John,
Although I've seen several of the other recent postings on this subject
where the builder did not feel the need to go ahead with this particular
mod, I am one who did, and I still feel good about doing it. And I do
recognize the "just a few grams here, a pound there" warnings, but I
still would do it again, from the pure logic of it. If I felt I would
be doing things like this at every turn it would be different. This one
made sense to me, and added perhaps 140-150 grams. And my main concern
was not torque on the bushings--but getting the bushings fixed in place
with the protective "tunnel" to avoid the inevitable banging of the
internal foam from time to time during rigging.
The bushing tube approach was devised by Nigel Graham (no kin). Nigel
posted it on the board for all last year sometime.
It seems to me that the added "connect" between TP1 and TP2 due to the
tube assembly will be worthwhile. The tubes work like a charm, and the
torque tube slides in straightaway and smoothly, with (1) little chance
of the individual bushings being misaligned or knocked out of kilter,
and (2) practically no chance of damaging the foam that is otherwise
right alongside the torque tube as it is inserted. The entire assembly
seems now to be much more soundly attached than it would have been
otherwise, with not that much weight added.
The tubes went together well, and went in without undue problems
(although it did take a bit of persistent roughing of the inside of the
lightening hole by rotating the tube repeatedly to get the hole sized
exactly right--see below). With regard to the venting, I made the
requisite holes with a hot welding rod through both sides of the
lightening hole, then did the same thing --below the lightening hole--
between the two adjacent holes. The inside of the TP5/6 tube is itself
vented into the TP1 lightening holes which are also cross-vented. So I
think it will be OK.
FYI, I did not feel the need to insert the steel torque tube into the
assembly during the internal rib layup on TP2, as the TP5-6 distance is
fixed nicely already, and although I did drill a 1-3/4" hole in my table
top to accept the short protuding end of the tube, I did not have the
torque tube size etc to contend with. I do suggest you leave a plug in
the end of the TP5-6 tube however to avoid even the chance of getting
epoxy in it as the fiberglass for the rib is lowered over the tube....
My only "difficulty" (and it was not that bad...) was that I probably
did not wrap the fiberglass tightly enough around the prepared/protected
stainless steel torque tube, causing the fiberglass bushing tubes (both
of them) to be just slightly larger than they might have been. That in
turn affected the ease of insertion into the round "lightening" hole in
the foam TP-2. I put a foam plug into each tube's outboard end, then
covered it and held it in place by masking tape. I inserted that end
into the foam hole and twisted radially, slowly at first, back and
forth, using the tube itself as a reaming tool to make the hole
perfectly sized. It resulted in a nice tight fit--I kept reaming until
I had a reasonably easy insertion, since the entire surface of the tube
was to be held in place by flox. Also, that allowed me to mark the
EXACT target location, 8 mm out on one end, 12 mm or so on the other, as
well as a good mark aligned with the hot wire slit above the hole. This
made it easy to replicate exactly where I wanted the tube to end up
during the final insertion and floxing.
When both tubes were in place, and before all began to set up, I
carefully inserted the tailplane torque tube into each of the now-placed
bushing tubes, all in their jigs. I then covered the TP2s with small
sandbags to be sure there would be absolutely no bowing or warping--even
though everything felt very stable without the sandbags. As a result
then I could check and double-check that I had good alignment on
everything--and then I left it alone.
Jim Graham, A101, N4EU
Here are Nigel's original instructions:
Improved TP5 TP6 bush bonding
Here are the details of a mod I carried out to improve the bonding of
TP5 and TP6 into the tailplane core. My original concern was that epoxy
never satisfactorily bonds to metal, particularly stainless steel (TP5
and TP6). This is further exacerbated by the fact that the build
instructions require only half of each bush at best to be bonded to the
rib. I also wanted to guarantee the alignment between the two bushes
without having to rely on a precariously balanced torque tube during the
layup of the rib. It struck me that if either of these bushes broke
away, (as has now happened) the tailplane could flutter in flight, with
disastrous results (this thankfully has not happened yet). My solution
was to encapsulate both TP5 & TP6 bushes into a BID tube, laid up
directly onto the greased TP4 torque tube. When cured, this simply
slides off the tube, and is then bonded into the TP2 foam core, the rest
of the build proceeds as per instructions.
DETAILS
The bushes TP5 and TP6 are inserted into the TP2 foam core to their
correct positions and the distance between the INNER edges measured
(note: since TP5s outer edge is angled, an accurate measurement between
outer edges cannot be guaranteed).
2. PREPARATION
Wrap a few loops of masking tape around the torque tube (TP4), angled
to align with TP5s outer (angled) edge. Grease the torque tube TP4 to
act as release agent and degrease and "rough" the outer surfaces of the
bushes to ensure as good a bond as possible, then slide TP5 onto TP4 to
abut with the masking tape. Accurately cut a strip of newspaper (across
two pages of tabloid) to be exactly the width of the spacing between the
bushes (from 1 above) and carefully wrap it around the TP4 tube,
abutting tightly against the inner edge of TP5 (it will stick to the
grease). Use a spray adhesive (or similar non-bulky adhesive) to secure
the last loop. This is done to provide a clearance between the torque
tube and "BID tube", ensuring loads are taken wholly by the bushes when
the structure is complete. Slide TP6 to abut with the edge of the
newspaper. The grease should hold it place. Add another strip of
newspaper (width not critical, but greater than 100mm) outboard of TP6
and secure. Wrap a couple of tight loops of "Cling Film" (Reynolds
Wrap) over the newspaper and both bushes (this will act as a release
agent). Take any round steel bar and carefully roll it around the edges
of TP5 and TP6. This will cut through the Cling Film, producing a
perfect fit. Discard the excess film covering the bushes. Using light
finger pressure, carefully add a fillet of grease to the edges of both
bushes, being careful not to get grease onto the bonding surface of the
bushes. This is done to ensue that epoxy cannot inadvertently bond the
bushes to the TP4 tube during laminating.
3. LAMINATING
Cut sufficient BID to roll 3 times around the torque tube and bushes,
and wide enough to cover TP5 and extend some 100mm past TP6. This is
done to encapsulate TP6 such that if the metal/glass bond should break,
the bush cannot move out of alignment. The extra length provides a
greater surface area, and hence, load path to/from the tailplane. The
BID is placed onto a sheet of polythene and wet out with resin. Using
the polythene, the BID is transferred to the tube and carefully wrapped
around tightly and the polythene removed. Lastly, a sheet of peel ply
is added to the entire outer surface. When cured, the assembly can be
slid off the torque tube the paper and cling film removed with tweezers,
and the ends trimmed to size. I weighed the finished assembly and found
it to be just 48 grams heavier than the weight of just the bushes.
3. ASSEMBLY
Remove the peel ply. The "bush tube" is assembled into the TP2 foam
core using flox, applied to the entire outer surface of the tube.
(note: if flox is applied to the inner surface of the foam core, it will
all be expelled as the tube is inserted). Allow to cure. From this
point on, the build proceeds as per instructions.
ADVANTAGES
1. When the ribs are laminated onto the "bush tube", an infinitely
better bond is achieved, since there is over double the bond area to
each bush, and the bond is glass/glass, and not glass/stainless steel.
2. When the TP1 core is assembled onto TP2, the extended "BID tube"
provides a spigot onto which TP1 accurately locates. All the builder
need do is rotate the latter to the correct alignment.
3. Rodent proof. If the aircraft is stored on farm land I considered
there would be a risk of mice nesting in the convenient torque tube
hole. (I have subsequently heard that one owner has experienced this
very thing).
CAUTION
Extra thought must be given to providing adequate internal venting since
the tube now fills the cavity in the tail plane core.
This procedure may appear long-winded when written down, in practice it
is straight-forward and took me only one evening per side to produce.
This mod increases the structural integrity out of all proportion to the
little additional weight.
--credit to Nigel Graham
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