This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

Sunday, June 05, 2016

When disaster struck - part 2

Disaster recovery

Just in case you might have missed it, I had a terrible construction mishap last year, and the time has come to fix it.

These images are still painful to look at

About 6" (15 cm) of damage on the left longeron

Same amount and type of damage on the other side

The foam and outer skin took a beating as well

Deciding how to fix the longerons was one of those things I didn’t want to do in a vacuum, on in a rush, so I polled as many Long EZ “experts” as I could find on this repair. I even had the head of CSA email Burt Rutan and Mike Melville with sadly to say, no reply. Not that I realistically expected one (though it would have been awesome)

Eventually, I offered as much money as needed to a well known engineer with extensive composite canard construction and modification experience. He graciously would have none of my money, offered expert advice, and cheerfully tried to put my many worries to rest. 

“I am very grateful for your input, Sir (you know who you are)!”

After all of this, I went on to build the Center-Section Spar instead, in order to create emotional and time distance from this problem, so that I could be more rational and deliberate when the time came, but even then, I secretly harbored serious reservations about the advice I had received. 

Like the doubting Thomas that I obviously am, I ended up trying to test those theories, albeit in a very unscientific manner, and I thought you might enjoy seeing what happened.

Let’s take a look at what this incredible epoxy our airplanes are built with allows us to accomplish…

The first "loading to fracture" experiment on longeron stock vs the cut-then-repaired longeron stock ended with the fracture of the pristine longeron, and the survival of the repaired one. 

Sacrificial stock for testing as well as repairs (this stuff ain't cheep!)

Cutting at a 15:1 ratio

The battle of repaired vs new begins...

Flox is the only bonding agent

Spending the night up

Ready to slug it out!
 I wish I had taken pictures of me climbing on these pieces. I stood with both feet in the middle of the repaired one, then lifted a foot, and... nothing. Then I did the same with the new stock, and when I lifted the foot... it just gave away.

As I said, unscientific and unrepresentative, but fun.

The results of this experiment were hard to believe 

I suppose Aviation Circular 43-13 really knows what it's talking about

According to AC43-13 this is the type of repair to be carried out on broken wooden wing spars, however it was explained to me that these longerons are nowhere nearly as stressed, nor similarly loaded, and was advised to use much less than a 15:1 scarf, and ditch the side reinforcements. 

In this 2nd totally unscientific, and once again hardly representative loading experiment, I have cut the spruce longeron stock (the broken one from the previous experiment) at a 6:1 angle (closely matching my future repairs), and used just flox to glue the pieces back together. 

If you have never tried doing this yourself, it is the most unbelievable thing you can imagine, I will let the following pictures speak for themselves...

Just a straight cut and some flox

Wanna bet?!

Let's just say... a lot!

Doubling the pressure (same force applied to half the area) on the poor joint

As you can imagine, I am obviously no longer worried about repairing my longerons this way. 

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