Combining Contemporary Technology with Traditional Metalworking Techniques
In the Museum’s Preservation and Restoration Unit, we have a variety of metalworking tools that allow fabricators to produce high quality work by hand. In combination, these tools often allow us to reproduce aircraft parts that may have originally been stamped out by machine in series. Using the equipment at my disposal in the Museum’s Mary Baker Engen Restoration Hangar, I recently fabricated missing parts for the Lincoln-Standard H.S. (modified J-1) open cockpit biplane currently undergoing restoration at our Steven F. Udvar Hazy Center.
The Standard J-1 type were used during World War I as training aircraft. After the war, these aircraft became available in great numbers to civilian pilots at low cost. They became popular with barnstormers and exhibition pilots of the 1920s and even served in productive roles such as aerial mapping and forest patrolling. The Museum’s Lincoln-Standard, a three-seat J-1 modified with a Hispano-Suiza engine (hence H.S.), was purchased by the Hunter brothers of Sparta, Illinois, in June 1924 from the Robertson Aircraft Company. Harlan A. “Bud” Gurney, friend and barnstorming partner of Charles Lindbergh, was sent to Sparta to teach the four Hunter brothers how to fly and perform stunts in the airplane. The brothers called it “Old Hisso” and it became a part of their Flying Circus. In total, this airplane had a 44-year service life and is an eclectic mixture of vintage and modern parts. With the goal of structurally stabilizing the airframe for long-term display, restoration specialists at the National Air and Space Museum are currently hard at work bringing the airplane back to a state that accurately reflects aircraft technology of the 1920s.
During restoration, it was discovered that the two steel brackets that attach the wooden supporting cross member for the passenger’s seat to the fuselage were missing. Luckily, we had surviving examples of these brackets from the pilot’s seat that we could use as models for the newly fabricated brackets. Examining the original brackets, we could see that they were formed by one piece of metal with great uniformity. These observations, coupled with the fact that approximately 1,600 J-1s were built during World War I, led us to conclude that the brackets were likely stamped out by a machine. Producing two more of these identical brackets by hand would be a great challenge.
Our goal was to fabricate two replacement brackets in the same manner, as the originals were made out of one piece of metal. Fortunately, we have access to a number of state-of-the-art tools in the Restoration Hangar that would make this feasible. One such piece of equipment is our OMAX waterjet. Using this 5-axis CNC machine would allow me to cut out two identical flat patterns that could then be bent into the same form as the original brackets.
To begin the project, I sat down with a set of calipers and measured every aspect of the original bracket. I took these measurements and created a CAD (computer-aided design) model that could be pathed as a cut file for the waterjet.
I then programed the waterjet to cut out a series of flat brackets out of mild steel the same thickness as the original brackets.
The next step was to come up with a way to bend the flat pieces of steel into bracket form. The original brackets had bends with tight radiuses and it was a challenge to recreate this by hand. We are again fortunate to have access to a Trumpf hydraulic press brake, but even this state-of-the-art piece of equipment has its limitations and would not be able to make all of the required bends. It would take a combination of technologically advanced equipment and traditional manually-operated metalworking equipment to achieve this series of bends. The tools needed included not only the Trumpf hydraulic press brake, but also a manual leaf brake and hydraulic shop press.
After experimenting with several test pieces, the bend sequence was determined. The first bend would be made using the leaf brake as it resulted in the tightest bend radius. The next few bends had to be completed using the Trumpf. Although the Trumpf tooling could not create as tight a radius as the leaf brake, its segmented tooling prevented the collision that would have occurred using the leaf brake for these subsequent bends.
The final and most challenging bend had to be made in the hydraulic shop press, as it was deep and too narrow for any of the other bending equipment to achieve. In order to produce this bend, I had to fabricate my own tooling.
I successfully bent up a number of brackets and chose the best two examples to hand over to the restoration specialists for painting and installation on the Lincoln-Standard H.S.
With the brackets installed, the Lincoln-Standard H.S. is a little closer to being fully restored. By combining contemporary technology and traditional tools, the fabrication possibilities are endless at the National Air and Space Museum.
Author: Meghann Girard, museum specialist of welding & fabrication.
Thank you to Tony Carp, museum specialist of aircraft restoration, for providing historical context for this blog.