What if the F-82 was based off the XP-51F/G instead of the P-51H?

[BarnOwlLover]

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?

 

[NTGray]

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?

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?

 

[BarnOwlLover]

The F/G were lighter (but shorter ranged) than the H, as the H carried heavier armament, more fuel, and was restressed to the 7.3G loading that the F/G were designed to would apply to the H's max combat weight. The F for example was nearly 2000 lbs lighter than the H depending on configuration. However, it gave up range since it didn't carry the fuselage tank the H did.

It should be noted that there was a single seat F-82 dedicated interceptor or long range single seat fighter was conceived at the early stage of its development, though it was never built.

Only big differences was what I pointed out as far as the F/G prototypes vs the H production model.

 

[MIflyer]

There were virtually no interchangable parts between the P-51H and the P-82 airframes. Of course the limited P-82 production used Merlin engines while the full production F-82's used versions of the V-1710, with the result that they sat around at Downey for years waiting on decent engines. In the Gringrich/Foreseten novel "1945", desperate to stop the German invasion of Great Britain, the V-1650's of P-51's in storage were used to replace the unreliable V-1710 engines of the P-82's, and with rocket boosters attached they were flown to England to fight the Me-262's.

The whole idea behind wthe P-82 was the same as behind the P-61E, a two seat very long range fighter so that at least one pilot could get some shut eye on the way to and from Japan on B-29 escort missions. So a lightweight version would not have made any sense.

 

[BarnOwlLover]

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).

 

[Shortround6]

You do know that the P-82 was about 7 ft longer nose to tail than most P-51s??

From a modeling website so it may not be accurate.

 

[BarnOwlLover]

I'm well aware that it was 57 inches (almost 5 feet) longer than the P-51H. What I do wonder is that as a single seat, single engine fighter would a single seat single engine F-82 one, gain weight from having a wing the span/area of the P-51 (which would be actually over 20% more area for weight vs the F-82), and how much would it lose adopting a P-51D or XP-51F/G or H length fuselage.

Asking from a weight and loading standpoint.

So, original point, what if the F/G was used to develop the basis for the F-82 instead of the H, and point two, could some stuff from the F-82 address the sole weakness from the H (landing gear) or reverse engineer a F-82 into a smaller single seat single engine fighter similar to the H in overall specs (namely weight and performance)?

 

[MIflyer]

I once tried to turn two Monogram 1/48 P-51D into a P-82. Then I found that merely stretching the fuselage would not work - an extended taper will not reach the required length before running out of room.

 

[PStickney]

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).

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.
No H models were deployed overseas - some did deploy to Alaska, but the need for them as Interceptors kept them in the U.S. They weren't deployed to Korea because the Dar East Air Forces had just finished converting their Fighter Groups to F-80Cs, and the storage depots in Japan and the Philippines were full of F-51Ds in flyable storage, with pilots and maintainers who were already familiar with flying and fixing them. While a lot of -51s were pulled into service at the start of the Korean War, their numbers peaked early, when they were standing in for jets that didn't have the endurance to fly Close Air Support over Korea from Japan, as Korean Bases were developed, and the F-84 was introduced in theater, most of the units that had pulled back F-51Ds reverted to their F-80s, After early 1961, the only F-51Ds were the 18th FBG. Wars are "Run what you brung". The D Models were already there, and peak of their involvement was in the early days, when the US was scrambling to get the job done. For the war as it was in Korea, the Hs had no advantages over the Ds, and the time that would have been spent getting Hs (Or, even more so, F-47s) IRANed (Inspect and Repair As Necessary), packed up for shipping, and sent across the Pacific, while also building a Maintenance and Supply pipeline would take away time, effort, and people needed to get the jets - F-84s and F-86s, more capable in every way except loiter time, and survivable against the MiGs that were showing up, deployed.

 

[MIflyer]

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.
'
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.

 

[PStickney]

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.
'
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.

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.
The Aneroid setup on the V1650 was set up so that the altitudes that it up-shifted was higher than the altitude that it shifted to low blower. This kept the engine from constantly shifting as it ascended and descended, or spent a lot of time at a shift point.
The V1710-143/145s of an F-82E or later had a main (Engine) stage with a 10.25" impeller diameter, and a 7.48:1 drive ratio.
The Aux Stage had a 12.18" impeller diameter, and a max drive ration of 8.08 I don't know what the minimum speed would be.
Nominally, it delivered a Military Power - Wet Rating of 1600 SHP at 3200 RPM / 74" at Sea Level, and 1250 SHP at 3200 RPM / 68" at 32,500' Manifold Pressure limits were 74" 0-5000', 69" 5000-15000', and 66
above that.

 

[Mike Williams]

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.
'

Spitfire IX pilots had a work around for that issue per S/L Robert Oxsbring:

 

[PStickney]

Spitfire IX pilots had a work around for that issue per S/L Robert Oxsbring:

View attachment 722115

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 P-51s had a 3 position switch that allowed the pilot to select Low Blower, High Blower, or Automatic operation.

 

[MIflyer]

The P-51s had a 3 position switch that allowed the pilot to select Low Blower, High Blower, or Automatic operation.

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 postwar P-51 manuals say that the longest range was attained by climbing to above 25,000 ft and manually putting the supercharger switch in ''Low."

 

[BarnOwlLover]

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?

 

[PStickney]

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?

The B Models had hydraulic boost on the elevator and rudder.
The E Models had hydraulic boost on elevator, rudder, and ailerons.

 

[BarnOwlLover]

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.

 

[PStickney]

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.

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)
Climb numbers are 4.5 minutes to 20,000', service ceiling (100 '/min 43,100', single engine ceiling 28,900'

Information from the USAAF Aircraft Characteristics and Performance Quarterly Chart, Experimental Aircraft, October 1945,

 

[BarnOwlLover]

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)?

 

[PStickney]

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)?

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.
Now - as to ratings. While these engines could develop 2200+ HP at Sea Level (0 feet), They're doing in in Low Supercharger Gear, so the supercharger pressure ratio, and thus the power consumed by the supercharger is less. (More for the propeller). at high altitudes, the supercharher can't maintain the manifold pressure, and power drops off. So, a second supercharger gear ratio is provided - At altitudes below the Critical Altitude for that power setting (The maximum altitude where the manifold pressure demanded can be met), The increased pressure ratio requires more power to drive it, and there's less for the propeller - That's why these engines have 1800 HP available at about 25,000'.

You have to be careful when comparing data, particularly USAAF/USAF data from this era. Before 1947-ish, the performance numbers in the ACP charts were based on the aircraft's clean takeoff weight. It was realized during the war that that wasn't very realistic - for example, there's not a lot of difference for an airplane with a small fuel load between takeoff and combat, so for a Spitfire or a Me 109, it doesn't make much difference. There is, however, a big difference between a P-51 or P-47 taking off from England with full internal and external fuel, and fighting over Berlin or Prague with 60% fuel. So, they developed the idea of Combat Weight (60% fuel + air-air weapons) to make comparisons in actual encounters more even.

Also note that in reality, all aviation questions should be prefaced with "That Depends". Trying to make a case on some specific stated or tested number - chasing 1 or 2 knots here or there, or a few hundred ft/minute of climb is irrelevant - No two airplanes of the same type perform exactly the same. In fact, the same airplane will perform differently on a different dat, or even a different flight. The best you can do is assume that the numbers are close enough within, say, +/- 5%.

Taking that into account, we see that the P-51H and XP-82 had nearly identical performance - or as close as doesn't matter. Of course, taking range into account...