the problem is. Sounds like the roof bracket provides
some of the strength to keep the top flanges from
twisting/buckling, so no cap required there. And the
bottom channels would be prevented from compressing by
the cap across the 2 channels. Would .020 be OK instead
of .032? That's what MAM's instructions had in the file
Roger sent me.
As it happens, I also found a longer piece of ST31 that
would fit here, so I might go your route instead of
putting an ST28 inside the existing ST31 as I mentioned
in a previous post.
Walter
-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Alan Hepburn
Sent: Monday, October 13, 2003 7:54 AM
To: murphy-rebel@dcsol.com
Subject: That darned aileron hanger
Gentlemen:
I've been thinking about that aileron hanger
area, and believe I have
gotten to the bottom of the design weakness
(by the way, I am an
engineer, though, electrical, not
structural). When a gust hits a
deflected aileron from behind, a torque is
applied to the torque tube.
If the control column is already against its
stop, the result of this
torque will be to either attempt to pull the
hanger out of the roof or
drive it up into the roof. It turns out the
latter is more of a
problem, as we'll see.
An upward force on the hanger will put the
top flange of the ST-31
channel in tension, and the bottom flange in
compression. Since the
flange is much weaker in compression, the
lower flange will tend to fail
by buckling, and it will tend to buckle away
from the top of the "U" in
the channel. With the current design, all
you've got to prevent this
buckling is that skinny little 5/8" x .020"
bottom flange on the ST-31,
since the hanger is pretty well free to
pivot about the roof bracket.
How to fix it? Well, you can see that on
the Elite, with no falpperons,
you've pretty well cut the forces by 50%
right there, so that may be why
Sean has never seen this problem on the
Elite. Next move, as some have
suggested, is to use thicker material in the
channel. This will improve
the situation, but adding a second channel
back to back with the first
one will work better (since the flange of
one of the channels will
always tend to get twisted in its stronger
direction. Tying the two
flanges together with a 0.032" cap will
stiffen things up immensely. It
more than doubles the thickness of the
flange, and provides a wider
member to resist the sideways buckling
force. If we make it maybe 2
1/2" wide that also will provide additional
resistence to buckling.
So, I'm proposing using two ST-31 channels
back to back (the extra
strength in the vertical channel web gained
by going to thicker material
isn't really required), and put a 2 1/2" x
0.032" cap across the lower
flanges of both, tied in to the flanges of
the roof bulkheads fore and
aft. The whole thing will wind up looking a
lot like the prototype
photograph, but with a rather wider cap
strip. My earlier idea of using
a piece of FUS-65 to tie the entire length
of the upper flange in to the
roof will also help, but not a lot since the
roof bracket already
provides a lot of resistence to buckling for
the top web of the ST-31
(by constraining its sideways movement).
Now, when you fix the weak link in a chain,
you have to be sure you
don't just move the problem somewhere else.
By stiffening the channel
section, the upward force on the hanger will
be transferred to to roof
bulkheads fore and aft. Since the hanger is
closest to bulkhead 9A,
that one will see most of the force.
Fortunately, it is extremely
strong already (because it has a 0.032"
cap), so there should be no
problem. Of course, tie the ST-31s to it
with clips to do a workmanlike
job.
Comments?
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