construction of aircraft
By Ron Alexander
During the fall of 1997, I wrote a
series of three articles on composite aircraft construction. These
articles provide an overview of composites as they relate to aircraft
building. The articles began with the October 1997 issue of Sport
Aviation. I am going to again focus attention on this popular method of
aircraft construction by discussing in more detail each step involved in
building a composite aircraft. A certain amount of review will be
necessary to achieve the goal of explaining the steps involved in this
type of building.
Once you have made the decision to build a composite aircraft, either a
kit aircraft or a plans-built, the first step is to set up your workshop
space, purchase the necessary tools, and organize materials and parts.
Once you have made the decision to build a composite aircraft, either a
kit aircraft or a plans-built, the first step is to set up your workshop
space, purchase the necessary tools, and organize materials and parts.
To begin this discussion it is important to note that you do not need a
pristine laboratory to build a composite airplane. Like most aircraft
building projects, if you have a 2-car garage you have what is needed. It
has been my experience that having your workshop in or near your home
solves two problems. First of all, you will be much more likely to spend
time on the project after getting home from work versus having to drive 30
minutes to another location. This equates to more hours on the actual
project. Secondly, your family is more likely to become involved. This is
very important if you are to successfully complete the project.
If you had an ideal composite shop you would have a "clean room" for doing
layups, cutting cloth, etc. and a "dirty room" for sanding operations.
Most of us do not have a partition in our garage so we must be careful
during our sanding operations not to contaminate our work. Sanding should
be accomplished after completed parts are cured and covered-not just after
doing a fresh layup.
You will need a table on which to cut your reinforcement fabrics (usually
fibreglass). Since most of your fabric will be cut on a 45-degree bias, it
may be handy to have a table set up just for that. You can shape the table
by cutting one end at a 45-degree angle to facilitate cutting on a bias.
The table should be wide enough to handle the fabric you will be using (60
inches should be enough). You should be able to unroll about 4-5 feet of
fabric on the table. You will want to place a hard plastic cutting surface
on the top of the table to allow you to cut the fabric with a cutting
blade. (More about cutting fibreglass later.) This material can be
1/8-inch thick high-density polyethylene or something similar.
Another table can be constructed to do your resin mixing and basic layups.
This table should be roughly 3 feet x 8 feet depending upon the amount of
space available. The length of the table needed will also vary with the
aircraft you are building. The table should be placed in an area that will
allow you to walk completely around it. In addition, some builders prefer
to have another smaller table dedicated to mixing resins. After completing
a part you should remove it from the area if at all possible or hang it
from the ceiling.
A large thermometer should be placed where you can view it along with a
humidity indicator. As you will learn, temperature and humidity control is
very important when mixing and working with resins. Ideally, you should be
able to control the temperature of your workshop. This, of course, is not
always practical. Place a large clock with a sweep second hand on the wall
where you can see it while working. The clock is always running on your
resins after they have been mixed. You will have only a certain amount of
time with which to apply the resin before it begins to gel. Of course, you
need a first aid kit and an eye wash station. The eye wash station must be
Proper ventilation of the work area is necessary. When working with resins
or when sanding you will want to move the air through the workshop space.
A fan can be set up to move the air outside the workshop. If you really
want to do it right mount an exhaust hood over your layup table. This is
not that difficult to do and is very effective in removing fumes created
from the resins when you are working with them.
Storage of materials, parts, etc. must be addressed. If you are building a
composite kit aircraft the pre-moulded parts must be carefully stored.
Wing panels, as an example, can bend and adapt to any shape to which they
are subjected. Warping can result from improper storage. The best way to
store parts is to simply leave them in the shipping crate in which they
arrived. You may also want to save the shipping materials from the crate
to use as padding, etc. for completed parts.
Resins should be stored in a warm area if at all possible. When the
temperature is less than about 65 degrees resins become thick. The colder
the temperature the more thick the resin. That means you will have
difficulty pouring the resins from their container. Several builders have
designed heated areas within their shops to store resins if the shop
itself is not maintained at a normal temperature. If resins are stored in
extremely cold temperatures they are susceptible to crystallizing. This is
not a major problem and can be corrected by placing the resin container in
a pan of water and heating the water to about 160 degrees F or so until
the crystals dissolve. Resins may be stored for several years prior to
being used. This is termed their "shelf-life". However, with epoxy resins
the accompanying hardener usually has a shelf life of less than one year.
Vinyl ester resins often have even less time for shelf life especially if
they have been promoted prior to shipment.
Most of the tools you will need to build a composite airplane are readily
available and somewhat inexpensive. The following is a partial list of
tools you will need:
Scales, mixing pump, or balance scales to mix resin
Saws hacksaw, coping saw, and pad saw
Electric hand drill
Grooved laminate rollers
Knives-including utility knife and large serrated knife
Other tools that are nice to have consist of a Dremel tool with bits for
shaping and cutting, a die grinder, drill press, band saw, rotary or
orbital sander and the list can go on. The tools I have mentioned are
specific to composite construction. You will also need basic tools and
usually some sheet metal tools for a small amount of riveting, etc. The
best way to determine the exact tools you will need is to read the kit
manufacturer's assembly manual or the designer's plans. They will almost
always provide you with a list of basic tools needed to construct their
Now that we have established what kind of workshop space you will need
along with several of the tools that are required lets get down to the
basics of construction. I will talk about each type of material used in
composite construction and how to work with each separate one. After we
have established a foundation, in up-coming issues I will discuss the
proper methods of doing a composite layup, methods of bonding and tape
glassing, forming hardpoints, post curing, and most of the activities you
will become involved in if you decide to build a composite airplane.
If you want a complete review of basic composites I invite you to read the
previous articles I mentioned in the beginning of this article. I do want
to briefly review some of the materials used in composite construction
with an emphasis on how to work with each one.
Let's begin with the core materials that usually consist of some type of
foam. Polystyrene is the first core material that will be discussed.
Polystyrene comes in large blocks and is normally used to form large
structures such as wings, control surfaces, etc. If you are building a
plans-built airplane you will build a large portion of the airplane out of
this material. Polystyrene can be cut with a knife, saw, or it can be
"hot-wired" into the shape of an airfoil. Usually the latter will be
called for in the plans. You can find plans for a "hot-wire" device in the
Rutan booklet called Mouldless Composite Sandwich Homebuilt Aircraft
Construction available from supply companies. This device is easily
constructed from common materials. Templates are made from the aircraft
plans you receive and are used as a guide in cutting the foam to proper
shape. One thing in particular when working with all foams and especially
with polystyrene foam, the cells or voids in the foam must be filled prior
to applying the reinforcement material. This is accomplished by mixing a
slurry compound or using a commercial filler manufactured by Poly-Fibre
called "SuperFil". This is the first step in the layup process that will
be discussed in detail later. It should also be noted that vinyl ester
resins will dissolve polystyrene foams therefore they are not used with
this type of core material.
Most of the kit aircraft use either polyurethane (urethane) or polyvinyl
chloride (PVC) foam. These foams come in different densities and
thickness. Usually the thickness will be from about one-quarter inch to
two inches or so. With most kit aircraft the large airfoils will be
partially completed and you will simply be required to construct ribs,
bulkheads, etc. and glue them in place. These foams are easily cut with a
knife or saw. DO NOT HOT-WIRE URETHANE FOAMS. They will emit poisonous
gases if hot-wired. They are also flammable. Do not burn the scraps of
material left over as the same gases are emitted. Sanding blocks are used
to shape foams. Band saws and routers may also be used to cut and shape.
Honeycomb cores are used in several kit aircraft. You will usually not be
required to work with this material, as the kit manufacturer will supply
the completed parts that use a honeycomb core.
This is a term used for the fabric materials found in composite
construction. We will find three different types of materials used in most
composite aircraft. They are fibreglass, Kevlar, and carbon fibre
(graphite). fibreglass is the most commonly used material. It has the best
physical characteristics at the lowest price.
Without going in to great detail, there are a few basic things you really
should know about fabrics. fibreglass is made up of filaments of glass
that are twisted together to form a yarn. This yarn, or fibre as it is
often called, is then woven into certain styles of fibreglass. When the
weaver looms fibreglass they use terms such as "warp", "fill", and
"selvage edge." See Figure 1. Warp defines the fibres that run the length
of the fabric as it comes off the roll. The warp direction is designated
as 0 degrees. Fill fibres run perpendicular to the warp fibres. They are
designated as 90 degrees. The fill fibres or threads interweave with the
warp fibres. Selvage edge is the woven edge produced by the weaver to
prevent the edges from fraying. Some of the new fabrics today appear to
not have a selvage edge. The edges have been stitched with a lightweight
With unidirectional fibreglass, all of the major fibres run in one
direction. All of the strength of the fabric is found in that one
direction. The fill often consists of threads designed to hold together
the glass fibres. A common term for this glass is "uni". It is
manufactured in both glass cloth and in tapes. A common style number used
by many composite airplanes is designated as 7715. This cloth is typically
used where the primary loads are in one direction such as a spar cap.
In this glass, the major fibres run in two directions, both the warp and
the fill. In other words, instead of using threads as a fill, glass fibres
are used. Thus we have glass fibres in both 0 degrees and 90 degrees. In
other words, the cloth has half of the fibres in one direction and half in
the other direction at right angles. This means that the cloth has the
same strength in both directions. This type of cloth is commonly called
"bid". Of course, there are many different styles and weaves that are
available. 7725 and 7781 are two very common cloths used in amateur-built
aircraft. In your plans they will often be referred to as bid cloth.
Bid cloth can be stitched together in more than one layer to form what is
known as biax cloth or triax cloth depending upon the number of layers
involved. The most important thing for you to understand it that you must
use the type and style of cloth called for in your plans. Do not
experiment with cloths. The designer has specified the cloth to use based
upon structural analysis. Use what they tell you to use.
Keeping it simple, I am not going to discuss all of the different weaves
of cloth, etc. that are available. You can read Andrew Marshall's book,
Composite Basics, for a good discussion of this. I want to concentrate on
the basics you need to know to safely build your airplane.
Handling & Cutting Fiberglas
First of all, you must be careful when handling fibreglass. Remember to
cut the glass in a clean area. Do not drop fibreglass on the floor. It
will be contaminated with dirt and debris. If your fibreglass gets wet do
not use it in the structure. Be careful when handling fibreglass as its
shape can be easily distorted. Mark the cloth using a Sharpie marker.
These marks will not show through the final finish. Your plans will
usually require you to cut your cloth at a 45-degree angle. This is done
to achieve maximum strength in the final structure. So we will usually be
cutting the glass on what is referred to as a 45-degree bias. You need a
sharpie marker, a straight edge, a measuring device, and a good pair of
scissors or a rotary cutter. When you make a cut, allowance for small
deviations is usually built into the dimensions. If you are within
one-half inch or so that should be good. As you make a cut the cloth may
slightly distort. If so, it can be carefully pulled back into its proper
shape by pulling on an edge. Cutting can be done using a good pair of
scissors or a rotary cutter or they are sometimes referred to as a roller
blade. Many people call this a pizza cutter-which is a term for the rotary
cutter-it is not a real pizza cutter. Get a rotary cutter from one of the
After you have cut the cloth to the proper dimensions, carefully roll it
into a fairly large roll. In other words, do not roll it tight. This is
the best way to transport the fabric to your structure. We will see how to
apply it later. If you pick it up by the ends it will distort and not fit
the area of the part correctly. It is also important to note that the
selvage edge must be removed prior to applying it to the structure. (Note:
this will not apply when using the type of fibreglass without a selvage
edge.) Cutting on a 45-degree bias will cause a certain amount of waste.
However, it is necessary that you cut this way to achieve maximum
strength. By the way, the angle is not critical. You do not have to
measure it accurately. Eyeing it will work fine. Let me emphasize that you
must cut the fabric in the orientation called for by your plans.
To emphasize the importance of the resin matrix I would like to quote
Andrew Marshall from his book Composite Basics. "Basically, the resin
matrix is the key to the whole operation of producing composite
structures. It was noted earlier that the resin matrix is the mass in
which the fibres exist, but the resin does much more than just contain the
fibres. Its primary job is to carry the load from one fibre to the next,
and from the bundles of fibres or groups of reinforcements into an
adjacent structure which may either be embedded in the composite during
manufacture, or adhesively bonded to it at a later stage. The resin
material thus distributes and transfers the load within the structure so
that each reinforcing fibre carries a proportional share of the load."
There are two types of resins that are most commonly used on composite
aircraft. They are vinyl ester resins and epoxy resins. I am not going to
discuss polyester resins, as they should not be applied on aircraft except
for very limited non-structural use.
Vinyl Ester Resin
This type of resin is used by several of the kit manufacturers. Vinyl
esters are low in viscosity making them easy to use. The cure time can
also be easily affected simply by adding more hardener thus speeding up
the cure time. Despite the cure time, hardened vinyl ester usually
exhibits consistent properties of strength and flexibility. Working time
with vinyl ester resin is dependent upon the ambient temperature and the
amount of catalyst that is added. Vinyl ester resin is less expensive than
epoxy and it will withstand high temperatures without post curing.
The negative side of vinyl esters results from the mixing process. Vinyl
ester resin must be "promoted" prior to mixing the catalyst. It is
promoted using a chemical called cobalt napthenate (CONAP). This chemical
must be added into the resin before catalyzing. Vinyl ester resin is
catalyzed using a chemical called methyl ethyl ketone peroxide (MEKP).
CONAP and MEKP mixed together prior to being placed in the resin can cause
a fire or explosion. You will not encounter this hazard as long as you
remember to place the CONAP into the vinyl ester resin prior to adding
MEKP. Extreme care must also be taken when using MEKP. This chemical is
very dangerous to the eye.
Overall, vinyl ester resins provide an easy to use, strong, high
temperature, and inexpensive resin. Skin irritation problems are also less
likely to occur than with epoxy resin. Just remember to take proper
precautions when you are mixing vinyl ester resins. Be sure not to mix
CONAP with MEKP and always wear a face shield when using MEKP.
Epoxy resin has come to dominate the aerospace industry and it is widely
used on custom-built aircraft. Epoxy resins differ from vinyl ester resins
in that they harden through a process known as "crosslinking". Epoxies are
packaged in two parts: a resin and a hardener. Unlike vinyl ester resin,
the mixing ratio of resin to hardener is critical. Adding more hardener
will not accelerate the cure time, in fact, it may seriously impede the
curing of the resin resulting in less strength of the final cured part.
Different types of epoxy resins are available. Again, use the type of
epoxy called for by the designer. Working time may be varied using
different types of epoxies. A 5-minute epoxy is commonly used to simply
hold two pieces together for further bonding. These epoxies set up within
5 minutes and should not be used for structural purposes. Structural
epoxies will have a working time of approximately 45 minutes depending
upon the type of epoxy and the ambient temperature.
Proper skin protection is a must with epoxies due to skin dermatitis that
can be caused by the chemical. In the next issue I will discuss how to
properly protect your skin from this problem. How to mix fillers and the
actual process of completing a composite layup will also be presented.
Many applications of composite construction require a filler material to
thicken and/or reduce the density of the resin mixture for various
purposes. The resulting mixture of the filler plus the resin is used to
form a fillet to provide a radius where two composite pieces are joined
together. Fillers are also used to seal the cells of foam. The slurry coat
is used to fill the cells with a lower density material than that of pure
resin. Fillers are also used to thicken a mixture so it can be applied
without running, to enhance the strength of resin material for structural
bonding, and to fill the weave of fabric during the composite finishing
process. Mixtures may also be used to fill any gouges or dents in the foam
core. Corners are also constructed using a filler material. Several
different filler materials are used with resins. The more popular ones
will be discussed.
Microballoons as they are often called are nothing more than very minute
spheres of glass, microscopic Christmas tree bulbs provide an accurate
analogy. This material is very lightweight and very easily suspended in
the air. Care must be taken when working with microballoons not to inhale
any of these glass particles. Quartz "Q cells" is another type of
microballoon called for in the plans of several kit aircraft. When either
of these forms of filler is mixed with a resin material the resulting
mixture becomes lighter in weight with less strength. This mixture is
commonly referred to as "micro". Micro is usually mixed in three different
thicknesses. First is a slurry consistency. This is usually a 1 to 1
mixture by volume of microballoons and resin. This provides a mixture that
is almost the same viscosity as resin by itself. Slurry is used to fill
the cells of the foam prior to applying the first layer of cloth. The
second type of micro is usually termed "wet-micro". It is thicker than
slurry and is used to join blocks of foam together. The mix ratio is
approximately 2-3 parts of microballoons to 1 part of resin. The third
type of micro is called "dry micro". This mixture requires about 5 parts
of microballoons to 1 part of resin and it is used as a filler material.
Micro must NEVER be used between plies of a layup as the final strength
will be severely decreased.
Flocked Cotton Fiber
This particular filler material, usually called cotton flox, is also mixed
with resin. It consists of finely milled cotton fibers that provide an
adhesive when properly mixed with a resin material. The mixture is termed
"flox". Flox is usually mixed about 2 parts of filler to 1 part of resin.
A popular use for flox is to reinforce a sharp corner to provide more
strength within that area. It is used in filling sections that require
structural strength. It has much higher shear qualities than micro but is
much harder and heavier.
As the name implies, this filler material is made by milling fibreglass
into a very fine consistency. Milled fibres have a higher strength than
cotton flox. The mixture of milled fiber and resin is used as a structural
filler. It is also often used to form a fillet that requires structural
integrity. Milled fibers and resin are used to form a "hardpoint" on a
fibreglass structure. The hardpoint is used to attach other structures to
the fibreglass. Care must be taken when working with milled fibre due to
the very fine particles of fibreglass that can penetrate the skin.
This material is the same as milled fibres, except it is available in
different lengths. This allows its use as a filler for very specific areas
where greater strengths are needed.
Cab-O-Sil is fumed silica that acts as a material to thicken a resin.
Small amounts should be used. Larger amounts can act to inhibit the curing
agents of some epoxies when used in concentrations greater than 15% by
weight. Using Cab-O-Sil simply keeps a resin from running when you are
applying it to a difficult area.
Poly-Fiber manufactures a substitute for dry micro called SuperFil. This
filler material is mixed to the exact same consistency with each batch. In
addition, it has talc added that facilitates the sanding operation.
SuperFil may be used as a filler for virtually any material including
metal, wood, and fibreglass. The epoxy in SuperFil has been optimized for
the filling process. Micro normally uses resin optimized for the
An important point-when you are mixing filler materials, always mix the
resin and hardener thoroughly prior to adding the filler substance.
A review of the safety issues involving composite construction is in
order. One of the most important issues regarding safety when working with
composites is skin sensitization. Many people become sensitized to resins.
This is more common with epoxy resin than with vinyl ester resin.
Regardless of the type of resin you are using you must protect your skin.
Wear long sleeve shirts and protect your hands using a form of glove. What
type of glove to wear is controversial. Many people can simply use a latex
type glove found in drug stores. However, a number of people are allergic
to the powder often found inside the latex glove. Vinyl gloves are
available and provide a very good alternative to latex. Rubber gloves are
used by many people who place a cotton liner inside the glove. Several
builders use barrier creams such as Invisible Gloves with success. No
matter what you use change gloves often or recoat with creams often. Never
wash your hands with solvents. Use soap and water.
Have adequate ventilation so you are not breathing the fumes from resins.
A small fan will assist in moving the air out of the area. You also should
wear a respirator. This is important when doing layups and also when
mixing fillers. Those tiny spheres of glass called microballoons will do a
number on your lungs if inhaled. Particles of fibreglass resulting from
sanding operations should not be inhaled.
Vinyl ester resins pose a different type of problem. They have chemicals
that should not be mixed together outside of the basic resin chemical. The
catalyst used with vinyl ester, MEKP, is destructive to the eye. A face
shield is preferable to use when mixing MEKP with the vinyl ester resin.
Again, skin sensitization is not as common when working with vinyl ester
as when working with epoxies.
Always acquire and read the Material Safety Data Sheet for the material
you are using. These MSDS sheets will explain the hazards of each type of
resin or solvent you are using.
Finally, mixing too large a quantity of a resin can cause a problem known
as exotherming. The exotherm process is a consequence of the chemical
reaction that takes place as a resin hardens or cures. This chemical
reaction causes heat to be generated which in turn speeds up the chemical
reaction causing even more heat to be generated. If you mix a large batch
of resin you can create an "out-of-control exotherm." The container
holding the resin will get so hot from the chemical reaction that you
cannot hold it. The resin may actually bubble or boil and you will see
smoke rise from the substance. You can prevent this by mixing small
quantities of resin (8-10 ounces by volume). If you see that you are
getting an out-of-control exotherm you should immediately pour the resin
onto a sheet of plastic. This will allow the heat to more readily
dissipate into the air. The exotherm process can actually cause a fire if
the container is thrown into the wrong place.
A similar type problem can occur when putting foam blocks together if too
large a micro joint is allowed. The foam is a good insulator and the heat
will build without escaping. This can melt the foam and cause a core void.
Now that we have set the stage and we understand some of the basics, let's
get to the fun part-doing an actual layup. First of all, what is a layup?
It is probably more accurately defined as a laminate. A laminate is one
layer of reinforcement material impregnated with resin and usually added
to a core material or to another layer of reinforcement material. This
process is commonly referred to as a layup. If you are building a
plans-built airplane you will become very proficient in doing layups. In a
plans-built composite airplane you actually build most of the parts of the
airplane and then bond them together. Building parts requires a lot of
layup work. On the other hand, if you are building a kit aircraft you
usually will only be required to bond the already completed parts
together. However, you will still use the layup procedure for many
activities on a kit aircraft.
The most important thing I want to recommend prior to our discussion is
for you to do practice layups before doing the real thing. Any experience
you can acquire doing basic layups will enhance the quality of your work
on the actual airplane. Attend one of the EAA/SportAir composite workshops
and make all of your mistakes while learning in a classroom setting. No
Before you actually begin the layup procedure you must be prepared. You
should have everything on hand before you begin. This means gloves,
respirator, mixing cups and sticks, scales or pump, squeegees, brushes,
rollers, etc. Be sure the squeegees you are using have a smooth edge. If
not, pass the squeegee over a sanding block to smooth it. The actual part
itself must be ready for the layup. The cloth should be cut and ready to
apply. The foam should be vacuumed clean of any debris. Temperature and
humidity control is important. Begin by heating the shop, if necessary,
and ensure the resin is warm (ideally 90 degrees F. or higher). The shop
should be cleaned if you have been doing a sanding operation. Control of
cleanliness is essential. If you are working on a large surface you may
want to have someone to assist you. This is a good way to involve a member
of your family. They can mix resins and maintain clean hands to move parts
or do other activities that require cleanliness.
If you are bonding parts together you may encounter peel ply that was left
in place by the kit manufacturer. Peel ply on a completed part is often
difficult to see. You must remove this peel ply material prior to
proceeding. The parts will not bond together if done over peel ply. The
parts that are supplied with a kit have usually been manufactured in a
mould and by the time you receive the part the resin has fully cured. This
is important to the builder because the surface of a cured part must be
prepared differently for an additional layup or bonding. This type of bond
is called a secondary bond. Secondary bonding is the process of bonding
together previously cured composite parts using a wet layup process. You
should prepare the part according to the instructions provided by the kit
manufacturer. This usually involves some type of sanding of the surface to
remove any glossy areas. 180 grit sandpaper is often recommended to abrade
the surface. Care must be taken to not damage any fibres.
Filling Cells of Foam
If you are doing a layup on a new piece of foam the cells of that foam
must be filled to provide enough surface area for the cloth to stay in
place and to achieve a strong bond. This also prevents excess resin from
flowing into the core material and adding unnecessary weight. Polystyrene
foam must be filled prior to application of the first layer of cloth. Some
of the high-density foams do not require this filling step. Again, follow
the directions of the designer. A slurry mixture of microballoons and
resin is generally used to apply this first coat of material. SuperFil may
be used very successfully to fill the cells on polystyrene foams. We will
discuss the mixing procedure for slurry later in the article.
Cutting the Cloth
This subject was discussed in the previous article. As a quick review, you
should use a Sharpie pen to mark cloth. Cut the cloth according to the
directions provided by the manufacturer. Usually this will involve cutting
on a 45-degree angle. Remember to be very careful with the cloth as you
are cutting it and while applying it to the structure. It is easily
damaged or distorted.
Now that we have everything ready to go we will mix the resin material.
Use only non-waxed cups usually the 8-ounce or 16-ounce size. Remember
that you are only going to mix small quantities. If you do mix any large
quantities the resin should be immediately poured into smaller containers.
A large amount of resin will create an acceleration of the chemical
reaction-hence an exotherm. Exotherm temperatures can easily exceed 200
degrees F. and may actually damage the foam core itself.
The total amount of resin to mix depends upon the weight of the cloth that
you are applying. You should try for a 1 to 1 ratio by weight of cloth to
resin. In other words, weigh the cloth you are applying and mix a
corresponding amount of resin. You will usually mix somewhere between
50-100 grams of resin at a time. If the kit manufacturer states that you
should use a resin pump then use that method to mix your resins. Be aware
that you should be careful of clogging or air bubbles that sometimes can
occur with a pump. Balance scales are also used to mix resins. The
important fact to remember is that you must be accurate in your mixing.
This is particularly true with epoxy resins. Do not adjust hardeners to
change cure rates in epoxies. The cure rate of vinyl ester resins is
easily adjusted during the mixing phase. Again, refer to the directions
for the specific resin material.
(I want to clarify a procedure mentioned in last month's article. If you
encounter a resin that has crystallized you can use the following
procedure to solve the problem. Put the can of resin in a container that
will not melt. Remove the cap of the resin can and place the can in heated
water to about 160 degrees for the length of time required to dissolve the
crystals. You can then safely use the resin after it has cooled.)
Back to mixing. After you have carefully measured the resin and hardener,
mix the two together for a minimum of 2 minutes. Take a mixing stick and
cut the end at a 90-degree angle so it will reach the corners of the
mixing cup. You must use a non-waxed mixing cup. Otherwise the wax from
cups will mix with the resin. Stir the mixture spending about 20% of the
time scraping the sides and corners of the cup to ensure adequate mixing.
Do not mix too aggressively, as air bubbles will form. If any air bubbles
form allow the resin to sit until the bubbles dissipate. Placing resin
with bubbles in suspension on a layup can create a void of resin in the
laminate. After you have completed mixing your resin leave a small amount
in a cup so it can cure. This will provide a good test to see if the resin
is curing properly. After a couple of days scratch the resin in the cup
with a knife. It should leave a white mark if it is suitably cured.
After the resin is completely mixed pour some of the resin over the
surface you are working on. Use your squeegee and spread the resin over
the surface. Then place the reinforcement cloth in place at the proper
orientation called for in the plans. Be very careful not to distort the
cloth. Use a squeegee and your protected hands to ensure the cloth is in
the proper place. Then, using a squeegee begin to press gently from the
centre of the cloth making sure you move the squeegee in the same
directions as the fibres of the cloth. Keep the fibres straight and press
the fabric into the resin while working the resin up through the cloth. Be
careful not to distort the fibres. You can use a brush and a roller to
assist in this process. After you have worked most of the resin through
the cloth pour on the remaining resin over the top of the cloth and work
it into the fibres. When the layer appears to have a nice even sheen that
is flat you have a good layup. You do not want any air bubbles. Work air
bubbles to the edge of the laminate to make them disappear. You can also
use a brush that has been trimmed to stipple resin into areas that do not
appear to have proper coverage or into problem areas.
If white spots appear in the laminate the cloth has not been properly wet
out. A lighter colour could also indicate an air bubble. Careful use of an
ordinary hair dryer will change the viscosity of the resin enough to allow
it to flow into certain areas. Do not hold the air dryer in one place for
any length of time-keep the hair dryer moving. Otherwise, it can create a
void if you leave it in one place.
When pulling the squeegee, excess resin will accumulate in front of it.
Scrape this off into the mixing cup. Pressure applied to the squeegee
varies with the type of resin, temperature, etc. Also, holding the
squeegee at a 45-degree angle or less will move less resin. Holding it at
90 degrees or more will move more resin. Remember that the clock is
running all the time on the working time of the resin. Normally, you will
have 30 minutes or so to work until the resin begins to gel. This of
course is dependent upon the type of resin, temperature, etc. Practice
will make this entire process easy and understandable. Again, do several
practice laminates prior to beginning on the actual structure. After doing
this you will easily perfect your own technique of doing quality layups.
Inspection of Laminate
The laminate should be thoroughly inspected for air bubbles, any trapped
air, excess resin, and of course dry areas or resin starved areas. Hold a
light at different angles to observe any problems such as resin starved
areas (not enough resin indicated by lighter colour) or resin rich areas
(too much resin indicated by darker or more glossy areas). When complete
the laminate should have a nice even sheen. Have someone else inspect your
work. They may see something you have overlooked. Inspect carefully for
any delamination problems.
I am attempting to convey to potential builders the very basic knowledge
necessary to construct a composite airplane. Composite building is not
difficult. It simply requires a fundamental knowledge of the basics. When
you undertake the building of a composite aircraft, the plans or assembly
manual will guide you through the process. The basic skills needed for
this type of construction consist of 2 primary items: knowledge of how to
do a basic layup and knowledge of how to bond pieces of material together.
Building a composite airplane from a kit is similar to building a model
airplane. You glue the pieces together. Now, obviously the gluing
procedure for an aircraft is much more critical and sophisticated than
with a model but the basic principles are very similar.
Peel ply is a polyester or nylon cloth material applied to the completed
laminate while the resin is still wet. This cloth will not adhere to the
layup thus allowing it to be peeled off at a later time, hence the words
"peel ply". The application of peel ply is suggested when you are going to
complete another laminate at a later time. If you are immediately going to
apply another layer of cloth this step is not necessary. Peel ply provides
an added benefit of absorbing excess resin from the composite skins.
Assuming you are going to apply another laminate later, or you are
completing the final laminate, you will want to place peel ply onto the
completed surface. Cut the peel ply to the proper size and lay it over the
laminate while the resin is still wet. One layer of peel ply is all you
will need. Use a squeegee and a brush to work the resin up through the
peel ply. You may have to add a small amount of resin to get the peel ply
to bond adequately to the laminate and to completely impregnate the peel
ply and thus fill the weave. After ensuring the peel ply is saturated onto
the layup, set the piece aside to cure. After the resin has cured you must
then remove the peel ply. This is very important! Failure to remove peel
ply will result in an unsafe bond of the next layer of reinforcement
material. (Note that a number of kit manufacturers will ship pre-moulded
parts that still have peel ply attached. It is imperative this be removed
prior to bonding the pieces together.)
After removal of the peel ply you will see that the laminate is very
smooth and requires little preparation for the next layer of cloth or for
the finishing process. The resulting surface is actually fractured
somewhat leaving it better prepared for additional bonding or painting.
Small glossy areas will be present on the peel-plied surface requiring
abrading with 180 grit sandpaper or Scotchbrite pads. Without using peel
ply, the composite surface will require extensive sanding or filling to
prepare it for bonding or painting.
Bonding is not a new process in aircraft building. In fact, bonding has
been used in aircraft construction since the very beginning. The technique
of gluing wood structures together has been used for years. Many of the
same gluing elements found in wood is also found in composites. The term
bonding, as applied to composites, is used to describe a common method for
joining composite structures. Bonding is the process in which previously
manufactured component parts are attached together during assembly of the
airplane. Bonding composites can also be compared to welding metal. It is
designed to be a permanent joining method. Several important points must
be considered in bonding. We must know how much strength is needed in the
joint, the bonding area required, what type of material must be used to
provide the adhesion, and the procedure used to apply the bonding
material. Preparing the surfaces that are to be bonded together is also
crucial. As stated earlier, the majority of composite kit aircraft require
some type of bonding procedure.
The first method of bonding used in amateur-built aircraft involves a
four-step process. The first step is to cut and trim the component parts
to get the proper shape and fit. The second step is to position the two
pieces together. This can be accomplished by using temporary jigs or by
temporarily gluing them together with a non-structural adhesive. Third, we
must fill any gaps that may exist as a result of butting the two pieces
together. The final step consists of actually creating the structural
joint using wet (resin laden) strips of reinforcement material (usually
fibreglass) bonded over the area connecting the two components together.
If we are bonding together two pieces that are perpendicular to each other
as in figure 1, then we must create a fillet.
The strength of a joint that is joined by a fillet is derived from the
reinforcement material and not the fillet itself. The fillet is needed to
prevent the reinforcement fibres from making a direct 90-degree bend
without any radius. Composite materials must have a bending radius just
like sheet metal. The number of strips of reinforcement material laid down
over the fillet determines the strength of the bond.
An example of the type of construction explained is found in mating a wing
rib to the wing skin. Another example is placing a bulkhead into a
fuselage. Both of these are common types of construction techniques used
when building a kit composite airplane.
The second method of composite bonding is termed "adhesive bonding".
Adhesive bonding involves assembling component parts together using a
structural adhesive in place of resins and fibreglass. Structural
adhesives range from pre-formulated, two part mixtures that are in paste
form to structural laminating resins that are mixed with flocked cotton or
milled fibre to provide the necessary strength. The first method of
bonding discussed uses laminating resins and reinforcement material to
create a bonding overlap. Adhesive bonding requires the bonding area to be
formed into the part when it is moulded. This is usually accomplished by
lowering one side of a part and raising a side of the second part. This
allows the two pieces that will be bonded to slide over each other
providing a precise fit. The joint that is formed when the pieces are
joined in this manner is referred to as a "joggle." With this type of
overlap the builder is required to lay down the structural adhesive and
apply some clamping pressure.
Some kit manufacturers prefer to combine both bonding methods to achieve
the greatest possible strength. The key to achieving strength in any joint
is to properly prepare the surfaces that will be joined. The laminating
resin or structural adhesive must bond well to the surfaces. The surfaces
must be cleaned properly and sanded.
You will often hear the term "secondary bonding" used in composite
construction. This type of bonding simply refers to the bonding together
of previously cured composite parts using the methods outlined above.
Secondary bonding is commonly found in most composite kit aircraft. It
requires proper surface preparation. Prepare the surfaces according to the
instructions provided by the kit manufacturer. Usually, the surface will
be abraded using 180-grit sandpaper or a Scotchbrite pad. Each of these
will provide the proper surface preparation without cutting or damaging
Steps of Bonding
When you receive your kit it will usually consist of many pre-moulded parts
that need to be bonded together. Sounds relatively simple-and it
is-providing you carefully follow instructions. You must first of all
remove any peel ply, prepare the surfaces, and then the pieces must be
properly jigged to maintain an accurate alignment. Then the actual process
begins. So, let's take the steps one at a time. We will use a simple "T"
bond of 2 pieces of material to illustrate the steps.
Most of the construction process of a kit aircraft involves secondary
bonding. This means it is critical to properly prepare the surface. With a
plans-built airplane or a kit airplane where you have just completed
building a part, the piece is already prepared for the bonding step.
Assuming you are working with pre-moulded parts, you must abrade the
surface to ensure an adequate bond. Failure to do so will result in an
unsafe bond. We have discussed this process earlier. Prepare the piece
according to the instructions of the kit manufacturer. They will usually
have you use sandpaper or Scotchbrite pads to scratch up the surface. 3M
Rol-loc disks also work very quickly to prepare glass surfaces for
bonding. You will want to make sure you have the proper fit between the
pieces. A certain amount of sanding may be necessary to ensure this fit.
You do not want any gaps between the pieces that are to be bonded
together. The pieces must then be thoroughly cleaned to remove any
contaminants. Often, residue from a mould release compound will be present
on the piece. This must be removed. Acetone is often recommended for the
initial cleaning followed immediately by a dry rag. The part should then
be cleaned with soap and water to remove any solvents and then dried.
Again, follow the directions of the kit manufacturer. I will amplify on
the cleaning process in the next article.
Tack the Parts Together
The next step in the bonding process is to mate the pieces together and
glue them in place using a non-structural glue. (Figure 3). This simply
allows you to begin the bonding process. You can use 5-minute epoxy, hot
glue, or instant glue to hold the pieces together. The parts only need to
be tacked in just enough areas to hold them in place. This is not the
final bonding of the pieces-it is simply a method of holding them together
while we actually complete the bonding operation. None of the glues
mentioned should be considered as structurally sound. Hold the pieces
together until the glue sets up. Figure 2 shows our 2 pieces glued
together using 5-minute epoxy. Assembly instructions will often require
the use of clecos, screws, or clamps to attach the pieces together for the
Note: As a reminder, remember to remove any peel ply that may be present
on the component parts prior to bonding.
Create a Fillet
Once the temporary bond has hardened, a fillet needs to be made. This
fillet provides a radius for the reinforcement material that will be
bonded on next. The fillet alone is not strong enough to bond the parts
together. Dry micro or SuperFil is used to make a non-structural fillet.
Structural fillets, if required, are made by substituting microballoons
with cotton flox.
Creating a fillet is relatively simple. Mix the SuperFil or micro and
place it in a sandwich bag or in the middle of a piece of plastic. Close
it up and snip a small hole in the bottom of the bag. (See Figure 4). This
is similar to a cake-icing dispenser. Now squeeze the mixture from the bag
along the corner area where the pieces are joined. A small amount is
sufficient. An optimal fillet will have about a 3/16-inch to 5/16-inch
After placing the SuperFil along the fillet area, take a tongue depressor
and smooth the mixture into the corner area. Rounding the end of a tongue
depressor with a pair of scissors will provide the exact size fillet you
desire. Use the tongue depressor holding it perpendicular to the fillet
and not leaned fore or aft. (See Figure 5). Remove any excess material
that may have formed near the fillet along the sides of the pieces. This
can be done using the tongue depressor. You do not want any micro or
SuperFil where the glass will be applied except at the fillet itself. The
completed piece should have the appearance of a smooth fillet. You are now
ready to bond the pieces using reinforcement material.
In our example, we are going to use fibreglass to complete the bonding
process of our two parts. This is often referred to as "tape glassing." On
your project, you will complete this process according to the
manufacturer's instructions. Usually at least 2-3 layers of cloth will be
placed between the two pieces. Once the glass tapes are in place, the load
path between the two pieces will be complete.
Wet layup strips of fibreglass cut at plus/minus 45 degrees are used for
bonding nearly all components together. The most simple and clean way to
make the layups is to pre-impregnate the material with resin while it is
between two sheets of plastic. Clean 1 or 2-mil plastic drop cloth
material works well for this. First, determine the total size for all
pieces you will need. Obtain a piece of fibreglass slightly larger than
this total size. Next obtain two pieces of plastic and cut them 3-4 inches
larger than the fibreglass both in length and in width. Draw lines, using
a Sharpie marker, on the plastic to form the necessary strips of cloth
that will be the exact length and width needed. Flip the plastic over so
the resin is not placed on the marks. Mix the required amount of resin
necessary to saturate the cloth. Pour the resin over the plastic and place
the fibreglass on top of the resin. Next place the second piece of plastic
over the resin.
Using a squeegee, work the resin into the fibres through the plastic. In
other words, you will be placing the squeegee on the plastic, not on the
cloth. This enables you to keep everything clean and neat. Wet out the
fibres completely just like any other layup. You can now pick up the
entire piece of material and handle it without getting resin everywhere.
The next step is to use standard scissors and cut out the tapes you will
need along the lines on the plastic. (See Figure 6). As you cut the
strips, draw the scissors slightly toward you. This will enable you to
make neat, easy cuts.
Next, lightly moisten the area to be laminated (on our "T") with resin
using a brush. This will ensure that the bond is not resin-starved. Remove
the plastic from one side of the tape. Place the strip down with the
remaining piece of plastic facing up. Use a squeegee over the top of the
plastic to remove any air bubbles and to smooth the resin evenly. After
the tape is in place you can then remove the top piece of plastic. The
process is then repeated for additional layers of cloth. (Be sure to
remove the plastic). Plans usually call for the pieces of reinforcement
material to be stepped out with succeeding layers. In other words, if the
first layer is 2 inches wide the next layer would be 3 inches wide. The
widest piece will be on the top.
Thoroughly inspect the piece for air bubbles and resin starved areas.
As you will see from the completed piece the tape is providing the
strength of the bond. This is a very efficient and effective method of
bonding two composite parts together. Again, it is a commonly used
technique for installing ribs in wings or bulkheads in a fuselage. Use of
the plastic is not necessary, but it does allow you to remain neat and
The final step is to place peel ply over the material. Laminate a strip of
peel ply over the surface and allow the resin to cure. This will eliminate
the sharp edges that will otherwise result from the fibreglass material.
Remember to remove the peel ply after the resin has cured.
Joggles are simply joints that have been pre-moulded to fit precisely
together. They overlap each other and are usually bonded together using a
structural adhesive. This type of construction is very common in the
mating together of fuselage parts. After bonding the parts together at the
joggle, reinforcement material is usually applied for added strength.
Often, you will be required to trim excess material off a joggle prior to
bonding. Usually you will place the two pieces together and then drill
holes to allow for the installation of clecos. (The same clecos used for
sheet metal construction.) Some instructions call for the use of clamps or
even strips of wood glued on the surface to hold it in place and to
maintain proper alignment. This will often be done in a jig to ensure
alignment of the parts.
After the pieces are mated together, and the proper fit attained, you will
then mix the structural adhesive. Structural adhesives are usually in a
thick paste form. They consist of a Part A and a Part B mixed according to
instructions. You want to be sure the ambient temperature is at least 60
degrees +. Most of the adhesives have a working time of 1-2 hours at 77
degrees F. Be sure you are ready to glue prior to mixing the adhesives.
Remove the clecos or other fasteners as you apply the adhesive to both
parts. Instructions will often tell you to replace the clecos with rivets
after applying the adhesive. The rivets are later drilled out after the
adhesive cures. The resulting holes are then filled. Fiberglas strips are
usually applied as a final step.
This provides you with a very basic idea of how to accomplish composite
bonding. The key to doing this correctly is to practice. Cut a few pieces
to form a "T" and bond them together until you perfect the process. This
will save you a lot of problems when you begin working on the real thing.
preparation of composite parts
Above, I outlined a brief procedure for preparing composite parts prior to
bonding. This step is most important and needs to be amplified. The
quality of a bond is directly affected by the preparation of the two parts
being joined together. If contamination exists on either part the bond may
be weakened even to the point of subsequent failure. Let me emphasize that
you should follow the directions found in the kit manufacturer's manual
regarding proper cleaning techniques. However, the preparation procedure
is important enough to warrant more detailed discussion.
First of all, when bonding to an outside mould surface (such as many of the
parts you receive from the kit manufacturer) cleaning and sanding of the
parts is always required. When aircraft parts are moulded a release agent
is applied to the inside of the mould itself allowing the part to be
removed when cured. This mould release agent must be removed prior to any
bonding activity. The agent is barely visible. Water will usually remove
this agent. After removal of the agent and any contaminants sanding is
Any surface that is smooth because of being next to a mould must be sanded
prior to bonding. Any primer that may be present must also be removed.
Sanding is generally the accepted way to prepare the surface. Opinions
vary on the proper grit of sandpaper to be used. Usually 80 grit to 180
grit is recommended. Our workshop experience has shown that 180 grit
sandpaper is usually satisfactory to prepare the surface. Use of 180 grit
will ensure the underlying fibres are not damaged or cut. The surface
should be thoroughly abraded (roughed) to completely remove any glossy
Abaris Training, located in Reno, Nevada, instructs the military,
airlines, and aerospace industry on composite construction and repair. I
consult with Mike Hoke, the President of Abaris, regularly concerning
composite construction. His company is considered to be one of the leading
composite training companies in the United States. The following quote was
taken directly from their training manual regarding surface preparation.
"High surface energy is the goal, not mechanical roughness. One must shear
up the top layer of molecules on the surface, creating many broken bonds,
without damaging or breaking underlying fibres. A water break test can be
used to determine surface energy. If surface energy is high, clean
distilled water will spread out in a thin uniform film on the surface, and
will not break into beads. If a water break free surface can be maintained
for 30 seconds, one has achieved a clean, high energy surface suitable for
bonding. If the surface is contaminated or at low energy, the water will
break into rivulets and bead up.
Note that tap water will not work. It is dirty enough to contaminate the
surface itself, and one will never pass a water break test using it.
It is important to note that the "high energy" condition, once achieved,
is short-lived. Within about 2-4 hours the effect is lost. In composites,
one should therefore wait as late as possible in the process before
surface abrasion is performed, so that all else is ready and the adhesive
can be quickly applied."
Dry the water off of the laminate with a hair dryer prior to applying the
adhesive. If it is wiped with a cloth it will likely contaminate the area
again. Do not use a heat gun for this process. The heat is too intense and
may damage the cured resin.
This process also applies to peel ply surfaces. Even though a peel ply
surface fractures the top layer of resin, it leaves a glossy, low energy
surface in the weave pattern of woven cloth. This must be abraded for
So, how should you clean parts prior to bonding? The best procedure is to
simply sand the surface, as discussed, and follow by a thorough cleaning
with soap and water. If you are using solvents, use them initially to
remove contaminants and then abrade the surface. Follow by soap and water
and then immediately dry using a hair dryer. Remember to begin the bonding
process within a few hours after preparing the surface.
Sometimes when working with epoxy resins, you may encounter what is
referred to as an amine blush. The development of an amine blush is most
visible under high humidity conditions. An amine blush is a surface effect
resulting from the curing agent reacting with Carbon Dioxide (CO2) in the
atmosphere rather than the epoxy resin. The by-product of this reaction is
a compound that forms on the surface of the curing resin and readily
absorbs moisture from the air. Under high humidity conditions, it will
cause white streaks to appear on the surface of the resin and the uncured
laminate. During cure, the white streaks usually disappear, but left
behind will be a greasy or oily residue. Sometimes, this residue appears
in the form of sweat like droplets. This residue is water-soluble and will
wash off with warm water. Depending on the severity of the blushing event
there may even be areas of surface tackiness. This tackiness is only on
the surface, and will not effect the overall properties of the cured
Amine blush must be removed before any additional laminates are initiated.
Sanding will remove blush but it will also quickly gum up your sandpaper.
Wiping the surface with a warm wet rag prior to sanding will reduce the
The best approach is to avoid amine blush altogether. Some resin systems
are inherently resistant to developing amine blush. And for others, it may
seem impossible to avoid it. But there are some things you can do to
minimize it greatly. Number one and foremost is - DO NOT use unventilated
combustion type heating sources to warm your shop. Gas or kerosene fired
salamander heaters produce copious amounts of CO2 and H2O. These are the
primary ingredients needed for producing an amine blush. So, use electric
heaters or ventilated exhaust type combustion heaters to keep your shop
You should avoid mixing resins or doing any layups if the temperature is
less than 65 degrees F. If you do a layup at this temperature you should
immediately move the part into a warm room for curing. Purchase a
thermometer and a humidity indicator and place them in your work area.
Avoid mixing resins and working with resins if the temperature is below 65
degrees F or if the humidity rises above 80%. The best solution is to
place an air conditioning unit in your workshop area.
You can reduce the susceptibility to blush in the following ways:
- Work in the prescribed environmental conditions.
- Use "dry" and ventilated heating sources
- Use peel ply. Amine blush usually forms on the outer-most portion of a
layup. By using peel ply the amine blush is removed when the peel ply is
- Cap all resins as soon as possible. This reduces their exposure to the
- Use a resin with demonstrated blush resistance. Some resins are more
susceptible to blushing than others blush.
Use of peel ply, purchasing a blush resistant resin, and working in the
right temperature and humidity will all work together to minimize amine
Often you will be required to mechanically attach another piece to a
composite structure. One method of doing this is to fabricate a "hardpoint".
If you mechanically attach a piece to a fibreglass part, the fibreglass
must be reinforced in the area where it will be fitted to accept the loads
imposed by the attachment. An example of a hardpoint is found on the
GlaStar airplane. A welded fuselage frame is placed inside a pre-moulded
fuselage shell. The two are attached using machine screws that are placed
through hardpoints fabricated in the fibreglass shell.
The most common method of fabricating a hardpoint is to route out a small
amount of foam core material between the inner and outer laminates of the
shell. See Figure 1. You must be sure not to remove any of the
reinforcement material on the outer and inner shells. A piece of piano
wire bent 90 degrees and placed in a drill works well for this step. The
core material may then be removed using a shop vacuum. After the core
material has been removed, a mixture of resin and milled fibre is injected
to fill the void. After the material is injected through the drilled hole,
a small piece of tape may be applied to keep the resin mixture from
escaping. After curing, this material provides the strength needed to
serve as an attach point. You must ensure that the entire area is filled
with material and no air bubbles are present. After the material
completely cures, a hole is drilled through the reinforced area to receive
the screw or bolt.
This is one example of a hardpoint. Various kit manufacturers use
different methods. Complete instructions on fabricating a hardpoint will
be included in your assembly manual.
Post curing is a process used to obtain increased strength from a resin.
If an epoxy resin is allowed to cure only at room temperature, its
ultimate strength is rarely achieved. Post curing will increase two
critical performance properties of an epoxy, chemical resistance and heat
resistance. Fuel tanks constructed using an epoxy will benefit
considerably from post curing. Post curing the entire airplane will
increase overall resistance to the heat build-up inside the airplane
resulting from the high temperatures found on any ramp in the summer. This
build-up of heat can reach the glass transition temperature causing a
weakened state of the resin itself.
To understand post curing, it is necessary to define the term glass