138 MODEL AVIATION
The Hole Shot project continues with the tail feathers
[[email protected]]
Control Line Aerobatics Bob Hunt
Also included in this column:
• The art of tack-gluing
• Barrel-hinging secrets
The stabilizer is cut from medium-hard 3/16 balsa; the elevator is cut from
3/16 medium-soft balsa. Notice the antiwarp end caps, also cut from 3/16 balsa
stock. Do not omit them.
This handy tool sands consistently deep hinge pockets
in movable surfaces. The plate the sandpaper is
mounted on is mounted to a plate that acts as a depth
guide.
The completed and hinged stabilizer-and-elevator assembly. Recessed hinges allow the
elevator to be positioned tightly against the stabilizer TE. A tight hinge gap means better
control!
(Editor’s note: You can follow along with
your own profile Hole Shot or get a set of
plans for it. Randy Smith at Aero Products
has plans and foam wing cores available for
this and other great Aerobatics trainers.)
IN THE LAST column we brought the Hole
Shot’s fuselage to the point where it was
ready to accept the wing and tail assemblies.
Let’s start this time with constructing the tail
feathers.
This model’s stabilizer and elevator are made
from simple 3/16 sheet balsa. The stabilizer
should be cut from fairly dense stock (8-10
pounds per cubic foot), and the elevator
should be made from medium to medium-soft
balsa (4-6 pounds per cubic-foot). Why? It is
extremely important to ensure that the
stabilizer will not bend excessively under
load.
Nothing will ruin a model’s turning
performance quicker than a stabilizer that is
too flexible. As it flexes under load, the
elevator tends to “oilcan,” or spring back
toward neutral. That means more control
needs to be given to achieve the desired turn,
but with more input comes more elevator
springing, and the result is a vicious circle.
Making the stabilizer from fairly stiff
balsa will not prevent it from bending some
under load, but it will bend much less than
one that is made from softer material.
Covering the stabilizer with the .2-ounce
carbon-fiber mat (as we did to the fuselage)
will further increase the part’s stiffness,
making it far less prone to bending under
load.
Start building the stabilizer and elevator by
accurately cutting the pieces from the
appropriate-density balsa.
There are 1/2-inch-wide, chordwise
stiffeners that will be glued to the tips of the
stabilizer and the elevator. These help prevent
the assembly from warping later; please do
not omit them! Cut the stiffeners slightly
wider from front to rear (chordwise) than is
required, and let them extend forward and aft
of the part to which they will be glued.
Use aliphatic resin glue to attach the
stiffeners. When the glue has dried, sand the
stiffeners flush with the front and rear edges
of the stabilizer and the elevator.
Tack-glue the elevator to the stabilizer
against a flat surface. This will allow you to
sand the entire assembly at one time, rounding
the LE and TE and the tips. The result will be
an assembly that looks like one smooth part
when it’s hinged together.
For those of you who are new to building, I
just introduced a new term: “tack-glue.”
Sometimes you will need to hold two (or more)
parts together for carving and sanding, and the
best way to do this is to put tiny dots of glue
every few inches along the mating seam. The
glue will hold the parts together well enough
for you to do the required work, and then you
carefully flex the assembly to break the glue
dots and separate the parts.
Sometimes you will be unable to flex an
assembly that is tack-glued, and in those
instances you will have to use a thin, sharp
knife blade to slit the glue dots apart. An
example of this would be tack-gluing balsa
blocks onto a fuselage crutch assembly for
carving and sanding. An assembly that size
will not be easy to flex, and you probably
shouldn’t try!
Okay, the stabilizer and elevator are tack-glued and sanded to shape. The edges should
have an even radius. Take your time here and
detail-sand the edges so they blend perfectly
with the upper and lower surfaces of the
assembly. When you are happy with the
shapes of the tips and the LEs and TEs, it’s
time to lay out the positions of the hinges.
There are several options for hinging the
elevator to the stabilizer. Barrel-type hinges
that have a pin between the two sections are
popular, and they are the type I recommend.
You can also use the hinge-point-type
hinges or the traditional cloth hinges. To
explain the installation of each of these types
would take more space than I have left this
time, so I’ll focus on the barrel variety. I use
the Great Planes Medium Nylon Pinned
Hinges (part GPMQ3971) on my airplanes.
You should use at least five hinges to
attach the elevator to the stabilizer. One hinge
should be positioned on the centerline of the
assembly, and there should be two more on
either side, equally spaced. Place the five
barrel hinges along the hinge line and space
them properly. The outside edges of the
outermost hinges should be approximately 1/2
inch in from the end of the assembly.
Once you are happy with the hinge
locations, use a soft pencil to make a light tick
mark on both sides of each hinge on the
stabilizer and the elevator top surfaces.
Transfer these mark locations to the elevator
LE and the stabilizer TE.
The hinges in the elevator will have to be
recessed to allow the elevator LE to almost
contact the stabilizer TE when the parts are all
assembled. It is imperative that the hinge
“gap” be as tight as possible, while still
allowing free movement up and down. Too
much gap will allow air to pass through, and
the result will be loss of control authority.
To recess the hinges, you will have to
make “pockets.” Mark the positions of these
pockets in the elevator LE, and use the #11-
blade knife to make a cut at each end of a
hinge location that is not quite as deep as will
be needed. Carve out some material between
these cuts, again being certain not to cut to the
depth that will eventually be required.
It’s time to make the first of our custom
building tools. The tool needed to complete
these hinge “pockets” is made from two
pieces of 1/8 plywood. One piece will be
approximately 2 inches long and a slight bit
wider than the hinges you are using. That is7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
I’m careful to keep the knife
perpendicular to the surface I’m slitting, and
I’m extremely careful to keep my fingers
away from the area I’m slitting in case I miss and the blade protrudes through the surface!
That can be painful and messy!
Once you have made the initial hinge slit,
you need to open (widen) it to accept the
hinge tang so it doesn’t spread the wood on
either side. Nothing looks worse than a
“bump” in the surface at each hinge location.
I use an X-Acto #27 saw blade to “pick”
out the excess balsa from the slitted areas until
I have achieved a slot that allows the hinge to
be installed without expanding the wood on
either side. I’m careful as I pick out the balsa
to make sure I’m not moving the centerline of
the hinge slot up or down. It must remain
centered!
Test-fit all the hinges in the slots, and then
assemble the elevator to the stabilizer without
using glue. Lay the assembly on a flat table,
and check to make sure there are no areas
where one of the surfaces is higher or lower
than the other at any point along the span.
Then disassemble the parts.
At this point you will need to either bevel
or round the elevator LE. On a thin elevator,
such as the Hole Shot’s, I prefer to sand the
LE round. On thicker elevators I prefer to
sand a bevel on the top and bottom of the LE.
The choice is yours, but if you don’t do one or
the other, the elevator will not move up and
down properly.
Each hinge will have to be prepared for
gluing. Place a small drop of oil on the end of
each hinge at the barrel. Work the hinge back
and forth until you can see the oil work its
way along the entire length of the pin/barrel
area. Clean off any excess oil with isopropyl
alcohol. The oil will prevent the glue from
getting into the barrel and impeding
movement.
Sand the surface of each hinge tang with
220-grit sandpaper, and then clean the tangs
with the alcohol.
There are several adhesives you can use to
glue the hinge tangs into the hinge slots. I like
Pacer’s Formula 560 Canopy Glue (PT-56). It will actually adhere to the plastic hinge tangs
and will not shrink. If you use a type of glue
that shrinks, it may draw in the balsa on
either side and leave a depression in the
surface that will require filling and sanding
during the finishing process.
Squeeze some glue into a hinge slot in the
elevator and wipe the excess off using a wet
paper towel. Insert a hinge into the slot, and
work it in and out a few times to ensure that
the glue covers the entire tang, with no gaps.
Again, wipe off any excess that oozes out of
the slot.
Leave the hinge barrel slightly forward of
the elevator LE; don’t push it all the way in
to the back edge of the pocket. Install and
position the rest of the hinges in the same
manner.
Here’s the neat part. Fold the tangs of all
the hinges to be 90° to the elevator surface.
Place the elevator, LE down, against a flat
surface and push down. The hinge barrels
will be pressed to exactly the same depth
along the length of the elevator, and the
result will be a perfectly aligned hinge line.
Let the glue dry overnight.
Squeeze some glue into each hinge slot in
the stabilizer, and slightly insert the elevator
hinge tangs into the slots. Place narrow
pieces of file-folder stock between the hinge
locations, and press the assembly together
against the file-folder material.
This will yield a narrow but consistent
hinge gap between the two surfaces. Check
to make sure the tips are aligned properly.
Wipe all excess glue away using a wet paper
towel, and let the assembly dry completely.
The first time you flex the hinge, you
may hear some glue cracking. This is
normal; some of the adhesive may have been
left on the barrel area. The oil you put on the
hinge barrel will prevent the glue from
permanently locking up the hinges. As you
work them back and forth, they will become
free and smooth.
Next time we’ll start on the wing. Till then,
fly Stunt! MA
(Editor’s note: Bob didn’t mention that
he has a series of DVD programs that covers
a wide range of CL Aerobatics modelbuilding
topics. One goes through the
hinging process he has described here. If you
are interested in obtaining this program, Email
Bob and request Stunt Flyer Video
Magazine issue Volume 2, Number 2.)
Sources:
Aero Products
(678) 407-9376
www.aeroproduct.net
Edition: Model Aviation - 2008/07
Page Numbers: 138,140,142
Edition: Model Aviation - 2008/07
Page Numbers: 138,140,142
138 MODEL AVIATION
The Hole Shot project continues with the tail feathers
[[email protected]]
Control Line Aerobatics Bob Hunt
Also included in this column:
• The art of tack-gluing
• Barrel-hinging secrets
The stabilizer is cut from medium-hard 3/16 balsa; the elevator is cut from
3/16 medium-soft balsa. Notice the antiwarp end caps, also cut from 3/16 balsa
stock. Do not omit them.
This handy tool sands consistently deep hinge pockets
in movable surfaces. The plate the sandpaper is
mounted on is mounted to a plate that acts as a depth
guide.
The completed and hinged stabilizer-and-elevator assembly. Recessed hinges allow the
elevator to be positioned tightly against the stabilizer TE. A tight hinge gap means better
control!
(Editor’s note: You can follow along with
your own profile Hole Shot or get a set of
plans for it. Randy Smith at Aero Products
has plans and foam wing cores available for
this and other great Aerobatics trainers.)
IN THE LAST column we brought the Hole
Shot’s fuselage to the point where it was
ready to accept the wing and tail assemblies.
Let’s start this time with constructing the tail
feathers.
This model’s stabilizer and elevator are made
from simple 3/16 sheet balsa. The stabilizer
should be cut from fairly dense stock (8-10
pounds per cubic foot), and the elevator
should be made from medium to medium-soft
balsa (4-6 pounds per cubic-foot). Why? It is
extremely important to ensure that the
stabilizer will not bend excessively under
load.
Nothing will ruin a model’s turning
performance quicker than a stabilizer that is
too flexible. As it flexes under load, the
elevator tends to “oilcan,” or spring back
toward neutral. That means more control
needs to be given to achieve the desired turn,
but with more input comes more elevator
springing, and the result is a vicious circle.
Making the stabilizer from fairly stiff
balsa will not prevent it from bending some
under load, but it will bend much less than
one that is made from softer material.
Covering the stabilizer with the .2-ounce
carbon-fiber mat (as we did to the fuselage)
will further increase the part’s stiffness,
making it far less prone to bending under
load.
Start building the stabilizer and elevator by
accurately cutting the pieces from the
appropriate-density balsa.
There are 1/2-inch-wide, chordwise
stiffeners that will be glued to the tips of the
stabilizer and the elevator. These help prevent
the assembly from warping later; please do
not omit them! Cut the stiffeners slightly
wider from front to rear (chordwise) than is
required, and let them extend forward and aft
of the part to which they will be glued.
Use aliphatic resin glue to attach the
stiffeners. When the glue has dried, sand the
stiffeners flush with the front and rear edges
of the stabilizer and the elevator.
Tack-glue the elevator to the stabilizer
against a flat surface. This will allow you to
sand the entire assembly at one time, rounding
the LE and TE and the tips. The result will be
an assembly that looks like one smooth part
when it’s hinged together.
For those of you who are new to building, I
just introduced a new term: “tack-glue.”
Sometimes you will need to hold two (or more)
parts together for carving and sanding, and the
best way to do this is to put tiny dots of glue
every few inches along the mating seam. The
glue will hold the parts together well enough
for you to do the required work, and then you
carefully flex the assembly to break the glue
dots and separate the parts.
Sometimes you will be unable to flex an
assembly that is tack-glued, and in those
instances you will have to use a thin, sharp
knife blade to slit the glue dots apart. An
example of this would be tack-gluing balsa
blocks onto a fuselage crutch assembly for
carving and sanding. An assembly that size
will not be easy to flex, and you probably
shouldn’t try!
Okay, the stabilizer and elevator are tack-glued and sanded to shape. The edges should
have an even radius. Take your time here and
detail-sand the edges so they blend perfectly
with the upper and lower surfaces of the
assembly. When you are happy with the
shapes of the tips and the LEs and TEs, it’s
time to lay out the positions of the hinges.
There are several options for hinging the
elevator to the stabilizer. Barrel-type hinges
that have a pin between the two sections are
popular, and they are the type I recommend.
You can also use the hinge-point-type
hinges or the traditional cloth hinges. To
explain the installation of each of these types
would take more space than I have left this
time, so I’ll focus on the barrel variety. I use
the Great Planes Medium Nylon Pinned
Hinges (part GPMQ3971) on my airplanes.
You should use at least five hinges to
attach the elevator to the stabilizer. One hinge
should be positioned on the centerline of the
assembly, and there should be two more on
either side, equally spaced. Place the five
barrel hinges along the hinge line and space
them properly. The outside edges of the
outermost hinges should be approximately 1/2
inch in from the end of the assembly.
Once you are happy with the hinge
locations, use a soft pencil to make a light tick
mark on both sides of each hinge on the
stabilizer and the elevator top surfaces.
Transfer these mark locations to the elevator
LE and the stabilizer TE.
The hinges in the elevator will have to be
recessed to allow the elevator LE to almost
contact the stabilizer TE when the parts are all
assembled. It is imperative that the hinge
“gap” be as tight as possible, while still
allowing free movement up and down. Too
much gap will allow air to pass through, and
the result will be loss of control authority.
To recess the hinges, you will have to
make “pockets.” Mark the positions of these
pockets in the elevator LE, and use the #11-
blade knife to make a cut at each end of a
hinge location that is not quite as deep as will
be needed. Carve out some material between
these cuts, again being certain not to cut to the
depth that will eventually be required.
It’s time to make the first of our custom
building tools. The tool needed to complete
these hinge “pockets” is made from two
pieces of 1/8 plywood. One piece will be
approximately 2 inches long and a slight bit
wider than the hinges you are using. That is7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
I’m careful to keep the knife
perpendicular to the surface I’m slitting, and
I’m extremely careful to keep my fingers
away from the area I’m slitting in case I miss and the blade protrudes through the surface!
That can be painful and messy!
Once you have made the initial hinge slit,
you need to open (widen) it to accept the
hinge tang so it doesn’t spread the wood on
either side. Nothing looks worse than a
“bump” in the surface at each hinge location.
I use an X-Acto #27 saw blade to “pick”
out the excess balsa from the slitted areas until
I have achieved a slot that allows the hinge to
be installed without expanding the wood on
either side. I’m careful as I pick out the balsa
to make sure I’m not moving the centerline of
the hinge slot up or down. It must remain
centered!
Test-fit all the hinges in the slots, and then
assemble the elevator to the stabilizer without
using glue. Lay the assembly on a flat table,
and check to make sure there are no areas
where one of the surfaces is higher or lower
than the other at any point along the span.
Then disassemble the parts.
At this point you will need to either bevel
or round the elevator LE. On a thin elevator,
such as the Hole Shot’s, I prefer to sand the
LE round. On thicker elevators I prefer to
sand a bevel on the top and bottom of the LE.
The choice is yours, but if you don’t do one or
the other, the elevator will not move up and
down properly.
Each hinge will have to be prepared for
gluing. Place a small drop of oil on the end of
each hinge at the barrel. Work the hinge back
and forth until you can see the oil work its
way along the entire length of the pin/barrel
area. Clean off any excess oil with isopropyl
alcohol. The oil will prevent the glue from
getting into the barrel and impeding
movement.
Sand the surface of each hinge tang with
220-grit sandpaper, and then clean the tangs
with the alcohol.
There are several adhesives you can use to
glue the hinge tangs into the hinge slots. I like
Pacer’s Formula 560 Canopy Glue (PT-56). It will actually adhere to the plastic hinge tangs
and will not shrink. If you use a type of glue
that shrinks, it may draw in the balsa on
either side and leave a depression in the
surface that will require filling and sanding
during the finishing process.
Squeeze some glue into a hinge slot in the
elevator and wipe the excess off using a wet
paper towel. Insert a hinge into the slot, and
work it in and out a few times to ensure that
the glue covers the entire tang, with no gaps.
Again, wipe off any excess that oozes out of
the slot.
Leave the hinge barrel slightly forward of
the elevator LE; don’t push it all the way in
to the back edge of the pocket. Install and
position the rest of the hinges in the same
manner.
Here’s the neat part. Fold the tangs of all
the hinges to be 90° to the elevator surface.
Place the elevator, LE down, against a flat
surface and push down. The hinge barrels
will be pressed to exactly the same depth
along the length of the elevator, and the
result will be a perfectly aligned hinge line.
Let the glue dry overnight.
Squeeze some glue into each hinge slot in
the stabilizer, and slightly insert the elevator
hinge tangs into the slots. Place narrow
pieces of file-folder stock between the hinge
locations, and press the assembly together
against the file-folder material.
This will yield a narrow but consistent
hinge gap between the two surfaces. Check
to make sure the tips are aligned properly.
Wipe all excess glue away using a wet paper
towel, and let the assembly dry completely.
The first time you flex the hinge, you
may hear some glue cracking. This is
normal; some of the adhesive may have been
left on the barrel area. The oil you put on the
hinge barrel will prevent the glue from
permanently locking up the hinges. As you
work them back and forth, they will become
free and smooth.
Next time we’ll start on the wing. Till then,
fly Stunt! MA
(Editor’s note: Bob didn’t mention that
he has a series of DVD programs that covers
a wide range of CL Aerobatics modelbuilding
topics. One goes through the
hinging process he has described here. If you
are interested in obtaining this program, Email
Bob and request Stunt Flyer Video
Magazine issue Volume 2, Number 2.)
Sources:
Aero Products
(678) 407-9376
www.aeroproduct.net
Edition: Model Aviation - 2008/07
Page Numbers: 138,140,142
138 MODEL AVIATION
The Hole Shot project continues with the tail feathers
[[email protected]]
Control Line Aerobatics Bob Hunt
Also included in this column:
• The art of tack-gluing
• Barrel-hinging secrets
The stabilizer is cut from medium-hard 3/16 balsa; the elevator is cut from
3/16 medium-soft balsa. Notice the antiwarp end caps, also cut from 3/16 balsa
stock. Do not omit them.
This handy tool sands consistently deep hinge pockets
in movable surfaces. The plate the sandpaper is
mounted on is mounted to a plate that acts as a depth
guide.
The completed and hinged stabilizer-and-elevator assembly. Recessed hinges allow the
elevator to be positioned tightly against the stabilizer TE. A tight hinge gap means better
control!
(Editor’s note: You can follow along with
your own profile Hole Shot or get a set of
plans for it. Randy Smith at Aero Products
has plans and foam wing cores available for
this and other great Aerobatics trainers.)
IN THE LAST column we brought the Hole
Shot’s fuselage to the point where it was
ready to accept the wing and tail assemblies.
Let’s start this time with constructing the tail
feathers.
This model’s stabilizer and elevator are made
from simple 3/16 sheet balsa. The stabilizer
should be cut from fairly dense stock (8-10
pounds per cubic foot), and the elevator
should be made from medium to medium-soft
balsa (4-6 pounds per cubic-foot). Why? It is
extremely important to ensure that the
stabilizer will not bend excessively under
load.
Nothing will ruin a model’s turning
performance quicker than a stabilizer that is
too flexible. As it flexes under load, the
elevator tends to “oilcan,” or spring back
toward neutral. That means more control
needs to be given to achieve the desired turn,
but with more input comes more elevator
springing, and the result is a vicious circle.
Making the stabilizer from fairly stiff
balsa will not prevent it from bending some
under load, but it will bend much less than
one that is made from softer material.
Covering the stabilizer with the .2-ounce
carbon-fiber mat (as we did to the fuselage)
will further increase the part’s stiffness,
making it far less prone to bending under
load.
Start building the stabilizer and elevator by
accurately cutting the pieces from the
appropriate-density balsa.
There are 1/2-inch-wide, chordwise
stiffeners that will be glued to the tips of the
stabilizer and the elevator. These help prevent
the assembly from warping later; please do
not omit them! Cut the stiffeners slightly
wider from front to rear (chordwise) than is
required, and let them extend forward and aft
of the part to which they will be glued.
Use aliphatic resin glue to attach the
stiffeners. When the glue has dried, sand the
stiffeners flush with the front and rear edges
of the stabilizer and the elevator.
Tack-glue the elevator to the stabilizer
against a flat surface. This will allow you to
sand the entire assembly at one time, rounding
the LE and TE and the tips. The result will be
an assembly that looks like one smooth part
when it’s hinged together.
For those of you who are new to building, I
just introduced a new term: “tack-glue.”
Sometimes you will need to hold two (or more)
parts together for carving and sanding, and the
best way to do this is to put tiny dots of glue
every few inches along the mating seam. The
glue will hold the parts together well enough
for you to do the required work, and then you
carefully flex the assembly to break the glue
dots and separate the parts.
Sometimes you will be unable to flex an
assembly that is tack-glued, and in those
instances you will have to use a thin, sharp
knife blade to slit the glue dots apart. An
example of this would be tack-gluing balsa
blocks onto a fuselage crutch assembly for
carving and sanding. An assembly that size
will not be easy to flex, and you probably
shouldn’t try!
Okay, the stabilizer and elevator are tack-glued and sanded to shape. The edges should
have an even radius. Take your time here and
detail-sand the edges so they blend perfectly
with the upper and lower surfaces of the
assembly. When you are happy with the
shapes of the tips and the LEs and TEs, it’s
time to lay out the positions of the hinges.
There are several options for hinging the
elevator to the stabilizer. Barrel-type hinges
that have a pin between the two sections are
popular, and they are the type I recommend.
You can also use the hinge-point-type
hinges or the traditional cloth hinges. To
explain the installation of each of these types
would take more space than I have left this
time, so I’ll focus on the barrel variety. I use
the Great Planes Medium Nylon Pinned
Hinges (part GPMQ3971) on my airplanes.
You should use at least five hinges to
attach the elevator to the stabilizer. One hinge
should be positioned on the centerline of the
assembly, and there should be two more on
either side, equally spaced. Place the five
barrel hinges along the hinge line and space
them properly. The outside edges of the
outermost hinges should be approximately 1/2
inch in from the end of the assembly.
Once you are happy with the hinge
locations, use a soft pencil to make a light tick
mark on both sides of each hinge on the
stabilizer and the elevator top surfaces.
Transfer these mark locations to the elevator
LE and the stabilizer TE.
The hinges in the elevator will have to be
recessed to allow the elevator LE to almost
contact the stabilizer TE when the parts are all
assembled. It is imperative that the hinge
“gap” be as tight as possible, while still
allowing free movement up and down. Too
much gap will allow air to pass through, and
the result will be loss of control authority.
To recess the hinges, you will have to
make “pockets.” Mark the positions of these
pockets in the elevator LE, and use the #11-
blade knife to make a cut at each end of a
hinge location that is not quite as deep as will
be needed. Carve out some material between
these cuts, again being certain not to cut to the
depth that will eventually be required.
It’s time to make the first of our custom
building tools. The tool needed to complete
these hinge “pockets” is made from two
pieces of 1/8 plywood. One piece will be
approximately 2 inches long and a slight bit
wider than the hinges you are using. That is7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
7/8 inch in the case of the medium Great
Planes hinges. The second piece of plywood
will also be 2 inches long, but it will need to
be wider than the first piece by at least an
inch.
Glue a piece of 220-grit sandpaper on the
face of the narrow piece of plywood and trim
it neatly to fit perfectly. Glue the narrow
piece of plywood on the center of the face of
the wider piece. The result will be a tool that
can be used to sand an exact-depth pocket at
each hinge location. The wider piece of
plywood acts as a stop to prevent you from
sanding the pocket too deep.
You can add a 1/8 plywood handle to the
back of this tool to make it easier to hold and
use. A look at the accompanying photo of
this tool will answer all your questions.
Scribe a line between the two transferred
tick marks at each hinge location. On the
elevator, these lines will be made in the
bottom of the pockets.
Several devices are available that will
scribe a mark exactly halfway between the
top and bottom surface of the parts to be
hinged. I prefer to use the edge of my finger
as a guide and draw the lines with a ballpoint
pen.
That method works fine if you have a
good eye and a steady hand. Just doublecheck
to make sure the lines you scribe are
centered and straight and that all the lines are
aligned from one end of the part to the other.
There are several hinge-slotting devices
on the hobby market, but I prefer to do it by
hand. (Hey, I’m old fashioned!) I use a #11
knife blade to carefully make slits at the
marked hinge locations. I make these slices
deep enough to accept the tang (which is part
of the hinge that will be inserted into the
wood).
I’m careful to keep the knife
perpendicular to the surface I’m slitting, and
I’m extremely careful to keep my fingers
away from the area I’m slitting in case I miss and the blade protrudes through the surface!
That can be painful and messy!
Once you have made the initial hinge slit,
you need to open (widen) it to accept the
hinge tang so it doesn’t spread the wood on
either side. Nothing looks worse than a
“bump” in the surface at each hinge location.
I use an X-Acto #27 saw blade to “pick”
out the excess balsa from the slitted areas until
I have achieved a slot that allows the hinge to
be installed without expanding the wood on
either side. I’m careful as I pick out the balsa
to make sure I’m not moving the centerline of
the hinge slot up or down. It must remain
centered!
Test-fit all the hinges in the slots, and then
assemble the elevator to the stabilizer without
using glue. Lay the assembly on a flat table,
and check to make sure there are no areas
where one of the surfaces is higher or lower
than the other at any point along the span.
Then disassemble the parts.
At this point you will need to either bevel
or round the elevator LE. On a thin elevator,
such as the Hole Shot’s, I prefer to sand the
LE round. On thicker elevators I prefer to
sand a bevel on the top and bottom of the LE.
The choice is yours, but if you don’t do one or
the other, the elevator will not move up and
down properly.
Each hinge will have to be prepared for
gluing. Place a small drop of oil on the end of
each hinge at the barrel. Work the hinge back
and forth until you can see the oil work its
way along the entire length of the pin/barrel
area. Clean off any excess oil with isopropyl
alcohol. The oil will prevent the glue from
getting into the barrel and impeding
movement.
Sand the surface of each hinge tang with
220-grit sandpaper, and then clean the tangs
with the alcohol.
There are several adhesives you can use to
glue the hinge tangs into the hinge slots. I like
Pacer’s Formula 560 Canopy Glue (PT-56). It will actually adhere to the plastic hinge tangs
and will not shrink. If you use a type of glue
that shrinks, it may draw in the balsa on
either side and leave a depression in the
surface that will require filling and sanding
during the finishing process.
Squeeze some glue into a hinge slot in the
elevator and wipe the excess off using a wet
paper towel. Insert a hinge into the slot, and
work it in and out a few times to ensure that
the glue covers the entire tang, with no gaps.
Again, wipe off any excess that oozes out of
the slot.
Leave the hinge barrel slightly forward of
the elevator LE; don’t push it all the way in
to the back edge of the pocket. Install and
position the rest of the hinges in the same
manner.
Here’s the neat part. Fold the tangs of all
the hinges to be 90° to the elevator surface.
Place the elevator, LE down, against a flat
surface and push down. The hinge barrels
will be pressed to exactly the same depth
along the length of the elevator, and the
result will be a perfectly aligned hinge line.
Let the glue dry overnight.
Squeeze some glue into each hinge slot in
the stabilizer, and slightly insert the elevator
hinge tangs into the slots. Place narrow
pieces of file-folder stock between the hinge
locations, and press the assembly together
against the file-folder material.
This will yield a narrow but consistent
hinge gap between the two surfaces. Check
to make sure the tips are aligned properly.
Wipe all excess glue away using a wet paper
towel, and let the assembly dry completely.
The first time you flex the hinge, you
may hear some glue cracking. This is
normal; some of the adhesive may have been
left on the barrel area. The oil you put on the
hinge barrel will prevent the glue from
permanently locking up the hinges. As you
work them back and forth, they will become
free and smooth.
Next time we’ll start on the wing. Till then,
fly Stunt! MA
(Editor’s note: Bob didn’t mention that
he has a series of DVD programs that covers
a wide range of CL Aerobatics modelbuilding
topics. One goes through the
hinging process he has described here. If you
are interested in obtaining this program, Email
Bob and request Stunt Flyer Video
Magazine issue Volume 2, Number 2.)
Sources:
Aero Products
(678) 407-9376
www.aeroproduct.net