The Heinkel He 219 A-2 Uhu (Eagle Owl) is often considered the best and most sophisticated night fighter flown by the Luftwaffe during World War II, and possibly the best night fighter of the entire war. Its advanced operational features coupled with its maneuverability and deadly weapons systems set it apart in German aviation history. The Museum’s He 219 is one of just three of its type that were captured by a team of intelligence officers in 1945 as part of “Operation LUSTY” (Luftwaffe Secret Technology). These three night fighters, along with 21 other captured German aircraft, were shipped to the United States for testing, with the He 219 A-2 eventually being transferred to the Smithsonian in January of 1949.
When it came time to begin restoring this significant piece of aviation history, restoration specialists at the Museum noted that the aircraft was complete, apart from its radar array. The Heinkel's radar was used to find enemy aircraft at night (it is the 4 tv-antennae looking arms out in front of the aircraft's nose in the image above). Being an extremely important component, the radar array would need to be re-fabricated so that the plane could be displayed accurately in the future.
Our restoration specialists are dedicated to finding the most historically-accurate solutions as possible when pieces are missing during restoration of an aircraft. It is important that aircraft are displayed as intact as possible with parts, original or replacement, that tell their stories— their function, use, and historic significance —accurately for the edification of the public. Unfortunately, there was very little information on the design of the masts and dipoles comprising the radar array. Unique to the He 219, these components do not appear in the spare parts manual for the aircraft. Fortunately, fragmented examples from a wrecked He 219 were recovered from the North Sea and loaned to the Museum’s restoration team for analysis. These fragments would serve as the best and only surviving examples from which the fabrication specialists could model their newly fabricated parts.
While fascinating, the fabrication of the radar array in its entirety is far too complicated to cover in a single blog . Here we will focus on the fabrication of the mast elbows, which were formed at the Mary Baker Engen Restoration Hangar using an innovative adaptation of established metalworking techniques.
Because the aforementioned loaned fragments were sheared off of the aircraft during its crash, it was difficult to determine their original length. In order to appropriately scale his newly fabricated parts, welding fabrication specialist Kenny Mills used measurements taken from the available loaned fragments along with archival images of intact radar array.
Once scaled, Mills had to determine how the elbows were originally made. He was told that they were hammered out by hand, but he wasn’t convinced; he noticed that many other parts across the aircraft had been stamped out using a punch and die—a process where a punch (the male or positive image of a particular part) presses a piece of sheet metal into a die (the female or negative image of a particular part) using hydraulic pressure, thereby forming that metal into the desired shape and structure. This would make the most sense considering that almost 300 He 219s were built during the war and there were four elbows per plane. It seemed impossible that, especially during wartime, such an enormous effort would have been made to hand-make each of these elbows. Examining the loaned elbow fragments, Mills could also plainly see that they weren’t hammered out—they lacked the characteristic hammer tracks you would see on a part made in this manner.
Given this overwhelming evidence, Mills decided to make his own punch and die to stamp out new steel elbows. He first consulted Die Design and Diemaking Practice ed. Franklin D. Jones where he read about the use of Cerrobend in diemaking for stamping out aluminum sheet metal parts. He thought he might be able to adapt this established process for stamping out his steel parts because Cerrobend, as an extremely soft metal alloy, would be easily formable.
The first step in Mills’ diemaking process was to make the punch. He meticulously measured the loaned, intact elbows, and made a pattern as well as a set of contour gauges. He drew this pattern on the surface of one inch thick steel plate and drew contours on either end. Then he used an oxy-acetylene torch to cut the pattern out of the plate. Next, he used a hand grinder to shape the punch according to the contour gauges he had made based on the measurements taken from the recovered elbows. He had to repeat this process to make both a left-side and right-side punch to form the two halves of each elbow.
After he was satisfied with the hand-ground punches, Mills needed to cast them to make the dies. To accomplish this, he built two steel boxes and suspended each punch in them. Next, melted Cerrobend was poured into the boxes around each punch. Once the Cerrobend cooled, Mills released the punches from the castings, revealing perfect negative forms.
Because Cerrobend is such a soft material, Mills was worried that the corners of the dies would deform with repeated use. To prevent this from happening, he plated the flat surfaces of each cast die with 1/8 inch steel, tacking the strips into place around the casted forms.
Mills couldn’t use what he had made just yet at there was no room between the punches and their castings to accommodate for the thickness of the steel that would be stamped out using these tools. The punches had to essentially ‘shrink’ an overall even amount in order to accommodate .050 steel. Mills knew he couldn’t remove this overall even amount of material on each punch by hand with any accuracy, so he handed the punches over to metal finishing chemist Dave Hendrick, who had a chemical solution. Hendrick placed the punches in a deoxidizing chemical bath for a calculated amount of time which allowed for an overall even amount of surface material to be removed from each shape.
With fingers crossed, Mills fabricated a jig to hold the punches in the hydraulic shop press. He first did a test using a thin piece of aluminum and it turned out exactly as he had hoped. He was then able to move onto punching the .050 steel halves of the Heinkel elbows. The steel-plated edge of the dies imprinted a line on each piece of steel which he could follow for an exact trim.
After trimming away the excess material, he put the halves together and gas welded them in the same manner as they would have originally been joined during war time. He made several pairs and picked the best four examples to use in the final Heinkel parts.
One of the most satisfying results of the process Mills developed was the characteristic wrinkle that was formed along the radius of each stamped elbow. This same wrinkle is present on the original recovered elbows from the wrecked He 219 that had been loaned to the Museum. His instincts had been correct all along, that these parts had been stamped out rather than hand-formed with a hammer.
The talented metal fabrication and restoration specialists in the Preservation and Restoration Unit accomplished nothing short of greatness in their extensive work re-fabricating the entire radar array of the Heinkel He 219. With historically accurate construction and craftsmanship, they have made this aircraft whole again so that current and future generations may learn from it.
Authors: Meghann Girard (Museum Specialist, Welding Fabrication) and Kenny Mills (Museum Specialist, Welding Fabrication).
Thank you to Rob Mawhinney, Preservation and Restoration Specialist, for providing background context for this blog.