KOBRA RC53 FUSELAGE CONSTRUCTION

The nose was designed to accommodate average engines in the recommended range. Certain engines of bulky configuration or long crank cases may be a little crowed. In this case it would help the fit to cut the fuselage sides out a bit longer at the front and also part FX. Check the fit of your engine before beginning.

77.

  1. Pin and glue the first top strip of 1/4" triangular stock in place on the fuselage side sheet.
  2. Pin and glue the first bottom strip in place.
NOTE: If the fuselage sides are bowed or warped, it will not be a problem. Pinning them down to a flat building board and gluing on the structure will flatten them out.

78.

  1. Pin and glue on the second top strip
  2. Pin and glue on the third top strip.
  3. Pin and glue on the second bottom strip.

79.

  1. Cut a piece of 3/16" sq. from the scrap remaining from the wing trailing edge to fit on the triangular stock.




80.

  1. Glue the 79a piece in place.
  2. Do the same for the next upright.
  3. Bevel the upper edge of FA to fit against the triangular stock and glue it on.

81.

If the 1/32" plywood doubler FD does not fit down against the fuselage side evenly, sand or bevel the edges until it does. (Triangular balsa stock varies in dimension slightly so it is not possible to die-cut the doubler to fit perfectly in every case without a bit of fitting.)

82.

Glue FD in place on the fuselage side.
CAUTION: Epoxy or gap-filling cyanoacrylate glue is recommended for FN. Water base glues such as Sig-Bond, Tite Bond, Elmer's etc. may cause curling because of the large area being glued. Spread a thin film or epoxy with a paddle. Don't use a large amount of epoxy glue - it will add weight to the model.




NOTE: Look ahead to picture 90. You will see pieces of 3/4" triangular stock being glued on the back of the firewall. These must be cut to fit against the 1/4" triangular stock on the fuselage sides. Do this now, on the sides, while it is easy, before they are joined together. Lay the pieces aside for use at picture 90.

83.

Cut the fuselage sides from the sheet with a modeling knife. Don't cut too close, leave a bit for sanding. Cutting too close can result in too deep a cut that is harder to fix than taking down the side a little with the sanding block.

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84.
  1. A small gap remains at the end of the front bottom triangular stock. Cut a small piece of 1/16" balsa sheet, thinned down to 1/32" thickness to fit the gap.
  2. Glue it in place.

85.

Pin the two fuselage sides together, outside to outside and match them by sanding as required to make them exact duplicates




86.

  1. Place the top view fuselage plan on the building board so that the cowl notch at the front of the fuselage is just over the edge. (Or you can put the entire plan on the building board and put scrap 3/32" shims under the fuselage sides.)
  2. Pin the sides in place on the plan, top side down against the plan.

87.

  1. Glue F-3 in place. Use a triangle to insure squareness. Pull sides together with tape. Minor adjustment in pinning down may also be required.

88.

  1. Glue F-2 in place.
    NOTE: While we put on the dowel doubler and drilled the dowel hole later on in the sequence, you may wish to do it now, before gluing F-2 into the fuselage. In this case, look ahead to pictures 98a and 98b.




89.

  1. Glue the F-1 firewall assembly in place.

90.

  1. Glue pieces of 3/4" triangular stock to the back of F-1.
  2. Cut away the stock to clear the blind nuts.

91.

Move the stabilizer mount portion of the fuselage assembly and plan off the building board until the sides will sit down on the board on the fuselage top. Pin down securely at F-3. Use vertical sided weights to pull the sides together at the rear and pin where necessary to hold the sides to the correct plan shape.



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92.
  1. Glue F-4 in place.

93.

  1. Cut a piece of 3/32" cross-grain sheet balsa and glue it on the rear of the fuselage.

94.

  1. Cut a piece of 3/32" sheet, with beveled edges to fit against the triangular stock and glue it against the back of F-4.




95.

Complete the bottom rear fuselage planking by gluing on pieces of 3/32" sheet balsa. Allow to dry before removing. This method of construction prevents the fuselage from bowing or twisting after being unpinned, even if the wood on one fuselage sode is harder than the other.

96.

Glue FX cowl pieces on the inside of the fuselage front.
STOP: Before preceeding go back and reread the "Tips On Tanks" section and install the tank hardware in the tank.

97.

  1. Fit the tank into the compartment.
  2. Glue scrap balsa blocks to each side of the fuselage.
  3. Don't get the blocks too tight - just enough to keep the tank from rattling around. The tank will need to be easily removable after the top and bottom of the fuselage are closed up, so make certain now that there is no obstacle to doing this. Note particularly the "bubble" on Sullivan tanks, which must clear the side blocks. The bubble is not used on the Kobra because the whole side of the tank is placed horizontal. The tank will work fine without the "bubble" being in use.




98.

  1. Glue F-2S, a 3/32" plywood square to F-2.
  2. Mark and drill the dowel hole. Before preceeding, you will need the completed wing.

99.

  1. Set the wing in the fuselage saddle. If it does not fit the saddle exactly, sand as required to make it fit. Check to see that the fuselage sits squarely on the wing.
  2. Hold the wing in position and mark the dowel holes in the wing by punching through the hole in F-2 with a 1/4" drill, a piece of tubing or a rod.
  3. Drill the holes in the wing out oversize - about 9/32" diameter - to allow some "wiggle" room during final positioning and gluing in of the dowel.
  4. Put a piece of wax paper over the face of F-2 and insert the dowel through the paper into F-2.
  5. Coat the hole in the wing with Kwik-Set Glue. Don't overdo the amount of glue.
  6. 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 hole. Allow the glue to set up firm, 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 hole is not completely filled with glue, fill it. If necessary, now that the dowel is set in place, you can cut away the wood around it to provide room for filling any remaining crack with glue.

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100.

  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.

101.

  1. Locate the position 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.21 drill. (5/32" is the nearest inch size equivalent.)


102.

  1. Run a 10-32 tap through the hole to cut threads in the wing bolt anchor blocks.
  2. Drill out the holes in the wing with a 3/16" diameter drill to pass the nylon wing bolts.




103.

We did not find it necessary to reinforce the wing bottom around the bolt holes, since the silked and doped finish was sufficient to harden the surface so the bolt heads would not sink into the wing. With plastic film covering, you may want to glue a plastic square from the landing gear cover scrap to the balsa for the bolt head to seat against.

NOTE:
Before proceeding, give some thought to the radio installation you will be using. The Kobra is not a large model with space to spare. But there will be no problem getting any standard set in the model if the installation is planned properly. The basic requirement is to put the rudder and elevator on the fuselage walls toward the back of the compartment. This leaves space between them for unhindered passage of the aileron pushrods and horns when the wing is on. Don't worry about putting this much weight that far back, the balance works out okay this way. In fact, the battery in the prototypes was mounted in the fuselage over the wing, not in the nose. Place the motor control servo just ahead of F-3, also on the wall of the fuselage. This leaves considerable remaining space for the receiver and battery. (The first prototype carried a big 7 channel receiver easily.) The plastic servo mounts were fastened to the fuselage sides with double-sided foam tape. It so happened that no Kraft mounts of the kind needed were on hand when the installation in the first prototype (the F-16) was done but it was found that Futaba "H" servo mounts fit the Kraft 14 servos fine, so they were used. Also, read the section on "Radio Hookup" included in these instructions.

104.

The engine and nose wheel pushrods should be installed now while access to the fuselage interior is easy. Use an extra length drill to make the holes for the nylon outer tubing. Or, put a point with a notch on it on a long piece of music wire and use that for a bit.

105.

This shows the nose wheel pushrod running to the rudder servo.

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NOSE GEAR INSTALLATION

The wheel collar suggested as optional for the nose gear is not furnished. File or grind a notch in the collar so it will fit down on the coil farther. The collar permits altering the height of the nose gear slightly if desired. Don't try to make large adjustments in nose wheel height with the wheel collar because the landing gear is more easily bent on a hard landing if the coil spring is located very far below the nylon nosewheel bracket. Large adjustments should be made by changing the wheel size.


106.

The front of the model with hookup of the nosewheel pushrod to a Goldberg PC-1 or Du-Bro 121 fitting on the steering arm.

107.

Futaba servo on an "H" mount for the motor control pushrod.



108.

Sheet the top of the fuselage with pieces of 3/32" balsa, crossgrained.


Notes On Loops

A true wing will perform perfect loops. A twisted wing will loop obliquely. One wing half being heavier than the other may also affect loop tracking. Side mounted motor may make one side of the model heavier than the other. Put weight in the opposite wing tip until balanced.
Should your model snap roll out of the top of a loop, it may snap in the direction of any twist in the wing, but the real reason for snapping is because of a stall. This is probably due to one or more of the following:
  • Airspeed too low.
  • C.G. too far back.
  • Pilot pulls too much elevator, a mistake aggravated by excessive elevator travel which makes the elevator more sensitive. Reduce travel of the elevator and use more care in transmitter stick movement.
  • Not enough power, too high a wing loading for the available power or both.


109.

Glue the 5/16" sheet bottom on the front of the fuselage. (Note: You may want to resin or otherwise fuel-proof the inside of the nose before doing this. We used Sig Epoxy, warmed to make it thin and easy to brush.)

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110. Mark the approximate shape of the corners of the model. (See the cross-section drawings on the full-size plan.)

111.

Cut the bottom nose block to fit between the FX pieces and carve out the interior to allow easy screwdriver access to the nose wheel steering arm setscrew.




112.

Glue pieces of 3/4" triangular stock on the front of the cowl.

113.

  1. Put the engine in the model with the spinner backplate installed.
  2. Mark around the spinner on the fuselage front.




114.

Carve the cowl to a pleasing shape. For practicality, this is as far as most builders want to go in cowling in the engine, so as to have easy access to the front end. On the prototypes however, a few more pieces of scrap wood were glued into the cowl on the top to provide some more carving area for a more completely cowled engine. We'll leave it to the individual builders to determine how practical or pretty he/she wants it to be. One observation we'd like to make is that in a small model like the Kobra, there is very little surplus room in the cowl. Changing the throttle linkage or the nosewheel steering is not as easy to do as in a model with a removable cowl. But once adjusted they do not need much attention.




115.

Place the canopy and plastic turtle deck on the top of the fuselage. Carefully center and position them and then trace around them with pencil. This will provide guide lines for installation on the 3/32" sq. gluing strips.
NOTE:
In reviewing our building sequence for the Kobra, we have decided that it is easier to round and sand the corners of the fuselage before the gluing strips are put on. Use the penciled guide lines to tell where to stop the shaping process. Photo 116 shows the rounded fuselage without the gluing strips in place. Do yours this way and ignor the fact that our fuselage still hasn't been rounded in the next few pictures.

117.

Glue the 3/32" sq. gluing strips on, allowing space for the thickness of the plastic parts between the line and the strip.

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118. The gluing strips must be trimmed and beveled to fit against the plastic part.

119.

Set the parts in place to check how well they seat on the gluing strips and correct as required for good contact.
NOTE: It is recommended that you do not glue the canopy and turtle deck to the fuselage until it is covered and/or at least partially painted. The canopy framing should be painted before the canopy is guled on.
See "Painting The Canopy" further on in these instructions.




At this point you will need the completed stab and fin assemblies.

120.

Pin the stab in place on the rear of the fuselage to check on how square it is with the wing. Correct any misalignment by sanding the fuselage. Do not glue the stabilizer on until later, after it and the fuselarge are covered.



121.

Temporarily pin the fin and rudder in place.

122.

Using the FT tail fillet patterns, carve the fillet blocks roughly to shape. (See plan for block layout drawing.)

123.

  1. Pin the blocks on each side of the rudder.
  2. Use the plastic turtle deck to shape the front of the blocks and the fins so the plastic will fit over them when in place.




124.

Trace the plastic turtle deck shape on the face of the block.

125.

Carve the block to shape. Glue it in place later, after the tail and fuselage are both covered and joined.

126.

  1. Put a piece of waxpaper against the front of the fuselage and mount the wing.
  2. Carve part WZ to fit on to the contour of the front of the wing and glue on.

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127.

  1. Cut WX pieces to fit against WZ as shown.

128.

  1. Remove a strip of planking between the two WX pieces.

129.

Insert a piece of 1/16" sheet between the WX pieces and flush with the center section planking.




130.

Carve the WX pieces to match WZ.

131.

Put the wing back on the fuselage and sand the wing cuff to match the fuselage shape.

132.

A Futaba "V" type servo tray is shown for the wing.
  1. The landing gear block was used for the rear mounting screws.
  2. A small scrap block of hardwood was glued to the bottom planking for the front mounting screws.



Other types of mounts may require a different placement of hardwood blocks for mounting the aileron servo.

133.

A Futaba FP-S16 servo is shown in the wing, hooked to the aileron horns.



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TAIL SURFACES

You have the option of building the model with either an F-16 styled swept fin or T-38 type of straight fin. The instructions printed on the 3/16" tail printed sheets indicate how to distinguish one tail shape from the other. However, we will show here the pieces for each style of tail. Study them before cutting on the sheet to make certain you are cutting on the right line. You may ask why there are so many parts, when it would be easier to cut the tail from a larger single piece. The reason is to provide a stiffer and more warp resistant tail.The different grain direction of the parts makes the finished surface stronger.

134.

These are the F-16 style printed parts. Sand the edges with the sanding block so that a good joint is obtained.









135.

  1. Glue the parts together on a sheet of wax paper to prevent them from sticking to the board.
  2. Add pieces of 3/16"x3/8" wood to the top of the fin and rudder. (Left over scrap from the wing spars.)
136. These are the T-38 style tail printed parts.

137.

  1. Glue the parts together.
  2. Add the 3/16"x3/8" scrap tips.
138.
  1. Glue S-1 and S-2 together.
  2. Add the 3/16"x3/8" scrap tips.

139.

Sand the printed lines off the tail parts with the sanding block.

140.

Round the leading edges of the fin and stabilizer with a sanding block. Carve and sand the rudder to the cross-section shape shown on the full size plan.

141.

The tail surfaces are hinged in the same manner as the ailerons. Cut the slots for the hinges, but do not glue the hinges in until after the tail parts are covered, either with silk or iron-on covering. It is particularly necessary with iron-on covering to cover before assembly so that the iron can be properly applied to the edges. (Some builders like to color dope and/or decorate the parts also before joining.) Join the tail parts together after covering is completed.
Cover the fuselage before gluing the tail to it, but cut away the covering to expose the bare wood in the spots where the tail is glued on. Always have wood-to-wood joints. Never have covering between the parts. Be sure that the stabilizer is lined up square to the fuselage. Coordinate the fitting of the stab with the fitting of the wing so that they are both aligned properly.

142.

If desired a nylon pushrod guide (not furnished) can be used in the pushrod rudder exit slot.
143. The elevator pushrod exits through the rear fuselage opening.

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RADIO EQUIPMENT

NOTES ON SERVOS
Most servos are standardized as to which direction they move in response to particular transmitter stick movement. Regardless of the direction of movement of the servo, you can adapt to it by moving the pushrod to whatever side of the servo output arm or wheel will give you the pushrod movement direction desired. Sometimes this requires that a pushrod brought down the side of the fuselage has to crossover to the inside of the servo output arm to get the desired direction of pushrod movement.

Some radio manufacturers make available reverse direction servos and often include one or more in an outfit for situations where the opposite direction of pushrod movement without changing servo sides is desired. For example, it is desirable to have the hookup for the pushrods to the nose on the outside of the servo so that the pushrod tubing need not be flexed as far as crossing over to the inside would require. At the same time the pushrods to the tail would be on the inside where there is plenty of room to maneuver it around. Several companies make reversing converters that can be plugged into a servo cord to reverse the direction of movement of a standard servo. But if you do not have a reverse servo it is quite possible to get along without it.

Which Side For The Rudder Pushrod?

The choice of which side of the fuselage the rudder pushrod will exit from is determined by the position of the throttle control arm on the engine to be used. If it is on the right (most common), use the servo nearest the right side of the fuselage for motor control. Use the servo nearest the left side of the fuselage for the rudder, with the rudder pushrod coming out to the left side of the rudder and the nosewheel steering arm hooked up on the left side of the nose gear bearing.

The photos show typical radio installations in the Kobra. There is enough room in the model for just about any standard radio though some of the larger size battery packs may be difficult to get into the nose under the tank. Neither of the prototypes required the battery to be in the nose for proper balance. If you have a large battery pack it may be advisable to finish the model completely except for permanent mounting of the radio components. Set them temproarily in place and test balance the model, shifting the equipment as may be required to achieve the suggested balance point. The best place for the battery out of the nose is either against the back of F-2 or the back of F-3.

As you can see, it is best to know in advance the radio and motor brand you will be using before you install permanent cable pushrods. Decide on which type of fittings you will use in the case of cable pushrods and have them on hand during construction because the type chosen will affect the location of the pushrod exit holes through the firewall, etc. The balsa pushrods to the rudder and elevator are not limited as to location and can be adapted to any of the types of connectors shown without preliminary planning.

Pushrods

1/4" square balsa sticks are provided to make the fuselage pushrods that run to the elevator and rudder. Bind the fittings to each end with heavy thread and epoxy glue. Use threaded rods with RC links at the tail end of the pushrods so that trimming adjustments can be quickly made. Straight pieces of 1/16" diameter wire are provided for the other end of the pushrods to allow hookup with the servo arm.

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Make the control surface ends of the pushrod first by attaching the RC link rod. Feed through the fuselage, hook the RC link to surface, cut the servo end of the balsa to the exact length needed, measure and install the servo connecting wire end. Some of the variety of detachable pushrod retainers for securing the pushrod wires to the servo that are available are shown here. Or you can make a "Z" bend in the wires to go into the servo. When a "Z" bend is used, the pushrod must be put into the servo outside of the fuselage and then threaded through the fuselage, which is more difficult to manage than the pushrod alone, as is the case when a retainer fitting is used.



SERVO HOOKUP ACCESSORIES
Sig Solder Clevises (SIGSH527) are dependable and very easy to install. If you are unsure what type of servo connector to use, try solder clevises.
A wide variety of servo connectors are available for attaching the servo end of each pushrod to its servo output arm. No servo connectors are supplied with this kit because modelers usually develop a personal preference for one type over another and often already have them on hand. Here are photos of a few types.
A Sig pushrod connector (SIGSH736) is especially handy for cable end attachment, as on the nosewheel steering arm, but can also be used on servos as shown. They are easily adjustable by loosening the screw and sliding the cable. Sig Nylon Pushrod Keepers (SIGSH184) or Goldburg Snap'R Keeper can be installed on a wire pushrod after it has been bent 90 deg. and inserted into the servo arm. Not recommended for cable pushrods.
Du-Bro Ball Links, which come in several different types (threaded, bolt-on, rivet etc.), gets the pushrod action up above the control arm so the pushrod can approach from a variety of angles without chance of interfering with the servo center post. It is good for cable pushrods. A fine adjustment can be made by screwing the end in and out. You won't need any connector for the wire pushrods if you can put a "Z" bend in the end of the wire. It's a little tricky to make a good clean "Z" bend, so practice with some scrap wire before trying it on your good pushrod. Not suitable for cable pushrods.

PREPARING CABLE PUSHRODS

To keep ends of cable from unraveling during handling, tin the end with solder. Use a non-corrosive paste flux (shown here is Kester, available at hardware stores) and rosin core solder. Use a very hot iron to heat the cable and then flow the solder completely through the strands.

When cool, grind or file the end smooth. Taper it down slightly so that it will go int the pushrod fittings and nylon outer tube easily..

After the proper length is arrived at, sweat solder the area to be cut so that it will not unravel while being cut. Cut through the tinned area with a good pair of side-cutting pliers, a cut-off wheel on a motor tool or a file.