> Hi Again,
> I am about to start the dreaded task of lift pins and wing incident setup.
I
> have two concerns. The first; how can I be absolutely sure I'm drilling the
> holes for the pins straight? I know what the book shows but has anyone
found
> a more precise way than the MK-I calibrated eyeball?
(snip)
Troy Maynor-120
Hi, Troy --
Not to dread; it's not bad at all, but yes the manual method is potentially
error-prone. Here's the method I used:
1. Set up wing so that the reference line between the spar pin bushes is
exactly level itself (lay a straight edge between spar pins to level).
2. If you have a drill with a bubble level, check it! They're only usually
accurate. If you don't have one, hot glue a circular bubble level or little
line level to the top of the drill. Level it with a real fat bit in the
drill, clamped level in a vise, and remember to hold the drill up a bit at
the back to adjust out bearing slop.
3. Instead of using tape to set the line parallel to the spar, tape a 1" wide
yardstick (ruler when doing the front-incidence block etc. is in the way),
parallel to the spar, and extend it out over the inboard edge so it will
cover about 2/3 of the drill (but not obscure the level). It may help to
drill some holes on the centerline of the yardstick, to sight down through to
a probable seam on top of the drill.
4. When drilling the holes, select intermediate size bits that match steel
rods you have or will buy. Cut to about 12" long. Use these to check level
and square with each size hole, since you can adjust a bit in drilling the
next size hole.
5. To check parallel to the spar, tape another yardstick to the top of the
spar. Measure the distance between the yardstick and the steel rod (can't
measure to the spar; the bid layers have tapered it).
6. You can still tap a cock-eyed thread in a perfectly aligned hole, so use a
bubble level on the tap to check level and keep sighting that's it's parallel
to the spar.
7. If you have a little bit of play in the threads when done (vacuum out the
crud), take advantage of it with a 12" long brass or ali tube from a hobby
shop that's a tight fit over the pin. When bonding in the pins, set the
final level and square as best you can and prop up or weight the rod while
curing. Easy does it though; these tubes bend easy.
8. There's a little bit of slop in the pins when they're in the blocks, so
when bonding them to the fuselage, use the really gooey wheel bearing grease
to help center them within the slop range.
On this topic, I'll pass along a method of checking that the incidence in
both wings matches, when bonding the wing pin blocks. Set up a wing to be
level at the incidence block. Take a piece of foam, 2" wide x 12" or so long,
and rough cut it to fit on the leading edge, with the foam wrapping around an
inch around to the bottom. Put release film on the wing; slop lots of Bondo
on the foam and place it on the film. After cure, grind the top of the foam
to be approximately level, and Bondo a perfectly flat strip of metal to be
exactly level. Make one for the inboard edge; one at 70% of span. Use these
to compare the level of each wing after you set the level using the incidence
blocks, as you may find that they aren't precisely the same. Halve out any
small differences, and I suggest giving preference at 70% of span, since
that's supposed to be the region of max lift pressure in the slow speed
regime approaching stall (on a tapered wing with washout, or so I read).
Note also that this method presumes the cores were set up accurately, since
the incidence blocks were set up using a much longer chord reference, not
just the leading edge.
(good luck on your other problem)
Regards,
Fred Fillinger, A063
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