Rebuilding an Extremely Rare Twin Mustang Fighter

Towards the end of the Second World War, as the United States considered their options for a possible invasion of Japan, there was demand for a new fighter that could escort long range bombers on missions which could see them travel more than 3,200 kilometers (2,000 miles) without refueling. In response, North American Aviation created the F-82, which essentially took two of their immensely successful P-51 fighters and combined them on the same wing. The resulting plane, of which only 272 were built, ultimately set the world record for longest nonstop flight of a propeller-driven fighter at 8,129 km (5,051 mi) and ended up being the last piston engine fighter ordered by the United States Air Force.

Today, only five of these “Twin Mustangs” are known to exist. One of those, a prototype XP-82 variant, is currently in the final stages of an epic decade-long rebuilding process directed by warbird restoration expert [Tom Reilly]. At the end of this painstaking restoration, which makes use of not only original hardware but many newly produced components built with modern technology such as CNC milling and 3D printing, the vintage fighter will become the only flyable F-82 in the world.

CNC milled replacement brake caliper

The project provides a fascinating look at what it takes to not only return a 70+ year old ultra-rare aircraft to fully functional status, but do it in a responsible and historically accurate way. With only four other intact F-82’s in the world, replacement parts are obviously an exceptional rarity. The original parts used to rebuild this particular aircraft were sourced from literally all over the planet, piece by piece, in a process that started before [Tom] even purchased the plane itself.

In a way, the search for parts was aided by the unusual nature of the F-82, which has the outward appearance of being two standard P-51 fighters, but in fact utilizes a vast number of modified components. [Tom] would keep an eye out for parts being sold on the open market which their owners mysteriously discovered wouldn’t fit on a standard P-51. In some cases these “defective” P-51 parts ended up being intended for the Twin Mustang project, and would get added to the collection of parts that would eventually go into the XP-82 restoration.

For the parts that [Tom] couldn’t find, modern manufacturing techniques were sometimes called in. The twin layout of the aircraft meant the team occasionally had one component but was missing its counterpart. In these cases, the original component could be carefully measured and then recreated with either a CNC mill or 3D printed to be used as a die for pressing the parts out of metal. In this way the team was able to reap the benefits of modern production methods while still maintaining historical accuracy; important on an aircraft where even the colors of the wires used in the original electrical system have been researched and faithfully recreated.

We’ve seen plenty of restorations here at Hackaday, but they tend to be of the vintage computer and occasionally Power Wheels variety. It’s interesting to see that the same sort of techniques we apply to our small scale projects are used by the pros to preserve pieces of history for future generations.

[Thanks to Daniel for the tip.]

60 thoughts on “Rebuilding an Extremely Rare Twin Mustang Fighter

        1. I can’t say for the P-82 but I can say for the P-51, after you get over the fact that you’re in a Mustang the next thing you have to get over is the noise and vibration. I can only imagine it as being worse in the Twin with the inner exhausts bouncing between the fuselages.
          Riding in a P-51 for about an hour and a half made me appreciate what WW2 escort pilots had to endure even if they didn’t have to face the enemy.

          1. I’ve flown in the backseat of a AT6D Texan (The trainer aircraft for the P51 Mustang). Would anyone have a noise comparison between that and a P51 (Louder or softer?)

          2. A friend of mine, Jerry Yellin, was a P-51 escort pilot and he told me after a flight, because of fatigue and numbness brought on by the vibration, he had to be carried from the plane. He has passed away now but a touching story of reconciliation can be found in his book “Of War and Weddings”.

    1. Well for one, the F-82 could carry a maximum of 1,816 gallons of fuel, compared to the maximum of 419 gallons by the P-51D. Wing area was also 408 sq ft compared to 235 on the P-51. More lift, more fuel.

      Interestingly, according to Wikipedia, the record distance attempt was also hampered a bit by the pilot forgetting to drop three of his four external tanks, likely having increased drag and decreased range by a good bit.

      1. The actuality of the record flight was that the three external fuel tanks would not disengage during the flight. Not because the pilot forgot to. I live near the Airforce Museum where the plane is displayed.

    2. I don’t think so because it seems to hold 6 machine guns and their ammunition. Maybe if it only has 1 pilot so the other fuselage can just hold fuel, but it doesn’t seem to be the case (i.e., it has 2 pilots). I’m baffled, too.

    3. Two engines would lessen the strain or requirements of the engine to produce adequate power to achieve and maintain flight. In addition, the two engines could be “trimmed back” so as to achieve a more economical burn of the fuel supply. There are also airframe related choices that may provide for lift benefits of this configuration as opposed to the original design.

    4. One other consideration is that to fly a long distance you need more than just fuel, you need a rested pilot, and the main reason for the F-82 was so that it became a twin crew aircraft. That way the pilots could spell each other on the controls.

  1. Hey Tom, Happy New Year to ya!

    In a semi-related concept of WWII Warbird, my fellow Commodore Engineer, Greg Berlin (He designed the disk drive we used on the C128 and did many other Amiga related things including the 4000), was the grandson of Donovan Berlin, the designer of the P40 Warhawk while working for Curtis.

    I had been to a warplane museum in Wright Patterson AFB in the 1960s and they had a model of the twin P51, always wondered what the story was.

    Bil

      1. You have that right. There are many other rare sights to behold there also not to mention that some of the docents that I met crewed on some of the hardware that is on display.

  2. It could fly further on account of being able to carry additional drop-tanks. In addition the fuselage is slightly stretched, allowing for larger in-body tanks.

    Further, the other big problem was pilot fatigue, and being able to rest one pilot at a time helped avoid that.

  3. from wiki: “… incorporated two P-51H Mustang fuselages lengthened by the addition of a 57 in (145 cm) fuselage plug located behind the cockpit where additional fuel tanks and equipment could be installed”

  4. I’ve not gone through the blog in detail, so please pardon my ignorance. I take it the original tech. drawings are no longer available? Otherwise, they can just re-manufacture the needed parts, can’t they? Stamped parts would be more difficult without the original tooling, but milled and cast parts should be doable, no?

    Anyhow, I’m much impressed with the whole thing. It’d be awesome to see it fly again.

    1. There almost certainly isn’t complete documentation available for such a rare aircraft, if it was ever fully documented to begin with. A lot of the wartime designs were rushed out at a pace that we just can’t even fathom today. Fighter planes were designed and put into production in literally weeks in some cases.

      But even if they had all the documents, it was very common for designs of that era to differ slightly from the real-world article. The design was like a guideline, they would make tweaks and changes during production that often didn’t get documented. Again, the pace that all this stuff was happening at has no analog in modern manufacturing and engineering.

      1. The single seater P-51 was famously designed and flown in 100 days. Even if we take into account that the world back then was much more prepared to accept danger and casualties even in peacetime, that’s quite an amazing feat. More so considering it turned out to be a more than decent aircraft.

        1. I’m pretty sure documenting tweaks is the only way you’re going to build a reasonable number of them. (Somebody found the complete plans for the de Havilland Mosquito on microfilm in 2017 – including ones that didn’t even make it to prototype stage. It does happen occasionally)

          1. I read that even though the plans for the F1 rocket engine are available no two were exactly alike. The engineers were making minor changes through the manufacturing process. It is the notes on these changes that are incomplete and the knowledge passed with many of the engineers.

          2. Ouch. A friend works in a place where the manufacturing side makes things to fit rather than to the plans. (Much to the annoyance to the designers. Especially when they need to create bits for refurb or require accuracy as well as fitting)

            And then when it gets fitted, it also gets redesigned. (Some changes necessary, some unnecessary)

            I had hoped it was a rare exception to the rule.

    2. One of the examples in the blog is the fairing for the exhaust. You can’t really make a complex part like that that was formed from a sheet without stamping it. You could certainly hack your way around it but these guys are after an identical reproduction — so it has to be stamped.

    3. My father used to build wood aircraft (among a ton of other things). One of the things he used to talk about was how a lot of next generation “kids” would forget how things were actually constructed in the first place. At the time, I really didn’t understand what he was talking about but after trying my hand at restorations myself, I finally understood what he was talking about.

      1. Damn it, I hit the wrong button. Continuing on….

        What he was telling me was, just because you have the original part, it doesn’t mean you know the correct process for recreating the part or even have the right tools. In other words, imagine if you have a shop but you lack a lathe, how would you fabricate a part made on said lathe if you you have no idea such a tool even exists?

        Then a new question will naturally arise out of that. If you use new tools/techniques to recreate the part, is the part technically a recreation? More importantly, does it still meet spec? What is the spec in the first place? How would you know if you don’t have the documentation to go with it? To put it another way, if the original part was say… milled from a single block and the new part is sintered, would that difference alter the desired performance?

        1. Very true. They finished building a steam train in Doncaster (North of England) in 2008 from complete plans. It took them 14 years. (Even then, apparently they spent 3 years planning before that)

  5. Interetingly, the P82 went back to the Allison engine that powered the early Mstangs. The B/C and D versions used British designed, Packard built Merlin. It was better at altitude but not quite as good at low altitude as the Allison. I am not sure how that works out for a long range escort. Mayby it got better mpg. I think the P82s saw action in Korea.

        1. The B-2 was not engineered from the Horten… The B-2 lineage developed in parallel with the Horten designs… The Northrop N9m test aircraft flew before the Horten 229 did, and one example remains airworthy at Planes of Fame in Chino, Ca… The N9m was developed into the XB-35, which was developed into the YB-49… The design was shelved due to stability problems until modern electronic aids were developed… This new electronics allowed for the development of the B-2, which matches the general size and design of the XB-35/YB-49 aircraft… The Horten program and the Northrop program have little to do with each other other than being flying wings…

  6. I saw one of these at Lowry Field Denver 1949 on takeoff ran off the Runway slightly with one wheel spun it around three or four times and it up down at the other end of the field and crashed and exploded pilot did not help his crew chief out trucchi’s however it did survive Happy New Year Howard k l e m m e t s e n

    1. Not correct. The reason that the switch to the Allison engine in part was that Packard didn’t want to and couldn’t afford to keep paying Rolls-Royce royalties for license building the Merlin engine in the United States.

  7. Note the way the props spin. The blades move downward in the center. They originally went up in the center. The engineers thought that going down would create too much down pressure on top of the center wing section and make it difficult to get it off the runway. Aerodynamics still had some tricks that scientists had yet to learn. Turned out to be very difficult to get off the ground, along with some other issues.

    So they swapped things left to right as seen in the photo and the F-82 became easy to take off and other issues with the original configuration went away.

  8. Tom Nardi you wrote a great article with the aircraft blog for source material. You caught things I missed after following it for a few years. Congratulations on a great job.

  9. I’m younger than this era. But I love the older fighter planes of ww2. My favorite being the f4u corsair. But I’m just in awe of this beauty. I saw them in books in school. But to see one live, and in person. Well that would be amazing. Don’t spend a lot of time , trying to figure it out. Just be amazed, like I am. Its beautiful. Signed a dumb kid.

  10. How fantastic would it be if all of these socialist liberal billionaires stopped funding all sorts of “nonsense causes” and redirected their spending to building new exact replicas of these pieces of aviation history?

  11. We had every North American Xp thru F82H drawing that ever existed. Over 50,000 on DVD’s. Also there was no fuel carried in either fuselage in any of the production aircraft with the exception of Betty Jo, the B model P82, 100 gallons behind each seat and 62 in the center gun bay. Also 450 under each wing for the Hi. to NYC 14 hour plus non stop trip. Good article Thanks

    1. Tom,

      is there any truth to story I saw somewhere that a guy at NAA was told to take the old drawings out to the burn barrel but instead put them in the trunk of his Chevy? (Things like this happen… when Chrysler survived its 1980 brush w bankruptcy the chassis department after laying off 1/2 the staff cleaned up and the boss there threw out all the old engineering drawings going back to the beginning of Chrysler and, before, to Maxwell.)

      Happy New Year to you and your team from Switzerland!

  12. MUCH more than a couple P51’s stuck together as noted above. The engines were set up to rotate in the opposite directions, but they had to change the timing. Both sides originally could fly the plane, but later models couldn’t.
    Shame, but it came around at the dawn of “the jet age”. Didn’t really get to show what it could do before being out done by the jets.
    Saw one in Harlingen, Texas at the CAF airshow in 85. VERY loud, but cool to watch. At that time it was painted in the night radar black with the red/orange lettering.

  13. Cool aircraft. Saw one in 1985 in Harlingen Texas at the CAF airshow. That one was painted in the radar black/red lettering. Sure was LOUD. As noted above, more than just two P51’s stuck together. Counter rotating props, knock down the torque/balance. Shame the “jet age” came along, the 82 never got to prove what it really could do.

  14. Damn. If the author could only keep from deviating to the F-82. The XP-82 was different from the production model. To include the B models and the two research frames, There were only 22 that are used the Packard Merlins. The rest used Allisons. There are also various other airframe differences.

    There is an XP-82 that is going through FAA certification in Douglas, Georgia. So far it is been 10 years in rebuilding. I have been lucky enough to see it being rebuilt. She is a beautiful plane.

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