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

[Shortround6]

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.

 

[Schweik]

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.

 

[wuzak]

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.

 

[Shortround6]

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.

 

[P-39 Expert]

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.

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.

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.

 

[tomo pauk]

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.

 

[P-39 Expert]

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.

 

[tomo pauk]

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)

 

[DarrenW]

r

....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.

 

[Elvis]

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

 

[wuzak]

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

...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.

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

 

[Shortround6]

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.

 

[tomo pauk]

... 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?

 

[Shortround6]

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.

 

[Schweik]

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

 

[wuzak]

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.

 

[Schweik]

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.

 

[tomo pauk]

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.

 

[Shortround6]

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.

 

[P-39 Expert]

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.