BUILDING ONE-PIECECANTILEVER BOX SPARS

THESPORTPLANEBUILDER

By Antoni (Tony) BingelisEAA Designee Program Advisor

8509 Greenflint LaneAustin, Texas 78759

I.T IS FAR more difficult to find sufficient work shopspace to accommodate a one-piece cantilever wing thanit is to build one.

A single piece cantilever wing spar is more difficultto construct than a solid wood spar, and more difficultfor some builders than for others, for a number ofreasons. But don't confuse the term 'difficult' with 'im-possible'. The degree of difficulty is an intangible thatwill vary greatly from one builder to another. It's likesaying it is difficult to get up early in the morning.(How difficult, how early and on what morning?)

Some aspiring builders may be needlessly awed bythe presumed difficulty of building a 'huge' box spar anddismiss consideration of a very desirable design consid-ering it to be beyond their capability to build. Not true!Not at all! Anyone who can build a fuselage or a tail as-sembly can build a cantilever spar and the wing as well.They can, that is, provided they can find a space largeenough in which to build a one piece wing spar, andlater to assemble the entire wing.

But how much space would you really need? Most as-suredly, a work area large enough to accommodate thewing span plus a couple of feet on either end. This re-quirement would indicate that a minimum work shoplength of 30 feet is essential. (Most cantilever designshave a wing span of approximately 26 to 27 feet.) Ofcourse, if your proposed project has a span of 30 feet,you will need a few feet longer space than that.

As for the work area width, the same basic require-ment exists. Remember, the wing will have to be assem-bled after the spar is built. And although the spar maybe built in a rather confined narrow work area, the wingassembly will require a larger obstruction free areaequal to the chord of the wing plus a couple of feet oneach side of the wing. Apparently we are describing awork area the width of a single car garage but consider-ably longer . . . say 30 feet long instead of the ordinary20 to 21 feet size. Some builders have been known tobuild a temporary enclosed frame and plastic extensionto their garage for this specific purpose. Although as-sembly of the entire airplane would not be possibleunder these minimum conditions, you could alwaysmove it out under the sun or stars for fitting purposes.

There you have it. Accept reality. If you can't ar-range for as much space . . . look for some other appeal-ing design, one that features a two or three piece wing.

O.K., so you do have the minimum space and the in-centive to construct that one-piece spar but are not quiteconvinced that you should attempt to build a big thinglike a box spar all by yourself. Well, why not? Is that allthat's standing in your way? Many others have done soand were pleasantly surprised to learn it really was nodifferent than building any of the other major compo-nents.

10 DECEMBER 1980

Divide and ConquerA large box spar is easy enough to build provided

you prepare for each step and sequence its building intosmall easily accomplished tasks something like the fol-lowing.

1. Build a suitable bench or work surface.2. Layout the spar's external shape and essential

dimensions on the work surface.3. Turn this outline into a jig by securing wood

blocks along spar outlines.4. Prepare the spar laminations, scarfing if neces-

sary, to obtain proper lengths.5. Assemble upper and lower booms by clamping

and gluing.6. Remove spar booms for clean-up and planing to

dimension. Taper, if necessary.7. Reinsert booms in the jig and fit and glue in stif-

feners at each rib location.8. Prepare the spar for plywood webs by surfacing

booms and stiffeners.9. Fit and prepare plywood webs by scarfing pieces

together.10. Glue the plywood web to the spar (one side only).11. Varnish or weatherproof the inside of the spar

and innersurface of the second web.12. Arrange for an FAA inspector to inspect the spar

interior and the closing plywood web.

The bench is of the type described in Figure 1 and the text.White blocks are nylon and glue will not stick to them. Jig isalmost completed for the Falco spar. Framed object atcenter of jig is the stabilizer. (No place to put it.) Note thestack of lumber of the saw horse ear marked for the sparflanges.

Na!

Alternative To Coretructhg A New Long Bench

<L

Renew Bench Top Witt) Masonite OrPlywood Panel If Necessary

Note- Additional Bench Legs And BracesNot Shown To Improve Clarity

Any Old Workshop OrGarage Bench

String S tre tched To EstablishA Temporary Reference Line

. The Spar - Prepare A Full Size Bench Layout(Use A Steel Tape For A ccura cy)

Add S tructura l Detail* To Bench Layout(As Dimensioned On Plan*) Mark Center Line

On Each Plank Edge

Scarf SplicesdS'l)

Scarf-Splice Laminations To ObtainLengths Needed

Mate Jig Blocks AndInstall Around OuteneOf Spar Layout

FIGURE I.

CANTILEVER S F fcR LAYOUT AND PREPARATIONSFOR CONSTRUCTION

SP O RT AVIATIO N 11

Scarfing the plywood webs. The drum sander is poweredwith an old washing machine motor and is mounted off theedge of the bench saw on hinges. A turnbuckle fastens un-derneath to provide adjustment of the scarfing angle. Ithelps to have a helper too.

do not have to have to build a one-piece 30 foot long per-fectly level bench especially for the purpose, althoughthere is nothing wrong with having such a luxurious ac-commodation. You might do as well with a couple of ex-tensions to your original bench in the form of long nar-row shelf-like surfaces (about 12" wide) supported ontheir own legs and braced sufficiently to keep them fromwiggling. Butt one end of each extension against thecenter bench and its rigidity will be assured. These twowork surface extensions should be secured to the centerbench at an angle to accommodate the spar's dihedral.

A string, a couple of strategically located nails, and alarge carpenter's square are employed to establish a ref-erence line (tip to tip), and a vertical line to locate thespar's centerline. All further layout measurements canbe made from these two references.

Your plans may give the spar's dihedral in degrees,inches or millimeters, depending on the designer's pref-erence. If you have the option, it would be more accu-rate to work with inches or millimeters in establishingthe dihedral angle than it would be to work with asmall protractor in an attempt to establish the correctnumber of degrees.

Double and triple check your dimension points everyway possible. This is no place to make a mistake inmeasuring.

Next, draw the outline of the spar directly on to thebench surface with a felt pen or a soft lead pencil andlabel each station number for easy reference. Later, thismay eliminate possible confusion in the assembly of theinternal parts.

As for getting the surface of the bench level, youmight find it easier to secure 12" long blocks (like smallrailroad ties) at each rib station and level them ratherthan the entire bench which may be on an uneven floor.

Stretch a string span-wise and sight across it to lo-cate the high blocks. Use a block plane to shave thehigh spots into alignment.

Alternatively, a long 2 x 2 wood straight-edge 8 feetlong can be coated with colored chalk and rubbed acrossthe surface of the blocks to detect the high spots.

13. Glue closing plywood web to the spar completingits construction.

14. Dimension the spar by beveling its top and bot-tom surfaces as necessary.

There you have it. Hardly a frightening undertakingwhen you undertake one task at a time. But certainly, itwill take a lot of preparation and work accompanied bycopious amounts of your spare time. It could easily takea month of persistent work to complete a spar workingon it a few hours each day.

Let's dispel any remaining doubts by looking intosome of the details involved in building a cantilever boxspar . . . in one piece tip to tip.

First, A Workbench . . . But What Kind?The type of workbench (or work surface) you will

need may, in part, be influenced by the method of con-struction and assembly you have to follow. Actually, youhave but two basic choices. Build the spar flat on itsside (on the bench), or build it upright with both tipselevated to form the dihedral angle.

The type of spar (framed box) found in Jodels areeasier to construct in an upright position using hingedextensions to support the outer portions of the spar.Otherwise, an ordinary bench will do for the conven-tional laminated boom type of box spar.

If you have any kind of a bench already, you mightbe able to use it as the center of your work surface. You12 D E C EMB E R 1980

The Assembly JigNothing can be simpler than jigging the layout for

spar assembly. All it takes is an armful of pre-cut blockswhich should be screwed or nailed securely to the benchalong the outline of the spar. These blocks must be tallenough to support the full width of the boom lamina-tions. Limit the dimensional thickness of the blocks to apractical minimum because your clamps must be largeenough to reach over a jig block, the spar laminationsbeing glued and a thin back-up block.

Building your spar over those leveled miniature rail-road ties gives you a few advantages you may not havehad with similar laminating jobs. The spar, being ele-vated somewhat above the bench, won't become asmessy from the squeezed out glue. Additionally, more ofyour smaller clamps will become useable. Even thoughthey have a shallow reach, you can slip them beneaththe spar assembly and use them to clamp the spar inmore places along the bottom edge. You can even viewthe clamped and glued results from beneath the sparwith a small mirror to assure yourself that everything isO.K. Finally, if you could only obtain nylon blocks to beused in lieu of wood blocks, the glue wouldn't stick tothem and you wouldn't have to use wax paper on eachblock to keep the spar from being glued to the jig.

Often, applied individual ingenuity will aid consider-ably in devising a practical jig for aligning and gluingthe spar laminations. Keep in mind, though, that thelaminations must be easily accessible for clamping.

Scarf Joint RequirementsIt is not likely that you can get spruce or Douglas

Fir spar stock in 30 foot lengths. You will, therefore, befaced with the need for making a number of scarf splicesto obtain the wood lengths required for the longer boomlaminations. Later you will again be making scarf jointsto similarly develop the necessary length in the plywoodwebs. These are the plywood webs which will cover thefront and back of the spar enclosing the box structure.

Scarf joints in laminated spar flanges (booms) are notmade steeper than 1 to 15. That means in a flangelaminate '/£ inch thick the length of the tapered jointshould be at least 7.5 inches long. (Plywood scarfs mustnot be steeper than 1 in 12, therefore, a 1/8" plywoodsheet would have a joint l'/i>" wide.)

Pay attention to the grain of both pieces which are tobe scarf joined. If there is any grain run-out in a board,its slope should be in the direction of the grain tominimize a cross-grain effect which would result in aweaker joint. Remember, butt joints are very weak, anda glued joint's strength is proportional to the grainangle. The strongest joints result when the grain in bothpieces runs parallel to the joint line.

According to old governmental guidelines, ANC-18,Design of Wood Aircraft Structures, for example, theminimum permissible longitudinal spacing of scarf jointsin adjacent laminations must not be less than 10 timesthe thickness of the thicker piece. Actually, it would bebetter to keep splices as far apart as is practical in adja-cent layers . . . especially in spar flanges. The number ofsplices in each layer is immaterial although logic andone's own addiction to the law of conservation of energywould serve to keep these to an absolute minimum. Donot locate scarf joints in critical areas such as fuselageand landing gear attachment points.

You might also note the fact that the further out-board a scarf joint is extended the lower the loads towhich it will be subjected. By the way, nails are neverused in a scarf joint intended for use in aircraft.

Laminating Spar Booms (Flanges)Most aircraft designs have some dihedral (usually

about 5 degrees) built into cantilever wings. As a result,when building a box spar, it is necessary to laminatethin layers of wood together to form the curved upperand lower booms for the spar. If no dihedral were re-quired then it would be possible to cut and shape theboom out of a solid piece of wood. When Douglas Fir isused, however, this might not be too good an idea as theFir often has buried pitch pockets which could weakenthe part. Laminating spars of thin strips, be they SitkaSpruce or Douglas Fir, minimizes this risk. Actually,any laminated unit usually results in a more uniformstructure free of hidden flaws.

Spar flanges are typically laminated of thin boardsapproximately 3/8" to '/£" thick because they can beflexed easily to obtain the necessary bend at the spar'scenter.

If two or more laminates are ripped from the sameboard, turn the alternate piece end-for-end before gluingthem to each other. This will guard against the possibil-ity that a localized defect or weak area in the wood willbe glued back into its original location.

Stack the pre-cut laminations for the upper boomand mark the spar's centerline on each. Number eachlamination at each end and at the centerline. Do thesame for the bottom lamination and you are ready to doa lot of gluing and clamping.

More next month.

Here's the objective. A completed cantilever box spar fresh out of the jig which was gleefully destroyed. Topand bottom flanges have yet to be beveled. Yes, it is a Falco spar.

SPORT AVIATION 13

BUILDING ONE-PIECE

CANTILEVER BOX SPARS

Part II

THE

S P O R T PLA N EBUILD E R

Bv Antoni I Tony t BingelisEAA Designee Program Advisor

8509 Greenfiint LaneAustin, Texas 78759

GLUE JOINT once assembled, as in a laminatedspar boom for example, must be assumed to be safelybonded. There is no practical way to test it withoutdestroying what you have done. Anyway, if you destroyevery joint you make to assure yourself it was a goodone, you will never finish the project.

Because you do not have a way of testing each jointfor its ultimate strength, you must be sure that thepreparation of each joint, and the gluing and clampingoperations are handled with care in order to obtain agood glue line.

One indication of a good glue line, of course, is thevisual evidence of glue squeezing out. However, thatmerely means the mating surfaces are generously coatedwith glue, but it does not indicate the condition of theglue mixture or that sufficient clamping pressure isbeing imposed.

Epoxy characteristically produces a better jointwithout heavy clamping as uniform contact of bothsurfaces with the glue is sufficient. However, if youare using a resorcinol glue, a clamping pressure of ap-proximately 150 psi is essential to produce a thin glueline and a safe joint.

A safe joint? You can ease your mind in this regardby making a few scrap wood test samples for each batchof glue mixed. These samples can be labeled and datedand, later, tested to destruction. If the wood fails ratherthan the joint, you may reasonably assume that theresults are a fair indicator of the quality of the jointsin the structure completed.

The alternative to making periodic test sampleswould be not to make test samples and assume that

because the glue joint was assembled while the condi-tion of the glue mixture was ideal, and because the tem-perature and the clamping pressures were adequateand evenly applied, and because this was done withinthe glue's working life . . . the joint must be good.

If, on the other hand, one or more of the assump-tions cited was less than ideal, it may now give you causeto wonder. The option is yours, amigo.

Laminating the Booms

They say the simple job often gives the most trouble.But since nobody says building a one-piece cantileverspar is simple, such an undertaking must not be as dif-ficult as is generally assumed. (How's that for reasoning?)

Actually, there is a degree of validity to that sortof reasoning. Particularly for the builder who has com-pleted the necessary preparations. He should find thatthe assembly of his spar will proceed with highly satis-factory and predictable results. But, let's get into a fewspecifics.

Before you get started with your largest major glu-ing effort be sure that you are really prepared.

Have you enough large clamps? Are they placed inthe approximate area where they will be used, and arethey opened to the approximate fit? How about back-up blocks for the clamps . . . do you have plenty? How'sthe air temperature . . . high enough for the glue beingused? Mixing cups, sticks and rags handily located?What about the jig? Look it over again. Will it be pro-tected from the oozed glue with wax paper or some othernon-stick provision? Are the centerlines marked on each

A one man operation requires application of a bit of ingenuity.Completed bottom spar boom Is clamped to the bench sawfor tapering. Opposite end is supported by rope from thegarage door track. Note jig table in the background.34 JANUARY 1981

Here is the opposite view of the bottom boom. The plan-form taper In the boom was cut on the bandsaw, feeding thatlong laminated thing thru the open window.

Making the blocks (stiffeners, bulkheads, dividers) that makeup the spar's interior. Some large blocks must be laminated.Large hole in this one is for the gear leg. To assure its align-ment the wood plug was used during gluing and clamping.Don't forget to pull it out immediately . . .

lamina (that's right — and the plural is laminae — Ilooked it up) on all four sides and arranged in propersequence for spreading the glue and quick assembly?

When you think about it, the building of a spar con-sists of a series of separate gluing sessions, each ofwhich can probably be completed in less than an hour.Actually, the spreading of the glue will probably takethe longest time in the process. (This is where a littlehelp from a friend could give you a real boost.)

Glues have a certain working life at a certain tem-perature. You must spread the glue and mate the pieceswithin this time limit to be assured of a good bond.

Often, the final readjustment of the clamps, a search-ing inspection of the glue line and cleanup takes longerthan the gluing process.

The two largest and perhaps most important of thegluing jobs in building a spar are definitely the lamina-tion of the upper boom and the lower boom. Don't starteither gluing job unless you have plenty of time.

Give the family instructions that you not be inter-rupted with phone calls and requests to come see thecute things the kitty-cat is doing, etc. . . . then makeone more pit stop and get to work.

It is necessary that the two full span laminae aswell as any other long ones be scarf joined to their re-quired lengths before undertaking the gluing assemblyof a boom. One concern in this regard is to maintain thealignment necessary in making the scarf joint. A slightmisalignment in joining the two pieces and the problemcreated will be difficult to overcome unless the widthof the boards allows plenty of surplus material for trim-ming. You can, for example, bracket the boards at eachend with headless nails to hold the correct alignmentwhile gluing and clamping the scarf joint.

It is well worth the time and effort required to layout the whole boom assembly once (without glue) fora trial run.

A complete boom can be assembled and glued in asingle work session and the other boom, the following day.Or you can glue a couple layers at a time, taking severa1

days to make one boom. Most builders prefer to go atit all at one time. The choice is yours. It should haveno effect on the quality of the completed spar, however,by spreading out the process much more work is in-volved. Instead of installing and removing back-up blocksonce, you will be doing the same thing several times.The recommendation, therefore, is to make one boomat a time if you can . . . progress will be speeded con-siderably.

With the exception of Aerolite, most glues lend them-selves to double spreading. That is, both contact sur-faces are coated with glue and clamping pressure isrelied on to squeeze out the excess. This is a far betterpractice than risking a starved joint when only onesurface is coated.

Coat each lamina with glue and stack it in the cor-rect sequence for assembly. It won't take you long tobecome aware of the fact that you have a lot of glue tospread and cannot afford to dilly-dally in applying it.An assistant could be very useful in speeding the glueapplication. A roller or squeegee may work well withsome types of adhesives but a thin flat wood stick willdo as well once you develop the knack for spreadingthe glue with it. Strive for an even coating. As soonas two planks (laminae) are put together, rub themagainst each other slightly to smooth the glue line andto work out entrapped air.

The multi-layered boom is now ready for the jig.Clamp it securely against the outside profile formed bythe jig blocks.

Be sure to align your reference lines and marksaccurately before tightening the clamps.

After an initial light clamping, you might begin atthe center of the spar and retighten each clamp slightlybecause the glue will have leveled, and some will havesqueezed out somewhat, reducing the initial appliedpressure . . . don't overdo it though. Check to see thateverything is right and that the laminae are not fannedor twisted toward the tips. Position each clamp so itexerts pressure along the center of the lamination, notnear one edge. Use clamps large enough to reach to thecenter area, otherwise the spar boom will be clampedlopsidedly.

After the glue has cured, remove the boom from thejig and set it aside until you have assembled the secondboom.

Allow each completed boom to set overnight (in thejig) before removing it. At least 24 hours must lapsebefore you undertake to clean up the booms. Then trimthem to size.

The completed booms will have a long graceful step-tapered appearance due to the greater number of laminaeused for the beefier inboard areas of the spar boom.These "steps" must be cut down to a uniform weight-saving-taper to eliminate stress points in the boomlaminations.

If the boom must be tapered in planform (as viewedfrom above) as well, mark the taper and cut it down ona bandsaw at this time. You will need a helper for this.

The second landing gear block is now fitted and all that re-mains to be done is to cut the 30 degree taper in each end.Blocks and stiffeners are now ready for gluing into the jiggedbooms.

SPORT AVIATION 35

(T) LAMINAT E UPPER (TOP) BOOM A G AINST UPPER JIG BLOCKS

2. LAMIN A T E B O TT OM BO OM A G AIN S T LOWER JIG BLO C KSNOTE-

C LAMP S AND WORKBENCH DETAILSOMITT E D F OR C LA RIT Y

(y)T A P E R B O TH BO OMS IN PLANF ORM IF RE QUIRED T O P , VIEW [

TAP E R BOTH BO OMS IN I PROFILE,

B O TT OM BOOM

R EIN F O R C E DPLYW O O DS TIF F E N E R

WO OD STIF F E N E RS(WITH LIG HT E NIN G H O L E S)

T A P E R BLOCKE N D S 30°

R EIN F O R C E DSTIF F ENER (BULKH E AD)

4 .) MAK E AND FIT SPAR STIF F ENERS AND BLO CKS

IN S E R T BO OMS IN SmR JIGGLUE IN BLO C KS AND STIF F E N E RSV A R NIS H (P O LY U R E T H A N E) ALL INT ERIO R A R E A S

(e j P R E P A R E AND INSTALLPLYW O O D W E BS

S C A R F JOINTS12 I MINIMUM)-

SCARF JOINT M E T H O D SHOW TO HANDLE REDUCTIONS IN

PLYWO OD WEB THICKNESS

Sf*R" WEB

ME TH O D (A) B E TT E R7

METHOD (5) O . K .

FIGURE I. SWR DETAILS '•'DIRECTIONOF BUCKLES

DIR E C TIO N OF PLYWO ODRA C E G R AIN

I would discourage attempting to do this by yourself.You might manage it alone, but why risk it? If yourworkshop space is limited, you can feed the boom intothe bandsaw and through an open window, otherwiseyou may have to take everything outside.

Finish the spar booms to dimension. Work slowlyand do not remove more wood than the plans dictate.A sharp plane will do the best job.

Re-mark the spar boom centerline and other refer-ences as each side is surfaced.36 JANUARY 1981

The completed booms are now ready for the returnto the jig. Clamp them securely against the outside pro-file formed by the jig blocks.

Internal Blocking and S tiffeners

Next you will have to make and install a numberof stiffeners (bulkheads or dividers). Their immediatepurpose is to precisely separate the spar booms to theexternal dimensions required for the completed spar.

Oh, happy day! The closing plywood web has just been gluedand Is being clamped using everything but the kitchen sinkas a weight . . . bags full of bolts, staplers, hammer, blockplane, drill guide, and plastic bags filled with sand. A fewclamps too.

After the clamping debris has been removed and the edgesof the spar cleaned of squeezed out glue its true form be-comes recognizable. The light areas on the spar have beenlightly sanded where ribs will be glued. The top and bottomof the spar must still be beveled to match the rib airfoil section.

However, in addition to separating the booms (flanges),these stiffeners have the function of improving the shearstrength of the spar's plywood webs and providing areinforced area for the attachment of the wing ribs.Bulkheads or stiffeners are located at each rib position.

Some stiffeners are made from solid spruce stockwhile some are made of plywood with stiffeners gluedto each side. They usually, but not always, fit acrossthe full width of the spar booms.

These bulkheads must be fitted accurately to insurethat the completed dimensions of the spar are exact.

Almost all stiffeners have lightening holes whichalso serve to interconnect the different compartments.This provides a means of equalizing atmospheric pres-sure with altitude changes . . . not a serious concernfor most homebuilts as they do not normally operateat high altitudes. Stiffeners, without lightening holes,should have small W vent holes drilled at the sparcenterline between the rib stations.

Glue the bulkheads in accurately because the wingribs will later be attached to these reinforced locations.

Finally, after double checking that all the blockingand bulkheads are well glued where they should be,weatherproof the entire spar interior with at least twocoats of polyurethane exterior grade glossy varnish.Brush it on. Spraying is not recommended because pene-tration will be poor.

Some builders prefer to use an epoxy such as T-88diluted approximately 25% with lacquer thinner. It isbrushed on in much the same manner as varnish. Thedifference, however, is that the mixture has the advan-tage of solidifying all cracks, gaps and crevices therebyeliminating possible trouble areas where moisture mightsettle.

The Plywood Webs

Prepare the plywood webs (skins) by scarfing thenecessary pieces together to get the length needed tocover the entire spar. It will be necessary to scarf threeor four pieces of plywood together for larger aircraft.Plywood skins are usually reduced in thickness towardthe tips. You will, therefore, have to scarf a thinnersheet to a thicker one in some locations. See Figure 1to see how this is accomplished. The scarf joints arenormally made with a 12 to 1 (minimum) bevel.

In earlier years, spars were built with the plywoodweb grain running in just about any direction the builderor designer took a fancy to. Many designs had squarelaid plywood while others were glued at a 45 degreeangle. As the popularity of using diagonal laid plywoodincreased, some discussion arose regarding the direc-tion of the face grain in the 45 degree web. It appearsthat a number of manufacturers as well as builders, havebuilt spars with the 45 degree surface grain running ineither direction. However, along comes the U. S. ForestProducts Laboratory, with the conclusion that the ply-wood, being stiffer in bending along the face grain,should offer greater resistance to buckling when theweb is laid with the face grain running across thebuckles. How much stronger a spar is with the webgrain laid 45 degrees according to this guidance (in-stead of being vertical or slanted in the opposite direc-tion) has never been made generally available to myknowledge.

Current practice does seem to favor laying spar ply-wood diagonally so its face grain is at right angles tothe direction of shear buckles . . . but refer to yourplans and lay the plywood web grain as indicated bythe designer. If he has not specified anything morethan "laid 45 degrees", the foregoing explanation maybe useful.

Avoid making scarf joints at landing gear attach-ment points and other highly localized stress points.

After scarfing the webs, clean them and mark thelocation of the internal structure on both sides of theplywood webs. The inside portion of the plywood websthat will not receive glue must then be weather proofedin the same manner as the spar's interior.

You are now ready to request a FAA inspection toobtain the O.K. to close the structure.

Closing the spar permanently with the long sloppyplywood web will again require the use of many clamps,possibly weights and sandbags, to obtain the desiredpressure on the glue line.

After allowing the glue to cure for 24 hours, yourfinal clean-up of the glue line may be followed by thebeveling of the top and bottom sides to make the sparconform to the airfoil section.

Do not varnish any of the spar's exterior surfacesuntil after the wing has been assembled because youwill still have plenty of gluing jobs ahead.

Stand back and take a look at your completed spar.Isn't it a remarkable sight?

SPORT AVIATION 37