The Sheet Metal Airplane
Sport Aviation - 9/97
By Ron Alexander
Once
you have decided to begin the adventure of building your own airplane,
you are faced with what type to build. The choices are almost
overwhelming. Do you build a kit airplane or buy a set of plans? What
type of construction? Composite, tube and fabric, wood, sheet metal or
a combination. In previous articles I have discussed the process of
building a tube and fabric airplane. This article will provide you with
an overview of the procedures involved in building your own sheet metal
airplane. Many high quality aluminum aircraft kits and plans are
available. Hopefully, after reading this article you will better
understand what is involved in building an all metal airplane.
Aluminum
airplanes have been flying for many years. Today they are considered
the standard type of construction for most factory built airplanes. The
aluminum alloys used in the construction of aircraft are very strong
and relatively light in weight. Sheet metal construction is used on a
large majority of aircraft in one form or another. Often tube and
fabric airplanes will have aluminum cowlings, fairings, etc. Even if
you are not considering construction of an aluminum airplane the odds
of using several sheet metal skills in building your aircraft are high.
Most builders will want to have at least a basic knowledge of sheet
metal techniques.
There are
five basic steps in the process of building an aluminum airplane. These
steps are: (1) Planning, (2) Basic building, (3) Assembly, rigging and
systems installation, (4) Inspection, certification, and test flying,
(5) Final painting. Some builders will elect to paint their aircraft
prior to their test flight while others will wait until they have flown
the airplane several hours to see if any additional rigging is needed
or any other problems surface that would require some disassembly.
There are also certain basic tasks involved in most sheet metal work.
These consist of cutting, bending, drilling, countersinking or
dimpling, and riveting. Each will be discussed later.
Let's
assume you have made the decision to build an aluminum airplane -
either from a set of plans or from one of the airplane kits that are
available. Step number one is Planning. I cannot overemphasize the need
to properly plan. Your success in completing your project is largely
dependent upon your planning. You certainly cannot anticipate every
problem that will arise but you can prepare yourself for most of the
stages of aircraft building. Begin by spending a lot of time reviewing
the aircraft plans and/or the assembly manual. A thorough study of the
manual prior to beginning construction will pay dividends throughout
the entire building process. You will know where to begin, what tools
will be needed, space required, safety considerations, etc. Resist the
temptation to uncrate the kit and start working. This is a common
occurrence and one that should be avoided.
Like
any other aircraft building project, an all metal airplane can be
constructed in a space the size of a two-car garage. Obviously, the
more space you have the easier it will be to work. Consideration should
be given to where to store the kit in addition to storage of the
materials that you will use. The kit will arrive in crates and usually
one of the crates will contain sheets of aluminum. I would suggest that
you leave the aluminum in the crate until you use it. Just be sure to
store the crate in a dry area. Aluminum sheets are very easy to damage.
More on that later. A workbench will be needed along with a work table
on which to assemble component parts. Organize your tools and your
hardware as much as possible before you begin. Remember to plan a space
to store completed parts prior to assembly. You will want them out of
the way so they will not be damaged. It may be necessary for you to
rent a hangar space at an airport during the final assembly stage. If
you are working in your basement or garage, noise will be a factor.
Drilling and riveting are not quiet activities. If you prime most of
your aluminum pieces prior to assembly, you must consider the paint
fumes and perhaps even construct a small paint booth out of plastic
sheets. Certainly, if you decide to paint your own aircraft you will
need to build a paint booth and the paint fumes will need to be
eliminated.
When we discuss
tools for sheet metal construction, the bottom line is you cannot have
too many. There is a tool you can purchase for every job. Most kit
manufacturers and designers will provide you with a list of necessary
tools. If they do not, the tool supply companies usually have assembled
a list of needed tools. It is not necessary to invest in a metal shear
or a metal brake. Both of these tools can be very expensive and they
also require a large amount of floor or table space. They are nice
tools to have, but not mandatory. Selection of a rivet gun is
important. You will be spending a lot of time with this tool so make
the selection carefully. Rivet guns are pneumatic tools and the two
most popular for our use are the 2X and 3X guns. The X simply has to do
with the length of the gun. A 2X gun is adequate for driving up to 1/8"
rivets. Above that size you will probably want a 3X gun. A 3X gun
basically hits the rivet slower and harder. Make sure when you squeeze
the trigger of your rivet gun you can vary the strength of the impact.
Try out a rivet gun before purchasing if possible. Squeezing rivets is
also an option. Several rivet squeezers are available, both hand
squeezers and pneumatic squeezers.
You
will need an air compressor capable of delivering 80-100 psi of air
pressure. Your air compressor will be a primary piece of equipment. You
will be using a lot of air tools. Tank size is just as important as
motor horsepower. The larger the tank the more volume of air stored.
This means you will not deplete your air source as rapidly when you are
using your pneumatic drill or a die grinder. Finally, when thinking
about sheet metal tools, think clecos. A cleco is a small metal holder
that is installed in a rivet hole to temporarily fasten two pieces of
metal together prior to riveting. You are going to need a large amount
of clecos, often 500+. They come in different sizes so be sure you
check your manual for the proper ones. Bucking bars, which are used for
riveting, are needed. You never seem to have too many shapes and sizes
of bucking bars. Once again, review the list of tools provided by the
kit manufacturer or tool company for a complete listing.
Two
other major factors are involved in building your airplane - time and
money. The time necessary to build your aircraft is dependent upon many
different factors. It is virtually impossible to define an exact amount
of time. A few elements to consider regarding time are: your technical
knowledge and skills, tools available, type of work you do, assistance
available, quality of assembly manual, family pressures, climate, plans
built or kit built, and on and on. As you can see, there is not a
simple answer for the question of "how much time." As a rule of thumb,
you can figure somewhere between 2,000 and 3,000 hours to assemble an
aluminum kit plane. Concerning cost, the price of the kit itself can be
easily determined from the manufacturer. However, in addition there are
a number of other costs that are not as easily calculated. Some
examples are shipping costs of the kit, cost of the engine, propeller,
avionics, instruments, upholstery, paint, etc. It is advisable to add
up the total cost as best you can so you will not be surprised with the
final number.
In previous
articles I have mentioned four primary reasons that contribute to low
completion rates of airplane building projects. Two of these reasons
are underestimating the total time and the total finances required.
With proper planning you can at least be somewhat prepared for what
lies ahead. As a review, the other two reasons are lack of family
involvement and inadequate technical knowledge. I feel it is essential
that you involve your family. Family members have a lot of
opportunities to assist with sheet metal construction. One of these is
riveting which is usually a two person activity. With the amount of
riveting necessary you will certainly learn to appreciate their help.
Obviously there are many other areas where assistance is necessary and
can be provided by family members.
Try
to gain as much technical knowledge as possible before beginning
construction. There are a number of videos and books available on sheet
metal construction techniques. You can also find builders' groups on
the internet for most airplane types. Individuals within these groups
are usually a very good source for information. Take advantage of EAA
Technical Counselors within your local EAA Chapter. Find someone else
who is building a similar type of airplane and spend as much time with
them as possible. Attend one of the EAA/SportAir workshops enrolling in
the sheet metal class. The training offered will provide you with basic
knowledge and allow you to construct a section of an airfoil out of
aluminum. Remember, lack of knowledge often is followed by a lack of
confidence in your building skills that can lead to shelving the
project.
Now that we are
adequately prepared to begin construction, let's talk about some of the
basics of sheet metal construction. First of all, I will discuss the
actual material used in sheet aluminum. Pure aluminum is rarely used in
sheet form because of its low strength and softness. Instead, alloy
elements are added to the aluminum such as copper, manganese, magnesium
and chromium. These alloys, along with heat treatment, will increase
the strength of the material. The total percentage of alloying elements
is usually less than 5-7%. One of the most common aluminum alloys used
in aircraft construction is designated as 2024-T3.
Copper
is the element used with aluminum for this alloy. It is further heat
treated to obtain optimum characteristics. This type of aluminum is
very strong and it is used mainly for structural applications. The T3
indicates the temper or type of heat treatment. 2024-T3 aluminum
usually has a coating of pure aluminum pressed on as a final layer to
prevent corrosion. This is known as an alclad surface. So when you
purchase aluminum for your project you will see the complete
designation of 2024-T3 alclad aluminum. Care must be taken to prevent
damage or scratches to the alclad surface. It is very easily damaged.
Another
type of aluminum used is designated 3003. This alloy contains manganese
and is used for construction of cowlings and for other non-structural
uses. 6061-T6 aluminum is often used to form attach angles, etc. It is
comprised of aluminum, silicon and magnesium. These three are the most
popular types of aluminum sheets that are used within our industry. The
thickness of aluminum sheets is measured in thousandths of an inch
width, .025 and .032 being the most common.
The
primary type of fastener used in sheet metal construction is the rivet.
Solid shank rivets are most commonly used in aircraft construction and
they consist of a head, known as the manufactured head, and a shank.
After driving the rivet, the resultant driven head is known as a "shop
head." This is simply the shank that has expanded as a result of the
riveting procedure. The most common rivet types that we will use are
the AN426 (MS20426) which is an aluminum alloy rivet with a 100°
countersunk head, and the AN470 (MS20470) which is also an aluminum
alloy rivet with a universal head. It is important to note that all
structural rivets have a small dimple in the middle of the head. This
serves both as identification and as a means to assist in removing the
rivet if that becomes necessary. Rivet diameter is measured in 32nds of
an inch and the length is measured in 16ths of an inch. The most common
rivet sizes used in custom built aircraft are 3/32 and 1/8 inch in
diameter.
Primary attention
must be given to safety during our planning stage. Eye protection is
absolutely essential. Drilling metal, grinding and other tasks, can
create metal shavings that can be thrown into an eye. Be sure to wear
adequate eye protection. A full face shield is recommended when you are
using a die grinder (high speed grinder). Metal shavings from the
aluminum along with pieces of fiberglass from the cutting wheel are
thrown with tremendous speed and impact. A high speed grinder can be
very hazardous if not used properly. Ear protection should be used
during several tasks such as riveting and drilling. When drilling, be
sure your fingers and hands are not in line with the end of the drill
bit. Disconnect the drill from its source of air prior to changing
bits. Avoid loose fitting clothing. Disconnect rivet guns from their
source of air before changing rivet sets. Avoid operating a rivet gun
unless the set is against a piece of wood or a rivet. If you pull the
trigger without having the set against an object, the set can become a
small missile.
As a final
reminder concerning the planning stage, use the checklist printed in an
earlier Sport Aviation that lists the items needed for final inspection
and certification of your aircraft. Begin preparing for the final
inspection from the onset of the project. In particular, the builder's
log needs to be started along with a review of needed paperwork.
The
Basic Building stage (this is what we have been waiting for) is next.
The manual has been reviewed so let's begin work. Begin with a small
part. Most kits start with a horizontal stabilizer or another section
of the tail. This allows you some practice to improve your skills
without the possibility of ruining a large component part. Most sheet
metal construction will be done in phases. Very simply, you will first
cut the metal and bend if needed. Next you drill rivet holes and
assemble the parts with clecos. After initial assembly, you will then
disassemble and prime the part if you so choose. You will also dimple
or countersink if you will be using countersunk rivets. You then
reassemble the unit with clecos and rivet it together. So, the steps
are:
1. Cut the pieces to size and bend if needed
2. Drill holes for rivets and cleco each hole
3. Disassemble the pieces
4. Deburr the holes
5. Countersink or dimple the holes
6. Prime the part - if required
7. Reassemble the pieces together using clecos
8. Rivet the pieces together.
Sounds simple enough, doesn't it? So we have some basic skills and tasks that are required. They are:
1. Cutting
2. Bending
3. Drilling
4. Countersinking and/or dimpling
5. Riveting
6. Removal of rivets
Yes,
removal of rivets. You will probably become very proficient at that
job. There are other tasks that are sometimes needed such as a
procedure called fluting. I will only discuss the basics listed above.
I
will begin with a brief discussion of cutting aluminum. Many tools are
available to cut metal. The easiest and most efficient way is to use a
large shear. Since many of us cannot afford to purchase this tool,
other cutting devices are available. The most common are snips that are
available in three types: right cutting (green handles), left cutting
(red handles) and straight cutting (yellow handles). Pneumatic power
shears and nibbles are also accessible. Needless to say, it is
important that you measure carefully prior to making a cut. Most of the
experts agree that you should use a fine point Sharpie marker to mark
and then make your cut about 1/4 to 1/2 inch away from that mark. This
will allow you to make a second cut closer and then file or polish to
the final dimension. Mistakes in cutting are costly. The old adage,
"measure twice and cut once," applies. After you have made your cut you
will want to smooth the edges of the cut piece using an edging tool and
trim the corners so you will not cut your hand.
Bending
aluminum can pose a problem, however, most parts supplied with a kit
plane are partially bent or pre-bent. Any bending required by most kit
manufacturers can be accomplished using a small brake that can be
constructed from wood. The kit manufacturer will usually show a drawing
of this brake in the manual. A sheet metal brake simplifies the bending
process and is very helpful although not necessary. If you cannot
afford a large brake, several smaller versions are available. A
detailed discussion of bending techniques is out of the scope of this
article. Suffice to say that you will encounter very little bending if
you are building a metal kit plane. A plans built airplane could be a
different matter.
The next
task is drilling holes for the rivets. The size of rivets to be used
along with the required spacing will be presented within your manual or
plans. (Remember, structural rivets will have a dimple on the head of
the rivet.) A pneumatic drill is the best tool for the job. Again, you
will want a good quality drill. You will want to control the speed of
the motor that is only possible with the higher quality drills. Most
people recommend using a pilot bit for the initial hole followed by
drilling with the next larger size bit prior to riveting. As an
example, a 3/32" rivet will not fit into a hole drilled by a 3/32" bit.
Drill bits are designated by numbers with a number 40 bit being the
next size larger than a 3/32" bit. So a number 40 bit would be used to
drill the final hole to accept a 3/32" rivet. If you are using
countersunk rivets, the dimpling process, which will be discussed
later, will often enlarge the hole to accept the rivet without
additional drilling. Support the piece you are drilling with wood or
particle board. Drilling aluminum causes a burr to form on each side of
the piece. These burrs must be removed by a process termed "deburring."
Failure to deburr could cause a separation between the two pieces being
riveted together or it could cause the rivet to not fit tightly. A
special tool is used for this process. Good technique is to drill the
pilot holes in both pieces, clecoing them together, then remove the
clecos and deburr both sides. Clecos are color coded according to their
size and they are applied using a special pair of pliers.
The
next step is to countersink or dimple if using countersunk rivets.
AN426 rivets require a 100° countersink. Dimpling is preferred over
countersinking but it can only be done on thinner metals - .040
thickness or less. Special dimpling tools are available that basically
consist of a set of dies (male shaped to match the rivet head and
female corresponding to the degree of countersink) which are squeezed
together with the aluminum in between. This will press the metal
surrounding a rivet hole into the proper shape to fit a flush rivet. It
is imperative that the rivet fit securely to achieve maximum strength.
The metal is stretched somewhat during this procedure usually opening
the hole to the proper size without additional drilling. Countersinking
is done with a special bit attached to a drill on thicker metals - .040
and thicker.
Next we are ready
to rivet. This is usually accomplished using a rivet gun or a rivet
squeezer. The rivet squeezer is preferred but its use is often limited
because of its design (jaw depth). The shop head (head resulting from
driving or squeezing the rivet) is much more uniform and balanced when
squeezed. Hand squeezers and pneumatic squeezers are available. Driving
a rivet using a rivet gun often requires an additional set of hands.
This is where you can bring the family together for some "quality
time." Riveting requires some practice to gain proficiency. Practice on
scrap pieces before working on the "real thing." The shop head that
results from driving or squeezing the rivet must meet certain criteria.
A properly driven rivet will have a shop head of at least 1-1/2 times
the diameter of the rivet shank in width and about 2/3 of the diameter
in height. Special tools are available to quickly check the rivet for
proper installation.
Removing
rivets is a skill you will learn and practice more than you would like.
We do make mistakes. When the rivet is installed improperly it must be
removed. When you remove the rivet you want to be sure you do not
enlarge the rivet hole. Also, if a rivet is improperly removed the
strength of the joint could be weakened. As mentioned, rivet heads have
a small dimple in them. Use your drill and place the correct size bit
into the dimple. It is best to use a bit one size smaller than the
rivet shank. Start the drill and simply drill off the head of the
rivet. The rivet shank can then be removed by driving it out using a
pin punch or by pulling it out using diagonal cutters.
The
question of corrosion proofing aluminum always arises. Aluminum will
not rust but it will corrode. The alclad layer on aluminum acts as a
shield to prevent corrosion. However, I would recommend that you prime
all surfaces for maximum protection. Some builders will actually prime
their sheets of aluminum prior to cutting. This serves to protect the
aluminum against scratches and nicks during construction. It is
certainly desirable to prime any 6061 aluminum parts or 3003. Neither
of these has an alclad coating which means they are more susceptible to
corrosion. Corrosion proofing new aluminum surfaces is fairly simple.
Clean the surface thoroughly using a cleaner and Scotch BriteTM pad
then apply a conversion coating such as Poly-Fiber E2300 or alodine.
The coating is necessary to provide the adhesion needed for the primer
to stick to the surface.
Etching
is not necessary on new aluminum. Next, apply a two-part epoxy primer -
one light coat will be sufficient. If the surface will receive paint at
a later date it is desirable to apply a somewhat thicker coat that will
provide a surface for sanding subsequent coats of paint. As an absolute
minimum apply a conversion coating for corrosion protection.
The
next step in our building process is the Assembly, rigging and systems
installation. Several kits require a certain amount of assembly and
rigging prior to completion of a component part. As an example, the
wings are often mated to the fuselage and the ailerons rigged prior to
final closing of the wing skins. Try to install as many systems as
possible before you assemble the aircraft. Fuel lines, hydraulic lines,
instruments, etc. can usually be installed. The tail is often assembled
and rigged first. Then, of course, the completed wings are installed.
At that time the engine, cowlings, canopy, etc. will be placed on the
aircraft.
Next comes the
fourth phase, Inspection, certification and test flying. This phase has
been covered completely in a previous article. The last phase is the
final painting of the aircraft. As previously discussed, some builders
will elect to paint the airplane prior to assembly, some will paint it
before they test fly the airplane, and others will test fly and then
apply the final paint. Any one of these methods is acceptable and it
depends upon your choice.
You
will find the building of a sheet metal aircraft to be a very
challenging and rewarding experience. Mal Harper of Griffin, GA is
building an RV-6 and is about 2/3 finished with the project. He is a
retired airline pilot who now has a considerable amount of time to
devote to the project. His wife, Mary, also provides a lot of
assistance. Mal currently has 1700 hours on his project. He is
fortunate in that he lives on a community airstrip where he has a
hangar and workshop. Plenty of space and nice tools make the job much
easier.
I have presented an
overview of building a sheet metal airplane. Certainly this is not
intended to be a technical article on sheet metal construction, rather
a summary of the most common skills required. Hopefully, the contents
will be useful to you if you are undecided about what type of airplane
to build or if you are simply undecided about building at all. I would
encourage you to begin the building process. I do not believe you will
find a more fulfilling project. The completion rate on sheet metal
airplane projects appears to be high. A large number are completed and
flying. Certainly the performance of the majority of these airplanes is
impressive and the cost is reasonable. What are you waiting for?
