KOBRA RC53 COVERING AND FINISHING

IMPORTANT: Don't skip covering the fuselage and tail just because they are solid wood. Painting them without covering first is not enough. They will be much more resistant to splitting and breaking on hard impacts if they are covered with something - Sig Silk, Silkspan, Sig Silray or iron-on covering material.
The manufacturer's directions for applying iron-on coverings are packed with the material. Follow these closely, for different types of covering have different iron-on temperatures and techniques of application.
Whatever kind of covering you desire to use, it will not conceal a rough framework. Sand carefully with fine sandpaper before beginning to cover.

Finishing The Plastic Top

The plastic top should be sanded to remove the gloss before it is painted. Don't use coarse sandpaper, which can cut deep scratches. These scratches may open up during doping (which softens the plastic) and become noticable. Instead use something like 220 3-M Tri-M-lte no load silicon paper to start and polish down with 360 Tri-M-lte or 400 wet paper before color doping.

The plastic top may be painted with Sig Supercoat Dope. Care should be used not to apply heavy, wet coats of dope. Put on light coats and allow them to dry thoroughly before applying a second coat. A spray gun is a good method of getting a good finish with a minimum amount of dope. Be especially careful with spray cans not to wet the plastic too much. Spary several light dusting coats with adequate drying time allowed. Plastic may also be painted with Sig Plastinamel, K&B Superpoxy, Hobbypoxy or Du Pont Dulux Enamel. Don't use other paints without testing first on scrap plastic.

Do not try to cover the plastic top with plastic film covering. The iron will damage the plastic.

Covering With Silk, Silkspan, or Silray

Although we refer to silk in the directions, all of these coverings are applied wet in the same manner as follows:
Brush an unthinned or very lightly thinned coat of clear Sig Supercoat or Sig Lite-Coat Dope over all parts of the framework that will contact the covering. When dry, resand with fine sandpaper, to remove any fuzz or raised grain. Brush on a second coat and sand again.

The bottom of the wing is a good place to start covering. Cut a piece of material about 1/2" larger all around than half of the wing, with the grain running lengthwise. (The grain of woven materials runs parallel to the finished bias edge.) Some builders next dip the piece in water and apply it to the wing. I find that the silk sticks together and takes a lot of pulling and smoothing to get it in place so we do it a bit differently as shown in the photo.

Pin the dry covering in place and "paint" the water on with a brush.

Go around the edges, pulling out wrinkles and stretching the material smooth. You need not pull it up drum tight, in fact going to this extreme is not advisable. Just pull out all of the wrinkles. Use pins, if necessary, to hold the silk smooth, though wet silk usually stays in place without too much pinning. We like to fasten one end - in this case the center joint of the wing - pretty firmly with pins so that you can pull against this anchored end in stretching the silk the long way.




Brush around the outside edge of the stretched silk with clear dope. The dope will soak through the material and adhere to the dope already dried into the framework.

Trim off the edges with a sharp blade. We find that a thin double-edged razor blade is ideal for this, but a single-edged blade does okay and you can't cut your fingers on it. On the bottom, trim off flush with the wing all the way around. Go over any rough areas or places that have not stuck down properly with more dope and press the loose spots down as the dope is drying and getting stickier.

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The top half is done in identical fashion except that the silk should be brought down over the edges instead of being trimmed off flush. On the front, lap the silk over the edge of the bottom, over-lapping about 1/8". At the back, bring the material down over the back edge of the trailing edge but do not lap it over the bottom covering.

Use the same process on the tail section and fuselage.

Allow the water to dry out of the wood before applying the first full coat of clear dope. On the open framework area of the wing, brush the dope on sparingly. If too much is applied, the dope will be rubbed through the material and will run down the surface on the inside and form a puddle. When these puddles dry, the large amounts of dope solids in them cause more shrinkage than the rest of the covering and a scarred area results. So apply dope very lightly the first time over. A second coat will seal most of the pores of the material and from this point, running through will not be a problem.

Use one or two coats of regular Supercoat clear on the wing to shrink the covering. After that, unless the covering is still not tight and unwrinkled, Sig Lie-Coat low shrink clear dope is recommended to help warping. The solid wood fuselage and tail can have Sig Lite-Coat from the beginning if desired. Sig Supercoat Color Dope has low shrinkage qualities.

A third coat of clear should provide a good base for color. Sand lightly when dry with 220 grit 3-M Tri-M-Ite no-load paper. Don't bear down on the edges of the ribs or the silk fibers will be cut through. The color dope may be brushed or sprayed.

Supercoat Color Dope should be thinned with Supercoat Thnner for brushing. This helps prevent brush marks and gives smoother coats. Flow on wet coats and avoid rebrushing back over an area aleady painted. If brush marks show, you need more thinner. For sparaying, thin dope about 50-50. Add more thinner if the dope does not go on evenly.

If high humidity causes the dope to "blush" or turn white, the best way to handle this problem is to wait until the humidity situation improves and apply another coat of dope. This will eliminate the blush. If it is necessary to dope during high humidity, Sig Retarder may be used in place of part of the Supercoat Thinner (amount depends on the humidity) to reduce the tendency to blush.

Painting the entire model white is recommended for a good color base, particularly when white is part of the color scheme. Color coats can be sanded with 360 Tri-M-Ite or 400 or finer wet paper. When using masking tape for trimming, seal the edge with a coat of clear dope to prevent the color dope from bleeding under the edge. Don't leave the masking tape on any longer than necessary. The longer it is on, the harder it sticks.

Patterns are shown on the plan for the curved decoration which are not on the decal sheets. Masking off these curved parts for painting is made much easier if 1/8" wide masking tape is used. This will bend around corners easier than wider tape. Strips can be cut off regular width tape with a straightedge. After the decoration is outlined, wider tape can be joined onto the 1/8" tape to block off the nearby areas not to be painted. We use paper taped on to match the rest of the model to shield for spray painting.

Another way the curved parts of the decorations can be applied is with the use of a mechanical drawing ruling pen to draw them on the model using paint in the pen instead of ink. Thin the dope slightly with blush retarder to slow the drying process and aid the flow of dope through the pen points. Clean the pen frequently with dope thinner and wipe on a cloth before reloading with fresh dope. Don't try to draw a thick line with the dope and pen but instead draw a thin line on each side of the desired pin stripe (about 1/8" wide were used on the original) and fill in between the lines using the pen free hand and opened up for a wider flow. If you have a steady hand, use a small brush. Use a French curve to outline curved parts of the decorations.


The ruling pen method is also handy to touch up any rough edges of a mashed decoration after the tape is removed.

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Complete the job with several sprayed coats of clear over the color scheme. This seals the colors and adds gloss. For best results, it is not a good idea to try and mix different brands of paint. Use Sig products from the start.

Painting The Canopy

We recommend Sig Plastinamel for painting the framing outlines on the canopy. Dope is very difficult to use on a canopy plastic because of its warping action. Epoxy paint can be used, but it does not stick on the plastic quite as well as Plastinamel. Sanding the gloss off the plastic will help adhesion. Other enamels and plastic paints probably can be used, but test in advance, because no assurance can be given for other types.

T-38 Style Kobra Bottom Color Scheme
The wing stripes ane blue and the center spar is red. The front stripe is 3" wide with a 1/2" white gap between it and the 2" wide rear stripe.

F-16 Style Kobra Bottom Color Scheme
The fuselage bottom spear is red. The nose color of the F-16 is black back to the rear of the canopy on top.

Decals

Stik-Tite Pressure Sensitive

Cut out the decals with a pair of sharp scissors. Leave about 1/32" to 1/16" of clear edge around the decal. Round the corners as you are cutting.

Wet the surface on which the decal will be placed with soapy water (use diswasher detergent). Place the decal on the model and squeegee the water from underneath with a balsa paddle. Allow to dry. This procedure will prevent air from being trapped underneath as is possible when the decals are applied dry.

A CURE FOR FUSELAGE WARPING
You may have noticed when a piece of balsa is doped on one side and not the other, it will curl. The same thing can happen on the fuselage sides, under the wing opening, particularly when you put on a number of coats. (The rest of the fuselage will not show this effect to any extent because it is four sided and cannot distort.) The effect isn't noticable until after full cure of the dope and aging, which may take several months. To prevent this from happening, give the inside fuselage a coat of dope every time you give the outside a coat. This has an added advantage in making the cabin area fuel proof. In addition, when the hardwood servo mounts are installed, have them a little over-long so that the cabin sides are bulged slightly outward.

Balancing
WHY MODELS MUST BE INDIVIDUALLY BALANCED
It is impossible to produce a kit that will automatically have the correct Center of Gravity (C.G.) position. Balsa wood varies in weight and it is easily possible for wood in the tail to be an ounce or more heavier or lighter than average. One ounce of extra weight in the tail has to be countered by about 3 ounces in the nose. Don't pile a lot of fillercoat or finish, use excess glue or make large fillets on the tail surfaces. The motor you choose, whether or not a muffler is fitted, the size and placement of your radio equipment, etc. all affect the balance. If you use an unusually heavy motor or muffler you may have to carry the battery in the radio compartment instead of the nose or even weight the tail. Don't consider that whatever C.G. the model builds out to as "good enough". Check carefully and make whatever adjustments that are required. With the C.G. properly located, a Sig design should fly with only minor trim changes required.

BALANCE POINT RANGE:
Test fly with the balance point located at 3-3/8" back from the front fuselage-wing joint. If your model will spin in both directions at this balance point it need not be moved any farther back unless experimentation with balance trim during aerobatic flying shows the need to do so. The Kobra flies best balanced farther forward than is common practice with this type of model. Do not assume that it should be balanced at some other percentage point because of your experience or that of expert fliers in your club with a farther back balance point. Like the Kougar (which is sometimes flown at a more conventional balance point by modeler's who do not believe our instructions - and subsequently have bad results including snap rolls at low speed), the Kobra is notably stable and forgiving during low air speed situations yet is fully aerobatic in the balance range indicated.

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.....Kobra flies best balanced farther forward than is common practice with this type of model......

Flying with the balance point any farther back than 3-5/8" from the front wing fuselage joint is not recommended unless you are an expert flier with a purpose for doing so.

Put a piece of masking tape on the bottom of the wing in the center. Mark the distances from the leading edge (the wing fuselage joint) on it. A balancer can be made from a triangular architect's scale placed on a block high enough to get the wheels clear of the bench. Shift the model back and forth on the edge of the scale until the balance point is found. Balance with an empty fuel tank but with all the other equipment installed and the model completely finished and painted.

If the tail hangs down at the desired balance point, it is tail heavy. Add lead or weight to the nose or shift the radio equipment as necessary to get it to sit level. Do not attempt flight in a tail heavy condition.

In addition to the fore and aft balancing procedure described above, the performance of manuevers is improved if the model is also in balance spanwise. For example, if one wing is heavy it may affect turning and loop tracking. Inset weight into the opposite wing tip to correct this problem.

Control Movements

Various brands of servo can give different control movement direction and amounts of travel. For this reason, follow the measurements when setting the Kobra up for flight. Shift the RC link to whatever horn hole will produce the amount of movement shown in the drawings. Measurements are made at the trailing edge of the control surface.

The control measurements are suggested as a beginning. Test flights may indicate a need for more or less movement, depending on individual model differences, center of gravity (C.G.) location, your personal preference, etc.


Should the servo output arm or wheel provided on the radio equipment not provide enough movement of the surfaces, accessory output arms that are longer are available for most radios, either from the manufacturer or other sources such as Rocket City.

It is not uncommon for the best elevator neutral position to test out to be slightly off from level. This introduces some nose up or down trim to keep the model from climbing or diving when the transmitter stick is in the center. The exact best neutral elevator position for each particular model must be determined during flight testing. With the model flying at about 3/4 throttle, feed in up or down trim in with the transmitter lever until the model flys level. Land and observe this position of the elevator. Adjust the elevator pushrod as required to keep the flight checked "neutral" position when the transmitter trim lever is returned to the center. Don't have excessive elevator control movement. If you are not using full stick movement to make the tightest desired maneuvers, reduce the movement until full transmitter stick travel is used.

The adjustable nylon fittings on the aileron horns supplied with the Kobra provide a fairly good range of possible movement adjustment. Screw the nylon fitting in toward the wing to get increased movement of the ailerons and out away from the wing to get decreased movement. Check your neutral position (or whatever amount of trim displacement you have from neutral) before altering the amount of up and down movement because these points will likely be changed by the raising or lowering of the nylon fitting and will need to be re-established by legthening or shortening the adjustable RC clevises on the aileron servo pushrods that snaps onto the fitting as required to return the neutral points to where they were originally.

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BALANCE IS PART OF THE TRIMMING PROCESS
The balance point we arrived at for this design is a good place to start when trimming out the model for top performance. However, it should not be considered the final and irrevocable location. Individual models built from the same kit are slightly different from each other. The incidence may be changed a bit, a small or large engine selected, the total weight varies - even the skill of the pilot has a bearing on just what should be the exact C.G. point. For example, when slightly nose heavy, the model is more stable and less likely to stall or snap roll from over-elevating. This also cuts down the reaction of the model to control movements which is good during test and practice flights to help prevent over controlling. But later, if extra sensitivity and quick reactions are desired for aerobatic performance, a position farther back may be desirable. So try different positions, but make the changes gradually, checking results and the effect of the change control responses and the performance of the model in the air at a good altitude.

Scale Details

The cowl, as described in the instructions and plan, was left open in the top for easy access. However, if you like a slicker appearance, and don't mind the tighter access to the interior, the cowl can be built up a bit more by gluing pieces of scrap sheet around the top before carving. The accompanying photo of one of the prototype models shows the effect this additional fill-in provides.



Above are the decal sheets for the Kobra. All color decorations not on the decal sheets were painted on the prototypes with Sig Supercoat dope. See the finishing section for details on how this was done.

In addition to the method described there, using 1/8" wide masking tape on the curves, it is possible to make good use of the common adhesive-backed vinyl shelf paper available at hardware stores. Using a new blade in a modeling knife, cut the color sweep on the T-34 version for example out of a piece of the shelf paper and use the mask remaining for spraying or hand painting the decoration on the tail. For models covered with plastic film, the fin flash could be cut from sheets. Or follow the manufacturer's directions that come with the plastic film for applying this type of decoration. On the original prototypes, a small wing fillet of Sig Epoxolite was installed. The main purpose of this was to make a good seal at the wing fuselage joint. It was put on after the model was silked and doped up to the point of color doping.

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Here is the procedure for making an Epoxolite fillet:
  1. Mask off the side of the fuselage where the Epoxolite will be applied. On a model the size of the Kobra it need not be a very wide strip - about 1/8" was used on the prototypes.
  2. Tape a strip of wax paper on top of the wing on each side where the fuselage will touch the wing.
  3. Apply a bead of Sig Epoxolite to the fuselage side bottoms.
  4. Put the wing on and bolt in place. This will cause the putty to squeeze out from under the fuselage onto the wax paper protective strips on the wing.
  5. Spread the excess putty onto the 1/8" strip of exposed fuselage side and start a rudimentary fillet shape.
  6. After the Epoxolite has gotton partly stiffened, shape the fillet further using a wetted finger or tool of the desired shape.
  7. Allow the Epoxolite to set up.
  8. As soon as it is hard, but not fully cured, remove the wing.
  9. Trim off the excess that squeezed into the inside of the fuselage.
  10. Even the outside edge of the fillet and shape the fillet with a round tool covered with coarse sandpaper - 60 or 80 garnet. If you use fine paper it will clog up to rapidly. Be careful not to sand through the masking tape protecting the fuselage, but sand down to it so that the tape can be peeled off, leaving a clean edge on the fillet.
  11. After the fillet has cured (24 hours), fine sand it and feather the edge formed by the masking tape into the fuselage side.
Don't get carried away with this idea, it can get heavy. Keep the fillet small. A model the size of the Kobra is a better flier if the wing loading is kept in a reasonable range.


FLYING

IMPORTANT: The Kobra is not a basic trainer, if you have no previous RC flying experience you cannot successfully fly a fast and responsive design like the Kobra, particularly on test flights. Do not attempt flying the model without the assistance of an experienced RC flier. Contact your local model club or ask your hobby dealer for the names of good fliers in your vicinity and a suitable location for RC flying.

Be certain to carefully range check your radio equipment and see how it operates with the engine running before attempting test flights. A lot of problems can be avoided if the engine has been well broken-in and the idle adjustment perfected on a test block or in another airplane before installation in this model.

Takeoffs with the Kobra from grass fields are easily made if the grass is not too long or the ground too rough. Generally a lot of elevator application is required for liftoff. Be prepared to relax control pressure particularly after becoming airborne so the climbout will not be too steep. On surfaced or smooth dirt runways less application of elevator will be needed.

Use the ailerons or rudder to keep the wings level and headed straight into the wind until about 75 feet of altitude is obtained. Keep first turns gentle and not steeply banked. Stay up wind of the transmitter. Use trim levers on your radio equipment where necessary to obtain straight and level flight with the control sticks in neutral position but don't attempt to make these adjustments until the model is at a good altitude. Throttle back at the altitude to find out the model characteristics in a gliding condition so that some indication is seen of what to expect during the landing approach. It is a good idea to make several practice landing approaches at a good altitude to get the feel of the model for this approaching critical maneuver. Make your final and complete landing approach while your engine still has plenty of fuel remaining so that the engine is not liable to stop before completion of the flight. This will allow application of power if the approach is being under shot. Notice the percentage of missed landings at an R/C field. Those undershot greatly outnumber those missed by overshooting. So if an approach that looks a little high is maintained, chances are good that a spot-on landing can be made.

After you get through the first flights you should begin to "trim" the model's control surfaces. If it is turning to the right, for example, with the stick in neutral, and you must move the transmitter trim lever to the left to make the model fly straight, then land the model and position the rudder to the left of center by turning the RC link on the pushrod one or two turns on its threads. Check in the air for the result. Repeat the process, if necessary, until the trim lever is centered when the model is flying straight with the stick in neutral.

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You may find that the reaction of the model is different to high and low power, requiring changes in trim lever position during flight, as for a landing approach. This is one of the controls you must learn to operate during practice flying, but it is not a critical matter at first since these minor corrections can be made with stick movement alone as you are steering the model along its course.

It is impossible to give exact directions for every case, since individually built models vary slightly and the engine used also affects results. But if the model is not flying in a satisfactory manner, the chances are it is not trimmed properly and should be adjusted accordingly. Do a little tinkering, a bit at a time. This is an instructive way to fathom the mysteries of perfect trim and in the process you can improve your flying performance considerably.

Grass Field Flying

If your flying field is not smooth, paved or closely cropped grass, lift off qualities on takeoff will be improved by bending the main landing gear forward so that the wheel position is up to 1" farther ahead than that shown on the plan. This makes it easier to rotate into lift off attitude as up elevator is applied during the takeoff run through the grass. The model will not stick on the ground quite as well using this forward landing gear position when landing on paved surfaces, but not to any troublesome degree.



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SIG MFG. CO., INC............Montezuma, Iowa 50171-0520


LIMIT OF LIABILITY:
In use of our products, Sig Mfg. Co.'s only obligation shall be to replace such quantity of the product proven to be defective. User shall determine the suitability of the product for his or her intended use and shall assume all risk and liability in connection therewith.