P-51D vs F/G vs H: how good was good enough/how good was actually great?

[BarnOwlLover]

Been thinking about some of the stuff I've read here and at places such as World War II Aircraft Performance, and though I'm a fan of the Mustang in general, I'm in particular a fan of the XP-51F/G and the P-51H. However, referring back to the last sentence, I do feel that I've been selling the P-51D a bit short. This is from reading basic stats about it's performance vs the more detailed info I've encountered since I started posting here.

Such as at 67" Hg the P-51D could climb at over 4200 fpm and have a top speed of 452 mph. The lightweights were great climbers, but I'd argue that they did maybe sacrifice a bit of practicality vs the D or even the H for their performance. Namely, their landing gear was a bit weaker, and the P-51's landing gear was the best part of it's ground handling. I do give them an edge in engine maintenance especially, though (along with the H and the F-82s).

But really, especially as the D was cleared for higher boost levels, was it really that far behind the lightweights or the P-51H in terms of overall performance? We do have to remember that weight helped with climb, but not speed. Most of the speed advantage for given power that the LWs and the H had was from refined aero, namely the revised radiator duct and wing profile.

So was I really selling the D short vs the LWs and the H?

 

[glennasher]

Look at the timing, the D/K was all that was really needed, as they were going to be replaced by jets soon enough anyway. While NAA stretched the 51 as far as they could, they knew writing was on the wall.

 

[pbehn]

Time is f the essence of the contract, so the legal term goes. From 1942 to 45 the Mustang Mk I and II were the aircraft of choice for the RAF in tactical recon. In 1943 the A-36 was one of the best fighter bombers in the allied armoury. In the India Burmah China theatre the P-51A was wanted because it could do missions others couldnt. In late 1943 early 1944 the P-51B was worth its weight in gold, the few available along with the few P-38s available were the only ones able to support the US deep penetration bombing strategy. The later versions were obviously better but by the time the P-51D was in service D-Day had happened and longer range P-48s were available.

 

[BarnOwlLover]

The reasoning I came up with this topic was that the stuff I've read here about the P-51B/D in interceptor trim (weight between 8600-8900 lbs) and higher boost numbers (75-80") could mean top speeds of over 450 mph and climb rates of 4800-4900 fpm. At 80", and at lower altitudes, there was actually little to choose between a P-51D and P-51H at similar weights.

And as far as the XP-51F being about 25mph faster than a P-51B/D on the same power, that was mostly aero (new wing profile, new radiator duct) vs light weight, which did help with climb (main thing that the Lightweight Mustang program tried to improve).

I would give the lightweight Mustangs an edge as far as maintenance, especially engine changes or engine work. But even then there were few complaints about the earlier Mustangs being difficult to work on.

But I do wonder if the LW P-51s (including the H) were really needed or wanted from a performance standpoint (not counting that jet fighters were on the horizon). The USAAF were interested in the F/G as interceptors, but they became of little more than academic interest due to the shift in wanting longer ranged fighters, hence the H was developed in preference. Also, the LWs were of interest in that their designs were influential on the F-82s.

So I guess what I'm asking is maybe the B/D undersold as far as maybe being interceptor aircraft from a performance standpoint vs the lightweights and the H given the timeline of how World War II actually unfolded?

 

[BarnOwlLover]

Also wondering (and possible debate for a different thread) about foreign interest in the P-51 (aside from Australian production of the P-51D by CAC), namely possible interest in the lightweights and the H.

 

[pbehn]

The reasoning I came up with this topic was that the stuff I've read here about the P-51B/D in interceptor trim (weight between 8600-8900 lbs) and higher boost numbers (75-80") could mean top speeds of over 450 mph and climb rates of 4800-4900 fpm. At 80", and at lower altitudes, there was actually little to choose between a P-51D and P-51H at similar weights.

And as far as the XP-51F being about 25mph faster than a P-51B/D on the same power, that was mostly aero (new wing profile, new radiator duct) vs light weight, which did help with climb (main thing that the Lightweight Mustang program tried to improve).

I would give the lightweight Mustangs an edge as far as maintenance, especially engine changes or engine work. But even then there were few complaints about the earlier Mustangs being difficult to work on.

But I do wonder if the LW P-51s (including the H) were really needed or wanted from a performance standpoint (not counting that jet fighters were on the horizon). The USAAF were interested in the F/G as interceptors, but they became of little more than academic interest due to the shift in wanting longer ranged fighters, hence the H was developed in preference. Also, the LWs were of interest in that their designs were influential on the F-82s.

So I guess what I'm asking is maybe the B/D undersold as far as maybe being interceptor aircraft from a performance standpoint vs the lightweights and the H given the timeline of how World War II actually unfolded?

There is no way a P-51B would be used as an interceptor it was specifically optimised for LR escort, there were 100 available at the start of big week, the P-51D was an improvement as a long range escort. What would they intercept and where?

 

[BarnOwlLover]

Maybe you should read some of the discussions I've had with drgondog about the qualities of the Mustang, not to mention that the original B/D models were developed before the USAAF wanted the 85 gallon fuselage tank added.

 

[pbehn]

Maybe you should read some of the discussions I've had with drgondog about the qualities of the Mustang, not to mention that the original B/D models were developed before the USAAF wanted the 85 gallon fuselage tank added.

I have read them, and I have read his book too. The P-51 could have been used as an interceptor, but in a different war. I ask again, where and when would it be used as an interceptor, and why, bearing in mind the Spitfires and Meteors available.

 

[BarnOwlLover]

Considering that the RAF used them to intercept V-1 flying bombs and they were the main fighter for the RAAF post war (until replaced by Vampires and CAC-built Sabres, CAC also built P-51s under license briefly post war), there was obviously some scope for them to be interceptors, especially given their performance on light fuel and 80"/25lbs supercharger boost. ANG units used P-51Ds and Hs as interceptors until replaced by jets. As did other European air forces post war who couldn't get or didn't want Spitfires.

You might as well also give the Brits grief for not using their Merlin P-51s for bomber escort at first. Other than V-1 interceptors, most Merlin Mustangs were first assigned as fighter bombers for the 2nd Tactical Air Force. Even the USAAF almost made that "mistake" when the first P-51B/Cs were delivered to the 9th Air Force, instead of the 8th's fighter units.

I've also read, speaking of CAC, that they wanted to license the P-51H as well as the D, but given that they never built any, either that's false or they ultimately turned down the license, given that they felt that the D on higher power ratings was enough until jets could take over the fighter role, and/or the H was sufficiently different from the D from a production standpoint.

Not to mention that with the H the USAAF didn't completely give up on the interceptor deal, since there were plans to use P-51Hs with reduced fuel tankage as kamikaze interceptors in late 1945 had the war continued.

 

[pbehn]

Considering that the RAF used them to intercept V-1 flying bombs and they were the main fighter for the RAAF post war (until replaced by Vampires and CAC-built Sabres, CAC also built P-51s under license briefly post war), there was obviously some scope for them to be interceptors, especially given their performance on light fuel and 80"/25lbs supercharger boost. ANG units used P-51Ds and Hs as interceptors until replaced by jets. As did other European air forces post war who couldn't get or didn't want Spitfires.

You might as well also give the Brits grief for not using their Merlin P-51s for bomber escort at first. Other than V-1 interceptors, most Merlin Mustangs were first assigned as fighter bombers for the 2nd Tactical Air Force. Even the USAAF almost made that "mistake" when the first P-51B/Cs were delivered to the 9th Air Force, instead of the 8th's fighter units.

I've also read, speaking of CAC, that they wanted to license the P-51H as well as the D, but given that they never built any, either that's false or they ultimately turned down the license, given that they felt that the D on higher power ratings was enough until jets could take over the fighter role, and/or the H was sufficiently different from the D from a production standpoint.

Not to mention that with the H the USAAF didn't completely give up on the interceptor deal, since there were plans to use P-51Hs with reduced fuel tankage as kamikaze interceptors in late 1945 had the war continued.

While the V1 and V2 threat was real the strategic bombing offensive was diverted to eliminating it, almost all types were tried out on diver patrols, not many P-51s though. It was useful research but limited in the case of the P-51.. Whatever the 9th and 8th were assigned P-51s for and why it wasnt as an interceptor

 

[ThomasP]

(you didn't see anything)

 

[BarnOwlLover]

That may be true, given that the P-51 (even in Allison form) had longer range than say the Spitfire and similar to or slightly better than the Tempest (and the Merlin versions were better even on wing tanks). However, I don't think that the RAF started to commonly use P-51B/D types on bomber escort until around the time that P-51Ds were delivered in numbers to the RAF (fall of 1944). Oddly, the 2nd TAF got a lot of the Mustang IIIs at first, and used them either as fighter bomber or fighter bomber escorts.

I do also remember that the units that got Mustang IIIs didn't particularity care for their planes being used for anti-V1 work because of them operating at 80"/25 lbs boost due to durability worried at the time.

 

[ThomasP]

65 Sqn was the first (I think) RAF unit to operate the Mustang Mk III (began working up in Dec'43), and was assigned to escort duties in Mar'44.

I am not sure, but I think the RAF stood up its first Mustang Mk IV squadron in Mar'45.

 

[BarnOwlLover]

We may have to ask one of the Mustang experts here. I know that the Ds got delivered to the RAF as early as fall of 1944, but that didn't necessarily mean they entered squadron service.

 

[pbehn]

That may be true, given that the P-51 (even in Allison form) had longer range than say the Spitfire and similar to or slightly better than the Tempest (and the Merlin versions were better even on wing tanks). However, I don't think that the RAF started to commonly use P-51B/D types on bomber escort until around the time that P-51Ds were delivered in numbers to the RAF (fall of 1944). Oddly, the 2nd TAF got a lot of the Mustang IIIs at first, and used them either as fighter bomber or fighter bomber escorts.

I do also remember that the units that got Mustang IIIs didn't particularity care for their planes being used for anti-V1 work because of them operating at 80"/25 lbs boost due to durability worried at the time.

It was a condition of the RAF getting P-51B and Ds that they were available for bomber escort of US forces.

 

[Geoffrey Sinclair]

The Mustang and Spitfire were fighters, interception and escort were missions they undertook, the common wisdom is the greater range made the Mustang the better escort and the better rate of climb made the Spitfire the better interceptor, but they could do both and who used which type post war comes down to things like ex RAF Spitfire units being transferred and what the aircraft cost to buy, given the end of Lend Lease. The US was not selling its latest types post war, if you wanted jets it was British.

The Australian production schedule as of 1 May 1943 was first Mustang production in June 1944, with 40 built by end 1944, monthly output to reach 35 per month in March 1945 and stay there, so that by end 1945 a total 445 Mustangs would be built, the order for 690 to be completed July 1946. Lots of tooling had been done before September 1945. The Australians turned the 100 kits supplied by North American plus some local production to replace a number of parts into 80 production Mustangs by end July 1946, production resumed in June 1947 and continued until July 1951, plus a straggler in April 1952, local production stayed in Australia during RAAF service, the P-51 sent to Japan and used in Korea were US manufacture. The result is a number of Australian built Mustangs survive on the warbird circuit. Australian Vampire production began in September 1949, but only 1 built, then 1 built every second month from January 1950 before it became more regular from July.

While Commonwealth Aircraft Corporation may have mooted the idea of the H model the amount of changes required would be extensive, the Australians had geared up to produce D models, jets were clearly on the way, the H was not better enough to justify the costs of retooling, plus in reality the late 1940's production was more geared to keeping a strategic industry alive. Enabling the later local production of more modern types, the Vampire, Canberra and Sabre.

The early decision the USAAF would be offensive and the RAF would handle defensive in Britain played a part in what the RAF P-51 units did in early 1944, along with the difficulty of crossing command lines for RAF fighters to do long range escorts, the reality the USAAF had lots of medium and light bombers to escort, the world wide shortage of Merlin Mustangs (1943 everyone wanted Spitfires, 1944 everyone wanted Mustangs) and the priority for Overlord.

The 9th Air Force in Britain started with 4 bomber groups, 1 fighter group arrived in December and being P-51 found itself mostly on longer range escort missions, by end February 1944 the 9th had 5 bomber and 5 fighter groups, end March 7 and 7, end April it had 10 bomber and 11 fighter, by end May 11 bomber and 18 fighter. And the newly arriving US fighters required more training for their intended ground support role than the already operational RAF fighter units but were being used at times for long range escorts.

On D-Day the RAF had 7 Merlin Mustang squadrons, 4 were then allocated to anti V-1 patrols, I do not have the number of sorties flown but according to Brian Cull in Diver! Diver! Diver! Tempests are credited with 851.75 V-1, Spitfire XIV 377.67, RAF Mustang 246.5, Mosquito XIII 223.5, Mosquito VI 173.5, Mosquito XVII 140 and Spitfire IX 116.25. The USAAF fighters did shoot down some V-1, but as targets of opportunity, not as part of the defensive patrols.

 

[drgondog]

The P-51B-5 and XP-51F using same engine 1650-3 @67"MP were presented in my book - pg 326.

The B was loaded to Fighter (100 gal) wing racks removed and flown at 8900+ pounds, Ditto for XP-51F with 105gal, at 6900+ pounds. Typo error by Osprey in the book, citing P-51B at 6900 pounds. 1650-3 @67"MP

Basic differences; Delta V= ~452 vs 465mph at 30,000 feet; 3600fpm vs 4700fpm. at ~12K.

The P-51B @72" was in mid 4000's ROC to nearly 6000ROC for the P-51F.

Practically speaking the P-51B would have been an excellent LR interceptor to anything Germany had except ME 262 or AR 234.

With the Merlin 100, the Mustang III zipped along near 410mph at SL.

As to why did anyone need P-51H? Well it was just better airplane at combat weight and far superior in ROC in Fighter Loading at same boost - and nobody knew the war would be over in 1945 when the contact was executed in spring 1944. Nor did anyone know that Iwo Jima would be captued to provide base for P-51 escort base, so the P-82 was authorized also.

 

[BarnOwlLover]

As far as the Merlin 100 in the B, probably the XP-51G with basically the same engine or the F running on 80"/25 lbs would be a good comparison. Same power with better aero should yield better speeds (as far as I understand, the XP-51G was at least estimated to do about 420mph at sea level on that power, and the H did do 400+ in USAAF testing).

Of course, the problem was that the F/G from what I've read would've had to be modified to carry 6.50s, and had directional stability issues and couldn't carry a rear fuel tank the size the USAAF wanted for long range bomber escort (all of which lead to the H being designed). USAAF I also think wanted the wings to be stressed to carry a 1000 lb bomb under each one. Of course, the armament and for sure the fuel capacity wasn't an issue in 1943 when the LW Mustang program started. But that does show how things can change when programs were ongoing. Similar happened with the Hawker Fury and Supermarine Spiteful, whose programs started at about the same time or even before the lightweight P-51s.

Of course, an interesting "what if" was if the P-51 was designed to British or a mix of British and American loading standards from the start. Of course, we do well know that didn't happen until the lightweight program was initiated.

 

[BarnOwlLover]

And of course, the whole lightweight Mustang deal happened because NAA became curious why the P-51 was heavier than the Spitfire. What I think would be interesting is why say the Fw-190 was as heavy or heavier than the P-51B/D in spite of much shorter range. I know that the engines used were heavier than the Merlin and the 190 usually carried heavier armament, but why the weight difference given the range differences?