MORE ABOUT GLUE By Bob Whittier writers and editors . . . who after all are huW hen man beings . . . apply themselves to the task of preparing material for publication, they make a sincere

effort to gather as much information as possible before sitting down at the typewriter. Literature prepared by reliable sources is studied, persons having knowledge of special subjects are interviewed, and the completed text is usually read by persons who know aviation thoroughly before being printed. But often when the resulting material is published and read by many thousands of men, it happens that unexpected additional sources of information are uncovered. In the case of SPORT AVIATION'S editors, at least, the reaction is to be properly appreciative and hasten to get the resulting new information into print. In the December issue there was an article entitled "Glues and Gluing". One of the points it made was that surfaces to be glued should not be sandpapered. Most men who have worked with glue, whether it be for cabinetmaking, boatbuilding or aircraft, have heard this advice and consider it sound. After all, it makes sense to assume that sandpaper dust and grit in the wood's surface will adversely affect the strength of the bond. One highly-regarded textbook on aircraft repair says "It has been found that sanding the surfaces, or otherwise roughening them to allow the glue to take hold, does not increase the strength of the joint". Another says, "Never sandpaper surfaces to be glued, as the dust clogs the pores

fibers and it can readily be seen how glue has trapped air bubbles. But, as the fibers are rather well covered by glue, adhesion is satisfactory. If the fibers are burred over, as would happen if the surface was rubbed with a blunt object or excessive pressure was applied during fitting operations, the air spaces will have an "inkbottle" shape as in the upper right panel. Capillary action will cause the glue to bridge over the tops of these spaces and the glue will contact only the top surfaces of the fibers. Glue contact is thus much reduced. The bottom panel shows how scraping the surface will bend the fibers all one way and in somewhat similar fashion prevent full contact. According to Ciba, this information was discovered before the Second World War and as a result of a number of investigations of serious accidents during the war, it became mandatory for wood joints to be sanded before gluing. The scarifying action of sandpaper, of course,

would avoid having too large a proportion of the fibers burred or bent. Seeking to double-check this rather upsetting information, your editors looked into "Wood Aircraft Inspection and Fabrication", ANC bulletin No. 19, a U. S. Government publication. Sure enough, hidden away in an obscure paragraph on p. 182 is a detailed discussion of

the subject. It is recommended here that garnet paper be used for the sanding job. The term "sandpaper" is highly misleading as it is loosely used to describe any abrasive paper. Common old-fashioned "sandpaper" with the familiar yellowish color is actually "flint paper" and the abrasive is soft and easily crushed into just the kind of powder which would spoil a glue line. But garnet paper is made with a much harder abrasive which will

Successive positions of meniscus

Good contact between glue and wood

Burred ends of capillaries prevent good contact

If the burrs are all one way good contact still prevented

not do this. Today we also have "sandpaper" made of silicon carbide and aluminum oxide abrasives, which are

fantastically hard. A light scuffing with one of these non-crumbling abrasives, using care not to cut into the wood, may therefore be regarded as a highly desirable procedure. ANC-19 also mentions that surfaces can be prepared for gluing by an application of water, followed by drying. This raises the wood fibers in ideal fashion. The Ciba literature uses this aspect of wood surfaces to highlight another important fact. If glue is mixed too thickly it has very little wetting power, and will not run down into minute depressions and wet the fibers as thoroughly as is required for safe bonds. A moment's

of the wood". Your editors felt they were passing along accepted and standard facts. But now we have at hand a copy of a pamphlet entitled "How Glue Sticks", which came in from the Ciba Company, Ciba Building, Montreal, P.Q., Canada. This firm represents an English adhesives manufacturer, Aero Research Ltd. Admittedly a more modern text, it takes exactly the opposite view, claiming that surfaces should definitely be sandpapered. The reasoning appears most sound, and is based on the fact that when glue is spread over wood, countless microscopic air bubbles are trapped

in the wood pores and affect the thoroughness with which the glue will coat the wood fibers.

In the accompanying sketch is shown an enlarged, idealized cross-section of a glue line. The tall conical shapes in the upper left panel represent upright wood 16

APRIL

1960

study of the sketch will show how much difference there will be between the wetting power of a thick glue mix and a thin one. But at the same time, a thin glue can be every bit as unreliable. Because it will dry so slowly,

there is more time for the wood's porosity to absorb the glue and produce "starved" joints. The glue is in the wood pores instead of at the interface. Mix glue exactly as the instructions say!

There is almost always some shrinkage when a glue sets up and this is why glue pockets must be avoided in aircraft work. Quite aside from the fact that glue holds

wood by bonding its fibers together, pockets weaken joints seriously because the shearing stresses set up when they harden and shrink are often great enough to cause self-propagating cracks. Continued on next page

Choosing Aircraft of an airplane color scheme should be T hebasedchoice not only on appearance but on durability, vis-

Color Schemes in this regard. If you plan to put a red stripe on a black

large aircraft factories avail themselves of the services of styling experts, the color combinations seen on production aircraft can be taken as guides to good taste. Often amateurs, in hopes of getting planes that look distinctive, select unusual color combinations which do not look so well at all out on the field because the colors fight or blend poorly. Different colors have markedly varying weathering qualities due to pigment characteristics. From the standpoint of visibility, durability and resistance to chalking, yellows and oranges seem to be superior. Whites, creams, light greens, blues and greys tend to chalk when planes are kept outdoors. When they are used, a few extra coats are suggested to delay the showing-through of undercoating. Waxing also helps appreciably. Reds and maroons fade with age. Maroons and dark blues have a marked tendency to become brittle and crack with age, and attempts at rejuvenation only aggravate the cracking because rejuvenator strikes into cracks and lifts their edges. Reds sometimes "bloom" after application, or develop a slight haze. Normal washing and waxing soon removes this. The covering power of different colors varies, some pigments being more transparent than others. Reds and maroons are quite transparent as compared to yellows and orange. If the first coat of any color seems to hide the aluminum dope poorly, one or two coats above the normal number is a good idea. "Bleeding" occurs when a trim color cuts into and raises an underlying color. Red is a notably bad offender

background, or a black stripe on a red background, a few coats of aluminum in between is insurance against bleeding. Using a dark trim color over a lighter, bleedprone color is another trick. Insignia red, Champion red, Berry red and Stearman vermillion are all bleeders. Pin striping often aids greatly in blending a trim color into the rest of the plane. A yellow stripe on a blue fuselage, for example, shows up more clearly and attractively if it is outlined with a red pinstripe. Red scalloping on the leading edge blends into a white wing with an "unfinished" look, and the addition of a black pinstripe adds the needed punch. It is very simple to put pinstriping on, contrary to popular opinion. Fuselage arrows, wing tip color bands, registration numbers and all other trim areas are outlined with masking tape and masking paper for spraying. If we want to put a black letter on a yellow wing with a silver pin stripe, first the body of the letter is masked out. A few coats of silver are sprayed around the border of the number. When thoroughly dry, narrow masking tape sold for this purpose is laid over the silver dope, the edge of the narrow tape being right up against the outlining tape. Then the black dope is sprayed onto the body of the letter. Pulling the narrow tape off reveals the silver pin stripe, and removal of the main tape and paper reveals the letter in all its glory, pin stripe and all. It is a good idea to remove masking tape as soon as dope loses its wetness. If it is allowed to stay on until the dope is hard, some of the trim dope may tear off with the tape. When tape is left on for a long time it sticks tighter and sometimes pulls all the dope off with it. Therefore it is best to remove tape within a day or two of application. Before spraying, push down tape edges firmly with the thumb to be sure of getting a clean-cut edge. A

More About Glue . . . ANC-19 points out that thin plywood is often made

of the Aerolite 300 glue distributed by Ciba. It is a resin powder which is mixed with water to form a honeylike

ibility and application problems. In the field of art and

decoration there are rules about which colors go well

together and which do not. This information is usually beyond the amateur airplane builder's reach but, since

of very thin veneer and unless real care is used, the pressure of the manufacturing process can force glue to

bleed through to the outer surfaces. This of course would seriously affect the ability of more glue to bond to the surface when the plywood is being assembled to an aircraft structure. When aircraft plywood is being made, the wood's moisture content is carefully controlled to minimize bleed-through. In addition, the heat and pressure of the laminating process can glaze the surface in about the same way as solid wood can be glazed or burred by dull tools. Metal cauls used to separate the several panels put into a press at one time may be waxed to prevent glue adhesion, and this wax can be transferred to the wood if care is not used. Most commercial plywood such as fir is machine sanded after lamination and when stacks of it are received from the factory it is common to find sandpaper dust on the surfaces. For all these reasons, therefore, it is now recommended that plywood surfaces be lightly sanded with fine garnet paper or equivalent before gluing. Of course, when any sanding is done the dust should be removed by some such manner as vacuum cleaning. A nice letter was received from member George Ryning of Calgary, Alberta, in which he speaks favorably

fluid with a shelf life of from about two weeks to three months. It is applied to one surface only, the mating surface being coated with a hardener. When the parts are brought together a very fast setting takes place; it will tear wood fibers in about ten minutes! This makes for speedy production, especially with a multitude of small parts such as gussets. However, George points out that this can be a disadvantage when working with large, awkward jobs unless extra hands are available. This glue can be used in temperatures down to 50° F. as compared to 70° recommended for many popular glues. In chilly garages and basements this can be a definite boon. There is a choice of slow, medium and fast acting hardeners, with setting times of from one to three hours before stressing depending on temperature. Calgary enthusiasts have used it in gliders and airplanes with much satisfaction. George also cautions against using any of the white colored glues now so common and so popular for cabinetmaking and similar uses. They are not recommended for aircraft use, particularly in the primary structure, because they do not stand up well to repeated stressing and humidity. • SPORT

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