region was not aproblem, but the gaps created by control deflectionsadded a great deal of drag. The airfoils were redesignedwith very thin trailing edges that successfully reducedthe control surface gap drag. Except for the analysis ofthese sections using simple airfoil design programs onApple Macintoshes, the airfoils were not tested beforethe first prototype was built. Truck-mounted tests ofthe glider suggested that the airfoils are working aspredicted, but accurate performance verification was notdone as it was not considered critical in the aircraft’sdevelopment. The main airfoil geometry is shown infigure 10. The maximum thickness is large and islocated quite far forward in consideration of the wingstructural constraints. The greatest challenge in theairfoil design was to achieve a very large CL range andto ensure that the section would operate efficiently withthe inboard flap. Figure 10 shows the section Cp

distribution at lift coefficients of 0.2 and 1.4. Thesection achieves laminar flow over approximately 25%-30% of its chord – not extensive by sailplanestandards, but a reasonable compromise to achieve theweight goals.

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Figure 10. Airfoil pressure distributions over a largerange of lift coefficients.

Structural DesignThe structure of the SWIFT is designed to meet thedemanding requirements of very low drag (fullycantilevered, accurate airfoil definition for laminarflow) and light weight. The wing structure uses a D-tube covering the first 25% of the chord with ribsextending from there back to the control surface hingeline at 75% chord. The prototypes were constructedwith an Aluminum D-tube and mylar covering toreduce costs and one-off construction time. This madeit possible to refine the design before committing tothe molds from which production versions were built.The basic structural concept is shown in figure 11.Using Kevlar skins and graphite spar caps, the vehicleempty weight is about 100 pounds, although with fullfairing, rocket-deployed recovery parachute, andinstruments the weight becomes somewhat greater thanwe had originally envisioned. In a light breeze,however, the wing lifts itself and the weight that mustbe borne by the pilot’s shoulders is quite manageable.

Figure 11. Structural concept.

The loads that need to be carried by the glider itself,though, are very large. Because of the low wing

loading and high design airspeeds, the effect of gusts isamplified. To comply with FAA sailplane criteria theglider must be capable of withstanding positive andnegative vertical gusts of 24 ft/sec up to VNE. Sincethe maximum speed of this sailplane is above 60kts,the required limit load is about 6 g's (see Fig. 13). Theprototypes were static loaded to 5 g's to be sure thatthey could be test flown. Subsequent successful staticload tests were made up to 9 g’s.

Figure 12. Initial V-n diagram. Note high gust loads.

Stability and ControlOne of the major goals of this project was to providevastly improved handling qualities to a foot-launchedglider, and many of the SWIFT's features weredesigned to improve stability and control. The large tipchord provides additional pitch damping to increase theaircraft's dynamic stability and reduce the possibility oftumbling in extreme conditions. It also gives theelevons increased control authority especially at lowspeeds. The use of aerodynamic pitch controls (actuatedby a side-stick controller) makes it possible to trim theglider over a very large speed range without large stickforces or low stability and gives the pilot positivecontrol in very rough conditions when weight shiftwould do little good. The stalling characteristics arealso improved by the moderate taper, high effectivetwist, and vortilons – vortex generators originallyinvented in the development of the DC-9 [10]. Thefixed pilot position and relatively short vertical c.g.offset, combined with the rigid character of the wing,make the pitch stability much more linear andpredictable than is usual for hang gliders.

We experimented with versions of the airplane thatused an inboard flap as the primary pitch controldevice. This offers some interesting possibilities fortailless aircraft control, avoiding the non-minimumphase response characteristic of elevons. Theperformance advantages of a larger inboard flap weresubstantial, however, and flight tests suggested that thetransient dynamics using elevons were acceptable, sothe inboard flap was used for pitch trim and glide pathcontrol, while elevons were connected directly to thestick.

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The airplane lateral dynamics were simulated usingvortex lattice codes and both linear and nonlinear batchsimulations. Lateral stability and controlcharacteristics are affected by winglet incidence andcant angle as well as by the wing twist. These werestudied in some analytical detail by Morris [11] andwere evaluated using two radio-controlled models andtest flights of several prototypes. Of particular interestfor this configuration is the variation of effectivedihedral and yaw stiffness with angle of attack, and thelarge increase in Cnp with angle of attack that producesadverse yawing due to roll rate.

Figure 13. Radio-controlled model for S&C testing

The half-span elevons provide the large roll controlmoments that could not be achieved with weight shift.These surfaces, in combination with the fixed wingletsproduce a nicely-coupled rolling and yawing motionwithout the delay or performance loss associated withdrag rudders or spoilers. The SWIFT's winglets arefixed surfaces, not rudders, although subsequentversions of the SWIFT have been produced withmoveable rudders as well. They increase the effectivespan of the wing, but more importantly interact withthe ailerons to produce favorable yawing moments andincrease the roll control authority.

To verify the basic computed stability and trimcharacteristics, the first glider was mounted on a truck,and was instrumented with a load cell to measure totallift. The glider was free in pitch so that stick andelevon positions for trim could be measured. Theglider was also covered with yarn tufts so that wecould observe where stall first began and adjust thevortilon position if necessary. Apart from some earlyseparation associated with large flap gaps (eliminatedin the second prototype), these tests held few surprisesand flight testing alone was used to verify performanceand lateral handling qualities.

Figure 14. Prototype on vehicle for ground testing.

Flying the SWIFT

Eric Beckman and Brian Porter made the first flightsfrom a 50 foot hillside in Marin County. The elevonsmade control on the ground very easy as the wingcould be rolled easily even in the very light breezesthat day. Despite the high wing loading of the firstglider, take-off was not difficult and a few test glidesshowed that the wing was stable and controllable witha glide that was noticeably better than previous aircraft.

Flights at Mt. Tamalpais and Ft. Funston on theNorthern California coast soon followed and we learnedmore about the glider performance and controllability.The first prototype had an excellent glide at highspeeds, but with the flaps deflected at low speeds, didnot do much better than good hang gliders in terms ofminimum sink. Roll response was also not as snappyas the pilots desired, so we took advantage of a hardlanding to retire the first wing and begin work on asecond prototype. The new wing had somewhat largercontrol surfaces, revised airfoils, improved wingletsections, and a bit less wing area. The first flights atFt. Funston proved that it was a big improvement.

After 10 or so hours of flying time we were quitehappy with the design. It had been flown in relativelyturbulent conditions, out-flew all hang gliders by awide margin, and proved to be a pleasure to fly. But thegoal was extended cross-country soaring, so the nextstep was to determine the wing's performance andcontrollability, in a true cross-country area: the OwensValley. With a complete pilot fairing, radios andinstruments, oxygen system, parachute, and water, theSWIFT took off at a gross weight of over 300 lbs fromthe 10,000 ft launch at Horseshoe Meadows. Hangglider pilots had begun launching at about 10 AM, butEric waited until most others had launched; he was inthe air at 11, late by Horseshoe standards. Flying northalong the Sierras, he passed most of the conventionalgliders. "I'm only getting to 13-5," Eric heard over the

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radio. "Yeah, no one seems to be getting any higherthan that," said another pilot. Eric replied, "I'm at 15-5and I haven't been circling." Just south of Bishop, Ericand the SWIFT crossed the valley to the WhiteMountains, passing the first of the hang gliders whohad a 1 hour head start. Continuing north passedBoundary Peak, Eric began to feel hypoxic. (We laterfound an obstruction in the oxygen system.) He decidedthat he should cut the flight short because of this andbegan his final glide. When he reached Minas he wasstill at 14,000 ft and went looking for sink. Finding abit, he lowered the flaps to act as drag producing divebrakes and landed. The first Owens Valley SWIFTflight covered about 140 miles. The idea of a true foot-launched sailplane had finally come to pass.

Figure 15. Prototype flies in the Owens Valley.After these very successful prototype flights, workbegan at BrightStar on production tooling and soonbeautiful white carbon and Kevlar SWIFTS were beingproduced.

Figure 15. Production SWIFT at Fort Funston.

The SWIFT was an unprecedented success in manyrespects. Almost doubling the performance ofconventional hang gliders at the time it was introduced,it dominated several competitions and was subsequentlyallowed to compete only in a separate class. Manypilots have flown the SWIFT for 100-200 miles.BrightStar manufactured SWIFTs in the U.S. and

granted a license to the Belgian company, Aeriane, tomanufacture SWIFTs for sale in Europe. Aeriane nowoffers powered versions as well and even a 2-placeversion for training. To date about 200 gliders havebeen sold – not a large number by hang gliderstandards, but far more than many expected would bepossible. There are now dealers and SWIFT schools inat least nine countries and a new generation of rigidwing hang gliders is becoming more popular.

Figure 16. Inside a 2-place SWIFT.

Conclusions and Future Directions

Although the SWIFT project has succeeded beyondmost of our expectations, it did not fulfill all of ourhopes. First, as a business proposition, it proved againthe old rule that a good way to make a small fortune isto start with a large fortune and invest in aviation.Hand-built composite aircraft are expensive to buildand despite a price significantly higher than aluminumand Dacron hang gliders, BrightStar made little moneyon each glider sold. More fundamentally,, the SWIFT,while providing a dramatic performance improvementover conventional hang gliders, was heavier and lessconvenient. Although it could be stored in a garage, itcould not be thrown on top of a car with several othergliders. So, for many hang glider pilots the additionalexpense along with the change in social aspects of thesport were not worthwhile. The idea of foot-launchingor landing at 25 mph was quite foreign to sailplanepilots and no marketing efforts were directed to thiscommunity, so it was not as widely adopted as somehad hoped. The SWIFT has, however, rekindledinterest in ultralight sailplanes and rigid wing hanggliders. BrightStar is currently marketing theMillennium, a version of the SWIFT that folds morecompactly and costs less to produce at the expense ofsome performance, and several other companies arenow marketing hybrid rigid wing hang gliders,cantilevered wings with flexible surfaces that can be

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folded. Several new ultralight sailplane designs arenow flying or under construction.

With refinements in aerodynamic control andcomposite structures, hang gliders will continue toevolve toward more soarable foot-launched sailplanes.If one does not constrain the designs to be able to belaunched like hang gliders, much higher performanceultralight sailplanes are possible. If an acceptablyreliable, convenient, and quiet propulsion technologybecomes more practical, a self-launching very lightsailplane may finally make Lilienthal’s vision ofroutinely soaring like birds a reality.

Acknowledgements

Much of the work described here has been accomplishedby students in the Aircraft Aerodynamics and DesignGroup at Stanford and by Brian Robbins, ErikBeckman, and Brian Porter of BrightStar Gliders. Inparticular, Steve Morris, a former Ph.D. student in thedesign group at Stanford, was responsible for many ofthe analyses and studies mentioned here and is stillworking with BrightStar gliders to perfect the foot-launched sailplane concept.

References

[1] Lilienthal, O., Der Vogelflug als Grundlage derF l i e g e k u n s t . Berlin: R . GaertnersVerlagsbuchhandlung. Translated by AW Isenthal:Birdflight as the Basis of Aviation . London:Longmans, Green, and Co. 1911.

[2] Kroo, I., Beckman, E., “Development of theSWIFT: A Tailless Foot-Launched Sailplane,”Hang Gliding, Jan 1991.

[3] Gannon, R., “Winging-It,” Popular Science, pp.60-65, March 1992.

[4] Porter, B., “The SWIFT – Dominating the Pre-Worlds,” Hang Gliding, pp. 28-33, Sept. 1992.

[5] Johnson, R., Soaring, June 1979.

[6] Kroo, I., “Hang Soaring – Cross Country SoaringPerformance of Hang Gliders,” Hang GlidingMagazine, pp. 42-44, April 1982.

[7] Gyorgyfalvy D. 1960. Performance analysis of theHorten IV flying wing. Presented at the 8th OSTIVCongress. Cologne, Germany.

[8] Lippisch, A., D.L.V. German Patent Spec. No.558959, 1930.

[9] Jones, R.T., Notes on the Stability and Control ofTailless Airplanes, NACA TN837, 1941.

[10] Shevell, R.S., Fundamentals of Flight, Prentice-Hall, 1983.

[11] Morris, S.J., “Integrated aerodynamics and controlsystem design for tailless aircraft”, AIAA PAPER92-4604, Aug., 1992.

[12] Kroo, I., Tailless Aircraft Design – RecentExperiences, in Symposium on Aerodynamics andAcoustics, Fung, K.Y.ed., pp. 205-228, WorldScientific, Singapore, 1994.