Steve Higginson has redrawn the plans for the Spirit of Zaic (Revision 2) having instituted a few minor changes that have proved beneficial from flight testing and regular flying (mostly with regard to towing). You can download the revised plan and accompanying text here
Revision 2. Steve Higginson has redrawn the plans for the Spirit of Zaic (AeroModeller 924 Nov/Dec 2013) having instituted a few minor changes that have proved beneficial from flight testing and regular flying (mostly with regard to towing). For explanatory photographs, please see the original article.

Thousands upon thousands of aeromodellers dreams of flight and the beauty of its art began with the designs of Frank Zaic. For so much that has been given, in some small way I can pay homage to The Master through the …

Spirit of Zaic FF Plan REV 2

The Spirit of Zaic

I believe there are very few aeromodellers out there who have not been influenced in some way by Frank Zaic. Frank and his brother John formed the Junior Aeronautical Supply Company, JASCO as it would become to be known, way back in 1933 as a way to supply the blossoming aeromodelling community in New York City with much sought after building materials.

During the 30s and post-war 40s, Frank and John, with the help of sister Christine would inspire JASCO to produce some of the most iconic glider designs in aeromodelling history. Through kits and Frank’s Junior Aeronautical Yearbook series of publications, the Zaics became the go-to source for quality products and accurate technical information. In fact, to this day, the Junior Aeronautical Yearbook series and other technical books like Glider Design and Circular Airflow are considered to be fountains of technical and reference material.

I have always been drawn to the purity and artistry of Frank and John’s designs. Pod and boom gliders, the beauty of the bird-like Wolf wing and those beautifully drawn plans with isometric views, have always spoken to me. As tribute to the Zaics and their legacy, The Spirit of Zaic is a compilation of design, construction, theory and science drawn from many Zaic sources to produce a compact 36-inch hi-start glider with the ability to be flown as a free flight or micro 2-channel radio assisted model.

My personal preference is for F/F, but the design was conceived with today’s R/C technology in mind and is easily adapted. Also, you will find the use of carbon-fibre and design features borrowed R/C sailplanes to affect the tail plane/DT. So, with that being said, let’s get started.

 

CONSTRUCTION

 

NOTE: The Spirit is intended as a project for mid to advanced level builders. Although the methods and materials for the build are standard and readily available, some phases of construction are complex and involved.

FUSELAGE

The fuselage is constructed very much in the Zaic tradition, with the exception of the boom, which is now of box construction utilizing square carbon-fibre pultrusion in place of the solid hardwood material so prevalent in many Zaic designs. The starting point is with the boom sides. I used 1/32˝ plywood on the prototype, but in hindsight, hard 1/32˝ balsa sheet would work fine and save a couple of grams of weight in the process. You may also notice (if you are familiar with high-performance R/C sailplanes) that the Spirit utilizes what is called a “flying tail plane” for pitch control or for the D/T mechanism.

The sides are mirror images of each other with the left side having a small service access panel under the tail plane. Its reason will become clear as we progress. Also, the holes for the D/T and auto rudder cables are specific for right and left sides. For the R/C version, the 2 holes on the left side just behind the wing T.E. are not necessary. Tape the two sides together and drill a 1/8˝ dia. hole for the tail plane pivot shaft bushing.  The pivot-bushing hole must be EXACTLY the same position on both sides or the T/P will not line up correctly. Drill a series of 5/64˝ holes to form the T/P sweep arc equidistance from the sweep shaft centre line (for R/C). For the F/F version, drill only half the arc below the centre line.

Cut two lengths of 1/8˝ sq. carbon-fibre pultrusion to the lengths shown on the plan. You will notice that the pultrusion has a hole running down the centre. This hole serves as the tube for the D/T and A/R cables to run through. For the R/C version, these are the pushrod tubes. Cement the 2 carbon tubes to the right boom side with epoxy or CA.

Cut the balsa parts for the tail plane riser making sure to make the 1/8˝ gap in the L.E upright for the 2-56 decalage adjustment screw if you choose to make the F/F version. The decalage adjustment screw is not necessary for the R/C version. For the F/F version a music wire loop must be fashioned and epoxied to the ½˝ x 1/8˝ tail post block as an anchor for the D/T tension spring. CA the tail plane riser parts into place making sure not to get CA into the carbon tube centre holes.

For the F/F version it is a good idea to fit the bell crank, carbon pivot shaft bushing and the tension spring so you can make any adjustments before mating the left side to the boom. Not to worry, the service access panel will allow you to do this afterward, but it is a bit easier not having to work out your connections using needle nose pliers.

Cut the service access panel out of the left side. Cut a nice fitting panel from 1/32˝ ply. As it will get a fair amount of use and balsa will not stand up. Cement a small filler strip of 1/8˝ sq. balsa upright between the two carbon tubes at the front of the boom. Block-sand the right side boom assembly to remove any high spots or glue globs to prepare for the left side.

Place a 3˝ length of 1/8˝ dowel into T/P pivot bushing hole to use as an alignment guide and cement the left side to the right side assembly. Align the pivot hole with the dowel and square in all directions. Make sure not to get CA into the carbon tube centre holes and do not cement the dowel. Hold the two sides together with weights until cement has dried.

When the boom is dry, true the edges with a sanding block and slide the T/P bell-crank through the access panel hole to test fit the bell crank with the 1/8˝ dia. carbon pivot bushing for freedom of movement. The D/T spring can be shortened to adjust tension after the model is assembled if need be.

Position and epoxy 2—1/8˝ dia. x 1/16˝ thick Neodium magnets to the inside of the service access panel cover in the positions indicated on the plan. When dry, place the magnet mates onto the cover magnets (they will find their polarity all by themselves). Place a small spot of epoxy on the magnets (DON’T BE SLOPPY) and position the cover in its place and push the cover down until it is flush with the left boom side and allow the epoxy to cure. When cured, you will be able to push a finger onto the front of the cover and the back edge will pop up and allow you to remove the panel cover. Put the boom assembly aside and move on to the pod.

Cut the 3/16˝ center pod “keel” to the shape on the plan and cement to the front of the boom assembly. Cut the pod sides for 7/16˝ balsa sheet/block to the shape indicated on the plan. Use the inside “keel” contour to mark the area on the sides to “hollow out” and carve or sand out material being very aware depth of cut and the finished contour you wish to achieve. When you are satisfied and confident you have not over carved, cement the right and left pod sides to the “keel”/boom assembly and set aside to dry thoroughly.

Make a paper template from the top view on the plan and use it to mark your pod block assembly on the top flat surface. Use a band saw or scroll saw to cut to the contour. Leave block material proud of the contour line and sand to final shape.

The pod cross sections on the plan are meant as guidelines to basic contour. Cut cardboard templates to use as your guide to shaping. Use rasps, whittling knives, sandpaper or your preferred method of shaping to form a beautiful teardrop shape. Do not finish sand yet. Just get it close. Cut the 1/8˝ sheet top/incidence plate and cement it to the top of the pod block.

This is where the finesse of shaping kicks in and your true artist emerges. Cut the ballast/radio compartment access hole in the top/incidence plate. Start small and enlarge to the final shape. Make the access cover plug from 1/8˝ Sheet and shape to fit the hole snugly. Cut the access cover from ½˝ block stock. Cut the block in half at the angle indicated on the plan. The back half will become the wing-fairing block. Cement the access cover plug to the bottom of the access cover making sure you have enough overlap to carve/sand to the upper pod shape.

When the access cover is dry, snap it onto the top of your pod assembly and carve/sand/whittle to shape. Now you can sand very close to the final shape, but leave a little meat on the bone for the final sand. Mark a line at the back of you access block across the top/incidence plate and remove the access block. Flat sand the top plate from that line back, to where the boom intersects the pod, to make the required +1.5° incidence for the wing. Make very sure to sand flat and straight.

The wing mounts using a keyhole anchor method. Therefore, you must carve out the keyway oval and anchor dimple in the top plate for clearance. Epoxy the threaded aluminium anchor nut into the dimple at a slight angle toward the rear of the fuselage. This angle allows the wing anchor screw to fit flat against the top of the airfoil at the T.E.

Cut the 1/32˝ plywood wing-mount plate to the pattern on the plan (make sure your keyhole and anchor holes are accurate, as they will determine the alignment of your flying surfaces). Epoxy the wing mount plate to the top plate paying particular attention to the centre line of the fuselage. Add fairing strips to the underside of the wing mount plate where it meets the pod sides and sand to a smooth concave shape. Now that the wing mount is secured, you may final sand the fuselage.

WING

We start with the spars. The inboard spar is laminated horizontally from 2—1/8˝ sq. hard balsa strips with a 1/8˝ x .05” carbon-fibre strip in between. The outboard spars are laminated from 2—1/16˝ x 1/8˝ hard balsa strips with a 1/8˝ x .05” carbon-fibre strip in between and capped on the inboard 3-inches with a 1/8˝ sq. hard balsa strip. This sounds a lot more complex than it is to build. This yields a very strong yet flexible spar that will put up with a lot of abuse.

Cement the spar assemblies with CA on a flat surface. I use a make shift press using two hard balsa sheets taped to the flat surface. With the first sheet held fast, dry assemble the spar components and press the second sheet tight against with the spar in between both sheets and tape the second sheet fast to the work surface.

Remove the spar and lay plastic wrap over the slot in the press. This will add a little more pressure. Reinsert the spar components making sure they are tight and flat against the work surface and run a bead of CA across the top of the spar and let it wick into the assembly. When the CA has set, remove the spar from the press and coat the entire spar with CA and let dry. Lightly sand the spar smooth. Cut the spars to length with the proper dihedral angles at the joints.

Now, we move on to cutting out the ribs. Fortunately, the whole wing is not tapered and you can duplicate about half of them. Use a nice 7-pound “C” grade 1/16˝ sheet and use the patterns from the plan. The flat area at the bottom of the fish-mouth for the L.E. makes cutting and assembly much easier and will be sanded to contour after assembly.

Cut wing tip parts from soft 1/8˝ sheet. Note the direction of the grain for the three pieces. Use as hard as you can get ½˝ x 1/8˝ T.E. stock than notch and cut to the shape indicated on the plan.

Pin the spars to the plan and lay out several ribs to establish T.E. placement. Pin the T.E. down and place the rest of the ribs in their proper locations. When all ribs are in place and you are satisfied with alignment, place a small drop of CA on every joint to secure. Cut L.E. lengths from hard 3/32˝ sq. balsa stock and cement in place. Cut the centre section 1/32˝ sheet planking, fit and cement in place. Install gussets in the required places.

When the wing assembly is dry, leave the centre sections pinned to the board and raise each tip panel 1 1/2˝. Cut out and install the tip dihedral braces and cement in place then clamp together until dry. Remove on side of the wing from the board and block up the other 1˝ at the outboard dihedral joint. Cement the centre dihedral joint and let dry thoroughly.

Invert the wing and install the centre dihedral brace. Cut the centre mount block from a piece of 3/8˝ x ¼˝ stock and centre drill a 3/16˝ dia. Hole and install the 3/8˝ aluminium screw post flange nut. It will stick out a fair amount, but you will cut that down before long. Cut the centre section filler strips from soft 3/16˝ balsa sheet. Butt the strips to the inboard side of the W2 ribs and cement to the outboard side of W1. There will be a peak at the centre rib joint. When dry, block sand the filler strips so they are flat across the centre and flush with the centre mount block. You will have to sand around the screw post flange nut but it is not too difficult.

Cut out the wing saddle plate from 1/16˝ ply. to the pattern on the plan. Check the fit and measure the height of each wingtip to make sure they are even. When satisfied, Epoxy the plate to the bottom of the wing centre section. When dry, you may cut the screw post flange nut and sand it flush with the saddle plate. Carefully drill a 3/16˝ dia. hole through the wing at the rear mount as indicated by the wing saddle plate. Double cement all joints and sand the wing smooth and ready it for covering.

TAIL PLANE

The fin and rudder are basic construction. The fin is largely 1/8˝ sq. stock and the rudder is 1/8˝ sheet. For the R/C version, you will make the rudderpost on the fin the hinge post and allow the entire rudder to move. The F/F version will use a simple auto rudder mechanism. See the plan for details. The auto rudder limit arms are 1/16˝ ply. 1/8˝ x 1˝. Round one end and drill and tap that end for 2-56 thread. Cement a 1/8˝ sq. x 5/8˝ length of med. Balsa to the other end. Bevel-sand the inside mounting edge so the limit arms will stick out at an angle when attached to the fuselage.

The tail plane is straightforward in construction and is designed to be a flying stabilizer and is the same for both the F/F and R/C versions. Cut all ribs to the patterns from the plan and set them aside. Cut and notch the T.E. stock and pin them to your board over the plan. Prepare each spar by laminating 2 lengths of 1/8˝ sq. med. hard stock to the configuration shown on the plans. Cut 1.32˝ sheet for the bottom root planking and pin to the board. At the root, you need to notch each spar to clear the 1/32˝ planking.

Use a couple of ribs to accurately place the spars and continue aligning the rest of the ribs. Cut out the wing tips from soft 1/8˝ sheet and fit them into place. Cut 2 lengths of hard 3/32˝ sq. stock for the L.Es and pin and when satisfied, CA all joints.

Installing the carbon fibre receiving tubes correctly is very important and it is something that cannot be done in the air. Cut lengths of tubing as indicated on the plan. Please notice there are 2 different I.D. for the tubing. The smaller I.D. is for the front location. Place each stabilizer next to each other as they will be when they are installed on the model. Slide each tube into there respective holes in the first 3 ribs. Cut lengths of carbon rod. 1—.072 dia. (the pivot spar) and 1—.052 dia. (the actuator spar) to the length that will be used in the model.

Use the spars as alignment tools by sliding them into each of the tubes. The stabilizers should rest flat on your building surface without any pressure. When satisfied, cement the tubes at the center rib with a little CA to hold them in place. Mix up a little Epoxy and cement the tubes in for good. Do not get any glue near the spars. You do not want to glue the spars into your tubes. When the cement has dried, remove the stabilizers from the board with the spars engaged and check the alignment to make sure they are true to each other. Cut the top planking from 1/32˝ sheet and install. Sand the root ribs and tubes flush and cement SR2 to each stabilizer.

Sand the tail plane and ready for covering. The final and rudder assembly can be cemented to the fuselage before covering and sanded along with the fuselage to ensure a slick joining of the surfaces.

FINISHING

The fuselage lends itself very well to painting. Just remember not to clog up the push rod tubes or D/T and Auto rudder tubes. The wing and tail plane can be covered with you favorite covering material so I will leave you to your own devices.

Rigging a D/T timer is a matter of mounting your choice on the left side of the pod ahead of the C/G. I personally like to coat my D/T and A/R thread with thin CA to give it a little stiffness. This makes running lines through tubing real easy.

Good luck and may the thermals always be with you.

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