RA Cores
R/C Combat Gremlin
Available online at http://www.racores.com/MiniGremlinInstructions.htm
with clickable color images.
All rights reserved RA Cores 2012
Produced by
Jim Reith
RA Cores
How the Mini Evolved......................................................................................................... 3
What you will need to
complete the kit............................................................... 3
Identifying the parts........................................................................................................ 4
As the kit comes out of
the box........................................................................................................... 4
Parts Bag #1.................................................................................................................................... 4
Parts Bag #2.................................................................................................................................... 4
Starting the Wing................................................................................................................ 5
Preparing the firewall.................................................................................................. 6
Assembling the Fuselage.............................................................................................. 6
Final assembly...................................................................................................................... 7
Wing Bolt Installation......................................................................................................................... 8
Servo Placement and
Installation.......................................................................................................... 8
Reinforcing Fiber Tape........................................................................................................................ 9
Elevon Hinging and
Installation.......................................................................................................... 9
Covering the Model........................................................................................................................... 10
Attaching the Fins............................................................................................................................. 11
Attaching the Elevon
Horns............................................................................................................... 11
Engine Installation............................................................................................................................ 12
Radio Connections............................................................................................................................ 12
Initial Flight(s).................................................................................................................... 12
Prop Selection................................................................................................................................... 12
Control Throws................................................................................................................................. 12
Setting the Center of
Gravity.............................................................................................................. 13
Launching........................................................................................................................................ 13
How the Mini Evolved
I
want to thank Steve Brian and Bob Barger for all their help in prototyping and
refining the Mini Gremlin. Both men are active full-sized Gremlin fliers and we
all live reasonably close together and so with some kit bashing and trash
mouthing we were able to develop an awesome little plane from our ideas over
4-6 weeks. We standardized on the Speed 400 size and went from there. 2S1P
800mah and 3S1P 500mah LiPo batteries were used, as well as Super Tigre and
E-Flite motors and some Turnigy outrunners and ESCs. HS-55 servos were
typically used in the wings. We found that the plane isn’t too sensitive to
weight so the various power systems all flew well. Pretty much whatever you
have on hand for Speed 400 motors or what your local hobby store carries will
work. Prop selection is a black art I left to Steve. We had a lot of fun, and I
really appreciate the help these guys provided. Thanks guys!
What you will need to complete the kit
2 small servos, HS-55 or equivalent
Speed 400 motor with firewall mounting hardware and
prop
Electronic Speed Control (ESC) 18 amp or equivalent
Battery(s) (2S1P 800mah or 3S1P 500mah used in
examples)
0.032 music wire and connectors for the elevons
Covering or Tape (see http://www.racores.com/CoveringWithTape.html)
Foam-safe glue or 5 minute epoxy or yellow woodworkers
glue
3/4” filament (fiber reinforced) tape
Identifying the parts
Please be sure that all
the parts listed here are in your kit and make note of which is which.
As the kit comes out of the box
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Contents
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Parts Bag #1
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Contents
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Parts Bag #2
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Contents
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Starting the Wing
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Find the wing cores and the 1/32” crossgrain plywood
wing center section doubler. |
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Spread
glue on the wing roots and align them and tape them across the fuselage gap
on top. Yellow woodworkers glue can be used as it will be stronger than the
foam it bonds to anyway. 5 minute epoxy can also be used if you are looking
to compete the plane in a single session. |
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Spread
glue on the crossgrain plywood wing center section doubler and align it on
the bottom of the wing (the side without the fuselage gap) with the center
seam of the wing and clamp or tape it to the underside. |
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Make
sure that the top of the wing rests flat on your building surface. Set the
piece aside for the glue to dry.
Preparing the firewall
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Find the 1/8” plywood firewall and take your motor’s
backplate mount and drill and attach it using blind nuts. Every motor is
different so use the template or actual mount of your motor for positioning.
Be sure to cut any holes for wire access, shafts and cooling that your motor requires. Depending on the
orientation of your mount you can either center a wire hole in the top of the
firewall or in one of the bottom diagonal corners so as to not interfere with
the key locking while assembling the fuselage. |
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If
your mount overlaps the firewall it may be necessary to remove or modify the
decorative extensions on the fuselage sides. These are really just in place to
give your Mini Gremlin the look of the PVC fuselage of the original.
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Once
you are happy with the position and attachment of the motor to the firewall,
remove the motor to continue with the fuselage assembly. |
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Assembling the Fuselage
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The fuselage key locks together. Find the two
fuselage sides, the fuselage bottom, the two fuselage top pieces, and the
firewall from the step above. |
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Dry
fit the fuselage pieces around the firewall to get a feel for how things fit
together. Notice how the firewall tabs fit in place, squaring up the fuselage
shape and aligning the firewall. If you use CA, you want to make sure the
initial pieces are square to end up with all the tabs aligning. My glue of
choice is 5 minute epoxy so there is enough time to get the alignment
correct. Tape the side seams of the fuselage with masking tape while dry
fitting and then lay the pieces flat and coat the seams with epoxy. Then wrap
the sides around the firewall. I omit the fuselage tops initially to better
spread the glue. If you do use CA/superglue, dry fit the entire fuselage and
then hit the tabs with the glue to lock it in place. I find the epoxy is a
little more forgiving. Use the firewall to square things and work back from
there with the final piece being the top back of the fuselage with the fin
spreaders. |
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Once
your subassemblies have dried, remove any tape or clamps you used to hold them
and round the fuselage edges with a sanding block to smooth out the tabs and
glue.
Final assembly
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Dry fit the fuselage into the wing saddle and make
sure there is a snug fit. The wing is cut slightly tight to allow for a snug
final fit. If it is too tight sand lightly to fit. |
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Wing Bolt Installation
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Fit the fuselage into
the wing and position it so it doesn’t overlap the rear edge of the wing.
Using a 1/8” drill bit, drill down through the fuselage and out the bottom
wing doubler near the rear of the fuselage, in the position marked with the
pilot holes in the fuselage bottom just in front of the fin spreader. Slip
one of the included 6-32 bolts through the hole for alignment and drill the
front pilot hole. Install 6-32 blind nuts (provided) on the underside of the
wing and screw in the provided nylon bolts from the inside of the fuselage.
These two bolts will hold your fuselage in place. |
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Servo Placement and Installation
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Mark the position of the servo wire holes in the
fuselage sides through the fuselage onto the wing saddle. Remove the bolts
and fuselage to continue. |
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Cut
out the servo holes with an X-ACTO knife and seat the servos so they are
flush with the top of the wing, with the arms pointing up and the wires
exiting in the area marked from inside the fuselage. Plug the servos into the
receiver and center the servos arms so they are perfectly vertical and have
clearance of the foam for full travel. |
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Apply
a strip of tape or covering over the servos to hold them in position. If the
holes are snug it should be unnecessary to glue them in place, making removal
easier in the future. |
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Reinforcing
Fiber Tape
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Apply a strip of 3/4”-1” fiber reinforced tape to
the wing from the fuselage saddle to the wingtip, around the tip, across the
bottom of the wing, around the other wing tip and back into the fuselage
saddle. This prevents the wing from breaking due to flight stresses flexing
the wing. Apply the tape smoothly and evenly to the high point of the airfoil
so the wing remains flat. This tape can also be secured with a low
temperature monocoat iron. |
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Elevon Hinging and Installation
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At this point you need to decide on how you are
going to hinge the elevons to the wing. The simplest method is to use the covering
material for the actual hinge. This works very well but requires some
preparation. Find the elevons in the package and bevel a 45 degree angle into
them with a straight edge and either a sharp X-ACTO knife or a sanding block.
Be sure to make a left and right elevon. (check the taper at the end of the
elevon to match the wing) |
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Apply
a strip of covering with a low temperature setting on your monocoat iron
around the rear surface of the wing to seal the surface and enable easier
attachment of the elevons. |
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Apply
a strip of covering to the hinge area of the elevons. Do this on the long
edge of your elevon and then attach the extended piece of covering to the
rear foam of the wing top, being careful to align the wing end edges. If you
are covering using the packing tape method from the website, apply the tape
to the elevon first and then to the wing. Fold the elevon flat onto the top
of the wing and apply covering to the inside of the hinge line securing the
surface vertically. |
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Covering the Model
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With the servos installed and the elevons attached,
it is now time to cover the model. For tape covering tips see our website
online at http://www.racores.com/CoveringWithTape.html
or cover with monocoat. Be careful not to limit the movement of the elevons. |
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At
this time you can also cover the fuselage if you desire. Originally we were
covering with white to simulate the PVC fuselage look of the original R/C
Combat Gremlin. |
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Attaching the Fins
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Bolt the fuselage to
the wing, passing the servo wires through the holes in the sides of the
fuselage. If necessary, notch the fuselage bottom to allow the wires to pass
by more easily. Attach the fins to the
rear fuselage spreader so the bottom edge of the fin meets at the
wing/fuselage joint. The fins can be attached with glue, tape or covering to
the spreader. Position the fins so the elevons are not limited by the rear of
the fin. Remove the fuselage
one last time and attach the bottom of the fins to the fuselage sides with
tape or covering. |
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Attaching the Elevon Horns
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Cut a small slice into the elevons in line with the
servo arms and glue the control horns to the elevons with epoxy (using a
toothpick applicator) or foam safe CA. |
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Engine Installation
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Mount the engine to the front of the fuselage
connecting the ESC through the wiring holes drilled in the fuselage. Check
the direction of thrust for your motor with your transmitter to be sure the
motor is pulling the plane. If not, swap any two motor wires to the ESC. When
snaking the wires from the ESC through, it can be helpful to slip them into a
soda straw as a guide. |
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Bolt
the fuselage to the wing to complete assembly. You may now trim the nylon
wing bolts even with the blind nuts. |
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Radio Connections
Attach the receiver to the servos and ESC/speed
controller. Put the speed controller and/or receiver into the nose of the
fuselage. If needed for balance later you can put the battery into the nose
compartment instead.
Initial Flight(s)
Prop Selection
Attach a propeller to the motor for proper CG
calculation. We found 2S1P flights were good with a 10x8 electric prop (GWS)
and the 3S1P flights were good with an 7x5E (APC) prop.
Control Throws
Attach the servos to the control horns with your
favorite method. We use 0.032” wire with 90 degree bends and keepers at the
horns and Mini E-Z Connectors to attach to the servo arms.
Adjust the elevons so that the bottom edge matches the
slope of the bottom surface of the wing initially to give a little “up” control
throw. Set the control movements for about 3/8” up or down travel with full
throw, The nice thing about the E-Z Connectors is that after you trim the
plane, you can recenter the trims by repositioning the rods. I use a pin into
the wing at the end of the rod on both wings and then recenter the transmitter
trims and adjust the rods back to the pins by loosening and retightening the
E-Z Connectors.
Setting the Center of Gravity
The CG location is 1” back from the leading edge of
the wing. Place a flight battery in the fuselage and check the balance. Slide
components forward and back to fine tune the balance. Once balance has been
achieved, attach the battery in place with Velcro. If different batteries are
used, check the balance with each capacity. The 500mah 3S1P and 800mah 2S1P
batteries we used were identical weights and only differed in thickness making
balance easy.
Launching
I find it beneficial to have someone else launch the
plane initially so I have my hands on the controls. After verifying that the
servos are moving in the correct direction, throttle up to about 3/4 throttle
and launch upward at about a 45 degree angle. This lowers any torque roll
tendency and gives the plane some initial altitude for trimming. The plane
should roll and loop without snapping out at full throws or you should reduce
them until it does. Once the plane is trimmed, you can launch it yourself with
confidence. If the CG is set properly, the plane will fly smoothly like it is
on rails. If you are using a prop saver hub you might want to use two bands to
secure the propeller. If you are using a shaft mount you might try dragging one wingtip on landing to
slide the plane sideways kicking the prop out of the way, and reducing prop
breakage on landing.