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SHEET METAL—A BEGINNER’S EXPERIENCE PART ONE BY RON ALEXANDER from Aircraft Spruce
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SHEET METAL—A BEGINNER’S EXPERIENCE PART ONE BY RON ALEXANDER

Walk up and down the flight line during any EAA fly-in and you will notice a large number of sheet metal airplanes. These aircraft reflect a technology that has existed for many years and it is obvious that many aircraft builders find this type of airplane attractive and practical to own. You immediately ask yourself—I wonder if I could build a sheet metal airplane? Do I have the needed skills to put one of these airplanes together? How did the builders who have successfully completed one of these airplanes begin?

Let’s assume that you have no knowledge of sheet metal construction. As a matter of fact, when the word “cleco” is mentioned, you think it is the name of a country music singer. In spite of your lack of knowledge, you want to build a sheet metal airplane. You have already decided on the specific type of airplane so after ordering the kit what is the next step?

I would suggest that before you begin unpacking those boxes that will arrive you gather some knowledge and create some skills. You actually have three good choices to help you get started. First of all, you can gain experience and knowledge working with another builder who is building a similar type airplane. Secondly, you can attend an EAA SportAir workshop on sheet metal construction. This will certainly provide you with the basics to get started in addition to giving you a comfort level with sheet metal tools. The third choice involves utilizing a builder’s assistance center. There are several of these around the country that are providing legal assistance for the inexperienced builder. Notice I said you have three “good choices”. That does not mean you cannot simply dive into the project yourself. Is that a wise decision? Probably not. Your chances of completing the project will dramatically improve if you receive some guidance.

Since I have first hand knowledge of a local builder’s assistance center I have the opportunity to interview and observe people who are arriving to build their RV tail assembly. The program is designed to provide builders with the sheet metal skills needed to build their entire airplane. Most sign up for the one week tail assembly program that allows them to learn the basics while actually constructing their tail kit. Recently I observed three people who were building their RV tail kit. Not one of them had any previous sheet metal experience. Each one asked Van’s Aircraft to ship their tail kit to the Georgia location. They personally arrived from various locations around the country to test the aircraft building waters firsthand.

There exist a lot of techniques and procedures for sheet metal construction that you will not find written. The advantage of working with an experienced person is that you will pick up many of these tips and techniques that will make the project easier and allow it to take on a more professional look. If you are doing the project by yourself you will probably learn many of these tips out of experience. The problem is going back and correcting or perfecting the work you have already completed.

Why start with the empennage kit? The beginning phase of any airplane construction project should start with a small piece. That will allow you to practice your skills without risking damage to a large part such as a wing or fuselage. In the case of the RV-7, all of the basic sheet metal skills needed for the entire airplane will be learned and practiced on the empennage kit. If you happen to damage the skin on the rudder, as an example, you simply order replacement skin and rivet it in place. No major delay in the project occurs and no catastrophe that can’t be fixed.

Okay, where do we start? Before you actually receive the empennage kit you should have the manual and plans in your possession. You should review both. Study the plans and note how they go hand in hand with the instructions in the manual. Spend some time trying to visualize what you will be doing. The RV-7 manual actually has a section on reading plans. Review that section. You will start with the horizontal stabilizer. Study the instructions and plans for it. Learn what a solid line on the plan means, a dashed line, etc...

When the boxes and you are both in place, the first step is to unpack and inventory the contents. No, we don’t get the rivet gun out and start riveting. Actually, that will be one of the last steps. Unpacking and inventorying accomplishes two objectives. First of all, you will discover any missing parts that can be ordered right away so you will not be delayed. Second, you will become familiar with all of the various parts as you identify them and organize them. Take out your plans and spend some time locating each part and where it goes. Notice that the parts have a protective layer of vinyl covering them. This serves to protect the metal from scratches, dings, etc. during assembly. The protective layer will be left in place during the first stage of construction.

Next, learn some of the basics of sheet metal construction. If you did not attend an EAA SportAir sheet metal workshop, I would suggest that you build a small practice project. Doing so will allow you to learn how to drill, cleco, rivet, etc. The builder’s assistance program actually devotes a few hours to a discussion of basic sheet metal construction. This is done before you begin working on the project. Building a sheet metal airplane, or pieces thereof, has certain steps that must be accomplished in sequence. Let’s review them.

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 needed. You then reassemble the unit with clecos and then 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
On modern kit aircraft, you will find that very little measuring and marking is required. This is particularly true with the quick-build option. Pilot holes have been drilled in the aluminum skins and on ribs, etc. by the manufacturer. The presence of these holes will save you a lot of time that would normally be spent measuring and laying out pieces. The proper size cleco will fit very snugly through the hole. On a RV project, most of the rivets used will be 3/32 or 1/8 inch in diameter. That means you will need to use either 3/32 or 1/8 inch clecos.

The first part of the empennage our three builders started assembling was the horizontal stabilizer. They reviewed the manual and plans to determine the proper sequence of assembly and what goes where before starting. It is very easy to get out of sequence causing major problems at a later step. Lay out the various pieces in front of you and identify where each goes. Actually mate them together to ensure the proper fit. What you are doing is building a skeleton frame and then attaching skin (aluminum) to it. Most of the parts are ready to assemble. On the RV-7, you will start by assembling the rear spar followed by the front spar.

Some parts, such as the ribs, must be prepared prior to assembly. These must be “fluted”. Fluting is not a term used in conjunction with a musical instrument. Rather, in this case, it is a sheet metal procedure used to prepare ribs prior to assembling them. During the manufacturing process, ribs may be slightly bowed. Before they are installed they must be straightened. This is accomplished by “fluting”. Fluting simply places small creases or “flutes” along the edge of the flange using special pliers. The flutes effectively shorten the flange and pull the rib into line.

Let’s continue to assemble the skeleton structure for the horizontal stabilizer. Identify and cleco together the parts in proper sequence as outlined in the manual. Clecoing is a procedure whereby special fasteners are used to temporarily hold aluminum pieces in place. The predrilled pilot holes will just allow the proper size cleco to pass through. A special tool—cleco pliers—will be used for this purpose. The pliers must be squeezed very tightly to allow the cleco to pass through the pilot hole. (Jack says it helps if you squeeze a tennis ball for a couple of months prior to beginning a sheet metal airplane—you then develop the strength for hours of clecoing). You are basically going to attach the front spar, rear spar, and remainder of the skeleton structure using clecos.

A minor amount of bending will be required on the front spar and on stiffeners that will be used. Does that mean you will need to purchase a fancy sheet metal brake? No, the bending can be done using a vise. Most kit airplanes are designed to be built without the use of expensive tools. It is nice to have a shear and brake but not absolutely necessary. Any bending required on most kit airplanes is 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. Suffice to say that you will encounter very little bending if you are building a metal kitplane. A plans-built airplane will be a different matter. In our next article we will continue the sheet metal building process by discussing how to assemble a horizontal stabilizer.