fiberglass repair?

On Monday, August 10, 2015 at 4:59:42 PM UTC-7, wrote:

Original poster- contact me off line and I will give
you some guidance without the panic mongering.

Seconded. This looks like it'd be a pretty straightforward repair, nothing to get too bent out of shape about. Hank will set you straight.

Thanks, Bob K.

That really does look relatively minor.
I have seen much bigger, and very poorly repaired damage where they were
discovered under decades old bondo and gelcoat. Having not caused any
problems for all of those many years of abuse and neglect...

I suspect most older glider structures are way over strength, because
they did not have the finite element analysis to say how strong to make
it - and newer ones are also over strength to get the required
stiffness/ crashworthiness.

That looks like it needs to be cleaned up over an appropriate area, and
the appropriate multi layer repair effected in accordance with the
manual. No big deal. (but quite a lot of effort)

Just saying.

On 2015-08-11 07:40, Bob Kuykendall wrote:
On Monday, August 10, 2015 at 4:59:42 PM UTC-7, wrote:

Original poster- contact me off line and I will give
you some guidance without the panic mongering.

Seconded. This looks like it'd be a pretty straightforward repair, nothing to get too bent out of shape about. Hank will set you straight.

Thanks, Bob K.


--
Bruce Greeff
T59D #1771

Is there a reason this damage cannot be repaired by putting simple multilayer patch (right size, of course) on the inside surface? With proper overlap it gives you original strength without time-consuming surface finish work. Why not take advantage that fuselage shell is not sandwich structure and inside of the shell is easily accessible?

At 08:52 12 August 2015, krasw wrote:
Is there a reason this damage cannot be repaired by putting simple
multilay=
er patch (right size, of course) on the inside surface? With proper
overlap=
it gives you original strength without time-consuming surface finish
work.=
Why not take advantage that fuselage shell is not sandwich structure and
i=
nside of the shell is easily accessible?


Yes. The manufacturer's repair manual does not specify this as an approved
method (it is, by the way, quite clear and helpful as a guide for repairs
like this).

.....and you would have the choice of either telling a future prospective purchaser that the glider had a non approved repair or else knowingly lying about it.

You still have the exposed crack on the outside to fill and finish.

On Wednesday, August 12, 2015 at 1:52:27 AM UTC-7, krasw wrote:
Is there a reason this damage cannot be repaired by putting simple multilayer patch (right size, of course) on the inside surface? With proper overlap it gives you original strength without time-consuming surface finish work. Why not take advantage that fuselage shell is not sandwich structure and inside of the shell is easily accessible?

The conventional reasoning for not going with a simple patch like that is that it results in a steep stiffness gradient at the site of the damage. The more widely accepted scarf repair, when properly done, restores not only the strength, but also the stiffness distribution of the original. That helps ensure that the repaired area will not be prone to delamination or cracking as it ages.

At issue is that gliders tend to be very limber structures--they bend a lot.. As they bend, some parts of them get stretched out, and other parts get pressed together. And where there are abrupt changes in stiffness, for example, where there is a change in sectional thickness, the stretching or shrinking happens unevenly. That uneven stress distribution can cause cracking in the most brittle layer, the gelcoat. Sometimes the cracking can go deeper, into the structure below, but that's pretty rare in a well-designed glider. Typically you'll see this effect in gelcoat cracks that form at the corners of airbrake boxes, along ribs and bulkheads and other internal reinforcements.

Thanks, Bob K.

On Wednesday, August 12, 2015 at 11:25:03 PM UTC-5, Bob Kuykendall wrote:
On Wednesday, August 12, 2015 at 1:52:27 AM UTC-7, krasw wrote:
Is there a reason this damage cannot be repaired by putting simple multilayer patch (right size, of course) on the inside surface? With proper overlap it gives you original strength without time-consuming surface finish work. Why not take advantage that fuselage shell is not sandwich structure and inside of the shell is easily accessible?

The conventional reasoning for not going with a simple patch like that is that it results in a steep stiffness gradient at the site of the damage. The more widely accepted scarf repair, when properly done, restores not only the strength, but also the stiffness distribution of the original. That helps ensure that the repaired area will not be prone to delamination or cracking as it ages.

At issue is that gliders tend to be very limber structures--they bend a lot. As they bend, some parts of them get stretched out, and other parts get pressed together. And where there are abrupt changes in stiffness, for example, where there is a change in sectional thickness, the stretching or shrinking happens unevenly. That uneven stress distribution can cause cracking in the most brittle layer, the gelcoat. Sometimes the cracking can go deeper, into the structure below, but that's pretty rare in a well-designed glider. Typically you'll see this effect in gelcoat cracks that form at the corners of airbrake boxes, along ribs and bulkheads and other internal reinforcements.

Thanks, Bob K.

Think you misunderstood his question, Bob. He was asking "Why scarf from the outside and make a very large area you have to re-finish? Why not scarf the inside, and have a small area that needs the more aesthetically time consuming finish work outside?" Inside is less "optically critical".

Eric says the manufacturer does not approve of such repairs. I have seen several manuals, and all seem to show scarfing to be done from the outside for a glass shell. However, they are also all showing the repair as a hole into something that you do not have access to the other side of as you close it up. In the cockpit, you would have this access. In the tailcone, you would not. And on the inner skin of cored structure, I have seen splices shown in both directions for the inner skin. Honestly, you are putting back the same number of plies, with the same orientation as what was there before the break. Can you tell me the repair will be weaker if the scarf is made from one side than the other?

The bigger thing is that it is MUCH easier to scarf a convex surface (outside of the fuselage) than a concave surface (inside of the fuselage).

Steve Leonard

On Wednesday, August 12, 2015 at 9:48:18 PM UTC-7, Steve Leonard wrote:
...Think you misunderstood his question, Bob. He was asking "Why scarf from the outside and make a very large area you have to re-finish? Why not scarf the inside, and have a small area that needs the more aesthetically time consuming finish work outside?" Inside is less "optically critical".

He uses the word "patch," but doesn't mention "scarf" anywhere, hence the general shape of my reply. I suppose that a scarf repair might be considered a patch, but a patch is not necessarily a scarf repair.

Thanks, Bob K.

A scab batch is just as strong as a scarf repair as long as the scab is as large as the scarf. So, a 1/8" skin with a 40:1 scarf would call for 5" all around the cut, with the same number of layers and orientation of the cloth. Sand the area with 40 grit, clean with acetone and lay up a scab patch on the inside and fill the cut from the outside with white filler.
Much to do about nothing!
JJ