The RA Cores COMBAT

 

 

AppleMark
A Fly-for-Fun flying wing.

48” Span.   AUW 3 lbs.

 

 

Designed By,

Eric Henderson for OS .25FP

And BEYOND!

 

 

                                          Produced by,

                                          Jim Reith and

                                          RA Cores

The most up-to-date copy of these instructions can be found online with expandable color pictures at http://www.racores.com/GremlinInstructions.htm

(Last updated - Thursday, August 29, 2013)


Full-size Fin Template

AppleMark


 

Table of Contents

 

Full-size Fin Template____________________________________________________________________ 2

Table of Contents________________________________________________________________________ 3

Introduction (the original from Eric Henderson circa 1992)______________________________________ 4

RA Cores introduction___________________________________________________________________ 4

Thoughts on getting started._______________________________________________________________ 5

Short Kit Instructions____________________________________________________________________ 6

Full Kit Instructions_____________________________________________________________________ 7

Building the fins_______________________________________________________________________ 8

Building the Wing______________________________________________________________________ 8

Almost-Ready-to-Cover Kit Instructions_____________________________________________________ 11

Completing the wing__________________________________________________________________ 12

Installing the Engine and Radio__________________________________________________________ 15

Setting up the CG_____________________________________________________________________ 17

Final Assembly_______________________________________________________________________ 19

Initial Flights and Trimming_____________________________________________________________ 21


Introduction (the original from Eric Henderson circa 1992)

 

Welcome to the Gremlin. The attached instructions have been produced in a sequenced order to help you build this plane as quickly as possible. It takes about ten hours to complete a Gremlin.

 

The Gremlin was originally conceived as a Streamer-Combat plane. It has proven to be very good for this purpose, but it has also proven to be a distinctly enjoyable and easy plane to fly.

 

The Gremlin holds level flight very well and has super stall characteristics. It will roll and loop with ease. Because of this, many people are deviating from the original design concept and using Gremlins to just have fun in the air.

 

A word of warning: if you are building this plane for Streamer-Combat please avoid putting too much effort into the finish of the model. Put a lot of work into a model and you will easily become attached to it. GREMLINS are designed to have disposable wings and are expected to be damaged in combat.

 

True combat flyers will barely sand anything!

 

Another great characteristic of this design is that it has also proven to be very tough, and has had no problems in being bounced on the runway or dumped in the long grass.

 

The plane has no undercarriage and will need a hand-launch. With an OS25 FP it will rise gently from a slightly upward, one-handed launch. To land, just cut your engine and glide onto the field.

 

Happy landings,

Eric

 

RA Cores introduction

 

The philosophy of the Gremlin was to have an inexpensive plane that was quick to build and could be standardized upon so pilots could test their skills and not the depth of their wallets in head-to-head competition. Originally we standardized on OS .25FPs as an inexpensive plain bearing engine used on the trainers of the time but any .25 to .40 size engine can be used. Check with your local clubs if you are going to be competing so you can meet their engine requirements.

 

The Gremlin was made national by the December 1992 construction article in R/C Modeler. Eric knew I was doing foam cores for my personal use and asked if I would be willing to do them for the construction article. I said sure and suddenly I was in business. I stopped advertising in the magazines in 1994 and have been selling kits through the website (http://www.racores.com/) and by word of mouth since. I am amazed at the longevity of the plane through all these years.

 

Feel free to email me (info@racores.com) with any questions or comments on these new instructions or any questions you have about the plane. Get out to the field and have a ball!

 

Jim

 

Thoughts on getting started.

 

 

You cannot use normal cyanoacrylate glue or fiberglass resin near foam. It will melt the foam causing a loss of adhesion. Do not attempt to use CA hinges on this project. For all glue joints I recommend yellow carpenter’s glue since it dries slowly enough to soak into the wood and foam and creates a bond stronger than the foam itself. To build your Gremlin you will need the following things:

 

Yellow woodworker’s glue such as Titebond or Probond

A small ruler and pencil for marking parts/cutouts

An Xacto knife

A small razor saw (for notching the plywood/basswood parts)

A drill and assorted bits

1/4-20 thread tap (for wing bolts)

sandpaper and sanding blocks as needed

Sticky masking tape (not low tack painters tape)

Radio gear (3 servos) including mounting hardware

An engine including prop and spinner and the mounting bolts for it (including 2 additional round headed bolts used to secure the rear of the motor mount plate)

A fuel tank that fits inside the fuselage (sized appropriately for the engine chosen)

2 nylon control horns

3 klevis rods

four 4-40 or 6-32 nylon bolts, 1” long, for attaching the fins and a thread tap of the same size

10 hinges

round toothpicks

3/4” or 2” wide filament reinforced tape such as 3M 8934 Economy Strapping Tape (for wing spar – do not omit)

covering material (check our website at http://www.racores.com/ for instructions on covering using colored packing tape)

 

Short Kit Instructions

Figure 1 - Contents of the Short kit box

 

Table 1 - Wooden parts you will need to supply to complete the short kit.

For pictures of each see the full kit wood bag contents in Figure 3 on page 8 below

Qty

Description

2

1/8" sheet balsa fins

2

1/8" x 1/2" x 5" sheet balsa fin caps

2

1/8" x 1" x 4 3/4" sheet balsa spar caps

2

1/8" x 2" x 10" sheet balsa wing tips

2

1/4” x 3/8” x24” balsa stick (medium to hard)

2

1/2” half round pine molding (24” lengths)

2

1/4" x 2" x 24" elevons (elevon stock)

2

1/4" x 2" x 9" fin wedges (elevon stock)

2

1/8" x 1" x 12" basswood spars

2

1/2" x 1/2" x 3"  basswood servo rails

1

1/2" x 1/2" x 6"  basswood rear bolt block

1

1/2" x 3/4" x 6"  basswood front bolt block

1

1/4" x 2 3/8" x 3 5/8" ply motor plate

 

1.  Collect the wood parts and cut to the sizes shown in Table 1 above using the fin template on the back of the instruction booklet cover.

 

2.  If basswood is not available the wing bolt blocks can be cut from 1/4” plywood used for the motor mount. Cut 5 pieces ½” x 6” and layer them as two and three pieces to get the appropriate thickness. The 1/8” x 1” x 12” wing spars may also be cut from 1/8” plywood.

 

3.  For all of the balsa parts the grain runs the length of the longest side.

 

4.  You will also need 1/4” x 2 aileron stock (two 24” lengths), two 24” pieces of 1/4” x 3/8” balsa trailing edge stick and 48” of 1/2” half round pine molding, used for the leading edge. Hard balsa can be substituted but the pine withstands string cuts much better.

 

5.  Continue with the Full Kit assembly instructions once this has been done


Full Kit Instructions

Figure 2 - Full Kit Contents

Table 2 - Wooden parts supplied in Full Kit bag

Qty

Description

2

1/8" sheet balsa fins

2

1/8" x 1/2" x 5" sheet balsa fin caps

2

1/8" x 1" x 4 3/4" sheet balsa spar caps

2

1/8" x 2" x 10" sheet balsa wing tips

2

1/4" x 2" x 9" fin wedges (elevon stock)

2

1/8" x 1" x 12" basswood spars

2

1/2" x 1/2" x 3"  basswood servo rails

1

1/2" x 1/2" x 6"  basswood rear bolt block

1

1/2" x 3/4" x 6"  basswood front bolt block

1

1/4" x 2 3/8" x 3 5/8" ply motor plate

 

Figure 3 - Contents of the Full Kit Wood Parts Bag


Building the fins

 

1.  Select the 1/8" sheet balsa fins, 1/8" x 1/2" x 5" sheet balsa fin caps, and 1/4" x 2" x 9" fin wedges from the wood parts.

 

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

 

3.  Glue the fin wedges even with the bottom of the fins with the wide part of the wedge away from the bottom of the fin and clamp to dry. The wedges are to make the tops of the fins angle outward from the fuselage. Be sure to make a left and a right fin


Building the Wing

 

1.  Select the 1/8" x 1" x 12" basswood spars, 1/8" x 1" x 4 3/4" sheet balsa spar caps, 1/8" x 2" x 10" sheet balsa wing tips, 1/2" x 1/2" x 6"  basswood rear bolt block, 1/2" x 3/4" x 6"  basswood front bolt block, 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.

 

2.  Mark a centerline on the wide sides of the spars and bolt blocks

 

3.  Dry fit the spars into the pre cut slots in the foam cores. Align the rear spar with the top surface of the wing and the front spar with the fuselage cutout maintaining the centerlines. Once the spars are in both wing halves and the root of the wing is pressed together, place the wing upside down on a flat surface so that the top of the wing is totally flat. Now mark the spars flush with the foam on the bottom of the wing.  Mark the fuselage cutout on the rear spar.

                                                                

 

4.  Cut out the spars on the lines marked.

 

 

5.  Position the bolt blocks so they are ahead of the spars on the wing and cut out sections 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.

6.  Disassemble the wing and spars and prepare to glue and assemble these parts. 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. Clean up any glue drips on the outside of the cores and then place the wing upside down again to make sure the top surface of the wing is flat. Smear glue on the bolt blocks and insert them into the proper holes. Using masking tape pull the joints tightly together.

 

7.  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

 

8.  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

 

9.  Hold the 1/8" x 2" x 10" sheet balsa wing tips to the end of the wing core and mark 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 once the glue is dry.

 

10.       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.

 

11.       Smear glue on and insert the 1/8" x 1" x 4 3/4" sheet balsa spar caps into the wing spar slots in the top of the wing. This will stick up from the wing once in contact with the spars and will be trimmed once the wing is dry. This is a good place to stop for the night so that the glue can dry.

 

12.       After the glue has dried, remove the tape from the wing and fins and trim the edges flush with the wing, Trim the leading edge for the fuselage cutout and the fins to their final shape.

 

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


 

Almost-Ready-to-Cover Kit Instructions

 

Figure 4 - Almost-Ready-to-Cover Kit Contents

 

Figure 5 - Almost-Ready-to-Cover parts bag contents

Completing the wing

 

1.  Wrap fiber reinforced packing tape span-wise around the wing in a continuous strip starting and ending in the fuselage cutout over the front spar/high point of the wing. Use 2” tape if available or multiple strips of ¾” tape. This tape provides the structural strength to the wing that provides 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.

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 them from fuel. 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 the wing 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 provides a low cost alternative to normal rolls of covering. See our website (http://www.racores.com/ CoveringWithTape.html) 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 fuel 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. Cover the fins at this point also.

 

 

6.  Add nylon control horns to the inner end of the elevons so that they are within the fuselage cutout and pointing up.

 

 

 

 

 

 

 

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.

 


 

8.  Your wing is now complete. Set it aside and work on the engine and radio installation in the fuselage next.

 

Installing the Engine and Radio

 

1.  Take the 1/4" x 2 3/8" x 3 5/8" ply motor plate provided in the plastic parts bag and cut and drill it to match your engine bolt pattern. Check the fit and alignment but do not install the engine at this point. Right thrust is not typically used with Gremlins.

2.  Place the engine mount into the front of the fuselage and with all the engine bolt holes overlapping plastic, mark and cut out the plastic to match the cutout between the engine bearers.

 

3.  Drill the engine holes through the plastic fuselage and drill two additional holes in the large flat section behind the engine. Attach the engine mount to the fuselage using these two rear bolt holes. Use round headed bolts so they don’t wear into your fuel tank with the vibration of flight.

4.  Assemble your chosen tank according to the manufacturer’s instructions and slide it into the fuselage behind the engine position. Be sure to bend the tubing to match the fuel feed on your engine’s carburetor.

 

5.  A flat battery pack can be placed under the fuel tank and foam rubber should be used to create a snug fit.

6.  Install the engine at this point with nylon insert locknuts through the plywood mount and plastic fuselage. Install a prop and spinner and the muffler for proper balancing in the steps below.

 

7.  Locate your three servos and using the manufacturer’s suggested hardware, mount them to the 1/2" x 1/2" x 3"  basswood servo rails. Standard servo installation with full size servos and a computer mixing radio is shown. Plans for a low cost sliding tray mixer are available in our hints and tips section on our website (http://www.racores.com/SlidingTray.pdf). DO NOT USE the Dubro V-Tail mixer as it will disengage under flight loads. Be sure to alternate the orientation of your output shafts so that you have clear access from the servos to the elevon horns. Align the arms for throttle servo access.  Place this subassembly in the fuselage for proper balancing in the steps below.

 

8.  Wrap and install your receiver and switch into the fuselage behind the fuel tank. It is a good idea to keep the switch out of the exhaust.

 

Setting up the CG

 

1.  Mark the CG on your wing next to the fuselage cutout at 1.75” back from the leading edge of the wing. This is 15% of the MAC (Mean Aerodynamic Chord) as used with flying wings. If you balance your Gremlin more than 2” rearward of the leading edge, the plane will be too unstable to be controlled.

2.  Temporarily tape the fins flat to the wing.

3.  Position the fuselage into the wing cutout. If possible put a loop of string around the fuselage at the CG location from step 1 above. Using scraps of wood wedge the fuselage in place snuggly enough so that it can be suspended by this loop of string. Move the fuselage and the servos inside the wing saddle forward and back until the model hangs level to slightly nose down at this point. Mark the CG on the fuselage.

 

4.  Mark the servo rail positions in the fuselage and remove the servos and rails from within the fuselage. With the fuselage and wing CG marks aligned, drill one front bolt hole through the fuselage from below. If you have threaded the bolt block, a 3/16” bit will pass through without damaging the threads. Just drill one of the front block holes at this point. Open the hole up to 1/4” and bolt the fuselage to the wing with that bolt only.

 

5.  Replace the servos inside the fuselage and double check the balance. If the balance is off, lengthen the hole into a slot and slide the fuselage forward or back until it is correct. Then remove the servos again and drill the other two holes. You do not want more than one hole enlarged into a slot or the fuselage will slide on hard landings, moving the CG and changing the trims.

 

Final Assembly

1.  Remove the fuselage from the wing and attach the servo rails to the sides of the fuselage. It may be helpful to slope the servos so the linkage will have a straight path to the control horns. Be careful to align the mounting screws with the gaps between the servos.

 

2.  Untape the fins from the wing and position them so the elevon cutout is over the hinge line.

 

3.  Drill through the fin and fuselage to attach them. Take care not to drill into your servos. Many of us have found that tapping the holes for 6-32 threads and using 6-32 nylon bolts works well as the fins will break free without damage on inverted landings or crashes.

4.  Attach pushrods from the servos to the elevon control horns. With the servos centered you want the top surface of the elevon to be slightly up from level. Control throws should be 3/8” from neutral to full deflection to start. You may want more eventually but that is a good test flight setup.

5.  Attach the throttle servo linkage to the engine.

 

6.  Wrap and attach the receiver to the plane. A tie wrap can be used to secure it to the servo rails of the throttle servo. If you attach the switch to the fuselage, be careful not to put it where holding the front of the fuselage to launch will cause you to switch the plane off.

 

7.  You may find it helpful when launching to have something rough on the sides of the fuselage in the area of the fuel tank. Bathtub non-slip strips or self adhesive wet/dry sandpaper are good choices. If you are going to be flying streamer combat, you might find it useful to bolt a spare round servo wheel to the rear fuselage section between the fins. This is a very easy place to loop the streamer string over.


Initial Flights and Trimming

 

1.  Get someone to help with the initial trim flight(s). If you aren’t comfortable trimming a new airplane, let a club instructor fly the initial flight. It will give you the opportunity to work on your launching technique. While someone comfortable with an aileron trainer can fly a Gremlin, it is much easier once the plane has been properly trimmed.

 

2.  It is important to have a reliable engine for your initial flight. Run a few tanks through a new engine at a rich setting before the trim flight.

 

3.  Once you have the plane set up and have verified the controls, hold the plane’s fuselage on the sides where the tank is located and in an underhand motion throw the plane out in front of you with a slight upward angle (maybe 10 degrees) at full throttle. If you are using a stock class engine, it might be wise to take a few trotting steps before releasing it. There is a launch video on our website (http://www.racores.com/images/HowToLaunch.MPG) that shows the technique.

 

4.  You may find it helpful to trim with a streamer installed. If you will be flying combat, it is certainly best to practice with streamers as it does change the characteristics of the plane, especially on landing.

 

5.  Once you have your Gremlin trimmed for hands off level flight, you should adjust your elevator control throws so that when you pull back for full up, you get a loop. If excess elevator is supplied it is possible for the Gremlin to snap out at the top of the loop. This can be distracting in combat where people tend to go from stop to stop on the sticks. If you have dual rates, set your low rate up to avoid this and set your high rate to maximum to play with stalls and spins.