Why did the P-40 lag behind?

[Brooke]

Ashley, it is an interesting post, and thought provoking.

[gyrene81]

Gyrene, even the most stripped down and weight-shaved P-40, the P-40L where they removed 30 gallons of fuel 2 guns and 1/3 the ammo from the remaining guns (200rpg only), gained a mere 4mph. And then it was so ineffective in that configuration at what it did they ended up putting all that back on for most of the airframes!! Weight was not the issue with the P-40. Or, perhaps it's better to say weight savings was not the solution.


The 109G-6 was a better machine than the G-2, for example, even though it was heavier. better radio, better guns, etc, made it a better killer.

i don't fully buy that krusty. aerodynamic deficiencies that limited it's potential aside, the added weight effectively decreased the potential performance gains that could have been seen with added horsepower...power to weigh ratio.

the p-40b with 6 guns had a 7,360lbs normal combat weight with a v-1710-33 @1040 peak hp, same as the P-40 which only had a combat weight of 6,787lbs...573lbs difference and the p-40b was 5mph slower.

the p-40e had the .30 cal guns replaced with .50 cal, combat weight 8,280lbs with a more powerful v-1710-39  1150hp engine, gain of only 90hp with an additional 920lbs...only 5mph faster than the p-40 and 10mph faster than the p-40b. if the more powerful engine had been added without the extra weight, the max speed would have been much higher.

jump to the p-40n-1-cu (first production run) with 4 .50 cal guns, v-1710-81 @1200hp, combat weight of only 7400lbs...50hp gain, 880lbs lighter and 33mph faster than the p-40e at 10000 feet. not outstanding by any means but still it shows what happens when you remove weight and add horsepower. then with the p-40n-5-cu they added 2 .50 cal guns and under wing racks and the combat weight jumped to 8,350lbs...an increase of 950lbs with no increase in hp and the top speed dropped 28mph.


the 109g-2 used the db-605a-1 1475hp peak and the 109g-6 used the db-605am 1800hp peak...

[PR3D4TOR]

Induced drag from lift (weight) is only a minor factor in total drag at high speed. Parasitic drag is far more important. The 109G-2 and G-6 have the exact same engine, delivering the same power. Only MW-50 equipped G-6's (G-14 in game) could produce 1800 PS. A good example is the La-5 and La-7. They cleaned up the La-5's airframe a bit and the result was the much faster La-7. As an experiment you can up a P-51 with 25% fuel and one with 100% fuel. Is one faster than the other?

[Vinkman]

The Allison had a two stage supercharger. What it did not have was a two speed supercharger. That is what limits operational altitude range. With a single speed, you have to choose a drive ratio for a particular altitude, which is almost always less than 15,000 feet.

Had the Allison been equipped with a two speed supercharger, it would have maintained sea level horsepower to a much higher altitude.

The other limiting factor would be the Curtiss propeller. The Curtiss propeller was both unreliable and inefficient.

Equip one of the lighter P-40 airframes with one of the late Allison engines, with a two speed supercharger, and add to that a four blade Hamilton Standard high activity paddle prop, and you have an entirely different aircraft.

I do not recall any two stage centrifugal supercharged allisons other than the P-63. The P-63 a single speed engine stage supercharger, and variable speed aux supercharger.  The P-38 was two stage, but of course the second stage was a turbocharger.

Which Allisons had two stage blowers?

[2ADoc]

Also the P-40 had a very thick cord wing, it was good for high lift but for speed it was not a good wing.  There was not much in the way of changes in the actual wing through the development of the P-40 series.  If you throw a bigger engine on a plane it does make changes but to improve the speed characteristics you must change the overall design.  Hence if you throw a 150HP engine on a Cessna 150 you do not change the speed of the plane that much, what you do is increase the takeoff weight, and decrease the take off distance, it is still a little over 100 Kts airplane.  If you remove the nose gear, put a tailwheel on the same 150, fundamental design changes, change the prop from a climb to a cruise prop, you add somewhere around 25 Kts.  Yu have to do changes to the actual fundaments of the aircraft, that is what didn't happen in the P-40 series.  

[Tupac]

Drag is a quadratic equation - if you want to go twice as fast you need 4x the horsepower. Little gains inHP don't do much if you don't lessen the drag

[Widewing]

I do not recall any two stage centrifugal supercharged allisons other than the P-63. The P-63 a single speed engine stage supercharger, and variable speed aux supercharger.  The P-38 was two stage, but of course the second stage was a turbocharger.

Which Allisons had two stage blowers?

No Allison used in WWII combat aircraft had two stage superchargers.

All P-39s and P-40s fitted with Allison engines were fitted with single stage, single speed superchargers. The P-40F's Packard Merlin had a single stage, two speed supercharger.

[Vinkman]

No Allison used in WWII combat aircraft had two stage superchargers.

All P-39s and P-40s fitted with Allison engines were fitted with single stage, single speed superchargers. The P-40F's Packard Merlin had a single stage, two speed supercharger.

Do you mean no "other" than the P-63? or is that a "combat" jab? 

 

[gyrene81]

No Allison used in WWII combat aircraft had two stage superchargers.

All P-39s and P-40s fitted with Allison engines were fitted with single stage, single speed superchargers. The P-40F's Packard Merlin had a single stage, two speed supercharger.

i understand the idea of two stage superchargers...high/low boost but, what's the deal with a two speed supercharger?

[Widewing]

i understand the idea of two stage superchargers...high/low boost but, what's the deal with a two speed supercharger?

A two stage supercharger compresses the air/fuel mixture twice via a main compressor and an aux compressor. There is always an after-cooler (sometimes called and inter-cooler) in between stages to reduce reduce the temperature of the mixture. A two speed supercharger has two separate gear sets and clutches (as well as neutral) for neutral blower, low blower and high blower.

[Widewing]

Do you mean no "other" than the P-63? or is that a "combat" jab?  

 

The P-63 had two completely separate superchargers, rather than a two stage unit... It was unique to the type, although the Republic XP-72 also employed a fluid coupling supercharger in place of the turbo in the preceding P-47s.

[gyrene81]

A two stage supercharger compresses the air/fuel mixture twice via a main compressor and an aux compressor. There is always an after-cooler (sometimes called and inter-cooler) in between stages to reduce reduce the temperature of the mixture. A two speed supercharger has two separate gear sets and clutches (as well as neutral) for neutral blower, low blower and high blower.

oh ok...that's easier to understand. thanks.

[Squire]

Just some comments on the original post.

The Hurricane was both lighter than the P-40 (@ 800 lbs) comparing a Hurricane II to a P-40E and had a better climb rate. It also had a lower wing loading and could out turn it. The P-40 series was faster in level flight which was its main advantage to the Hurricane if you are comparing the two. The P-40 was the later design. The P-40B came into USAAC service in the summer of 1940 some 3 years after the Hurricane entered service with the RAF in pre war 1937.

The reason that combat planes lasted longer in service depended on a number of factors. Each country was different. Its not just a case of saying one plane lasted longer ergo it was somehow better or worse (or the same). The A6M Zeke series was still in combat in August of 1945 but I would not use that as an example of its superiority over other types.

In the case of the British they made the decision to replace the Hurricane as a front line fighter in 1941 and replace it with the Spitfire and other types they were working on like the Typhoon. In the USA, the decision was made in 1942 to replace the USAAF P-40 with the P-47 and P-38, and later the P-51B/D. Like the British it would take time for that to happen and in the meantime the factories were still producing Hurricanes and P-40s. It takes time for industry to catch up to the demands of wartime.

Both fighters soldiered on in the China/Burma/India (CBI) theater fighting the Japanese untill 1945. In the case of the Hurricane the Mk.IIc it was being phased out in favor of Spitfires but not completely replaced the last WW2 air combats of any great note taking place in the Arakan region of Burma in 1943. The RAF flew it in some fighter-bomber units right untill wars end in the CBI where it did see sparodic air combat from time to time. In the case of the P-40 it was largely phased out by the end of 1944 with the P-40N being the last variant to see combat scoring its last kills in January 1945 in China. The last US Warhawk unit in China converted to Mustangs in June 1945 a few months before the war ended. In both cases they ended up fighting in a theater that gradually had very little air combat as Japanese fortunes declined. "Last kills" were more a function of luck than anything else in the waning days of Japanese airpower in the region.

[Ardy123]

Beyond the differences in simple form drag of the fuselage and the engine/cowling/cooling system, and the ability of the P-51 cooling system to actually generate *thrust*, there were also the airfoils.  

The P-40 warhawk used a semi-symmetrical 1930's airfoil, while the P-51 Mustang had an early example of a laminar-flow airfoil...

P-40

http://commons.wikimedia.org/wiki/File:NACA_2215.svg


P-51

(Image removed from quote.)

There must be much more than that, there are many '30s designs that had considerable performance boosts as the war progressed. The first one that comes to mind is the 109s. They had many bulges (g6.g14) etc.. yet tacking on an even bigger engine enabled them to keep the planes mph competitive.

109s had y-clark airfoils (another '30s trend) where the 190s had a 2300 series airfoil, so laminar flow isn't the whole story. I'd even venture to say, laminar flow enabled the p51 to have great range as it was less draggy, but both the 190s and 109s had similar speeds (late war variants), and neither had laminar flow (they also didn't have the range as they had larger, more gas hungry engines and smaller fuel tanks).

[33Vortex]

I'd say that the P-40 lagged behind for two main reasons:
1. The lack of a two stage supercharger in the initial offerings, which casued a perofrmance hit that really screwed with it.
2. The development of the P-51 overtook the need to continue tweaking the P-40.

This ^

Sometimes you realize that the mistakes made in a design are so fundamental that you need to rebuild from scratch. The Mustang was a more modern design and showed more promise. There's no point in beating a dead horse. The Mustang was very much living up to its name at the time though it took a lot of development before it came to life in the successful and most widely known P51 iteration.