Shear Web Layup

[spitzy]

I completed my shear web layup and found there was delamination around the corners in some spots.

 

I started to sand out the bad areas and plan to complete the 4 unidirectional ply layup followed by on ply of bid. This is per the instructions to make a repare that is 100% of the origional strength.

 

Given this is the main load bearing member in the aircraft, I've been having second thoughts about doing this.

 

1) Will the extra thickness or bump under the spar cap caused by the imperfection in my shear web reduce strength?

 

2) Should I trust a repare in a part like this?

 

I have a feeling that this wing spar was designed to handle a load that is WAY above any actual load experienced to complensate for any small deviations in strength due to the manufacturing process.

 

Any thoughts?

 

I'm considering finishing this one, measuring the breaking point of the thing to satisfy my own curiosity. Then I'll either buy or build another one for my plane.

[Jon Matcho]

It's hard to say without seeing it in person, and since I'm not an ME you'll have to disregard anything I say and form your own opinion.

 

But...

 

I have a feeling that this wing spar was designed to handle a load that is WAY above any actual load experienced to complensate for any small deviations in strength due to the manufacturing process.

I agree. Unless the delamination was caused by an exotherm, the strands are all there and just need to be held in place. Ah, but you've already sanded? I would wonder too about the strength of the repair, but not necessarily dismiss the part. But again, I'm not in your shoes.

 

I'm considering finishing this one, measuring the breaking point of the thing to satisfy my own curiosity.

That would be interesting. It might test to demonstrate less strength than a completed wing, because it's just part of a bigger system. Still, it would show a minimum strength.

[Wayne Hicks]

I completed my shear web layup and found there was delamination around the corners in some spots.

 

-----> Do you really mean delamination (as in, the glass cured as it should but has somehow separated itself from the underlying foam), or do you really mean air bubbles (as in, the glass cured with an air bubble underneath. The glass has raised up in relation to the glass that surrounds the air bubble.)?

 

-----> If it's truly a delam, just drill two small holes at the delam. Inject raw epoxy or inject a wet slurry into one of the holes to fill the void under the glass. The other hole lets the air evacuate as you inject the epoxy or slurry. Stop injecting when full.

 

----> If it's an air bubble, sand away the glass over the air bubble. Feather back the surrounding glass. Patch over it with the same number of plies ensuring you have at least a one-inch overlap. See Chapter 3 and Chapter 25. Those chapters specify how to make corrections. Sounds like you're aware of this and have read these chapters from your previous post.

 

Given this is the main load bearing member in the aircraft, I've been having second thoughts about doing this.

 

----> Don't have second thoughts. Just make the repair.

 

1) Will the extra thickness or bump under the spar cap caused by the imperfection in my shear web reduce strength?

 

-----> It depends. Believe it or not, delams, air bubbles, and sad-throughs up to the size of a 25-cent piece (a quarter) are acceptable as long as there are no other air bubbles of that size close to it. On the other hand, the plans also state that the UNI fibers (implied, and the spar caps) are supposed to be straight to within a sixteenth of an inch. If your air bubble is under where the spar caps will go and is taller than a sixteenth of an inch, then you theoretically need to fix the air bubble. Else, your spar cap will be too thin above the area of the air bubble.

 

2) Should I trust a repare in a part like this?

 

-----> Yes! Just think of how many one-inch overlaps are joining the rest of the plane. They are still flying and are not dropping out of the sky.

 

I have a feeling that this wing spar was designed to handle a load that is WAY above any actual load experienced.

 

-----> Well, RAF designed the wings and the construction methods to be tolerant of builder variances. RAF didn't say anything about being tolerant of builder mistakes. I think not. RAF expected their builders to fix their mistakes. And they outlined the process for doing that. If you're building a Cozy, I can tell you that Nat just followed along with the RAF building methods. He never tested the Cozy wing to failure. So we don't know how much load they can actually carry.

 

Any thoughts?

----> I'm assuming you have air bubbles. Maybe the edges weren't radiused enough before glassing? Just make the repair and press on.

[Waiter]

In regards to quality of workmanship:

 

"Your very best is just barely good enough"

 

Waiter

[Jon Matcho]

I agree w/Wayne -- it should be very salvageable.

 

"Your very best is just barely good enough"

I don't like that quote at all. For someone who might be reading this who has not yet stepped into mouldess composite construction, that would likely scare them away.

 

Let me put it this way... I have seen some kit parts that I wondered if the goal was to make as heavy and messy as possible, but taking a step back I realized the parts would fly just fine.

 

I'm a fan of "good enough, ship it!"

[Drew Swenson]

If I remember right, Waiter's quote is actually a Burt Rutan quote---and it pertained to the canard.

[Waiter]

If I remember right, Waiter's quote is actually a Burt Rutan quote---and it pertained to the canard.

Thats exactly right. I believe his reference was to people like me who are new to this building technique, we haven't yet accumulated the years of knowledge and experience. So Burt challenged us to do our very best, and not accept mediocre.

 

 

When I stand back and look at this piece of art that others call an airplane, and realize that "I" did this, with my own two hands, WOW; I didn't know I had it in me.

 

Thank you Burt for the plans, and the challange to do my very best.

 

Waiter

[Jon Matcho]

Obviously Burt's words worked for Waiter, and no offense intended to anyone, but I still don't care for the quote.

 

I too had zero composite experience, but can follow the plans and do know my way around a screwdriver. My point is that if someone who sits in an office all day, such as myself, can do it, then so can "you".

 

[Waiter]

My point is that if someone who sits in an office all day, such as myself, can do it, then so can "you".

ME TOO. :)

 

Waiter