Stressed Skin Repair Question

[RPM the A&P]

Hi,



I'm looking for input on the best way to layout a repair of some "hangar
rash" on the tail boom of my Taylor Coot amphibian project.



The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
the tail boom on this aircraft. The part was dropped and sustained a crease
roughly perpendicular (but not quite) to the length of the tube.



I have removed the displaced material and now have an opening 4.1in long and
1.1in wide.



Since I'll be joining two sheets of .065, I've selected -6 rivets. I also
realize that I'll need to slip roll the patch.



I'm now trying to apply the guidance for stressed skin repairs found in AC
4313-1B around paragraph 4-58 figure 4-16 and tables 4-9 through 4-11 (note
that there is apparently no table for 6061 T6).



Here is a link to an image showing the current state to the repair:



http://rilinterface.com/Cootnest/Images/Building/TailTube/DSCN0997.jpg



I'd love to get folks' input on how best to lay out the repair for this.



Thanks!



Russ

http://rilinterface.com/cootnest

[John Kimmel]

RPM the A&P wrote:
> Hi,
>
>
>
> I'm looking for input on the best way to layout a repair of some "hangar
> rash" on the tail boom of my Taylor Coot amphibian project.
>
>
>
> The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
> the tail boom on this aircraft. The part was dropped and sustained a crease
> roughly perpendicular (but not quite) to the length of the tube.
>
>
>
> I have removed the displaced material and now have an opening 4.1in long and
> 1.1in wide.
>
>
>
> Since I'll be joining two sheets of .065, I've selected -6 rivets. I also
> realize that I'll need to slip roll the patch.
>
>
>
> I'm now trying to apply the guidance for stressed skin repairs found in AC
> 4313-1B around paragraph 4-58 figure 4-16 and tables 4-9 through 4-11 (note
> that there is apparently no table for 6061 T6).
>
>
>
> Here is a link to an image showing the current state to the repair:
>
>
>
> http://rilinterface.com/Cootnest/Images/Building/TailTube/DSCN0997.jpg
>
>
>
> I'd love to get folks' input on how best to lay out the repair for this.
>
>
>
> Thanks!
>
>
>
> Russ
>
> http://rilinterface.com/cootnest
>
>
I recommend using 2024-T3 for the repair doubler, putting it inside, and putting a filler on the outside. This is not because the 2024
is stronger, but because it will be much easier to buck large, hard rivets against a harder material than 6061. The heads go against
the 6061 outer skin, you'll need a 5x rivet gun and a really heavy bucking bar. You can also use fresh icebox rivets (-D, -DD) or heat
treat AD rivets, (if you're daring enough). The most important thing is to thoroughly prime and seal the skin and repair doublers
before riveting them together. Rivet spacing in 43-13 seems really close to me, compared with actual rivet spacing in aircraft or in
aircraft structural manuals. I'm too lazy myself, but MIL-HDBK-5 will give you FAA acceptable numbers for calculating the number and
size of rivets needed for the repair.

Your photo really sucks, by the way. Without knowing the location of the damage on the aircraft, there's no way to determine the load
path. With digital cameras and unlimited web space, there's no reason not to upload a couple hundred photos of the aircraft to a
Google photo album.

--
John Kimmel


I think it will be quiet around here now. So long.

[Stealth Pilot[_2_]]

On Sun, 13 Jul 2008 17:21:39 -0700, "RPM the A&P"
> wrote:

>Hi,
>
>
>
>I'm looking for input on the best way to layout a repair of some "hangar
>rash" on the tail boom of my Taylor Coot amphibian project.
>
>
>
>The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
>the tail boom on this aircraft. The part was dropped and sustained a crease
>roughly perpendicular (but not quite) to the length of the tube.
>
>

If it were my money on the aircraftand if I owned it.
I'd throw out the tube and replace it.

2 reasons.
the rivet holes will reduce the strength of the tube no matter how you
lay out the repair.

aluminium and particularly 6061 is subject to fatigue.
you cant ever get away from this because it is an inherent
characteristic of aloominum.
these tubes are not the optimum shape structurally and have stresses
that arise from poor resolution of the loads on them.

I WOULD REPLACE THE TUBE.

ymmv
Stealth Pilot

[RPM the A&P]

Thanks for the tips, John!
I'll certainly take a look at that mil-spec

I'd like to offer one point of feedback. The tone of your comment, "Your
photo really sucks, by the way," is outside of what I would consider
productive.

The photo was offered merely to show the relationship of the damaged area
relative to axis of the part. To clarify, I'm looking for help interpreting
the AC 4313 guidelines for patch size, rivet count, and rivet spacing. That
said, if a qualified aerospace engineer steps forward to offer assistance
with a deep analysis of this tailboom component and its repair options, and
he or she would find it helpful, I can certainly provide numerous high
resolution images from my self-hosted IIS box.

- Russ

"John Kimmel" > wrote in message
...
> RPM the A&P wrote:
>> Hi,
>>
>>
>>
>> I'm looking for input on the best way to layout a repair of some "hangar
>> rash" on the tail boom of my Taylor Coot amphibian project.
>>
>>
>>
>> The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
>> the tail boom on this aircraft. The part was dropped and sustained a
>> crease roughly perpendicular (but not quite) to the length of the tube.
>>
>>
>>
>> I have removed the displaced material and now have an opening 4.1in long
>> and 1.1in wide.
>>
>>
>>
>> Since I'll be joining two sheets of .065, I've selected -6 rivets. I
>> also realize that I'll need to slip roll the patch.
>>
>>
>>
>> I'm now trying to apply the guidance for stressed skin repairs found in
>> AC 4313-1B around paragraph 4-58 figure 4-16 and tables 4-9 through 4-11
>> (note that there is apparently no table for 6061 T6).
>>
>>
>>
>> Here is a link to an image showing the current state of the repair:
>>
>>
>>
>> http://rilinterface.com/Cootnest/Images/Building/TailTube/DSCN0997.jpg
>>
>>
>>
>> I'd love to get folks' input on how best to lay out the repair for this.
>>
>>
>>
>> Thanks!
>>
>>
>>
>> Russ
>>
>> http://rilinterface.com/cootnest
>>
>>
> I recommend using 2024-T3 for the repair doubler, putting it inside, and
> putting a filler on the outside. This is not because the 2024 is
> stronger, but because it will be much easier to buck large, hard rivets
> against a harder material than 6061. The heads go against the 6061 outer
> skin, you'll need a 5x rivet gun and a really heavy bucking bar. You can
> also use fresh icebox rivets (-D, -DD) or heat treat AD rivets, (if you're
> daring enough). The most important thing is to thoroughly prime and seal
> the skin and repair doublers before riveting them together. Rivet spacing
> in 43-13 seems really close to me, compared with actual rivet spacing in
> aircraft or in aircraft structural manuals. I'm too lazy myself, but
> MIL-HDBK-5 will give you FAA acceptable numbers for calculating the number
> and size of rivets needed for the repair.
>
> Your photo really sucks, by the way. Without knowing the location of the
> damage on the aircraft, there's no way to determine the load path. With
> digital cameras and unlimited web space, there's no reason not to upload a
> couple hundred photos of the aircraft to a Google photo album.
>
> --
> John Kimmel
>
>
> I think it will be quiet around here now. So long.

[RPM the A&P]

Thank you for your recommendations.

Agreed. I would love to replace the boom and I have tried to do so for some
months now. Regrettably, they no longer appear to be in production. FWIW,
the original listed in the Coot bill of materials was a
Reynolds-manufactured irrigation pipe.

A number of aluminum distributers have informed me that there are no drawn
tubes in this diameter and wall thickness, or anything close to it,
currently available from any manufacturing source.

Additionally, the rolled and welded tubes that I have located are
unacceptable due fabrication quality, alloy, or both. In fact, I purchased
one tube sight unseen from a former Coot builder, but it turned up with such
extreme tooling (roller) and welding marks that I have reassigned it to
become a pair of reflecting telescope bodies and nothing more. Fortunately,
it only cost me $20. :-)

Finally, after contacting all the folks that I have been able to find in
North America who have been know to have spare Coot components, I've still
failed to turn up an acceptable replacement.

Fortunately, although this boom will be enclosed in a tail fairing, the
repaired area can be made highly inspectable.
For now, I am still planning to pursue a repair.

However, based on your recommendation, let me expand the scope of this
request...

If anyone on the group is aware of a source of drawn tubing that's 116 inch
long, 10in OD, ~.063in wall thickness, and that's made out of 6061-T6,
6063-T3, or 5051-T3, I'd love to hear about it.

Thanks!

Russ


"Stealth Pilot" > wrote in message
...
> On Sun, 13 Jul 2008 17:21:39 -0700, "RPM the A&P"
> > wrote:
>
>>Hi,
>>
>>
>>
>>I'm looking for input on the best way to layout a repair of some "hangar
>>rash" on the tail boom of my Taylor Coot amphibian project.
>>
>>
>>
>>The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
>>the tail boom on this aircraft. The part was dropped and sustained a
>>crease
>>roughly perpendicular (but not quite) to the length of the tube.
>>
>>
>
> If it were my money on the aircraftand if I owned it.
> I'd throw out the tube and replace it.
>
> 2 reasons.
> the rivet holes will reduce the strength of the tube no matter how you
> lay out the repair.
>
> aluminium and particularly 6061 is subject to fatigue.
> you cant ever get away from this because it is an inherent
> characteristic of aloominum.
> these tubes are not the optimum shape structurally and have stresses
> that arise from poor resolution of the loads on them.
>
> I WOULD REPLACE THE TUBE.
>
> ymmv
> Stealth Pilot

[cavelamb himself[_4_]]

RPM the A&P wrote:

> Thanks for the tips, John!
> I'll certainly take a look at that mil-spec
>
> I'd like to offer one point of feedback. The tone of your comment, "Your
> photo really sucks, by the way," is outside of what I would consider
> productive.
>
> The photo was offered merely to show the relationship of the damaged area
> relative to axis of the part. To clarify, I'm looking for help interpreting
> the AC 4313 guidelines for patch size, rivet count, and rivet spacing. That
> said, if a qualified aerospace engineer steps forward to offer assistance
> with a deep analysis of this tailboom component and its repair options, and
> he or she would find it helpful, I can certainly provide numerous high
> resolution images from my self-hosted IIS box.
>
> - Russ
>

He's just like that most of the time, Russ.
But while crass, the comment was correct.

The thing I'd worry about on a Coot tailboom is WHERE that cutout was
located. Not knowing where the damage is, it's hard to make a valid
sugestion.

Way aft is probably not too bad, but up front? I'd probably agree about
replacing it.


FWIW

Richard

[cavelamb himself[_4_]]

RPM the A&P wrote:

> Thank you for your recommendations.
>
> Agreed. I would love to replace the boom and I have tried to do so for some
> months now. Regrettably, they no longer appear to be in production. FWIW,
> the original listed in the Coot bill of materials was a
> Reynolds-manufactured irrigation pipe.
>
> A number of aluminum distributers have informed me that there are no drawn
> tubes in this diameter and wall thickness, or anything close to it,
> currently available from any manufacturing source.
>
> Additionally, the rolled and welded tubes that I have located are
> unacceptable due fabrication quality, alloy, or both. In fact, I purchased
> one tube sight unseen from a former Coot builder, but it turned up with such
> extreme tooling (roller) and welding marks that I have reassigned it to
> become a pair of reflecting telescope bodies and nothing more. Fortunately,
> it only cost me $20. :-)
>
> Finally, after contacting all the folks that I have been able to find in
> North America who have been know to have spare Coot components, I've still
> failed to turn up an acceptable replacement.
>
> Fortunately, although this boom will be enclosed in a tail fairing, the
> repaired area can be made highly inspectable.
> For now, I am still planning to pursue a repair.
>
> However, based on your recommendation, let me expand the scope of this
> request...
>
> If anyone on the group is aware of a source of drawn tubing that's 116 inch
> long, 10in OD, ~.063in wall thickness, and that's made out of 6061-T6,
> 6063-T3, or 5051-T3, I'd love to hear about it.
>
> Thanks!
>
> Russ
>
>

It might be more productive to ask the composite guys about how to make
a glass copy...

[Morgans[_2_]]

"cavelamb himself" > wrote

> It might be more productive to ask the composite guys about how to make a
> glass copy...

Good point. It would be possible (if this suggestion would fit into the
design) to make the replacement tapered towards the end, which would
probably allow the weight of the glass replacement not exceed the original
tube's weight.
--
Jim in NC

[[email protected]]

On Jul 13, 5:21*pm, "RPM the A&P" > wrote:

>
> The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
> the tail boom on this aircraft. *The part was dropped and sustained a crease
> roughly perpendicular (but not quite) to the length of the tube.
>
> I have removed the displaced material and now have an opening 4.1in long and
> 1.1in wide.
--------------------------------------------------------------------------------

I'd go with the patch. But I'd like to know more about that portion
of the structure, especially with regard to the condition of the
surrounding area. Depending on what's already there and the condition
of the tube, I'd probably try to distribute the load a little better
-- might come out looking more like a sleeve.
-R.S.Hoover
PS - John got it right. Seems kinda casual to risk your life on a
snap-shot... for all we know there could be a bullet hole on the other
side :-)

[Mark]

This is a 10-inch diameter tube. How far from an open and accessible end of
the tube is the damage located? It may not be possible to reach the patch
on the inside of the tube to properly buck the rivets...

The photo and description indicate the damage is across the tube. 4.1-inches
is a quite a chunk of the total circumference (13%) to removed and then
attempt to bridge with a riveted patch. The cited sections of AC43.13 may
not be appropriate repair guidelines for this particular tube structure.

If the builder's handbook does not provide repair guidelines for this tube,
you should probably hire a qualified aircraft structural engineer to develop
a suitable repair technique that the experimental/homebuilt inspector (FAA)
will accept.


> wrote in message
...
On Jul 13, 5:21 pm, "RPM the A&P" > wrote:

>
> The damaged area is on a 10in diameter .065in 6061 T6 tube that serves as
> the tail boom on this aircraft. The part was dropped and sustained a
> crease
> roughly perpendicular (but not quite) to the length of the tube.
>
> I have removed the displaced material and now have an opening 4.1in long
> and
> 1.1in wide.
--------------------------------------------------------------------------------

I'd go with the patch. But I'd like to know more about that portion
of the structure, especially with regard to the condition of the
surrounding area. Depending on what's already there and the condition
of the tube, I'd probably try to distribute the load a little better
-- might come out looking more like a sleeve.
-R.S.Hoover
PS - John got it right. Seems kinda casual to risk your life on a
snap-shot... for all we know there could be a bullet hole on the other
side :-)