Tip: Liquid Shim

[Indigo]

It has been awhile since i have purchased some adhesive from them, the quantity size is usually one quart size but i have purchased 50 gram and 100 gram packs of certain styles of Hysol products. You will have to call and tell then what product you are ordering and see what size they come in.

[hevansrv7a]

I needed some between the forward skin of the tip up canopy frame and the bent tube that supports the aft portion of the glaresield, at the outboard portions of the bends. JB Weld works well and doesn't drip.

[JDAviator]

When I worked for Cessna Aircraft in Citation factory flight fest, we used a Hysol product called EA9309. It was more of an adhesive than shim, but was used for both. It was available in a small quantity (two part plastic packet), probably a couple of ounces at the most. Remove the divider, give it a 10 second zap in a microwave to allow it to mix easier, mix the envelope, cut the corner and use. When it had set, it too was workable either by sanding or filing. Neat stuff.

Sure wish I still had access to their amost expired shelf life stock...

[DanH]

Re; riveting through a West System and flox mixture, why not a simple homebuilders test? Mix an epoxy and flox sample, put some 1/8" layers between aluminum coupons. Next day bandsaw them into 1/2" wide strips. Beat he*l out of them with a flush set and back rivet plate, see if the filler falls out.

For those who have not done the Rutan thing, edges and sharp corners in a moldless composite get what is known as a flox core. It serves as an internal bond filet as well as greatly improving impact strength. Years ago I rolled some flox/epoxy balls using excess Safety-Poxy from a layup task. Two days later I put them on the concrete floor and beat them with a claw hammer. That exercise convinced me; flox/epoxy toughness is very good.

[gmcjetpilot]

Sounds Great and all the applications sounded like non-structural, but if you are talking airframe, metal to metal use metal shims. Its a lot of work to grind down aluminum to a tapper or to a supper thin gauge but its needed for the strength of the joint. Two reasons, clamp up strength and bearing strength (edge of hole).

In general shims increase the eccentricity of the "load path" and are not desirable, but if gap is not too great than effect negligible.

[AZtailwind]

General Dynamics- St Loius.....
Now what was that stuff that was used to bond F-16 wings in the late 80's-90's it came in gallons and had a shelf life....???
JB weld !!

[Rick6a]

Originally Posted by AZtailwind

General Dynamics- St Loius.....Now what was that stuff that was used to bond F-16 wings in the late 80's-90's it came in gallons and had a shelf life....??? JB weld !!

Actually it is the F/A-18 Hornet you are referring to, and the company is (was) McDonnell-Douglas. The following procedure is a general overview and is the same for both the upper and lower wing skins. A skin is temporarily fitted to the structure which is oriented in the vertical position. Using a magic marker, an outline of the structure is made on the inner skin surface, a graphite composite material ranging in thickness from 1/4" to 3/4". The outlined area sort of resembles an enormous 2"-4" wide grid pattern when you are done. Do not think of the wing structure in the conventional sense....ribs lined up symetrically along a main spar and trailing edge.....it is a much more complicated structure than that. The skin is then removed and within the confines of the outline, the skin surface is roughed and cleaned to enhance the holding power of the liquid shim material. The structure itself is given a liberal application of parting agent, modeling clay used to keep the shim out of certain areas. Liquid shim is mixed and manually stuffed into Semco cartridges. It is generously applied to the skin using a spreader nozzle fitted to the Semco making sure to cover all areas within the magic marker outline. Using a hoist because the skin is very heavy, it is carefully refitted back into place and held fast using a series of 3/16" draw buttons which are basically clecos on steriods. Naturally, much of the liquid shim gets squeezed out from between the structure and the skin. After the shim dries, the skin is drilled for all those fasteners, many hundreds of holes. After the drill-out is complete, the skin is pounded free of the structure using a dead blow mallet. If the parting agent did its work, the skin will break loose relatively easily. What follows is not fun. The skin is secured to a holding jig to more easily remove all that excess liquid shim squeeze out from it. Most often, a die grinder fitted with a 80-120 grit sanding disk is the tool of choice. The liquid shim is feathered to a smooth transition everywhere, including burrs that might occur around all those previously drilled holes. It is obvious that the liquid shim remaining is relatively thick in some areas, thin to non existant in others. All traces of parting agent are removed from the structure while the skin is countersunk. How would you like to machine countersink graphite wing skins 8 hours a day generating lots of itchy smudgy black graphite dust while doing so? The smallest countersink is 3/16" and the largest is 5/8". Every hole location on that skin is numbered and charted. A QA inspector will hold you to close tolerances on every single hole you countersink. Finally the skin is permanently reattached to its mating structure. The holes are then filled with fasteners. That is, in a nutshell how we did it.

[Gsuit]

Originally Posted by DanH

Re; riveting through a West System and flox mixture, why not a simple homebuilders test?

I mixed up some home brew liquid shim (West epoxy and flox) to fill the KNOWN ISSUE (Van's!!) of the gap that occurs on tip up canopy frames between the tube and top skin. After curing, I drilled, countersunk, and riveted. All came out very smooth and I'll wager to any structural analyst to try and break this stuff!!