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

The Kobra is a sort of Kougar, Jr. with it's own individual lines and character. Like it's bigger brother, it is fast and maneuverable, so we don't recommend the design as a beginner's trainer. Although very aerobatic, it is also stable and easy to manage. The built-in washout is automatically incorporated as the wing is built from the kit parts. This decrease in wing incidence at the tip as compared to the root rib helps prevent tip stall and improves slow speed flight performance.

The .25 size engines used in the prototypes gave excellent stunt capability and good vertical maneuvers. Takeoffs were made from a grass field without difficulty. A .19 would be okay for sport flying from smooth surfaces and as a transition trainer. A .30 cu.in. engine provides high speed action with air-show type aerobatics. A mild .35 will give similar performance and a hot one produces spectacular results. Any larger engine is not required and is not recommended.

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 sections of the fuselage. We occasionally get suggestions that our instructions 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 the the wing......etc." and a lot of jumping back and forth between assemblies with no consistant pictorial progression. Also a pre-selected building sequence by our choice might not suit your workshop space and time allotments. 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 was chosen as the best way to explain the building of each major assembly and is not intended to be followed exact one-two-three fashion. Start on the wing at No.1 and after doing as many steps as is convenient, flip over to "Fuselage Construction" and do a step or two or three, then over to "Tail Assembly" 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 and mounted wing before the fuselage can be entirely completed.

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The way to understand these relationships is to read the instructions completely and study the full size plan before beginning to work. 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. The 3/16" square sticks are intentionally oversize (actually 13/64") to allow for shaping the trailing edge to correct airfoil contour. Sort the different sizes of sticks into individual piles to avoid confusion during building.

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. Pins can be pushed through all pieces in the kit without any lasting damage. Don't be afraid to use plenty of pins when planking. The holes will fill up during sanding and doping. Use Sig-Bond for general construction except where the instructions call for epoxy.

A single edge razor blade is best for cutting sticks. Use a modeling knife for cutting out sheet balsa parts.

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.

CUTTING OUT PRINTED PARTS

A jig saw is best for the job. Cut just outside the lines, leaving all of the black line on the part. When fitting the part into place in the model, use the sanding block to bring the edges to an exact fit. If a modeling knife is used to cut the parts, don't cut too close to the lines - leave some extra wood outside the line. True up and finish the edge with the sanding block.

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.

USE ENOUGH GLUE!

Instant glues assemble a framework quickly but may not be good enough to withstand engine vibration if improperly used. Thin cyanoacrylate should only be applied to perfectly fitted joints, otherwise use gap-filling types. If you assemble with instant glue, we think it is a good idea to go over the framework with a normal slow dry glue like Sig-Bond and fillet all of the joints on the outside. One mistake often made is inadequate amounts of glue on large mating surfaces to completely cover them when the parts are joined. A structure does not have full strength unless all of the parts are glued to each other. For example, the wing spar webs must not just be glued to the spars but the outside wing sheeting must be glued to the ends of the webs as well as to the spars.

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WING CONSTRUCTION

1.

  1. Cut the bottom wing spar from the 3/16"x3/8" balsa strip stock. Cut it to exact length on the plan and save the scrap ends for later use on the tail.
  2. Pin it in place on the plan.
2. Pin and glue the ribs in place on the bottom spar.

3.

Make certain that the rib tabs are contacting the building board. This assures accurate alignment of the built in washout.




4.

  1. Add the top spar. Cut it to exact length and save the scrap ends.
5.
  1. Pin and glue the 3/16" sticks into the trailing edge notch. Save the scrap ends for use in the fuselage,

In designing the Kobra it was discovered that standard 3/16" sq. sticks were not quite large enough for the wing trailing edge. We cut special 13/64" oversize sticks for the kit but refer to them as 3/16" sq. on the plan and in these instructions, since the appearance is about the same as a stock 3/16" piece of wood.



NOTE: Do not press down hard on the jig tabs during this and subsequent steps. They will break if overloaded. Support the rib with your fingers when pushing in pins.

6.

Carefully bevel and even the front edges of the ribs so they will contact the leading edge accurately.

7.

Using a straight edge, cut the taper into the leading edge stick as shown in the drawing. The reason we do not cut this taper in at the factory is because of the tendancy of balsa sticks to bow when they are cut from the raw wood block. Tapering them just before building into the structure avoids this problem.



8.

  1. Glue and pin the tapered leading edge in place.

9.

  1. 3/32"x2-3/4"x12" wood (it may sometimes be 3" wide) is provided for the wing spar webs. Mark the depth required for each rib bay and cut off the strip. The grain runs vertically.

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10.

Fit the strip into the bay by acrefully trimming the end.



11.

Glue in place.

12.

  1. Repeat the process in the next two rib bays.
  2. The last web is cut from scrap 1/16" sheet wood.

13.

Trim the tops of the web strips flush with the spar.







14.

Using the dihedral guage, mark the leading edge for trimming. Place the bottom corner of the guage on the centerline.

15.

  1. Do the same thing with the spars.
  2. Repeat the process on the trailing edge.
Remove the first wing half from the board and repeat Steps 1 through 15 on the opposite wing plan half. Leave one wing half pinned to the board on the plan. Now proceed as follows:

16.

  1. Pin a 1-7/8" high squared block to the opposite end of the plan.
  2. Have the back corner right on the trailing edge line so that the tip rib can be accuractely positioned front and rear.
17 Set the loose wing half in place on the block and check the fit at the center section. It may be necessary to touch up the spar ends for good fit.
18 Use epoxy glue to join the wing halves.

19.

  1. Lay out the pattern for the front dihedral brace on the 5/16"x1"x3-7/8" piece of balsa provided.
  2. Saw out the brace.

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20.
  1. Fit the brace into place by touching it up with a sanding block.
  2. Glue it to the leading edge.
21. Fit the two center section ribs into place. They will need trimming on the front to fit against the brace. We did not do this on the die-cut because the brace will vary in position according to the individual construction.

22.

As the ribs progress toward the tip there is a slight protrusion of the spar at the rear because of the washout. Trim rear of the spar to conform to the rib contours and touch up with a small sanding block.




23.

Glue pieces of 3/16" sheet scrap to the trailing edge in the center section as shown. Shape the rib and trailing edge contour. This provides a filled in area for later drilling of the wing bolt holes.



24.

Trim the trailing edge to airfoil contour.

25.

Touch up with a sanding block.

26.

Trim the leading edge to airfoil contour and touch up with a sanding block.

27.

Complete the spar webbing in the center section, using a 3/32" scrap from the rib WS die-cut sheet.




28.

Glue a strip of 1/16"x1" planking on the trailing edge. (Save the scrap ends.) Have the wing pinned down to a flat surface and make use of the jig tabs. This is a critical piece of wood in locking in the washout angle. If you put it on with the wing off the jig tabs and building board, you could twist the structure out of alignment. Don't use a water base glue which may bow or warp the wood. We used epoxy which definately locks in the washout. Gap-filling cyanoacrylate "super" glue would also be a possibility on this piece. Look at the plan and picture 58 and see where plywood part WP will be glued on. Don't glue on this area so that it will later be easier to trim out the opening for WP in the 1/16" trailing edge planking.

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29. Remove the jig tabs from the bottom of the ribs.

30.

  1. Glue on the bottom 1/16"x1" planking strip. Save the scrap end. Be careful not to twist the structure when pinning the planking in place.
31.
  1. Glue the leading edge 1/16"x1" planking to the leading edge. Allow to dry.




32.

  1. Apply glue to the ribs and curve the leading edge sheet into place, holding it down with a pin at each rib. If it will not curve into place dry, dampen the top of the sheet with water to make it more pliable.


33. Repeat the process on the opposite side of the leading edge.

34.

  1. Glue the landing gear blocks into the wing.


35.
  1. Glue P-2 in place on the landing gear block at rib 2.
  2. Glue P-4 in place at Rib 4.
36.
  1. Glue the 3/8"x5/8"x3/4" hardwood landing gear anchor block in place on top of the landing gear block against P-2.

37.

With an epoxy glue joint where the landing blocks join, the center section of this model is probably strong enough. For those who worry about possible bad joints we show a joint brace WL to be made from scrap 3/32" plywood. To install, cut a slot in the center ribs 3/32" wide on each side of the landing gear block.



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38. Epoxy glue the braces to each side of the block.



NOTE:
Look ahead to pictures 132 and 133 for further information about the wing aileron servo compartment and the installation of plastic servo mounts.

39.

Add the 1/8"x5/8" leading edge cap to the front of the wing.

40.

Mark the flat area on the front of the center section and saw it off.

41.

Shape the leading edge cap to airfoil contour.

42.

Begin sheeting the center section bottom with 1/16"x3" sheet wood.

43.

Complete the bottom planking.



44.

  1. Cut out the center ribs to provide space for the servo installation. Length depends on your servo size.
  2. Add ribs WS.

45.

  1. Make a rear for the servo compartment from 3/32" scrap.



46.

Sheet the top of the center section with 1/16" sheet, leaving the servo compartment open.

47.

Complete the top planking.

48.

Sheet the tips on top and bottom with 1/16" sheet balsa.

49.

Cut a piece of 1-3/32" wide trailing edge stock provided. Sand it as shown by the drawing so as to center on the tip rib and glue in place.

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50.

Trace the wing tip pattern on the 1/2"x1-1/8"x6-1/2" wing tip blocks. Carve the block to this line.



51.

Glue the blocks to the tip ribs.

52.

Carve to the shape of the tip rib.

53.

Carve the tip to a rounded shape and sand smooth.

54.

Using a 1/8" bit, drill through the landing gear anchor block. The hole should be vertical to the bottom face of the landing gear block. Cut a radius in the groove hole to pass the corner of the bent wire.



55.

  1. Fill in the end of the block groove with scrap to provide an anchor point for the covering.




56.

Fill in the landing gear block groove in the center section with scrap balsa.



57.

  1. Cap the top of each spar with a strip of 1/16"x1/4" balsa.
  2. Cap the top of each rib with 1/16"x1/4" balsa.
58. Remove the top center-section sheeting over the trailing edge for installation of the WP plywood parts.

59.

Position the WP part so that it is in line with the top surface of the wing. Use a ruler to check this.

60.

Glue one of the WP pieces in place.

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61.

Add the second WP part.


62.

Turn the wing over and drill holes to pass the aileron horns.
NOTE: Keep the horns close together in the middle so that the aileron pushrods will have no trouble clearing the two servos mounted on each side of the fuselage. Remember that there will be two nylon fittings on the horns. You may need to bend the horns slightly for parallel position.


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.

63.

  1. Pin the horns in position so that they line up with the center of the wing trailing edge.
  2. Glue the tubing to the wing and WP.

64.

  1. Cut pieces of 1-32" x 1" trailing edge stock.
  2. Carve out the fronts of the pieces to pass the tubing and horn.
  3. Glue them to WP and the wing. Be careful not to get glue in the tubing.

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65. Trim down the trailing edge stock as required to match the wing contour.

66.

Drill a hole in the aileron to accept the horn.

67.

  1. Cut a slot in the end of the aileron in which the horn wire will be inset.
  2. Cut slots in the ailerons and wing for the hinges.
    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.
  3. Glue the hinges into the ailerons and allow the glue to set up.
  4. Glue the aileron hinges into the wing and simultaneously glue the aileron horn to the aileron.
  5. Check positioning and clearance carefully before the glue sets, making sure the aileron is correctly located.








FIREWALL ASSEMBLY


68.

  1. Smooth and even F-1A and F1-B with the sandpaper block.
  2. Glue them together with epoxy glue, as shown in the drawing, to make a firewall. If they should be warped, clamp them together with "C" clamps or put the assembly in a vise while the glue is setting.
  3. Mark the vertical and horizontal datum lines.

69.

  1. Place the motor you will use on the firewall or measure the mounting width required.
  2. Draw accurate guidelines, centered on the vertical line for the motor mounts.

70.

  1. Put a mark on the sides of the mounts, lined up with the top of the mounts, so that they may be accurately located on the firewall horizontal line.
  2. Mark the mount holes on the firewall.



71.

NOTE: Read "Tips On Tanks" over before cutting out the tank hole.
  1. Locate the center of the tank cap hole and draw a 7/8" circle on the wood.
  2. Drill a series of holes on the inside of the circle.

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72.
  1. Break out the wood and sand the edges smooth with sandpaper wrapped around the dowel.

73.

  1. Drill out the motor mount holes with a 7/64" bit to pass the 4-40 bolts.
  2. Position the nylon nose gear bearing on the firewall, punch the holes with an ice pick or awl and drill out with a 7/64" bit to pass the 4-40 bolts.
  3. Turn the firewall over and drill out the backs of the 7/64" nosegear bearing holes with a 9/64" bit to take the shanks of the 4-40 blind nuts. To complete the holes, take a modeling knife and round off the edges on the back of the firewall so that the rounded off part of the blind nut will fit down into the hole when it is pulled tight against the firewall.
  4. Drill out the backs of the motor mount holes in the same way.





74.

Be sure and epoxy the blind nuts to the back of the firewall so that they will not come out later when it may be necessary to take off the mounts. Work some under the nuts but don't get epoxy into the threads of the bolts. Pull the blind nut points tight into the wood with the bolts before the glue sets up. With the mounts and nose gear bracket in place, cut off the mounting bolts for both flush with the face of the blind nuts on the back of the firewall. This is to prevent any chance of the bolt ends puncturing the tank or rubbing on the batteries.

75.

  1. Put on the spinner backplate that will be used on the motor. (Note: some backplates have a recess in the back as does this Goldberg spinner used on the prototype Kobra. This is why the measurement must be taken from the spnner backplate itself and not the prop drive washer on the motor.)
  2. Position the motor on the mounts so the spinner backplate will be 3-1/4" from the face of the firewall. It is handy to tack the motor in position with some spots of epoxy, brought up over the edge of the engine to grip it good or put a strip of double-faced masking tape between the engine and the mounts. This will keep it from slipping during the next step.


76.

With a punch or sharpened piece of 3/32" wire, center punch the motor mounting bolts. (Hint: If you are not used to doing this sort of job, don't try to punch and drill all four holes at once. Punch and drill only one hole. Then put the motor back on the mounts, secured by the first bolt. Punch and drill a 2nd hole, repeat the procedure, then a third hole, etc. With this process you are much less likely to make a drilling mistake that will ruin the mounts.) Drilling our mounts will not be a problem if a good quality high speed drill bit is used, operated at neither too fast or too slow a speed, lubricated and with moderate pressure.


IMPORTANT: If you are going to tap a hole in the mounts for mounting the engine, you will need a No.43 drill. Tap out this hole with a 4-40 tap. Be very careful, aluminum galls the tap easily, it may jam and break the tap. Back it out frequently and clean the fragments out of the tap threads. Use a special aluminum tapping fluid or kerosene to lubricate.

SIGSH104 4-40 x 1/2 Socket Head Bolts are recommended for mounting the motor. They are not furnished because some builders may not have a tap. If you do not have access to a tap, then drill a 7/64" hole and use 4-40 bolts long enough to go completely through the mount. Fasten them with nuts and lock washers. It helps with this method to file a flat place on the tapered bottom of the mount so the nut and washer will seat squarely.

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TIPS ON TANKS

IMPORTANT: To prevent fuel spray from staining the canopy, run the tank vent line out of the bottom of the cowling. Fill the tank through the muffler pressure line or needle valve line.

We occasionally receive suggestions from builders that a removable hatch be designed into the model for access to the gas tank. Our opinion is this is not the best method in most cases. The hatch opening makes the nose weaker and there is no good way to keep oil leaking in around the hatch. A method of fastening has to be built into the fuselage to hold a hatch in place.


Modern plastic tanks are virtually indestructable under normal use and bursting or cracking is almost unknown. If you use Sig Heat Proof Silicone tubing (which will not harden or deteriorate in fuel) in the plastic tank, the tank will seldom have to be removed. We have models in which tank has been installed for three or four years without ever needing removal. So it is quite practical to put the tank in semi-permanently. Check the models at a contest - you'll find that the majority have sealed noses, as does this kit.

Read this before you drill the 7/8" hole in the firewall.
Some fliers prefer not to bring the tank cap through the firewall as is shown in the construction sequence in these instructions. Instead they drill two holes for the vent tubes only and make the vent tubes long enough to extend through the firewall. This method requires little sealing but it is more difficult to install and remove the tank. The best way to manage this is to feed long pieces of fuel line through the holes and attach them to the tank in the cabin area. Steer the tank into the nose as the tubes are pulled back through the holes. If you are undecided as to which method you should use, our advice is that large hole installation shown in the construction pictures is the best for beginners.

Put scrap wood supports under and at the back of the tank. The front is supported by the 1/4" hole in the firewall. Seal the tank cap in the hole with G.E. Silicone Bathtub Seal (available at hardware stores) or Devcon Seal-It. Put an oil-proof finish on the firewall and in the hole before sealing the tank cap. Get some of the sealer on the sides of the hole and also put a bead over the edge of the cap at the front. Should you need to remove the tank, break out the scrap wood supports in the rear and push out the silicon rubber seal around the front cap. Reach into the fuselage and guide the tank outside.

Some builders, after putting their receiver battery in a plastic sack, taping it shut, wrapping it in a foam rubber package and stuffing it into the nose under the tank, then stuff paper toweling or foam rubber in to fill the nose compartment and keep everything firmly in place.

After installation, put fuel tubing on the vent tube and run it to the outside of the cowling on the bottom, so that fuel overflow is not blown over the wing-fuselage joint, where it may leak into the fuselage. The best way to fill the tank is to take off the fuel line to the needle vlave and pump the fuel in there until it runs out the vent. Be sure and use a filter on your fuel supply can, and it is a good idea to have a filter between the tank and the needle valve also.

Pressure Feed

If the engine you are using is equipped with a muffler pressure tap, make use of it for a more even fuel feed and reliable operation. The hookup for pressure feed is shown in the picture. To fill the tank, remove the fuel line from the engine and pump the fuel in. When the tank is full, it will overflow through the muffler pressure line. Use transparent or translucent fuel line so you can see the fuel starting to overflow when the tank is full. Should some fuel happen to get in the muffler, drain it out before starting the engine. Do not try to fill the tank in reverse from the pressure line, the tank will not fill properly and fuel may be forced into the engine.