When you buy an all-metal homebuilt kit, you generally get a whole lot of cut sheet metal parts, usually with rib and bulkhead flanges already bent for you, and several large sheets of aluminum that have been bent with beautiful leading edge curves. These tail and wingskins are often the largest parts in the box, and when builders have problems during construction, they dread making a mistake that might lead to replacing one of these skins because they can just see the dollar signs associated with shipping such pieces one at a time. And trying to bend a long, large-radius piece using home tooling seems impossible—but only because they haven’t seen it done!
One of the neat things about visiting kit manufacturers is the chance to see how they make parts. In a recent visit to Hummel Aviation, makers of the Hummel Bird, UltraCruiser, and H5 kits, we got a chance to observe a very simple technique that they use for making these skins with long, beautiful bends—and you’re going to be surprised at just how easy they have made it. In fact, you’re going to be shocked at just how simple it really is. All you need is a flat table, a few strips of 1×2-inch wood to make a frame the size of the table, some heavy-gauge plastic painter’s sheet, and an old vacuum cleaner.
Building the Table
The Hummel vacuum table has a sheet of plywood for a top, with a hole cut near one end to fit the suction hose from an old, tired home vacuum. Inserted into the top of the hole is a PVC angle (available at almost any hardware store) to help direct the airflow through the part. Their frame is a simple rectangle, big enough to enclose the largest part that needs to be bent—in most cases, a leading-edge wingskin. The heavy, translucent plastic is wrapped around the frame at the edges and stapled in place. The frame is then hinged to the back of the table to allow it to be flipped up. Frankly, the technique would work without the hinges, but they probably only cost a couple of bucks at the hardware store, so why not make your vacuum table the envy of the neighborhood?
Making the Bend
With the table complete, you take the flat skin that has been cut to size and curl it around into a teardrop shape, taping the edges together with duct tape. If this is going to be a small-radius part, like a stabilizer skin, place it in the vacuum table so that the taped edge and the future leading edge are parallel to the tabletop and located near the vacuum hole angle fitting. Now close the lid, making sure that the vacuum hole is not blocked, and turn on the vacuum.
Watch what happens next: The plastic is sucked down against the tabletop and workpiece, then slowly, the workpiece is squeezed down by the plastic sheet. The vacuum is not all that powerful when measured in psi—but multiply the psi by the area of the piece, and figure out the forces involved! You’ll be surprised at just how flat it becomes. When it appears that the compression has ceased, turn off the vacuum and open the lid; the new leading edge will have taken a nice set, and the final curvature will be determined when you attach the ribs that go underneath the skin.
If you need a large-radius curve for a wingskin, insert a suitable round pipe (PVC will do) into place before beginning the operation. The table will still suck it down tight, but this time, around the pipe, making for a much more gentle bend. With the vacuum turned off, you’ve got a wing leading edge! The key to remember is that you are not trying to bend to specific molded shape—you are just putting in a nice curve and letting the ribs set the shape during assembly.
At some point, the work piece will stop collapsing—that is time to turn off the vacuum, as the work has been done.
With the vacuum turned off, the plastic will billow, releasing the workpiece, which will have taken a nice set.
With the tape removed, the piece will expand, but the leading edge has been formed and will conform to the ribs when assembled.
So the next time you are deciding whether or not to replace a damaged skin, and you find shipping costs becoming a driving factor in your decision, remember that “rolling your own” is a very viable option. And, of course, if you’re building from scratch, or to plans, the vacuum table could quickly solve a great number of forming problems. This is true homebuilding at work. Not only do you make the parts—you make the tools.
Photos: Paul Dye