# Why didn't Allison quickly develop a one-stage 2 speed Supercharger for the P-40



## oldcrowcv63 (Nov 12, 2013)

(Don't know if this has been covered elsewhere in the forum.)

Thinking of the time-frame of 1941-42. I understand the emphasis on the Turbocharger short-circuiting the development of the two-stage unit but was an upgrade to a two speed unit for the Allison 1710-39 engine beyond Allison's capability or were their other technical issues that prevented or slowed development of a two-speed, one stage unit. I've seen documentation (referenced in this forum) that seems to suggest Allison 2-speed units did appear somewhat later in the war but were not used on any fighter aircraft until much later. 

Seems like a 2 speed unit would have been pretty useful on both the P-40E subsequent non-Merlin powered models and for the P-39 as an interim fix. ???


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## wuzak (Nov 12, 2013)

Considering the route they took for the 2 stage engine was to have a variable speed auxiliary stage supercharger, I wonder if that is a route they could have taken to improve the single stage engine's performance?

Neither a variable or 2 speed drive for the supercharger will help much with the altitude performance - unless the fixed gear ratio used was a compromise so as to not take away too much low altitude performance.


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## Shortround6 (Nov 12, 2013)

Two speed does very little for altitude use. The big gain is usually in reducing the power needed to drive the supercharger at low altitude buy using a low gear, This also heats the intake charge less and allows the throttle to be opened more reducing pumping losses.

Best illustration is the Merlin III and the Merlin X. 
Merlin III used an 8.588 gear with a 10.25 in impeller for an impeller tip speed of about 1151fps. Power was 1030hp at 16,250 ft (no ram). Take off was 880 hp. 

Merlin X used a 6.389 low gear and a 8.75 high gear. Impeller speeds were about 857 fps and 1173fps. Power was 1075hp for take-off, 1130hp at 5250ft in low gear and 1010hp at 17,750ft no ram. 

Since the power needed to drive the supercharger goes up with the square of the tip speed low gear needed about 53-54% of the power that high gear needed and since about 30-40 percent of the power used is turned into heat _over and above_ the heat resulting from the compression of the air take-off/low altitude performance can be much improved. 

On the high end the supercharger is limited to a tip speed _below_ supersonic (around 1100fps at standard sea level pressure and temperature) and while the conditions inside the supercharger allow for slightly higher tip speeds there is a definite limit to how fast you can turn the impeller. IF the tip speeds go supersonic inside the supercharger it sets up shock waves (little sonic booms) that severely impact airflow through the supercharger. 

The vast majority of two speed engines in the US ( P&W and Wright radials) used their two speed superchargers to pick up several hundred hp at low altitudes. Critical height ( or full throttle height) in high gear usually being under 15,000ft. 

Once Allison got the 9.60 gears to work ( the gears had to be made wider to stand the load, the original thickness 9.60 gears broke) the Allison was giving about as good as it was going to get with _that particular_ supercharger (inlet, impeller, diffuser). 
Allison had to redesign the gear case and change the casting patterns (molds) to accommodate the wider gears.

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## GregP (Nov 12, 2013)

Allison was a relatively small firm. They were ramping up to produce engines at the time, and tyhey asked their primary customer, the Army / Navy, twice if they wanted to fund the development of a 2-stage supercharger. The Army / Navy answered “no” twice. Allison had no other market since the US government owned the V-1710 design, and they could not afford to produce expensive new designs on their own.

So the US government got exactly what it ordered … engines by specific part number. And the government never did authorize Allison to develop an integral 2-stage supercharger on Department of Defense money, so they never got one from Allison.

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## wuzak (Nov 13, 2013)

GregP said:


> Allison was a relatively small firm. They were ramping up to produce engines at the time, and tyhey asked their primary customer, the Army / Navy, twice if they wanted to fund the development of a 2-stage supercharger. The Army / Navy answered “no” twice. Allison had no other market since the US government owned the V-1710 design, and they could not afford to produce expensive new designs on their own.



The US government did not own the V-1710 design.

They were, however, the only customer for the engine, since the civilian aircraft market was either for cheap, lower powered engines or sewn up by Pratt Whitney and Wright. In any case, the V-1710 was not available for airliner development before WW2.

Thus, without prospects for orders of 2 stage or 2 speed engines Allison did not put their own money into developing them (Allison was small, but they were owned by GM at that time, and if a business case for a 2 stage or 2 speed engine could be made I'm sure GM would have funded it).




GregP said:


> So the US government got exactly what it ordered … engines by specific part number. And the government never did authorize Allison to develop an integral 2-stage supercharger on Department of Defense money, so they never got one from Allison.



Allison did develop a 2 speed supercharger. I am doubtful that the Army or government specified that it had to be a remote unit - as in not an integral 2 stage supercharger like the Merlin's. I would think that Allison's ethos of manufacturability and inter-changeability led them to develop the remote system, whereby a single stage unit could be made into a 2 stage unit by simply bolting on some new parts.

The initial 2 stage units had the liquid to air intercooler, and the carburettor mounted on the auxiliary stage supercharger. Then they moved the carby back to the engine stage supercarger - but that required a different carby with the intercooler. And then they ditched the intercooler and had the same base engine as for a turbocharged installation.


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## GregP (Nov 13, 2013)

The US Government absolutely DID own the V-1710 design and production authorization.

They funded the development with rights and it was proprietary to the Department of Defense.

Wayne, I clearly stated "integral 2-stage supercharger." Allison never developed one. The auxilliary unit was external to the engine, not integral. Joe has two ready for overhaul and sale, should anyone be interested.


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## tomo pauk (Nov 13, 2013)

Where we can read that US Government was the owner of V-1710 design?

Hi, Oldrcrow,



> I've seen documentation (referenced in this forum) that seems to suggest Allison *2-speed units did appear* somewhat later in the war but were not used on any fighter aircraft until much later.



Maybe you mean '2-stage units'?


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## oldcrowcv63 (Nov 13, 2013)

tomo pauk said:


> Where we can read that US Government was the owner of V-1710 design?
> 
> Hi, Oldrcrow,
> 
> Maybe you mean '2-stage units'?



Tomo, 

Check out:

ModDesig for the engine models described in our forum's thread:

http://www.ww2aircraft.net/forum/engines/allison-v-1710-supercharger-impeller-15753-2.html

On our forum from a discussion in 2008:

Specifically the engine models listed in Kool Kitty89's post number 16:

V-1710-57 -131 although I may have misunderstood the labeling in the chart and Kittys description. It may be the single stage, 2 speed unit was rarely used operationally and was more a developmental step toward the purportedly _two stage_ system used in the P-63 and F-82? 

Also, cryptically I found this obscure reference on: 

The Allison V-1710 V-16 Engine

to a to a planned upgrade of the DC-3 called the DC-8 Skybus:

"_This low-winged contra-rotating pusher transport powered by two V-1710's was announced by Douglas Aircraft in October 1945 as the "Transport of Tomorrow" -- the post-war successor to the DC-3. This airplane had a drive configuration similar to the XB-42 but placed the engines under the floor in the passenger compartment. It's 39,500 gross weight was sufficient to accommodate 48 passengers in a pressurized cabin sitting five abreast. The plane would have been 50% faster than the DC-3, at half the operating cost. The engine was expected to be a derivative of the V-1710-E29, but would have *achieved the required altitude performance* with a *single-stage two-speed supercharger,* *an Allison first*. The airlines were concerned about the maintenance costs of the unconventional propulsion arrangement and chose to purchase conventional Convair 240 and Martin 2-0-2 airlines. The DC-8 designation would be reused for the company's successful first jet transport a decade later._"


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## oldcrowcv63 (Nov 13, 2013)

Thanks SR6. That's just the kind of detailed answer I was looking for and also to Wuz and Greg for their added perspective.


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## Conslaw (Nov 13, 2013)

The Allison 1710 was designed with a simple integral supercharger because it was contemplated to be augmented by an external turbocharger when necessary. No external turbo would be necessary for use by airships. By late 1941, the P-47 program crossing over from paper to metal, and its turbocharged R-2800 was extremely promising. The P-38 was flying at nearly 400 MPH with turbocharged 1710s. The P-40 was a stopgap plane to be used until the turbocharged fighters could be delivered in quantity. The one-stage two-speed supercharger was also bested at this time by the two-stage, two-speed supercharger in the F4F Wildcat and similar unit in the prototype F4U Corsair. In essence, the Air Force knew that the single stage two-speed supercharger would not be good enough, and better alternatives were already in the production pipeline. By the way, at this time two-stage two-speed superchargers, probably weren't a realistic option either. There were a shortage of two-stage supercharged engines available to Grumman for its Wildcat production, so some planes had to be delivered with a single-stage unit. These were the maligned F4F-3A models.

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## Shortround6 (Nov 13, 2013)

It seems there are a few misconceptions. The Allison was NOT designed as an airship engine. Allison did get money form the Navy for an airship _version_ after the Army told them they had no money for a development contract at that time. The Airship version had NO supercharger of any kind. 
Turbo Allisons had been used in at least one (or more ?) Bell Airacudas and the XP and YP-37s. 
The lot of early turbo engines used 6.23 gears on the engine driven supercharger and when Allison switched to an "altitude" rated engine without turbo they went to 8.80 gears. They lost of bit of low altitude power doing so but this was masked by the adoption of 100 octane fuel and the fact that the Allison was not used in bombers. 

As for the US government "owning" the Allison design, I would certainly like to see some sort of proof. 

1. Getting permission to export military goods is different than the goverment owning the design.
2. Getting permission to 'bump' delieveries to a later date than called for in a contract in order to fill foreign orders is not evidence of government "ownership".
3. The Government did NOT present Allison with plans/blueprints for the V-1710 and ask/tell them to build it. This last is closer to what they did with the 'Continental Hyper engine' with much of the 'theory' and initial specifications/drawings coming from the army with Continental just building what they were told. 
4. The Government did enter into development contracts with Allison, paying *ONLY* for work or tests *successfully completed*. Engine fails on test Allison has to fix/repair engine out of it's own pocket and retest before it gets paid the _original contract amount._ 
5. By the Spring of 1939 the Government *OWES* Allison over $900,000 dollars for work already done.
6. To get permission to export the engine Allison has to forget about the governments $900,000 debt.

There is little wonder that Allison wasn't falling all over it self developing new versions at it's own expense. Until the Spring of 1939 the Government (read Army) was neither paying it's bills or ordering engines in quantities large enough for Allison to make a profit ( GM had already sunk over 1/2 million into Allison).


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## tomo pauk (Nov 13, 2013)

oldcrowcv63 said:


> Tomo,
> 
> Check out:
> 
> ...



Okay, thought it was a typo.
I've checked out the Vee's for victory book, and here is what it says about the two. 
The -57 was intended for the Curtiss' ill-fated P-53 fighter, using the supercharger desinged by Birmann's TEC (Turbo Engineering Company; the supercharger itself was NOT of turbo variety here). Impeller was bigger, 10.25 in, same as Packard Merlin V-1650-1. The inlet elbow was also new, as it was the carburetor ("Holy Carburetor"). Test engine was completed on April 24, 1942. The gearing was 'Allison typical', using 6.44:1 (as early engines used on P-38 ) and 8.80:1 (as on early P-39 engines, and most of the P-40s). Power at altitude was 1150 HP at 16000 ft; almost as good as with V-1650-1, and slightly better than Allisons with 9.60:1 gearing that were to be produced from second half of 1942 on - in other words, there was no great advantage going for the -57, and as such was canceled.

The -131 (V-1710-G3) was intended for use on variants of the C-54, the 1st engine tested in December 1946. There is also shown the advantage of having a slightly bigger impeller, 10.25 in diameter.



> Also, cryptically I found this obscure reference on:
> 
> The Allison V-1710 V-16 Engine
> 
> ...



If we discount the -57 as being a co-effort of Allison and TEC, and if the engine was really there in late 1945, then it will really be the 'Allison's 1st' 2-speed engine.

One thing to add - in Feb 1941 (before the USAC's XP-51 flew), the NAA (Lee Atwood) wrote to the GM and Air Corps recommending the Allison to develop a 2-speed supercharger for engines for the future V-1710s for P-51; NAA's engineers considering back then that a perspective Continental V-1430 would be a difficult thing to install in the P-51, due to it's intercooler.

All in all, considering that a two-stage engine was in works, there was no much point in a two-speed supercharger, since it would not improve P-40s main failing, namely the weak hi-alt performance.

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## oldcrowcv63 (Nov 13, 2013)

Conslaw, The P-47, F4U and even the P-38 were years from operational deployment in the Fall of 1941 when it began to dawn on USAAF pilots flying the P-40E that they would be essentially ineffective in intercepting high flying bombers (above ~25,000 feet). As you point out, the F4F was handicapped by insufficient numbers of doubly supercharged radial P&W 1830's. Indeed, the P-40 was a stopgap plane until deployment of the P-47 P-38, but it was an inadequate stopgap so the question as stated was based on the assumption that a single stage, 2-speed Supercharger might have been quickly developed for the P-40 that would have incrementally improved its high altitude performance in the time-frame of 1941-42. Instead, the USAAF fielded the P-40K in the Summer/Fall of 1942, which did nothing to improve the aircraft's high altitude performance but did provide additional performance for the overweight aircraft to get off the ground. SR6 has provided the rationale for why it wasn't pursued: little benefit to be achieved, and of course the fact that at about the same time, the single staged, two-speed variant of the Packard-Merlin engined P-40(F) was coming on line which was perhaps a far simpler solution to the problem than a dedicated Allison development.


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## tomo pauk (Nov 13, 2013)

The main 'driver' for a two speed V-1710 might've been a bomber, not a fighter. However, historically the V-1710 overwhelmingly served in fighter planes, so that 'driver' was not existing.


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## GregP (Nov 13, 2013)

You can read about it in Dan Whitney's Vees for Victory and in Graham White's Allied AIrcraft Piston Engines of WWII. In both it is clearly stated that the Allison V-1710 was funded by the US Navy and the US Army Air Corps. In those days, NOBODY in the government would fund a development of a military engine and leave the result in private ownership. If they funded it, they owned it and the rights. If Allison had funded it, and had then offered it to the government, and if they accepted ... they'd still have a provision for government approval for sales and exports to other than US-military customers. It works that way today, too. The only way for Allison to have retained control of the design and manufacturing rights would have been for Allison to fund it, sell to private individuals and/or other countries, and THEN have the US government express an interest. Allison would then have been in a position to accept or reject government control by saying "no" to the controlling provisions. That didn't happen.

If the US Air Force of then or now funds an aircraft, they own the rights and would have approval oversight for any sales to outside entities. Ask Boeing or Lockheed-Martin! If you win the lottery, go try try to buy an F-22! You can't do it and also can't buy the engines or any other parts. Same as in pre-WWII days. 

All the government would have had to do to deny Allison sales was to say "no!" That is defacto ownership, just as it is in 2013.


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## rinkol (Nov 13, 2013)

tomo pauk said:


> The main 'driver' for a two speed V-1710 might've been a bomber, not a fighter. However, historically the V-1710 overwhelmingly served in fighter planes, so that 'driver' was not existing.





The original intent seems to have been that a turbo-supercharger would be used if high altitude performance was a priority. In this case, the turbo would be the first stage in a two stage configuration where the second was the mechanically driven supercharger. Since the turbosupercharger provided variable boost, the mechanical supercharger could get by with a single speed and was also suitable for use by itself at lower altitudes. The mechanical supercharger used in most of the V1710 variants seems to have been physically integrated with the engine block and adding a two speed drive would have required major changes. It probably seemed best to continue the original strategy where types such as the P-38 were produced with turbosuperchargers for applications where high performance, particularly at high altitudes, was a priority while types such as the P-39 and P-40 would be produced for low altitude operations. This avoided production disruptions at a critical stage of the war.


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## P-39 Expert (May 13, 2018)

The main purpose of low gear in a two speed engine is to keep the pilot from overboosting the engine at takeoff.
Allison accomplished this in their single speed engine by throttling the engine at TO and gradually opening the throttle as altitude was gained and air became thinner. 
By mid '42 the automatic boost control (automatic manifold pressure regulator) automatically prevented the engine from exceeding a set manifold pressure at any altitude. So the pilot could select full throttle and not overboost the engine at any altitude.
This single speed arrangement provided plenty of power at lower altitudes (peak WEP was under 9500') and adequate power at higher altitudes since the single speed was in effect high gear. It was also lighter and simpler than a two speed arrangement.
The auto boost and variable speeds of the turbo and mechanical auxiliary stage rendered a two speed Allison unnecessary.


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## swampyankee (May 13, 2018)

Even today, defense contractors do not have unlimited rights even to products developed at their own expense; this was actually tightened under the Reagan administration, when the DoD had to pass on _any_ article submitted for publication even if no military application was considered.

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## tomo pauk (May 13, 2018)

P-39 Expert said:


> The main purpose of low gear in a two speed engine is to keep the pilot from overboosting the engine at takeoff.
> Allison accomplished this in their single speed engine by throttling the engine at TO and gradually opening the throttle as altitude was gained and air became thinner.
> By mid '42 the automatic boost control (automatic manifold pressure regulator) automatically prevented the engine from exceeding a set manifold pressure at any altitude. So the pilot could select full throttle and not overboost the engine at any altitude.
> This single speed arrangement provided plenty of power at lower altitudes (peak WEP was under 9500') and adequate power at higher altitudes since the single speed was in effect high gear. It was also lighter and simpler than a two speed arrangement.
> The auto boost and variable speeds of the turbo and mechanical auxiliary stage rendered a two speed Allison unnecessary.



Actually, the main pupose of a low gear was to increase power at lower altitudes, especially the take off power. We know that V-1710s with 'low' gearing were making better power down low than those with 'high' gearing. Everybody was using throttling, not just Allison on their engines. Turbo was not an easy strap-on piece of gear on aircraft that usualy used V-1710, and variable-speed drive was pretty late in game vs. 2-speed dives used on many engines already from late 1930s.
Granted, since the 1-speed V-1710 was rarely powering bombers (apart from the A-36), lack of 2-speed S/C wasn't that much of a problem. Lack of mechanically-driven 2-stage S/C, that would've required a 2- or variable-speed drive to be efficient was a more pressing need. Even the 9.60:1 drive was too late to matter for crucial 1942 year.


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## wuzak (May 13, 2018)

P-39 Expert said:


> The main purpose of low gear in a two speed engine is to keep the pilot from overboosting the engine at takeoff.
> Allison accomplished this in their single speed engine by throttling the engine at TO and gradually opening the throttle as altitude was gained and air became thinner.
> By mid '42 the automatic boost control (automatic manifold pressure regulator) automatically prevented the engine from exceeding a set manifold pressure at any altitude. So the pilot could select full throttle and not overboost the engine at any altitude.
> This single speed arrangement provided plenty of power at lower altitudes (peak WEP was under 9500') and adequate power at higher altitudes since the single speed was in effect high gear. It was also lighter and simpler than a two speed arrangement.
> The auto boost and variable speeds of the turbo and mechanical auxiliary stage rendered a two speed Allison unnecessary.



You can keep saying this utter bollocks, but it won't make it true.

Single stage Merlins had automatic boost control. They tended to have less power than a V-1710 at low altitudes, but more at high altitudes.

The 2 speed Merlin XX had more power down low than the similar single speed 45, and had more power up high. And the Merlin XX had automatic boost control too.

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## swampyankee (May 13, 2018)

Allison was _totally_ dependent on USN and USAAF business; Curtiss-Wright and Pratt & Whitney had large commercial customer bases. One effect of this was that both companies had larger engineering staffs and likely better field service operations and broader access to vendors. 

It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.


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## tomo pauk (May 13, 2018)

swampyankee said:


> Allison was _totally_ dependent on USN and USAAF business; Curtiss-Wright and Pratt & Whitney had large commercial customer bases. One effect of this was that both companies had larger engineering staffs and likely better field service operations and broader access to vendors.
> 
> It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.



Allison didn't sold a singe engine to the USN, IIRC, so dependance on that service didn't existed. Allison certainly developed a two-stage supercharged versions of the V-1710 (a year or more before Jumo and Daimler-Benz did it for their service engines), where 1st S/C stage was driven via variable-speed drive. More than 1500 of 2-stage V-1710s were produced until end of 1944.
Unfortunately, while the USN materially supported P&W (and Wright?) to develop 2-stage versions of their engines much before the US entered the war, the AAC/AAF didn't saw fit to do the same with even basis V-1710, let alone it's 2-stage version. Rather, they burned a lot of money into hi-per engines' money pits in the late 1930s.


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## tomo pauk (May 13, 2018)

wuzak said:


> Single stage Merlins had automatic boost control. They tended to have less power than a V-1710 at low altitudes, but more at high altitudes.
> 
> The 2 speed Merlin XX had more power down low than the similar single speed 45, and had more power up high. And the Merlin XX had automatic boost control too.



Depending on a version, there were 1-speed Merlin versions, in production and service, with a bit more power than V-1710s down low. Eg. Mk. 32 and 34 with 1640 HP at 2000 ft; 1625 HP for take off.
Re. Merlin XX and 45 hi-alt performance, the difference was in a few HP?


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## Shortround6 (May 13, 2018)

P-39 Expert said:


> The main purpose of low gear in a two speed engine is to keep the pilot from overboosting the engine at takeoff.
> Allison accomplished this in their single speed engine by throttling the engine at TO and gradually opening the throttle as altitude was gained and air became thinner.
> By mid '42 the automatic boost control (automatic manifold pressure regulator) automatically prevented the engine from exceeding a set manifold pressure at any altitude. So the pilot could select full throttle and not overboost the engine at any altitude.
> This single speed arrangement provided plenty of power at lower altitudes (peak WEP was under 9500') and adequate power at higher altitudes since the single speed was in effect high gear. It was also lighter and simpler than a two speed arrangement.
> The auto boost and variable speeds of the turbo and mechanical auxiliary stage rendered a two speed Allison unnecessary.




You have brought this up before, it was wrong then, it is wrong now and it will be wrong regardless of the number of times you bring it it up.
Please see post #3 in this thread for an explanation of 2 speed superchargers. 
Please note the Merlin III was using an automatic boost limiter well before the Allison ever made it into mass production as were a number of other European engines (like the DB 601). 
WEP was _APPROVED _in the Fall of 1942, a bit late to be sure, but way to late too have any effect on planning of engines/superchargers and accessories. 

The Germans had built engines in WW I that required a restricted throttle opening at low altitudes. No supercharger but high compression ratio in the cylinders.
Throttle had to be restricted for the same reason, to keep engine from destroying itself at low altitude. as the plane climbed into thinner air the throttle could be fully opened and the engine mad more power than an equivalent engine with a lower compression ratio. I am not trying to take anything away from the people at Allison but I have never seen any claim that they invented automatic boost control or even promoted for the use you are claiming.

I would note that on some engines (like Wright R-1820s) the two speed drive weighed a whopping 30lbs. To me 30lbs for an extra 100hp (or more) for take-off is a pretty good trade.

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## Shortround6 (May 13, 2018)

tomo pauk said:


> Allison didn't sold a singe engine to the USN, IIRC, so dependance on that service didn't existed.



Allison may have sold 2 engines to the Navy for airship use but the Airships crashed/were removed from service before delivery could be made. I would note that Allison had designed and built some drive systems for Navy airships earlier. That is extensions and angle drives. 



swampyankee said:


> It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.



The Corporate Master at GM was interested in making money, much like the Corporate Master/s at Continental and Lycoming. Allison during the 30s was making a nice profit with engine bearing division, they may have been making money (or at least not losing much) doing specialty engineering. However the V-1710 project was a money pit that GM had already funded to the tune of over 1/2 million dollars with no pay-off insight until the fighter order of April 1939. The Army was over $900,000 in arrears for work already done at this point and GM was considering shutting the whole project down. Not increasing company funding for new variations no matter how forward looking they may be.

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## P-39 Expert (May 15, 2018)

Shortround6 said:


> You have brought this up before, it was wrong then, it is wrong now and it will be wrong regardless of the number of times you bring it it up.
> Please see post #3 in this thread for an explanation of 2 speed superchargers.
> Please note the Merlin III was using an automatic boost limiter well before the Allison ever made it into mass production as were a number of other European engines (like the DB 601).
> WEP was _APPROVED _in the Fall of 1942, a bit late to be sure, but way to late too have any effect on planning of engines/superchargers and accessories.
> ...


If I'm wrong then the AAF would have directed Allison to produce a two speed engine. They didn't. Allison estimated a low gear power increase of around 100hp which wasn't viewed as being worth the effort.


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## Shortround6 (May 15, 2018)

Do you have any documentation?

AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters
Bristol was using autotatic boost limiters in 1939.
The First Zero's used a single speed engine with an automatic boost limiter and yet went to a two speed engine, still with an automatic boost limiter.

The list goes on and on. The US was simply late to the game with automatic boost limiters, it wasn't a break through in technology.

Allison and the USAAF got lucky. They started with 100 octane fuel and didn't have to put up with the limits that 87 or 91 octane fuel imposes. The luck continued when the development of 100/130 fuel meant that they could use 30% higher pressure in the cylinders for WEP (or slightly higher military power than before) without having to resort to a more sophisticated supercharger design.

Difference between a Merlin III and a Merlin X using the same supercharger design except for the superdrive and gears was close to 200hp when using 87 octane fuel.
The P & W R-1830 picked up 150hp for take-off using low gear over what it could make if using the the high gear only.

To give an idea of what was going consider the comparison of the V-1710-33 (long nose in P-40) and the V-1710-27/29 used in the YP-38s and some of the other early P-38s. The -33 used 8.77 supercharger gears and the -27/29 used 6.44 gears.

......................V-1710-33...............V-1710-27/29
BHP.................1040..........................1150
RPM................3000..........................3000
[email protected]*..........41.9..........................39.7
Mixture temp.... 196...........................112
Friction HP*.......290..........................240
Indicated HP......1330........................1390

The Map is at 70 degrees F inlet temperature going into the carb.
The mixture temperature is the temp in fahrenheit in the manifold after the supercharger.
Friction hp includes the power needed to drive the supercharger.
Indicated HP is the power produced in the cylinders.

Please note the increase power needed to drive the supercharger with the 8.77 gears. Please also note the decreased mixture temperature.
Of course a plane using 6.44 gears and no turbo would have had truly atrocious altitude performance.

I would note that later pilot's manuals for the P-38's with -49/53 engines show 1240hp for take off using 44.5in of MAP. compared to the 1150hp at about the same pressure in the -39 engine. The -49/53 engines used 7.48 gears. Bumping the pressure up to 47in gave 1325hp which the -39 engine (and other 8.80 supercharger ger engines ) needed 51in to match.
Again the lower gear ratio engine cannot maintain it's better power as high as the 8.80 gear engine.
Please note the the later Allisons used stronger construction and were able to withstand the higher boost pressures of WEP ratings much better.

The Automatic boost control did not make 2 speed superchargers obsolete. Allison got "lucky" in that better fuel allowed higher boost pressures at low altitudes and stronger construction (better materials/techniques) allowed the engine to survive the increased power.

Remember, each and every use of WEP required notation in log books, extra maintenance (even if just checking for metal in the oil and more frequent spark plug changes) and if used enough, pulling the engine before normal life expectancy for overhaul. 

Automatic boost control DID NOT,
Lower intake mixture temperatures.
Require less power to drive the supercharger.
Reduce cooling load.

Allisons were never used in a production bomber or transport where long hard take-offs and climb outs would be the normal operating procedure.
Weirdly enough, despite the "wonders" of the automatic boost control, at the end of the war Allison built 8 two speed, single stage engines to power a planned version of the C-54 transport. They used a new 10.25in impeller supercharger and were rated at 1600hp/3200rpm/SL/61.7in in low gear for take-off and military power of 1220hp/3000rpm/15,500ft. Low gear was 7.48:1 and high gear 9.60:1.


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## wuzak (May 15, 2018)

Shortround6 said:


> AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters



At some point the 2 speed engines also got automatic gear change.




Shortround6 said:


> Difference between a Merlin III and a *Merlin XX* using the same supercharger design except for the superdrive and gears was close to 200hp when using 87 octane fuel.



Do you mean Merlin X?


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## wuzak (May 15, 2018)

P-39 Expert said:


> If I'm wrong then the AAF would have directed Allison to produce a two speed engine. They didn't. Allison estimated a low gear power increase of around 100hp which wasn't viewed as being worth the effort.



The AAF were still very keen on the turbo to provide the altitude performance for the V-1710.

IIRC, Curtiss had to request an altitude rated engine (ie one that wasn't using a turbo). But the altitude rated engines didn't have every high critical altitudes. In that sense the 2 speed drive may not have made much difference.

Gaining altitude performance in a 1 speed supercharged engine led to a loss of low altitude performance. So if Allison could have produced a better, higher altitude supercharger, the 2 speed drive would have been beneficial.


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## Shortround6 (May 15, 2018)

wuzak said:


> Do you mean Merlin X?


yes, thank you, I will edit the post.


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## Shortround6 (May 15, 2018)

wuzak said:


> The AAF were still very keen on the turbo to provide the altitude performance for the V-1710.
> 
> IIRC, Curtiss had to request an altitude rated engine (ie one that wasn't using a turbo). But the altitude rated engines didn't have every high critical altitudes. In that sense the 2 speed drive may not have made much difference.
> 
> Gaining altitude performance in a 1 speed supercharged engine led to a loss of low altitude performance. So if Allison could have produced a better, higher altitude supercharger, the 2 speed drive would have been beneficial.



Timing is everything.
The Allison was very bit as good at altitude as the DB 601 in 1939/40 (The DB 601 got a bit better later) and every bit as good or better than any mass produced Hispano V-12 or derivative in 1939/40/41. It was also just as good as the American radial engines, the R-1820 and R-1830, and just as good or better than the Italian radials and Japanese radials of 1939-40-41. Unfortunately the game changed from late 1939 and into 1940. Planes got heavier as more "stuff" was added. 
Hooker changed the game with the Merlin. The Germans revamped the 109 and cut drag a lot while gaining very little weight. 

In 1939 and for most of 1940 Nobody had an engine with a higher critical altitude than around 14,000ft (correction welcome) aside from the Merlin. 
And the Merlin III as you say, payed a price for it's altitude performance in poor take-off power.

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## P-39 Expert (May 16, 2018)

Shortround6 said:


> Do you have any documentation?
> 
> AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters
> Bristol was using autotatic boost limiters in 1939.
> ...


Documentation was from Vees for Victory/Whitney. Only about 100HP increase at SL/Low Alt. Not worth the time and trouble.


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## tomo pauk (May 16, 2018)

P-39 Expert said:


> Documentation was from Vees for Victory/Whitney. Only about 100HP increase at SL/Low Alt. Not worth the time and trouble.



What page(s) of the book would've that be? What versions are compared?


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## tomo pauk (May 16, 2018)

Shortround6 said:


> ...
> Unfortunately the game changed from late 1939 and into 1940. Planes got heavier as more "stuff" was added.
> Hooker changed the game with the Merlin. The Germans revamped the 109 and cut drag a lot while gaining very little weight.
> 
> ...



Germans also quickly improved the DB 601 series, 1st with clearance of the 601A for 2600 rpm*, together with introduction of the 601N (not a jewel of reliability, but then neither the V-1710 was without problems), then in mid-1941 introduction of 601E, as well as uprating the 601A and 601N to 2800 rpm*. Plus better intake and prop with each engine? 
For the V-1710 in non-turbo application, it was just barely a service-worthy engine in 1939, unlike the DB 601A, let alone the Merlin III (while the RR already has in production the 2-speed Melin X).

*above the rated altitude, ie. direct improvement of altitude performance.


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## GregP (May 16, 2018)

Regarding post#7, Tomo, if you can find copies of the contracts for V-1710 development, especially the early ones, you will see the government defacto retained the rights to the design, and was to specifically authorize any changes they wanted to fund. If Allison themselves made improvements, these improvements were part of the design, but Allison retained the right to use improvements on other Allison-owned and/or developed projects.

The government specifically retained the right to purchase by specific part number, and they did. When they contracted for, say, V-1710-89/91 engines, Allison was constrained to produce -89/-91 engines without modifications, unless called out specifically. You can call that whatever you want. I call it design ownership. The government also retained rights to approve or deny V-1710 exports and use on other-than-government projects, and mandated cooperative production allocation so production could be assured for each project. Effectively, the government had ownership of that particular design, but Allison's name was on it.

Allison wisely insisted on only Allison in-house manufacture, but was constrained otherwise as to what they could sell from existing production lines. Allison was free to experiment on in-house prototype engines, but they didn't have many resources for that or money from GM to expand development staff. So, development was fraught with delays. A small staff restricts the number of projects that make progress.

In hindsight, Allison could have done things differently, but may also have perished in the great depression of 1929 had not the Navy been interested enough to fund development of the V-1710. So, the Navy found a buyer's market and Allison accepted the contracts eagerly, little realizing how they would be constrained later by these same contract clauses.

Live and learn, is the old saying.


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## Schweik (May 16, 2018)

oldcrowcv63 said:


> was based on the assumption that a single stage, 2-speed Supercharger might have been quickly developed for the P-40 that would have incrementally improved its high altitude performance in the time-frame of 1941-42. Instead, the USAAF fielded the P-40K in the Summer/Fall of 1942, which did nothing to improve the aircraft's high altitude performance but did provide additional performance for the overweight aircraft to get off the ground.



That's pretty funny that a P-40 is overweight but the P-38 and P-47 aren't?

P-40K used the low altitude power, especially when overboosting to 60" (which was secretly agreed to by Allison before Dec 1942) gave them ~1,500 + hp at 2,000 feet so that was quite useful in Theaters like the Russian Front (which is why so many Soviet pilots did so well with the P-40E and the P-40K in particular) and also in North Africa, and to some extent Burma. 

In the Pacific, like at Darwin when the Japanese level bombers were sometimes coming in at 30,000 feet the altitude limit on the P-40s was a major problem, though they did work out effective tactics anyway. It did a lot better against tactical aircraft like dive and torpedo bombers such as at Milne Bay.

As Shortround noted, the V-1710-73 in the P-40K had a strengthened (first peened, then tempered) camshaft which allowed overboosting at higher levels and for longer periods. The gasoline quality also made a big difference though I don't understand precisely by what mechanism. From reading Allison memos it sounds like they are saying the better fuel didn't run as hot which seems counter-intuitive.

The later P-40 engines with the 9.6-1 gear ratio such as the V-1710-81 on the P-40M etc. had a bit higher altitude rating but were riskier to run at high manifold pressure.



> SR6 has provided the rationale for why it wasn't pursued: little benefit to be achieved, and of course the fact that at about the same time, the single staged, two-speed variant of the Packard-Merlin engined P-40(F) was coming on line which was perhaps a far simpler solution to the problem than a dedicated Allison development.



The Packard Merlin P-40F raised the effective ceiling of the P-40 to 20k feet, which helped somewhat in North Africa but was still generally insufficient in terms of altitude performance. tey did use them to escort medium bombers like the A-20s, B-25s and B-26s, but escorting the heavy bombers was left to P-38s. Interestingly Packard strengthened the V-1650-1 / Merlin 28 used for the P-40F/L by coating the bearings with super hard iridium, and these too were overboosted quite a bit in the field.

P-38s however, while they found a niche early on in the Pacific, and were the best high altitude escort available in the Med and Europe until the P-51, more generally simply did not live up to the hopes of USAAF planners in the Med which is one of the reasons why the P-40 continued in use for so long. That along with the range limitation on Spitfires and the almost complete failure of P-39 units in the Med forced the USAAF and RAF to keep equipping units with P-40s until 1944 (and 1945 for RAF and in the Pacific / CBI Theater). The P-47 was what ultimately replaced the P-40 in the Med. As a radial engined plane it with a heavier bomb load it was a better fighter-bomber in many respects and it also had the altitude capacity to escort heavy bombers in theory, though in practice they still used the P-38s for that mostly I think until the P-51s were available.

The decision to use the single stage Merlin XX as the basis of the P-40's "Merlin 28" instead of something more like the Merlin 60 series is allegedly why Don Berlin quit Curtis aircraft company. I do think it was a major mistake though it's basically a question of quantity vs quality. Merlin 61 were not being produced by Rolls Royce until March 42 and I'm not sure when the V-1650-7 became available from Packard I'm guessing a bit later that year. 1942 was a very tough year for the Allies and the V-1650-1 powered P-40, for all it's limitations, was definitely needed and played a crucial role in the capture of Tunisia, the reduction of Pantellerina, the invasion of Sicily and the invasion of Italy through Anzio. The Spit V was a better interceptor but didn't have the legs for most escort missions. The Spit IX was the best air superiority fighter they had by far but there weren't enough of them, and all Spits were a bit too fragile for ground attack.

I think the Russians actually preferred the P-40K though with the Allison engine because they were fighting down low for the most part. They got about 100 P-40Fs but sent them right to PVO units (air defense). Allison engine P-40s seem to also have been preferred in the CBI.

One question I have is why weren't the Allison engined P-51s a bit more widely used in the Med? They did have them but were using them as dive bombers and recon planes mostly.

It seems like one answer to the 'supercharger dilemma' would have been to produce some V-1650-1 / P-40Fs while also diverting at least some of the V-1650-7s to P-40s along with the P-51s. The P-51s were better in many ways but a P-40 with a V-1650-7 probably would have been a pretty dangerous opponent for the Luftwaffe. I guess it was basically a question of prioritizing Strategic bombing over the (stalled out) Tactical campaign in Italy.

S


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## Shortround6 (May 16, 2018)

Schweik said:


> That's pretty funny that a P-40 is overweight but the P-38 and P-47 aren't?
> 
> It seems like one answer to the 'supercharger dilemma' would have been to produce some V-1650-1 / P-40Fs while also diverting at least some of the V-1650-7s to P-40s along with the P-51s. The P-51s were better in many ways but a P-40 with a V-1650-7 probably would have been a pretty dangerous opponent for the Luftwaffe. I guess it was basically a question of prioritizing Strategic bombing over the (stalled out) Tactical campaign in Italy.



Power to weight is what makes an aircraft "0ver weight". 
Once you get a Allison P-40 to 15,000ft or so you have an 1150hp engine in an 8,000lb plane and as you go higher it gets worse. 
P-47 had 2000hp for a 12,500lb plane and had that power o weigh ratio at 25,000ft and a bit above. 
P-38 likewise a had a much better power to weight ratio at altitude due to the turbos. A late model P-38 had four times the power at 25,000ft that a P-40N did. It sure didn't weigh four times as much. 

A P-40 with a V-1650-7 might have been a very dangerous opponent but it would have required quite a number of changes evenif you could actually get your hands on a quantity of V-1650-7s that weren't needed for Mustangs. You need the 6-70 heavier propeller, the engine weighs about 150lbs more than a V-1650-3 and the cooling system (includes intercooler for the 2 stage supercharger) weighs about 350-360lbs more than the cooling system in the P-40F.
given enough time they could have sorted everything out but why jump through all those hoops to build an inferior airplane? Just build more P-51Ds.
By the spring of 1944 P-40 production was winding down and they were building roughly twice as many P-51s per month as they were P-40s.


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## wuzak (May 16, 2018)

Schweik said:


> The decision to use the single stage Merlin XX as the basis of the P-40's "Merlin 28" instead of something more like the Merlin 60 series is allegedly why Don Berlin quit Curtis aircraft company. I do think it was a major mistake though it's basically a question of quantity vs quality. Merlin 61 were not being produced by Rolls Royce until March 42 and I'm not sure when the V-1650-7 became available from Packard I'm guessing a bit later that year. 1942 was a very tough year for the Allies and the V-1650-1 powered P-40, for all it's limitations, was definitely needed and played a crucial role in the capture of Tunisia, the reduction of Pantellerina, the invasion of Sicily and the invasion of Italy through Anzio.



The V-1650-1 started production in 1941, ramping up into 1942. Note that most of the single stage Merlins built by Packard (known as Merlin 28) went to the British for Lancasters and Hurricanes (built in Canada).

The V-1650-3 may have started production in late 1942, but was well into 1943 before they began to appear in quantity. I believe this is what held up production of the P-51B.

The V-1650-7 appeared later in 1943.

Why the V-1650-3 and V-1650-7 didn't appear in P-40s is simple - they were spoken for.

Berlin had left Curtiss long before the V-1650-3 was in production. Possibly before the V-1650-1 was in production.


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## wuzak (May 16, 2018)

Schweik said:


> Interestingly Packard strengthened the V-1650-1 / Merlin 28 used for the P-40F/L by coating the bearings with super hard iridium, and these too were overboosted quite a bit in the field.



The bearing materials were standard US practice at the time, so it made sense to use them instead of starting another process to make the same spec as Rolls-Royce were using. The main reason for the iridium seems to be that they prevented corrosion, though they did have slightly better strength.


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## Milosh (May 16, 2018)

Schweik said:


> The Spit IX was the best air superiority fighter they had by far but there weren't enough of them, and all Spits were a bit too fragile for ground attack.



The Spit IX and later the Spit XVI were the main a/c used by the 2TAF in NE Europe. They could be dived up 60 degrees.


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## KiwiBiggles (May 16, 2018)

Milosh said:


> The Spit IX and later the Spit XVI were the main a/c used by the 2TAF in NE Europe. They could be dived up 60 degrees.


Careful, you're using facts to destroy a widespread article of faith. _Everyone_ knows that the Spitfire was fragile; in fact, it's astonishing they ever made it into the air.

Tactical Spitfire squadrons in Italy, probably using Mk.Vs, used to dive-bomb at an angle of 85 degrees.


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## Shortround6 (May 16, 2018)

Schweik said:


> I think the Russians actually preferred the P-40K though with the Allison engine because they were fighting down low for the most part. They got about 100 P-40Fs but sent them right to PVO units (air defense). Allison engine P-40s seem to also have been preferred in the CBI.
> 
> One question I have is why weren't the Allison engined P-51s a bit more widely used in the Med? They did have them but were using them as dive bombers and recon planes mostly.



Production numbers and spare parts answer those questions. There were about 1560 P-40Fs built, that includes 250 Kittihawk IIs. This compares to the roughly 2900 P-40D&Es and 1320 (?) P-40Ks. The P-40L was built to the tune of about 700 compared to about 600 Ms (including lend lease) and then we get into the Ns starting in March of 1943. these run to around 5000. 
There simply weren't enough Merlin P-40s to scatter around to too many different theaters. The US screwed up and only ordered around 20% spare engines (or equivalent spares?) instead of the more customary 50% spares. The British tore down around 600 used Merlin engines to provide the US forces in North Africa with spare parts. 

sending a few squadrons of P-40F/Ls to lesser theaters without adequate spare parts (not just the engines, the Merlin and Allison needed different radiators and oil coolers) is going to result in a lot of effort for not much result. 

The numbers tell the story of the Allison P-51/Mustang/A-36 too. 
There were roughly 1580 Allison Mustangs built. The British got the first 620 (-2) the Americans got 500 A-36s with dive brakes and the rest were somewhat divied up. There simply weren't enough to go around to everybody who wanted them (British were still using two squadrons on VE day). 
Now please note that there were 3 different Allison engines used in the P-51. The 500 A-36s got a special low altitude model engine that would have limited them more than the other Allison engines. 

The 500 A-36s kept 3 fighter bomber groups (9 squadrons?) in action for quite some time, over a year?


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## Shortround6 (May 16, 2018)

British try using bombs as air to air weapons as Spitfires are too fragile for ground attack  

Yes the Spitfire was not as desirable as some other aircraft for ground attack (Typhoons with hundreds of pounds of extra armor?) and P-51s were more susceptible to ground fire/battle damage than P-47s but both Spitfires and P-51s performed many ground attack missions.


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## Schweik (May 16, 2018)

Shortround6 said:


> Power to weight is what makes an aircraft "0ver weight"
> Once you get a Allison P-40 to 15,000ft or so you have an 1150hp engine in an 8,000lb plane and as you go higher it gets worse.



That's part of it (Power to Weight) and with the turbo, especially useful at high altitude - but there is another factor, wing loading, which is an issue at medium altitude and increasingly important as you get lower and lower. Seeing as how P-47s were used so much in the Fighter Bomber role and even P-38s were used that way as well (and therefore could be engaged by enemy fighters at low altitude), this was an issue.

This is one of the main reasons that neither the P-47 nor P-38 were particularly popular with our allies and saw little postwar service. I think the English accepted a few hundred P-47s which were immediately sent to Burma to be used as bombers in lieu of Hawker Hurricanes. The Soviets accepted a small number, which they evaluated. They appreciated the big engine and all the space and nice construction standards, but noted that at low altitude the aircraft performed aerobatics "reluctantly" and in the words of the Soviet Test pilot, while nice in many respects, the P-47 was "not a fighter". The UK had a similar opinion of the P-38 I believe.

With a leaden wing loading combined with a huge draggy fuselage and a weight of 12,700 lbs the P-47 is not the ideal candidate in a dogfight at low altitude, regardless of how much power that huge turbo has. Until they got the paddle blade props it didn't climb well either. Can you do impressive things with it anyway? Certainly, you can work around the weight - but I think that does qualify as _overweight_. At 53 lbs per square foot wing loading the P-38 was even less agile than a Thunderbolt at a low or medium altitude dogfight (at least P-47s had good roll), which is why they had so much trouble in the Med and ETO. Not even getting into the compressability, heating, engine, turbo, intercooler, etc. problems.



> P-38 likewise a had a much better power to weight ratio at altitude due to the turbos. A late model P-38 had four times the power at 25,000ft that a P-40N did. It sure didn't weigh four times as much.



"At altitude" being the key phrase here. P-38 didn't even have an impressive HP to weight ratio (0.16) compared to say, a Spitfire Mk V (.22) - in fact it's about the same as a later model P-40.



> A P-40 with a V-1650-7 might have been a very dangerous opponent but i.



I'll just skip the 'what-ifs' with you guys lol...

S


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## Schweik (May 16, 2018)

Shortround6 said:


> British try using bombs as air to air weapons as Spitfires are too fragile for ground attack
> 
> Yes the Spitfire was not as desirable as some other aircraft for ground attack (Typhoons with hundreds of pounds of extra armor?) and P-51s were more susceptible to ground fire/battle damage than P-47s but both Spitfires and P-51s performed many ground attack missions.



You have an excellent command of the obvious, and of the literal.

I'm well aware Spits were used for ground attack - the whole DAF was, every aircraft type pretty much - I just said they were a bit too fragile for it, which is true. Not the ideal bird for that job. This was also the feeling of most Spit pilots. Spitfire was basically an interceptor, that was what it was best at.

S


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## Schweik (May 16, 2018)

Shortround6 said:


> Production numbers and spare parts answer those questions. (snip) There simply weren't enough Merlin P-40s to scatter around to too many different theaters.



But production doesn't tell the whole story and in fact proves misleading. They may have built 5,000 P-40Ns but by the time the N came out, the urgency for using P-40s was over. The P-40F and L models, conversely, were available at the most critical moment. They had enough of them to be the main variant in use by no less than 5 USAAF fighter groups in the Med / Italian Theaters: 33 FG, 324 FG, 325 FG, 57 FG and 79 FG were all mainly using merlin engined P-40s, as well as some P-40K.



> sending a few squadrons of P-40F/Ls to lesser theaters without adequate spare parts (not just the engines, the Merlin and Allison needed different radiators and oil coolers) is going to result in a lot of effort for not much result.



5 Fighter groups with 3 squadrons each = 15 squadrons, (that's a little more than 'a few'), operating during the most critical months on the war, they played a key role in finally and decisively securing the English supply route from India, taking one of the major Axis powers (Italy) out of the war and forcing vast amounts of German resources to be redirected to cover their southern flank, not a "lesser Theater" in my opinion, though I know some perceive it that way.

Conversely what, precisely, was happening in North West Europe in 1942 and early 1943?

The Theaters of significance in 1942 were Russia, the Pacific and the Med. Probably in that order. The Battle of Britain was a key battle but that was long over. I don't think Dieppe qualifies as "THE" major front line event of the war in that era. YMMV.



> The numbers tell the story of the Allison P-51/Mustang/A-36 too.
> There were roughly 1580 Allison Mustangs built. (snip)
> 
> The 500 A-36s kept 3 fighter bomber groups (9 squadrons?) in action for quite some time, over a year?



Actually again, not of that actually tells us much of anything. It's a large number of planes which they seemed to mostly use for recon - not that recon was unimportant - it was very important and recon planes seemed to almost always get shot down, so a fast one like the P-51 / A-36 certainly had an important role. I'm just wondering why they weren't using them more on fighter sweeps over German airfields like they did with the P-40s and Spit Vs.

S


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## Schweik (May 17, 2018)

My guess on the P-51A / A-36 is that they didn't have too many of them in the Med or Italy and were probably using most of them for recon and "army cooperation" stuff from the UK.


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## Milosh (May 17, 2018)

Spitfire fragile? The Spit had a higher dive speed than the Mustang.

The A-36's 1st mission was June 6 1943 attacking Pantelleria. The 27th and 86th FBG flew mission in the MTO. The 311th FBG flew in the Far East. They flew some 23,373 combat sorties and delivering over 8,000t of bombs. They shot down 84 e/as for the loss of 177 to all types of enemy action.

The P-51/Mustang Ia had a camera mounted but not the A-36.


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## Shortround6 (May 17, 2018)

Schweik said:


> I'll just skip the 'what-ifs' with you guys lol...



I like a good what if. 
However when we start talking about 1943/44 engines in 1942 aircraft it starts getting strange.
The P-40 was going to be 20-40mph slower than P-51 using the same engine so it is a little hard to to figure out why they would take the best engine and put it in the 2nd best airframe.
The British did do this with the Hurricane II and the Spitfire but then it was a case of improving the Hurricane or facing a real fighter shortage in the winter of 1940/41. SPitfire production not being large enough. The Americans were not faced with such a production problem in the summer/fall of 1943 (or later?) when the V-1650-7 became available.


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## Shortround6 (May 17, 2018)

Schweik said:


> But production doesn't tell the whole story and in fact proves misleading. They may have built 5,000 P-40Ns but by the time the N came out, the urgency for using P-40s was over. The P-40F and L models, conversely, were available at the most critical moment. They had enough of them to be the main variant in use by no less than 5 USAAF fighter groups in the Med / Italian Theaters: 33 FG, 324 FG, 325 FG, 57 FG and 79 FG were all mainly using merlin engined P-40s, as well as some P-40K.
> 
> 
> 
> 5 Fighter groups with 3 squadrons each = 15 squadrons, (that's a little more than 'a few'), operating during the most critical months on the war, they played a key role in finally and decisively securing the English supply route from India, taking one of the major Axis powers (Italy) out of the war and forcing vast amounts of German resources to be redirected to cover their southern flank, not a "lesser Theater" in my opinion, though I know some perceive it that way.



The P-40Fs were sent to the NA/Med in 1942 as that was, as you say, the most important theater at the time, at least for the US. They were not used in Europe (England) at all. 
They were also not used (at least in any numbers) in the CBI or Pacific theaters, which for supply purposes are different. Those are the lesser theaters I was referring to. The CBI and Pacific using different supply routes and depots for the most part. 







> Actually again, not of that actually tells us much of anything. It's a large number of planes which they seemed to mostly use for recon - not that recon was unimportant - it was very important and recon planes seemed to almost always get shot down, so a fast one like the P-51 / A-36 certainly had an important role. I'm just wondering why they weren't using them more on fighter sweeps over German airfields like they did with the P-40s and Spit Vs.



As I said, there weren't enough of them. The British kept the vast majority of the Mustang Is in England and while they called it reconnaissance, many of them engaged in low level strafing from the beginning. The next batch was the P-51/Mustang IA with 20mm cannon and there were only 150 of them. 93 go to England and 57 stay in the US, 55 of which become F-6A photo-recon planes. Then comes the A-36 production batch. Deliveries start (at the factory) In Oct of 1942 which is a bit late for operation Torch, at least the initial stages. The 310 P-51As are what is left after the initial order for 1200 is cut in Dec of 1942 with the 890olane difference to be delivered as P-51Bs with Merlin engines. However that cut was made before the first P-51A rolled out the factory door. It takes until May of 1943 to complete the 310 P-51As and a P-51 sitting on a ramp In Los Angles is weeks/months away from seeing combat in any theater. 50 of the P-51As are supplied to the British as replacements for the 57 P-51s taken by the US. 
Timing of aircraft into service squadrons took a while. The British had four squadrons of Mustang Is at Dieppe and it took until Jan 1943 to get 15 squadrons into service. The British operated a peak of 21 squadrons of Mustang Is. But notice that the last Mustang I was delivered/accepted at the factory in July of 1942, so 618/620 planes accepted by July of 1942 but only 4 squadrons in service at the beginning of Aug.


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## P-39 Expert (May 17, 2018)

The AAF made their decision in '42 that the P-38 and P-47 would be the main combatants and the P-39, P-40 and P-51 would be mainly for export to our allies and help out with ground support.
Neither the Lightning nor Thunderbolt panned out as superplanes for the AAF.
The P-38 had the obvious flaws of poor diving characteristics and below average maneuverability which put it at a disadvantage in Europe where the opposition was just as fast. It excelled in the PTO because of a 70mph speed advantage.
The Thunderbolt didn't want to climb or turn and had the endurance of a Spitfire. 
Both were enormously expensive compared to other fighters and neither was available in 1942 (except for the P-38 at the very end).
Why in the world didn't the AAF prioritize two stage Allison production for the P-39, P-40 and P-51? Turn these planes into high altitude planes and their utility to the AAF goes through the roof.
But the big question is why the AAF insisted on making the P-39, P-40 and P-51 so damn heavy when the engine they were designed around only produced 1150-1200HP at TO? At 6# per HP those planes should have weighed 7200# when they actually weighed 7650, 8400 and 8600#. 
Either a two stage engine or a significant weight reduction program would have turned those planes into tigers.


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## Shortround6 (May 17, 2018)

P-39 Expert said:


> The Thunderbolt didn't want to climb or turn and had the endurance of a Spitfire.


We have been over this before, the P-47 had much greater endurance than Spitfire, roughly double on internal fuel. 


> Why in the world didn't the AAF prioritize two stage Allison production for the P-39, P-40 and P-51? Turn these planes into high altitude planes and their utility to the AAF goes through the roof.


Because the early Allison two stage was a piece of crap. Talking about the early test rig/s. You can "_prioritize" _whatever you want, without sufficient engineers and a good basic design you have crap for output. The first proposal/development contract was in Dec 1940 and used an auxiliary stage supercharger using the smae sized impeller as the main engine and used a fixed speed (single) supercharger drive to the auxiliary supercharger. At the time Allison was building about 250 engines a month and desperately trying to increase production of engines for P-38s, P-39s, P-40s (all different models) and work on a few other experimental projects. What gets _DE-prioritized_ ? weight given is 1545lbs and output is whopping 1150hp at 21,000ft. I think we can see why they weren't jumping all over this thing. 
The F9R engine was proposed to the Navy with a weigh of 1510lbs and 1125hp at 18,000ft using a two stage supercharger. 



> But the big question is why the AAF insisted on making the P-39, P-40 and P-51 so damn heavy when the engine they were designed around only produced 1150-1200HP at TO? At 6# per HP those planes should have weighed 7200# when they actually weighed 7650, 8400 and 8600#.
> Either a two stage engine or a significant weight reduction program would have turned those planes into tigers.



In part because the US believed in self sealing tanks and pilot protection, in part because US strength standards were higher than some other countries and in part because the US was saddled with the .50 cal machine gun and it's ammo. A P-40D with four guns and 250rpg was carrying 621lbs of guns, ammo and gun equipement. A Spit with two 20mm and four .303s was carrying about 650lbs of guns/ammo, extra equipement unknown. 
How much below four .50 cal do you want to go? Please note the Spit was carrying 243bs of ammo, 1000 rounds of .50 cal weighs 300lbs. 
Yes the US planes were over armed but their is a limit as to how low you can go.


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## Barrett (May 17, 2018)

Just FWIW: The JSF program is now in its 21st year, coming up on 11 since first flight. Contrary to what L-M, the services and DoD claim, not one F35 is FMC (that's fully mission capable.) The marines have been lying about its status forever, beginning with the first one that rolled into the chocks with the training squadron at Yuma. The AF recently had to announce that none of the first block (a coupla hundred airframes) will ever be FMC because they were delivered AND ACCEPTED as-was. Costs too much to upgrade them. 

A few years ago the AF secretary expressed a revelation when she said "Maybe we shouldn't buy airplanes until they're finished testing." Yathink? That's why F35 remains in low-rate production: the law requires successful completion of operational test & development, and the thing is nowhere close.

JSF is a prime example of corporate welfare, which is no accident because subcontractors are strewn across more than 80% of congressional districts. (Same with F22, as I recall.)

I could go on & on & on, but being naval-centric I'll note that the USN's C model spent *2 1/2 years* getting the tailhook to engage a wire. A test pilot told me that it only had a 10% arresting rate during land-based "roll in" tests. That's on the most expen$ive Fighterjet of all time, and we've been hanging hooks on airframes since 1922! 

What should we do? Aside from massive fines and maybe prison time, cut back JSF to sustainable levels and buy upgraded "legacy" jets like F15/18, AND INVEST IN ECM. But the AF has sacrificed EW on the Stealth Altar, leaving only the navy and marines with that capability. Still, it should be a no-brainer: you cannot add on stealth (which gets degraded just flying around, let alone basing in blowing sand & dirt--and heavy rain--and in-flight refueling.) ECM is much-much-much less expensive, which is probably why we don't have enough of it!

I'm a taxpayer. I want a refund...


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## P-39 Expert (May 17, 2018)

Shortround6 said:


> We have been over this before, the P-47 had much greater endurance than Spitfire, roughly double on internal fuel.
> 
> Because the early Allison two stage was a piece of crap. Talking about the early test rig/s. You can "_prioritize" _whatever you want, without sufficient engineers and a good basic design you have crap for output. The first proposal/development contract was in Dec 1940 and used an auxiliary stage supercharger using the smae sized impeller as the main engine and used a fixed speed (single) supercharger drive to the auxiliary supercharger. At the time Allison was building about 250 engines a month and desperately trying to increase production of engines for P-38s, P-39s, P-40s (all different models) and work on a few other experimental projects. What gets _DE-prioritized_ ? weight given is 1545lbs and output is whopping 1150hp at 21,000ft. I think we can see why they weren't jumping all over this thing.
> The F9R engine was proposed to the Navy with a weigh of 1510lbs and 1125hp at 18,000ft using a two stage supercharger.
> ...


A clean Thunderbolt (as all were until August '43) had barely the same range as a clean Spitfire especially in the ETO where they had to operate at max continuous.
Regarding the two stage Allison, 1180HP at 21500' was a tremendous increase over single stage models that would develop less than 700HP at that altitude. The Merlin 61 only developed 70HP more at 23000'. 
Regarding weight, the Germans (and British) were able to manufacture fighters with self sealing tanks, armor and heavy firepower at much better P/W ratios. What is the use of pouring resources into a fighter if it can't climb high enough to engage the enemy?

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## tomo pauk (May 17, 2018)

Schweik said:


> My guess on the P-51A / A-36 is that they didn't have too many of them in the Med or Italy and were probably using most of them for recon and "army cooperation" stuff from the UK.



P-51As and A-36s flew about zero combat sorties from the UK. A-36s were used exclusively in the Med, P-51As in the Med and CBI.


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## Shortround6 (May 17, 2018)

P-39 Expert said:


> A clean Thunderbolt (as all were until August '43) had barely the same range as a clean Spitfire especially in the ETO where they had to operate at max continuous.



Do you have some documentation of this? Comparing endurance, range, radius and combat/escort radius is very difficult unless you are sure you are comparing the same things. I would note however that a clean P-47 rarely had to cruise at max continuous (rich mixture) as that is 2550rpm and 42in map. At low level the P-47 could suck down a lot of fuel but at 25,000ft &altitude used for figuring escort radius it could do 360mph at 2500rpm and 38in map. Since that is about 30mph faster than US planners figured on (they figured 210mph IAS) you can drop down to 225mph IAS at 25,000ft at 2350rpm and 36in MAP and still be going faster than the planners need and drop fuel burn from 190gph tp 145gph. Max lean is 2250 and 32in MAP (105 gallons per hour) , The chart is missing a column but at 200mph IAS at 25,000ft the P-47 was supposed to need 2150rpm and 31in map and burn 95 gallons an hour. 
Do you have any figures for the Spitfire when cruising at those altitudes and speeds? 


> Regarding the two stage Allison, 1180HP at 21500' was a tremendous increase over single stage models that would develop less than 700HP at that altitude. The Merlin 61 only developed 70HP more at 23000'.



Yes 1180hp at 21500 was a very large improvement however you have underated the Merlin considerably. The Merlin 61 was good for 1390hp at 23,500ft. 
So Allison power is down to 84% and 2000ft lower rather than the 94% and 1500ft you are claiming. 
I would note that the V-1650-1 in the P-40F was rated at 1120hp at 18,500ft with it's single stage supercharger which basically comes down to the early two stage Allison going to a lot of complication and trouble for 2500-3000ft of altitude. Later ones got better.

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## Schweik (May 17, 2018)

While we are on the subject of superchargers, how did the Germans manage to get such good high altitude performance with the DB 601 series, just a lighter plane? Multi speeds? Something different about the impeller? I know water injection came a bit later (I think) and they didn't use two stage superchargers.


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## wuzak (May 17, 2018)

Schweik said:


> While we are on the subject of superchargers, how did the Germans manage to get such good high altitude performance with the DB 601 series, just a lighter plane? Multi speeds? Something different about the impeller? I know water injection came a bit later (I think) and they didn't use two stage superchargers.



They used a variable speed drive for the supercharger, and improved the supercharger itself over time. The supercharger may have even got bigger in later versions.


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## Milosh (May 17, 2018)

wuzak said:


> They used a *variable speed drive for the supercharger*, and improved the supercharger itself over time. The supercharger may have even got bigger in later versions.



I thought everyone knew that.


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## Schweik (May 17, 2018)

Milosh said:


> I thought everyone knew that.



Forgive my ignorance I'm not really into Luftwaffe planes that much. When it comes to the Axis kit I've always been more drawn to the Japanese and Italian planes, though granted some of the nicer of theirs also used the DB 601.

S


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## Shortround6 (May 17, 2018)

For the 109 they started with a small/light plane. They improved the supercharger, one change increased the number of vanes. 
Another major advantage was they used a bigger displacement engine that didn't use as much boost. If you are taking in air at 15in of pressure (sea level being 30in rounded off) and you are only trying to make 42.6in of manifold pressure you can use a less sophisticated supercharger than the guys who are taking in the same 15in air and trying to squeeze it to 60 in or higher. 
Late model DB605s in 109s used the supercharger from the DB 603 engine. Helped at high altitude, cost around 50hp (?) at low altitude.


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## Schweik (May 17, 2018)

Shortround6 said:


> The P-40Fs were sent to the NA/Med in 1942 as that was, as you say, the most important theater at the time, at least for the US. They were not used in Europe (England) at all.
> They were also not used (at least in any numbers) in the CBI or Pacific theaters, which for supply purposes are different. Those are the lesser theaters I was referring to. The CBI and Pacific using different supply routes and depots for the most part.



Well, they did use the P-40F and / or L in the Pacific without a doubt. For example according to the caption these were with the 44th fighter squadron at Guadalcanal







This one stuck in the mud, similar markings, is aalso clearly a Merlin engined P-40F or L






These are also Pacific, different markings, I think 18th Fighter Group (same group as the 44th)










These look like the same squadron as the last set. So I would guess probably at least 3 squadrons.

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## wuzak (May 17, 2018)

Shortround6 said:


> Another major advantage was they used a bigger displacement engine that didn't use as much boost. If you are taking in air at 15in of pressure (sea level being 30in rounded off) and you are only trying to make 42.6in of manifold pressure you can use a less sophisticated supercharger than the guys who are taking in the same 15in air and trying to squeeze it to 60 in or higher.



That is an important point.

Increasing boost, for a given supercharger, tends to lower the critical altitude.

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## Shortround6 (May 17, 2018)

Schweik said:


> Well, they did use the P-40F and / or L in the Pacific without a doubt. For example according to the caption these were with the 44th fighter squadron at Guadalcanal....................
> These look like the same squadron as the last set. So I would guess probably at least 3 squadrons.


Thank you , I stand corrected.

This site says 2 squadrons, the 44th and 68th. 

http://fighter-collection.com/cft/curtiss-p-40f-warhawk/

other websites are not helpful as they either skip over the use of the P-40 by these squadrons etriely or don't give the variants used.


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## P-39 Expert (May 18, 2018)

Shortround6 said:


> Do you have some documentation of this? Comparing endurance, range, radius and combat/escort radius is very difficult unless you are sure you are comparing the same things. I would note however that a clean P-47 rarely had to cruise at max continuous (rich mixture) as that is 2550rpm and 42in map. At low level the P-47 could suck down a lot of fuel but at 25,000ft &altitude used for figuring escort radius it could do 360mph at 2500rpm and 38in map. Since that is about 30mph faster than US planners figured on (they figured 210mph IAS) you can drop down to 225mph IAS at 25,000ft at 2350rpm and 36in MAP and still be going faster than the planners need and drop fuel burn from 190gph tp 145gph. Max lean is 2250 and 32in MAP (105 gallons per hour) , The chart is missing a column but at 200mph IAS at 25,000ft the P-47 was supposed to need 2150rpm and 31in map and burn 95 gallons an hour.
> Do you have any figures for the Spitfire when cruising at those altitudes and speeds?
> Per the chart in the pilot's manual, the P-47B/C held 305gal less 45gal for TO&Climb to 5000' leaves 260gal divided by 190GPH (max cont. at 25000') gives 1.4HR x 360mph = 504miles divided by 2 = 252miles radius (barely over the German border) before deducting any reserve for combat (15min) or landing reserve (20min). Now we have 1.4HR-35minutes = .8HR. .8HR x 360mph = 288miles divided by 2 gives 144miles radius. Not even across the channel.
> Now, you may not spend the whole trip at maximum continuous, but if you are over occupied Europe in 1943 you had better PLAN your mission on being at max continuous since that was the most heavily defended airspace in the world at that time and you better be going as fast as possible to avoid becoming a statistic.
> ...


Later versions of the Allison got better too. And the Allison was about 200# lighter (including aux. stage) than the Merlin, had fewer parts and was tougher by passing a 150 hour test when the Merlin only had to complete a 100 hour test. The two stage Allison would have turned the P-39, P-40 and P-51 into much improved planes at high altitudes.

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## tomo pauk (May 18, 2018)

P-39 Expert said:


> Later versions of the Allison got better too. And the Allison was about 200# lighter (including aux. stage) than the Merlin, had fewer parts and was tougher by passing a 150 hour test when the Merlin only had to complete a 100 hour test. The two stage Allison would have turned the P-39, P-40 and P-51 into much improved planes at high altitudes.



The V-1710 types featuring the auxiliary stage weighted ~1540 lbs, add an extra ~100 lbs for the extension shaft as found on the P-63. No intercooler present. The V-1650-3 and -7 were at 1690 lbs, featuring an intercooler (= more power), and were shorter by 10 inches.
The V-1710 prototype with intercooler added another 4 inches to the length (total of 102 in), weighting now 1750 lbs. The V-1650-9 weighted 1745 lbs (length 89 inches).
But I agree that the US fighters, originally outfitted with 1-stage V-1710, would've gained plenty of performance already past 15000 ft.

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## P-39 Expert (May 18, 2018)

tomo pauk said:


> The V-1710 types featuring the auxiliary stage weighted ~1540 lbs, add an extra ~100 lbs for the extension shaft as found on the P-63. No intercooler present. The V-1650-3 and -7 were at 1690 lbs, featuring an intercooler (= more power), and were shorter by 10 inches.
> The V-1710 prototype with intercooler added another 4 inches to the length (total of 102 in), weighting now 1750 lbs. The V-1650-9 weighted 1745 lbs (length 89 inches).
> But I agree that the US fighters, originally outfitted with 1-stage V-1710, would've gained plenty of performance already past 15000 ft.


Thought the extension shaft was considered part of the plane, not the engine.


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## tomo pauk (May 18, 2018)

P-39 Expert said:


> Thought the extension shaft was considered part of the plane, not the engine.



Nope - the data sheets for the P-63s list the engine at 1620-1710 lbs, plus up to 114 lbs worth of 'engine accesories' The remote reduction gear is also listed under the 'engine'. Engine controls at 40 lbs, water injection system (without the ADI liquid) at 50 lbs.
per America's hundred thousand, pg. 410
(weight of lubricating system at ~135 lbs, cooling system at a bit under 350 lbs, all figures without respective liquids; vs. P-39: 56-63 lbs and 322-326 lbs for respective systems; engine ~1400 lbs)

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## DarrenW (May 18, 2018)

r


Schweik said:


> ....but there is another factor, wing loading, which is an issue at medium altitude and increasingly important as you get lower and lower.....



Excellent point Schweik. A great example of how wing design could effect aircraft of similar size, weight, and drag would be to compare low altitude handling of the Thunderbolt verses the Hellcat. Both were equipped with the R-2800 and both were rather large and heavy fighters, but the differences in their overall maneuverability at low to medium altitudes was like night and day. The Hellcat was endowed with a greater wing area which helped to reduce wing loading and was one of several ingredients that made it far more agile than the Thunderbolt, save for roll rate. The larger wing and thicker airfoil also provided extra lift which added to the ability of the Hellcat to out climb the Thunderbolt, at least the earlier versions without the paddle blade propeller and water injection. Even take-off performance between the two was starkly different; the Thunderbolt using over twice the distance of the runway before it's wheels left the ground. Several seasoned Japanese pilots are quoted as saying that the Hellcat was very maneuverable and could dogfight with a Zero, unlike other American fighters which commonly used hit-and-run tactics in order to achieve their kills.

Get above 20,000 feet and it's a totally different ball game, where the virtues of the Thunderbolt's turbo could be fully realized. A difference of some 300+ horsepower allowed the Thunderbolt to gain a lot of ground on the Hellcat, making it the better choice for high altitude combat IMHO.

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## Elvis (May 25, 2018)

Didn't read through the entire thread, but I didn't see anyone mention one glaring problem with the 2 stage SC in the P-40...






...where you gonna put the second stage?
You either have to lengthen the airplane in front of the cockpit, or move the pilot back....at that point, you've thrown off the balance of the airplane.
Granted, we're not talking about a huge amount of space, but I don't see ANY room, whatsoever.


Elvis

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## wuzak (May 25, 2018)

Elvis said:


> Didn't read through the entire thread, but I didn't see anyone mention one glaring problem with the 2 stage SC in the P-40...
> 
> View attachment 494763
> 
> ...



Perhaps it is not necessary to read the whole thread, but the title instead:

Why didn't Allison quickly develop a *one-stage 2 speed Supercharger* for the P-40

In any case, the XP-40Qs were fitted with a 2 stage V-1710, and were based on P-40K or P-40N airframes. The engine was fitted by lengthening the fuselage ahead of the firewall.

Curtiss XP-40Q Fighter


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## Shortround6 (May 25, 2018)

A lot of things _could _be done, the question often overlooked is at what cost, both money and lost production. 
The latter becomes important because at the time the P-40Q was built (middle of 1943 for the first) the P-40 was on the way out. It was being built in large numbers for Lend-lease but the US was taking rapidly decreasing numbers into US service. Every successor to the P-40 that Curtiss had tried had failed or were failing (P-60 saga) so even a major modification to the P-40 looked good to Curtiss if it would sell. With P-38s, P-47s and P-51s coming out of the factories in larger than ever numbers retooling the Curtiss factory might not have been that big a deal.
Compare this to 1941 when it was pretty much the P-40 or nothing. 2248 P-40s built compared to 926 P-39s, 207 P-38s, one P-47 and 138 Mustangs. Navy got 324 F4Fs. 
The P-40Q used the long rear fuselage developed/introduced in 1942, it moved the radiator/s to the wings, it extended the forward fuselage. Nothing earth shattering but even if some sort of two stage engine had been available (actually worked) in 1941, early 1942 could the US afford the loss of several hundred P-40s while the changes were made to production tooling? P-40Qs also went to four guns and still weighed about 9,000lbs. 

About the best that could be hoped for was that Allison copied the Supercharger on the Merlin XX. This would have given planes of close to the performance (maybe less/maybe more but only a few percent difference) of the P-40F in much larger numbers. 

Keeping the Allison supercharger and adding a 2nd gear wasn't going to do much for altitude performance over the later 9.60 gear engines. Perhaps you could have used a higher gear ratio but you are getting into diminishing returns. Perhaps another 1-2,000ft of critical altitude with the low gear restoring take-off performance? 

A Merlin XX had 1280hp for take off at 12lbs boost compared to a Merlin 45 having 1185hp for take-off at 12lbs. High gear in the XX was just a bit more than 45 and gave 1490hp at 12500ft at 16lbs vs 1515bp at 11,000ft at 16lbs. 12lbs is roughly 54in and 16lbs boost is roughly 62 in. The Hooker modified supercharger on the Merlin XX and 45 was a generation ahead of the one used on the Allison. 

A P-40 with a set of 7.48 gears down low would certainly have some impressive low altitude performance and the 9.60 gears to allow some mid altitude performance.

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## tomo pauk (May 25, 2018)

Shortround6 said:


> ... The Hooker modified supercharger on the Merlin XX and 45 was a generation ahead of the one used on the Allison.
> ...



Perhaps it was 'just' bigger, with a greater capacity for airflow per unit of time, while of same generation?


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## Shortround6 (May 25, 2018)

Same 'generation' to Allison would be the Supercharger on a Merlin III or X. 
6lbs boost (42in?) to 16,250ft (1030hp) compared to the Allison -81 with 9.60 gears making 1125hp at 15,500ft with 44.5in pressure. This is with whatever improvements Allison had made over the earlier superchargers. The -33 was not anywhere near as good, 1140hp at 12,000ft at 42in? Not all the improvement coming from just the gear change. the rotating inlet guide vanes helped. 

I can't find my book with altitude pressure charts to work out the pressure ratio of the different superchargers.


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## Schweik (May 25, 2018)

Forgive me if I'm wrong but I think V-1710-81 came out kind of late, like midway through 1942 or later?


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## wuzak (May 25, 2018)

Schweik said:


> Forgive me if I'm wrong but I think V-1710-81 came out kind of late, like midway through 1942 or later?



That's SR's point. The -81 was a couple of years later than the XX/45 and had the performance of a Merlin III.


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## Schweik (May 25, 2018)

wuzak said:


> That's SR's point. The -81 was a couple of years later than the XX/45 and had the performance of a Merlin III.



And yet the V-1710-33 seems to have been producing 1580 hp routinely at low altitude in the actual field, not to mention the -73.... (at 60" Hg per the Allison memo) could a Merlin III do that?

Not suggesting Allisons were better than Merlins because I don't think they were - the altitude performance was very important, but low altitude performance also had some uses.

I also don't think the -33 was equivalent to a Merlin III frankly.


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## tomo pauk (May 25, 2018)

Shortround6 said:


> Same 'generation' to Allison would be the Supercharger on a Merlin III or X.
> 6lbs boost (42in?) to 16,250ft (1030hp) compared to the Allison -81 with 9.60 gears making 1125hp at 15,500ft with 44.5in pressure. This is with whatever improvements Allison had made over the earlier superchargers. The -33 was not anywhere near as good, 1140hp at 12,000ft at 42in? Not all the improvement coming from just the gear change. the rotating inlet guide vanes helped.
> 
> I can't find my book with altitude pressure charts to work out the pressure ratio of the different superchargers.



I was trying to point out that (not just) early Merlins have had a bigger S/C than (not just) early V-1710s, rather than it was somehow a more refined piece of kit. The bigger S/C being a crucial thing for altitude performance, and Hooker's much improved intake before the S/C cancelled many of losses thus further improving the all-around and hi-altitude performance starting with Merlin XX.


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## Shortround6 (May 25, 2018)

Tomo's last post is part of it. The pressure ratio of the supercharger on the Merlin XX/45 was significantly better than the pressure ratio of the Merlin III, Allison and DB 601 superchargers. It could flow both volume and pressure (they are somewhat related but not quite) and the pressure was higher at the same altitudes than the competing superchargers. 
In working as a firefighter our pumpers used centrifugal pumps, water is NOT compressible so I have to be careful about trying to transfer things over 
But a pump rated at 1500 gallons per minute at 150lbs pressure was only good for 750 gallons per minute at 250lbs pressure, not 300lbs pressure. Older pumps with two impellers instead of single impellers (much lower powered engines) could be changed over from operating in series (like a two stage aircraft engine supercharger) to get pressure or operating in parallel to get large volume at low pressure. I can tell you that getting the last 10-20lbs pressure out of the pump called for a lot more throttle/rpm than getting a 10/20lb rise in the middle of the gauge 
Air is compressible and you can squeeze more air into the same volume, but you can still get into diminishing returns, a lot more impeller rpm for not a lot of increase in either pressure or volume. Centrifugal pumps are not positive displacement. If the output is blocked or restricted the pump continues to turn and excess water or air simply bleeds back around the impeller to the inlet side. This results in things heating up considerably. 

the best single stage compressor that I know off that saw use in WW II ( I don't really know what late war P & W R-2800s could do) was the Merlin 46. Which used a larger diameter impeller, circular arc inlet guide vanes and a modified diffuser compared to a Merlin 45. 
It could maintain 9lbs boost (48in?) to 22,000ft. This cost quite a bit of power at lower altitudes though.


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## P-39 Expert (May 25, 2018)

Schweik said:


> Forgive me if I'm wrong but I think V-1710-81 came out kind of late, like midway through 1942 or later?


The 9.6 geared engine was first tried in early '42 with a pre production batch of 25 that went into the P-39J. The service life of the new gears was too short, as predicted by the 150 hour test. The gears were redesigned (widened) and production engines started coming out in August with the first installation in a production plane in November '42 with the P-39M. The P-40M followed. The big performance boost came the next month in the P-39N with the same engine re-designated -85 with the introduction of the 2.23 reduction gear. Over 7000 P-39N & Q were produced using this engine.


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## Elvis (May 25, 2018)

wuzak said:


> Perhaps it is not necessary to read the whole thread, but the title instead:
> 
> Why didn't Allison quickly develop a *one-stage 2 speed Supercharger* for the P-40
> 
> ...


Some of the conversation spilled over into mentioning the Two Speed-Two Stage setup and that is what I was addressing.
Apologies for not being clearer on that.
As for the OP's question, I got into a discussion with someone here many years ago and they were quite knowledgeable about engines and blower systems.
One thing about single stage two speed systems is they are not as efficient as two stage-two speed systems.
It is better to _re-pressurize_ the air going into an engine, rather than trying to increase the pressure put out by a single blower by simply increasing the speed of the impeller.
....then there's the whole intercooler/after-cooler idea...
However, from I did see so far, it seems we've touched on all that already.


Elvis


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## wuzak (May 25, 2018)

Schweik said:


> And yet the V-1710-33 seems to have been producing 1580 hp routinely at low altitude in the actual field, not to mention the -73.... (at 60" Hg per the Allison memo) could a Merlin III do that?
> 
> Not suggesting Allisons were better than Merlins because I don't think they were - the altitude performance was very important, but low altitude performance also had some uses.
> 
> I also don't think the -33 was equivalent to a Merlin III frankly.



I should have been more clear - the supercharger performance was similar to the Merlin III's supercharger.

The boost limits of the Merlin III was not limited by the supercharger, but the strength of the engine itself. The -33 was a later engine than the Merlin III, IIRC. Certainly the Merlin III didn't see widespread use beyond 1940.


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## Shortround6 (May 25, 2018)

Ok, found the book and did a bit of number crunching.
What I was looking for was the pressure ratio of the *single stage *superchargers. How many times the supercharger could multiply the pressure of the incoming air.

Engine................altitude................MAP...............pressure ratio
Allison -33..........13,200.................38.9in.................2.14
Allison -39..........11,700.................44.6in.................2.31
Allison -81..........15,500.................44.5in.................2.69
Merlin III............16,250.................42in....................2.69
Merlin X (HG)......17,750................41.5in.................2.74
Merlin 45.............18,000.................48in...................3.61
Merlin 45.............11,000.................62in...................3.13
Merlin XX.............18,500.................48......................3.23
Merlin 46..............23,000................48......................3.96
P&W R-1830.........14,500.................39.....................2.26
R-2600 B..............12,000.................44.5in................2.34
DB601..................14,750................39in...................2.29
DB605A................19,000................39in...................2.72
DB605A................18,700................42in...................2.88
DB605DB..............19,700................54in...................3.85

As can be seen from the pressure ratios the Allison was right in with the other American single stage superchargers.
There are at least 3 different early DB601 engines with critical altitudes from 3700 to 4500 meters, I used the highest one and figured 1.3 ATA as 39 in.
Some rounding off was done.
On the world stage the Allison was doing all right except when compared to the Merlin.

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