# Mustang Mk I/P-51/P-51A (Allison engined) vs. P-40



## gjs238 (Apr 21, 2010)

Mustang Mk I/P-51/P-51A (Allison engined) vs. P-40: Which was the better plane?
(Certainly the P-51 had greater design potential.)
Was the early P-51 "underappreciated" by the USAAC?
Should P-51 production have been ramped up earlier?
Should the P-40 had been supplanted by the P-51?


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## Colin1 (Apr 21, 2010)

Not much in it for range, armament and low-altitude combat manoeuvrability
The P-51 could, of course, choose to not stick around if he didn't want a fight.

I'll need to get home first but I don't believe the USAAC were interested in a British-spec fighter at all, initially.

The P-51 probably did have greater design potential but it doesn't alter the fact that Curtiss-Wright more or less had the P-40 airframe marking time right up until the end of the war. They could have worked on getting the ventral cooling arrangement to work instead of selling it to North American and it wouldn't have taken a rocket scientist to do something about that undercart arrangement which, by the standards of 1943 airframe design, was obscene. Nothing to stop them from tidying up the canopy and, for what it was worth, adopting a laminar flow wing now that the undercart arrangement has been dragged into 1943.

It did replace the P-40, didn't it? Not wholesale across the board, but the Brits were convinced by NAA that they could give them something better than the P-40 and the AVG eventually transitioned to P-51s.


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## billswagger (Apr 21, 2010)

In some reading on the MTO, the A-36A is mentioned by a few pilots and it was considered to be superior to the P-40. 

Perhaps its lack of numbers had more to do with the availability of the P-40 at that time where as the A-36 was primarily a low flying recon plane that had been developed into a dive bomber A-36A. 

It had some deficiencies similar to the P-40 in its altitude performance and in part due to the light construction of the aircraft, carrying bombs and diving sometimes meant the wings fell off in pull outs. In those situations, the issue seems to have been centered around the dive flaps, so they actually prohibited the use of them and diving too steeply when bombing. Still, pilots mention it was a very effective plane for that roll. It may have contrasted with the P-40s trim issues while diving, needed a heavy foot to balance the yaw. If this was not present in the A-36, i could see why they might like that plane more. 

As for design potential, i think both planes had room to expand upon, but it was just a matter of finding an engine that could offer better performance. Once those engines were built i think the features of the P-51 started to show greater promise than the P-40. 
There are still a handful of pilots who prefered the P-40 for its superior maneuverability, something i find a bit surprising when talking about these aircraft but as we all know, the higher and faster the planes fought, maneuverability takes a back seat to speed and energy. The P-51 is clearly the better design for that. 



Bill


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## tomo pauk (Apr 21, 2010)

P-51 all the way - it really extracted every mph out of non-turbo Allisons, without sacrificing anything worth talking about.


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## marshall (Apr 21, 2010)

tomo pauk said:


> P-51 all the way - it really extracted every mph out of *non-turbo Allisons*, without sacrificing anything worth talking about.



I think that the Allison engines in P-51 had a single stage supercharger.


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## tomo pauk (Apr 21, 2010)

That's why I've said non-turbo (= not having a turbo-compressor, so the one that uses exhaust gasses to turn a turbine therefore the compressor in common shaft); the Allison of P-51 (and P-40 P-39) used engine-driven compressor


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## marshall (Apr 21, 2010)

My bad I understood that you wanted to say that there was no forced induction at all.


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## Jabberwocky (Apr 22, 2010)

P-51 easily.

Superior speed, range, maneuverability and visibility. 

Comparative tests against the P-40 (Allison and Merlin engined variants) found the aircraft to be 15-25 mph faster below 20,000 ft, better climbing than the Allison engine P-40s up to 15,000 ft and generally more pleasant and less pilot intensive to fly. The P-51A showed none of the nasty trim and dive characteristics that the P-40 did.

The USAAF did consider the P-40 to be a marginally superior close in dogfighter, but noted that the P-51 could "engage or break off combat at will" in a dogfight with the P-40. 

The Allison powered P-40s only increased their superiority over the P-40 as the war progressed. By 1943, Allison powered P-51s were in the order of 20-45 mph faster than similarly powered P-40s,


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## Colin1 (Apr 22, 2010)

Jabberwocky said:


> The Allison powered P-40s only increased their superiority over the P-40 as the war progressed


?


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## Jabberwocky (Apr 22, 2010)

Colin1 said:


> ?



The Allison powered P-51's gained very little weight over the basic design during the war and their handling characteristics and range remained very much the same.

The less slippery and streamlined P-40 did not respond as favourably to more powerful Allison engines, and so Curtiss was forced to put the aircraft on a diet, several times, and make corrections to the aircraft (shortening/lengthening the fuselage, adjusting the tail) to rectify handling deficits. The resulting P-40N was a much improved aircraft, but it still wasn't a match for the P-51A.

The mid-war P-40s suffered from slowly degrading performance, particularly top speed, while the P-51 performance in both speed and climb steadily improved over 1942 and 1943. The P-51As with the RAF used for low altitude intrusion initially did around 377 mph. By mid-1943, RAF trials have them performing at around 410 mph, only slightly less than the best USAAF test speed of about 415 mph.

In comparison, the RAF always had trouble replicating the speed and climb performance that the USAAF got out of their P-40s. 

While the USAAF reports a maximum level speed of 362 mph for a P-40E (and Curtis data shows 366 mph), the RAF Kittyhawk I (essentially the same aircraft) maxed out at 344 mph. The same with the P-40N; the RAF could get theirs to 362 mph, well down on the best speed of 378 mph that the USAAF tests show. 

RAAF testing shows a similar story: their P-40Es maxed out at 338 mph in testing, with some tests having them struggle to make it much past 330 mph. Their best test for P-40N is 364 mph.

This could be down to equipment, paint and finish differences and the like, but from my reading the P-51 usually seems to meet its earlier testing, while the P-40s tend to fall short of the test data, a situation that is often repeated with Spitfire testing as well.


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## Colin1 (Apr 22, 2010)

Jabberwocky said:


> The Allison powered P-51s


Gotcha
I think you typo'd 



Jabberwocky said:


> The Allison powered *P-40s* only increased their superiority over the *P-40*...


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## ppopsie (Apr 22, 2010)

Allison Mustang's only snag; just 1500+a were built in all variants.


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## drgondog (Apr 22, 2010)

ppopsie said:


> Allison Mustang's only snag; just 1500+a were built in all variants.



The orders to NAA were higher but switched to P-51B in late 1942..


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## claidemore (Apr 22, 2010)

Jabberwocky said:


> This could be down to equipment, paint and finish differences and the like, but from my reading the P-51 usually seems to meet its earlier testing, while the P-40s tend to fall short of the test data, a situation that is often repeated with Spitfire testing as well.



Often repeated in Spitfire testing? Can you give us examples?


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## zoomar (Apr 22, 2010)

I'm interest in people's thoughs. If the British Purchasing Commission never went to NAA for its own plane and just ordered more P-40's, what possible effect might the non-existence of the Mustangs have had on USAAF procurement. Would the P-38 and P-47 be the only"modern" planes around which the USAAF would structure its strategy in the ETO? Would more effort be put into upgrades of the P-40, such as the P-40Q. Would more effort have been to perfect and freeze for service introduction the Curtiss P-60? Might the P-63 be looked at more seriously for US service? 

In a way the P-51 is one of the most accidental planes.


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## Colin1 (Apr 22, 2010)

zoomar said:


> I'm interest in people's thoughts. If the British Purchasing Commission never went to NAA for its own plane and just ordered more P-40s, what possible effect might the non-existence of the Mustangs have had on USAAF procurement. Would the P-38 and P-47 be the only"modern" planes around which the USAAF would structure its strategy in the ETO? Would more effort be put into upgrades of the P-40, such as the P-40Q. Would more effort have been to perfect and freeze for service introduction the Curtiss P-60? Might the P-63 be looked at more seriously for US service?
> 
> In a way the P-51 is one of the most accidental planes.


There's no way of telling
in this alternative time-line whether NAA would have just sat on it. It could be argued that they would not. The British Purchasing Commission came to buy P-40s and NAA told them unequivocally that they could build them something better; it is quite feasible to suggest that, eventually, they would have said the same thing to the USAAF. The question is, built to US specifications rather than British, would we have the same plane and would the paths of the Merlin and the Mustang ever have converged? Again, probably, I doubt the Purchasing Commission would overlook the new fighter and once on UK shores in whatever its new form would be, it would attract the attention of Rolls-Royce, Air Ministry Procurement and the RAF and someone would come up with the idea of dropping a Merlin in.

The absence of any P-51 might actually have had the reverse effect on Curtiss-Wright, who were already in a state of torpor over replacement designs for the P-40. With no threat from NAA, they have even less call to animate themselves. The XP-40Q was not so much a Curtiss-Wright venture as an after-work project by a bunch of dedicated engineers, they were very poorly supported by their parent company - without the P-51 the half-spark that almost created the P-40Q might never have sparked at all.

Still don't see the P-63 getting the call to arms, range was ever the issue.

The P-51 the most accidental of planes? Yep, history is fragile.


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## billswagger (Apr 22, 2010)

I really don't think the P-51 full filled any roll that could not be done with other fighters other than the long range escort roll. 
Its possible that if the P-51 was not designed then long range P-40s or P-60s could've seen development. They would probably end up looking more like P-51 by design. 

I find it hard to think the P-51 was accidental. It was another airframe that showed better performance with a similar power plant as the 40. I don't think that is something they could ignore. 

Bill


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## Jabberwocky (Apr 23, 2010)

claidemore said:


> Often repeated in Spitfire testing? Can you give us examples?



Lots of RAF tests (and RAAF tests) have Spitfires that are down anywhere from 10-25 mph in top speed over the original test flights.

Some of this is added weight or new equipment, or poor fit and finish of paint and panels or just general war weariness of the airframe/engine. An aircraft is not a static thing. It gets damaged, worn out. Pilots leave scuffs on the wing surface, the aircraft skin gets bent, ailerons droop in the airflow, gaps appear between panels, ect, ect. 

In testing of a ten different Spitfire Mk Vs by the RAF in late 1942, none of the aircraft could exceed 370 mph. The best aircraft did 369 mph (just a few mph short of initial testing) while the worst did just 352 mph (close to 20 mph short of initial testing, due to an underperforming engine). The ten aircraft together had an average best to speed of 360 mph.

There is an early 1944 (maybe late 1943?) RAF test of an in squadron service Spitfire Mk Vb where the aircraft was reporting a max speed of 357 mph, about 14 mph slower that the Mk Vb flight tests of about three years earlier, despite the subsequently higher-power ratings for the Merlin.

So, they got to fixing this aircraft. They serviced the engine, scraped the wing leading edge and repainted it, waxed the aircraft, filled the panel gaps, cute down the chute ejectors for spent shells, changed the type of rear view mirror for the aircraft, fitted multi-ejectors in place of the fish-tail exhausts. 

All these little detail improvements resulted in the aircraft gaining 28 mph in top speed, with the aircraft maxing out at 385 mph. That's about 14 mph faster than the initial MK Vb flight tests, and 10 mph faster than the Mk Va tests. 

The RAAF saw similar results with their Spitfire Mk Vc aircraft. The aircraft were initially underperforming, topping out at about 355-365 mph, thanks mostly to some of the tropical modifications to the aircraft, particularly the air filter. The RAAF modified the filters, exhausts and made some detail changes, and soon enough they had their Mk Vcs operating past 370 mph, with one topping out at 378 mph.

All this goes to show just how important mechanics in WW2 were, and how they are the unsung heros of fighter ops.

If you're in the RAF in 1942 and you're facing 400 mph FW 190s and 380-390 mph Bf-109Fs, would you rather your aircraft maxes out at 352 mph or 375 mph? 

It doesn't sound like much when you're doing 300 mph plus, but that 20-25 mph difference might just be enough to get you home in one piece. If you're running for your life after a bad bounce from some FW-190s, that little advantage may mean the difference between your pursuers turning back mid-way over the Channel, or shooting you down as you go feet wet getting the hell out of France.

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## Colin1 (Apr 23, 2010)

billswagger said:


> I really don't think the P-51 fulfilled any role that could not be done with other fighters other than the long-range escort role.
> 
> Its possible that if the P-51 was not designed then long range P-40s or P-60s could've seen development. They would probably end up looking more like P-51 by design.
> 
> I find it hard to think the P-51 was accidental. It was another airframe that showed better performance with a similar power plant as the 40. I don't think that is something they could ignore


P-38?

Where would the necessary fuel be situated on a 'long-range P-40' and why would the removal of the P-51 from the timeline suddenly make the P-60 a going concern?

Yes, it was an airframe that showed better performance with similar power than the P-40 - once it came into existence. Prior to its existence, you have to ask yourself why the Purchasing Commission should listen to a company who had never previously built a fighter, claim that they could come up with something better than the not-brilliant-but-proven P-40 and build it for them in 120 days.

That's the 'almost accidental' bit


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## MikeGazdik (Apr 23, 2010)

IF the P-51 was not created, the P-47 'N" model would have been needed and thus created sooner. The P-47 would still be the backbone of the 8th AF. Without the Mustang, the U.S. and Republic Aviation would have needed the longer range Thunderbolt sooner. The P-38 would also naturally get a bigger role, but I don't see anything changing as far as its development. About the time the Mustangs were coming in, the J and L model P-38's were starting to prove themselves, but were already on the way to being replaced.


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## Colin1 (Apr 23, 2010)

MikeGazdik said:


> IF the P-51 was not created, the P-47 'N" model would have been needed and thus created sooner. The P-47 would still be the backbone of the 8th AF. Without the Mustang, the U.S. and Republic Aviation would have needed the longer-range Thunderbolt sooner.
> 
> The P-38 would also naturally get a bigger role, but I don't see anything changing as far as its development. About the time the Mustangs were coming in, the J and L model P-38s were starting to prove themselves, but were already on the way to being replaced.


Agreed on all but one point
with the P-51 not around to hog resources and Lockheed looking to keep pace with the latest Axis developments, who knows, the P-38K might have had her day.


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## MikeGazdik (Apr 23, 2010)

Colin1 said:


> Agreed on all but one point
> with the P-51 not around to hog resources and Lockheed looking to keep pace with the latest Axis developments, who knows, the P-38K might have had her day.



Very, very good point!


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## billswagger (Apr 24, 2010)

Colin1 said:


> P-38?
> 
> Where would the necessary fuel be situated on a 'long-range P-40' and why would the removal of the P-51 from the timeline suddenly make the P-60 a going concern?



i gather that the P-60 was over shadowed by the performance of the P-51, and if the Mustang was never built then the P-60 may have received more attention. The long range P-40 is also pure hypothetical, but it would probably end up resembling a P-40Q in some ways and fitted with wing tanks. Its actually hard to conceive with out thinking it would eventually turn out like a mustang. 

What's sort of been over looked thus far is how the planes actually compared from a tactical, practical and combat perspective. 
I seem to remember reading something about the actual time to altitude, something like 10,000ft, from engine start up that put the P-51 towards the back of the pack. Part of the advantages of the P-40 in an actual combat zone may have been its ability to get up and go with out a lengthy take off procedure. Why the P-51 took so long to get up in the air I'm not sure. 

Any thoughts on that?


Bill


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## tomo pauk (Apr 24, 2010)

I bet Russians would've loved the P-51A - faster than P-39s they liked, but without vices...


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## drgondog (Apr 24, 2010)

billswagger said:


> i gather that the P-60 was over shadowed by the performance of the P-51, and if the Mustang was never built then the P-60 may have received more attention. The long range P-40 is also pure hypothetical, but it would probably end up resembling a P-40Q in some ways and fitted with wing tanks. Its actually hard to conceive with out thinking it would eventually turn out like a mustang.
> 
> What's sort of been over looked thus far is how the planes actually compared from a tactical, practical and combat perspective.
> I seem to remember reading something about the actual time to altitude, something like 10,000ft, from engine start up that put the P-51 towards the back of the pack. Part of the advantages of the P-40 in an actual combat zone may have been its ability to get up and go with out a lengthy take off procedure. Why the P-51 took so long to get up in the air I'm not sure.
> ...



Depended on the engines 

The basic engine for the P-51 and P-51A and Mk I was the 1710-39 with a blower gear ratio of 8.8:1 

The A-36 got the P-38 V-1710-87 w/o the turbo or the high altitude pressurized ignition system - but had a 7.48:1 Blower ratio and was a better low altitude engine (also better than P-40).. this engine developed +200hp at Military power more than the -81 below but had a lower critical altitude than the -81.

The Mk II got the V-1710-81 with a 9.6:1 and developed better power above 8,000 feet - This one had a climb to 20K at 6.9 minutes at 8630 pounds - almost 200 more than the P-40F.


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## JoeB (Apr 24, 2010)

tomo pauk said:


> I bet Russians would've loved the P-51A - faster than P-39s they liked, but without vices...


The Soviets received 4 ex-RAF Mustang I's; including 3 which used at the front for evaluation by 5th Guards Fighter Regiment in fall of 1942. They didn't like the plane and recieved no more.

An interesting P-51A and P-40 comparison is combat results in Burma in1943, per "Air War over Burma" which were surprisingly unfavorable for the P-51. In the course of that year P-40's achieved a kill ratio of 21:7 v IJA Type 1 (Oscar) units in Burma, mainly in interceptions of escorted bombers, also some hunting of Type 1's which were in turn hunting transports flying 'Over the Hump' to China. P-51A's and P-38's, mainly on escort missions over Rangoon scored 6 known kills against a mix of Type 1, Type 2 (Tojo) and Type 2 two-seat (Nick) units; losses aren't recorded for a few of those Japanese units but the USAAF fighters only even claimed a handful more victories than 6, v loss of 8 P-51A's and 2 P-38's. It wasn't directly comparable operations to the P-40's and isn't a very big sample, though it was against very similar opposition including some of the same Japanese fighter units the P-40's faced. It doesn't seem to indicate any significant advantage though of the P-51A over the P-40 v that type of opponent, at least.

Joe


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## tomo pauk (Apr 24, 2010)

Tnx for sharing 

What were the main complaint vs. the Mustang I the Russkies tested?


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## Juha (Apr 24, 2010)

Hello Tomo
the main complains by Soviets were lack of horizontal and vertical manoeuvrability, even if dive characters were good, and lack of acceleration. They saw that the power loading was too high. But plane was easy to fly.

IIRC Japanese thought at first that P-51A was only a new version of P-40, nothing spectacular.

Juha


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## billswagger (Apr 24, 2010)

drgondog said:


> Depended on the engines
> 
> The basic engine for the P-51 and P-51A and Mk I was the 1710-39 with a blower gear ratio of 8.8:1
> 
> ...



So you think the model with out all the high altitude fittings was quicker to get into the air?

BTW, i've seen no direct comparison as far as time to altitude concerning the P-40 and P-51. And to clarify, I mean from engine start up, not from take off. These sort of differences can really make a difference in a combat zone.


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## Glider (Apr 24, 2010)

They would have bilt more P47, after all the P47 was a considerable improvement on the P40 so why not.


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## Shortround6 (Apr 25, 2010)

billswagger said:


> BTW, i've seen no direct comparison as far as time to altitude concerning the P-40 and P-51. And to clarify, I mean from engine start up, not from take off. These sort of differences can really make a difference in a combat zone.



It doesn't make much difference unless you are trying to set up an air defense system with a limited warning system and few, if any standing patrols. 
It makes no difference to offensive operations, fighter sweeps or bomber escort missions.

It varies with climate conditions. England vs North Africa or South Pacific vs Aleutian Islands.

If quick response was really needed planes were either started and left idling for periods of time or started, warmed up and then shut down periodically.


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## Schweik (Jun 20, 2018)

I know this is an old thread but it came up on my feed so thought I should mention it here -

Many people have said, and some even said during the war, that the Allison engined Mustang was a much better plane than any P-40. It was certainly faster, climbed better, and had better range. However there is one issue with that.

*P-40 had a much better combat record than the Allison Engined P-51*. P-51A and A-36 were getting slaughtered by Ki 43s in China and Burma. Didn't do very well in Italy either except as bombers, and in fact the A-36 squadrons in Italy were drawn down due to high casualties and replaced with ... P-40s.

There was only one Allison Engined Mustang Ace, an American flying A-36s in Italy. By comparison there were 18 USAAF P-40 Aces in the Med, 45 in the CBI, 40 in the Pacific, and 46 RAF / Commonwealth P-40 Aces including at least 8 double and 4 x4 Aces (all of those numbers only counting confirmed victories while flying P-40s). Not sure how many in Russia but it was a lot, probably more than Commonwealth. There were 4 x4 P-40 Aces Russia and 12 double aces.

This is not just a matter of there being many more P-40 squadrons - if you look at it on a per-sortie basis, the P-40s come out way ahead. Nor the nature of the missions, the majority of P-40 sorties were fighter-bomber missions too. In fact quite often specifically dive-bombing.

Apparently one of the issues with the early P-51s was that the ailerons weren't designed right and they fixed this in the P-51B

S


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