Glue for Aircraft Construction

Glues and Gluing Procedures:

The following is an excerpt from both the Zing and Cloudster building plans (as of 7/20/2014) : 

All wood surfaces to be glued must be free or dirt ,sawdust, and oil. Wipe the surface with tack cloth or similar. Try to use freshly cut parts, and avoiding ‘sanding’ the end of the piece your gluing, especially with fine sandpaper as this prevents good penetration into the wood. The dust can clog the pores in the wood and greatly reduce glue penetration.  Surfaces cut with a sharp blade or saw are good, the pores are open. Both surfaces can be roughened with a quick pass of a rasp. This gives the glue a better surface for gripping and more surface area to work on. The dust produced is not fine enough to clog the wood pores and vacuums away readily.

When gluing the joint be sure to coat both surfaces with a liberal amount of glue before joining together. When the parts are clamped together, excess glue should squeeze out around the entire parameter of the joint. Though perfect tight fits look very nice, they are not needed and can sometimes be a problem with epoxy, so don’t get it too tight. You can end up squeezing out most of the epoxy and have a very weak joint that looks nice. Excess glue shouldn’t be completely wiped away, as you need glue at the glue line to be absorbed into the wood. If you can see the glue line between the two pieces, the odds are that you still have a nice film of glue in the joint. This is important to avoid starved glue joints. Make practice joints and test them to destruction.  Make some test pieces with different clamp pressures, and see how they break.  That will give you a very good feel for the correct amount of pressure. The failure should be in the wood each and every time.  Failure can cross the glue line, but the glue line cannot be the failure point.

All joints should be clamped until glue or epoxy dries. All gussets should be clamped or stapled. A light duty staple gun such as the Arrow JT21 works very well. Staples should be removed from all gussets. An exception can be made with the wing rib gussets, as those staples can be left in place if desired. However, removing them does save weight! Areas where joints and gussets are always visible such as the cockpit tend to look nicer if very small brass aircraft nails are used to secure the gussets during gluing. Emphasis must be placed on “small nails” as their job is only to lightly secure the gusset to the joint until the epoxy or glue dries. After that they are just left there for looks, and should not be dug out. Their purpose is not structural, and using large nails will actually weaken the structure.

When choosing glues, find one that is readily available, and one that you are positive you can trust with your life. Remember, it’s just wood and glue holding you up! Aircraft failures have been recorded where the cause was cited to be poor glue selection during construction. Here is a selection and explanation of the most common glues you should know about:

  • System Three T-88 Epoxy is a 2-component, epoxy/polyamide adhesive and is the most popular and likely the best choice for gluing wood aircraft (and boats). With a tensile strength of 7,000 PSI we highly recommend its use when building one of our designs. T-88 has exceptional adhesion to most clean surfaces including wood, fiberglass, concrete, aluminum, steel and many plastics. It’s a two-part epoxy mixed at a 50/50 ratio (or 1 to 1) by volume. It is somewhat idiot proof as the ratio can be slightly off by a small percentage and still produce good results. This only means that you are safe to eye ball the 50/50 mixture and thus don’t need fancy measuring tools.  However, some builders still choose to make this ratio perfect.  At 70°F, T-88 will harden in 6-8 hours and will reach functional strength in 24 hours. T-88 has been specifically formulated to cure as low as 35°F without reduction in strength; however, the cure time required is increased to approximately one week. You should always wear thin gloves when handling. It is important to note that if you live in the North West where Douglas Fir is plentiful and common in aircraft construction, there are special instructions for its use with T-88. See the T-88 Materials Data Sheet.  Also Do not substitute look-a-likes or hobby glues under a different name.
  • Resorcinol glue or known under some brand names as Weldwood Resorcinol Glue or Weldwood Marine Resorcinol Glue. This type of glue is probably of the first of the glues used in aircraft construction that is completely waterproof and resistant to elevated temperatures. It is used commonly in boat building. It is great for gluing joints exposed to severe service conditions. Resorcinol glue withstands temperature extremes from sub zero cold to tropical heat, as well as exposure to either salt or fresh water immersion. However, unlike epoxy, it does not have gap filling properties, requiring joints to be close fitting and clamped under pressure to achieve good results.  Well fitted joints are important. Before epoxy came around this was the preferred glue by many builders. A great glue for laminating several wood pieces.
  • Plastic Resin Glue (Urea formaldehyde) or known by the brand names Weldwood or Panite. It has been used for many years with good success. It usually comes with hardener in the dry powder mix. Mixing with water is easy and activates the hardener and prepares the product for use. It once was one of the standard glues used on a good majority of homebuilts. It is another good glue for doing laminates of several wood pieces. It dries super hard which reduces spring back and it won’t creep. It has a long open time (20 minutes or more). It will have varying drying times depending on the manufacturer.  Plastic resin glues also clean up with water.  One of downsides to plastic resin glue is the odor and safety. The ingredients used to make the glue can be harmful if inhaled. It’s important to work in a well-ventilated area and wear a respirator or protective face mask. It also weakens at high temperatures, yet so do many other glues. It is available at many local hardware stores. However, it must be used at a 70 deg. minimum temperature for success.
  • Polyurethane glue common under brand names such as Gorilla Glue, Excel, Titebond, Probond, or Bolderbond has been a rising alternative that a few people have used with good results. The glue comes pre mixed and is easy to use. Apply glue to one surface and dampen the other. When brought together a chemical action occurs curing the glue. It swells and foams quite a bit while curing. However, for show quality building the foam is quite unsightly, so don’t use it in any finished areas where joints will be visible (like the cockpit). However, this type of glue is not recommended by Simplex Aeroplanes for wing spars or any fuselage joints, though it possibly is a good alternative when building wing ribs or tail feathers. You should always wear gloves when working with polyurethane glue as it is hard to remove with soap and water, and almost impossible to remove when cured. It is highly moisture resistant and the brand gorilla glue is now available at every hardware store, most supermarkets, and even walmart. (Note: Gorilla glue and Titebond also market another type of “wood glue” which is completely different, and is simply standard carpenter glue)
  • Common carpenter glue  (polyvinyl acetate)  that is yellow or white in color from any of the various brands (titebond, elmers, gorilla) is unacceptable in aircraft use. It softens too easily with heat and  joints can creep when subjected to constant stress. The manufacturers also specifically state in their material data sheets that this glue is Not for structural or load bearing applications. This phrase alone should make you reconsider its use.  It also requires a good joint fit for strength. Though strong when used for home projects, it simply isn’t designed to support aircraft loads, and thus should be avoided for use on your airplane. In some cases it is accepted for non structural areas such as turtle decks, fairings etc.

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