Rick6a
Well Known Member
I adopted a technique long used on the combat aircraft production line that served my homebuilding purposes very well. During the construction process, even military aircraft frequently have situations in which gaps between mating parts occur and must be shimmed prior to permanently fastening together. Particularily problematic is a "tapering gap" condition. Frequently, mating the wheel pants to the brackets exposes this condition and I'll address that in a moment.
We've all seen RV's with puckers in the fuselage skin, usually at an intersection of longeron and bulkhead or stringer. This is caused by riveting the gap closed rather than taking the time to shim out the open void between the parts. On the production line, a worker can easily spends hours and hours with a piece of aluminum shim stock and sanding disc in an attempt to fashion a shim, tapered or otherwise to fit acceptably within the confines of the gap. In an effort to reduce manhour costs and enhance quality, a space age material was developed to address this tedius process. I don't know what the actual material was, we production types simply referred to it as "liquid shim." While the exotic material we used at McDonnell-Douglas is not generally available, I often made up an ersatz home brew which consisted of nothing more than West System epoxy with lots of cotton flox mixed in (roughly to the consistency of peanut butter) to make it structural. The process goes far faster than it sounds. After masking off the local area with bits of tape, the mixture is forced into the gap between the parts (i.e. bulkhead and skin) until some of it oozes from around the parts and out the rivet hole. Wearing latex gloves helps. The liquid shim will find its way throughout the void and fill in the gap. Using a plastic scraper, clean up the excess ooze-out from around the general area to avoid excessive sanding later. After allowing the mixture to cure overnight, the hole is then reopened by passing a drill bit through it, the countersink retouched and finally the rivet shot. There you have it...a perfectly pucker free exterior skin.
Among the most important applications for liquid shim I found was precisely fitting the wheel pants to their mating metal brackets. On my RV, virtually all of the brackets met the interior of the wheel pants with a very noticeable and often tapering gap. It is tempting to simply torque down the screws mating the pants to the naturally flexible brackets but I believe all this really does is "preload" the brackets with an undo and constant stress and doing so is quite likely to be a major reason why so many builders have reported cracking of the brackets over time. To address this common problem, an after-market stainless steel type wheel pant bracket became available. By shimming the brackets to the pants precisely in the first place, I believe the problem would go away. That's only my opinion though.
Some builders have used a variation of this technique to precisely "float" hinge halves into place particularily on the flaps and cowling. In fact, I long used a similiar technique to help insure interchangeability between avionics doors on the T-45 nose cone. It was required you had to be able to remove the piano wire holding the hinge halves together with slight finger pressure alone, no needle nose pliers or vice grips allowed, and demonstrate this with any random door pulled from stock. Believe me, it's not as easy as you might suppose. Just imagine removing a flap from your RV and having it fit the RV parked next to you! Liquid shim proved to be the only realistic way to consistently maintain random door interchangeability in part by liquid shimming its mating hinge half to the structure on the nose cone door thus assuring repeatability.
There you have it....an easy building technique I'm sure you can find any number of situations to use it in.
Rick Galati RV-6A "Darla"
We've all seen RV's with puckers in the fuselage skin, usually at an intersection of longeron and bulkhead or stringer. This is caused by riveting the gap closed rather than taking the time to shim out the open void between the parts. On the production line, a worker can easily spends hours and hours with a piece of aluminum shim stock and sanding disc in an attempt to fashion a shim, tapered or otherwise to fit acceptably within the confines of the gap. In an effort to reduce manhour costs and enhance quality, a space age material was developed to address this tedius process. I don't know what the actual material was, we production types simply referred to it as "liquid shim." While the exotic material we used at McDonnell-Douglas is not generally available, I often made up an ersatz home brew which consisted of nothing more than West System epoxy with lots of cotton flox mixed in (roughly to the consistency of peanut butter) to make it structural. The process goes far faster than it sounds. After masking off the local area with bits of tape, the mixture is forced into the gap between the parts (i.e. bulkhead and skin) until some of it oozes from around the parts and out the rivet hole. Wearing latex gloves helps. The liquid shim will find its way throughout the void and fill in the gap. Using a plastic scraper, clean up the excess ooze-out from around the general area to avoid excessive sanding later. After allowing the mixture to cure overnight, the hole is then reopened by passing a drill bit through it, the countersink retouched and finally the rivet shot. There you have it...a perfectly pucker free exterior skin.
Among the most important applications for liquid shim I found was precisely fitting the wheel pants to their mating metal brackets. On my RV, virtually all of the brackets met the interior of the wheel pants with a very noticeable and often tapering gap. It is tempting to simply torque down the screws mating the pants to the naturally flexible brackets but I believe all this really does is "preload" the brackets with an undo and constant stress and doing so is quite likely to be a major reason why so many builders have reported cracking of the brackets over time. To address this common problem, an after-market stainless steel type wheel pant bracket became available. By shimming the brackets to the pants precisely in the first place, I believe the problem would go away. That's only my opinion though.
Some builders have used a variation of this technique to precisely "float" hinge halves into place particularily on the flaps and cowling. In fact, I long used a similiar technique to help insure interchangeability between avionics doors on the T-45 nose cone. It was required you had to be able to remove the piano wire holding the hinge halves together with slight finger pressure alone, no needle nose pliers or vice grips allowed, and demonstrate this with any random door pulled from stock. Believe me, it's not as easy as you might suppose. Just imagine removing a flap from your RV and having it fit the RV parked next to you! Liquid shim proved to be the only realistic way to consistently maintain random door interchangeability in part by liquid shimming its mating hinge half to the structure on the nose cone door thus assuring repeatability.
There you have it....an easy building technique I'm sure you can find any number of situations to use it in.
Rick Galati RV-6A "Darla"
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