Stabilator Construction (2 of 2)
This page was last updated Sunday May 1, 2005
Click on any photo to enlarge it.
Continued from Stabilator Construction (1 of 2).
2005-Mar-13:
With the four centre ribs installed, I glued the centre trailing edge in place.
I used hobby push pins through 1/32" holes in the trailing edge to keep things
lined up, and elastics to keep the trailing edge pulled tightly against the
backs of the ribs.
Later the same day, I installed the next pair of ribs, using
pins to keep them lined up with the trailing edge.
I also started work on the anti-servo tab, cutting out its leading edge from
1/2" resawed pine, and ribs from 1/4" okoume marine plywood. I cut all the
ribs at once on the bandsaw from a stack of 1/4" blanks bolted together.
Warning: I am deviating from the plans for the
anti-servo tab. The plans call for an all-aluminum tab, the design of which I
was not happy with. I thus designed an all-wood tab (incorporating some ideas
suggested by fellow VP-1 builder Todd Currier), so any information about my tab
will not apply if you are following the plans. If you do choose to build a
wooden tab, be sure to do all the analysis to ensure it won't compromise the
airworthiness of this airplane. Please read the
disclaimer.
2005-Mar-17:
Over the course of a few days, I glued the remaining ribs in place followed by
the leading and trailing edges. When gluing the outboard ribs, I used a piece
of aluminum angle placed across the trailing edges of the ribs to ensure they
were lined up. By viewing this from behind at just the right height, I could
line it up with the spar to ensure that I wasn't building a warp into the
stabilator.
The only tricky part about gluing on the leading edges was figuring out how to
clamp them. Eventually I settled on a combination of clamps and elastic bands
as you can see in the picture.
When gluing the trailing edges, I used a level to ensure that they
were parallel to the top of the spar (after checking that it was
level from front to back). At this point, the whole assembly
was really starting to look like part of an airplane!
2005-Mar-22:
A few more days of work saw the installation of all the gussets, short pieces
of quarter-round pine, and the pine filler strips between the ribs above and
below the spars. The centre section trailing edge gussets were a challenge to
clamp, due to the non-standard angled trailing edge.
The speed at which I could work was basically limited by the number of clamps I
had available. The photo shows just about every suitable clamp that I own.
2005-Mar-23:
The next step was to finish shaping the leading edge, which I did in
stages, starting with a hand plane, and finishing with a reasonably long
sanding block.
To check that the shape was correct, I traced around the leading edge of
my rib template to produce another template that would fit precisely over
a properly shaped leading edge. Frequent checking with this template showed
me where to remove more material, and more importantly, where I had removed
enough.
After shaping the leading edges, I proceeded to glue on the counterweight ribs,
making sure they were aligned with the tip ribs and with each other. I checked
their relative alignment by sighting across them from one tip to the other, and
then double checked with a level.
2005-Mar-25:
Once the epoxy had set from gluing on the counterweight ribs, I carefully
removed the cured excess squeezed out epoxy with a chisel.
I then cut and shaped the tip rib gussets from 1/16" aircraft plywood, and
installed two of them. Instead of using a saw to try to make precise cuts in
the thin material, I cut it with repeated passes of a heavy duty knife with
snap-off blades. Using a steel rule, this made a very straight and smooth
cut.
At first I tried to use clamps to hold them in place,
but then decided to try some small tack nails that I had on hand (these are
tapered like carpet tacks, but thinner). These worked extremely well. Because
they are tapered, they hold the plywood down even without driving them all the
way in, which in turn makes it easy to pull them out later.
2005-Mar-26:
The next day, I pulled out the tacks from the first pair of tip rib gussets,
and glued on the other pair.
The soon-to-be-installed leading edge sheeting overlaps the tip rib gussets,
which results in a few places where there would be a step in the gluing
surface. To avoid this, I made some ramps (from failed scarfing attempts -
never throw anything out) and glued them where the tip gussets lie on the pine
spar filler blocks.
2005-Mar-27:
Once the second set of gussets had cured, I pulled out their tacks. At this
point I also varnished some parts of the leading edge cavity that would be hard
to reach once the leading edge sheeting was applied to one side.
2005-Mar-31:
Finally, it was time to cut out the leading edge sheeting from 1/16" birch
aircraft plywood. Again, I used a knife to cut the sheets to size, allowing
for about 1/2" overlap on the pine leading edge.
I also sanded down the ribs a bit to be completely flush with the pine filler
blocks.
2005-Apr-02:
I was very apprehensive about gluing the plywood to the stabilator. My
preliminary dry fitting showed that it was difficult to keep the leading edge
of the plywood firmly against the pine leading edge material.
After some final sanding of the pine, I mixed up a batch of T-88, and set to
work applying it to all the surfaces that the plywood of the upper right
leading edge would contact. I then started tacking the plywood to the
stabilator, starting with a few tacks into the pine filler strips, and then
working my way forward along all the ribs at once.
When I finally got to the leading edge, my apprehension was vindicated, as
I was unable to keep the wood tight against the pine leading edge, even with
numerous tacks. Unfortunately, it's no fun to tell oneself, "I told you so!".
To resolve the problem, I cut some 1/2" wide nailing strips from 1/16" plywood.
I then nailed the tacks through this, all the way in, so the heads were on
the surface of the nailing strip. This proved sufficient to hold the wood in
place.
2005-Apr-03:
Pulling up the nailing strip proved more difficult than I thought it would
be. I had envisioned just lifting it up, and having all the tacks come popping
out. Instead, the strip came off in pieces, but it was fairly easy to then
pull the tacks out with pliers.
2005-Apr-04:
I glued on the second (upper left) leading edge sheet using exactly the same
technique as the first, since it did work.
2005-Apr-05:
I didn't have a lot of time today, so I just pulled out tacks and did a bit
of sanding.
2005-Apr-07:
By now, I was getting a bit fed up with the difficulty of getting the sheeting
properly glued down, so I varnished the inside of what I'd done so far, and
while that was drying, constructed a jig to bend the leading edge to shape.
Also today, I glued the anti-servo tab's spar/leading-edge to its bottom
sheet, once again using tacks to hold things together until the epoxy cured.
2005-Apr-09:
I soaked about a 2" wide strip along one edge of each remaining leading edge
sheet in hot water for an hour, and put it in the jig for half a day. Masking
tape was used to hold the sheet as it appears in the photo (the clamps and
aluminum level were there to force a slight warp out of the jig).
While waiting for the two leading edge panels to dry in the bending jig,
I pulled the tacks from the anti-servo tab, and cut out the end ribs and
trailing edge filler strip for it.
2005-Apr-10:
More varnishing of the interior of the stabilator leading edge and the now bent
leading edge sheets. In hindsight, I should have marked the areas to be glued
(and thus not to be varnished) before gluing on the opposite
sheets, since I could have just held the sheet in place and marked around the
ribs and other internal structures. Instead, I had to make careful measurements
to determine which areas of the curved sheets to varnish.
I applied two coats of varnish to all interior surfaces. To make it easier
to see where I'd varnished already, I mixed about a tablespoon of blue
Tremclad (an oil-based rust paint) into the Varathane to give it a slight tint.
2005-Apr-11:
Today I did something new, which was to work with aluminum. I cut the
anti-servo tab control horns from 1/16" (0.063) 2024-T3, using a 1/4" 14
tooth-per-inch bandsaw blade. This cut the metal so easily that it was
difficult to make a smooth cut, because with any sideways pressure, the
saw would just go ahead and cut in that direction. However, I managed
to stay outside the lines.
I used my benchtop disk sander, then a Dremel tool with a fine sanding
drum, and finally emery cloth to achieve the final shape. It is important
that there are no nicks in the edges, since cracks could form from them.
2005-Apr-12:
I installed the first of the pre-bent leading edges, and this worked a lot
better than working with flat plywood. Rather than applying the epoxy to the
structure, I applied it to the sheet, being sure that there was epoxy
everywhere that there was no varnish.
I still used nailing strips, but instead
of tacks, I tried 3/8" staples with my electric staple gun. This proved more
than adequate to hold the plywood in place against the pine leading edge. To
make the nailing strip easier to remove, I put a strip of filament tape on the
back side of it.
Note: Unless you are building your VP-1 as a
Canadian Basic Ultralight as I am, you will require an inspection before
closing in the leading edge.
2005-Apr-14:
With the epoxy cured, I pulled the nails and staples out of the leading edge
sheet, and glued on the remaining sheet, using the same technique as above.
2005-Apr-15:
This day saw a number of unrelated tasks. I drilled and deburred the holes in
the anti-servo tab horns (one 3/16" hole for the control rod, and two 1/4"
holes to fasten it to the centre section assembly - see below). I then cleaned
and primed the horns with zinc chromate primer (except in the area where they
would be epoxied into the centre section assembly).
Still on the subject of the anti-servo tab, I cut the washers that would be
used in the centre section to match the shapes of the ribs. I used bolt cutters
to get the rough shape, and then cleaned them up with a file.
I pulled the nails from the last leading edge sheet, and then planed and
sanded the plywood to fair smoothly into the pine leading edge over a distance
of about 1/2" (about the amount that the plywood overlaps the pine).
2005-Apr-16:
After a bit more planing and sanding of the leading edge plywood, I assembled
the anti-servo tab centre section. The five layers of 1/4" plywood and the two
horns were sandwiched together with epoxy, and clamped using the same AN17-4A
bolts that will remain in the assembly. I applied a bit of Teflon grease to the
bolts so I could still remove them.
While the centre assembly was curing, I installed the other anti-servo tab
ribs, end ribs, and trailing edge filler. Clothespins proved useful as clamps
for the latter job.
2005-Apr-17:
I cut pieces of extruded aluminum hinge for the anti-servo tab: two 4" pieces
and one 5" piece for the centre section (so enough of the hinge would clear
the centre assembly). I drilled and deburred the 3/16" bolt holes in the
hinges, and then in tab leading edge. To line these up, I turned tab over,
placing the 1/16" plywood top sheeting under it. The hinges were then held
against the spar/leading-edge with the hinge loops resting on the workbench,
and the hinge outlined with a pencil. I then turned the tab right-side up,
clamped the hinge in place where marked, and drilled through the holes into
the wood. Unfortunately, it's hard to take pictures while doing this.
I also glued the centre assembly (with the horns) in place today, with the
bolts removed.
2005-Apr-19:
It soon became apparent that the 3/16" double-lug anchor nuts would not fit
over the two centre-most holes, so I elected to use 10-32 blind nuts (also
known as T-nuts) in this location. To allow for this, I enlarged the diameter
of these holes to 1/4" to a depth of 1/4" from the inside.
Again using a knife, I cut out the anti-servo tab plywood top sheet, and
slotted it for the horns to pass through.
Finally, I varnished all the inside surfaces of the tab, the bottom of the
top sheet (only where no glue would be applied), and inside all the bolt
holes.
2005-Apr-20:
After applying another coat of varnish to all interior surfaces and bolt
holes, I started installing the anchor nuts. The holes in the lugs were not
quite large enough for #4 x 1/2" stainless steel wood screws, so I drilled them
all out with a 3mm bit (half way between 7/64" and 1/8"), and deburred the
holes.
Each anchor nut was temporarily held in place using a 3/16" bolt and washer
from the outside. Then, 1/16" holes were drilled through the centre of the
lug holes, and the #4 screws installed after dipping them in varnish. By the
way, this really illustrated the usefulness of Robertson (tapered-square-drive)
screws. They fit snuggly enough on the screwdriver bit that they could be
dipped while on the screwdriver.
The two blind nuts in the centre were installed by just pulling them tight
using a 3/16" bolt and washer. Because these aren't lock nuts, I will use
threadlocking compound when I reinstall the hinge after the plane is covered,
and will placard the hinge to be sure this is done if the hinges are ever
removed and reattached.
I think I will also drill an additional (fifth) hole in the hinge, and a
corresponding hole in the spar/leading edge and centre section, and insert
a #10 stainless steel pan head wood screw, dipped in varnish.
Once all the anchor nuts were in place, I installed the centre section bolts,
which are responsible for transferring the control forces from the horns to the
structure of the tab. The epoxy probably helps with that too, although I would
certainly not rely on it, since it is under shear load, and is glued to
aluminum.
In the photo you can see the whole thing just before the top sheet was added.
Even without the top sheet in place, this wooden anti-servo tab had a lot
of torsional stiffness.
I proceeded to glue on the top sheet, applying epoxy to all the areas that
weren't varnished. I used tacks at the leading edges and rib locations, and a
large number of plastic spring clamps along the trailing edge. Clamping the
trailing edge to the workbench ensured that it would set up perfectly straight.
Note: Unless you are building your VP-1 as a
Canadian Basic Ultralight as I am, you will require an inspection before
closing in the tab. This applies if you are building the aluminum tab too.
2005-Apr-23:
The next step was to install the corresponding hinge halves on the stabilator
trailing edge. To do this, I first attached them to the anti-servo tab hinges
and then marked the horizontal location of each hinge half on the stab trailing
edge. Next, I removed the hinge pins and aligned the hinge loops to be flush
with the top trailing edge gusset.
I used small wood screws through some of the holes to hold the hinges in place
while drilling 3/16" bolt holes through the other holes. After each bolt hole
was drilled, I inserted a bolt, removed one of the wood screws, and drilled
the next bolt hole.
Next I removed all the bolts and varnished inside all the bolt holes. While I
was at it, I varnished the main stabilator hinge bolt holes too. A cotton swab
was handy for this.
2005-Apr-24:
Not much time today, so I just applied the second coat of varnish in all
the bolt holes.
2005-Apr-25:
Using the same technique as on the anti-servo tab itself, I installed the
anchor nuts for the hinge bolts. This was made a bit more difficult by having
to work between the top and bottom trailing edge gussets.
2005-Apr-26:
Finally, the moment of truth. I bolted the hinge halves onto the trailing
edge and attempted to hinge the tab. Unfortunately, it was a very tight fit
and I couldn't get the hinge pins all the way in. A bad sign.
Then it occurred to me that the bolts became marginally narrower just below
their heads, so that even though they were a snug fit when part way in, there
was a bit of play once they were all the way in (but not yet tightened down).
So, I loosened off all the bolts about half a turn, and everything fell into
place.
After retightening the bolts, the anti-servo tab operated smoothly with no
binding.
My only disappointment was a slight misalignment (about 1/10") between one side
of the tab and the stabilator trailing edge when the other side is held aligned
with the trailing edge. After fretting over this for a while, I realized that
most of the trim tabs on Cessnas I've seen were more misaligned than this, and
that it is nothing to worry about.
Finally, I installed a pair of #10 x 1-1/4" stainless steel wood screws
through the counterweight ribs as per the plans. I predrilled the holes,
countersunk them, and dipped the screws in varnish before installing them.
Remaining tasks:
-
Constructing the balsa channel for the anti-servo tab control
rod. I will wait until the main hinge bolts are in their final
positions before doing this, which in turn requires a plane to
hinge the stabilator to.
-
Replacing two of the hinge bolts. I ran out of stainless steel
ones, and don't like the idea of ordinary steel bolts on
aluminum. I painted the offending bolt heads red to remind me.
-
Devising a method to secure the hinge pins. Either safety wire
or some sort of removable obstacle at each end of the hinge are
my current preferences.
-
Installing that extra hinge attaching screw in the centre of the
centre hinge on the anti-servo tab.
-
Placarding the two centre bolts to require thread locking
compound.
|
|
Stefan's General Aviation Web Site
|
E-mail Stefan
|
|
|
|
Disclaimer:
Although every effort has been made to ensure accuracy and
reliability, the information on this web page is presented without
warranty of any kind, and Stefan Vorkoetter assumes no liability for direct or
consequential damages caused by its use.
I am not a flight instructor, aircraft designer, or aircraft mechanic,
nor any other professional in this field. I am merely using this site
as a medium for sharing my experiences and the things I've learned.
It is up to you, the reader, to determine the suitability of, and
assume responsibility for, the use of this information.
Copyright:
All materials on this web site, including the text, images, and HTML
mark-up, are Copyright © 2008 by Stefan Vorkoetter unless
otherwise noted. All rights reserved. Unauthorized duplication
prohibited. You may link to this site or pages within it, but
you may not link directly to images on this site, and you may
not copy any material from this site to another web site or
other publication without express written permission. You may make
copies for your own personal use.
|
|
