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?

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BarnOwlLover

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Nov 3, 2022
Mansfield, Ohio, USA
I was able to read an excerpt from a book on the development of the P-51H Mustang, and it stated that the XP-51 series that begant the H's development was designed for 6+Gs in flight for loading (basically British recommendations during World War II for fighter aircraft), while the USAAF preferred 7.33Gs (of note, all P-51s from the NA73X through the D/K were designed for 8 Gs at approx. 8000lbs gross TO weight). Is there any truth to that statement?

Also, it does bring up a point on how strong is strong enough for a World War II era fighter. If the Spitfire was designed to what the book alleged was RAF standards, I'd have to argue aside from landing gear that the Spitfire was more than strong enough, given that at least one was dived at more than 600mph (corrected for compressiblity) with no damage, albeit on accident in 1952 over Hong Kong.

And it's been written on here that the 8 G for 8000 lbs for the P-51D dropped as low as 6.3 Gs depending on gross weight, so not too far from Spitfire/LW Mustang standards for their gross weights.

Thus, is the statement that I'm referring to about the XP-51's correct or in error? And what G loading standards were too much, just right or not enough for the era?
 
Hey BarnOwlLover,

I ran across this ARC (Aeronautical Research Council) report a few years ago. In brief, the British installed g-meters in a bunch of aircraft types and recorded the maximum +/- g-loads on the airframes during missions. The results are interesting.

re your question "what's a good G loading for a World War II fighter?"

Check out the Tables 1-3 on pages 15-20 in particular, where the maximum and average positive and negative g-loads actually encountered are listed. They do need to be read with the understanding that the American fighter types (Thunderbolt, Mustang, Lightning) and the Spitfire were performing different missions to a significant degree. It implies in the text of the report that the P-47s missions were primarily high(er) altitude bomber escort, the Mustang (Allison powered variants) missions were lower altitude recon and ground attack, and the P-38s missions were (all?) unarmed reconnaissance. The Tables give data for the Spitfire Mk IX only, so presumably the missions were shorter range escort and all around fighter/intercepter.
 

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Still wondering if the info in that book excerpt is accurate about the 6+G rating of the lightweight Mustangs prior to the P-51H. This was from the book "North American P-51H Mustang (Air Force Legends)", and the copyright of the chunk of it I read was 2000. So the info could be out of date. Or it could've been misinterpreted, as those were apparently British loading standards, or maybe what the Spitfire was designed to. Or the LW Mustangs were designed to approximate the weight of the Spitfire IX while carrying more fuel.

Anyone know which points here may be right or wrong?
 
This is a very complex subject, most of your questions have been answered on other threads, maybe if you read them again and then again as I have at times you will see the answers.
 
I have searched for further details, and I have to say that either I'm not good at searching on here, or the info is pretty widely scattered, or both.

I'm still trying to find all the info I want and decipher what's accurate and what's not or outdated into.

Basically, I'd like to add this and see if it makes sense as far as trying to get some info that the most accurate. The P-51 from the NA73X, though the Mustang I to the P-51D/K (and presumably M) were designed to the 8g at 8000lbs standard. The P-51H/L and presumably the F-82 were designed to 7.33g at max normal combat weight. The book excerpt that I mentioned said that the XP-51F/G was designed to 6g, under which conditions is not clear.

Now, I need to make it clear that I'm not an expert in such areas of design. I'm also unclear, given various sources that I've read on here and else where that typically suggest that the F/G were designed to the same load limits as the H, or British standards (whatever they were during World War II). Just trying to sort out which is correct, which isn't. Not to mention that I've never been able to find much detailed info on the XP-51 lightweights. I've found quite a bit on the H, but not that transitional planes between the D and the H. Which probably shouldn't be surprising, given that relatively few photos or articles/books exist.
 
OK, a "non-technical" hip-shooting summation of my understanding:
Despite being a British project, as you say, the original Mustang was designed to USAAC standards. I don't know whether Schmued et al just thought it was a good idea, or whether they didn't want to give the American brass that easy reason to disqualify it from consideration in case of future developments.

From that origin through the "main branch" of Mustang evolution (B, D), my impression- and it is primarily only that- is that they did their best to keep up as the flight envelope and especially gross weight kept increasing. However, all of this was in the context of urgency of time and of minimizing production disruption, so it was mostly "spot fixes". Certainly there's relatively little structural change from Mustang I through the B/C, somewhat more in the D, but then it also incorporates a growth of armament in the wing and the first big change to the basic fuselage.

Meanwhile, one of the biggest complaints about the Mustang was its relatively poor rate of climb (compared to a Spitfire?), which was largely blamed on weight. While the B/D Merlin development was going on, NAA was therefore quite concerned about controlling the weight of the new versions. (I'm not too confident, but I think some efforts at weight reduction were also made on the P-51A, which was virtually a parallel development.) The arrival of Faber's Fw 190 in the UK in June turned up the heat on weight consciousness, and some of the trade-offs and structural methods evident in the 190's design suddenly became things for Allied aircraft designers to consider.

While adapting the existing airframe to the Merlin, and further evolving to incorporate improved view and other desired features, Schmued, and others, therefore began to ALSO think about what a "clean sheet" Merlin Mustang could be like. Part of the early studying in this direction was a structural weight comparison/analysis between the Mustang and the Spitfire. (Note that this was well underway BEFORE Schmued's trip to England in early '43.) It was logical, in the interest of weight reduction, to design to the reduced factors of British standards. (I have a vague idea that NAA got US approval to do this, at least for the purpose of this experiment.)

Now when it came to the P-51H I think the AAF re-asserted their own desires (stresses and increased armament) which started pushing the weight up again. I'm on shaky ground with that statement, though...
 
The P-51 up through the D model had a G-limit that varied with weight. It was 8.0 g up to 8,000 pounds, and was 64,000/ gross weight for weights above 8,000 pounds.

So, at 10,000 pounds, for instance, that works out to 6.4 g load limit. For 7.33 g's, that works out to 8,732 pounds.

That comes from a P-51D pilot's operating manual.

Bob Chilton, the test pilot for the P-51H, said his favorite Mustang was the P-51F with the Allison V-1710-119 in it. Can't quite remember why, but he did specify that one as his personal favorite P-51 ride.
 
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The P-51 up through the D model had a G-limit that varied with weight. It was 8.0 g up to 8,000 pounds, and was 64,000/ gross weight for weights above 8,000 pounds.

So, at 10,000 pounds, for instance, that works out to 6.4 g load limit. For 7.33 g's, that works out to 8,732 pounds.

That comes from a P-51D pilot's operating manual.

Bob Chilton, the test pilot for the P-51H, said his favorite Mustang was the P-51F with the Allison V-1710-119 in it. Can't quite remember why, but he did specify that one as his personal favorite P-51 ride.
This reflects the P-51 moving into a different "territory", more like that of a bomber, where max loads change through the mission. When all fighters re doing the same thing, with approximately the same internal fuel then things are comparable. With the P-51 it could carry a huge amount of internal fuel to the combat and since its whole operating procedure was ro preserve internal fuel a "normal" combat load could be considered to be 200+ gallons.
 
The P-51 up through the D model had a G-limit that varied with weight. It was 8.0 g up to 8,000 pounds, and was 64,000/ gross weight for weights above 8,000 pounds.

So, at 10,000 pounds, for instance, that works out to 6.4 g load limit. For 7.33 g's, that works out to 8,732 pounds.

That comes from a P-51D pilot's operating manual.

Bob Chilton, the test pilot for the P-51H, said his favorite Mustang was the P-51F with the Allison V-1710-119 in it. Can't quite remember why, but he did specify that one as his personal favorite P-51 ride.
Didn't the F use the same V-1650-3/7 Merlin as the P-51B/D? I've also read that Chilton (like many others at NAA) didn't like the two stage Allisons.

Still doesn't quite answer my question about load standards for the F/G and how it translated to the H (and maybe the F-82). The Lightweights are the missing link on how we got from the D/K to the H, and little has been written about them and I haven't seen much technical documentation on them, be it written or items such as plans or design schematics.
 
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You are so right, the third prototype of the lightweight Mustangs was powered by the ALlison V-1710-119 and was called a P-51J. Chilton mentioned the Allison -119 engine variant as his favorite. The "F" part was my error. Unfortunately, the Allison V-1710-119 was never put into quantity production, but it had an F-series type nose case and a 2-stage supercharger. 1,900hp/3,200 rpm/78.0 in MAP/S.L. and 1,200hp/3200rpm/58.0 in MAP/30,000 ft according to enginehistory.org. Definitely an impressive engine, but came right as interest in jets was on a steep rise while interest in military pistons was on a similar decline.

The F, G, and J were basically the same aircraft with different engines. All had a new wing, with an NACA 66,2-(1.8)15.5 a=.6 at the root and an NACA 66,2-(1.8)12 a=.6 at the tip. The P-51J was loaned to Allison. None went into production, but they were definitely performers.

Quite a bit of the lightweight Mustang design features went into the P-51H. It was a performer, too, hitting 472 mph at 21,200 feet. That's pretty quick. It also climbed at more than 5,000 feet per minute at lower altitudes and was still climbing at 4,000 feet per minute at 18,000 feet. Again, pretty good for the time and is about even with a Spitfire XIV climb rate. That means the P-51H was a decent step up from a P-51D as far as performance goes, particularly in climb at high power settings.
 
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Actually, a P-51H was tested by North American in 1946 in interceptor trim (clean, gross weight of about 8500 lbs), and it hit 487 mph on 90 in/Hg at 25,000 ft. This was also with a fully functional ADI system (a problem that took nearly a year to resolve), by which time the use of the H as an interceptor was academic with war's end and the focus on jets. However, in the same trim, climb rates of over 6000 fpm were also realized. These charts can be found at World War II Aircraft Performance, as well as some information on the lightweights when tested.

Granted, the P-51F was flown with a Packard Merlin rigged to give 90 in/Hg boost in an attempt to reach 500 mph (the G with the Merlin RM 14SM was also tried for this), and it topped out at about 493 mph, the G reportedly 498 mph. This is per the book Building the P-51 Mustang section on the LW development.

Also of possible interest may be this article: The Mustang that might have been

I do have to admit that the info and such floating around about the LW Mustangs is IMO fairly meager. Which is why I'm looking for like books and other good sources of info on them.
 
I was able to read an excerpt from a book on the development of the P-51H Mustang, and it stated that the XP-51 series that begant the H's development was designed for 6+Gs in flight for loading (basically British recommendations during World War II for fighter aircraft), while the USAAF preferred 7.33Gs (of note, all P-51s from the NA73X through the D/K were designed for 8 Gs at approx. 8000lbs gross TO weight). Is there any truth to that statement?

Also, it does bring up a point on how strong is strong enough for a World War II era fighter. If the Spitfire was designed to what the book alleged was RAF standards, I'd have to argue aside from landing gear that the Spitfire was more than strong enough, given that at least one was dived at more than 600mph (corrected for compressiblity) with no damage, albeit on accident in 1952 over Hong Kong.

And it's been written on here that the 8 G for 8000 lbs for the P-51D dropped as low as 6.3 Gs depending on gross weight, so not too far from Spitfire/LW Mustang standards for their gross weights.

Thus, is the statement that I'm referring to about the XP-51's correct or in error? And what G loading standards were too much, just right or not enough for the era?
Regarding the XP-51J:
North American Engineering Report No. NA-8033, Model Specification for the Model XP-51J Airplane, dated1-1-1944, on page19 says the following about "Structures."
Criteria for Design/Flight Maneuver Factors: Design Gross Weight 7,550 lbs, +7.33 g, -3.67 g. Diving speed limit is 475 mph IAS.
Dan Whitney
 
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.
 
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.
What was the Spitfire dive speed limit according to the pilot manual. Wasnt the Spitfire dived to 600MPH fitted with a prop that could be feathered just for that purpose and dived under very controlled, non combat conditions?
 
There were those trials, and one that was dived at those speeds on accident over Hong Kong in 1952:

"On 5 February 1952, a Spitfire 19 of 81 Squadron based at Kai Tak in Hong Kong reached probably the highest altitude ever achieved by a Spitfire. The pilot, Flight Lieutenant Edward "Ted" Powles,[132] was on a routine flight to survey outside air temperature and report on other meteorological conditions at various altitudes in preparation for a proposed new air service through the area. He climbed to 50,000 ft (15,000 m) indicated altitude, with a true altitude of 51,550 ft (15,710 m). The cabin pressure fell below a safe level, and in trying to reduce altitude, he entered an uncontrollable dive which shook the aircraft violently. He eventually regained control somewhere below 3,000 ft (910 m) and landed safely with no discernible damage to his aircraft. Evaluation of the recorded flight data suggested he achieved a speed of 690 mph (1,110 km/h), (Mach 0.96) in the dive, which would have been the highest speed ever reached by a propeller-driven aircraft if the instruments had been considered more reliable.[129]"
 
There were those trials, and one that was dived at those speeds on accident over Hong Kong in 1952:

"On 5 February 1952, a Spitfire 19 of 81 Squadron based at Kai Tak in Hong Kong reached probably the highest altitude ever achieved by a Spitfire. The pilot, Flight Lieutenant Edward "Ted" Powles,[132] was on a routine flight to survey outside air temperature and report on other meteorological conditions at various altitudes in preparation for a proposed new air service through the area. He climbed to 50,000 ft (15,000 m) indicated altitude, with a true altitude of 51,550 ft (15,710 m). The cabin pressure fell below a safe level, and in trying to reduce altitude, he entered an uncontrollable dive which shook the aircraft violently. He eventually regained control somewhere below 3,000 ft (910 m) and landed safely with no discernible damage to his aircraft. Evaluation of the recorded flight data suggested he achieved a speed of 690 mph (1,110 km/h), (Mach 0.96) in the dive, which would have been the highest speed ever reached by a propeller-driven aircraft if the instruments had been considered more reliable.[129]"
In an "uncontrollable dive"!!!!
 
The P-51 up through the D model had a G-limit that varied with weight. It was 8.0 g up to 8,000 pounds, and was 64,000/ gross weight for weights above 8,000 pounds.

So, at 10,000 pounds, for instance, that works out to 6.4 g load limit. For 7.33 g's, that works out to 8,732 pounds.

That comes from a P-51D pilot's operating manual.

Bob Chilton, the test pilot for the P-51H, said his favorite Mustang was the P-51F with the Allison V-1710-119 in it. Can't quite remember why, but he did specify that one as his personal favorite P-51 ride.
Greg - Chilton and Barton despised the NA-105B P-51J (with Allison). Chilton's favorite Mustang was the NA-105 P-51F with 1650-3 engine. The Two P-51Js had a total of 5 troubled hours when they gave them to Allison.
 
It DOES seem meager, especially given the raw performance of the type. I'd think EVERYONE who might have been a potential customer would have been interested, but the remaining documentation seems meager, as you say.
Greg - Chilton and Barton despised the NA-105B P-51J (with Allison). Chilton's favorite Mustang was the NA-105 P-51F with 1650-3 engine. The Two P-51Js had a total of 5 troubled hours when they gave them to Allison.

Hi Bill. Was going on a talk given by a historian at a museum. Didn't check it out personally before posting. Do you have a good source for Chilton stories? Curiosity only from me. I never really looked at the F, G, and J because they made only two each (I believe the S'Ns were 44-76027 & 44-76028 for the J's), and never got deployed anywhere, so didn't affect any outcomes.

I've seen a report saying the P-51J was a great performer, but the report was short summary of performance; nothing in much detail. It's been awhile, but it seems like it was maybe two paragraphs. Would be nice to see some actual flight test reports.

My friend Joe Yancey has some parts from a -119 Allison ... a one-off manifold going into an intercooler instead of the large intake tube and a supercharger housing that looks different from all other ALlison housings I have seen including the G-series housings. It appears to be a 2-speed unit and definitely fits a V-1710 power section.

Cheers.
 
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It DOES seem meager, especially given the raw performance of the type. I'd think EVERYONE who might have been a potential customer would have been interested, but the remaining documentation seems meager, as you say.


Hi Bill. Was going on a talk given by a historian at a museum. Didn't check it out personally before posting. Do you have a good source for Chilton stories? Curiosity only from me. I never really looked at the F, G, and J because they made only two each (I believe the S'Ns were 44-76027 & 44-76028 for the J's), and never got deployed anywhere, so didn't affect any outcomes.

I've seen a report saying the P-51J was a great performer, but the report was short summary of performance; nothing in much detail. It's been awhile, but it seems like it was maybe two paragraphs. Would be nice to see some actual flight test reports.

My friend Joe Yancey has some parts from a -119 Allison ... a one-off manifold going into an intercooler instead of the large intake tube and a supercharger housing that looks different from all other ALlison housings I have seen including the G-series housings. It appears to be a 2-speed unit and definitely fits a V-1710 power section.

Cheers.
SDAM interview of Chilton, April 27 1986. They have a copy but nobody I have contact with knows where to find it.
 
I was able to read an excerpt from a book on the development of the P-51H Mustang, and it stated that the XP-51 series that begant the H's development was designed for 6+Gs in flight for loading (basically British recommendations during World War II for fighter aircraft), while the USAAF preferred 7.33Gs (of note, all P-51s from the NA73X through the D/K were designed for 8 Gs at approx. 8000lbs gross TO weight). Is there any truth to that statement?

Also, it does bring up a point on how strong is strong enough for a World War II era fighter. If the Spitfire was designed to what the book alleged was RAF standards, I'd have to argue aside from landing gear that the Spitfire was more than strong enough, given that at least one was dived at more than 600mph (corrected for compressiblity) with no damage, albeit on accident in 1952 over Hong Kong.

And it's been written on here that the 8 G for 8000 lbs for the P-51D dropped as low as 6.3 Gs depending on gross weight, so not too far from Spitfire/LW Mustang standards for their gross weights.

Thus, is the statement that I'm referring to about the XP-51's correct or in error? And what G loading standards were too much, just right or not enough for the era?
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.
 

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