P-40 Warhawk/Kittyhawk

[Elvis]

Ivan,

While I haven't checked out that particular aspect myself, have you investigated the Perils P40 Archive Data website?


Elvis

 

[MikeGazdik]

Speaking of the thrust line change, was there any validity to the USAAC wanting to change the Allison engine prop gearing? I have read that the change reduced the load on the front of the crank, when changing to the design of the P-40E and beyond. (P-38 was also changed)

I have never read anything other than it was changed because the USAAC believed it should be done. But in my opinion, the USAAC was hardly ever absolutely right on any of thier pre-war beliefs.

 

[Ivan1GFP]

Hi Elvis,
I HAVE checked the Perils P-40 stuff. Nothing conclusive there that I can find either. A simple stated number doesn't really help much because I have several of those. The best that I have found is the P-40D schematic hosted here because it gives a break down of the lengths of various parts.

Hello MikeGazdik,
I remember reading somewhere that the earlier reduction gear had a fairly low limit on the amount of torque it could handle. When the engines became more powerful, the older gearing could not reliably handle it, so the profile changed to the P-40D and later.

- Ivan.

 

[Elvis]

Ivan,

Re: prop gearing

That sounds familiar to me, too, but the way I understand it, the actual gear ratio didn't change, only the "stoutness" of the gear assembly itself.
Whether that meant bigger bearings and/or bigger gears, I don't remember now.
Do you know if there was an actual change to the ratio or was it just a change to a more robust assembly?


Elvis

 

[V-1710]

The early V-1710-C as used in the P-40/40B/40C used an internal spur gear on the prop shaft with a 2:1 propeller reduction drive ratio. The centerline of the prop shaft was 3-3/16" above the crank certerline. It was thought this design would provide for better streamlining, though it did cause a certain amount of trouble during development due to high bearing loads and lubrication requirements. The V-1710-F as used in the P-40D and later Warhawks used a simple external spur gear propeller shaft drive, still with a 2:1 ratio. The propeller shaft ceterline was now 8-1/4" above the crank. The two principle reasons for the change was the external spur gear was stronger and more reliable than the internal spur gear, and it was easier to engineer a left hand rotation version for the P-38.

 

[MikeGazdik]

The early V-1710-C as used in the P-40/40B/40C used an internal spur gear on the prop shaft with a 2:1 propeller reduction drive ratio. The centerline of the prop shaft was 3-3/16" above the crank certerline. It was thought this design would provide for better streamlining, though it did cause a certain amount of trouble during development due to high bearing loads and lubrication requirements. The V-1710-F as used in the P-40D and later Warhawks used a simple external spur gear propeller shaft drive, still with a 2:1 ratio. The propeller shaft ceterline was now 8-1/4" above the crank. The two principle reasons for the change was the external spur gear was stronger and more reliable than the internal spur gear, and it was easier to engineer a left hand rotation version for the P-38.

Geat info! My question is this, was it the belief by the USAAC that the internal spur gear was weaker than the external spur gear, or was it a fact that Allison had found themselves? Like I said alot of what the Army Air Corps pushed for or believed in during the pre-war period was suspect at best. If that theory was from Allison, or backed up by other aeronautical / engine designers than I fully respect that theory.

 

[V-1710]

Geat info! My question is this, was it the belief by the USAAC that the internal spur gear was weaker than the external spur gear, or was it a fact that Allison had found themselves? Like I said alot of what the Army Air Corps pushed for or believed in during the pre-war period was suspect at best. If that theory was from Allison, or backed up by other aeronautical / engine designers than I fully respect that theory.

The internal spur gear required almost 1/3rd. of the engine oil flow at 75 p.s.i. to be properly lubricated. The V-1710 C was rated at 1150 h.p. with a max r.p.m. of 3700, and it is my understanding the limiting factor in the C's rating was the internal spur reduction gear. There seems to be some evidence the internal spur gear caused additional crankshaft stress too. Wright Field asked Allison the feasability of changing to an external spur gear as early as 1935, but the change required a redesign of the crankcase and crankshaft. This was eventually tested in 1937, and the design was used in the V-1710 F. So, it would seem that the idea to change the design came from the A.A.C..

 

[Elvis]

V-1710,

Thanks for the reduction gear info.
Yeah, I didn't think the ratio changed.

Earlier in this thread, I saw a question pop up a few times that I don't think ever really got answered.

Why wasn't the supercharger on the Allison developed to further the engine's performance?

We already know why the turbo portion of the supercharger was only used with the P-38 and how the general feeling towards the Allison seemed to change, once it was found the Merlin could be successfully adapted to both the Mustang and the P-40.
However, once all P-M production had switched over to series 60/61 engines, I think this is when the Allison found its way back into the P-40 (because it was easier and quicker to relegate all P-M production to the higher performing series, thus discontinuing the older 20 series engine that was used in the P-40).
So, since the P-40 was once again "saddled" with the Allison, why send pilots out in a plane that lacked the neccessary engine performance to make it...competitive?
Ok, so it can't have the turbo, but why couldn't an alternate supercharger be developed to help the engine make a little more power, at least at altitude, where it counted?
It could've been as easy as a larger impeller (which I think more than a few of us had mentioned before).

Does anyone know if any tests were carried out with a larger supercharger? (and not the "Q" version, a larger version of how the engine had been outfitted, pretty much all along)



Elvis

 

[V-1710]

Good question. I have never found a definitive answer as to why a 2-stage blower was not developed earlier when the turbocharged version turned out to be only feasable in the P-38. By the time Allison got around to the V-1710-93, the Packard built Merlin had already satisfied the requirement. My guess would be that the AAC may have felt the single stage V-1710 was wholly adequate for low altitude fighters such as the P-39, P-40, and P-51/51A, and thus there was good reason to continue to produce it for those aircraft. An interesting post-war side note- North American Aviation was developing the P-82 Twin Mustang as a long range escort fighter, and with the end of the war in sight, the decision was made to equip it with 2-stage Allisons. The reason was that when the war was over, Packard would have to pay Rolls-Royce a very substantial royalty for every Merlin it produced. The P-82 eventually did see service as a night fighter in Korea, and most versions were powered by 2-stage V-1710-143/145

 

[claidemore]

There was always a need for low alt optimized engines, both Allison and Merlin. For example, most of the Spitfire Mks had low alt versions.
The P40 design had already shown that it did not get significant increase in performance by the installation of the more powerful Merlin, so developing a more powerful Allison for a limited increase in perf, simply wouldn't make sense.
There was plenty of low alt fighter bomber work to be done, which the P40 had proven very suitable for, and there were plenty of excellent performing high alt designs (P51/P47/Spitfire) to fill the escort and interceptor roles.

The question really isn't 'why didn't they develop the Allison further for the P40', but 'what need would have been filled if they had?'.

 

[vanir]

From the fantastic link Elvis posted earlier in the thread, I noted the RAAF routinely modified the boost regulator for 70" Hg on the F3R and F4R and from an Allison document to the effect it seems restricted themselves under advisement to 60" Hg for the F20R due to likely engine damage with the impeller upsizing.
Nevertheless Allison states in excess of 1580-1780hp were the ratings "for an average engine" under these high boost conditions and considering the P-40M has a rated altitude around 4000m this seems to put it on equal terms with the LF MkVb Spit becoming available at the time.
It would seem as a fighter-bomber in ready numbers you couldn't do much better, I'm wondering if this was the reason the relatively few MkV Spits delivered to the RAAF were sent to the mainland for training squadrons whilst the P-40 remained in front line service.

 

[Elvis]

Excellent points, everyone.
I think that answers my question, however, in regards to your question, Claidmore, maybe I should've written "why didn't they develop the Allison further for the P-51?"...or at least left the turbo on the engine, which begs another question...

...but that's getting off topic.




Elvis

 

[Shortround6]

Excellent points, everyone.
I think that answers my question, however, in regards to your question, Claidmore, maybe I should've written "why didn't they develop the Allison further for the P-51?"...or at least left the turbo on the engine, which begs another question...

...but that's getting off topic.




Elvis

The "turbo" was never "ON" the engine. The "turbo" was always mounted some distance away, it weighed somewere inthe area of 120-150lbs (without piping/ducts) and was about 2 feet across. See the P-37 toget an idea oh how much room was needed for the turbo. Turbo also needed an intercooler to be really effective. More space, weight and drag. Thousands of cubic ft per minute have to go through the intercooler (or over it, see P-38 ). Turbos only show any advantage at altitudes above 20,000ft and the higher above that the more advantage they show. 25,000-35,000 is were they really come into their own.

If what you want is a low altitude engine a single stage, single speed supercharger works just fine. any extra stages just add weight and bulk with no gain in power. One might note the low altitude Spitfires that had their impellors cropped to about the diameter of the Allison's impeller. One might also note that they used a single speed, single stage supercharger. Many Beaufighters also had impellors cropped and gear boxes locked in "M" gear for low altitude work. The British designating superchargers as "Ground", Medium or moderate (4000-8000ft?) and High altitude.
As a further note the Difference between a single stage Merlin, as used by the P-40 and a two stage Merlin as used by the P-51 was 8 in in length and about 250lbs dry weight on the engine. This does not include the weight of the inter-cooler radiator (Merlins used liquid cooled aftercooler vrs the air-cooled intercooler the americans used on the rest of their engines) or the coolant. Could the P-40 move it's firewall back 8in to maintain the CG or would other changes have to be made? Could the P-40 afford the extra 300-4000lbs of weight subtracted from it's gross weight? Or can the extra 3-400lbs just be accepted. The extra Power will only be useable somewere above 10-14,000ft.

 

[Elvis]

...It appears I've been busted by The Semantics Police.
Ok, how about if I change that to "with", instead of "on".
Better?

...and what I'm trying to do is to take an existing design and make it an "all-altitudes" fighter, at least on par with the Me-109.
Clay Allison thinks re-fitting the P-40 with a R-R Griffon would do the trick (essentially creating a sort of "American Me-109"), and he may actually be onto something.
Personally, I think the Allison was already capable of creating enough performance to make it a more worthy adversary of the 109, than it actually was.
What I noticed was that the Allison was originally designed to be a turbo-supercharged engine from the get-go and that those engines, such as the V-1710-89A, had very good performance numbers that could've made the P-40 into a better performing airplane at altitudes over 15K feet, while keeping the increase to the engine's weight (compared to the single stage, single speed version) as minimal as possible.
I believe the increase was only listed at around 100-200 lbs., at the most, according to the specs seen at enginehistory.org.

Another thing that struck me the other day, was to simply increase the c.r. of the engine, while utilizing a two-speed, intercooled version of the supercharger that was actually used on the P-40.
I sat down and ran the numbers last night.
The rule of thumb that I used was that there is a 4%-6% increase in horsepower for every point of compression increase (a little fact I read in Hot Rod magazine years ago).
Erring on the lower side of that scale, I found that if the c.r. were raised to 8.15:1 (from 6.65:1), the resulting HP increase would be 6%, or a raise from 1150HP to 1219HP.
@ 25K ft., I believe most of the engines that were actually used in the P-40 were rated at 800HP.
So HP @ 25K ft. is about 69.5% of take-off HP.
If you factor in the 6% increase in HP, that nudges that rating to 848HP.
If the second speed of my proposed two-speed supercharger were enough to double the speed of the impeller (compared to the initial gear ratio that would be used at lower altitude), and that resulted in cutting the HP @25K ft. differential in half, then, with the S.C. in "high gear" and the engine running the higher c.r., would result in a HP @ 25K ft. of 1033.5.
At that point, I believe high altitude HP rating is about 84.7% of the take-off rating.
This is an improvement of 15.2% of the rated HP of the engine at altitude, comared to its take-off HP.

So what does this all mean?
Either nothing at all, or a marked improvement of high altitude performance, without loss of low altitude performance.

Would that improvement put it on par with an Me-109, at that altitude?
Maybe, maybe not, but one would hae to see some improvement over the form of the engine that was actually used, and even though Pilot Skill had a lot to do with the outcome of a lot of those sorties, you have to admit that the more you start off with, the more you can do.



Elvis

 

[Clay_Allison]

...It appears I've been busted by The Semantics Police.
Ok, how about if I change that to "with", instead of "on".
Better?

...and what I'm trying to do is to take an existing design and make it an "all-altitudes" fighter, at least on par with the Me-109.
Clay Allison thinks re-fitting the P-40 with a R-R Griffon would do the trick (essentially creating a sort of "American Me-109"), and he may actually be onto something.
Personally, I think the Allison was already capable of creating enough performance to make it a more worthy adversary of the 109, than it actually was.
What I noticed was that the Allison was originally designed to be a turbo-supercharged engine from the get-go and that those engines, such as the V-1710-89A, had very good performance numbers that could've made the P-40 into a better performing airplane at altitudes over 15K feet, while keeping the increase to the engine's weight (compared to the single stage, single speed version) as minimal as possible.
I believe the increase was only listed at around 100-200 lbs., at the most, according to the specs seen at enginehistory.org.

Another thing that struck me the other day, was to simply increase the c.r. of the engine, while utilizing a two-speed, intercooled version of the supercharger that was actually used on the P-40.
I sat down and ran the numbers last night.
The rule of thumb that I used was that there is a 4%-6% increase in horsepower for every point of compression increase (a little fact I read in Hot Rod magazine years ago).
Erring on the lower side of that scale, I found that if the c.r. were raised to 8.15:1 (from 6.65:1), the resulting HP increase would be 6%, or a raise from 1150HP to 1219HP.
@ 25K ft., I believe most of the engines that were actually used in the P-40 were rated at 800HP.
So HP @ 25K ft. is about 69.5% of take-off HP.
If you factor in the 6% increase in HP, that nudges that rating to 848HP.
If the second speed of my proposed two-speed supercharger were enough to double the speed of the impeller (compared to the initial gear ratio that would be used at lower altitude), and that resulted in cutting the HP @25K ft. differential in half, then, with the S.C. in "high gear" and the engine running the higher c.r., would result in a HP @ 25K ft. of 1033.5.
At that point, I believe high altitude HP rating is about 84.7% of the take-off rating.
This is an improvement of 15.2% of the rated HP of the engine at altitude, comared to its take-off HP.

So what does this all mean?
Either nothing at all, or a marked improvement of high altitude performance, without loss of low altitude performance.

Would that improvement put it on par with an Me-109, at that altitude?
Maybe, maybe not, but one would hae to see some improvement over the form of the engine that was actually used, and even though Pilot Skill had a lot to do with the outcome of a lot of those sorties, you have to admit that the more you start off with, the more you can do.



Elvis

I think that having some "Interceptor Hawks" with larger impellers for higher altitude work alongside the regular Warhawks would have been a good stopgap solution until more flexible all-altitude fighters became available.

 

[vanir]

I think you'll find raising the compression will cause you to recalibrate the boost governer and return lowered throttle heights. The way you'd compensate this is a much larger diameter, lower pressure supercharger and/or an increase in engine capacity. You'll also have to play around with valve and ignition timing a fair bit, even combustion chamber and piston crowns and in those days...without computer modelling to help...

One historian said simply, in that period it took a new fighter two years to develop from drawing board to prototype, whilst it took a new aero engine six years.

Using the Merlin 20 series as roughly comparable to the F20 except for two speeds, even in the high gear the Allison only loses out around 4000ft of throttle height from the Merlin. A recalibration for higher CR could kill this tiny advantage, which for the most part could be made up for with ram and experienced piloting anyway. My impressions from various RAAF documentation is the P-40M and N were about as good as the P-40F, with cruise-altitude performance the most marked difference in the field. The P-40M was used in far greater numbers by the RAAF than the F, of which we used only a handful (I think it might've been a parts availability thing..but vets have said the Allison was very highly regarded and very well suited to conditions).

 

[Clay_Allison]

I think you'll find raising the compression will cause you to recalibrate the boost governer and return lowered throttle heights. The way you'd compensate this is a much larger diameter, lower pressure supercharger and/or an increase in engine capacity. You'll also have to play around with valve and ignition timing a fair bit, even combustion chamber and piston crowns and in those days...without computer modelling to help...

That's just what I was thinking in the last post, making a P-40 with the above changes and perhaps some load lightening for rate of climb (reducing guns to 4, etc) would have given Warhawk pilots in North Africa for instance a better shot at the 109.

 

[Shortround6]

Sorry about the "sematics" Elvis but the point is that the turbo-charger used up an awful amount of volume inside a fuselage. And the weight was not a small thing either. See the Republic P-43 Lancer which used a basic engine the same as many P-36 models.

For aircraft use compression ratio in the cylinder and boost fight each other. Raising either one increases the risk of detonation. For a given type of fuel if you raise the compresion you have to lower the boost. The Allison used a higher compression ratio than the Merlin, this did give it better fuel compsumption numbers. the Higher compression engine will have higher thermal effiencincy. But by using less boost it means that less fuel/air is going through the engine so there is less power. It was estimated that the 6.65 compression ratio in the Allsion limited it to about 10% less power than the Merlin with it's 6.0 compression ratio. The Allision's extra 60 cu. in. cut the actual difference to about half of that though.

As far as your two speed super charger idea goes it is a question of when.

There is a very definite limit as to how fast you can spin impellors. both from the exploding them due to centrifugal force and air flow. Just like propellors, having the tip speed of the impellor exceed the speed of sound does strange things to the airflow in the supercharger.

IN 1939-40 most peaples superchargers were doing good if they had a pressure ratio of 2.3 to 1. That is 69 in of manifold pressure at sea level. the problem is with altitude. The Early Allisons were limited to 42in of MP so they were using a pressure 1.4 already at sea level. At 10,000 ft the air is about 75% as dense as it is at sea level so you need a supercharger that can deliver a 1.33 ratio to get you sea level pressure. multiplying this times your 1.4 PR used at sea level means you need 1.86 pressure ratio to keep your sea level T-0 power rating. going to 20,000ft where the air is 50% as dense as sea level means you would need a supercharger with a pressure ratio of 2.8 to get 1040 hp from the Early Allison at 20,000ft. By 1941-42 superchargers were available with pressure ratios of 3 to 1. This is a limitation of the compressor and not how it was driven.
It also takes an awful lot of HP to drive a supercharger delivering these kinds of pressure ratios. This was a large part of the reasoning behind two speed drives. By using a lower gear at low altitudes were the high pressure ratio was not needed the impellor turned slower, used up much less power ( allowing more to go to the propellor) and heated the intake air less resulting in denser air and less problems from detonation. In some cases two speed superchargers were as much about getting better take-off performance as they were about getting better high altitude performance. British bombers got the first 2 speed supercharged Merlin Xs not because they gave 1,010HP at 17,750ft compared to the Spitfires 1,030HP at 16,250 ft but because they allowed 1,075hp for take-off compared to the Spifires Melin III's 880hp take-off rating.
The Allisons used a compromise setting/gear ratio that gave them not quite as much altitude as the Merlin III but but better take off and certanly much better altitude performance than the low gear in the Melrlin X would have given.

An easy way of getting a ball park estimate on engine power at altitudes is to graph the engine power. Plot your known power at altitude point and then draw a line to Zero Hp at 55,000ft.

 

[Elvis]

Very informative comments, guys.
Thanks for chiming in.

BTW, it seems I failed to mention that I was actually thinking more along the lines of the later M and N model P-40's, which would've used the "-81/99" engines, so comments about "early P-40's" doesn't really apply.
My apologies for not making that more clear, earlier.

I'll grant you that some tweaking would need to be done with a raise in c.r., but the 8:1 range isn't an "outlandish" figure for a supercharged engine, however, I wouldn't want to go much higher.
That's why I picked it.
There'll have to be some testing and maybe a slight redesign to the cylinder head (i.e., possibly better sealing), but the fact remains that a higher c.r. will give increased effciency to an engine, all the way across its rpm spectrum.
Also, that higher c.r. I gave isn't much more condusive to detonation, than the lower ratio...anyway, those engines did have a water-injection system, to counter detonation at times when the "WEP" button was pushed, so there's an added "anti-detonation device" at your command.
As for the two-speed comments, there's a "time-to-altitude" chart over at Peril's P-40 page, that the RAF ran and I noticed that the P-40 seemed to achieve its best climb when going from 5k-10k feet (approx. 3k ft./min., IIRC), however performance fell off sharply once 10K feet was surpassed.
If the transmission were atmospherically controlled, so that the second (higher) speed was engaged at somewhere around 12k-15k feet, wouldn't performance at those higher altitudes improve, due to increased delivery rate of fuel/air and maintaining cylinder pressure (compared to say sea level to 5k feet)?
I think maybe the point some you may not have noticed (and I admit, I probably wasn't too clear making it, as well) is that I'm not against tempering that higher impeller speed, so as to retain the best engine performance at the higher altitude.
"Halving/doubling" the figures I posted earlier just made the math easier.

...and Clay, you made a good point, by splitting up the existing inventory of P-40's into low altitude and high altitude configurations, but I keep going back to the thought that if a pilot is engaged in a fight, he'd want to have the plane perfom as good as possible, regadless of altitude, thus the idea of an "all-altitude" P-40.
Something that would perform as well at 25,000 feet as we already know it did at, say, 2500 feet.
I'd just rather an enemy not have an altitude advantage over me.
I don't know, I'm not a pilot, just what I'd want, if put in that situation.
Am I wrong in assuming that?


Elvis

 

[Elvis]

Clay and Vanair briefly discussed putting a larger supercharger on the Allison engine.
I too was once on that bandwagon, stating in the past that moving up to an impeller size of around 12-15 inches would give improvements in high altitude performance.
However, I'm now seeing a bigger picture, which is that it may not be financially feasible to produce a separate s.c. just for that one application.
I think implementing a two-speed gear box to the already existing s.c. would allow for a less expensive (and possibly less intesive) R&D phase, and I'm betting that production costs would not only be lower but would allow for an existing unit (the supercharger itself) to be used for more applications, making it "look' more versatile in the eyes of the US government...who, after all, are footing the bill for all of this.
So what you'd have is one s.c. unit with two different gear assembly's available for it. It may even be possible to make them 'removable", so that one could swap out the single speed gear set for the two-speed gear set.
I do think though, for the purposes of this aircraft, that the largest incarnation of that s.c. (which I believe was 10.2") would be the best platform from which to work the mods from.

Just a comment from the other end of the discussion.



Elvis