7/27/2019 1961_08_10

1/8

Geodetic Aircraft StructureBy Keith D. Powell, EAA 1939

T he "Player" homebuilt sport plane is in its twenty-first year of f lying, has appeared at three Nat ionalEA A Fly-Ins an d copped first place fo r spot landing inth e 1960 f l ight events. Innumerable pilots ( inc ludingthe author) have tasted the thrill of their first flightin command of a homebuil t aircraf t , and have been al-lowed to join Earl Player in m an y enjoyable hours oft rouble free ai r t ime in this durab le product of a "Pio-neer" homebuilders skill brought into being for thefabulously low sum of $500.00 cash outlay.All this is leading up to the reason fo r th is article.To provide information on construction principles andto i l lustrate th e value of wooden geodetic s t ructure fo rth e amateur . As a practical approach to solving th eaverage income homebui lders biggest problem, funds over

an d above living expenses, the economy and proven qual-ities of the geodetic diamond mesh s t ructure has longbeen overlooked or jus t not k n o w n about by the growinggang of present day enthusiasts . This article was pre-pared to help in the latter respect.We are indeed grateful to Messr's. Player and Thai-m an fo r their invaluab le assistance in prepar ing thisarticle. Without them it could not have been written.Ou r present day organizat ion owes it s very existence toth e efforts of such stalwart pioneers as Bogardus, Yates,Long, Ruper t , W i t t m a n , T h a l m an , Player and m an y othersof the early restricted era. Knowing just two of themhas been a rich exper ience an d if EAA ever inaugura tesa Hall of Fame they deserve proper recognition.To give the reader a brief history of our subject,let's see what others have done. The British World WarII Vickers Armstrong "Wellington" bomber was a wellk n o w n exponent of geodetic structure. The Wellingtonwas famed for its load lif ting capacity and durabilityThe metal riveted and bolted mesh s t ructure could bepeppered with flak and cannon shell holes an d still hangtogether.Geodetic aircraf t s t ructure was used in the U. S. bythe "father of geodetic homebuilts" , George Yates ofBeaverton, Oreg., as early as 1927. His first ship, the"Stiper", was constructed in 1930 using ',4 in . diametersteel tubing welded at each crossover of th e geodeticmesh. It was still f lying in 1938. The "Stiper" was atwo-seat tandem parasol. Later effor ts during the 1930's

included the midwing Salmson powered, single-placeOregon "0", several low-wingers and a maximum ef fo r tin design dur ing the period was the building of 2 low-wingtwin engine ships powered by 40 hp Continentals (Fig. 1).

The "Stiper" was the only Yates design using metalstructure. A convert to the wooden materials qualit iesof l ightness and strength, ease of fabr icat ion an d econo-my, all the rest were constructed of wood an d glue usingmetals only at vi tal stress points; engine mounts , f i t t ings ,l and ing gears, etc. M r. Yates developed geodetic for notonly th e fuselage structure but used it in the en t i re air-frame; wings, fins and control surfaces. Some Yateswings were spar-less using light internal members only toform th e geodetic latt ice airfoil. Photos of some uncover-ed Yates wings show the spar to be built up truss of lightspruce indicating the geodetic carried the major portionof th e fl ight loads.

An evident takeoff of the Yates low-wing geodeticdesign, "The Plxweve CT-6" two-seat tandem trainer ap -peared in 1941 an d according to specifications performedwell on a 75 hp Cont inen ta l (Fig. 2) . Other develop-

Fig. 1. Yates Twin.

Fig. 2. Plxweve C T-6ments by Mr. Yates and the fate of these unique aircraf tis unknown to the au thor and additional information bysomeone closer to their development and use would bewelcomed.

In the mid 1930s information on Mr. Yates' systemintroduced Earl Player and Harry Tha lman of Salt LakeCity to its promising features and both were soon en -tangled in their own separate backyard "basket weave"projects. The "Player" fuselage w as assembled in thealley behind Earl's home incidentally. How "backyard"can you get? Anyway the oft used excuse, "I don't havea place to bui ld" doesn't seem to stop some hardy persons.Earl's ship grew from th e applicat ion of several de-signs p o p u l a r d u r i n g the thirties. The wing was bui l t fromplans of the Long "Longster" appearing in an early"Mechanix Illustrated Flying Manual ." The tail planeswere modified from a cracked "Curtiss Jr." and thefuselage of pr ime interest to us, is his own design inwooden geodetic (see Fig. 3 cu t aw ay ) .As an i l lustrat ion of how the amateur bui lder an d

designer ca n resolve aerodynamic layout problems byContinued on next pageSPORT AVIATION 17

7/27/2019 1961_08_10

2/8

Fig. 3. Wm. Player's "Player" Sport. The "Player" is a single-place ship with half para-sol and half shoulder wing, geodetic fuselage. Designed and built by Wm. E.Player of Salt Lake City, Utah- Aptly named, the ship is a frisky, fun to fly sportster.1. Aluminum cowling - 65 hp Continental.2. Conventional strut braced wood wing, Clark "Y", fa-bric covered.3. Plywood cockpit framing. Originally an open job,the sliding canopy is shown in the open position.4. Wire braced tubular steel tailplanes.5. Pietenpol tailwheel.

6. All wood geodetic fuselage- Plywood former rings de-creasing in thickness from nose to tail, (i.e.) 1 in. fire-wall, 3/4, %, etc. Four % sq. longerons, bucket pilot seat,other internal details conventional. Fabric over longi-tudinal fairing strips.7. Cub type, streamline tubing landing gear.

using a proven designs features, leaving the headachesfor non-conformists who want something "way-out"; Earladapted the "Corben Super Ace" general layout, basicfuselage dimensions and station locations in the designof his airplane. The wing placement and other layoutdetails also are the same as the "Super Ace".

The "Player" was test hopped in 1940and exceptfor a forced four year storage period during World WarI I, has been active ever since; culminating her existenceby bringing home the bacon from the 1960EA A nationals.The Thalman midwing took to the air in 1941andthrough its outstanding performance, demonstrated herdesigner-buiders self taught engineering prowess andthe benefits of wooden geodetic. A single-seater power-

ed by the five-cylinder Velie 55 hp engine, Harry's brain-child clipped along at 130 mph top speed, 120 cruise andlanded on high elevation air strips at 38 mph. Take offand climb (1500 fpm) were fabulous for the low horse-power due primarily to the tapered 41 ft. span, high as-pect ratio sailplane like wing employed. See Fig. 4 -three view.This wing, as did the rest of the airframe, incorpor-ated the diamond mesh, glued spruce strips. A box sparof full span and the geodetic monocoque made the wingfully cantilever and clean. The entire design displayedThalmans' devotion to aerodynamic cleanliness. Thefuselage carried thru the radial engine's circular cowlingcross section ending in a pointed tail-cone. In contrast

to most homebuilders who want just a sportplane fea-turing proven qualities for Sunday flying, M r. Thalman

went out to achieve a flying machine of superior per-formance with an original design.A true experimenter and progressive backyard en-gineer, Harry tried two different empennage designs andseveral minor modifications for performance improve-ment on this ship. The original configuration mountedthe stab and elevators on the fuselage center line (alsothrust line). Final configuration changed the ThalmanT-3B to a "T" tail with the horizontal surfaces mountedon the vertical fin tip. The smoother air flow over thehorizontal surfaces gave better in-flight performance butcaused a loss of control effectiveness during take-off.The original configuration with the elevators in the propblast was found best; at least for a ship with low stallspeed and the desired short field, high cruise performancequalities sought and achieved in the Thalman design.

Having proven his basic theories, Harry began con-struction of his second midwing in 1946. This ship wasto be the ultimate in aerodynamic efficiency, a func-tional, economical mode of transportation for four per-sons. One which would enlarge on the hi-speed, versuslow-speed compromise block that has faced the airplanedesigner since the Wright Brothers first took wing.Using the same basic design as the T-3B with thesailplane like wing mounted just above the thrust line,Harry incorporated a manually retractable tri-cycle land-ing gear and among other innovations, concentrated onthe elimination of an ever present turbulence and drag

producing feature of the conventional airplane. Thecontinued on page 2218 AUGUST 1961

7/27/2019 1961_08_10

3/8

-CREAM, -BCD-WINSFig. 4. NX28374 Thalman T3-B

SPORT AVATION 19

7/27/2019 1961_08_10

4/8

7/27/2019 1961_08_10

5/8

Thalman's T-3 and T-4

Photo from Roy Millard CollectionThalman and T-3B in flight.

Photo from K D. Powell CollectionThalman T-4, 4-place midwing, all-wood geo-detic with latest configuration, 270 hp Lycoming.

Photo from Roy Millard CollectionThalman T-3B Note forward sliding canopy a la ArtChester style, Racey look and fabulous high altitudeperformance.

Harry ThalmanThalman T-3B

Photo from Roy Millard Collectionset to go in his homebuiltall-wood geodetic midwing.

Photo from Roy Millard CollectionHarry Thalman and the "Thalman T-3" in original con-figuration, before "T" tail wheel pants, etc., all silver.

Photo from Roy Millard CollectionThalman T-4 interior of fuselage from rear seat aft.

Photo by K. D. PowellThalman T-4 geodetic fuse construction details.SPORT AVIATION 21

7/27/2019 1961_08_10

6/8

7/27/2019 1961_08_10

7/8

form the curved surfaces of the component under con-struction, little imagination is needed to relate this tothe mathematical definition of a geodesic line, (i.e.)The shortest line lying on a given surface and connect-ing two given points. Here the member (geodeticstrip) actually replaces a line."From the above we can see how our structure, whichtransfers loads from member to member on a criss-cross"great circle" route, derived its name.To most, the engineering theory and mechanicalfunction of the geodetic form seems mysterious andcomplicated. Fortunately this is not true but is in factthe essence of simplicity and directly related to a wellknown structure. The following analysis by Chapter58's chief Aeronautical Engineer, Lt. Rod Huggelman,sheds the cloak of mystery. Lt. Huggelman says, quote:"Nature has endowed the insect with a most excellentstructure. The lowly ant for example has an exoskeletalstructure of amazing lightness and strength. This closedshell structure is called monocoque in the field of air-craft structures. I t has probably the highest strength toweight ratio and is widely used today in aircraft and

missiles.Any homebuilder who has tried to cover compoundcurves with a sheet of plywood has already had experi-ence with one of its primary short comings. Anotherdisadvantage is often its extreme rigidity. Rigidity isgenerally an asset but such structures are not normallyable to take high shock loading concentrated in a smallarea since they cannot easily distribute the stress. Anegg shell for example can take amazing loads properlyapplied while a sharp blow with a pointed object willeasily crack it.Often it is possible to compromise ultimate strengthand rigidity in monocoque structures by perforating the

shell with holes. The flexibility thus provided will en-able the structure to better handle shock and impactloads. By increasing the shell thickness we can restorethe ultimate strength of the structure while still main-taining some desirable flexibility, although at a slightweight penalty.The geodetic or "basket weave" structure is simplyan extension of the perforated monocoque structure.However, it is much less expensive, simply constructedand unrestricted by compound curves." Unquote.The last sentence should be very appealing to theamateur.The geodetic wooden aircraft structure as used inthe Y ates and Thalman aircraft undoubtedly reacheda high point in perfection and have contributed proofof service durability and performance.Let's check some features.Weight - A light airframe allows greater pay loadand/or more speed, better climb, etc. with lower horse-power and consequently increased economy. (Ask SteveWittman about this. No formula was more successful.)In this respect, geodetic will go all-out.The average basic geodetic fuselage should notweigh over 40 pounds. Y ates built a cantilever wingpanel that weighed only 24 Ibs. Strength to weight

features are evident and undoubtedly better than mostconventional light aircraft structures.

Strength - Before starting design or constructionof wooden geodetic, or any other wood aircraftstructure, a familiarization study should be madeof wood materials and their application. Severaltexts are available including Manual 18 and es-pecially recommended are the Munitions BoardAircraft CommitteeBulletins ANC-18, "Design ofWood Aircraft Structures", and ANC-19, "WoodAircraft Inspection and Fabrication," availablefrom Aero Publishers.Chapter 3 of ANC-18 entitled "Methods of StructuralAnalysis" contains a comprehensive presentation onengineering data for wood aircraft structures includingthe monocoque and semi-monocoque plywood stressedskin structure. The application of stress analysis anddesign features of the monocoque shell are the basisfor geodetic airframe engineering as used and recom-mended by geodetic exponents.The book "AirplaneDesign" by K . D. Wood, 6th edi-tion, 1941 offers our only known information in stressanalysis and preliminary design for geodetic structures.The book gives examples and mathematical equa-tions for suggested geodetic engineering and is recom-

mended for reference here. In his book Mr. Wood statesthat published data for design or stress analysis of geo-detic structures did not appear to be available at thattime. This seems to be the case at this late date also.The following recommendations are taken from his book:Quote: (1) "To arrive at preliminary design andstress analysis it is probably conservative to design anequivalent monocoque fuselage and then select geodeticmembers of such size and spacing as to make the latticecage have the same weight as the monocoque skin.This procedure has been used at Purdue University witha resulting margin of safety in excess of 50% for plywoodconstruction.(2) For structural analysis, a lattice cage fuselagemay be regarded as a series of triangular frames withimaginary bulkheads and pin joints at all intersections.In such a framework the load which can be carried byone of the compression diagonals determines the strengthof the structure in torsion and bending." Unquote.For Mr. Thalman's explanation of the mechanicalprincipals he references a tube, likening it to a fuselage.Imagine the tube without any internal bracing and madeup of one set of equally spaced strips spiraled only oneway around its diameter and length. If you twisted thetube in the direction of the spirals it would decreasein diameter. Twisting it the opposite direction causes itto enlarge in diameter. Now imagine a second layer

of strips wound in the opposite direction over the firstto form the diamond mesh lattice. Now twisting ineither direction causes an opposing reaction and thetube is rigid and strong.Monocoque, in nature, the structure is a strong, com-pact, torsion resistant component braced in all direc-tions, yet strange as it may seem, it is also elastic innature, shock and engine vibrations are effectivelydampened. Thalman says the fuselage he builds could betwisted one quarter turn before failing. This elasticityis a prime strength feature. Standard airframe struc-tures may receive a peak stress amount and will "give"very little before failure. Under the same energy, thegeodetic would "give" more and not reach the breakingpoint. We hasten to add that this apparent "limberness"continued on next page

SPORT AVIATION 23

7/27/2019 1961_08_10

8/8

GEODETIC AIRCRAFT STRUCTURE . . .Continued jrompreceding pagedoesn't mean the wings flap or the tail shakes however.Ever see the undulations of the "Helio Courier" tailduring taxiing or the "T-Crafts" shuddering when theengine is started? None of this is apparent in our struc-ture.

Safety - This same elasticity affords a structurewith the progressive failure features so necessaryto safety in a crash. For those fearful of the us-ual splinter hazards associated with wooden air-craft crackups, the springy strips tend to bendand break outward eliminating the occupantspearing danger.Both M r. Player and Mr. Thalman have experiencedaccidents causing damage and verify the damage re-sistant features and another inherent valuable trait ofour subject. Ease and economy of repair. Damage isusually slight and the splicing in of a few spruce stripsis much easier than the usual procedures with sheet metalor tubing when an undercarriage is damaged or a wingtip hooks in a snow bank on take off run, slammingthe ship to a stop in the snap of a finger.Durability Trapped moisture in an airplanestructure can raise havoc with sometimes irre-pairable damage resulting. In steel-rust, alu-minum-corrosion, and wood-rot, glue joint sep-aration, etc. Since our structure is of wood it'svery gratifying to know that the physical makeupof the diamond mesh eliminates any possibilityfor trapping moisture. No trouble should be ex-perienced except in an unusual case in par-ticular design or partial use of conventionalwooden gusseted structure (i.e. Jodel. Pietenpol,etc.).The "Player" was recovered after fourteen years

of outside exposure in 1958 and the only spot showingdeterioration was the lower portion of the bulkheadat the tail wheel where water from the entire fuselageinterior drains to. The drain hole evidently had pluggedwith mud or was slightly misplaced.To be concluded in the August issue watch for theconstruction tips to be included.

AUTHOR'S NOTE: The general nature of this presenta-tion required brevity. Therefore much was left out to keepthe article "magazine" size. One important part not cov-ered was the requirement for GOOD glue joints at eachcrossover of the geodetic mesh strips and former to geo-detic cage junctures. Good glue joints are important inany wood airplane structure but they are particularly sowith geodetic since they absorb or transfer the compres-sion stresses.

Not much is known nor can it be put down as exactfact in engineering formulas for geodetic or for that mat-ter monocque design of any kind.Thalman proved his structure by static loading thewing, lever twisting the fuselage section and FLIGHTTESTING (about 600,000 miles on the T-4). Thalman alsostates that he doubts that there is anyone who can ac-curately stress analyze geodetic construction "on paper".Recent information points out that ANC-18 and 19Bulletins and K . D. Woods' "Airplane Design" are out ofprint. However there are rumors that the A NC Bulletinsare going to be published in sectional form and that

24 AUGUST 1961

Keith D. Powell"Airplane Design" will be reprinted some time this fallin a 1961 version.Further information indicates that it will be availablefrom the University of Colorado's campus book store"On Campus", Boulder, Colorado. Perhaps sufficientinquiries from EAAers would hasten the publication ofthese valuable texts. Keith D. Powell

Welding DemonstrationThe monthly meetings of Detroit Chapter #13usually include a practical demonstration of some phaseof homebuilding srt for the benefit of the more inex-perienced members. Here we see Phil Austin, Head Weld-ing Instructor of the Detroit Board of Education fromTrombly Trade School, showing how to weld aircrafttubing. This meeting was held at the H & S PropellerShop, 25210 Ryan Road, Warren, Michigan. Mr. Stanleykindly offered their facilities for this meeting, and mem-bers also had a chance to tour the shop where equipmentis available to handle anything from the simplest lightplane props to huge turboprops.

Photo by Robert F. PouleyPhil Austin, Head Welding Instructor of the DetroitBoard of Education from T r o m b l y Trade School.