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Sunday, October 18, 2015

Center section spar - part 5

Changing the layup schedule of Step 4 (slightly)

What I outline here are my reason for doing something different than the plans. I do not imply or suggest that anyone do the same. Your life depends on following the plans, not on what I chose to do with my plane.

Make no mistakes about it, the plans specifically states Step 4 should be done as one layup. 

However… I will try to rationalize this away.

In case you haven’t noticed, Step 4 is a pretty big job, and having a few assistants would allow it to go more smoothly, and take less time.

Having only occasionally been able to work with helpers, mostly due to my weird schedule, and the local scarcity of such creatures, I knew going into this that I could not do it all in one take. Although I was concerned about it, the reality of my situation had me question this requirement.

You see, this is a very important structural layup, and I have no skills to second guess the designer. Still, I couldn’t help thinking that the statement in question seemed very generic and slightly out of context, and contained no explanations of why it extended its veil of authority over the entire inner structure. Maybe it could have been a little more specific as to which part really needed to be done at once, and which ones perhaps didn’t.

Clearly the thick and convoluted layups #3 and #4 needed to be done at the same time, since they created the base for the aluminum attach points. The better chemical bond of an “all at once” layup might be more advantageous here, but the reasons that requirement extended to the entirety of layup #2 eluded me.

After all layup #2 is just a single ply of BID covering the whole inner surface of the spar, and overlaps itself many times due to the relative small size of the BID roll, compared to the spar it has to cover. 

Since when had a single BID layup become so critical that had to be done all at the same time?

Perhaps the wording sequence of Step 4 had been slightly unfortunate, and maybe a more creative interpretation would have allowed for an improved building cycle for the unassisted builder, while at the same time not giving up the required extra strength of the “all at once” construction.

After much pondering, I came up with an alternate layup schedule that would allow me to perform this step in 2, to as much as 4, separate glassing sessions, apparently without sacrificing strength.

My idea was to subdivide layup #2 in such a way to make sure that the unavoidable BID overlaps would always occur outside of the main layups #3 and #4.

4 separate BID sections making up my layup #2 (each number represent a possible separate working day).

The peel-plied overlaps occur away from any major layups under the metal attach points (LWA1)

Using this new schedule, the fiberglassing of each hard point would still be “laid as one” on a fresh layup #2, but these could happen on as many as 4 separate days. 

The departure from the plans would only be in the manner in which the single BID (layup #2) covering the whole interior, overlapped itself to make up a single BID sheet. Because single BID plies are relatively weak structures to begin with, I saw no point to the requirement that these overlaps be done all at the same time.

While I could not find anything wrong with this approach, I decided to ask around. 

Lacking the engineering skills, and since other builders liked my idea, I turned to a very conservative, well known Aeronautical Engineer that specializes in composite structures. He has build these planes, and is very well regarded in the canard circles, but shall remained unnamed since I didn’t ask for his permission to publish his name and comments. He also liked my idea, and described the single BID ply as perhaps a way to “stabilize the foam”.

Happy with the outcome of my consultation, I modified the schedule for my Center-Section spar. My new plan would be to start at one end, and go as far as possible, then peel-ply the BID transition, and pick it up again another day. I reckoned I might be able to do 2 quarter sections per day, and be done in two days, depending on how much time this really took, while retaining the ability to split it up into up to four days, if I needed to.

Next time it will be “fiberglassing time”.

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