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Heinkel He 219 A-2 Uhu (Eagle Owl) is generally regarded as the best and most advanced night fighter flown by the Luftwaffe during World War II, and may also be the best night fighter in the entire war. Its advanced operating characteristics, coupled with its maneuverability and lethal weapon system, make it unique in German aviation history. The He 219 in the museum was one of only three such aircraft captured by a group of intelligence personnel as part of "Operation LUSTY" (Secret Technology of the German Air Force) in 1945. The three night fighters, along with the other 21 captured German aircraft, were transported to the United States for testing, and the He 219 A-2 was finally transferred to Smithsonian in January 1949.
The file photo of Heinkel He 219 shows the complete radar array. (NASM (Si-2006-20931))
When it was time to start repairing this important piece of aviation history, the museum's restoration experts pointed out that in addition to the radar array, the aircraft was completed. Heinkel's radar is used to find enemy aircraft at night (in the picture above, it is 4 TV antennas with arms outstretched in front of the nose of the aircraft). As an extremely important component, the radar array needs to be remanufactured so that the aircraft can be accurately displayed in the future.
Our repair experts are dedicated to finding the most accurate solution possible when parts are lost during aircraft repair. It is important that airplanes display the original or replacement parts as completely as possible to accurately tell their stories—their functions, uses, and historical significance—in order to educate the public. Unfortunately, there is very little information about the design of the masts and dipoles that make up the radar array. These parts unique to the He 219 will not appear in the aircraft's spare parts manual. Fortunately, a fragment sample of a wrecked He 219 was found from the North Sea and lent it to the museum's restoration team for analysis. These fragments will serve as the best and only surviving examples from which manufacturing experts can simulate their newly manufactured parts.
Although fascinating, the manufacturing of the entire radar array is too complicated to be covered in a blog. Here, we will focus on manufacturing mast elbows, which are innovatively adapted from Mary Baker Engen Restoration Hangar using proven metal processing technology.
Since the above loaned fragments were cut from the plane when the plane crashed, it is difficult to determine their original length. To scale his newly manufactured parts appropriately, welding manufacturing expert Kenny Mills used measurements taken from available loaned debris and archived images of the complete radar array.
Once scaled, Mills must determine how the elbow was originally made. Someone told him that they were knocked out by hand, but he didn't believe it. He noticed that many other parts on the plane were stamped out with punches and dies-a process in which a punch (a positive or positive image of a specific part) presses a metal plate into a mold (a female or negative image) Specific parts) use hydraulic pressure to shape the metal into the desired shape and structure. Considering that nearly 300 He 219s were manufactured during the war, and each aircraft had four elbows, this would make the most sense. It seems impossible, especially in wartime, it takes such a huge effort to make each of these elbows by hand. Examining the loaned elbow fragments, Mills can also clearly see that they were not hammered-they lack the characteristic hammer marks you see on parts made in this way.
In view of this overwhelming evidence, Mills decided to make his own punch and die to stamp out the new steel elbow. He first consulted the editor of Die Design and Diemaking Practice. Franklin D. Jones read there about using Cerrobend to stamp aluminum sheet metal parts in mold making. He thinks he might be able to stamp steel parts using this established process, because Cerrobend, as an extremely soft metal alloy, is easy to form.
The first step in the Mills mold manufacturing process is to manufacture the punch. He carefully measured the entire borrowed elbow, and made a pattern and a set of outline gauges. He drew this pattern on the surface of an inch-thick steel plate and drew outlines on both ends. Then he used an oxyacetylene torch to cut out patterns from the board. Next, he used a manual grinder to shape the punch based on the contour gauge he made based on the measurements taken from the recovered elbow. He had to repeat this process so that the fists on the left and right formed two halves of each elbow.
After he was satisfied with the hand-ground punches, Mills needed to cast them to make molds. In order to do this, he built two steel boxes and suspended each punch in them. Next, pour the molten Cerrobend into the box around each punch. After the Cerrobend cooled, Mills removed the punch from the casting, revealing a perfect negative.
Since Cerrobend is a very soft material, Mills is concerned that the corners of the mold will deform with repeated use. To prevent this from happening, he plated 1/8 inch of steel on the flat surface of each mold and fixed the steel strips around the mold.
Mills could not use the product he had just made because there was no space between the punch and the casting to accommodate the thickness of the steel stamped with these tools. In order to accommodate 0.050 steel, the punch must essentially "shrink" an overall uniform amount. Mills knew he couldn't manually remove this overall uniform amount of material from each punch accurately, so he handed the punch to Dave Hendrick, a metal finishing chemist, who had a chemical solution. Hendrick placed the punch in the deoxidation bath for a calculated time so that an overall uniform amount of surface material can be removed from each shape.
With fingers crossed, Mills made a jig to hold the punch in the press in the hydraulic workshop. He first tested it with a piece of thin aluminum, and the result was exactly what he hoped. He was then able to continue punching the 0.050 steel half of Heinkel's elbow. The plated steel edge of the mold has a line printed on each piece of steel, and he can accurately trim it according to this line.
After trimming off the excess material, he put the two halves together and gas-welded them in the same way they were originally connected during the war. He made several pairs and chose the best four examples for the final Heinkel part.
One of the most satisfying results of the process developed by Mills is the characteristic wrinkles formed along the radius of each stamped elbow. The same wrinkles appear on the original elbows recovered from the wrecked He 219 loaned to the museum. His intuition has always been correct, these parts are stamped out, not handmade with a hammer.
The talented metal fabrication and restoration experts in the protection and restoration department have achieved extraordinary results in the extensive work of remanufacturing the entire radar array of the Heinkel He 219. Relying on the historically accurate structure and craftsmanship, they made this aircraft complete again, and current and future generations can learn from it.
The Heinkel 219 airframe on display at the Steven F. Udvar Hazy Center in Chantilly, Virginia. (NASM)
Authors: Meghann Girard (museum expert, welding manufacturing) and Kenny Mills (museum expert, welding manufacturing).
Thanks to Rob Mawhinney, conservation and restoration expert, for providing background to this blog.
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