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The Sheet Metal Airplane
By Ron Alexander (originally published in EAA Sport Aviation, September 1997)
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:
4. Countersinking and/or dimpling
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 Brite™ 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?