the dangers of assumptions

For quite some time now I've been helping Ron rebuild the wings on a 701 - using this as a learning experience towards building my 750.

In an attempt to ramp up our progress on the repair so we can get back to building new airplanes, we've been working hard at the flap extension.

It's become a huge quagmire.  NOTHING that the original builder matches the plans.  We could forgive a little tolerances here and there - IF THE TOLERANCES WE EVEN CLOSE TO WHAT THEY SHOULD BE! Even worse, it seems to have been a real terminal case of TLAR.... "That Looks About Right".

We've been building any new parts for the repair and extension exactly as the plans call for.  However when we add those as components of the flap extension, they seem to create more problems because the "good parts" of the original builder are not "good parts" after all.  Assumptions...

For example, we've had to add 3 rear ribs and a couple of nose ribs.  Perfectly sized parts according to the plans.  But the originals are several millimeters bigger than the plans, so the new extension skin doesn't fit right, it won't wrap tightly down to the new ribs, nor will it remain straight (leading and trailing edges).  There is no way to predict this as none of the "changes" the original builder made are to scale (nothing is larger or smaller by the same amount). Assumptions...

What does this mean for us?  Hours of thinking and rethinking on how to get the job done.  I know the extension is our idea, but really that should be just more materials - if the dimensions were right (or at least close) on the original, we wouldn't be fighting at each and every step of the build.... sigh.

We'll keep working on it and I'm certainly learning where to watch tolerances.  So far the pieces I've made for my 750 are real accurate to what is shown in my plans, so I'm happy.  Guess I'll have to wait and see how they go together in assembly but I have much more confidence than I do with this 701.

Speaking of assembly, someone on the Zenair builders website was kind enough to email me a copy of the latest revisions to my 750 STOL plans.  I have "Edition 3" plans, but with any design they are updates and additions made by the Zenair factory.  Most of these are small changes made to make things incrementally better and of those most come from builders suggestions.

Having a look a the 20 or so pages of updates he sent me initially made my mind spin:

Typical page of revisions. Part numbers, page reference, new page references and dates... quite overwhelming at first glance. Red marker for "need this new page for my plans", yellow for "my plans have this but more questions", and green for "my plans already contain this information"

After pondering these and truly wondering what the hell I was thinking when I decided to scratch build this airplane, I decided the best way was to set aside some time at the dining room table and make notes on my plans of the updates:

The entire exercise took me most of a Sunday morning with breaks here and there for food, water (and sanity), but I think I have a good handle on what is important and what isn't.  I probably have more freedom to decide what changes I want to incorporate as I am still early in my build and I'm making my own parts so unchanged parts can be modified to match up with the new designed parts easily.

A lot of the changes are simply cosmetic and make the 750 STOL have more part commonality with the more recently introduced 750 Cruzer model.

Onwards fellow builders and thanks for continuing to follow along.