Lightweight Mustang (XP-51F/G/J vs H) in flight G loadings question/in general, what's a good G loading for a World War II fighter?

[drgondog]

So that's a pretty low dive speed limit, given that Spitfires could be dived at over 500 mph without issue (and at least one was dived at over 600 mph without damage to plane or pilot). I wonder what gives there. It should also be noted at the 8g/8000 lbs limit the P-51D's g limit for combat weight drops to as low as 6.3 gs, and had no trouble with high speed dives.

BTW, I wish that such details were accessible and such for all the late Mustangs and even the Twin Mustang. Lots of tech drawings and info about earlier variants, not nearly as much as the variants after the D model.

475 Indicated Air Speed, not TAS

 

[BarnOwlLover]

First. As stated before, All NAA aircraft design standards (until, and exclusively for, the LW Fighters) were per Materiel Command Airframe Standards. As a side note, Lee Atwood's first job after graduation from Univ. Texas was as a structural engineer at MC before later being recruited by Kindelberger at Douglas Aircraft. Atwood was Chief, Structureses under Kindelberger before both left for the new NAA. The same standards at Douglas and Boeing and Curtiss, etc - were adopted at NAA.

The primary load condition was largest permissible Angle of Attack loads at Design Gross Weight = 8G Limit and 12G (1.5x Limit) Ultimate. The "1.5" was basically the common 'yield' point for aluminum, at which aluminum under applied stress begins to move away from constant deformation vs stress to the plastic range.

The genesis of the LW Fighter change from 8Gto 7.33G was to match the Supermarine/RAE standard following the weight fraction comparisons between the P-51B and The Spit IX, said report delivered in November 1942. The NA-5567 Report was dated 23 November 1942.

NA-105 (XP-51F), 105A (G).105B J) and NA-126 (P-51H) all had the same 7.33Limit/11Ultimate load specs but to the different Gross Weights. Each Gross Weight for the specification was max interal loadout.

The reason the D was so much lower ultimate (~6.26 at 10,200 pounds w/maxinternal loadout), is that NAA continually upgraded without performing the necessary full part by part stress analysis as features like external racks, etc were added in previous models so the design GW remained the same (8000 pounds) as NA-73 for 8GLimit/12GUltimate.

Save the landing gear, the P-51H was the strongest of all the Merlin Mustangs.

I do not know whether P-82 specification calls for LW structures allowables - or revert back to AAF standards. Perhaps Dan Whitney kows.

So basically the problem wasn't the 8g vs 7.33g standards in and of themselves, but how it was applied. Both standards revolved around designing around a specified weight, and for the NA-73X this was 8000 lbs gross weight. As you mentioned before, this wasn't so much a US Armed Forces standard, but an accepted industry standard that was common until at least the Vietnam War.

As you just mentioned, it seems that each time the P-51 was upgraded, probably due to time and production constraints, they didn't do a full restress of the airframe and most components. Hence, as weigh went up, max loading dropped until the weight dropped. Granted, the lightest weight I've seen for a Merlin Mustang (until the lightweights) was the 8600 lbs gross weight for a XP-51B set up essentially as an interceptor. 8600 lbs is a bit over the 8000 lbs original intended design limit, but was probably as close as the Merlin Mustangs got to that initial intended design protocol until the LWs.

It seems that the big advantage of the LW Mustangs wasn't the 7.33g limit (though it allowed for some weight savings), but how it was applied. Instead of being stuck at 8000 lbs like the old 8g limit was, the 7.33g applied to the intended normal max gross weight of the aircraft. Granted, this still went down for absolute max gross weight (ie, max overload), but was still an improvement at normal max TO weight. And as weight drops (like the H shedding nearly a 1000 lbs between max internal fuel vs being set up as an interceptor), the permissible load should increase in theory.

I'd bet that even if the LW Mustangs were designed for 8g for their intended gross weights, they'd still be lighter and stronger than the B/D models per pound. Not to mention that overall airframe optimization brought about other improvements regarding aero (supercharger intake, radiator duct), servicing (cleaner engine installation, simplified gun/ammo installation), and for the H especially, being able to carry a fuselage tank without significant CG/directional stability issues.

I wonder how say the Spitfire or the Me-109 dealt with loading vs weight as those aircraft packed on the pounds because of development (especially, though the P-51 family was the example, this was also a general question overall).

 

[GregP]

SDAM interview of Chilton, April 27 1986. They have a copy but nobody I have contact with knows where to find it.

What is SDAM? San Diego Air Museum?

 

[BarnOwlLover]

I think so, they also have a pretty good photo archive.

 

[drgondog]

What is SDAM? San Diego Air Museum?

yes

 

[GregP]

I spoke with Joe Yancey today about the P-51J and the Allison V-1710-119, which was a G-series engine. He had some interesting insights.


Initially, the supercharger drive was a helical-cut gear and the drive gear and had a boss in the center with a bored hole in it. A pinion shaft was fitted there and Joe says the clearances around the pinion shaft were excessive. I put one together and you could feel the slight wobble/play in the shaft/gear combo. The helical drive gear caused the bearing it rode on to spall after some hours of operation due to pressure inward from the helix angle. Joe has a modern "fix" including a machined new bolt-in center mount with almost no clearance, together with a new shaft which includes propeller-type splines … with a good fit eliminating the "play."


The "fix" back then on the XP-51J was relatively simple. They went from helical cut gears to straight-cut gears, and eliminated the helical gear tendency to push hard against the bearing while wobbling, causing it to spall. After that "fix," the supercharger gave MUCH less trouble, evidenced by Allison internal communications about the subject, which Joe has in his files. He said he'd look for it, but not now since he is in the middle of a couple of V-1710 overhauls that actually pay money for his company. Money-producing effort is better than no money-producing effort, or so he says. I concur. The -119 primary supercharger was a 9.5" unit that went from 7.48 : 1 to 8.10 : 1, and the auxiliary-stage supercharger had a 10.5-inch impeller at 7.64 : 1.


Joe's says that once they went to straight-cut gears, most of the supercharger gear issues reduced greatly and the P-51J flew just fine on it with Allison test pilots. Interest in jets was running very high, and interest in pistons was dying off at relatively the same rate, so the P-51J didn't get much attention. Neither did the P-51H, which was derived from the lightweight P-51 prototypes, evidenced by the low production numbers. What they really wanted was to get the war over and stop spending money that fast. We could stand some that today!

In fact, they gave at least one P-51J to Allison, and Allison corrected the supercharger issues. I say "gave" because they "loaned" it but never really asked for it back during the war. Joe has parts from the -119 for two engines except for the intercooler cores. He has the intercooler boxes, so someone could have him make two -119s if they made two intercooler cores. He has the aux-stage S/Cs, too. He has his modern "fixes" for both engine part sets ready for use, should someone want a 1900+ hp Allison. Joe is the same guy who ran a hybrid F/G-series Allison at Reno in 2018 and won Bronze and Silver with it pulling 110" MAP in a Yak-9 called "Full Noise," flown by Grahame Frew. The engine still runs just fine, but did not run in the Gold race, which was run with a stock F-series Allison.


So, according to Joe, the P-51J ran just fine after it went to straight-cut supercharger drive gears and had some hours of trouble-free operation once the S/C bearing spalling was fixed.

Here's a stock Allion F-series unit built by Joe Yancey:

The Gold decal on the valve cover says, "Allison 1710 V12 by Joe and Pat Yancey." Below is the 2-stage Allison, very close to the V-1710-119 configuration used in the P-51J, complete with 10.5-inch aux S/C:

The -119s installed in the F-82 were the original design, still with the helical-cut supercharger gear sets ... as ordered by the USAAF. They gave trouble in-service due to the main supercharger bearings getting spalled ... very much as expected. There were two "fixes" at the time. 1) go with straight-cut main S/C gears, or 2) go back to 7.48 : 1 main supercharger gears, neither of which the USAAF elected to do at the time. It was easier to blame Allison, let the troubles continue, and then get jets, which is what they wanted to start with, and got in due time.

Some years later, they used the same process when they wanted Boeing B-52s instead of Boeing B-47s. They let the B-47s continue to have issues that could have been fixed and then retired them when they got B-52s. All the issues with the B-47 except the range and relatively low g-limits were fixable ... but not what was wanted by the then USAF.

 

[BarnOwlLover]

What of the other issues that the 140 series V-1710s had? Namely burnt pistons and valves, detonation issues, restriction to 61 in/Hg, and fouled plugs? Reportedly NAA fixed this with an improved backfire screen, but Allison never adopted it.

IMO, from what I've read, an integral two stage supercharger like the Merlin probably would've still been better. Then again, the Allison should've been a world class design if they did fix the supercharger stuff. First, sticking to a single stage supercharger for so long/hoping that turbos would work (which only worked on the P-38) didn't help, plus other issues that have been mentioned with the attempts at two stage Allisons over the years.

Fact does seem, though, that Allison and maybe the USAAF are equally to blame for the F-82's issues, but it's also (IMO) telling that NAA didn't have a high opinion of Allison management, considering that GM owned Allison and NAA was majority owned by GM until 1948.

 

[GregP]

I've seen two G-series units built up and running, and neither had any issues at all. Burnt pistons are usually mixture-related, and anyone who has some time in the airplane and has any sort of mechanical aptitude can see if the engine is running rich or lean by looking at the exhaust color and shape. I have it in a manual, in color, too, and have seen the very things in the pictures coming out of the exhaust stacks. It isn't hard to see and correct, and I'm a rookie at it. It can be adjusted ... that's what the mixture lever is for. Detonation can be mixture or temperature-related ... or even fuel quality related since the compression ratio is not too bad at 6.00 : 1. I'm guessing pilot error or running fuel with too low a performance number. Can also happen if you descend from altitude and forget to make the mixture richer on the way down. Can happen to ANY engine at high altitudes when descending.

Not really sure since the ones I have seen run ... ran perfectly. One won Reno Bronze and Silver, and still runs fine. It also doesn't have burned pistons even though it got a bit hot due to a smallish radiator in the Yak-9.

Not saying they didn't have issues at all. I'm saying I can't confirm or deny the issues or the solutions, but I KNOW they run great when properly operated. Doesn't mean I'm claiming the pilots back then didn't have trouble, just to be clear. I'm sure they did.

I've seen a modern Merlin, in P-51D, fail at 25 hours (main bearing bits in the oil filter). Doesn't mean Merlins are bad engines ... they aren't. In fact, they are great engines. But, yes, they had a few issues, too. Likewise, a few issues with Allison G-series engines doesn't mean they were duds as engines. It means they had a few issues as-operated at the time. ALL piston engines had a LOT of issues when compared with the labor involved in running a jet engine.

 

[BarnOwlLover]

Basically, if the F/G/H were at least "strong enough", what was the minimum practical loading a fighter could be designed for during World War II (general question)?

 

[pbehn]

Basically, if the F/G/H were at least "strong enough", what was the minimum practical loading a fighter could be designed for during World War II (general question)?

To answer that, you need to define "strength" in great detail.

 

[GregP]

Strength could be the static G-load demonstrated, the calculated flight load limit, the design dive speed, or the ability to survive a snap roll above slower speeds. A strong airplane, g-load wise, might also have wings that could become a bit aeroelastic and generate roll reversal ... while still being technically strong. So, some definition might definitely be in order.

So, pbehn, what type of strength would you be asking about with your question above? Nothing more than curiosity on my part. I'm guessing a 7.3-g airplane is strong enough since they flew them in combat very successfully. But, hey, maybe a 6.3-g airplane is ALSO "strong enough." I doubt any WWII fighter could sustain more than that for a complete 360° circle anyway. Maybe it is strong enough.

I'd defer to an aeronautical engineer. I started out as one, but then switched into electrical engineering and my aerodynamics thinking is several decades old, and I never made a living at it. My class only designed one airplane before I switched into electrical engineering. It was an aerobatic aircraft. Looked sort of like a CAP 20 with semi-elliptical surfaces and tail feathers. Definitely not military. We designed it for ±12-g even though nobody ever pulled -12-g, at least deliberately.

 

[BarnOwlLover]

Our of curiosity, since max loading drops once a plane is loaded above its design weight, does the reverse happen as it loses weight or is loaded to less than design weight (as in the P51H as an interceptor, 8450 lbs vs the design weight of 9450 lbs).

 

[drgondog]

Basically, if the F/G/H were at least "strong enough", what was the minimum practical loading a fighter could be designed for during World War II (general question)?

"The minimum combination of acceleration related forces and dynamic pressure loads only 'bend', but not 'break' the airplane in the execution of planned maneuvers/performance envelope" - might be a start.

BTW, the P-51J spec called for top speed (level) of 495mph TAS at 22K with 150 octane fuel - which is beyond the contemplated top speed for NA-73, including dive. The fact is however, that the Mustang did dive well past that speed in practice - demonstrating that airframe structures design to both Limit and Ultimate Stress factors was conservative.

 

[BarnOwlLover]

"The minimum combination of acceleration related forces and dynamic pressure loads only 'bend', but not 'break' the airplane in the execution of planned maneuvers/performance envelope" - might be a start.

BTW, the P-51J spec called for top speed (level) of 495mph TAS at 22K with 150 octane fuel - which is beyond the contemplated top speed for NA-73, including dive. The fact is however, that the Mustang did dive well past that speed in practice - demonstrating that airframe structures design to both Limit and Ultimate Stress factors was conservative.

Was that applicable to both the original designs and derivatives, and the lightweights? I know that the Spitfire gets a lot of flak for being "flimsy" (though landing gear aside, pilot reports, combat damage and the speed it could achieve in dives does suggest far otherwise), but the Hawker fighters (Hurricane, Typhoon, Tempest, Fury/Sea Fury) were designed for those same standards as the Spitfire (and the LW Mustangs), and few have accused Hawker's fighters of being weak or flimsy, and that's even with the Typhoon's tail troubles early in its life.

But it does seem that it's not so much the limit that a plane is designed to--that's only part of the story. It's also in how it's applied. The P-51D it seems would've been insanely strong if it was designed for 8g load at 10,000 lbs (which was close to max take off weight without external stores), especially given that a load limits decrease with weight, they should increase as weight goes down.

 

[drgondog]

Was that applicable to both the original designs and derivatives, and the lightweights? I know that the Spitfire gets a lot of flak for being "flimsy" (though landing gear aside, pilot reports, combat damage and the speed it could achieve in dives does suggest far otherwise), but the Hawker fighters (Hurricane, Typhoon, Tempest, Fury/Sea Fury) were designed for those same standards as the Spitfire (and the LW Mustangs), and few have accused Hawker's fighters of being weak or flimsy, and that's even with the Typhoon's tail troubles early in its life.

But it does seem that it's not so much the limit that a plane is designed to--that's only part of the story. It's also in how it's applied. The P-51D it seems would've been insanely strong if it was designed for 8g load at 10,000 lbs (which was close to max take off weight without external stores), especially given that a load limits decrease with weight, they should increase as weight goes down.

The D operated on 8G for 8000 pounds, it was only rated at 6.27GLimit at 10,200 GW (no external drop tanks)

 

[GregP]

Hey Bill,

Do you know the ultimate fates of the XP-51F, G, and J? One of the J models was supposed to have been at Chino for some time before it "disappeared," but I don't recall having seen a summary of what happened to any of them. One guy told me they actually made six XP-51Js, but I have only ever seen serial numbers for two of them (well, actually 4 serial numbers, but the first two of them were supposed to have been "cancelled") along with three Fs and two Gs, so I am inclined to believe two Fs until I hear differently.

Let's see, I have the serials as:
XP-51F: 43-43332, 43-4333, and 43-43334
XP-51G: 43-43335 and 43-43336
XP-51J: 44-76027 and 44-76028 built; with 43-43337 and 43-43338 being cancelled and maybe allotted to the P-51H series. Nobody seems very sure, but I don't show any Hs with 43-xxx SNs.

I have the first J model making only seven flights by NAA (Joe Barton for the first one) and the second one making 2 flights by NAA before they were both given to Allison to test the V-1710-143 engine to be used in the XP-82. I have not seen reports or summaries of those tests.

 

[BarnOwlLover]

The D operated on 8G for 8000 pounds, it was only rated at 6.27GLimit at 10,200 GW (no external drop tanks)

I meant if it was designed for 8g at 10,000 lbs. Which would've taken quite a bit or foresight, considering that when the NA73X was designed, things like the two-stage Merlin was barely a thought, and the thought of a long range single engine, single seat fighter was essentially much the same.

Is it safe to assume that since max load goes down as weight goes up (without restressing), that max load can increase if weight goes down? IE, if the standard Mustang lineage was designed for 8 g at 8000 lbs (as in the estimated approx. gross weight of the NA-73X/first generation Allison P-51s/Mustang I and II), which if NAA was able to lighten the P51 below 8000 lbs but still with the 8g limit at 8000 lbs?

Granted, it does seem that at 5-6g the planes were fairing better than the pilots, since P-51 and P-47 pilots were among the first to suffer from grey out and even sometimes (worst case) G-LOC, hence they were among the first to use G-suits, which are standard issue for most fighter pilots since the start of the jet age following World War II.

 

[BarnOwlLover]

Hey Bill,

Do you know the ultimate fates of the XP-51F, G, and J? One of the J models was supposed to have been at Chino for some time before it "disappeared," but I don't recall having seen a summary of what happened to any of them. One guy told me they actually made six XP-51Js, but I have only ever seen serial numbers for two of them (well, actually 4 serial numbers, but the first two of them were supposed to have been "cancelled") along with three Fs and two Gs, so I am inclined to believe two Fs until I hear differently.

Let's see, I have the serials as:
XP-51F: 43-43332, 43-4333, and 43-43334
XP-51G: 43-43335 and 43-43336
XP-51J: 44-76027 and 44-76028 built; with 43-43337 and 43-43338 being cancelled and maybe allotted to the P-51H series. Nobody seems very sure, but I don't show any Hs with 43-xxx SNs.

I have the first J model making only seven flights by NAA (Joe Barton for the first one) and the second one making 2 flights by NAA before they were both given to Allison to test the V-1710-143 engine to be used in the XP-82. I have not seen reports or summaries of those tests.

A chunk of a XP-51G fuselage and some other parts (rudder, engine mounts, canopy, some other small parts) is in private ownership for at least 25 years (probably longer) and has been subject to a long term restoration to hopefully flying status. There used to be a Facebook group about it (still online), but it hasn't been active for 2-3 years with no major updates since about 2020 or so.

 

[Geoffrey Sinclair]

USAAF RC-301 reports, the format of the reports change at times, omitting or listing contract number and the program the aircraft were ordered under. A contract number was issued but not formally executed for a while.

January 1943, no mention of XP-51F or later program.
(no February 1943 RC-301 report available)
March 1943, 2 XP-51F under 1943 experimental program, contract number AC-37857, listed as uncontracted.
(No RC-301 reports available April to June)
July 1943, program changed to 3 XP-51F and 2 XP-51G no number contract or program details, listed as uncontracted.
August 1943, program changed to 5 XP-51F, no contract number or program details, listed as uncontracted.
January 1944, program changed to 3 XP-51F, 2 XP-51G, 2 XP-51J, no contract number or program details, listed as uncontracted.
(no February 1944 RC-301 report available)
April 1944, program changed to 3 XP-51F, 2 XP-51G under 1943 program, 2 XP-51J under 1944 program, no contract number details, XP-51F and G now on approved formal contracts, J on approved letter contract.
(no May or June 1944 RC-301 report available)
July 1944, program changed to 3 XP-51F, 2 XP-51G, under 1943 experimental program, contract AC-37857, 2 XP-51J under 1944 experimental program, contract AC-37857 Supplement 4, all now on approved formal contracts

The USAAF lists 43-43337/43338 as Cancelled Contract(s) and 43-43339/43340 as Higgins Aircraft C-46A-1

43-43334 to Britain, arrived August 1944, on RAF Charge as FR409 20 November, officially Off Charge 18 February 1947
43-43336 to Britain, arrived June 1945, on RAF Charge as FR410 14? June, officially Off Charge 26 June 1947

G-suits were around and used in WWII, according to the experiments there was a range where pilots would begin to experience G problems with 6G about the limit where everyone did, the experimental RAAF suit could push tolerance toward 9 or 10 G, leading to worries about pilots over stressing the aircraft, to which a test pilot replied he preferred to be aware of the aircraft falling apart.

 

[GregP]

Hi BarnOwlLover,

Thanks for the data.

As Bill said above, the P-51D was designed for 8.0 g at 8,000 pounds. To find the G-load at heavier weights, you use the formula:

design g-load = 64,000 / gross weight

Edit: At lighter weights, the limit is still 8.0 g.

So, at 10,000 pounds, the P-51D is a 6.4 g airplane with an ultimate safety factor up to 9.6 g (50% safety factor). The safety factor does not mean you can pull 9.6 g at 10,000 pounds and be "OK." It means there should be no airframe damage up to 6.4 g at 10,000 pounds and, if you pull between 6.4 g and 9.6 g at that weight, you MAY get plastic deformation damage and likely will as you increase the g-load. The airplane should stay together and remain flyable up to 9.6 g at 10,000 pounds, but anything more than 9.6 g and it could break. If you were to actually pull 9.6 g at 10,000 pounds, the airplane might remain serviceable and it also might be scrap. Generally, you'd expect damage that would make the airplane doubtful to remain in service. There are records of P-51s coming home from Europe with a few more degrees of wing dihedral than stock after over-g, and they were scrapped on landing. Undamaged, parts were salvaged, but the airframe was basically unflyable thereafter.