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For the benefit of those of us who are not intimately acquainted with the important differences between the F/G model and the H, could you explain them, and explain why in your estimation the F/G might have served as a better baseline? How does a "heavy" interceptor differ from a "pure" interceptor? What would this have meant for the F-82?To begin with, basing the F-82 off the P-51H was extremely sensible. The P-51H was a high performance escort fighter that had the speed and climb abilities of a genuine interceptor (and can even be set up as a pure interceptor). Hence, why not take the lightweight Mustang that made it to production and use that as the starting point for the F-82?
However, if you wanted a F-82 that was a pure heavy interceptor (the XP-51F/G were biased towards being interceptors with long range escort being a secondary consideration), why not start with the F/G models?
If that's the case, how would it look, how might it perform, and what armament and items would it carry in combat?
F-51Hs didn't start showing up in the Air National Guard until late 1949-1950. Prior to that, 1945-1950, they were used by the Regular Air Force. The reason that they were generally tasked as Interceptors was simple - they were the only piston-engined fighters (Other than, eventually the F-82) that could reliably scramble, climb to altitude, and get into firing position on a B-29 type target with GCI radar detection at 150-200 miles (Typical for the time). They saw a lot of use -unlike the P/F-51Ds there weren't a lot in storage that could be pulled out as War Reserve airplanes. As the Regular USAF F-51Hs were replaced by jets, the ANG started receiving them in '49. With the onset of the Korean War, and the need to, among other things, expand Continental Air Defense, activated Air National Guard Fighter Interceptor Squadrons received F-51Hs as more jets entered the inventory. ANG F-51Hs didn't leave service until 1955.This may derail the thread, but what if the P-51H was "half" a F-82? The P-82B had an unloaded weight of about 13,400 lbs and a gross weight of 22,000 lbs. Divide that by half and you get 6700 lbs and 11,000 lbs, both are lighter than the same figures for the P-51H. Also, that might leave room to fix the P-51H's main problem, which was relatively weak landing gear that tended to want to collapse in ground loops and would've made operations in the Pacific Theater difficult (that issue combined with low production numbers and desire for the ANG to use them as interceptors in the short term lead to them not being used in the Korean War).
The shift point for the 2-speed blowers varies with the power setting (Commanded Manifold Pressure and RPM), and more importantly, the gear ratios of the different supercharger speeds. The V1650-7, for example, has lower Critical Altitudes than a V1650-3, due to the lower gear ratios, and thus lower pressure ratios of the blower - however, since the blower isn't drawing as much power out of the engine's total output, you get more horsepower at those critical altitudes.In WW2 the V-1650's in P-51's were set to switch from low to high supercharger speed at 18,000 ft, this point being detected by an aneroid device. Given the possible tolerances in settings this meant that when a Mustang unit was in a climb the formation got very ragged as the superchargers switched to high speed at slightly different times.
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I recall reading about a combat between a P-51D and a FW-190D. The P-51 pilot said the FW pilot clearly knew the Mustang well and tried to fight at around 18,000 ft, with the result that the supercharger kept switching speed, requiring him to jocky the throttle a lot. Maybe this is why the postwar P-51D pilot's manual says they changed the supercharger switch point to being determined by ram air pressure rather than altitude.
The F-82 used a "hydromatic transmission" approach for the auxilliary supercharger, so there would not have been a switch point. I will have to take a look at the P-51H and P-82 manuals and see what the switch point was for those.
Spitfire IX pilots had a work around for that issue per S/L Robert Oxsbring:In WW2 the V-1650's in P-51's were set to switch from low to high supercharger speed at 18,000 ft, this point being detected by an aneroid device. Given the possible tolerances in settings this meant that when a Mustang unit was in a climb the formation got very ragged as the superchargers switched to high speed at slightly different times.
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Yep. The Spit and other Brit 2-speed Merlin installations had a switch that would hold the blowers in Low (M or Moderate Supercharge), so you could hold the blower shift off.
The standard P-51 High Blower switch was spring loaded; you had to hold it in High with one hand, which made it very hard to fly. Some P-51's flying out of Iwo to escort B-29's had that switch modified so they could switch it to High and not have to hold it. The Japanese knew full well that if they stayed below 15,000 ft the Mustangs were operating at the upper limit of their Low Blower range. Of course, on daylight raids with the B-29's at 25,000 ft the Japanese fighters were no threat if they were 10,000 ft lower, but it was a difficult thing for Mustang pilots to fly all that way and not go after targets being dangled below them.The P-51s had a 3 position switch that allowed the pilot to select Low Blower, High Blower, or Automatic operation.
The B Models had hydraulic boost on the elevator and rudder.Small overall F-82 question. I know that the Allison models (E/F/G/H) got powered ailerons to help with overall roll rate and to deal with asymmetric roll due to rolling about one of the two fuselages. Did the Merlin versions (XP-82, P-82B) ever use or at least test them?
Best Data that I have for the XP-82 - Vmax 482 mph / 25100' on 3600 HP. That would correspond to a V1650-11 Wet War Emergency (90" MAP/ 3000R) rammed, 2 engines (1800 each)Is there any info out there in terms of performance data from manuals and such on the F-82B/XP-82? Namely speeds and climb rates for different configurations. I found some for the P-51H at World War II aircraft performance issued to SAC units and ANG units post war. I'm mostly comparing and contrasting the Merlin powered Twin Mustangs with the P-51H and DH Hornet.
The -9/11/21 was basically the same engine - with the same ratings. The same fixes would work on both. ADI (Anti-Detonant Injection) when done right, isn't as simple as spraying water in and cooling the charge air flow. At high manifold pressure settings, the engines are running very over-rich, with the excess fuel serving to absorb heat. To get the best power using ADI, you need to "de-rich" the fuel-air mixture for more efficient burning. This requires a lot to tuning and testing - an aircraft engine operates over a great range of air densities, and it all has to be covered. That can be hard to do with a mechanical system. You also need to be able to deal with the situation when the ADI supply runs out - you suddenly go from a snarling powerhouse to a large mass of broken metal. It's like setting a bomb off inside the engine.Odd thing there is that the P-51H and P-82B/XP-82 ran basically the same engines as far as set up and ratings. And the P-51's V-1650-9 Merlin was ultimately capable of 2200+bhp (I've read anywhere from 2200-2280) on WEP/combat power with ADI at 90" boost. One, I've never found anything about he Merlin Twin Mustangs using ADI, and two, the P-51H's ADI and boost control system took until late 1945/early 1946 to work all the bugs out (that's how long it took the P-51H to achieve 487 mph in interceptor trim). This makes me wonder if the F-82's Merlins didn't have the same problems as the -9 initially did with the ADI/boost control combination, though the power rating of 1800bhp vs 2200+ makes me wonder what made the F-82 faster in that configuration.
Also in regards to climb, the SAC manual on the P-51H did mention climb rates at combat weight of 5200-nearly 5500 fpm. The excerpt from a F-82 book that I'll be buying soon suggests 4900 fpm climb rate. These seem to be initial climb rates at or near sea level.
I know that the P-51H was obviously much lighter than the XP-82/P-82B. But which had the better power to weight ratio (which largely governs climb), and which had the better aerodynamics for its size (which governs speed along with power output)?