According to Gordon Elliot White's book on the Wasp Major the contra-prop gearbox and drive system for the XB-35 worked fine on the test stand, the problems started when it was mounted on the aircraft. White posits that the issues were caused by where they were mounted and that they were a pusher configuration. Being mounted with the thrustline above the surface of the wing, which put the propellers in clean air at the top of their arc but at the bottom subject to the assymetrical airflow off of the wing that resulted from the changing thickness of the airfoil and the sweep of the trailing edge, which meant that the speed of the air traveling over the surface changed as you went outboard from the centerline of the aicraft. As a result, as the propellers rotated, the loading was constantly changing. It's possible that staggered positioning of the propeller assemblies, the inboard being forward of the outboard, may have played a role. He also points out that changing to the single rotation four-bladed propellers didn't eliminate the vibration, it just reduced the performane of the aircraft.
something I forgot to add, the P-47 at 72" of MAP was good for 2800hp, but there is evidence that they ran more power than this in combat. At minimum the P-47Ms got juiced up to a documented 76" (to their apparent detriment) and I've heard more than one tale of Jugs running 90-100 inches of mercury in the direst conditions. the R-2800 is easily past 3000hp at such a setting. Republic tested the R-2800 on an engine stand at 3600hp for some 250 hours straight
Probably a P-47 could have carried at least 6 but it would have required a redesign of the wing internals and I don't think it was ever a priority. 8 fifties is getting to be quite a lot to even consider upgrading, especially when you consider the jug had nearly infinite ammo and the M3 browning at ~1200rpm came out late in the war. Furthermore the US Navy was enamored with them but by the time the USAF decided to pursue 20mm cannons the P-47's successor's successor's successor (F-100) was already underway. Here was a one-off experiment with 2 under the wings.
Looking at the photos of the wing structure of this restoration back in 2020 it doesn't appear to me that converting to 20mm cannon would be a big deal.Probably a P-47 could have carried at least 6 but it would have required a redesign of the wing internals and I don't think it was ever a priority. 8 fifties is getting to be quite a lot to even consider upgrading, especially when you consider the jug had nearly infinite ammo and the M3 browning at ~1200rpm came out late in the war. Furthermore the US Navy was enamored with them but by the time the USAF decided to pursue 20mm cannons the P-47's successor's successor's successor (F-100) was already underway. Here was a one-off experiment with 2 under the wings.
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here is the room in the wings for them to play around with. the wing spars seem to be designed around the length of a .50 bmg shell
I doubt that the combination was especially successful. The succeeding P-60E used a conventional propellor with the R-2800. If a simpler. lighter, cheaper, more readily available four-bladed standard propellor could handle the R-2800's power, the contrarotating type would not seem to offer any advantages.,
Ab ad about Curtiss electric propellers in "Aviation" February 1942
6 fifties of the new M3 variant had the firepower of 8 of the old M2 guns. That would make the P-47 lighter and maybe increase its roll rate since the outer 4th gun and its ammo would be deleted. The XP-72 was to carry 6 M3 guns or 4 37mm cannons.
Maybe, but in practice P-47 pilots always elected to carry 8 guns, even when allegedly some P-47Ns received the M3 in the final days of the war. There's no such thing as overkill!
