P-40 Warhawk/Kittyhawk

[Shortround6]

We may have three problems here.

1. The single stage supercharger just doesn't work that well over 20,000ft now matter how many gears or what gear ratio you use or drive it with. The late model Allisons used higher pressure ratios at take off than the early versions (mainly becaue they could use improved fuels), the -73 vesion in P-40K ran at 51in of boost for take off and 1325hp and 60in of boost at war emergency for 1550hp at sea level for pressure ratios of 1.7 and 2.0 respectfully. going to even 20,000 feet at those power levels means you need pressure ratios from your supercharger of either 3.4 or 4.0. The higher the pressure ratio the more power it needs to drive it AT ALL altitudes and throttle settings. Two stage superchargers can give the same pressure ratios with less power input and less heating of the intake charge. Or even more boost for the same drive HP.

2. Increasing compression gets more work or power from the same amount of fuel. Great for cruising or for engines without superchargers. An engine running at 60in of MP is putting 20% more fuel and air through the engine per minute than an engine running at 50in of MP. so 20% more gross power, deduct the extra power needed to run the supercharger at that level and you have your net or propellor HP. ANYTHING that gets in the way of getting the MAXIMIUM amount of fuel and air through the engine per minute is going to hurt PEAK power. AN 8 to 1 compresion ratio would seriously limit the ability to use high manifold pressure. Allison themselves dropped the Compresion ratio of the later two stage engines.

3. Can the P-40 ever be made competitive with the Me 109? the basic weight of most P-40s (the E through Ms) is about 6900-7000lbs? That is no guns, fuel, ammo, pilot etc. vrs the normal loaded weight of the 109 at just about the same 6900-7000lbs. Loaded the P-40 is going to go about 1500lbs more. even allowing the P-40 a bit less becasue of it's larger full tanks that is a lot of extra weight to try to compesate for. even Calling the 109 at 6900lbs and the P-40 at 8000lb for combat weight you are going to need 1677hp at altitude to equel the power to weight ratio of the 109G. Yes the P-40 may have some advantages but it was never going to have the straight line speed and climb of the 109. unless it had an egine with much,much more power.

If you can get a copy I would strongly suggest "Vee's For Victory!" by Daniel Whitney. See if yoiu can get it from a library if nothing else.

I used the early Allison and Merlin engines as an illistration of effects of different setups because at the time there was little difference in the performance of the superchargers on the two engines. Later superchargers with much differnt pressure ratio capabilities and efficiencies would tend to cloud the results.

 

[vanir]

According to a letter from Allison Division dated late 42 the only differentiation made between F3R and F4R engines was the automatic regulator. MAP ratings were revised and new ones cleared, partly because squadrons in the field were already ignoring Allison released operating guidelines and running both these engine types at as much as 66" and 70" MAP on 100/130 fuel.
My impressions are the revised MAP ratings are common to all F-series engines retroactively and ranged from 44" take off and 52" WEP to 46" and then 50" take off with 57" and then 60" WEP, with engine speeds for take off revised from 2800rpm to 3000rpm and probably a few more figures tossed in there as it seems to have been a work in progress through 1942.

There was a tremendous change with the F20R onwards due to the higher ratio and Allison was particularly concerned about engine overspeeding associated with extreme (unsanctioned) boost pressures being used in the field. Their final limitation was around 50" take off with 60" WEP although 46" for take off and 57" WEP was preferred for all F-series engines during 1943 (and if you asked they'd probably prefer you used the higher ratings on the 8.8 ratio F-series only).

At least this is what I've gathered so far. And from a mechanical standpoint I can see no reason personally why you wouldn't apply anything from the F4R to the F3R as the only change is an introduction of an auto boost regulator I assume absorbed from Packard's Merlin license.


It does seem to me the ability to move a larger volume of fuel per cycle transfers to increased throttle heights, aside from the simpler perspective of compensating reduced air pressure with increased boost pressure.
Here is what I'd do in hindsight with the Allison,
raise the impeller diameter to 9.75" and keep the 8.8 ratio
revise chamber and port design, including crowns and manifolding
revise valve and spark timing and bump max nom op speed to 3200
raise stat comp to 7.5 with a dynamic around 6.7
aim for a 60" take off and a 70" WEP on 100/130, looking for a throttle height of 12K ft with 60" (18K ft with 56")

Let a 109 cop 1750hp at a 3600m combat height and 1550 at 5000m coming into 1943. I'm going for something that'll give a 190A pause. Well...at least that's how I'd put it to the commitee for war funding.

As far as I can tell the major problem with the Allison is there's no real power available over about 5000ft as it stands (it was capable of tons below that). I don't think it ever needed to perform up near 20K ft, but 12-18K feet is definitely typical air combat height and it was running out of puff there (ie. could basically perform at climb/combat without any extra reserve which is what made it seem inadequate). To me the Allison seems like it just needed to be pimped a little, and I think the ratio jump to 9.6 from F3/4 to F20~ was the wrong approach, tackling this by force feeding instead of a general efficiency revision.

The Allison F3/4R was a brilliant motor. Tell me a Merlin can do 70" on 100/130 without holing pistons.

 

[Shortround6]

This does tend to paid to the super P-40 therory.

Even the Flying Tigers were using 58" of boost instead of the book 40.6" on their C series engines. 1600hp up to 1,700feet. that is 1,700 ft not 17,000ft.

So when reading combat reports of fights at low altitudes the pilots may have been using hundreds more horespower than the book figures say they had.

While the engines seemed to stand up to this abuse fairly well and Allison themselves may have wished (at times) for higher book limits early in the war, some pilots and mechanics pushed things to an extent that gave Allison fits. Some peaple report using 62" on these early engines but that would require over speeding the engine to 3200RPM. This is supposed to give 1700hp but that is a 6.25% gain in power for a 14% increase in bearing loads and internal friction.

There is an awful lot of things happening with engines and fuel in just a few years that mean that just because something was done in 1943 that it could have been done in 1941 or even in parts of 1942. For instance 100 octane fuel. In 1939-1940 British and American 100 octane fuel was not the same gasoline. British fuel would allow much higher boost pressure than American 100 octane ( by british and american I mean the specifications the fuel was manufactured to) under rich mixture conditions. the two fuels were just about the same under lean conditions. British fuel would eat or disolve gaskets and seals in some american fuel systems and self sealiing fuel tank liners. It discovered that British 100 octane acted like 120 octane (or more properly performance number) when run rich. From this they developed a 100/125 octane fuel that was soon replaced by 100/130 but it took a while for the 100/130 designation to catch on so you will reports (especially from squadrons) that refer to 100 octane when they mean 100/130. A few batches of American fuel (before America got in the war) actually dropped from 100 octane lean to 90 something rich.
This may have something to do with the conservative ratings for American engines in regards to boost in the 1941-early 42.

Another thing is that the engine manufacturer was trying to ensure long engine life. While an under performing engine is a liability in combat, and engine that fails in flight isn't much good either and changing engines every 50-100 hours doesn't do much for operations compared to an engine that will go 150-200hrs.
Once WER ratings were officially allowed for the Allison the company recommended the following :

1, Engines should be removed and replaced at the end of ten hours total time at War Emergency Rating.

2 , New exhaust spark plugs should be installed prior to any expected WER operation to avoid running plugs which may have previously become partially lead fouled. Such plugs may form lead globules which lead to immediate pre-ignition which causes piston and ring failure.

3, Intake plugs should be replaced at the end of one hour of WER operation, or after ten hours of total operation during which at any time, the engine operated at WER settings.

 

[Elvis]

I don't know why you guys want to concern yourselves with variations that were superceeded by the mid point of the war, but whatever.
I'm trying to work with the a later versions, mainly the "G" series motors, which had the advantage of a 7-main block and a counterweighted crankshaft.
This allowed Allison to remand their prior recommendation of max. engine speed, going from 3200 to 3600 rpm.
Those changes would allow the engine to work better under heavy load, and even allow it to sustain heavier loads, easier....not to mention smoother operation.

Shortround - Weight and balance stats for a P-40N, dated 12/18/42 - http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/P40N-1 Weight spec.pdf
Unloaded weight was 6120. Loaded was apparently 8000.

I still say upping the c.r. and implementing two-speed gearbox is a step in the right direction, however, I will agree with vanair, in that it should also include a change in manifolding and possibly porting and definately some sort of intercooler.

Lastly, according to some of the info given by Shortround6, some improvements in the ignition system seem to be in order, as well.



Elvis

 

[Shortround6]

Well you would need a time machine for the "G" model Allison to do any good for the P-40.

By the time the G models were running on the test stands the P-40 was not only past it's prime, it was beyond "stick a fork in it, it's done". It was throw the scraps to the dog, done.

ALL Allisons used 7 main bearings and in fact the 12 counter weight crankshaft could be fitted to E and F series engine blocks.

The first true G series engine didn't run until late 1944. Only 3 or 4 were built. The next the "G" the G-1RA didn't hit the test stands till March of 1945. The G-2 model was never built and the G-3R model (8 built) got to the test stands in December of 1946.

It is true that the G-6 model did complete a 150 hour test in October of 1946 but I think you get the point.

By the way, the ONLY versions of the "G" that used higher than a 6 to 1 compression ratio were the -97 (G-1R) mentioned above (3-4 built), the -131 ( G-3R) built for a couple of modified C-54 transports and G-5 model of which none were actually built.

Upping the compression ratio is definatly the WRONG way to go if you are looking for power and not economy.

P-40N was the stripped, lightened version (using aluminum radiators and oil coolers and two less guns amongst other modifications) that isn't ordered until almost 2 months after the Americans start test flying Merlin engined Mustangs. The engines in the final P-40N-40 production block used the 12 counter-weight crankshaft.

Point of listing Allison recomendations was that the use of WER power levels came at a cost. Practically any engine that used those high boost settings came with similar instructions.
Use of ADI carried similar or even more maintainence requirements.
Later post-war commercial engines managed to over come some of them.

 

[vanir]

That info on British-American fuels makes very good sense. Australian and Middle Eastern squadrons attributed with using extreme boost pressures of 66-70" could've been using either, whilst Stateside the published restrictions were probably wiser.
And I think these operating conditions were a matter of sheer desperation, I know that at the time the elite Lae Zero squadron was overflying Port Moresby taunting local pilots with aerobatics and then shooting down any attempt at interception. Hell, I'd tell my mechanics to bugger the documents and just get me more power too.
Whilst in the Middle East you've got the F and G series 109 and then the appearance of the Fw190 over Tunis at the end of 42.
Plus in both these places you've got short, rough fields in poor conditions with a high emphasis on army support operations. You want a bomb load off the ground as soon as possible, and you want a very fast escape and a good low altitude climb rate to get you out of trouble.

I think perchance, if the extreme boost potential at very low altitude of the P-40 had been compromised at this stage for improved high altitude performance, it would've been a less valuable type. Once again I do think the best improvement to the P-40 would've been simply raising throttle heights by only a few thousand feet and maintaining that bulletproof reliability.
Leave the high altitude work for the turbosuperchargers and newer airframes. Besides, as air superiority slipperiness goes, the P-40 is just frumpy.

 

[Elvis]

By the time the G models were running on the test stands the P-40 was not only past it's prime, it was beyond "stick a fork in it, it's done". It was throw the scraps to the dog, done...Upping the compression ratio is definatly the WRONG way to go if you are looking for power and not economy.

Being "past its prime" is the whole point of trying to improve performance in the first place.
If its performance was already on par with the 109, then this whole discussion would be moot to begin with.
I still think you're wrong about upping the c.r., as it would make the engine more efficient. That would improve both power AND economy, however, I guess we'll just have to agree to disagree on that point.
I'm not saying the jump would be "huge", but even that small bump would be helpful to the engine's performance.


Elvis

 

[Shortround6]

The P-36/P-40 airframe was just too dated to really bother with by 1943. No real shame in that. There were plenty of other 1934 era aircraft that didn't last any longer. And plenty that didn't last anywhere near as long. The fact that 1 or 2 others did last that long doesn't mean the Curtis was bad, just not as lucky.
With the rapid advances in aerodynamic knowledge, structural design and even new materials NO designer could see 6-9 years into the future. They had to make a best guess and hope they guessed right. In fact, I would bet that any designer who's 1934 design (of any type of aircraft) was still being manufactured in 1943 was as surprised as anybody else because he had probably designed (at least in his head or on napkins) any number of "NEW, IMPROVED" versions or entirly new planes based on new knowledge.

Trying to equel the performance of the 109 with a larger, heavier aircraft means you need a proportionately more powerful engine. In America in late 1941, early 1942 you have two choices. The Wright R-2600 or the P&W R-2800. You do have to have a prototype flying at least 6 months to year before the plane/ engine combination really sees squadron service. To equel a 109G in 1943 you don't need 1500HP at 20,000ft, you need more like 1800-2000hp at 20,000ft.
While the P-40 may have had certain advantages over the 109 it always had a larger fuel capacity (advantage range/endurance-disadvantage size of airframe to house/carry larger tanks) and from the F model of the 109 the P- 40 carried a heavier installed weight of armament. In some cases a much heavier installed armament. A 109G with a single MG 20mm gun and two 13mm MGs had 184lbs worth of guns. A P-40 with even four .50s had over 250lb of guns. Since installed weight (mounts, heaters, feed boxes, access hatches etc) can be 30-50% higher than the gun weight WITHOUT AMMO we can see that the P-40 was at a disadvantage weight wise from the start.
While Allison started investigating two- stage supechargers in 1940 they really didn't get any flying hardware until 1943. Without the 2nd stage the Allison was going to have trouble making enough power at altitude (20,000ft? and higher) to compete.

If you can show me how I am wrong about the compresion ratio question please do so.
Please remember however that aircraft engines are going to behave rather differntly than modern auto engines.
Since heat in the cylinder is a major part of detonation the largerdiameter cylinders of the aircraft engines are harder to cool, less serface area to volume than smaller car cylinders. Car engines seldom run at full boost for anywhere near the time aircraft engines did. Modern superchargers may create less heat for the same amount of boost (higher effiency ) than the older superchargers. And of course 40-60 years more knoweldge in combustion chamber shape.

 

[Elvis]

Shortround,

Took a look at 109 performance over at the Aviation-History page and I see your point.
By the time of the 109G, the aircraft were of similar weight (I'm thinking P-40N), but the DB605 was making much more power than the Allison ever did during the war, aiding in that plane's performance advantage over the P-40.
However, we all know that the Allison eventually eclipsed those engine performance figures later on, so coaxing 1800HP, or so, would definately be in the realm of possibility. I just seems that Allison didn't have the R&D to fall back on, yet.
However, at that time, it seems an 1800HP Allison probably wouldn't be ready for production until around the end of the war, and by that time, it would be more prudent to install such an engine in the P-51.
FWIW, I found a trivial, but interesting, fact the other night.
It appears the Allison's 1150HP rating yields a very similar HP/cu.in. rating as the 605 was, at the time it was rated @ 1475HP.
Both work out to about .67+ HP/cu.in.

As for increasing c.r., I don't see how it couldn't be beneficial.
One is basically getting a better "quench" of the f/a mixture in the TDC area and that always yields better engine efficiency because you ensure a more complete burn (in conjunction, upping voltage of the ignition system, coupled with a little more gap in the plug doesn't hurt matters, either).
Supercharging can yield a similar effect, because of the rise in cylinder pressure, the its not quite the same thing.
Again, the bump in c.r. I'm proposing is a small one, because I am being mindful of engine construction, changes in ignition timing and detonation.
Even though the HP rating jump is also fairly small (only 5.4%, if the jump is to a straight 8:1 c.r.), its something, and that's better than nothing.

However, all that being stated, I think the "best" thing we possibly could've done, at that time, was to try installing an engine of larger displacement into the P-40, along with making some aerodynamic improvements (such as laminar flow flight surfaces).
I'm thinking along the lines of Clay Allison's query about replacing the Allison with a Rolls Griffon....but that's a discussion for another thread.



Elvis

 

[Vincenzo]

for info on 109 one of best it's Kurfurst - Your resource on Messerschmitt Bf 109 performance

 

[Shortround6]

In 1938-40 the NACA did a series of tests on factors that affected detonation. they found in no ranked order.

compresion ratio
inlet air temperature
inlet air pressure
fuel/air ratio
spark advance
size and shape of the combustion chamber
Temperature of the combustion chamber and cylinder walls
engine speed.

I am sure that other countries were doing simaliar testing.
According to some sources compression ratio acted as a multiplier. Whatever the incoming charge temperature was ( and it could be several hundred degrees from the supercharger) was multiplied by the compression rato, maybe not a straight "times' the comprsion ratio but a higher compresion ratio heated the charge in the cylinder to a higher temperature before the ignition system fired.

If the charge self ignited before the spark pug fired things could get very strange, what is even worse is true detonation were instead of the charge igniting in one spot and flame front traveling across the piston top the charge being so close to auto-ignition temperature that even the pressure wave of the moving flame front causes the rest of the fuel/air charge to all burn at once.

In aircraft sized cylinders this could get VERY interesting. Bristol once blew a cylinder off a test rig, through the building roof and completly clear of the building. Not likely to happen with an enbloc V-12 but kind of points out why nobody wanted to happen in the air.

As I have pointed out bfore the increase in compression, while it did increase engine efficiency also lowered the amount of boost that could be used. Amount of boost is almost directly related to power output at a give rpm because it is directly related to the amount of fuel/air going through the engine. Discounting pumping losses or power to turn supercharger a 10% increase in manifold pressure means a 10% gain in power.

Some of the studies are available on the NACA/NASA server.

 

[Elvis]

for info on 109 one of best it's Kurfurst - Your resource on Messerschmitt Bf 109 performance

THANK YOU!
I've been trying to find that site for months, but couldn't remember what it was called and subsiquent Google searchs came up with everything but that site.
-------------------------------------------------------

Shortround,

I don't know what else to tell you, other than, like anything, there are tolerances that one can work within.
Detonation can be quelled via a number of techiniques, the most common of which is reworking of the ignition timing and camshaft timing.
Again, for the small bump in c.r. that I'm proposing, I think the engine could handle the amount of boost it was already receiving, and then some.
Look at the Hydroplane racers that used those engines, almost exclusively, from the 1950's through the 1980's.
They would often throw on multiple turbo applications, spin those motors faster than anyone in their right mind would've during the war, and especially in the later "piston-era", would use nitrous oxide injection on a regular basis in order to increase power.
...and many of those engines would last at least a season or two before they needed any kind of major work done to them.
A lot of those engines simply wore out, before they "broke", as you propose, mainly because parts became increasingly harder to find.
I just think you're making more out of the c.r. bump than what would actually transpire.





Elvis

 

[Vincenzo]

np, it's must help

 

[Shortround6]

Well in racing boats you can swim home if the engine blows up
not really an option in an airplane.

The US wanted a test engine to survive 7 1/2 hours at the WER before they would sanction the use of that power level.

And as I have pointed out before even Allsion lower the compression of just about all their two stage engines. In fact of the 763 G series engiens that left the factory only about 13 had a compresion ratio higher than 6 to 1 and 8 of those engines were installed on re-engined DC-4 airliners( C-54s) called the Xc-114 and the YC-116 wer fuel economy was more important than sheer power.

What we know now may not have been known then.
Many aircraft engines used a fixed ingnition timing or sometimes a two position timing, one retarded for starting and the normal position for all other running. Yes this did limit performance but it was also one less thing to go wrong. Or more than one thing since the insulating ability of air diminishes with altitude and some engines resorted to pressurised magnetos to help out. one less rotaing or moveing seal.

Rolls kept the same compression from first Merlin to last while going from 6 lbs boost to 25lb and even the war commercial versions stayed at 6 to 1.

6 to 1 is not a magic number it is just what ROlls used and the two stage Allisons. Other engines used different compression ratios but then they had different bore to strokes, different cooling, lower peak boost and other differences (including ingnition timing and cam timing)

 

[Clay_Allison]

1. The single stage supercharger just doesn't work that well over 20,000ft now matter how many gears or what gear ratio you use or drive it with. The late model Allisons used higher pressure ratios at take off than the early versions (mainly becaue they could use improved fuels), the -73 vesion in P-40K ran at 51in of boost for take off and 1325hp and 60in of boost at war emergency for 1550hp at sea level for pressure ratios of 1.7 and 2.0 respectfully. going to even 20,000 feet at those power levels means you need pressure ratios from your supercharger of either 3.4 or 4.0. The higher the pressure ratio the more power it needs to drive it AT ALL altitudes and throttle settings. Two stage superchargers can give the same pressure ratios with less power input and less heating of the intake charge. Or even more boost for the same drive HP.

Not necessarily. Higher altitude performence can be gained at the COST of poorer low altitude performance if you use a larger impeller. I now believe that the engine change wasn't necessary so much as putting one production line to work making "Interceptor" Hawks built for high altitude supercharging and maximum weight savings for rate of climb wherever they could get it.

 

[Shortround6]

Not necessarily. Higher altitude performence can be gained at the COST of poorer low altitude performance if you use a larger impeller. I now believe that the engine change wasn't necessary so much as putting one production line to work making "Interceptor" Hawks built for high altitude supercharging and maximum weight savings for rate of climb wherever they could get it.

That may be true but how much low altitude performance do you want to give up?
You still have to take-off and climb to combat altitude.
Some of the later P-40s did trade 125hp at take-off for an extra 100 or so hp at 20,000ft. if you want several hundred HP than that several thousand feet higher then you are going to loose several hunderd more HP at sea level. Take- off and low altitude climb using less than 1000hp isn't going to impress too many peaple.

 

[Elvis]

Higher altitude performence can be gained at the COST of poorer low altitude performance if you use a larger impeller. I now believe that the engine change wasn't necessary so much as putting one production line to work making "Interceptor" Hawks built for high altitude supercharging and maximum weight savings for rate of climb wherever they could get it.

That may be true but how much low altitude performance do you want to give up?
You still have to take-off and climb to combat altitude.
Some of the later P-40s did trade 125hp at take-off for an extra 100 or so hp at 20,000ft. if you want several hundred HP than that several thousand feet higher then you are going to loose several hunderd more HP at sea level. Take- off and low altitude climb using less than 1000hp isn't going to impress too many peaple.

Thus my suggestion for the adapting a second gear to the existing supercharger setup.
This way you don't loose anything down low and gain some up on top.
Bumping the c.r. only aids overall engine efficiency across the engine's entire powerband, but my main point (before bringing up the c.r. issue) was that the addition of a second gear would help the plane perform better at altitudes above 12K-15K feet.



Elvis

 

[Elvis]

...something else that just struck me, but I've heard from others in the past.

What if a two-speed/two-stage S.C. were added to the Allison? (I'm thinking about the "-81" and "-99" engines, in this particular case)

Would that be a more "compact" system than re-installation of the turbo that was used in the P-38?


Elvis

 

[Shortround6]

Thus my suggestion for the adapting a second gear to the existing supercharger setup.
This way you don't loose anything down low and gain some up on top.
Bumping the c.r. only aids overall engine efficiency across the engine's entire powerband, but my main point (before bringing up the c.r. issue) was that the addition of a second gear would help the plane perform better at altitudes above 12K-15K feet.



Elvis

No you gain very little on top Or at least over 18-20,000ft.

Later single speed Allisons could give over 1100hp at 15,500ft. using 44in of boost. You are already operating at a pressure ratio of 2.6. even a better supercharger that could give a presure ratio of 3 to 1 at the same or better efficency so that the charge temperature doesn't go up will only give you about 50.7in of boost for about 1300hp at that altitude.

You are going to gain a lot on the low end however. THe above Allison was good for 1200hp at take off so by using a second gear you could go back to the 1325HP take off rating of the lower altitiude Allisons. THe better supercharge would really improve things at 4-7000 ft for WER.

Pulling a stiffer gear dosn't solve the limitations of the compressor itself. Maybe with a stiffer gear you could pull 1100hp at 16,500-18,000ft instead of 15,500ft but that isn't enough to turn the plane into a high altitude fighter.

Try checking out the performance of single speed single stage Merlins and two speed single stage Merlins. Leave out the two stage merlins (the 60 series and 70 series engines) . Try comparing the Merlins from the same year and see what you get.

 

[Shortround6]

...something else that just struck me, but I've heard from others in the past.

What if a two-speed/two-stage S.C. were added to the Allison? (I'm thinking about the "-81" and "-99" engines, in this particular case)

Would that be a more "compact" system than re-installation of the turbo that was used in the P-38?


Elvis

Yes it was more compact but it was still over a foot longer than the regular Allison. IF you are using the Allison system. You are also not using an inter-cooler which does tend to limit the potential. Allison used water injection on the P-63 set-up with this engine. This is also the set-up used in the P-40Q.

If you are using a Merlin type set up it is a bit shorter but you still need the Inter-cooler and the inter-cooler radiator.