How to construct a s..

c
RAFTSMAN'S CORNER
Edited by Chuck Larsen
How To Construct A Scarf Joint Repair To A
Wood Wing S p a r . . . In The Wing
By Tom Glenn, EAA 127063
1010 13th St.
Bellingham, WA 98225
You own an airplane with wing spars
formed of wood. Following an annual
inspection your Inspector announces,
"I've found some bad news in the right
wing — a crack in the rear spar about
3-1/2 feet from the outer end." (I am
quoting from an actual experience with
my own 1946 Taylorcraft BC12D
floatplane last spring.)
You climb the stepladder, flashlight
in hand, to peer through the 3-1/2 inch
inspection hole on the underside of the
wing just outboard of the rear strut attach point. At first you see nothing; then
as your eyes adjust and focus, you
sweep the light beam back and forth
and . . . oops! there it is, a crack leading
away from a bolt hole.
To the unskilled eye, the task of repairing the crack looks (first) impossi-
ble, (second) bewildering and (third)
costly. Wrong on all counts. Here's how
to proceed.
My partner in T-Crafl N39976 is a
machinist, skilled in mechanics and
metal working with some aircraft experience. I am a long-time amateur woodworker. We combined our skills to repair
the cracked spar ourselves, did a firstrate job and saved a good deal of
money.
Looking back over the job, we observed a number of lessons we learned
whose explanation here might save
other enthusiasts time, money and anxiety. Here is our step-by-step procedure
for performing the job successfully.
First, work out an overall program
with your FAA Inspector. Level with him.
He must supervise your work. Tell him
exactly what you propose to do and how
you will proceed. Get his agreement before you begin any work. Agree on time
intervals at which he will inspect your
progress. Then follow your program.
The "Bible" for the performance of all
aircraft repair work is FAA Publication
AC 43.13-1 A. It contains a wealth of information on all aspects of aircraft rehabilitation and repair. Chapter 1 deals
with wood structures. Our description
given here tells you how we applied the
techniques and methods set forth in
Chapter 1.
Another reliable guide will be the
nearest EAA Chapter. Members of EAA
have earned a wide reputation for being
skilled, dedicated and generous with
their time and counsel. Ask for help.
STEP ONE: Remove the wing and
set it up flat on padded supports in as
roomy and well-lighted workshop space
as you have available. In our region that
means inside a building and in winter,
heated (you will need a modicum of
temperature control in STEP TWELVE).
STEP TWO: Like a surgeon, you will
need to "open up" the patient. If your
SKETCH* A*
OF"
IW
60 MAY 1985
g
SPAR.
wing covering is serviceable, it will not
be necessary to remove and replace it
entirely. Instead, slit the covering on
both upper and lower surfaces so that
it may be laid back to reveal the cracked
spar and to give you just enough room
to perform the work.
STEP THREE: The miracle you are
about to perform is called a Scarf Joint
built inside the wing without removing
the entire spar. (See Sketch A) Scarfing
is a method of removing an unsuitable
portion of a wood beam or spar and replacing it with a sound piece using a
glued joint whose strength is equal to
that of the original spar.
One key to this process is correct location of the joint. So now, with the wing
structure laid bare before you, study the
crack and the spar to locate the best
spot for the repair. This will be determined by the total length of the scarf
joint including reinforcement, or cheek
plates, all of which must fall between
two adjoining wing ribs (this will avoid
having to remodel a wing rib where it
attaches to the spar). So select a rib
bay inboard of the crack which will produce a scarf cut to remove the least
existing spar wood while getting rid of
all the unsound material. To calculate
the length of the scarf joint, move on to
STEP FOUR: Lay out lightly in pencil
the line of the scarf joint on the edge of
the spar, following the guidelines in
Chapter 1 of AC 43.13-1A. In Figure 1.4
we find the critical proportions of the
scarf slope to be: optimum (recommended) — 12 to 1 and minimum 10
to 1. Apply these proportions to your
specific situation. For example, let's use
a spar thickness of 3/4 inch. The optimum scarf length would measure 3/4"
x 12 or 9". The minimum allowed is
3/4" x 10 = 7-1/2" long. Your aim is
to attain the longest joint up to 12 to 1
which your rib bay will permit.
Note that the scarf can slope in either
direction, toward you from left to right,
or away. Which is determined by how
you will operate the hand saw to make
the scarf cut in the existing spar. Make
it easy on yourself, think ahead: how
will I make the cut?
Now calculate the dimensions of the
two reinforcing or cheek plates, one for
each face of the spar. Again, consulting
Fig. 1.4 in AC 43.13-1 A, thickness must
be 1/4 of the spar thickness: in our
example, 3/4" x 1/4 = 3/16" thick.
Width is same as spar. Overlap at centerline of spar is 3/4". Overlap at joint
end must be at least five times spar
thickness, or 5 x 3/4" = 3-3/4" (optimum is 6 x 3/4" = 4-1/2"). So, total
length (acceptable) of each cheek plate
is (a) one-half scarf length, from STEP
FOUR — 3-3/4"; (b) centerline overlap
— 3/4"; (c) end overlap — 3-3/4" = 8-1 /
4" (or 9" optimum).
Total length of the optimum completed scarf joint is (a) cheek overlap x
2 (each end) — 9"; (b) length of scarf
-•
IS'CLBAH.
, — */,i CHEEK. PLATE, a tteoln
\
\
f— etfOi&.-O
\
\
\
-
-•—^
'itCTAI. WlMfi
— - ..- — — |
t »CAB^
_______
_ . . . - . . - _ _. . ...
,^.
/
/
—
RJB
^
MEIV «PAR
PI6CE
^-J
-J-i
^*
^T
TJV
8*'
SKETCH *B'
PLAN OF SCARF JOINT
(12:1) — 9". Total is 18" (a length seldom attainable in a rib bay of a light
aircraft).
The minimum acceptable joint length
is (a) cheek overlap x 2 — 7-1/2"; (b)
length of scarf (10:1) — 7-1/2" =15".
You must find 15" of clear space between ribs but won't need more than
18". Take the longest available space
up to 18". Let's say you have 16" available (see Sketch B). Use it all and your
scarf will produce a slope of 8-1/2" divided by 3/4" = 11-1/3:1, considerably
better than the 10:1 minimum.
To complete the layout, draw the line
of the scarf on the upper edge of the
spar using a sharp-pointed 2 or 3H pencil and a line on each face of the spar
where the scarf line intersects it. Your
sawcut will follow these three lines.
STEP SIX (The Secret): Construct a
simple two-piece, glued, wooden jig to
accurately guide the sawcut along your
scarf line (see Sketch C). The lower
piece of the jig clamps to the near face
of the spar; the upper piece guides the
saw. We made ours of maple and the
saw guide part was 2" thick and as long
as the rib bay to the left of the sawcut
would permit, to give the saw a firm
backing to work against.
Faces must be true and square. Join
the two pieces by using a good shop
glue like Elmer's Carpenter's Wood
Glue.
STEP SEVEN: Clamp the saw jig
onto the spar, on the scarf's inboard
portion, so that the saw guide aligns
perfectly with the scarf line you have
drawn. Position the jig so your sawcut
will barely save your pencil line on the
inboard portion of the scarf. Position the
two clamps so as not to interfere with
strokes of the saw.
We next rubbed powdered graphite
on the saw guide face of the jig to make
the saw slide easily.
STEP EIGHT: Make the sawcut. This
will take concentration to hold the saw
blade firmly against its guide (with
another block of wood held in place by
the finger tips of the left hand) and patience because space limitations shorten your saw strokes and your progress
will thus be slow. The secret of a proper
cut is to diligently keep the flat of the
saw completely and continuously
against the guide face of the jig as you
stroke.
A word on saws: first choice is a
Japanese cabinetmaker's saw (Ryoba
Noko Giri), a fine-toothed, thin-blade
crosscut which cuts on the pull stroke.
This saw, in the hands of a careful workman, will give the truest cut. Second
choice is a regular back-saw, similar to
a hand mitre saw, deep enough to cut
through the spar before bottoming on
the underside of the "back" or thickened
upper edge. The standard hand crosscut or rip saw is not the best tool for this
job. A thin, wide, fine-toothed saw is
needed; and even though the cut to be
made is almost parallel to the grain, a
crosscut works better.
If you use a back-saw note that, as
the sawcut proceeds, the jig can be removed and the depth of the guide block
lowered by ripping off successive layers
1/2" to 3/4" thick from the upper surface
using a table saw; this will enable the
back-saw to cut deeper before bottoming. With no more than about one-fourth
of the cut remaining, the jig can be removed altogether; the sawcut in the
upper 3/4 of the spar will thereafter provide its own guide to the remainder of
the cut. Be careful to follow the two end
guidelines on opposite faces of the
spar.
Any cleanup of the face of the scarf,
to remove ridges and otherwise make
it flat and true, can be done with a
cabinet scraper, a simple and remarkably effective and little used hand tool.
Be careful not to injure the feather end
of the cut. DO NOT SAND any faces to
be mated by gluing.
STEP NINE: Remove the scrap end
of the spar from the wing. All fittings
and their fastenings, including the
wingtip frame, attached to this piece of
the spar must, of course, be removed
first. It can then be eased out of its position in the wing. Save it as a pattern for
the replacement piece.
STEP TEN: Acquire a suitable piece
of spruce or other acceptable wood
specie and construct the replacement
piece. Follow the guidelines of Fig. 1.2,
SPORT AVIATION 61
Ch. 1, AC 43.13-1A for material choice.
Also, the September 1984 issue of
SPORT AVIATION contains an excellent article evaluating alternative wood
species for use in aircraft structural
components. Your local aircraft builders
supply house will be a reliable source
of counsel and materials, or one advertising in SPORT AVIATION. I know of
only one manufacturer today still offering aircraft quality sitka spruce in the
various sizes and any quantities required by homebuilders, name and
address furnished upon request.
Suitable aircraft quality plywood for
the cheek plates can similarly be obtained. Chamfer the ends of each plate
at a 45 degree angle.
Remember, when you lay out the new
spar piece, the cracked piece you removed is the thickness of the sawcut
(kerf) short of the true length needed.
The matching scarf can be produced
on the new piece by use of any of several woodshop machine tools such as
a large-diameter table saw or a
bandsaw. It can also be done by hand,
as was the scarf in the wing. In any case
use an accurate jig and enjoy all the
working space you will have while cutting this half of the scarf joint.
Once cut, the new scarf piece can be
slipped into position in the wing to verify
precise fit to its mating piece. Treat the
feather ends gently.
STEP ELEVEN: Glue the scarf. Use
a glue which conforms to Para. 6 of Ch.
1, or another suitable product (epoxy)
developed since that date of publication
(1972), one which is recommended by
your aircraft building supply house and
approved by your FAA Inspector.
Carefully follow the manufacturer's
directions. Line up the two mating
pieces carefully. Use wood pads under
all clamp faces and wax paper between
pads and work. Wipe away all wet glue
dribbles. Maintain temperatures and
clamp pressures as specified. Keep
glued surfaces meticulously clean.
Don't skimp on the details.
A tip to hold the scarf accurately in
place while setting up the clamps: drive
a steel brad into each end of the scarf
1 -112" in from the feather ends and midspar. Drill guide holes slightly undersize
first. Length of brad is 1/8" less than
thickness of spar; drive brads so that
both ends lie below surfaces of spar
and thus will not interfere with gluing of
cheek plates. Putty the two brad holes
not covered by cheek plates in opposite
faces of spar before varnishing.
Glue the scarf itself first. After it has
set, clean away any hardened glue dribbles and glue the cheek plates (one at
a time is easier than two at once). Clean
up again and lightly sand in preparation
for...
STEP TWELVE: Coat all new and reworked old wood with top-quality spar
varnish. Again, follow manufacturer's
directions. Allow first coat to dry
thoroughly before applying second coat
(Ch. 1, Section 2, AC 43.13-1A).
STEP THIRTEEN: Reassemble and
fasten all interior fittings and elements.
Have work inspected officially, then proceed with recovering in accordance with
Ch. 3 of A/C 43.1 B-1 A. Replace the
wing, restore the aircraft to airworthy
condition and present for final inspection.
This repair, properly done, will serve
as well as a complete new spar and
save you time, money and a great deal
of extra work.
Tom recommends the use of the
Japanese cabinetmaker's saw. His
source of supply is the Japan Woodworker Catalog, 1731 Clement St.,
Alameda, CA 94501, phone 415/5211810.
CLAN/PS
TO GUEAC. SAW STROKES
WOOD PADS UNDER CLAMP
FACES — FROM "SCRAP
•SAW
LIKJE OF SCARF
S KETCH *C"
OF
2.- Pi ECE
WOOD CTlS TO <3«JIDE
(T) CUAMPJKJfi BLOCK
(2)
62 MAY 1985
SAW
(3UIDE &L.OGK