 |
Matronics Email Lists
Web Forum Interface to the Matronics Email Lists
|
| View previous topic :: View next topic |
| Author |
Message |
GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
|
 Posted: Fri Apr 14, 2006 9:35 pm Post subject: Cracks in aft fuselage stiffeners |
 |
|
The torque box is made up of the rear bulkhead (where the vertical spar
attaches) and the bulkhead where the forward vertical attachment attaches. This
is also the torque box used by the original Traveler for the attachment of the
forward portion of the horizontal (as well as the forward vertical
attachment).
A couple of years ago, when this whole cracking problem became an issue, I
sent drawings to an engineer to have him come up with a probable cause for the
cracking. It was his opinion that the torque box, still acting as a torque
box, was racking during a sudden input to the rudder (possibly to counter a roll
input, shear, wind gust etc.).
His conclusion was that when the rudder is moved quickly, the vertical moves
(in the opposite direction) and loading the aft bulkhead. Since the rear
horizontal spar is also attached it (the rear bulkhead), it responds to the
rudder input. The inertia in the 12 foot horizontal resists the rudder input
and the forward horizontal spar and it's attachment to the fuselage through the
subject vertical spar support angle and the lower horizontal stiffener respond
slower than the rear bulkhead. Since the torque box is made up of the rear
bulkhead and the forward bulkhead (where the original horizontal attached), it
racks and causes stress risers at the junction (sharp corners) where the
vertical spar support angle and the lower horizontal stiffener are bonded.
He cautioned that once the joint (or the lower horizontal stiffener) cracks,
the load is then transferred through the skin to the forward bulkhead.
Either way, the skin carries a fair portion of the twisting imposed by the rudder.
What you say about the stiffeners being there to prevent the skin from
bucking is true. However, I think the stiffeners also help to act as a frame to
transmit loads forward. Not a well designed frame, but a frame just the same.
If the frame had been completed up to and under the ELT panel assembly,
tying both vertical spar supports together at the top as well, it would have been
a lot stronger torque box. The double I have in mind would help transmit the
twisitng of the vertical spar support angles to the stronger forward
bulkhead.
I've seen a lot of the horizontal stiffeners buckled. I have only seen one
with a crack in it. Do I think it's a big problem? No. Do I think
debonding, removing and then replacing the the same pieces is the right approach?
No. Do I think adding a double that is radiused in the corner and transmits
the load the the entire portion of the tail assembly is a better idea? Well,
yes, that's why I want to do it. If the DER and structural engineer agree,
I'll do it.
Gary
| | - The Matronics TeamGrumman-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?TeamGrumman-List |
|
_________________
Gary
AuCountry Aviation
Home of Team Grumman |
|
| Back to top |
|
 |
gilalex(at)earthlink.net
Guest
|
| Posted: Fri Apr 14, 2006 10:28 pm Post subject: Cracks in aft fuselage stiffeners |
|
|
Example can be seen here...
I believe my buckling is typical and follows what Gary describes.
http://home.earthlink.net/~gilalex/Tiger/
However, my horizontal showed some hangar rash and denting, and the tail
tie down ring was bent back a bit.
Excess loads could have come from other sources than the one Gary mentions...
It was repaired (?) by replacement of the stiffeners per the SAIB, but
since no-one really knows what did it, can it be called fixed?
It will be interesting to see if any of the repaired ones buckle a second
time.....
gil in Tucson
| Quote: | I've seen a lot of the horizontal stiffeners buckled. I have only seen one
with a crack in it. Do I think it's a big problem? No. Do I think
debonding, removing and then replacing the the same pieces is the right
approach?
No. Do I think adding a double that is radiused in the corner and transmits
the load the the entire portion of the tail assembly is a better
idea? Well,
yes, that's why I want to do it. If the DER and structural engineer agree,
I'll do it.
Gary
|
| | - The Matronics TeamGrumman-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?TeamGrumman-List |
|
|
|
| Back to top |
|
|
flyv35b(at)ashcreekwirele
Guest
|
| Posted: Sun Apr 16, 2006 6:58 am Post subject: Cracks in aft fuselage stiffeners |
|
|
I don't really think the buckling is typical. Out of all the planes I have
looked at (maybe 20) I have only seen one that had any buckling and it was
less than what Gil has pictured. It also had some tail damage and was most
likely backed into a hangar wall, etc. This is what I think causes this
particular buckling, i.e., a horizontal load applied to the end or near the
end of the horizontal stabilizer.
If you put your hand inside the fuselage up in the corner near the top of
the vertical angle and the horizontal ELT mounting bulkhead you will feel
quite a bit of flexing and movement when someone pushes a little bit (20 lb.
at the most) on the end of the stabilizer in a horizontal direction.
What I have noticed in numerous planes is a small crack propagating out of
the radius for the small tab that is bent horizontally and riveted to the
ELT bulkhead at the top corner of the vertical angle. I think this crack is
due to the flexing and a stress concentration in this area. I cannot see
any real structural significance to this small crack and I doubt if it will
continue to propagate. If the vertical angle was not attached to the ELT
bulkhead with this small tab I don't think the crack would occur. The
vertical angles are bonded full length to the fuselage side skin and can
transfer vertical loads from the front spar of the stabilizer due normal
vertical loading but are not intended for the flange to be bent fore and
aft from a horizontal load resulting from the stabilizer hitting something
way out at the tip.
The other loading is a torsional load from a large rudder deflection, maybe
at high speed possibly, that is input at the rear bulkhead where the
vertical spar attaches and tends to twist the fuselage. This is possibly
what might cause cracking of the vertical and horizontal angles where they
overlap, although hitting the tail on something might cause this as well.
At any rate these are very thin angles and wouldn't seem to offer much
support for the "box" section compared to a real bulkhead with both OD and
ID flanges like you typically see on fuselage bulkheads.
I don't see that anything is really accomplished by removing the vertical
angles and bonding and riveting on new angles other than to just replace
parts that cannot be straightened. Adding another doubler angle to the
existing one and maybe a formed corner doubler would seem to strengthen the
corners and the box much better and be easier to accomplish. But what do I
know since I'm not an aeronautical or structural engineer (just a BSME and
A&P).
Cliff A&P/IA
---
| | - The Matronics TeamGrumman-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?TeamGrumman-List |
|
|
|
| Back to top |
|
|
GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
|
| Posted: Sun Apr 16, 2006 11:12 am Post subject: Cracks in aft fuselage stiffeners |
|
|
In a message dated 4/16/06 7:58:46 AM, flyv35b(at)ashcreekwireless.com writes:
| Quote: | f the vertical angle was not attached to the ELT
bulkhead with this small tab I don't think the crack would occur.=A0
|
You are absolutely right. The stupid little tab is TOO small. That entire
ELT plate should be attached to the vertical angles. The corners should be
radiused. The DER and I came up with a radius of 3 inches for the corner
doublers.
Gary
| | - The Matronics TeamGrumman-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?TeamGrumman-List |
|
_________________
Gary
AuCountry Aviation
Home of Team Grumman |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
