Howdy Burt, I Fixed Your Puny Spar
 2006.03.17
 

The way I heard it, back in the 80s a fellow told Burt how he just didn’t trust the main spar. It looked a little dinky. So he beefed it up a little, adding extra layers to the spar caps. Sounds reasonable enough. More is better. What could it hurt?

Burt congratulated him for building a three-G spar.

I read in the early CPs of a wing loaded to 18 Gs before it started to fail. But this man’s innocent (ignorant) ‘strengthening’ actually created a hard, non-flexing element in the previously extremely strong, uniformly flexible wing and spar unit. It has to ‘give’.

You can do a similar demo. Take a thin length of wood like a flexible yardstick and play with it and bow it good and get a feel for it and maybe measure the bow flex at the center, like where an arrow would go.
Add another similar layer in the middle, simulating the strengthened spar. Now try to get it to bow to the same curve again. Snap. Right at one of the new stiffer, “stronger” stress points. Gee. That’s a lot better design.

Some of us would struggle some just to do this yardstick flex demo right, much less redesign a spar. Hey, maybe we could get someone to come up with a strong, flexible, light weight, spiffy, well proven wing and spar design so we could just go fly…?

I fly with no structural concerns of my own making. The bulkheads and spars and such are per plans. My only changes would involve things like using LongEZ foam, glass and gear improvements. I have specifically benefited operationally from Burt’s elegant design, rather than something “better”.

OK, so LESS must be better?
A builder recently said he is planning on building the lightest airframe ever and asked if I knew of any special tricks to reduce the epoxy weight. I don’t. I use a stipple brush and squeegee as best I can, as described in the documents. I do go back after an hour or two and dab a paper towel in low spots where epoxy is pooling. The large areas inside the fuselage pieces are prone to this.
I just don’t gravitate toward using aggressive techniques that in some way or other result in epoxy being drawn out of the weave. Wonder where the “starve” point is? I am not referring to vacuum bagging. I have never done it. There are others than discuss that appropriately.
I don’t know how far one can push or pull the epoxy deprivation effort. I just told him I wish him well and will look forward to congratulating him after he has outrun everyone. Proof’s in the pudding.

Carbon Fiber
Another builder mentioned having purchased some carbon fiber for use in the spars. After reading one of these Chronicles he was having second thoughts and asked if this was wise. I suggested staying precisely with the plans. He will have plenty of other uses for the carbon fiber.
I don’t know anything about how to make Burt’s design better. Only prettier maybe, and heavier. And less efficient.

Several years ago there was a magazine article published by a fellow that built his plastic plane, maybe a Lancair, using carbon fiber instead of fiberglass. It ended up weighing a few pounds more than standard.

It could be surmised that since I have done things with my cowls and prop, that I also surely would have “improved” the big things with carbon. No, somehow I draw a very distinct difference with the things I have done versus the primary structure. I guess I appreciate the overall design and “fit” of the airframe elements.

Also, an experienced prop guy mentioned the problems you can create when ignorantly using carbon on props. Whereas fiberglass has some give, carbon fiber doesn’t stretch much and will fail. His discussion was not technical and didn’t give me anything useful as far as the “how to”, rather just a warning that using specialized materials has two sides. Props are black magic.

Aluminum and Carbon Fiber
I heard that while they were in the middle of building the globe circling Voyager, one day Burt asked in passing about the glass layer that separated the aluminum wing fittings from the carbon fiber spars and structure. After a moment of unblinking silence, it was clear that no one had placed the protective layer there. The aluminum was steadily deteriorating.

If you draw a straight line with a pencil on a thin sheet of aluminum, later you can go back and easily fold it right on that line and break it. The carbon eats the aluminum. I understand that Burt did some quick calculations on how long the Voyager wing fittings had been built and gave the team a drop dead date for when the fittings would have deteriorated too much. They had to fly when they did.

If all else fails, read the directions
There are basic documents required for building an EZ. Anyone building or flying an EZ is at a disadvantage if they haven’t studied them in their entirety, and even have them handy for perusing in the porcelain library. A little here, a little there.
There is a balance in there somewhere between Burt’s fierce early focus versus important improvements like the canopy catch and gear/throttle/canopy warning setup, early fire warning wires… (have heard of a trial by fire out in the field, successful). Burt’s instructions in the Yellow Composite Handbook give instructions on getting the right amount of epoxy wetness in the cloth. Pretty straight forward.

I can’t believe that everyone hasn’t heard about the total foolhardiness of changing the layers to Burt’s design. But evidently there are those that have done it, are doing it, and even folks planning to do it.
Any dark camo airframes out there? The wings would look real good with some melted radical swooping anhedral.

This is right in there with washing your skin with harsh chemicals like MEK. Or even breathing the fumes. It permeates right to the Liver. Has everyone read Burt’s admonishment to get that stuff completely out of your shop?

Is anyone slabbering micro (little hollow balloons?) in crazy places like between structural spar layers so they will be more even and look better. I’ll be back in a minute, I have to go outside and scream.

I feel a definite advantage in having read all of the CPs and Plans and addendums. Simple understanding of the genesis prevents mindless errant temptations, like the dark camo paint that will allow your wings to warp. While it is OK initially to have a goal to remove the cowls on your recently purchased EZ only once a year during the annual, a reading of your basic documents will dictate otherwise.
I would like to think that even if I hadn’t read the basics early on, I would have had the curiosity and discipline to garner their important details anyway. Many of these Chronicles don’t include exact detail, in hopes it will interest you to invest a few minutes in the source and get Burt’s and Mike’s intent.

Having read his stuff, at least I won’t walk up to Burt and say something stupid, like “Howdy Burt, in our hanger there are two more VariEzes going together!”
Again.

Bill James, Fort Worth VariEze

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