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This section on building tips sponsored by D'Best Co. and CENTRAL HOBBIES |
MK Control Horn Installation
by Eric Hawkinson, D'Best Co.
If you are not familiar with the MK horns, they are the tapered aluminum horns that are most favored due to their resistance to fatigue and also the ball-bearing clevis that is available to use with them. I've used these horns and clevises for years and feel that they are the best in terms of reliability and precision as well as being maintenance free.
The basic steps (except for leaving the dowel ends untapered) are also great for installing any type of horn in a very rugged and worry free manner.
Step 1
I used to just insert the short dowel that holds the horn into
the control surface. But on a couple of occasions I had slop
develop from the dowel compressing the foam and balsa around
its perimeter. So now I install all of these dowels into a 1"
balsa block with its grain running perpendicular to the surface.
I've never had one of these installations loosen up.
Start out by cutting two pieces of 1"x1"x whatever
balsa block, long enough to go from top to bottom of the surface
plus a bit. Use a fine pen to carefully trace the outline onto
the top surface of the elevator (in this example, same for any
control surface). Make sure that the FRONT of the block sticks
out a bit from the hinge line, 1/16" is fine. Using a scroll
saw with the table set at 90 degrees to the blade, with the elevator
on the shuck, carefully make the cutout slightly undersize and
then sand to a perfect fit.
Step 2
Now fit the balsa blocks, grain vertical, into the cutouts you
made. Mark the top and bottoms with a fine felt pen around the
3 sides of the block where they meet the sheeting. Use your scroll
saw to cut the blocks almost to the line you marked on each end.
Step 3
Now use a fairly slow epoxy or yellow alphiatic glue to glue
the blocks into the surfaces. Make sure that the ends are slightly
above the control surface to allow sanding to a perfect fit.
Let the glue cure.
Step 4
Take some masking tape and "surround" the block to
prevent marring of the control surface. Now use a straight sanding
block and coarse paper (80-120) to sand the blocks down to the
tape. You'll probably tear up the tape a bit while sanding. Don't
forget to sand the front edge also.
Step 5
Now remove the tape and use a straight block with finer paper
- say 240 or finer - and sand the block smooth to the surface
on all three sides. It should be very smooth and flush as in
photo number five.
Step 6
Now you need to locate the hole that you'll drill for the dowel.
Using the little aluminum "washer" that comes with
the MK horns, position it so that the front edge of the washer
is set back about 1/8" - 5/32" from the leading edge.
This will vary depending on how much taper you'll be sanding
into the leading edge of the surface. You want a minimum of 1/16"
of balsa left in front of the dowel at the top and bottom after
you sand the bevel, but you want the clevis pin as near the hinge
line as possible to minimize any differential. With the washer
in position, use a center punch to hold it in place while you
trace around it with a dark, fine-tipped pen.
Step 7
Now we need to make the hole for the dowel. DO NOT use a drill
bit for this! And do not drill it by hand. The only acceptable
method for best results here is to use a drill press. If you
don't have one, get all of your surfaces and dowels ready and
then take them somewhere where you can borrow a drill press.
Your "bit" will be a sharpened brass tube. I use 1/2"
diameter dowel and thus 1/2" diameter (OD) brass tube. Carefully
sharpen the inside edge with a sharp hobby knife, and file an
edge on the outside. Cut the length as desired, 4 inches is plenty.
Now chuck the brass tube into the drill press and adjust the
table to be perpendicular to the tube. Put the control surface
ON THE SHUCK - the leading edge should be vertical to your flat
surface. Now very carefully align the brass tube with the circle
you marked on the control surface. Look at it from front and
side, and make a "shallow" start. If it's not in the
right place, move your surface and try again. When you get it
right, use light pressure to continue the cut through the entire
block.
Step 8
Now you should have a perfectly smooth hole in your block, perfectly
vertical and perpendicular to the surface. Admire it for a moment,
then proceed to ...
Step 9
Now we need to get those dowels ready to glue in that perfect
hole you just drilled. Please note that we will cut the dowels
with the ends SQUARE to the length - there is NO bevel. (A big
plus, in my mind!)
Cut off several inches of 1/2" dowel and test its fit into
the hole in the block. Is should be fairly snug. I've found that
it's common for the dowels to be slightly oversized, which is
great - just chuck it up in a drill and carefully run it while
wrapping fine sandpaper around one side. When the diameter is
perfect, unchuck it and place it in the surface. Carefully mark
the intersection of the dowel with the top and bottom surface
at the REAR side of the dowel. That's the length we'll use. Yes,
this means that the FRONT side of the dowel will be slightly
recessed into the block. Using a miter box, carefully cut the
dowels (at 90 degrees) to the exact length you have marked.
There are two ways to go for drilling the hole you need for the
3mm bolt that attached the horn. I prefer to use my vise with
the jaws that hold tubing/rods perfectly vertical. If you don't
have one of those, then glue the dowels into the wood blocks
now, paying attention to get the ends flush with the rear of
the hole. Use good epoxy for this step.
Regardless of whether you glue the horns into the surfaces first
or drill them before gluing, you need to mark the position for
the hole you'll drill. Again, simply use the aluminum MK washer
and center it as perfectly as possible on the end of the dowel.
Now use a center locater of some kind (Great Planes sells a fine
one for under $10) to start/mark the hole location. Again using
the drill press, drill the hole for the 3mm bolt. If you are
doing this after gluing the dowel in, be sure to use the shuck
again to ensure that the hole is drilled vertically!
Step 10
If you haven't already, glue the dowels into the wood blocks
now, paying attention to get the ends flush with the rear of
the hole. You should have a part that looks like photo number
10.
Step 11
The only gripe I have about the MK horns is that the bolts provided
are NOT of any use for any pattern plane I've seen - they simply
are not long enough to go through the surface. So order up some
3mm x 30mm bolts and custom cut them for the length of the surface
you are building. You want as much thread as possible going into
the horn portion of the horn without bottoming out. Order a few
3mm nuts too, and put a nut on the bolt before you cut it - this
way after you cut and file the bolt you'll make some "sorta
threads" when you remove the nut, allowing the bolt to start
into the horn without cross-threading.
Photo 11 shows the final installation from both sides, before
beveling the leading edges. After the plane is painted, the horns
will be re-installed using some loctite and that will most likely
be the last maintenance they ever need. If you are not glassing
and painting, then before covering use THIN foam-safe CA to seal
the balsa, letting it penetrate into the ends of the block (top
and bottom) thoroughly. If you glass and paint, just give a good
coat of thinned epoxy finishing resin to those ends and it will
soak in and harden the block nicely.
Until next issue, keep the faith - and "Do It" with Precision!
D'hawk