Friday, November 17, 2006

Glue Wars

Like most conflicts, the Glue Wars are spawned by ignorance. The War begins when someone says they use Brand X glue. No matter what type it is, someone is sure to declare THEY use Brand Y because it’s STRONGER, and anyone who uses anything else is not only a stupid person but probably performs unnatural acts with goats.

All of which may be true, except for the part about the ignorance.

The truth is, all modern adhesives are stronger than wood. Nowadays we don’t select an adhesive for its strength, we choose the glue that most closely matches our particular needs.

Trying to keep your Dream Machine under a thousand dollars? Then you’re probably using Weldwood ‘Plastic Resin.’ At about four bucks a pound and available from most hardware stores, it is the only aircraft-certified adhesive commonly available to all. Of course, you must keep the temperature of the glue-line above 70* Fahrenheit or the stuff will dry out before it can harden chemically.

If you’re forced to do your woodworking at lower temps and can’t build a heat box (*) then you’re probably using one of the many epoxy-based adhesives. The same would be true if your main constraint is limited time, in that you’d naturally select an adhesive having a fairly quick cure. The opposite would be true when laminating a spar or wing-tip bows, where you may need an hour or more to apply the glue to all of the surfaces before you can begin clamping-up. In that case you’d probably select a glue with a slow cure and long ‘open’ time. The message here is that regardless of your situation, there’s a glue that will do the job.

Some builders find the 5:2 ratio of ‘Plastic Resin’ to water to be a challenge. Others find the 10:1 ratio of FPL-16A beyond their abilities and even the 1:1 of T-88 something of a trial. Aerolite is too viscous; Hughes too runny; Excel One can’t be any good because it’s too easy to use (!!). Or their shop is too something to use Brand X (hot, cold, damp, dry, dusty... you fill in the blank). The reasons a builder selects a particular glue are as varied as the builders themselves but once they settle on what works best for them, an odd thing happens.

With one airplane’s-worth of experience under their belt the typical homebuilder becomes an expert on his particular choice of glue, wood and fabric. The sad part is that they often say as much :-)

Unlike stupidity for which there is no cure, ignorance - our normal state - may be altered by access to information. If your total gluing experience has been limited to one type of adhesive, go find another and make yourself some test-blocks. Make a few notes describing how you prepped the surface, prepared the adhesive, clamped the coupons, how long you allowed them to cure and under what conditions. Then break them and record the results. You may use the standard shearing test, like the Forest Products Laboratory, estimating the amount of wood adhesion using a loup having a graduated reticle (machinist supply houses carry such things). Or you can do some scarf joints and use a bending tester. Or both.

Making up glue blocks and breaking them is such a trivial task that most homebuilders consider it a waste of time. But as your notebook grows with information derived from different glues you’ll discover that the integration and comparison of that information yields a wealth of knowledge based on actual experience. All for a few bucks and a little time. Asked to make a prop for a Jenny or repair an ash interplane strut, you’ll know which glue will work best.

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“What’s that on your ribs, resorcinol?” The Instant Expert asks.

To which you reply, “Howabout them Chargers, eh?”

“I only use T-88,” he insists, chin sticking out to here, just daring you to say otherwise.

“That new quarterback seems to be doing okay,” you respond.

“It’s a lot stronger than them other glues,” he says, rather plaintively.

“Didja see what they’re charging for gas out at the airport now? They must think we’re made outta money.”

And about there the Instant Expert mutters, “Best damn glue in the world,” and wanders off toward the coffee machine.

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It takes two to start a war.

Build yourself an airplane. Go fly. Leave the bullshit to the bean-counters.

-Bob Hoover

-PS. Need a heat box? Odds are, you’ve already got one. Cool days, I put glued parts inside my Volkswagen bus. In the winter the temperature inside the bus is rarely less than 80 degrees.

I’ve got a small bathroom off the shop. With the door closed and the light on, the place stays warm enough to cure ‘Plastic Resin’ over night. If the weather is really cold I’ve got a cardboard spray-booth in the shop I can seal up. A light bulb in a recessed ceiling fixture, the kind with a built in thermal switch, will keep the spray booth above 70* on the coldest southern California night. Larger parts, such as spars, ailerons or even an entire wing, can be maintained at glue-cure temperature by covering them with a tarp and putting a couple of lights inside.

Why would you use Weldwood ‘Plastic Resin’ when you’ve got a good low-temp epoxy on the shelf? Because ‘Plastic Resin’ is certified. If you’re doing a repair on a certified airframe, even something relatively minor like mending an aileron, you owe it to the customer (and the next mechanic down the line) to use only certified materials. Check the specs. Most of the adhesives available today are NOT certified for use on airplanes.

Repair or fabrication of a homebuilt is a different story. You’re the Mechanic-in-Charge; whatever adhesive you use is your decision. But don’t fall in love with the stuff. Some wars are worth the fight but glue isn’t one of them.

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