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Before
starting my spars I have calculated their heights where the inner and
outer spar ribs will be positioned. The spars are angled from
front to back so the height at the back differs from that at the front.
On this forward spar, I have plotted the highest side (aft).
I then rip the timber into shape but do not angle the blade.
The angle at the tip of the spar differs from the middle and
I
have no one to help change the angle as I move the timber through.
After this I machine the spar to 5/8" thickness. |
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This
is the ripped aft spar showing the side that faces forward.
This
is the highest side. The height at the inner rib is 52.8mm
and
35.6mm at the outer. The NACA plans from
Mark Langford's site, give the angles and from them I have calculated
the heights on the opposite sides of all three spars. |
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This is
what I have worked out.
FORWARD
SPAR LENGTH = 90
inches
HS forward spar aft face
height at inner rib = 59mm, outer rib =
32mm
HS forward spar forward face
height: inner rib = 59mm less 3.9o =
56.8mm
:outer
rib = 32mm less 7.8o =
27.6mm
AFT SPAR LENGTH = 90
inches
HS aft spar forward face
height at inner rib = 52.8mm,
outer rib = 35.6mm
HS aft spar aft face
height: inner rib = 52.8mm less 5.1o =
50.0mm
: outer
rib
= 35.6mm less 4.5o = 33.0mm
CENTER SPAR LENGTH = 84
inches.
HS center spar forward face
height at inner rib = 59.5mm,
outer rib = 42.2mm
HS center spar aft face
height: inner rib = 59.5mm less 3.0o =
57.8mm
:
outer
rib = 42.2mm less 2.9o = 40.6mm
I have then
plotted the lower (angled down) height on the reverse side. |
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Using
the hand plane I shave the angle down to the marked line on the lower
side without removing timber from the high side. The best way
to
do this is to use a permanent marker and run it down the high side of
the 5/8" face. When planing I know to not shave below this
marker
line or the one on the lower side. |
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The
ribs have been accurately cut from the plans and stuck to 2.5mm plywood
with spray adhesive. This dries pretty quickly and I then use
a
jigsaw to cut the ribs, leaving the bulging supports. To
remove
the spar "cutouts", I have pre-drilled some holes so I can get the
jigsaw started. |
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This
is the result after accurately jigsawing and some sand papering.
Taking time with the jigsaw and using a steady hand reduces
the
amount of sanding. |
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The basic
skeleton of my horizontal stabilizer is now finished. |
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I
received the hinge components from AS&S and had to
assemble
and marvel at how smooth they were in some scrap timber. The
hinges are the "Dr. Dean" type which can be seen on Mark Langford's
site here. It seems most
builders agree these are a more workable alternative than the Rand
Robinson examples. |
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In
order to line them up perfectly, the spars were clamped together but
the center spar was moved along 5.5mm from its center position.
This is because the holes for the eye bolt and rod end
bearing
are offset as you can see in the photo above. The two spars
are
then pre-drilled all the way through with a 2mm bit to act as a pilot
hole for the 3/16" hole that follows. |
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Everything
lines up perfectly and the hinges move freely with no binding which may
well have been an issue if I didn't clamp the spars and make sure the
holes were exactly centered length ways down the elevator. I
didn't finish off the hinges with the nylon locknuts as I need to
disassemble it now and add 1/4" plywood stiffeners and put the ribs
back on. I wanted to be sure the holes were spot on......and
they
are. |
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At
the tips of the elevator I have built in allowances for the balance
horns. Extra ribs were made and epoxied into position.
The
tip of the horn will sit behind the leading edge spar. While
the
epoxy dried, I used scrap plywood to give a 2.5mm gap all around.
The inner two trailing and leaded edge rib tips will be
removed
prior to gluing in the foam. |
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I
looked around and believe it or not I don't have a hacksaw blade to
removed the majority section of the humps. So I used my
electric
plane and then sanded the last of the excess plywood to leave a perfect
airfoil. |
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After
cutting out the ribs I realized that the airfoil templates made no
allowance for the 1/4" plywood stiffener that is added to the center
spar. A quick rework with the jigsaw and they were
ready.
As the inner three hinge rod end bearings secure through the
stiffener, the bolts need to be a little longer. AN3-13A
bolts do
the job here. |
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I
did some testing gluing urethane foam together. Initially I
used
expanding foam. No matter how thinly I smeared it or the
amount
of pressure I applied to the foam, the expanding foam did just
that...expanded. The spray adhesive I used to glue the paper
templates to the plywood is also a urethane glue according to the can.
Months ago I used it on some scrap foam just for fun.
When
I went back to the foam it was stuck together very well. I
carved
and sanded it through the glue layer and it was like sanding one solid
piece. No hard glue ridge to get in the way. So I
have
decided to use it in preference to the expanding foam. |
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After
removing all the humps I used the jigsaw to cut the ribs that needed
removing from the leading and trailing edges. I also cut the
balance horns free of the leading edge spar. The elevator
still
moves very smoothly. |
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Foam
has been cut and placed in position ready for the expanding foam to
adhere it to the spars and ribs. The lower right corner of
the
picture shows the 60 grit sandpaper I stapled to the bench.
It is
perfect for sanding straight edges on big pieces of foam. |
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Once
the big sheets of urethane foam had dried after using the spray
adhesive, I was able to prise them apart so I had to rethink the
process. What I came up with was to use the expanding foam
and
then clamp the sheets in a makeshift press. It worked a treat. |
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I
was not happy with the lack of control over the expanding foam when
applying it to the precut urethane sheets and then sliding them into
position against the spars and ribs. Because the sheets are a
tight fit, the expanding foam get scraped and leaves a layer on the
timber edges. What I did was use a wood rasp and carve a
shallow
channel into the edges to be glued. I then mix the part A and
B
foam and place it in the channel. The urethane foam gets
placed
in position before any serious expanding takes place and when it does,
it is far less likely to ooze beyond the timber edge. |
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To hold the
urethane in place I have just used some timber straight edges and
clamps to keep it from expanding off the timber. |
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It
takes a little time to sand the foam down. The "sanding
stick"
has to rest on the ribs at all times to avoid sanding below the airfoil
shape. |
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I
ran into a problem on one of the leading edges. The expanding
foam used to join the urethane became exposed when sanding and as we
know, it is harder than the surrounding foam. It's quite a
large
seam and I have concerns about its stability when exposed to heat so I
have decided to remove the leading edge and do it again. |
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The good
leading edge came up a treat. |
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The
elevator including the balance horns are finished and ready for the
carbon fiber cloth. I must remember to balance the elevator
first. |
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So
I could work on the stabilizer without having to move it, I built a
very simple rack which has some checks removed for the spars and is
screwed into the overhanging loft. I am using the plastic
sheet
layup method for fiber glassing. The plastic hung over the
stabilizer and marked with a texta 1" outside the desired line. |
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The
bottom layer of light weight fiberglass cloth is cut to the marked line
on the plastic as is the top layer of carbon fiber. The
carbon fiber I am using a 5.8oz flat tow weave. After
thoroughly wetting out the cloths with resin on the plastic using a
squeegee, I lift the whole lot and place it in position on the
stabilizer which has been already been coated with a slurry of 50%
microspheres, which are a dark purple color, and 50% resin.
Once
again squeegeeing the layers through the plastic brings them into close
contact with the foam. |
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When
satisfied that the layers were well contacted with the foam, I slowly
peeled the plastic off and then squeegeed again. The layers
were
then covered with peel ply which will be removed when the resin is dry,
hopefully leaving a pinhole free surface. |
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Look
at the mess on this side. The micro-slurry in my mixing
container
started to really heat up after about 1 hour , at which stage it began
to thicken too. I thought I had enough time to
finish the
foam coating before the slurry became unworkable. WRONG!
It
went from the consistency of paint to that of cheese in less than 2
minutes, in the process tearing at the foam and then leaving solid
blobs everywhere. My only option now is to wait for the other
side to cure and then remove the foam from this side down to half
thickness. I'll then replace it and finish off the carbon
fiber
layup. Right now I have to head back out to the garage and
trim
the edges with a razor. Its been about 3 hours. A
lesson
learned. |
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This
is the same side just swapped around in the rack so I could do the
carbon fiber layup on the other stabilizer. I dug out all
that
hard resin and removed enough urethane to accept a fresh 1"
thick piece. |
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The
purple microsphere slurry was applied to the elevator and then resin
was squeegeed into the plastic/fiberglass/carbon fiber layup.
This was placed on the elevator and squeegeed to the foam
before
removing the plastic. A further squeegee was done to the
fiberglass surface and then the peel ply applied. That was
until
an hour later when I was inside and realized that I had mixed the
hardener-resin at 5% instead of 5:1. I had added 5ml of
hardener
to 100ml of resin instead of 20ml. Needless to say I went
straight back outside and peeled off the layup and redid the whole
thing. 24 hours later the carbon fiber is still a sodden mess
on
the garage floor. |
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The
stabilizer is finished but for the seals which will cover the
gap at the hinge area. |
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The gap
seal profiles have been stuck to 3/8" plywood and carefully cut out
with the jigsaw. |
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I
have cut a length of 2" thick foam and pressed it onto the elevator eye
bolts which then mark where I need to remove some foam so they are
exposed for connecting to the rodend bearings down the track. |
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Using
a round metal file, I have bored through the foam to the thickness of
the washers. I have created "pockets" for the semicircle gap
seal
profiles by pressing them straight into the foam and then removing
them. The "pockets" will be filled with expanding foam before
I
put the profiles in permanently. |
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The
foam is ready to be bonded to the spar. All the pockets are
cut,
holes bored and my rasp channels have been made along the length to
avoid that expanding foam overflow ooze. |
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Expanding
foam is doing its job of sticking the urethane to the spar.
When
set I will sand down and around the profiles creating that perfect
elevator gap seal. |
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After using
the wood rasp to shave off the bulk of the foam, I
continued sanding with the straight edge down to the plywood profiles
as well as sanding the reverse side of the elevator. I have
intentionally left out the two triangular pieces that define the rudder
arc from the center of the elevator. I will wait until the
rudder
is in position and then shape them perfectly for minimal clearance and
reduced drag. |
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I
have removed 1" of foam from the elevator trailing edge using a
straight edge as a guide. This will be filled with flox to
create
a rock solid trailing edge before covering with the carbon fiber. |
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The
stabilizer needs its gap seals too. I have glued a length of
foam in place and held with clamps over night. |
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The alloy
straight edge is clamped in place and I use the carving knife to remove
the bulk of foam. |
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Having
done that, I grabbed the roll of tape to use as the sanding block.
Puts a perfect curve on the gap seal. Doing it
seemed a
better option than gluing tiny guide blocks into place. |
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Here's
the top gap seal finished. The foam angle becomes more acute
towards the center of the stabilizer as per the guide blocks on the
plans. |
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I
want to have a way to inspect the hinges on my pre-flight walk around,
so cut out inspection holes which are wide enough to insert and remove
the AN3-7 bolts. I am still undecided whether to cover the
holes
with clear plastic, like from a coke bottle, or to cover with them with
carbon fiber which I can paint and hinge with tiny hinges which I can
pick up from a modeling shop. |
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I
have decided to experiment with making the elevator tab control horn
out of carbon fiber instead of aluminum. I have cut the rough
shape out of some scrap foam. |
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Using
the pocket knife and very gentle work with sand paper, the control horn
was crafted to the required 3/4" height. Having read Kent
Paser's
book "Speed with Economy", drag reduction has been foremost on my mind
when building. Accordingly, the control horn has been formed
to
be tear dropped shaped and is 3.7 times as long as it is wide which is
the accepted ratio when fairing in something which protrudes into the
air stream. |
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So
here it is. I know it is only 3/4" tall and the amount of
drag
reduction will be absolutely minimal but hey, I am having fun. |
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I
have used two layers of carbon fiber and one layer of light weight
fiberglass cloth. The press has been set up to mimic the
position
the control horn will be in when in place on the tab. The
flat
piece of cured carbon fiber has had a slit cut into it with the Dremel.
The layers covering the horn pass through that slit and
spread
out to the sides. The horn and flats which will be under the
tab
skin have three layers of peel ply to give a nice finish and soak up
excess resin. |
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Well here
it is finished. All .92 grams! A hole will be
drilled for a brass sleeve which will be floxed in. |
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In
order to carve out the holes in the stabilizer that the elevator
bearings will poke through, I used a router bit in the drill press.
|
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When
I am done with all the bearing holes, I'll bolt the stabilizer and
elevator together and check everything for alignment before lining out
the inspection holes with flox. |
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Everything
lines up beautifully. After
cutting away the excess over hanging carbon fiber with the jigsaw, I
sanded the elevator trailing edge to make it perfectly straight.
The job was made easy by using the length of stapled down
sand
paper. The trailing edge is about 2mm thick and hardened by
the
underlaying flox. |
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This
is my third attempt at getting the gap seals right. The first
attempt left too much of a gap even when the elevator was fully
deflected. The second attempt was not much better.
For the
third try, I floxed the foam onto the stabilizer and gradually carved
out the middle until I was able to bolt on the elevator. I
then
slipped strips of sand paper between the foam and elevator and
proceeded to saw it back and forth from both sides, gradually moving
the elevator towards its 30o
up and 20o
down limits. When I sanded the surface of the gap seals, the
tips
of the foam became so thin that they crumbed away. I had
spent
around 15 hours trying to get these gap seals right. Sanding
away
the flox from the timber every time I decided to start again was a
killer. |
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I
cruise around the web checking out as many homebuilt planes as
possible, not just KRs but all kinds and I came upon a guy who is
building an F1 Rocket and saw the way his gap seals were made.
Still not happy with the way my gap seals were progressing
I have decided to make mine along similar lines which in fact is also
similar to the Mylar aileron gap seals on Troy Petteway's KR2.
The new gap seals are made out of one layer of carbon fiber
and
one of
the light weight fiberglass. This was sandwiched between
plastic
and then put into a timber press to cure over night. |
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They
have been epoxied to the edge of the stabilizer and at this stage will
extend all the way over the leading edge of the elevator.
They
are pliable enough to flex when the elevator moves yet solid enough to
not flap in flight but only flying will tell. If all is
successful, I will have eliminated any gap all together. |
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I
think the gap seals came up pretty well. They will need to be
trimmed yet but I couldn't wait to see how they looked with the
elevator attached.
So here it is down. |
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.......and
up. |
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Sanding
the stabilizer shows any low spots that need filling. Low
spots
show up as they are darker than the surrounding high areas that have
had the top layer of resin sanded. As per good advice, the
spar
was sanded just a little lower before applying the foam. This
leaves a slight valley that is easy to fill after the carbon fiber and
resin has set. It is too easy to have a slightly high spar
after
sanding the foam. You can't sand the carbon fiber over a high
spar as you will cut straight through it. So the only thing
left
to do is then add filler to either side to smooth the airfoil shape.
This adds unnecessary weight but more importantly, changes
the
airfoil shape ever so slightly. |
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I
have used a stiff micro, that is resin and hardener mixed and then
microspheres added until the mixture has the consistency of stiff
whipped
cream. The micro is used here as a light filler.
The
closest stabilizer has already had one coat sanded and a second touch
up application has just been applied. The stiff micro is very
light and easy to sand. This stage makes all the hard work
look
"shabby". That is until the painting has finished.
Creating
a perfectly smooth surface now is going to pay dividends at that time. |
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Some
time has passed since the previous photo was taken and in that time I
have noticed that the gap seals had an ever so slight concave arch so I
decided they had to come off and be done again. |
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To
avoid any convex or concave shape to the seals, I decided to fill the
gap with foam and shape it to the exact contour I wanted.
Using
scrap foam strips, I just slid them along the gaps which sanded the
base to a snug fit. Small blobs of expanding foam were then
used
to temporarily hold them in place. |
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The foam
was then very carefully sanded to give a perfectly continuous shape
between the horizontal stabilizer and the elevator. |
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I
then place a length of "Glad Wrap" on top of the foam and then a layer
of CF and peel ply, all of which is pulled taught at either end by the
clamps. |
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The
"Glad Wrap" and peel ply were removed as well as the temporary urethane
foam. The new gap seal is held in place with the
resin/hardener
mix and then trimmed so that it just touches the elevator when it is
deflected up 30o.
I
have done away with the idea of bridging the entire gap for the moment
as I have had concerns about the paint rubbing off. When it
painted of course. |
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As the rudder
is now sufficiently complete, I am able finish off the inner elevator
angles. As with all my foam I cut channels with a wood rasp
into
which the expanding foam can expand instead of forcing the urethane out
of position. |
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The
inner "wedges" are stuck in place and have been sanded even with the
upper and lower surfaces of the elevator. Prior to this I
held
them in place and moved the elevator full up and down, sanding off the
inside at the same time to allow freedom of movement where the wedges
near the fuselage. This inside edge has already had the
carbon
fiber applied. |
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The
rudder is then hung on its hinges and held firmly against the foam.
A few pulls of the sand paper and the foam is perfectly
shaped to
the contour of the rudder. |
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The
upper and lower surfaces were layed up with carbon fiber, allowed
to dry and be sanded before finishing off the thin inner
edges.
Flox was used where the CF meets CF at right angles.
This
makes the corners rock hard. |
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This is the
match I was after when the rudder is at full deflection. |
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The
4130N tube connects the elevator horn to the bellcrank at the forward
spar. The tube has been tapped at both ends and MW-3 rod end
bearings screwed in. |
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The
bellcrank and hinge have been made from 0,125" 6061-T6 aluminum
extrusions. The lower point on the bellcrank will connect
directly to the rear spar bellcrank via the 4130N tube and rod end
bearings. |
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At
the rear spar I have just placed the bell crank into position to check
for free movement. Since taking this photo I have decided to
use
a 5/8" 6061-T6 aluminum push rod tube from here to the rear bell
crank.
The 4130N tube would flex too much over the 7 feet distance. |
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The
5/8" push rod tube has a wall thickness of 0.049" which is the
correct match for the threaded end fitting. I have used 3
rivets
to hold it in place and the MM-4 rodend bearing with lock nut is
screwed on. |
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As
this rod end bearing takes an AN4-7A bolt, I had to drill 3/16" hole
out to 1/4" on the bell crank which left just enough room between the
bearings. |
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The does
away with the cables that would normally run between aft spar bell
crank and the one at the rear. |
