The Rapier Mk. IV
for "SSC" combat


**** NOTE: These instructions are currently being re-written. Right now, they are half-done, so read carefully and contact me if you have any questions. And check back later for updates.
Kit parts: Needed to complete: Recommendations: Coroplast Parts:
  1. Cut out the parts from the 4mm coro sheet. You will need 2 fuse side panels, a horizontal stab/elevator, and a vertical stab.
  2. The exact shapes are not critical. However, do not make the tail surfaces too small. The vertical stab is large to help with handling when a streamer is hanging on the wingtip and also with spin recovery after a mid-air collision.
  3. Horizontal Stab Dimensions
  4. Use the Vertical Stab Pattern if desired. Horizontal stab, showing hinge and reinforcement
  5. Pick a flute in the horizontal stab to become the elevator hinge. I make the elevator 2" wide.
  6. Glue a fiberglass rod into the flute in front of the elevator hinge
  7. Glue a bamboo skewer in the flute behind the elevator hinge.
  8. Cut out one side of the hinge flute. Fold the elevator back and forth until it moves freely. A picture is worth a thousand words here.
Wing Construction:

1.Trim TE of foam cores to the 5/16" thickness point. Find the point on the core where the TE is 5/16" thick and mark it. I use the 5/16" balsa TE stock, and just mark where it matches. Using a straight edge and a sharp razor knife, trim the TE, sand straight if necessary. Wing TE marked and cut to size
2.Sand LE round. Use the sanding block and 60 grit paper, sand the LE to a nice round shape. Don't worry about taking off too much material. The exact shape is not critical, but a nice blunt LE helps delay stall, which means tighter turning. Wing LE sanded to shape
3.Epoxy TE stock to core. Cut the TE stock to the proper length. Use masking tape to hold the wood to the core until set.
4.Match core roots. Trial fit the ends of the wing cores together. Since the wing is constant chord, pick the ends that fit together best as the root. Sand as necessary to make a straight, flat wing with no dihedral. Mark the cores so you can tell which is the root later.
5.Mark spar slot locations. Measure from the TE (since the LE is sanded to a round shape, it's not good to measure from), and mark where the wing spar slots will be cut. Note that the spars are NOT directly above each other. This offsetting of the spar seems to make the wing more damage resistant. Mark a line down the core to aid in cutting.
  • Top spar 6.25" from TE
  • Bottom spar 5.75" from TE
  • 6.Cut spar slots. I use a Dremel tool with a 1/8" cutting bit and a cutting attachment. I set the depth at a hair deeper than 1/8". Some guys have good luck using a soldering gun and straight edge. A table or bench saw can also be used, as many saw blades cut out about 1/8". The slot does not have to be absolutely perfect. I cut by eye, just running the cutting bit along the edge of the line. I cut along the front part of the top spar line, and to the rear of the bottom one, which has the effect of staggering the spars just a little more, but it's not critical. Spar slots cut in to wing cores
    7.Sand balsa TE to shape. Sand the TE stock down so that it follows the top and bottom surface of the wing. In a perfect world, the TE should wind up close to 1/4", but it's not critical. You can do this step earlier, but I used 15 min epoxy in step 3, and it's just now solid enough to sand. You can also do this later, but it's a pain once the wing halves are joined.
    8.Epoxy wing cores together. Make sure the TE, LE and spar slot all line up, then epoxy the cores together. Use masking tape to hold everything in place until set. Wing cores epoxied together and held with masking tape
    9.Cut fiberglass rods 62" long if necessary. The spars should be 1" short of the wing tip on each side. This helps reduce damage in a cartwheel. Save any excess fiberglass rod for use later.
    10.Spray the wing with 3M-77. Use a light misting from a foot or more away. The glue is heavy, so go easy on it. The current 3M-77 uses acetone as a propellant, and acetone can eat the foam. However, if the can is held far enough away, the acetone will not harm the foam. You should make a few practice passes on the wing shucks to get a feel for how to spray the glue. Note, it's ok to let the glue dry before the next steps.
    11.Glue in spars using either polyurethane or epoxy.
  • Apply a bead of polyurethane glue in to the spar slot on the top of the wing.
  • Mist the spar with water, and push the spar fully into the slot.
  • Run a strip of strapping tape (bi-di tape is best) from wingtip to wingtip over the spar. Pull the tape as tight as possible, some tension helps strengthen the wing.
  • Repeat the above steps for the bottom spar
  • Then place the wing back in the shucks, place on a flat surface and apply weight to keep the spars from lifting out of the slot.
  • 12.Mark and cut recesses for the servo-mounting rails. I use 1/8" aircraft ply cut 3/8" wide to mount the servos, you may prefer something else. If using a 3/8" servo rail, it should be positioned 1/4" back from the top spar. Make sure the servo will not hit the bottom spar. Mark the outlines of the servo rails and the servo on the wing and cut the recesses. Then epoxy the servo rails in place. I again use the Dremel tool to cut the foam. Aileron servo mount locations marked, and mounts installed
    12.Glass wing center section. I use a 5" wide strip of heavy (5 oz.) glass cloth. Using two separate pieces, start with the bottom surface of the wing, overlapping approximately 1-2" around the LE and TE. Then do the top surface, again overlapping. This creates a strong double layer on the LE and TE to help prevent the wing rubber bands from crushing the foam. If less (or no) fiberglass is used, some sort of reinforcement should be added to the LE and TE to prevent crushing.
    13.Cut out hole for aileron servo.
    15.Strapping tape LE and TE. Apply a strip of tape wrapped around the TE and another wrapped around the LE. Strapping tape applied over spars, LE, and TE
    16.Cover wing with colored tape. Apply the tape from wing tip to wing tip, working from the TE toward the LE. The wing can also be covered with low-temp covering such as Ultracote or Econokote. I believe that tape is stronger and more durable than the heat shrink coverings. And most brands of tape will actually shrink with heat application.
    17.Cut ailerons to length. The aileron should start 2-1/2" from the center of the wing.
    18.Bevel aileron LE. I hinge the ailerons at the top, so I cut a single 45 degree bevel.
    19.Cover aileron with Bi-di tape. Run a strip of packing tape from tip to tip, wrapping around the aileron TE. Then run a second strip wrapping around the LE. The tape will dramatically stiffen the balsa aileron to prevent flutter, and makes the aileron a lot more damage resistant. Ailerons beveled, then reinforced with strapping tape
    20.Hinge aileron with strips of Bi-di tape. Cut 18 strips of tape approximately 1/2" wide by 2" long. Attach tape sticky sides together, overlapping about 1/4" or so, giving 9 hinge strips. Run the tape from the top of the wing down under the aileron. The next strip should go the opposite direction. Put one set of 3 at each end of the aileron and one set in the middle. Aileron hinged with strapping tape
    21.Cover hinge top and bottom with colored tape. Flip the aileron up on top of the wing and run a strip of tape down the aileron. Then flip the aileron down and hold it in the full down position and apply a strip of tape down the aileron.
    22.Cover any exposed clear packing tape with scraps of colored tape. Extended exposure to sunlight can cause the clear tape's glue to break down. The underside of the wing is fine, as it's not exposed to the sun as much.
    24.Install aileron servo, control horns, and pushrods. Aileron servo and pushrods


    Fuselage Construction

    1.Mark fuse center line. Measure and mark the centerline on the fuse blank. Mark the center at the front and rear of the blank and draw a straight line between them. Note that it's possible that the foam isn't exactly straight, so the line may not be in the center all the way down. That's ok, it will get fixed later, and this is a combat plane after all. But make sure the centerline is straight.
    2.Mark the following points along the center line, measured from the front:
    • 3.5" - back of the fuel tank
    • 6" (approximately) - front of battery compartment cutout
    • 10.5" (approximately) - back of receiver compartment cutout
    • 13.5" - front of elevator servo (assuming standard servo)
    • 17" - front of vertical stab slot
    • 18" - front of tail mount reinforcers
    • 19" - back of tail mount reinforcers
    • 20.5" - back of vertical stab slot
    3.Taper fuse from back of receiver compartment mark. I cut the foam with a long knife and sand to shape.
    4.Cut firewall reinforcement bevels on the front of the fuse core for the triangle stock. Front of fuselage cut for firewall reinforcement and marked for fuel tank
    5.Install triangle stock firewall reinforcements.
    6.Poke a series of pinholes through one side of the coro side panels and fuse bottom. A "wood pecker"-type tool works well, but a T-pin or knife can be used. The goal is to allow some glue to go through the plastic to create a better bond.
    7.Glue on coro side panels and fuse bottom. I prefer polyurethane glue, but epoxy works too. First glue on the side panels put on the fuse bottom. Hold everything together with tape, clamps or weights. I wrap the fuse in masking tape. Before the glue is dry, do the next step. Glueing on fuselage sides and bottom
    8.Glue the 1/8" fiberglass fuse reinforcing rods into the flutes of the coro side panels. This is best done before the glue from the prior step drys. The rods should go into flutes just below the lowest point of the wing saddle. You want the rods as high as possible on the fuse, but you don't want to cut in to the rods later, when you cut the wing saddle out of the coro. Leave the rods flush with the front of the fuse.
    9.Optional: Use clear or colored packing tape to tape over the seams between the side panels and the fuse bottom. Pull the tape tight around the corner from the side to the bottom of the fuse. This will "round" the fuse slightly, and help prevent the seam from opening and allowing oil and dirt inside the fuse.
    10.Drill 1/4" holes through the fuse just in front of the wing LE and just behind the wing TE. The hole should be just below the fuse reinforcing rod. Epoxy 4.5" long 1/4" fiberglass rods in place. Wood dowls may also be used, but may shear off in a mid-air, fiberglass rods have proven to be worth the extra weight.
    11.Mark and cut two recesses for the throttle serve mounting rails. The servo should sit just behind the front of the wing saddle. If the servo is too far back, the low point of the wing may interfere with the servo. Epoxy the mounts in place.
    12.Cut out hole for throttle servo.
    13.Trace the outlines of the fuel tank, battery, and receiver on to the fuse. The back of the fuel tank should be 3.5" from the front. The receiver and battery locations depend in part on the specific battery and receiver being used.
    14.Cut out radio and fuel tank compartments. Note that in this picture, the radio compartment is one large cutout. This is the older design. the fuse is stronger if two smaller, separate cutouts are used, one for the battery and one for the receiver. The separate cutouts also help keep the battery from damaging the receiver in a crash. Fuel tank, servo, and radio compartments cut out
    15.Cut holes for the elevator servo mount rails. For ease of construction, you can mount the rails flush with the top of the fuse, or you can recess the servo rails into the fuse to allow only the servo arm to be above the fuse top.
    16.Epoxy in the elevator servo mounting rails. As an option, you can glue in some scrap foam over top of the elevator mounting rails if desired. This fills in the fuse around the servo better, which keeps out oil and dirt, and may add some strength.
    17.Cut a firewall to fit, and epoxy it in place.
    18.Fuel proof the wood firewall and the first inch or so of foam with epoxy.
    19.Cut the 1/4" HDPE to fit your engine.
    20.Cut the aluminum angle to length. The bottom piece should be a hair shorter than the width of the HDPE mount or the firewall. The top piece should be the same, but may need a corner cut off to allow clearance around the throttle pushrod. Engine Mount
    21.Assemble the engine mount from the HDPE and 2 pieces of aluminum angle. 3 4-40 size screws are sufficient to hold the mount together.
    22.Screw the engine mount to the firewall. I use 4 #4 wood screws. Important! The engine mount should be located as low on the firewall as possible. This is to get the engine's carb down compared to the center line of the tank. If the engine is mounted in the center of the firewall or higher, then the engine may go lean during a high-G turn. Engine Mount
    23.Drill holes through the firewall and foam for the fuel lines.
    24.If using an internal throttle servo, drill a hole through the firewall to the throttle servo for the throttle pushrod. I find that using a nyrod throttle linkage makes installation go easily through the foam, and allows some flex in the event of a mid-air.
    25.Place the fuel tank into the tank compartment and hook up fuel lines.
    26.Wrap the front of the fuse in colored tape. The tape holds in the fuel tank in, keeps fuel out of the foam, and reinforces the fuse. As an option, use some clear tape instead of the colored tape to allow part of the tank to be visible. This makes it easier to fuel and verify that everything is working correctly. Front of fuselage with wing dowl and tape over fuel tank
    27.Tape over the top of the fuse with colored tape to protect the foam from fuel.
    28.Install the radio and engine.


    Tail Installation

    1.Mark the location of the slot for the vertical stab. The slot will be 4mm or 1/8" wide and 3.5" long. It should be centered on the fuse center line, and 17" from the front of the fuse core. Vertical tail slot and tail reinforcer slots
    2.Mark and cut out the slots for the tail reinforcement plywood. These slots should be 1/8" wide (one pass with a dremel cutting tool), 1" long, and 1.5" deep. They should be 18" back from the front of the fuse, and on either side of the tail slot.
    3.Mark and cut the slot for vertical stab. I use a Dremel with a long cutting bit, but it can be done with a knife or heated bit of music wire. The slot should be the length and depth of the base of the vertical stab, and about 4mm or 1/8" wide. A single pass with a dremel cutting tool is about right. Coro scrap placeholder in tail slot and plywood reinforcers
    4.Cut a scrap of coro the same size as the base of the vertical stab to be a stab placeholder. Wrap it in wax paper to keep it from getting glued in the fuse, and insert it into the vertical stab slot. Now use epoxy or polyurethane to glue in the plywood tail reinforcers on either side of the vertical stab placeholder. To ensure a solid glue joint, use clamps or tape to hold everything together while the glue dries. Closeup of plywood tail reinforcer
    5.Remove the vertical stab placeholder and as much of the wax paper as possible.
    6.Cut the horizontal stab slot from the back of the fuse up to the tail reinforcers. The slot should be cut in the flute directly below the fuse reinforcing rod. Ensure that the stab sits level when viewed from behind, and is parallel to the bottom of the fuse. Horizontal tail slot with retaining rod hole and tail retaining screw
    7.Measure and mark the center line of the horizontal stab. Place the fuse on top of the horizontal stab, with the elevator hinge line just behind the fuse, and perpendicular to the center line of the fuse. Trace the outline of the fuse on to the horizontal stab.
    8.Now cut out the notch in the horizontal stab. When done correctly, the horizontal stab will "key" in to the fuse. The hinge line will be just behind the back of the fuse, and the stab will be level when viewed from behind and square to the fuse centerline. Horizontal stab cutout to match fuse
    9.With the stab on the fuse, insert a piece of 1/8" fiberglass rod in a flute in the stab, forward of the stab slot on the fuse. The selected flute should be towards the center of the plywood tail reinforcers glued in the fuse earlier. Push on the rod to mark the side of the fuse.
    10.Use a 1/8" drill to drill through the fuse. More than anything else, this hole will set the angles of the horizontal stab. Make sure the hole is square to the fuse center line and level with the bottom of the fuse. Use a sharp drill bit to avoid reaming the hole in the plywood out, you want as snug a fit for the 1/8" stab retaining rod as you can manage.
    11.Insert the vertical stab into the vertical stab slot. Make sure it is fully seated. Drill through the hole from the prior step to put a hole through the vertical stab.
    12.Use a pencil or knife to mark the portion of the stab that is blocking the horizontal stab slot. Remove the vertical stab and cut the marked area out of the vertical stab. (or, just cut the vertical stab while it's in the fuse by cutting along the top and bottom of the horizontal stab slot).
    13.Drill a pilot hole for the stab retaining screw. through the plywood tail reinforcers and the vertical stab near the top of the fuse. A servo screw or other wood screw works well.
    14.Insert a bamboo skewer in to a flute in the vertical stab. If everything worked correctly, there should be a flute between the hole for the stab retaining rod and the pilot hole for the retaining screw.
    15.Install the vertical and horizontal stabs. When installing the vertical stab, tighten the stab retaining screw enough to cause the fuse to clamp the tail in place but not enough to crush the coro. (Though the bamboo skewer in the vertical stab should prevent the crushing and provide something solid for the plywood to clamp in place.
    16.Optional: Glue a piece of triangle stock to the side of the fuse below the horizontal stab to add some stability. Push the triangle stock up against the bottom of the stab, but be sure not to accidentally glue the stab in place. Be sure to fuel proof any exposed wood.


    Setup and flying
    1. Set the ailerons with 1/8" of reflex. Reflex means that both ailerons are set slightly "upwards". It's measured at the hinge line. Set a straight edge on top of the wing, and measure the gap between the straight edge and the aileron hinge line. If the plane shows a tendency to hunt in pitch (ie, the nose pitches up and down on its own and won't stay in level flight, or the nose suddenly darts up or down), increase the reflex a little at a time. If you have it correct, the plane should easily track hands-off straight and level.
    2. Aileron throw is as much as you are comfortable with. The pushrod layout will naturally give some differential. 3/8" travel upwards is a good starting point, and increase it from there.
    3. CG should start at 2.5" - 2.75" from the wing LE. However, CG should be determined by flight testing, as proper CG is partly a pilot preference issue.
    4. Lateral balance is important. If the plane is not balanced laterally, it will snap out of tight turns earlier than it otherwise might.
    5. Elevator throw, start at 3/8" upwards and work from there. Elevator throw should be adjusted to produce the tightest possible turn without snapping out of a loop or turn in either direction. Elevator throw and CG are closely related when trying to achieve a tight turning radius.
    6. Be sure the engine can be stopped from the transmitter.