Sig Mfg. Co., Inc...401-7 South Front Street....Montezuma, Iowa 50171

The King Kobra was designed to fill requests for a .60 sized airplane like our popular .40 - .50 size Kougar. Following the Kougar philosophy, it is a compact model. Because of this efficient size, the King Kobra is capable of excellent aerobatic performance without necessarily needing a tuned pipe and/or retracts. We know there will be some builders who will want to use these high performance extras so a section is included in these instructions to guide them. The kit, however, is intended to produce the fixed gear version as it will be built by the majority of buyers. Any extra parts or materials needed for retracts or tuned pipe installation are not supplied.
It should be obvious that this is not a model for novice fliers. You can't go straight from a high wing trainer like the Sig Kadet to the King Kobra without a lot of assistance from an instructor. But if you travel the full 3-step "Learn To Fly RC The Sig Way" program or its equivalent first, the transition to the King Kobra's high speed and responsive performance will be easy.
Since most builders of this kit will probably have had some experience we were tempted to short cut the instructions. This was not done. Some skilled fliers may not have had much prior building experience and beginners at both building and flying will probably be constructing the model for future use. Other readers with considerable expertise may feel they can skip the instruction book. Our advice is the same as to the amateur. Read it all before beginning. There are some essential facts mixed in with the more elementary, don't get bored and miss these.

ABOUT THE BUILDING SEQUENCE

The quickest and most efficient way to complete a model is to work on several pieces at the same time. While the glue is drying on one section you can start on or proceed with another part. Work can even go forward on several sections of the same assembly at the same time, such as the front and rear of the fuselage. We occasionally get suggestions that our instruction books should be in exact step-by-step building sequence. But this would result in many sentences starting, "While the glue is drying on the fuselage, move to the wing etc." and a lot of jumping back and forth between assemblies with no consistant pictoral progression. Also, our pre-selected building sequence might not suit your workshop space or time allotments.

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Therefore, we feel the present system of covering main assemblies in a unit works out best for the majority of kit builders.So keep in mind that the numbering sequence used in these instructions were chosen as the best way of explaining the building of each major assembly and is not intended to be followed in exact one-two-three fashion. Start on the wing at No.1 and after doing as many steps as is convenient, flip over to the next main heading of "FUSELAGE CONSTRUCTION" and do a step or two there, then over to "TAIL SURFACES" and so forth. You will, of course, arrive at points where you can go no farther until another component is available. For example, you need a nearly completed wing before the fuselage can be entirely completed. And you will need both the wing and stab to fit the wing and tail saddles on the fuselage and align them to each other. The way to understand these relationships is to read the instructions completely and study the full size plan before beginning to work. Think ahead! Any reference to right or left refers to right or left as if seated in the cockpit.

SOME BUILDING SUGGESTIONS

Cut all long pieces of balsa first, followed by medium lengths before cutting up any full-length strips into short pieces.
A piece of Celotex-type wallboard or foam board makes a handy building board, into which pins can easily be pushed. Lay the building board on a table with a flat and untwisted top. Don't be afraid to use plenty of pins when planking. The holes will fill up during sanding and doping. Use Sig-Bond glue for general construction except where the instructions call for epoxy.

YOU CAN'T GET ALONG WITHOUT A GOOD SANDING BLOCK

An indispensable tool for proper construction is a large sanding block sized to take a full sheet of sandpaper. Use several wood screws along one edge to hold the sheet in place. Use the block to bring all parts and sticks to final, exact fit. We recommend 80-grit garnet paper for use on the block during general construction. You can switch to 100-grit, followed by 220 silicone paper for final finish just before covering.

In addition to the large block, there are places where a smaller one is handy. Also, a sandpaper "file" can be made by gluing sandpaper to a flat spruce stick for working tight-places.

A modeling knife or jig saw can be used for cutting out printed parts. Don't cut too close to the lines - leave some extra wood outside the lines. True up and finish the edges with a sanding block as you are fitting the parts together. Don't force die-cut parts from the sheet. Use a modeling knife to finish freeing them.

ABOUT PRINTED WOOD PARTS
Some years ago we had kits featuring die-cut parts in both thick and thin sizes. If the thick parts were cut from dry wood, the wood often crushed or crinkled on the edges, even when using a brand new die. If the thick parts were cut from wet wood there was an improvement - though many of them still crushed - and the swelled wet wood parts changed shape after drying, making them inaccurate. So we asked modelers if they would rather have the parts printed on the wood instead. They could be cut in a few minutes with a saw or modeling kinife and thus avoid any "die-crunching." Most voted in favour of this idea.

To answer the question we are sometimes asked - no, we do not print parts on wood to save money. It is actually more expensive to print the parts using a silk screen press than it is to run an equivalent sheet through our automatic feed die cutting machine. If we hand-sawed the parts it would be even more expensive and the labor cost would have to be added to the kit price. We believe that most modelers would rather cut their own out and save the cost. Since there are not many thick parts in our average kit, it really doesn't consume a lot of the total building time for the builder to do the parts.

NOTE:
Some of the pictures in the instruction sequence are of the Kougar because the operation being performed is identical on both models. Also, some of the King Kobra pictures may show a retract gear installation in the background. Ignore these areas if you are using a fixed gear, the construction step shown in the picture is the same for both types.

WING CONSTRUCTION

1.

True up the edges of the twelve sheets of 1/16"x4"x30" wing planking wood by trimming where necessary, using a metal straightedge as a guide. Use the sanding block for final touch-up.

2.

Tape 6 sheets tightly together with strips of masking tape.

3.

Turn over and open up the joints, with the masking tape serving as a hinge. Put a bead of Sig Bond in each of the seams and close the joint.

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4.

Lay the sheets flat. Scrape off the excess glue with a squeegee made from a balsa scrap. Finish glue cleanup with a damp rag. Weight down the sheets on a flat surface and allow to dry thoroughly.

NOTE:
The grain of the wing skin sheet can be on either parallel with the leading edge or with the trailing edge. We have tried it both ways and on this shape wing can see no appreciable difference in ease of application. Both ways are shown. Use your own preference. There is a little more leeway for error in placement on the edges of the piece with the trailing edge method. And, it seems to us easier to keep that critical trailing edge true when it is placed down and rolled parallel to the grain.

5.

Sand the skins smooth with a sanding block.



6.

Wing Skin Sheet

Cut one 6-sheet piece diagonally in two, with untaped side up, as shown by the dotted line marked "A".

Cut the other 6-sheet piece diagonally in two, with the untaped side up, in the opposite direction as shown by the dotted line marked "B". This provides 4 wing skins with the untaped, rougher glue seam on the outside surface of the wing. The smoother, taped side should be used against the foam wing for best adhesion of the skin to the foam. The rougher, outer glue seams can be sanded down partially with a sanding block before application of the skins and completed during final sanding of the skin on the wing.


CAUTION: Use only Sig Core Bond, Sig Kwik-Set, Sig Epoxy Glue or Sig-Bond Glue on the foam wing cores. Model cement such as Sig-Ment, dope and fiberglass resin will attack and destroy foam. If you use any product other than those listed, test them on a scrap of foam.

7.

Sand any irregularities or cutting wire marks from the cores with a sanding block.

8.

Glue the 1/2"x1-1/2"x2" balsa blocks WLB to the flat front of the core.

9.

Save the foam blanks to use as a cradle for sanding later. Saw WLB off flush with the core leading edge.

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10.

Finish WLB with a sanding block.

11.

Carve WLB to airfoil contour.

12.

Hold the cores together at the center joint. If there is any mismatch in the airfoil shape, sand as required to make them fit smoothly together. Done this way, little matching will be required after planking. Refer to the dihedral view on the plan. Check out the cores as shown there before proceeding. Do not cover the cores until you are certain they match properly.
NOTE: If you plan to have retract gears, turn to the section called "Retracts" and install the gear mounts in the wing at this time. Then preceed with step 13.




13.

  1. Sig Core Bond is recommended for applying the wing skins. This is a special adhesive, light and strong, that is ideal for use with foam. As experienced modelers have found, many foam wing glues contain very volatile solvents. When using these glues, if the wing skin is put on before the glue is absolutely dry, the still evaporating solvents are trapped in the assembly and quickly attack and destroy part of the foam core, ruining the wing. Sig Core Bond doesn't cause this type of damage so it is ideal for beginners at foam wing sheeting in addition to being a superior adhesive. If directions on the can and these instructions are followed, it will result in a perfect wing sheeting job.
  2. Shorten the bristles of an ordinary 2" house paint brush to about 1-1/2" in length. This stiffens the brush and makes it easier to spread the glue evenly.
  3. Apply a thin, even, full coverage coat of Core Bond to both sides of the foam cores. Avoid heavy spots. These are inclined to skin over, leaving wet spots underneath that won't stick down properly after the wings are skinned. Stand the cores on end to dry. (The cores should be coated first because they take slightly longer to dry than the wing skins.)
  4. Coat the wing skins with Core Bond.
  5. Allow the cores and skins to dry completely. This generally takes about one hour. In conditions of high humidity it may take somewhat longer. It is best to join the parts as soon as they are dry, since if they are allowed to lay around for a long period, they will not stick together as well.
THE SECRET OF A
PERFECT FOAM WING
It's a simple matter of a FLAT table. Most tables are not flat, as can be seen by checking them with a good straightedge. If a foam core is covered on a bowed or twisted surface, then the wing will be bowed or twisted. And a table that checks out true but is flexible and will yield as you press on it will also spoil a wing. The ideal working surface is a sheet of plate glass. Or, it is possible to find a thick piece of plywood that is perfectly true.
Like balsa blocks, foam blocks sometimes have built-in internal stresses and the core bows slightly when cut out of the block. Skinning on a flat surface, in the sequence shown in the pictures, will correct minor bows.
Incidently, the washout in the King Kobra wing is cut right into the foam core. The tip section is higher at the trailing edge compared to the center section trailing edge. The washout will take care of itself. No blocking up or other steps are required of the builder. Proceed with wing construction as if it were a standard wing.

14.

Hold the trailing edge of the foam core in position just above the wing skin and lower the edge only onto the skin. Make sure it is properly aligned before contact is made because it cannot be removed and repositioned after contact is made. Press down along the trailing edge to make sure it is making good contact and is flat against the table.

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15. Roll the core down onto the sheet with a rocking motion.

16.

Continue rolling the core onto the sheet until the leading edge is attached.




17.

Turn the core over and firmly rub down the wing skin sheeting with the flat of your hands to insure that the balsa skin is firmly attached to the core. Repeat this process from time to time as you proceed with the next steps. This seals down any areas that may have separated during handling. After sheeting the first side of a core, check for straightness. A bow can sometimes be corrected by hand pressure and twisting the assembly before proceeding.


18.

Rough trim around the edge.

19

Repeat Step 14, 15, 16 and 17 on the other side as steps 19 to 21.




22.

Trim and sand the edges of the sheeted foam cores. While the regular sanding block can be used, note how useful an extra long block is for this purpose. The one shown is made from a section of aluminum channel extrusion - with sandpaper glued on using sanding disc adhesive. This handy specialized glue is available at hardware stores and lumber yards.



23.

Glue on the 5/16"x1" leading edge.



24.

Glue on the 1/4" x 5/8" trailing edge.

25.

Carve the leading edge and trailing edge roughly to contour.

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26. Sand to exact shape with the sanding block. A pencil line drawn down the center of the leading edge from root to tip will help get the shape true all along the wing.
NOTE: If you are using retracts, skip Steps 27 and 28.

27.

  1. Epoxy glue the anchor block to the grooved block.
  2. Cut out the balsa sheeting above the landing gear block slots in the foam core. The slots may be located by pressing on the sheeting or by use of the waste block from the foam core. Cut the holes in the sheeting out undersize at first so that the opening can be trimmed down carefully for an exact fit around the landing gear blocks.
  3. Excavate the foam out of the pre-cut cavity to accomodate the anchor block. The best way to cut foam is with a brand new, sharp modeling knife whittling blade. Or you can heat an old blade in a flame and hot cut the hole.
  4. Epoxy glue the landing gear blocks into the wing. Should there be any areas in the cavities which do not fit snugly against the blocks, fill these voids with a mixture of epoxy glue and scrap foam which has been crumbled into bits.




28.

Position the landing gear and drill a 5/32" diameter hole into the gear block and anchor block. CAREFUL! It is easy to slip and go clear through the wing. (Put a collar of tape on the bit as a guide.) Trim the edge of the hole so that the radius of the wire at the bend will fit down into it. The gear should fit into the block snugly, but not so tightly that it will jam in the block. You may want to remove it later for straightening after a hard landing. Place a nylon landing gear strap held on by NO.2 screws across the gear at each end to retain the gear in the groove.

29.

The angle already cut into the ends of the foam wing halves sets an approximately correct dihedral angle. To check it, set up the wing halves as shown in the drawing on the plan. Sand the wing ends (Photo 29) as required to make the center joint fit correctly together. Glue the halves together with Sig Epoxy Glue or Sig Kwik-Set Glue. Use plenty of glue where the balsa sheeting meets so that the joint between the two halves is completely filled. Be certain that the leading and trailing edges are lined up exactly so that no twist between the two halves is built into the wing. Mark center lines on the ends of each panel before joining and match the lines when joining. If you have the wing sitting on a true, flat surface, a further check on twist can be made by putting center marks on the tips also and measuring from them to the table as a second reference.

30.

Glue the wing tip block in place. If you wish to save weight, tack glue it on so it can be removed for hollowing.




31.

Carve the tip block to contour. (Seen from TE side).

32.

Hollow the tip with a router.

33.

Cut out the inset holes in the top wing sheeting for the 1/16"x2-1/4"x2-1/4" plywood tabs called PW.

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34.

Epoxy the PW tabs in place, using a ruler to line them up with the wing top surface.

35.

Drill holes in PW for aileron horn.

36.

Epoxy the brass tubes to the wing trailing edge, using pins to hold the horn wires in alignment. I find that gluing the tubes directly to the wing puts the horn a little too close. A small scab of 1/64" plywood was used behind the brass tubing in the photo to move them out a bit. Or, you can shim out the horn wires with pieces of balsa or cardboard and fill the small gap behind the tubing with epoxy glue.




37.

Hollow the fronts of 1/2"x1-5/16"x2-5/16" balsa blocks WTB to fit over the brass lube bearing and horn wire. Epoxy in place. Don't get glue into the bearing.
NOTE: Before the next step is done you will need the fuselage with the rear bottom just behind the wing shaped and the wing mounted in final position.

38.

  1. Set the wing into the fuselage and mark a line on the back of the WTB blocks to indicate the fuselage outline.
  2. Carve the WTB blocks to shape.
(NOTE: Pictures 40 and 41 show the fuselage bottom block in place on the fuselage but it is best NOT to have it installed when Step 40 and 41are done. Access to the dowels is much easier when it is not in the way.)

39.

Replace the wing on the fuselage and finish sanding the joint so that they blend together smoothly.




40.

Put a point on the 1/4" diameter wing dowels and install in the fuselage.

41.

Put the wing on and push the points into the leading edge.

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42.
  1. Drill the holes in the wing out oversize - about 9/32" diameter - to allow some "wiggle" room during the final positioning and gluing in of the dowels. Dig out a little foam just behind the leading edge so the glue will form a "collar" to lock the dowel to the balsa.
  2. Put a piece of wax paper over the face of F-2 and insert the dowels through the paper into F-2.
  3. Coat the holes in the wing with Kwik-Set Glue and put enough extra glue in the holes to fill the gap between the oversize holes and the dowels. Don't overdo the amount of glue.
  4. Put the wing in place and secure it in position with masking tape. Hold the fuselage vertically to keep the glue from running out of the dowel holes. We mix some micro-balloons with the glue so it is not so runny. Allow the glue to set up firm, but not fully cure, just in case it may have stuck the wing to the fuselage in some leaky spot. Remove the wing. If the dowel holes are not completely filled with glue, fill them. If necessary, now that the dowels are set in place, you can cut away the wood around them to provide room for filling any remaining crack with glue.




This step must be done before the fuselage top is glued to the fuselage.

43.

  1. Put the wing on the fuselage again with a piece of wax paper between it and the fuselage at the back.
  2. Epoxy the wing bolt anchor blocks in place against the fuselage sides.
NOTE: We tried the block farther forward on some prototypes so you may see it in a slightly different location in other pictures. The way shown here, in the corner, is strongest.

44.

  1. Locate the positions of the wing bolt anchor blocks on the bottom of the wing. (Remember that the wing bolt holes are drilled at an angle so that the heads of the bolts will end up flush with the surface of the bottom of the wing.
  2. Drill a hole through the wing and on through the anchor blocks with a NO.7 drill. (13/64" is the nearest inch size equivalent.)
  3. Run a 1/4-20 tap through the hole to cut threads in the wing bolt anchor blocks.
  4. Remove the wing and drill out the holes in the wing only with a 1/4" diameter drill to pass the nylon wing bolts.
NOTE: An alternate method of making the area covered in Steps 45, 46 and 47 is shown in the "Retracts" section. If you do not like carving and fitting balsa blocks, the alternate method can be adapted to the wing fairing shape needed here.

45.

  1. Put a piece of wax paper between the wing and fuselage (which now has the bottom front block installed and shaped) at the front.
  2. Shape a piece of scrap balsa block to fit down into half of the cavity. Make a matching block for the other half.




46.

Carve the blocks roughly to shape so that the contour of the fuselage is carried on to the wing.

47.

  1. Glue the blocks to the wing and fine sand the shape as shown.
  2. Fill any small remaining gaps with Sig Epoxolite or a mixture of Sig Kwik-Set glue and micro-balloons or talcum powder.

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48.
  1. Cut a cavity in the wing for the servo. Size will depend on the servo and/or mount.
  2. Cut strips of 2" fiberglass tape for both sides of the wing center joint.



49.

We use regular Sig Epoxy Glue (not Kwik-Set Glue) for applying the fiberglass tape, since it is thinner and easier to spread out smoothly. It will be even easier to spread if you warm the mixing container by setting it in hot water for a few minutes to raise the temperature of the glue. But work quickly, for the glue will set up much faster than normally when warmed.
  1. Coat the wing center with glue.
  2. Lay the tape on top of the glue.
50. Glue hardwood mounts for the servo into the cavity. Coat the entire inside of the cavity with epoxy glue to prevent the foam from being damaged by fuel or dope.

51.

The plastic servo mount in the photo is fastened to hardwood mounts with No.2 screws. Other brands may need a different arrangement.




52.

  1. Draw a centerline on the front of the 7/16"x1-1/4" shaped aileron stock and shaping lines on top and bottom, as indicated.
  2. Carve and sand the front of the aileron stock to shape, so that it can move without bumping the aileron. (Check before gluing.)
  3. Slot the aileron to receive the aileron horn wire.
  4. Drill a 3/32" hole in the aileron to take the arm of the wire.

NOTE:
At this point it is best to cover both the wing and the ailerons before gluing the ailerons to the wing. This is particularly advisable in the case of plastic film covering so that access to the edges of the parts is open to the iron.



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53.
  1. Glue the hinges into the ailerons and allow the glue to set up.
  2. Put a 1-1/2" wide strip of wax paper about 3" long, between the wing and the aileron horn wire to keep the epoxy glue that is put into the slot and hole in the aileron from being squeezed onto the wing during assembly. Then proceed with gluing the aileron hinges into the wing at the same time the horn wire is glued into the aileron.
  3. Check positioning and clearance carefully before the glue sets, making sure the aileron is correctly located.
  4. Just before the glue sets up, pull the ends of the strip of wax paper over the horn wire, squeezing the glue into a rounded shape and forming a skin over the wire.
  5. After the glue stiffens, any excess that is squeezed up over the aileron when the wax paper strip was pulled back over the horn can be picked or trimmed off before it is fully cured.

54.

  1. Screw the nylon connectors supplied in the kit onto the threaded aileron horns.
  2. Hook the servo to the aileron horns with the rods and RC links.
  3. A servo connector can be used at the other end instead of a "Z" bend, if desired. The Sig SIGSHl84 connector is shown here but is not furnished in the kit.





INSTALLING EASY HINGES
Using a No.11 X-Acto blade (or similar) cut a slot approximately 1/2" in depth and slightly wider than the hinge. After all slots have been cut, insert an Easy Hinge halfway into each slot in one of the pieces to be hinged. Then carefully slide the matching model part onto the other half of the hinges. You'll find it easiest to slide the part onto the hinges at an angle, one hinge at a time.
At this point the surface to be hinged is attached but not glued. Align the two surfaces and adjust the gap between them as required. For best control response, the gap should be as small as possible but big enough to allow the control surface to move to the maximum deflection that you will require.
Place three or four drops of any brand of cyanoacrylate adhesive (thinnest variety) directly onto the Easy Hinge in the gap. You will notice that the glue is quickly wicked into the slot as it penetrates both the wood and the hinge. Continue this process, gluing the same side of all of the hinges. Then turn the surfaces over and repeat the gluing process on the other side of each hinge. After the glue has cured, approximately three minutes, the joint can be flexed. You may notice a slight stiffness in the joint. This can be eliminated by flexing the surface to full deflection each direction a couple of dozen times. Don't worry about shortening the life of the hinge as they are almost indestructible.