July 2007 29
BY JASON NOLL
Any size model will
benefit from having
a tight hinge line
CORRECTLY HINGING and gap-sealing
your International Miniature Aerobatic Club
(IMAC) or 3-D airplane with Robart or
hinge point-style hinges can be a frustrating
experience. I hope this article makes it
easier for you and helps you through that
stage of building an airplane or assembling
an ARF.
This method will set you up for a greatlooking
hinge line and will allow you to do
a terrific finishing job on the gap-sealing
process. There are many ways to achieve
these goals, but the one I will share with
you works best for me.
I like to use Robart hinge points because
they provide a strong, slop-free joint that I
so often see broken on models that were set
up with flat-style hinges. The inboard or
outboard portion of the surface is where the
torsional loads are highest, and I’ve learned
that the hinge points hold up better. All that
combined with ease of gluing the hinge
point-style hinges in place makes this
method an excellent way to end up with an
excellent-looking aircraft.
I recommend that you use the Robart
3/32” Hinge Point (item 305) on 10- to 40-
size models, the 1/8” Hinge Point (item
307) on 1.20-size models, and the 3/16”
Super Hinge Point (item 309) on 30-40%
models.
If you are unfamiliar with the gapsealing
method and its benefits to you as a
pilot, a “gap” in your model’s control
surface is the distance between the control
surface TE and LE. This gap allows air to
travel through while you are flying,
creating a high risk of flutter: a violent
oscillation of your aircraft’s control
surface that can cause it to depart from the
airplane.
Sealing the hinge gap will improve
control-surface effectiveness. By not
allowing the air to pass through the gap,
you are essentially making that air work
against your control surface, allowing it to
be more responsive and more efficient. The
pictures show correct and incorrect gaps.
Making It Easy: You will need the
following tools to do this job.
• Drill
• Motor tool
• Drill bit (The 11/64 size is right for the 3/16-
inch-diameter hinge points used in this
demonstration. A slightly larger bit may be
required, depending on the wood’s
hardness.)
• “Pine tree”-shaped sanding bit.
• Soldering iron with an old bullet-point tip.
Once you have located all your tools,
mark the centerline on the LE of the control
surface (aileron, elevator, or rudder) and on
the TE of the flying surface (wing,
stabilizer, or fin) if it’s not prebeveled. For
this article I prebeveled surfaces, and that
will act as my centerline.
Mark the desired hole locations for the
hinge points. As a good rule, on 35%
airplanes you should have a hinge every 4
inches on the ailerons and every 3 inches on
the rudder and elevators. For 40% airplanes
you should have a hinge every 5 inches on
the ailerons and every 3 inches on the
rudder and elevators.
Carefully center-grind each location with
the point of the “pine tree”-shaped sanding
bit. This will create a small pilot hole you
can follow up with an 11/64 drill bit. The
pilot hole prevents the drill from wandering
or sliding off center; otherwise, that
particular hinge may not line up and may
even cause binding when the control surface
is moved.
Drill the center marks to the desired
depth. If possible, run the bit all the way in,
stopping just shy of the chuck.
Now you have the hard part behind you.
Relocate your motor-tool bit. Push it in
slightly farther to allow for half of the hinge
joint to sit just below the surface. This will
minimize the possibility of a gap between
the surfaces.
Be careful to make the depth the same
on all points. If you sink the bit too far, it
could cause binding and reduce the effective
gluing area. It is crucial that the pivot point
on all the hinges align.
After you have made all the hinge
points, cover the model. If you are building
an ARF you will start from here. Do not try
to cover down into the holes with your iron
or attempt to cut out with a knife.
Once the model is covered or the ARF is
out of the box and you are at the hinging
Hinging for IMAC and 3-D
07sig1.QXD 5/23/07 1:54 PM Page 29
30 MODEL AVIATION
Control surfaces deflected at high angles
need extra support to handle the abuse of
3-D demands.
Precise hinging not only allows a surface to work better, but it puts less stress on the
servo and linkages.
The goal is a tight hinge joint. Double
bevels permit deflections of 45° or more.
A hinge gap as shown is unacceptable.
Diverging air into the gap will promote
flutter and surface failure.
Match the marks on the control surface
and the flying surface. The bevel should
be on center.
Tools for installing point-style hinges. Use a soldering iron with an expendable tip. An
11/64 drill is used for the 3/16-inch-diameter hinges.
With the motor tool’s speed on low, use
the “pine tree”-shaped bit to make a drill
center point.
Photos by the author
07sig1.QXD 5/23/07 1:57 PM Page 30
July 2007 31
The 11/64 hole is tight for the hinge. It
should be deep enough to accept the
hinge up the pivot centerline.
With the hot tip of an old soldering iron,
poke through the covering hiding the
hinge pocket.
Cover the aircraft, leaving the hinge
pockets sealed over until you are ready to
install the hinges.
Now is a good time to lubricate the hinge
joint. Securely glue hinges and install the
control surfaces.
Rotate the iron so it melts the covering.
The excess will seal and coat the bare
wood.
The finished hinge hole should be
completed as shown. Do all the hinge
points in the same manner.
The center of the hinge pivot point is on
the center of the bevel point. Dry-fit the
surfaces before covering.
Through the center point, drill an 11/64
hole through the hinge material as
straight as possible.
Use the “pine tree”-shaped bit to hone
out the opening just enough to give the
pivot point clearance.
07sig1.QXD 5/23/07 2:08 PM Page 31
32 MODEL AVIATION
The covering iron’s temperature is lower
to prevent damaging the composite
fuselage at the rudder hinge line.
Matching covering is used to seal the
hinge gap. A 1-inch-wide strip is typical
for a 30%-40% model.
Crisply crease the sealing strip down its
length. The adhesive side should be facing
outward.
Fully deflect the control surface. Cut the
sealing strips into lengths that fit exactly
between the hinge points.
With a trim iron on low heat, adhere the
covering inside the hinge bevel. Work
one side at a time.
You can use scraps of covering to transfer
the color scheme through the hinge
cutout.
Sealed hinge gaps will make any model, such as this
Somenzini Yak, easier to trim and longer lasting.
07sig1.QXD 5/23/07 2:17 PM Page 32
stage, locate the soldering iron with the
old bullet-point tip. Go ahead and get it
warmed up.
Use a pen to mark each of the
previously drilled holes hidden below the
covering. Carefully sink the soldering iron
into the holes. Give it a twist or two and
move it around slightly if your iron tip is
smaller than the hole diameter. This will
create a nicely cleaned and sealed hole.
Once all the holes in the surfaces are
prepped, glue the hinges in place
accordingly. Be sure that the hinge joints
are pushed together tightly to eliminate
any possible gap. Be sure to monitor them
during the drying process. Sometimes
they slip a bit; the epoxy heats up and
thins as it cures, creating almost a
lubricant.
Now that you have all the hinges
glued in place, you can start the gapsealing
process. Measure the distance
between each of the hinges. Cut a strip of
covering roughly 1 inch wide, depending
on the size of the bevel. Cut individual
pieces to match the measurements you
made previously.
Remove the backing from the strips of
covering. Fold each piece in half and make
a sharp crease. Place the folded piece in
the crevice of the bevel and between the
hinge points.
Be sure to have the surface fully
deflected when ironing the folded piece
into place. Control-surface travel will be
limited if the surface is not fully deflected,
which will lead to the gap seal’s failure
because of excessive force applied by the
control-surface servo. Continue to fill the
spaces between the hinge points and the
ends of the hinge line.
You’re finished! Now you have a clean,
airtight control surface that will have a
positive effect on your flying and last the
life of the model. MA
Jason Noll
[email protected]
Sources:
Horizon Hobby
4105 Fieldstone Rd.
Champaign IL 61822
(217) 352-1913 or (800) 338-4639
www.horizonhobby.com
Robart Manufacturing
Box 1247
Saint Charles IL 60174
(630) 584-7616
www.robart.com
