The RA Cores

GremTwin

 

Hyperlinked version available at http://www.racores.com/GremTwinInstructions.htm

With clickable images that expand in color

{Copyright 2011-2013 RA Cores, Southbridge MA - All rights reserved}

 

Produced By

Jim Reith

RA Cores

http://www.racores.com/

Full Kit Instructions......................................................................................................................... 3

Building the Fins.......................................................................................................................... 3

Building the Wing........................................................................................................................ 3

Building the Fuselages................................................................................................................. 6

Almost-Ready-to-Cover Kit Instructions....................................................................................... 10

Completing the Fuselage Pods................................................................................................... 10

Completing the Wing................................................................................................................. 11

Final Assembly.......................................................................................................................... 13

Final Balancing.......................................................................................................................... 15


Full Kit Instructions

Building the Fins

1)  Select the 1/8" sheet balsa fins and 1/8" x 1/2" x 5" sheet balsa fin caps from the wood parts.

2)  Glue the fin caps across the top edge of the fin and tape to dry


Building the Wing

1)  Select the precut 1/8" lite plywood spars, 1/4” plywood canopy handle, 1/8" x 2" x 10" sheet balsa wing tips, 1/2" x 1/2" x 3"  basswood rear bolt blocks, 1/2" x 3/4" x 3"  basswood front bolt blocks, 24” pieces of ¼” x 3/8” x 24” balsa trailing edge stick and 48” of ½” half round pine molding from the wood parts. Take the foam cores out of the core beds. Locate your 18”-24” servo extensions.

2)  Take the two spars and transfer the top fuselage side slot locations to the bottom edge of the plywood for the location of the holddown blocks later. Transfer them to the bottom of the rear spar also.

3)  Dry fit the spars into the pre cut slots in the foam cores with the notches facing up. Align the spars with the top and bottom surfaces of the wing cores.

4)  Once the spars are in both wing halves and the root of the wing is pressed together around the canopy handle, position the bolt blocks so they are ahead of the spars on the wing and centered on the fuselage side slot markings. Cut out sections of foam for them to be embedded in the wing with a flush fit to the bottom of the fuselage cutout. Cut carefully so there is a snug fit for good glue contact.

5)  If you are going to want to paint your 1/4” canopy handle, now is the time before it is glued into the wing and overspray gets on your covering.

6)  Disassemble the wing and spars and prepare to glue and assemble these parts. Thread the servo extensions through the tunnel behind the front spar from fuselage cutout to cutout. Be sure both extensions are oriented the same way. The plug end defines your receiver location. I find it easiest to smear yellow woodworker’s glue on one half of the main spars and insert them into one wing core, aligning properly and then smear glue on the portion sticking out and the root wing surface and push the halves together around the 1/4” plywood canopy handle.

7)  Clean up any glue drips on the outside of the wing. Smear glue on the bolt blocks and insert them into the proper holes. Using masking tape pull all the joints tightly together.

8)  Smear glue on the leading edge of the foam core and attach the ½” half round pine molding centered on the core with masking tape. Repeat with the second piece.

9)  Smear glue on the trailing edge of the wing core and attach the ¼” x 3/8” x 24” balsa trailing edge stick holding it in place with masking tape. Repeat with the second piece.

10)      Cut and assemble the balsa wedges into the wing behind the rear spar to blend the fuselage gap in this area as the fuselage will not extend behind the rear spar.

11)      Trim the leading and trailing edges flush with the end of the wing cores.

12)      Hold the 1/8" x 2" x 10" sheet balsa wing tips to the end of the wing core and trace around them to get the shape of the wing transferred to the wood. Cut slightly to the outside of this line so there is a slight edge to shape/trim once the glue is dry.

13)      Smear glue on the end of the wing and position the wing tip on it and attach with masking tape. Repeat with the other wing tip. If there are any gaps with the leading edge or spars, fill these with scraps from the wing tips.

14)      You can continue with the building of the fuselages at this point but be careful that any wet glue doesn’t stick the fuselages in place. Be sure to remove the fuselages from the wing when you stop for the night so they don’t get glued in place by mistake.

15)      After the glue has dried, remove the tape from the wing and fins and trim the edges flush with the wing. Trim the fins to their final shape.


Building the Fuselages

1)  Select the precut 1/8" lite plywood fuselage sides (4), the 1/8" lite plywood fuselage firewalls (2), the 1/8" lite plywood firewall keyed alignment pieces (4), the 1/8" lite plywood rear keylock (2), 1/4” plywood bolt block pieces (4) and the 3/32” balsa sides (4) and crossgrain sheeting (12) from the wood parts.

2)  The two fuselages can be built pretty much identically with the exception of the servo position being mirrored. There is no difference in the sides until you cutout the servo openings. It is strongly suggested you drill the firewalls to mount your motors while you have them out of the plane.

3)  Select the servos you will use for the elevons and cut the holes out so that the servos fit through the sides from the inside with the servo ears bolted to the inside. They are designed to be in the fuselage sides between the bolt block positions. I prefer to put them on the inside of the pods for looks from the side but they can be mounted on the outer sides also to better center the control horns in the elevons. Center the servo from top to bottom in the fuselage side. Since all the sides are the same, just cut two without worrying about left and right before assembly. Access to attach the servo will be from the bottom of the fuselage before it is bolted in place.

4)  Place the 1/4” plywood bolt blocks on top of the basswood blocks in the fuselage cutouts on the wing with the notches away from the spars.

5)  Slip the fuselage sides into the wing saddle being sure to have a left and right side (servos either both facing inward or outward) and keyed into the front spar to help with alignment. Slip the sides over the bolt block but do not glue at this time.

6)  Mark the servo cutout on the side of the fuselage saddle so you can cut clearance later.

7)  Glue the firewall and its keylock parts into the front of the fuselage sides using yellow woodworkers glue and masking tape.

8)  Glue the rear fuselage keylock piece into the top of the fuselage behind the main spar and hold it in place with masking tape.

9)  One at a time, gently remove the fuselages from the wing and glue and tape the hold down bolt blocks into the fuselage now.

10)      Glue pieces of the 1/4” triangle stock in place to reinforce these bolt block joints. Let the glue dry at this point so you can remove the tape to put the balsa fuselage sides on.

11)      Once the glue is dry remove the tape and locate the balsa fuselage sides. These fit in place keylocked into the firewalls and extend 3/32” above the top of the plywood. For alignment I use a knife blade in the hatch slot near the rear to align it with the one in the plywood. Smear glue on the side and glue it in place and tape or clamp. Repeat for all 4 sides. Allow to dry.

12)      Once the fuselage sides are dry, glue the crossgrain 3/32” balsa top and bottom onto the pods and allow to dry. Cut a small hole over the rear bolt location so you have access. Make sure it is large enough for the nylon bolt head to seat on the block.

13)      Trim the balsa sides to match the servo cutouts where it overlaps the plywood side.

14)      With the fuselages now complete and dry, use a sharp knife and remove the pod hatches by gently breaking the tabs holding them in place and cutting straight across the tops to connect the sides. I use a magic marker on the inside of the pod and hatches to indicate pairings and orientation.

15)      Locate the 4 long strips of 1/16” plywood and the 2 short pieces. Glue the long strips to the inside long sides of the hatches you just cut free so it extends half way below the hatch to act as a guide when they are in place. Attach the small piece so it protrudes as a tongue forward on the hatch to lock the front in place.

16)      Continue following the Almost-Ready-to-Cover kit instructions for completing your plane.


Almost-Ready-to-Cover Kit Instructions

Completing the Fuselage Pods

1)   If you are starting an almost ready to cover kit here, decide on your servo positions and cut out the servo openings at this point.

2)   Install the servos in the fuselage pods from the inside. I use 2-56 bolts with washers and position the servo shaft/arm towards the front.

3)   Slide the pods into place making note of where the servos come in contact with the foam and cut some clearance so the pods seat flush. You may also need to bevel the plywood pod sides where any glue fillets are on the wing bolt blocks.

4)  Remove the pods from the wing and remove the servos and cover the pods as desired.

5)   The servos and motors can be reinstalled at this point. Some method for securing the back of the hatch should be installed at this point. Dubro makes a nice hatch latch or Velcro or magnets could be used. Since this is your battery access I tend not to use screws for speed of access.

Completing the Wing

1)  Wrap fiber reinforced packing tape span wise around the wing in a continuous strip starting and ending on either side of the center handle. Dip into each fuselage cutout. Position the tape just in front of the front wing bolt blocks. Use 2” tape if available or multiple strips of ¾” tape. This tape provides the structural strength to the wing and insures its durability. It allows the wing to flex but prevents the foam from being pulled apart. The tape forces the opposite side to compress and foam rebounds nicely from compression. This is VERY important. Do not skip this step.

2)  Tapering the outer third of the elevons can reduce the occurrence of flutter in flight. My preference is to trim the outer 8” of the elevon from 2” to 1.5” at the tip. Trim the ends of the elevons to match the rear taper of the wing. This can be done using the core beds as a template.

3)  Hinge the elevons to the wing. My preferred method to do this is to first cover the trailing edge of the wing and the leading edge of the elevon to seal and to give a good seal to the covering. Be sure the tapered end of the elevons is towards the wing tip.

4)  DO NOT USE CYA hinges in this step. Cut about 5 hinge slots into the trailing edge of each wing panel and leading edge of the elevon and assemble them with the chosen hinges. Flex the hinge line to be sure you have appropriate movement that does not bind. Using a 1/16” drill bit drill holes through the trailing edge and hinge and pin them in place with round toothpicks. Do the same on the elevon side. Trim the toothpicks flush with the surfaces.

5)  Cover the wing at this point. A low cost covering solution is 2” wide colored packing tape. This is a low cost alternative to normal rolls of covering. See our website (http://www.racores.com/) for more information and suppliers. You should start at the trailing edge and work forward overlapping about 1/8” on the previous strip to avoid air getting under the edge and lifting the tape. Large designs that are different patterns on the top and bottom help with the orientation of the plane in the heat of combat.

6)  Add nylon control horns so they line up with the servo locations you have chosen on the fuselage pods.

7)  Drill and tap the wing bolt holes. Drill using a 3/16” bit and then tap 1/4-20 threads for the nylon bolts provided. These taps, or 1/4-20 blind nuts, are available at your local hardware or hobby store. Align the holes to be centered under the predrilled bolt blocks within the assembled fuselage pods.

8)  Cover the two 1/16” plywood fin spreader pieces. Leave the center glue points and the outside edges bare.

9)  Glue the spreaders in place being careful to mount the top spreader at a right angle to the canopy handle.

10)      Take the fins and hold them in place and sand the contour of the wing into the bottoms so they sit flush without wiggling.

11)      Cover the fins at this point.

12)      Hold the fins in place again with the elevon bevel ahead of the hinge line about 1/4” and mark the location of the fin spreader contact. Cut the covering so you have a clean wood to wood glue joint. Glue the fins in place to the spreaders watching the symmetry of the alignment on each side.

Final Assembly

1)  At this point we need to assemble the plane and sort out the radio connections. Bolt the two fuselage pods onto the wing with the servos on the sides that match the horns on the elevons. Be sure to bring all the wires forward into the hatch area so they are accessible.

2)  Connect the ESCs to the motors. On the side without the receiver, plug the ESC and servo into the extensions making note of which extension is which.

3)  On the receiver pod side, plug the ESC into the throttle channel and the servo into either the aileron or elevator channel (we will sort that out in radio setup).

4)  Plug the servo extension for the other pod elevon servo into the open elevator or aileron channel. (do not plug in the other ESC yet)

5)  Program your transmitter for “Delta” wing configuration and using one battery in the receiver pod, bind the model.

6)  With your trims centered, install the servo arms in a vertical position and install the pushrods to the elevon horns.

7)  Test the controls. Check that both elevons move up on up elevator and that the elevon on the side you turn towards comes up on aileron movement. If “Aileron” is backwards but “Elevator” is correct, swap the servo plugs for those channels and reverse both servos on the transmitter

8)  The proper initial adjustment is for the rear edge of the elevons to be 1/8”-3/16” above perfectly centered. The bottom of the wing and the bottom of the elevon should also be flat. Each elevon should raise or lower 3/8” from center with full stick travel.

9)  Unplug the motor battery at this time. Remember to remove the binding plug from your receiver.

10)      At this time you need to decide if you want to play with differential thrust or not. (You can change this at any time.)

a)  No differential thrust

¨    Set your rudder channel to zero travel

¨    Mix the throttle channel to the rudder channel at 100% mix (so they both have the same travel)

b)  Differential thrust

¨    Mix the rudder channel to the throttle channel 100%. The direction of this mix will be determined when we test since different radios have different settings.

¨    Mix the throttle channel to the rudder channel at 100% mix

11)      On the servo extension for the other pod ESC, lift the lock for the RED (positive voltage) pin and remove the pin from the connector. Put a piece of heat shrink over the exposed pin. It is not good to try to power the receiver from two separate BECs. Plug the extension into the rudder channel.

12)      Secure the plane with the propellers able to turn freely and turn on your transmitter. Plug the motor battery into the ESC in the receiver pod. Then plug the motor battery into the ESC in the other pod. Always plug in the receiver pod first and unplug it last so the other ESC sees a valid signal on start up.

13)      Slowly advance the throttle stick until one of the motors starts turning. If it is the receiver pod motor, adjust the subtrim on the rudder channel until the other motor starts up as well. The point is to make both motors start and stop at the same time. If the non-receiver motor starts first, turn back the rudder subtrim until they both start simultaneously.

14)      Holding the plane up by the canopy handle, advance the throttle to half. If you chose differential thrust, move the rudder stick and see if you have the mixes in the correct direction. If not, reverse the mixes.

15)      With just the throttle stick, you should feel balanced and equal power over the entire throttle range. Shutdown the throttle and disconnect the motor batteries (non-receiver pod first)


Final Balancing

1)  Install Velcro in the bottom of the pods behind the motors and in front of the wing. I generally find that it sticks better if there is a coating of glue on the bare wood first.

2)  Place batteries in the pods on the Velcro and close the hatches. Do not connect the batteries, this is just to determine the CG

3)  With a piece of string or wire through the hole in front of the canopy handle, suspend the model and check the balance. The model should hang level or slightly nose down for proper balance. Like the original Gremlin, this point is 1.75” back from the front of the leading edge. Move the batteries as necessary to get proper balance.

Get to the field and

fly!!