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?

[GregP]

Hi Sinclair,

Thanks.

Where did you find those? Very curious. Sources are one of things we all want to see!

 

[special ed]

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.

A bit of trivia. Higgins Marine, in New Orleans, was contracted to build C-46s in their facility in the city. Two were completed as large assemblies and shipped to Curtiss Wright St.Louis for final assembly. This was obviously going to be difficult so the plan was dropped. Another bit of trivia: Higgins built and flew a nice co-ax helicopter in 1943.

 

[special ed]

Another bit of trivia about Higgins Marine in New Orleans. The workforce was fully integrated, black, white, male, female working side by side. Trivia lost in time.

 

[drgondog]

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?

Airframe design is complicated enough without introducing unknown mission requirements beyond those specified as 'Acceptable'. NA-73X was built to original specs at 8000 pounds with full armament (to include 20mm) but with only 150 gal fuel. Immediately thereafter, as RAF/RAE engaged with NAA for production requirements, the addition of self sealing tanks and desired internal fuel from 150 to 170gal alone increased the GW by several hundred pounds.

Notably NA 73XX built same as NA73 for Static tests FAILED between Limit and Ultimate Load tests with some buckling experienced in shear panels aft of wing, indicating deformation (minimally) in longerons. This was in December 1940 an drove NAA to apply redesigned patches for NA-73 production articles #11 through #320. That is the reason that #1 through #10 were declared unsuitable for combat ops. That said, the Brits ignored the 'advice' and did just fine in the low level Army Co-operation role. Ditto AAF and NACA as XP-51 #4 and #10 served on through dive testing and manuever testing thrugh EOW and XP-51 41-038 lives today at EAA.

Yes, safe to assume that if the Mustang was operating Under 8,000 pounds, the allowable Limit Load of 8G could be Increased marginally. That said, the -51D was at 7900 pounds without fuel or ammo. The P-51A with half fuel was at 8,000. The P-51/F6/Mustang I with half fuel was at 8.000.

The addition of the Merlinand increasing internal fuel from 170gal to 184 to 269 w/fuse tank added 1000 pounds - More guns and ammo increased the P-51D GW over the 51B. NAA and AAF-MC knew this but accepted mission creep and solved by introducing the V-n diagram in the Operating Manual to explain the consequences of increasing Gross Weight.

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.

The RAFused G-suits prior to 8th AF. In fact the first G-suits in fall 1944 were RAF types. While dive pull out is a factor in AoA Limit Loads, high G turns were equally considered, hence the illustration of N (load factor) versus speed on left side of the chart. High G is directly associated with High CL and AoA and instantaneous high G was frequently attained with a snap roll - or very hard bank to turn. Both those manuevers also put, then as yet unknown,huge asymetric loads on the empennage.

 

[drgondog]

A bit of trivia. Higgins Marine, in New Orleans, was contracted to build C-46s in their facility in the city. Two were completed as large assemblies and shipped to Curtiss Wright St.Louis for final assembly. This was obviously going to be difficult so the plan was dropped. Another bit of trivia: Higgins built and flew a nice co-ax helicopter in 1943.

To annotate Sinclair's RC-301 info.
NAA began XP-51F with NA-105 charge number on 1-2-43 re: AC-37857 for three XP-51F airframes with 1650-3 engines

(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.

NAA began XP-51G with NA-105A charge number on 7-2-43 re: AC-37857 amended for three XP-51F and two XP-51G (with R-R R.M. 14 S.M.).

Formal Meeting between NAA (C.E.Patterson Preliminary Design/XP-51F/G Projects) , AAF-MC (Col Roth – Chief Experimental Airplane Section) , Allison (Ed Newill, R.M. Hazen) (O.E. Hunt and Don Berlin) on September 15, 1943. Discussion – High Performance LW w/Allison 1710-119 engine. Tentative agreement to begin detail design after NA-105A package released t Experimental Dept.

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.

NAA began XP-51J with NA-105B charge number on 1-2-44 re: AC-37857 amended for three XP-51F, two XP-51G and two XP-51J

(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

In each case, as in many previous and succeeding authorizations for NAA to begin work, Letter contracts whether for Material Change Requests or for new airframe, were issued either as Supplements to existing contract like P-51B and protortype P-51D as addendums to NA-91 AC-30479. For XP-51F, the initial authorization was from H.H. Arnold to Kindleberger via phone conversation when discussing the NAA LW Fighter proposition following the November, 1942 P-51B vs Spitfire IX Weight Comparisons. The NA-105 team was immediately organized in late December 1942 and Letter Agreement followed on Jan 2, 1943.

One may only wonder why it was difficult for AAF team issuing RC-301 to gather contract number data.

 

[BarnOwlLover]

One question: if the 8g at 8000 lbs weight was figured as 64,000 divided by gross weight, what mathematics were used for the British/lightweight P-51 standards for the 7.33g loading at X gross weight (I suck at math, by the way)?

 

[BarnOwlLover]

Airframe design is complicated enough without introducing unknown mission requirements beyond those specified as 'Acceptable'. NA-73X was built to original specs at 8000 pounds with full armament (to include 20mm) but with only 150 gal fuel. Immediately thereafter, as RAF/RAE engaged with NAA for production requirements, the addition of self sealing tanks and desired internal fuel from 150 to 170gal alone increased the GW by several hundred pounds.

Notably NA 73XX built same as NA73 for Static tests FAILED between Limit and Ultimate Load tests with some buckling experienced in shear panels aft of wing, indicating deformation (minimally) in longerons. This was in December 1940 an drove NAA to apply redesigned patches for NA-73 production articles #11 through #320. That is the reason that #1 through #10 were declared unsuitable for combat ops. That said, the Brits ignored the 'advice' and did just fine in the low level Army Co-operation role. Ditto AAF and NACA as XP-51 #4 and #10 served on through dive testing and manuever testing thrugh EOW and XP-51 41-038 lives today at EAA.

Yes, safe to assume that if the Mustang was operating Under 8,000 pounds, the allowable Limit Load of 8G could be Increased marginally. That said, the -51D was at 7900 pounds without fuel or ammo. The P-51A with half fuel was at 8,000. The P-51/F6/Mustang I with half fuel was at 8.000.

The addition of the Merlinand increasing internal fuel from 170gal to 184 to 269 w/fuse tank added 1000 pounds - More guns and ammo increased the P-51D GW over the 51B. NAA and AAF-MC knew this but accepted mission creep and solved by introducing the V-n diagram in the Operating Manual to explain the consequences of increasing Gross Weight.

The RAFused G-suits prior to 8th AF. In fact the first G-suits in fall 1944 were RAF types. While dive pull out is a factor in AoA Limit Loads, high G turns were equally considered, hence the illustration of N (load factor) versus speed on left side of the chart. High G is directly associated with High CL and AoA and instantaneous high G was frequently attained with a snap roll - or very hard bank to turn. Both those manuevers also put, then as yet unknown,huge asymetric loads on the empennage.

One issue is that when the NA-73X was designed (and hence basically the early Allison P-51s) was that there was really no two stage Merlins (the were maybe a thought at Rolls-Royce, but were far from even prototype stage in the fall of 1940). Adding the second supercharger stage to the Merlin alone added about 300 or so pounds vs the single stage version. Trivia: the Merlin now was about as heavy as a single stage R-R Griffon, which itself faced similar weight creep due to going to the two stage route.

Also, in 1940, few fighters weighed 8000 lbs gw under normal conditions (this would be true within a year for the early Fw 190s, which suggests that fighters being designed new as "light fighters" in 1940 were growing themselves, let alone "heavy" fighters with 2000+ hp engines from the start like the R-2800 powered P-47s and the Hawker Typhoon/Tempest line). However, weight creep was basically an accepted fact. The Spitfire V was several hundred pounds heavier than the I or II, and the IX/VII/VIII were about a 1000 lbs heavier than the V. And that was over the course of 1940-1942.

Obviously, operational requirements, more power, and more armament almost always resulted in weight increases. Of course, that could be headed off as NAA did with the LW Mustangs (culminating in the P-51H) though a detail redesign to reduce weight, though again, the LW P-51s were redesigned to British 7.33g at GW standards. And also, the XP-51F/G were intended to be GP/interceptor fighters (like the original P-51B/D, pre-fuselage tank), though they again carried more internal fuel (again, pre-B/D model fuselage tank).

Issue ultimately was the desire for increased range, increased armament (XP-51s had 4 .50s, though I do believe an interceptor version of the F was intended to use 4 20mm cannons?), and I did read that the F/G was stressed to carry 500 lb bombs (though I think it was stated here they were ultimately cleared for 1000 lb'ers) resulted in the H production model being 1500-nearly 2000 lbs heavier than the F/G, plus re-stressing to keep weight creep from compromising the 7.33g limit at normal gross weight (about 9500 lbs for the H on max internal fuel, without external stores).

Still, with the H you got a faster, faster climbing, faster accelerating, lighter and ultimately stronger per pound P-51, aside from maybe the landing gear.

Granted, these are all points we and others have been over before to one degree or another.

But basically, weight creep is inevitable, but I guess it's down to either what the customer is willing to accept, and/or how the engineers can head it off/reign it in. Interestingly, after he left North American, Ed Schmued designed the F-5 for Northrop, which was an attempt to address Cold War era weight creep and complexity of many jet fighters of the era.

 

[Geoffrey Sinclair]

As noted before Air Force Historical Research Agency, files ADM-1 to 37, ADO 1 to 7, ADT-1 to 20, 22 to 50 are the US(A)AF delivery logs. More detail: incomplete, most comprehensive from 1941 to the late 1940's, earliest USAAF serial 38-471, last 75-125, includes some missiles when they were given serials, includes some foreign orders, some RAF serials between AE711 and JX269.

Actually found I did have the February 1943 RC-301 and it mentions the 2 XP-51F.

One may only wonder why it was difficult for AAF team issuing RC-301 to gather contract number data.

The formats of the reports change over time, as people decided what was key information, after the block number system came in the production reports included that information for a while. In February 1943 the layout of the experimental types report is designation, maker, factory, then 8 columns of data, in July 1944 it is designation, maker, factory, then 11 columns of data, in July 1945 it is designation, maker, factory, then 12 columns of data, including that the XP-51 was a no cost contract, the XP51F to J was a fixed price one. At times they decided the contract number was not needed.

 

[BarnOwlLover]

I was reading though one of my recent book purchases last night, and again, it implied that the XP-51F/G were designed to different loadings than the 7.33g that the P-51H was. But reading and looking at performance reports, I do wonder if the performance at the end of the day between the XP-51F/G and the P-51H was really all that different?

To compare as much as apples to apples as possible, I did find a graph for estimated performance of the P-51G at a gross weight of almost 7900 lbs (probably a realistic combat weight), and it's rate of climb maxed out at about 5700-5800 fpm. That's not far from what the P-51H was capable of on WEP in interceptor trim (ranging between 5480-5850fpm depending on condition). Not to mention that the book I read last night hinted that the USAAF was possibly looking at removing fuel tanks from the P-51H (fuselage tank and one wing tank) to make it a point interceptor against IJA and IJN conventional and Kamikaze attacks.

Also, the XP-51F the RAE evaluated had a gross weight of 7850+ lbs in interceptor trim. I really hope that volume 2 of the P-51B Bastard Stepchild book will shed some more light on this chapter, given that the LW Mustangs, the P-51H and the XP-82/P-82B Twin Mustang will be a focus.

 

[drgondog]

I was reading though one of my recent book purchases last night, and again, it implied that the XP-51F/G were designed to different loadings than the 7.33g that the P-51H was. But reading and looking at performance reports, I do wonder if the performance at the end of the day between the XP-51F/G and the P-51H was really all that different?

To compare as much as apples to apples as possible, I did find a graph for estimated performance of the P-51G at a gross weight of almost 7900 lbs (probably a realistic combat weight), and it's rate of climb maxed out at about 5700-5800 fpm. That's not far from what the P-51H was capable of on WEP in interceptor trim (ranging between 5480-5850fpm depending on condition). Not to mention that the book I read last night hinted that the USAAF was possibly looking at removing fuel tanks from the P-51H (fuselage tank and one wing tank) to make it a point interceptor against IJA and IJN conventional and Kamikaze attacks.

Also, the XP-51F the RAE evaluated had a gross weight of 7850+ lbs in interceptor trim. I really hope that volume 2 of the P-51B Bastard Stepchild book will shed some more light on this chapter, given that the LW Mustangs, the P-51H and the XP-82/P-82B Twin Mustang will be a focus.

They (XP-51F/G/J and P-51H were all designed to Limit loading of 7.33G and ultimate = 1.5xLimit (11G) - but for different comat gross weights.

I devote time in the book covering the Gensis and development of the XP-51F because the first flight was in February 1944 and the end of my book was D-Day. I present the performance testing against light P-51B with each having 1650-3 engine at 67". According to Chilton, the 'hot rod' at 75" was his favorite Mustang to fly... that comment is anecdota as I don't have published source.

 

[pbehn]

They (XP-51F/G/J and P-51H were all designed to Limit loading of 7.33G and ultimate = 1.5xLimit (11G) - but for different comat gross weights.

What does that mean to a pilot? Can a pilot tell what "G" load he has? I can understand that at low levels he has a great idea of the difference between 1, 2 and 3 G but at higher levels it varies between person to person and day to day. Humans are notoriously bad at gauging such things. Maybe BiffF15 could give some insight.

 

[drgondog]

What does that mean to a pilot? Can a pilot tell what "G" load he has? I can understand that at low levels he has a great idea of the difference between 1, 2 and 3 G but at higher levels it varies between person to person and day to day. Humans are notoriously bad at gauging such things. Maybe BiffF15 could give some insight.

Well, Biff will give you an Informed opinion on high G.

That said, most WWII pilots never flew sustained G above 4-5G. The G level relates to Allowable stress (for 2024 dural/aluminum or steel components/assemblies/fittings, etc)- around the Yield Point at which the material has a permanent set if exceeded. Ultimate was simply a 'guess' of 1.5 factor theoretically with safety margin before breaking the airplane. The issue with both Yield Point and Ulimate is best guess because so little was known about aeroelasticity - or even had a proven grasp on both lift related aero loads as well as asymmetrical loads imposed during manuevering flight. The highest G 's experienced wee usually high AoA during terminal dive pull outs and the highest combined and oft exceeded rolling/skidding during dives - most ended many feet under the turf or scattered far and wide as the airplane disintegrated.

 

[BiffF15]

What does that mean to a pilot? Can a pilot tell what "G" load he has? I can understand that at low levels he has a great idea of the difference between 1, 2 and 3 G but at higher levels it varies between person to person and day to day. Humans are notoriously bad at gauging such things. Maybe BiffF15 could give some insight.

To be honest it's tough to tell what G you are at, and I've tried guessing. The G suit starts to inflate in the Eagle at about 2.5 - 3, and is full inflated shortly thereafter. At about 5 you have to start working (perform your anti-G straining technique), and by 7.5 to 8 it starts to hurt.

I'm in agreement with Bill that it probably wasn't often that guys pulled max Gs back in the day. Modern jets have G meters but I'm not sure when they became prolific in planes.

Fresh pilot, good nights sleep, hydrated will have a 2-3 G tolerance advantage over himself hungover with min sleep and dehydrated. Or so I've heard…

 

[pbehn]

To be honest it's tough to tell what G you are at, and I've tried guessing. The G suit starts to inflate in the Eagle at about 2.5 - 3, and is full inflated shortly thereafter. At about 5 you have to start working (perform your anti-G straining technique), and by 7.5 to 8 it starts to hurt.

I'm in agreement with Bill that it probably wasn't often that guys pulled max Gs back in the day. Modern jets have G meters but I'm not sure when they became prolific in planes.

Fresh pilot, good nights sleep, hydrated will have a 2-3 G tole advantage over himself hungover with min sleep and dehydrated. Or so I've heard…

Thats what I thought, in WW2 with no G suits and the usual seating position in the middle of a 6 hr mission everyone would have a different idea of what it is. I also presume hurtling towards the ground or being under fire affects your judgement?

 

[BarnOwlLover]

They (XP-51F/G/J and P-51H were all designed to Limit loading of 7.33G and ultimate = 1.5xLimit (11G) - but for different comat gross weights.

I devote time in the book covering the Gensis and development of the XP-51F because the first flight was in February 1944 and the end of my book was D-Day. I present the performance testing against light P-51B with each having 1650-3 engine at 67". According to Chilton, the 'hot rod' at 75" was his favorite Mustang to fly... that comment is anecdota as I don't have published source.

I know what one of my next book purchases will be. So we know if there's a second book where it'll start (around D-Day), but any idea where it'll end?

Also, looking at more charts of the XP-51G and the P-51H, and it does seem that for a more apples to apples comparison (especially since all else being equal the G should've weighted a bit more than the F based on local strengthening for the RM 14SM Merlin, and the engine being a bit heavier than the V-1650-3/7), it seems that at the end of the day, a P-51G offered maybe marginal performance improvement over the P-51H in interceptor trim (about 10 mph faster, slightly higher or at least similar climb on WEP), but with the penalties of reduced max range on internal fuel, reduced armament, somewhat reduced versatility and reduced directional stability.

Still, I wonder what an intermediate between the XP-51G and the P-51H could've done as a cannon armed interceptor?

 

[drgondog]

I know what one of my next book purchases will be. So we know if there's a second book where it'll start (around D-Day), but any idea where it'll end?

Also, looking at more charts of the XP-51G and the P-51H, and it does seem that for a more apples to apples comparison (especially since all else being equal the G should've weighted a bit more than the F based on local strengthening for the RM 14SM Merlin, and the engine being a bit heavier than the V-1650-3/7), it seems that at the end of the day, a P-51G offered maybe marginal performance improvement over the P-51H in interceptor trim (about 10 mph faster, slightly higher or at least similar climb on WEP), but with the penalties of reduced max range on internal fuel, reduced armament, somewhat reduced versatility and reduced directional stability.

Still, I wonder what an intermediate between the XP-51G and the P-51H could've done as a cannon armed interceptor?

The second book starts in late 1942 when both the improved sliding canopy and the 6-gun wing emerge from Preliminary Design and the Kindleberger/Arnold conversation about LW Mustang spawn the Xp-51F. The six gun wing is accepted by AAF in January, 1943 and planned for 'super P-51B/C' as P-51D/E via NA-106/NA-107. The "Cockpit Enclosure, Sliding" emerges from A6M like configuration to full blown canopy without ribs in March 1943, with mods of P-51B-1 fuselage starting in July 1943.

P-51D & E were dropped in favor of P-51D-NA and P-51D-1-NT, originally planned for insertion for P-51D-NA after P-51B-5-NA 43-6712, but various delays between airframe strucures analysis and subsequent changes to the new fuselage with cut down after deck pushed the introduction insertion well into B-10 block.

As of now it ends with P-51M. Having difficult time finding ecessary flight test reports for either Merlin or Allison P-82.Nor can I find the NA-125 or NA-123 P-82 Model Airplane Specifications. So much was destroyed by Rockwell.

 

[Peter Gunn]

*SNIP*

As of now it ends with P-51M. Having difficult time finding ecessary flight test reports for either Merlin or Allison P-82.Nor can I find the NA-1250 or NA-123 P-82 Model Airplane Specifications. So much was destroyed by Rockwell.

AAARRGGHHH!!!

 

[BarnOwlLover]

Too bad that you're having so much of a hard time finding stuff about the early development of the F-82. Makes me wonder if Boeing might have anything left, or if the XP-82 restoration project can help. Also might explain why I've been having trouble finding data about XP-82/P-82B testing and such by either North American or the USAAF. I do know that photos and such of wind tunnel models and mock ups of the P-51H and XP-82/P-82B exist, but without the tech reports and info the only paint part of the picture.

Sort of ironic, though, that so much stuff involving the LW P-51s still exists, though. I'd bet that the photos of Bob Chilton running up a XP-51F might be the closest we'll get to a LW Mustang being caught in flight.

 

[BiffF15]

Thats what I thought, in WW2 with no G suits and the usual seating position in the middle of a 6 hr mission everyone would have a different idea of what it is. I also presume hurtling towards the ground or being under fire affects your judgement?

G suits showed up at some point but don't know to what level they were issued.

Should you get into a fight your adrenaline level would spike through the roof, and that's regardless of where you are in the timeline. Depending on the individual, time compression or dilation will or can occur. Being under fire or having a face full of ground rush will get your attention. Some guys are as cold as ice and unfazed (don't lose their perspective or focus) regardless of their task loading while others are not as capable. It's a spectrum and is definitely influenced by experience. It's also task prioritization, some are better than others.

 

[PAT303]

Being under fire or having a face full of ground rush will get your attention. Some guys are as cold as ice and unfazed (don't lose their perspective or focus) regardless of their task loading while others are not as capable.

I'm in the second group, if I'm getting fired at or hurtling towards the ground I'm hiding behind the armor with poo coming out.