Combat-Vet P-51D Sierra Sue II - A New Benchmark
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Thread: Combat-Vet P-51D Sierra Sue II - A New Benchmark

  1. #1

    Combat-Vet P-51D Sierra Sue II - A New Benchmark

    The restoration of the combat-vet P-51D "Sierra Sue II" was completed a couple weeks ago, and it raises the benchmark in authentic Mustang restorations to a new height. It is the most authentic Mustang restoration to-date, and I thought I would make a thread about it for those that may be interested.

    This aircraft was originally assigned to the 402nd Fighter Squadron, 370th Fighter Group, of the 9th Air Force. The aircraft was the personal mount of Lt. Bob Bohna, who would name the aircraft "Sierra Sue II". The aircraft entered service in the European Theatre in March of 1945, and the aircraft joined the Swedish AF following WWII, as Fv26152. In the mid-1950's, the aircraft went to Nicaragau, where it served that country's air force until 1961 when it was put on display. In 1970, the aircraft was returned to the U.S., and somewhat rebuilt into a racer, #19. In 1977, the aircraft was purchased by Roger Christgau, of Minnesota, who had the aircraft returned to general stock-configuration and painted, loosely, as it had been during WWII, as "Sierra Sue II" (as well as a few markings on its starboard-side, as a tribute to its Swedish AF service). Roger Christgau was well-known throughout the midwest for his air show displays flying "Sierra Sue II". He owned the aircraft from 1977 all the way until 2011, when, in failing health, he sold the aircraft to Paul Ehlen (also from Minnesota), who is no stranger to the warbird scene. Ehlen had the aircraft sent to the then newly founded Aircorps Aviation, a restoration company located in Bemidji, Minnesota, for a ground-up restoration to 100% stock condition, to make the aircraft just as it was in April 1945. Aircorps specializes in super authentic warbird restorations, and through its employees and owners, has a tremendously large span of experience in authentic warbird restorations. Just prior to the start of the restoration of "Sierra Sue", Aircorps was involved in the detailing of the combat-vet P-51D "Twilight Tear" restoration - the aircraft, that for a weekend, they were able to mount actual functioning .50-cal machine guns to, and fire them off into a purposely-built gun range/pit (I'm sure most/all have already seen the YouTube video).

    From the very start of the restoration, Aircorps Aviation got a hold of all of the original NAA P-51 blue print microfilm and digitized and cataloged it all on their computers. All of the original drawings have on them the serial numbers from the production blocks they each applied to, and, as also utilized at the NAA factories, they recreated the P-51D production flow-chart for the specific production block that "Sierra Sue" had been part of (an early P-51D-20-NA), providing a specific plan (mapped out to each drawing/group of drawings), down to every last detail, exactly all that was in the aircraft originally (which differed, sometimes quite a bit, from one production block to another), and how each part was to be finished at the specific time the aircraft was manufactured, as specified in the drawings and via North American finishing specifications. Within the airframe itself, all of the details from the areas that still had yet to be touched since original manufacture were recorded and matched in exact detail. Unlike most all of the other very authentic Mustang restorations, which use modern paints to match the original primer and paint shades, in the restoration of "Sierra Sue" they used primers and paints matched to the actual chemical make-up used during the war, with the results being a look which is quite a bit different than the modern paints - rather than being smooth, the period-specific primers and paints have a slightly rough finish, much like an authentic Army Jeep or tank finish. The finish can also scratch/scuff and stain quite easily, and thus the finished restoration is not void of such imperfections, just like the aircraft were new from the factory. On all of the ribs and other parts which were dipped in primer at the factory, rather than sprayed, there are streaks in the finish just as there originally was. There are also some big voids of primer in regions of the aircraft where from the factory the parts were simply left bare metal. A lot of the metal throughout the aircraft (either new or original) were covered or re-covered in the same period-authentic Alclad watermarks as there was on the aluminum when new in '44 - which has been done to a limited degree on a few other past restorations, but not to the extent as seen on "Sierra Sue". The result is that, even in the areas where there was an effort to remove them on the exterior, as there would have been at the factory, remnants still remain, as there would have been (rather than a perfect clean finish). All of the paint applications, such as the cockpit interior green paint application, were applied only at the correct intervals in the rebuild as to the point that those paint applications were made at the factory - the result being, that since the cockpit wasn't painted interior green until after the fuselage was assembled, there is ghosting (just as can be seen in preserved examples) where the paint cannot easily reach past all of the already assembled brackets and ribs (where the paint can't reach is either primer or bare metal depending on how each part was individually finished prior to the fuselage being assembled), and there is also over-spray in other areas where the same level of care (or lack there of) was made in keeping the paint spray contained. Some of the parts added only after the assembled cockpit was painted, are left in the finishes that they were - either simply primer coat only, or a different shade of interior green (from a different batch), or no paint/primer at all. In a similar fashion, all of the stencils that had been applied originally as ink stamps (such as the vast majority of all of the exterior stencils) were applied as such in the restoration, actually using a full collection of the original stamps - the appearance of them, with their imperfections, is another detail that is not found on any other warbird restoration. Other stencils (such as in the cockpit) are either applied as transfers or as sprayed-on stencils as per original, and not always perfectly aligned (as preserved examples show). A few of the really cool and rare stencils, found on the armored firewall panels and the armor plate in front of the engine, are those which were applied by the companies those parts were originally manufactured by, stamped out right before shipping them off to North American. Unlike the other Mustang restorations, which have used modern rivets since the original rivets are no longer produced, Aircorps contracted with National Rivet, based in Wisconsin, to make the rivets to original specifications, just as the company had done during WWII. As such, National Rivet had to make the proper tooling (which they still had the drawings for) and produce whole new production runs of those original wartime rivets. Just as during original wartime production, the rivets come in different finishes, with clear anodized, yellow iridite/dichromate, blue anodized, and red anodized (which I think is limited to exploding rivets). At the factory, the silver, yellow and sometimes blue rivets were all poured together into the employees trays and pockets, so just as how Mustangs and other bare-metal aircraft of the period looked out of the factory and in combat, there is a random variety of silver and yellow rivets all over the exterior, as well as some blue and red. Most/all of the screws and bolts are either dyed red, orange, or in some case blue, as per the original factory inspection process - a process that was done in order to quickly check if they were defective or not. In all of the areas where parts are spot welded together, acid is brushed on the metal, just as done at the factory in a quick/non-precise manner, to etch the metal, making it clean/imperfection-free and thus quickly ready for spot-welding - that is what all of the bright areas on the metal are from. Another real stand-out/rare aspect of this restoration, over all, is that all of the fittings for the plumbing throughout the airframe are original wartime-correct AC fittings rather than the current standard AN fittings as typically found in most Mustangs today. All of the bare skins aren't polished, but just left in their Alclad finish as would have been the case from the factory. All of the other authentic details imaginable are there - full armament, the gun solenoids wired-up to the trigger, the original fuselage fuel tank (always one of the first things to go in the immediate post-war years) with all of the supporting balsa wood wedges and phenolic liners, all of the period radios, a completely stock/unmolested cockpit that matches in every detail how it looked in early '45 (which differs from one production block to another), all of the armor plating is in it, all of the wiring is fabric covered, laced, and marked-up as per original (with the modern FAA-required protective coating beneath), and all of the authentic inspection stamps and grease pencil signatures throughout. In order to fly the aircraft today, legally, it has a couple of very small/basic modern avionics heads (simply just a com and a transponder) fitted to temporary mounts on each side of the gun sight mount, which as such, does not change in anyway the complete authenticity of the cockpit. There are only a few details that remain to be added in the cockpit (such as an N-9 reflector gun sight), but there is still almost a whole year to go until Oshkosh 2015.

    When you look at the details of this restoration, you can more easily see/understand how the manufacturing process dictated how each part/assembly was finished. For instance, in authentic restorations and period photos, you always see the insides of the main landing gear doors, the insides of the clamshell doors, and in many cases the insides of the tail gear doors, left in bare metal. On the "Sierra Sue" restoration, the gun bay doors and ammunition bay doors are also bare metal on the insides (where as in any other P-51D restoration, these always are primered). The reason why these assemblies are all left bare of primer, is because all of them have spot-welding, and at the factory, you couldn't primer the panels and then spot-weld them together. The only way in which those assemblies would have been primered, is if there was enough time during production to take the completed assembly, after spot-welding and riveting, and spray it. In the midst of the greatest production need, there was no time for that - it was only later, towards the end of production (late 1945), that those assemblies began to be primered, sometimes. Also, when you look at the steel rub strips on the landing gear clamshell doors on the restored "Sierra Sue", they are finished in zinc chromate, where as on all other restorations, these are always left bare. As per original production standards, you couldn't have bare steel touching bare aluminum, as it would cause dissimilar metal corrosion. Early on, these rub strips were always primered, for that reason, and wartime color photos of P-51D's (and earlier Mustangs), tend to show these rub strips finished in that way, most of the time. Even in circumstances where the rub strips are bare during WWII, the sides that came into contact with the aluminum gear doors would have likely been primered. (On the "Sierra Sue" restoration, the steel rub plates even have the period-correct watermarks from the production plant where NAA sourced the steel - the Carnegie-Illinois steel plant.) Furthermore, you'll see on the insides that there was a 'seemingly' random patchwork of yellow and green primer. As per original results during wartime production, if you applied only a single coat of zinc chromate primer, the finish was yellow. If you applied two coats of zinc chromate, the finish would appear green. As all aluminum parts only required one coat, they typically always are yellow, and because all of the magnezium parts required two coats, they always typically appear green. However, if it was decided that an aluminum part needed another coat, for some reason related to production results, it too would be green as a result. Still, in other cases, due to production demand, some alumnum parts/panels were used with no primer at all, as long as they still had their alclad finish intact.

    Of special note... both the combat-vet "Sierra Sue II" and the combat-vet "Twilight Tear" (another super-authentic restoration, save for the polished metal), were restored by many of the same guys, and were very close to one another from the factory as they were of the same production block, thus their configuration, such as the precise details of their cockpits, are essentially the same. These being early P-51D-20-NA's, they don't have the rocket stub mounting holes/brackets nor the rocket controls panel in the cockpit. It wasn't all that long before this production block that D-models were still being manufactured with manual primer pumps and during the production of these aircraft the instrument panel bases were still being produced with the holes cut in them for the manual primer pump/lever despite the fact that they were no longer equipped with them, but rather an electrically controlled pump - the result is that, for a while, a simple cover plate had to be made and screwed into place, from the factory, covering that hole in the panel. Such a small, but authentic detail is seen on both of those restorations as per original. By mid-way through D-20 production, that detail would disappear as the area of the panel was redesigned to mount both of the oxygen gauges (flow and pressure) side-by-side. The design and layout of the engine and armament control panels on these aircraft is also unique, being of the type seen on all early/mid production D-models (and much the same as was on all B/C's) up until mid-late production D-20's. When the mid-late production D-20's came, they introduced the rocket controls panel (and mounting holes/brackets in the wings for the rocket stubs), which required the redesign of the engine and armament control panels to what you see most of today in later-variant D-models. They also both have a unique/authentic placement of the gun circuit protector shield plate, which was re-positioned on mid-late production D-20's and all remaining production versions. Both of these aircraft, when manufactured, also came from the factory fitted with the N-9 reflector gun sight and ball-tip throttle lever, as also seen on all previous D-model production. However, they differ in this regard through their restorations - where as "Sierra Sue II" will have the stock from the factory N-9 gun sight and ball-tip throttle, "Twilight Tear" was restored as if it had received the K-14 gun sight mod kit in the field (as originally), and is fitted with all of the K-14 accessories. The K-14 wasn't introduced into factory Mustang production until mid-late D-20's. These early D-20's also were not manufactured with the AN/APS-13 tail warning radar, which didn't come about until sometime during D-25 production. Between the restorations, "Sierra Sue II" is configured as stock, without it, where as "Twilight Tear" is restored with it, with it having been originally installed in the field. And another small detail which is exactly authentic about the restored cockpits in each, is the specific manifold pressure gauge. Generally it used to only be accepted that the 75"-span MP gauges were authentic to wartime use, however, documents/specifications have since proven that those gauges were only installed in Mustangs up until D-15 production, and from D-15 production onward, they were all fitted with 100"-span MP gauges, just as "Sierra Sue II" and "Twilight Tear" have. That change came about after the introduction of higher octane fuels in mid-late 1944.

    To cap-off the restoration, even the nose art (typography, names and lady artwork) were done with enamel paints and brush, and with the same style/fashion, as it was done originally in the field during WWII.

    First up, here are some video links related to "Sierra Sue II"...be sure to watch each in the highest resolution.

    Here are both videos from the first test flight, from the pilot's perspective. The final details of the restoration and the test flight were conducted at the Bemidji airport. The test pilot was the well known Doug Rozendaal - which was extra special, as "Sierra Sue" was the first Mustang Doug ever sat in, 25 years ago! He also got to fly with Roger Christgau before Roger retired from flying.
    Part 1: https://www.youtube.com/watch?v=dqM8qqIq_Ss
    Part 2: https://www.youtube.com/watch?v=cqSVllwMB_g

    Here is a walk-around video from shortly before the wings and fuselage were mated, providing some nice close-up views of the restoration:
    https://www.youtube.com/watch?v=a6pylSQlINs

    And here is a video from its first engine start up and engine runs...note the owner doesn't quite look the part in his business clothes, but certainly does look like the experienced warbird pilot he is, once in his flight suit (Paul Ehlen used to own the P-51D "Little Horse", and also owns one of the most authentic AT-6's flying today):
    https://www.youtube.com/watch?v=cwj-66GDS0A

    Here are some recent photo albums of the completed aircraft, courtesy of Max Haynes. A couple weekends ago, they had a private unveiling of the aircraft, at the Austin, MN airport, where Roger Christgau (who passed away in 2012) used to keep the aircraft. It was well attended by everyone who has been involved with or touched by this aircraft over the years.

    https://www.facebook.com/media/set/?...1586385&type=1

    https://www.facebook.com/media/set/?...1586385&type=3


  2. #2
    Here are some of my favorite photos from the restoration over the past few years which have been shared by Aircorps Aviation, and highlight the level of detail in the restoration.

    Just as can be seen in color photos of Mustangs (and other aircraft) from WWII, the rivets are not just silver, but a mixture of silver and yellow, and in some cases blue, and assorted randomly, as although they came from different batches, they were all poured together in the employees trays and pockets.



    The fuselage is assembled with each part either primered, painted or not primered/painted at all, as per original, and only then is the cockpit sprayed interior green.






  3. #3
    Some more:

    Note the streaks in the (double coat) primer finish, due to the part being dipped in primer, rather than sprayed.








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    Note, on the yet to be filled and painted wing, the authentic mixture of silver anodized, blue anodized, red anodized, and yellow iridite rivets, as well as red-dyed screws (from the inspection process) and areas of spot welds with acid etching of the metal in those areas.








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    In a small confine of one of the wings, now completely buttoned-up, are the signatures of the all of the Aircorps employees, done with grease pencil just as if they had signed it at the factory in 1944.


  10. #10
    These photos were taken by Bryan Darnell:






  11. #11
    And one final photo, this one taken from the back-seat of an SNJ, by Bob Canepa.


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