Maneuverability vs Speed

[Shortround6]

I also think that the British got some early-ish Mustangs (don't remember the precise mark) which had V-1710-73, which is the same engine as the P-40K. Quite robust, ~1500 hp even without overboost, which apparently they did for more power (up to ~1700 hp) down at sea level, according to the American wartime memo on the British use of the Mustangs.

The -73 engine is not listed in most of the sources on the Allison engines for the Mustang. Like "Vee's for Victory".

The P-40Ks don't hit the production line until May of 1942.
The P-51's were 55 aircraft held back from the British order for 150 planes (Mustang IAs) placed on July 7th 1941, First plane from this order was delivered in July of 1942.
The A-36 hits the production line in Oct 1942 after being ordered on April 16th, 1942.
The P-51A's were ordered on June 23rd 1942 to the tune of 1200 planes. Contract was cut back to 310 planes in Dec with the remainder to be built at P-51Bs.
British got not quite the number of Mustang IIs (it is a bit higher or lower) out of the P-51A batch that the US swiped from the Mustang IA batch.
The P-51A/Mustang IIs got the same engines as the P-40M/Ns of the time.

Now we have a discrepancy between the memo and Allison factory figures for power, compounded by the USAAF figures.

The 1500hp figure is overboost, no matter who's figures you are looking at. The question is how much overboost was allowed when and by who.

Once you are using more than about 44.6in of MAP the Allison was being over boosted. It may have stood up to it very well and Allison may have been lobbing for higher boost pressures but that is what the USAAF approved for "military power" and anything that exceeded that was overboost.
The exception were for the P-38s where the F & Gs were allowed 1325hp at 47in "military" and the H & Js were allowed 1425hp at 54in. "military".
Of course the P-38 engines used lower engine supercharger gears which makes up some of the difference. (engine can put more power into the prop because it is using less to drive the gear driven supercharger).

Maybe a few British Mustangs got -73 engines, strange things happened in the war, but most of the Mustangs Is had come off the lines before the -73s showed up and the -73s were ending their production run before the Mustang II/P-51A started their run. Allison having switched over to the -81 engines. Putting US ordered -73 engines into British ordered Mustang IA airframes in the spring/early summer of 1942 would have been a bit strange.
British only got about 21 of the P-40Ks unless there were in theater transfers.
British got 595 out of 600 P-40Ms but they had -81 engines.

Now for "official" over boost it seems that the -39 engines were allowed 56in at some point,1490hp at 4300ft (US didn't allow over boosting until Oct 1942)
The -73 engines were allowed 60in (1580hp at 2500ft), the -87 engines in the A-36 were allowed 52in ( 1500hp at 5400ft?) and the -81 engines were allowed 57in. (1410hp at 9,500ft)
What some squadrons did may be another story but going over those limits meant the pilots were acting as test pilots as far as the USAAC was concerned.
What the Allison rep said may have been different but what the Allison rep said may not have been a good defense at a court marshal for destroying US government property (engine/plane) in defiance of USAAC instructions/directions.

WEP is over boost.

 

[GregP]

I show the V-1710-73 as rated at 51.0 in Hg for takeoff, at 1,325 hp and 42.0 in Hg at 12,000 ft at 1,150 hp Military and 37.2 in Hg MAO at 10,000 ft at 1,000 hp normal rated power.

I show it as a P-40K engine and also for P-38 SN 40-744, which is a reasonably early model (P-38-LO). The early models could not use more than 1,050hp or the wing leading edge intercoolers would not cool the intake air sufficiently and the temps would start to rise quickly ... which is why they engines were restricted in MAP use. This limitation went away when the P-38J and later models adopted the larger beard radiator setup and had different intercoolers. By then, they were using -89/-91 (F17R/L) engines.

 

[Wild_Bill_Kelso]

The -73 engine is not listed in most of the sources on the Allison engines for the Mustang. Like "Vee's for Victory".

The P-40Ks don't hit the production line until May of 1942.
The P-51's were 55 aircraft held back from the British order for 150 planes (Mustang IAs) placed on July 7th 1941, First plane from this order was delivered in July of 1942.
The A-36 hits the production line in Oct 1942 after being ordered on April 16th, 1942.
The P-51A's were ordered on June 23rd 1942 to the tune of 1200 planes. Contract was cut back to 310 planes in Dec with the remainder to be built at P-51Bs.
British got not quite the number of Mustang IIs (it is a bit higher or lower) out of the P-51A batch that the US swiped from the Mustang IA batch.
The P-51A/Mustang IIs got the same engines as the P-40M/Ns of the time.

Now we have a discrepancy between the memo and Allison factory figures for power, compounded by the USAAF figures.

The 1500hp figure is overboost, no matter who's figures you are looking at. The question is how much overboost was allowed when and by who.

No, that is incorrect - it's the official WEP setting and it's in the manual, at 60" Hg.

Once you are using more than about 44.6in of MAP the Allison was being over boosted. It may have stood up to it very well and Allison may have been lobbing for higher boost pressures but that is what the USAAF approved for "military power" and anything that exceeded that was overboost.

I'd say that is a weird definition of overboost. 44.6" Hg was below the takeoff power setting in most cases (depending on when the manuals are made)

The exception were for the P-38s where the F & Gs were allowed 1325hp at 47in "military" and the H & Js were allowed 1425hp at 54in. "military".
Of course the P-38 engines used lower engine supercharger gears which makes up some of the difference. (engine can put more power into the prop because it is using less to drive the gear driven supercharger).

Maybe a few British Mustangs got -73 engines, strange things happened in the war, but most of the Mustangs Is had come off the lines before the -73s showed up and the -73s were ending their production run before the Mustang II/P-51A started their run. Allison having switched over to the -81 engines. Putting US ordered -73 engines into British ordered Mustang IA airframes in the spring/early summer of 1942 would have been a bit strange.
British only got about 21 of the P-40Ks unless there were in theater transfers.

They had more than that, or the Aussies did which is maybe not the same thing, but that has little bearing on the Mustang, it's just that they had the -73

British got 595 out of 600 P-40Ms but they had -81 engines.

Now for "official" over boost it seems that the -39 engines were allowed 56in at some point,1490hp at 4300ft (US didn't allow over boosting until Oct 1942)
The -73 engines were allowed 60in (1580hp at 2500ft),

Right, which makes it an official WEP setting. Overboost is when you go over the allowed / designated boost limits including for 'war emergency' power.

the -87 engines in the A-36 were allowed 52in ( 1500hp at 5400ft?) and the -81 engines were allowed 57in. (1410hp at 9,500ft)
What some squadrons did may be another story but going over those limits meant the pilots were acting as test pilots as far as the USAAC was concerned.
What the Allison rep said may have been different but what the Allison rep said may not have been a good defense at a court marshal for destroying US government property (engine/plane) in defiance of USAAC instructions/directions.

WEP is over boost.

I think we may be talking about two different memos. There is the Allison memo and there is this one which was specifically about mustangs:

E-GEH-16

In particular this part:

"In view of the British operation and the fact that we have an approved war emergency rating on the 1710-39 engine of 56", it is suggested that immediate steps be taken to remove the automatic boost controls from our P-51 airplanes in this theatre and that the instrument dials be marked with the proper lights. The British have operated at full throttle at sea level (72" Hg) for as much as 20 min. at a time without hurting the engines. According to them, the Allison is averaging 1500 hours between bearing failures as compared to 500 to 600 hours for the Merlin. The Allison, they have found, will drag them home even with the bearing ruined."

 

[Geoffrey Sinclair]

P-40 Delivery allocations,

537 K, 115 M USAAF
44 K, 96 M RAAF
30 K, 10 M Brazil
339 K, 94 M RAF
170 M Britain For Russia
15 M RCAF
14 K, 15 M China, later reallocated
22 K, 35 M RNZAF
314 K, 50 M USSR

Total 600 K-1 "export model", 700 K-5 and later, 600 M. RAF mixture, 191 P-40K-1, 52 P-40K-5, 46 P-40K-10, 50 P-40K-15, 50 P-40M-5, 44 P-40M-10

Early Mustang allocations
55 P-51, 260 P-51A USAAF
1 P-51 USN
92 P-51, 50 P-51A RAF
Total 148 P-51, 310 P-51A

USAAF Model Designation Army Aircraft 11th edition, engines. P-40E, E-1 V1710-39, P-40K, K-1 V1710-39, P-40M to N-16 V-1610-81, P-40N-20 to N-35 V-1710-99, P-40N-40 V1710-115. XP-51, P-51 V1710-39, P-51A V1710-81, A-36 V-1710-F21R (RAF document V-1710-30 = F3R?)

May 1942 P-40K-1 production begins (1 acceptance)
June 1942, P-40E-1 production ends
July 1942, NA-73/Mustang I production ends, P-51/Mustang IA begins
August 1942 P-40K-5 production begins
September 1942 P-40K-1 production ends (1 acceptance), P-51 production ends
October 1942 A-36A production begins
November 1942 P-40M production begins
December 1942 P-40K production ends (3 acceptances)
February P-40M production ends
March 1943 P-40N and P-51A production begin, A-36A production ends.
May 1943, P-51A production ends.

Interceptor Fighters for the Royal Air Force 1935-45 M.J.F. Bowyer
Mustang I V-1710 F3R, 1,150 HP at 12,000 feet, 3,000 RPM, 6,288 pounds tare, 8,625 pounds loaded, including 140 gallons of fuel, 370 mph at 15,000 feet radiator shutter closed, 8.65 minutes to 16,000 feet, 24.6 minutes to 28,000 feet service ceiling 30,000 feet.

Mustang I fitted with V-1710 F21R (supercharger gear now 7.48:1 versus 8.8:1) 377.5 mph at 4,000 feet, service ceiling 28,000 feet.

Mustang II V-1710 F4R engine, 8,200 pounds loaded, 401 mph at 4,400 feet, and 409 mph at 10,000 feet with the F20R.

The book reproduces the top speed versus altitude for AG315 with the F3R and AP222 with the F4R, plus time to climb for AG315.

 

[Shortround6]

No, that is incorrect - it's the official WEP setting and it's in the manual, at 60" Hg.

It is the official WEP setting from a manual dated/updated as of April, 25th 1944.
What was the WEP setting in June/July of 1942?

Right, which makes it an official WEP setting. Overboost is when you go over the allowed / designated boost limits including for 'war emergency' power.

No, over boost is when you exceed the Military power by any amount and for any period of time.
The Pilot is allowed to use Military power at anytime during the flight and even multiple times during the flight (subject to temperature limits) WITHOUT special maintenance procedures or even notifying the ground crew or maintenance/technical officer.

If the pilot uses WEP at all, forms must be filled out, the maintenance/technical officer notified, and depending on circumstances, maintenance procedures done. Might be simple, like taking an oil sample, might be a bit more, might be changing when the next spark plug change is done. Number of minutes at WEP are calculated into figuring when the engine is to be pulled for overhaul. The squadron maintenance/technical/engineering officer based on experience and bulletins/notes/memos will be adjusting the expected overhaul life of the engines in the squadron. Like dirty/sandy conditions.
Going beyond the WEP settings calls for even more scrutiny. Since not all squadrons had the same access to spare engines and even minor parts in all theaters and time periods what was done or allowed varied considerably.

US planes often had a tell-tale on the throttle in form of a small wire. If the pilot landed with the wire intact (Military power was not exceeded) then maintenance proceeded as usual.
If the wire was broken then all the forms and questions must be taken care of. It was all or nothing according to wire, there were not multiple wires and there was no timer.
The squadron maintenance/technical/engineering officer, after talking to the pilot, may decide to skip some procedures if the pilot can claim the time spent in WEP was short in time and low in pressure, But it is not up to pilot. Since the officer has no way of telling, once the wire is broken, either how long the higher boost was used or what the pressure was communication and honesty were key. A number of engines failed in the next flight or two after being "over boosted" in the early part of the war. Letting your squadron mate go up in a plane the day after you over boosted the engine and lied about it ?
The tell tail wire only limited the travel of the throttle handle, it did not limit the actual boost of the engine.

Since the throttle could give you, on the Allison -73, 44-46in at FTH it could give you a bit more at low altitudes and indeed, at take off, could give the 51in or so for take-off power.
The thicker, higher pressure air at low level (sea level) would give higher pressure at the same rpm and throttle opening.
Some times the Take-off power, while using more pressure, was done at lower rpm.
the -33 engines were rated at 1040hp for take-off at 40.6in but at 2800rpm. in one chart.
Military power was 1040hp at 37.2in at 14,300ft at 3000rpm.
OR
Military power was 1090hp at 38.9in at 13,200ft at 3000rpm.

 

[Wild_Bill_Kelso]

It is the official WEP setting from a manual dated/updated as of April, 25th 1944.
What was the WEP setting in June/July of 1942?

No, over boost is when you exceed the Military power by any amount and for any period of time.
The Pilot is allowed to use Military power at anytime during the flight and even multiple times during the flight (subject to temperature limits) WITHOUT special maintenance procedures or even notifying the ground crew or maintenance/technical officer.

If the pilot uses WEP at all, forms must be filled out, the maintenance/technical officer notified, and depending on circumstances, maintenance procedures done. Might be simple, like taking an oil sample, might be a bit more, might be changing when the next spark plug change is done. Number of minutes at WEP are calculated into figuring when the engine is to be pulled for overhaul. The squadron maintenance/technical/engineering officer based on experience and bulletins/notes/memos will be adjusting the expected overhaul life of the engines in the squadron. Like dirty/sandy conditions.
Going beyond the WEP settings calls for even more scrutiny. Since not all squadrons had the same access to spare engines and even minor parts in all theaters and time periods what was done or allowed varied considerably.

Right, we have discussed this before. It's interesting and important. I make the distinction between WEP vs overboost, because WEP settings are usually approved as 'safe' for the engine, based on testing. Of course, that doesn't mean it was 100% safe. Engines broke or even caught fire blew up fairly routinely in military service. They were basically racing engines, very veyr powerful ones at the limits of the technology of the day, and were as temperamental as the engines on a racetrack.

Overboost means the pilot took the boost setting past what was recommended / allowed emergency maximum. You are right that a 1944 WEP setting was not the 1942 official setting. The 1944 setting came from the overboosting routinely being done by P-40 pilots in the Pacific and the Mediterranean. Not sure if it was also done by P-39 pilots, (I assume it was), and I know the P-38 had specific and unique issues. As we know via the famous Allison memo, Allison and the War Department and the units basically worked out what was the 'best practice', though of course if the pilot was fighting, or running, for his life, guidelines may likely go right out the window.

The original maximum boost settings from the early manuals was too low, given the speed and performance of enemy aircraft, and people who stuck to those guidelines often paid with their life. They appear to have gone right past it within weeks or even days of combat in many cases. Pilots from the Australian 75th Sqn in New Guinea described boosting at 70" in their very first air battles with Zeros. In the MTO Theater, also Australian and then some British and South African units were doing this with their P-40D Kittyhawks after about 5-6 weeks.

I believe over time, the maximum boost settings (WEP) were used routinely while the overboost (beyond WEP) ratings were used less often (only in true life or death emergencies), except in cases like with the British recon Mustangs where the unit had carefully worked out how far they could push things in specific conditions.

US planes often had a tell-tale on the throttle in form of a small wire. If the pilot landed with the wire intact (Military power was not exceeded) then maintenance proceeded as usual.
If the wire was broken then all the forms and questions must be taken care of. It was all or nothing according to wire, there were not multiple wires and there was no timer.
The squadron maintenance/technical/engineering officer, after talking to the pilot, may decide to skip some procedures if the pilot can claim the time spent in WEP was short in time and low in pressure, But it is not up to pilot. Since the officer has no way of telling, once the wire is broken, either how long the higher boost was used or what the pressure was communication and honesty were key. A number of engines failed in the next flight or two after being "over boosted" in the early part of the war. Letting your squadron mate go up in a plane the day after you over boosted the engine and lied about it ?
The tell tail wire only limited the travel of the throttle handle, it did not limit the actual boost of the engine.

The big scorch mark and often leaking oil also gave this away I think

Since the throttle could give you, on the Allison -73, 44-46in at FTH it could give you a bit more at low altitudes and indeed, at take off, could give the 51in or so for take-off power.
The thicker, higher pressure air at low level (sea level) would give higher pressure at the same rpm and throttle opening.

And at a bit higher altitude when coming out of a dive, apparently

Some times the Take-off power, while using more pressure, was done at lower rpm.
the -33 engines were rated at 1040hp for take-off at 40.6in but at 2800rpm. in one chart.
Military power was 1040hp at 37.2in at 14,300ft at 3000rpm.
OR
Military power was 1090hp at 38.9in at 13,200ft at 3000rpm.

Seeing as takeoff power was routinely used ... for takeoff, especially when carrying bombs and / or external fuel tanks, I would think the wire would be set at least for 51" or whatever takeoff power was. Otherwise you aren't going to know much about "overboost" / WEP because the wire is already going to be broken before the plane is 100' off the ground .

 

[Mike Williams]

Pilots from the Australian 75th Sqn in New Guinea described boosting at 70" in their very first air battles with Zeros. In the MTO Theater, also Australian and then some British and South African units were doing this with their P-40D Kittyhawks after about 5-6 weeks.

Can you provide sources? (I do have the 2 SAAF engineering report on "overboosting" during Nov. 1942.) Thanks.

 

[Wild_Bill_Kelso]

It is mentioned (rather vague as far as unit details) in the famous Dec 12, 1942 Allison overboosting memo, and some specific pilot memoirs and interviews. One 75 RAAF pilot at Port Moresby described his first combat with a Zero (paraphrasing) as a brief fight in which he shot at some enemy planes, followed by a desperate power-dive and escape from three zeros in which "the manifold pressure went all the way past the red line and almost down to the middle again" ... during which he finally managed to disengage, with several bullet holes in his aircraft. He said when he landed he smoked a whole pack of cigarettes one after the other. I have that interview somewhere but would have to spend a bit of time running it down.

The relevant quote from Allison is:

"In the past week we have received reports from both the Middle East and Australia indicating the use for considerable periods of time of very high aminfold pressures on V-1710-F3R and F4R engines, (AAF models -39 and -73). From Australia we quote:

"Some pilots admit operating for prolonged periods at around 70" Hg (20 lbs / sq. in.) of boost", and from the Middle East our Representative who just returned advises they are resetting boost controls to 66" Hg (18# sq. in.) maximum boost pressure."

It also notes, 'this company has agreed to the war emergency peration at 60" manifold pressure and approximiately 1570 H.P. at 3000 RPM. As can be seen from the atached chart... on the average engine 66" boost is approximately 1745 B.H.P. at sea level or 1770 H.P. at 2000 feet and can only be optained either under ramming flight conditions at 3000 R.P.M. or by overrevving the engine it can be obtained to considerably higher altitude. "
So that indicates that War Department / AAF had already agreed to 60" Hg WEP rating by Dec 1942, and some units were actually reconfiguring their boost controls for 66" Hg by that time. Gives you some idea what the combat conditions were like. I think boosting to 70" was definitely increasing engine breakdown rates.

You also see a lot of comments like this one from an interview with Bobby Gibbes:

"We were a little disappointed when we first got the Kitty, we thought it'd be way ahead of the Tomahawk. In actual fact, it was a little bit better." ... "However, later when we got our Kittyhawks running properly - were getting better performance - they were a better aeroplane."

This corresponds, if you look at the combat record, with very heavy losses in Jan-March 1942, but then lower (like half) losses from April onward. And also the introduction of the higher WEP boost rating of 54, 56, or 57" Hg in P-40 flight manuals.

That interview was on this site, but unfortunately the site is down right now.
http://www.3squadron.org.au/subpages/AWMGibbes.htm

It may be here one of these interviews Wing Commander Robert Henry Maxwell Gibbes

 

[Wild_Bill_Kelso]

Found another Gibbes interview, this one is really good, not precisely the same but covers some of the same ground Australians at War Film Archive

 

[Shortround6]

The original maximum boost settings from the early manuals was too low, given the speed and performance of enemy aircraft, and people who stuck to those guidelines often paid with their life. They appear to have gone right past it within weeks or even days of combat in many cases. Pilots from the Australian 75th Sqn in New Guinea described boosting at 70" in their very first air battles with Zeros. In the MTO Theater, also Australian and then some British and South African units were doing this with their P-40D Kittyhawks after about 5-6 weeks.

1942 was also a time of great change in the available fuel supply. For most (all?) of 1941 the US was using US 100 octane fuel which was around 100/98-104 in extreme, most of the time it was 100/100. The British were already using 100/115-120 during the BoB. Trouble is nobody really knew it in 1940 and for most of 1941. The rating scale had not been invented yet. The US came out with a very short lived 100/120 fuel followed by a bit longer lasting 100/125 fuel spec which was superseded by the joint British/American 100/130 fuel specification ( the first one with 3.0 CCs of lead). Tomahawks In Britain were given the BoB fuel when they arrived and in 1941 they were getting much better fuel than the US was giving their fighters. When the British got 100/130 fuel to Africa I don't know. Could very well have been before Hawaii and the Philippines got it. The Flying Tigers were supplied with British fuel. It may have been dirty or contaminated but was more Knock resistant than US fuel.
The British were running 54in (or close) in the BoB and were moving towards 60in in 1941. The Merlin used lower compression in the cylinder and could use a bit more boost than the Allison. So the US was running too low a boost limit in 1941 and most of 1942, however it took a while for the US Forces to catch up and evaluate the new fuel/s. This was not helped by changing the fuel specification several times (100/130 with 4.0 CCs of lead and then 100/130 with 4.6 CCs of lead, these are not the only differences but are shorthand for specifications that allowed for a number of changes). Changing the specifications for a fuel type does not change the fuel already in shipment/storage.
Units in NA supplied by the British in 1942 probably had little trouble with boost, They may have had trouble with other things. Units in the South West Pacific may or may not have had trouble? Who were they getting their fuel from? By the summer of 1942 most of the old US stuff should have been gone or stayed in the US for training.

I believe over time, the maximum boost settings (WEP) were used routinely while the overboost (beyond WEP) ratings were used less often (only in true life or death emergencies), except in cases like with the British recon Mustangs where the unit had carefully worked out how far they could push things in specific conditions.

The high boost settings (over the WEP range) were only useful at very low altitudes and in certain conditions. If you are flying around at 0-2000ft you aren't escorting (or intercepting) bombers at 12,000ft.
For the P-40s with 8.80 gears you had 44.6in at 11,700ft (not including RAM), you had 56in at 4300ft (not including RAM)and you had under 70in at sea level (not including RAM).
Mustangs were around 30-40mph faster (?) and had more RAM at any given altitude. If you know the Pressures at two altitudes you can plot it on graph for all altitudes.

Now for the P-40s and Mustangs you had 3 and 2 engines (not counting the A-36) and once the US figured that 100/130 was here to stay and not a passing fad the P-40s use a 56in limit on the -39 engines, a 60in limit on the -73 engines (which were sturdier) but the 60in was only good at 2500ft or lower without RAM and then with the 9.60 gears they got the 44.5in at 15,500ft and the 57in at 9,500ft. Because the higher gear ratio (faster spinning impeller heated the intake charge more) ran a higher risk of detonation they had to lower the boost limit back down from the -73 engines.
I would also be a little careful of applying a boost limit you could use over the English Channel and NW Europe to a boost limit you could use in North Africa or New Guinea/Solomon's. Having your carb intake air running 30-40 degrees hotter is pushing you closer to limit. Also note that often the cooling system was not designed for the increased heat output. Level flight may be OK, using the extra power in a climb may not be.

Seeing as takeoff power was routinely used ... for takeoff, especially when carrying bombs and / or external fuel tanks, I would think the wire would be set at least for 51" or whatever takeoff power was. Otherwise you aren't going to know much about "overboost" / WEP because the wire is already going to be broken before the plane is 100' off the ground

To take our P-40K as an example, the engine would make over 1600hp just sitting on the runway at something close to 64-65in IF you opened the throttle all the way. So the throttle, was partially closed to block part of the airflow. Depending on how the throttle and linkage were set up the Throttle lever would not break the wire at or near take off, Please note that this may change from model to model or even within a model. Allison Mustangs had either no boost limiters, Claudel-Hobson boost limiters or Delco boost limiters. The last two did not operate exactly the same, one of them had wire inside the throttle lever housing that had to be replaced by a ground crewman.

I would also note that while P-40Ns with engines rated at 1200hp for take-off used up to 52in off boost to get their 1200hp they were not officially allowed to use anymore what ever the load. What they did in combat may be another story but even with a pair of 225 gallon ferry tanks take-off was supposed to have been done at 52in, despite a 4500ft ground run.

You also see a lot of comments like this one from an interview with Bobby Gibbes:

"We were a little disappointed when we first got the Kitty, we thought it'd be way ahead of the Tomahawk. In actual fact, it was a little bit better." ... "However, later when we got our Kittyhawks running properly - were getting better performance - they were a better aeroplane."

This does not tell us anything.
Did they change the spark plugs?
Did they re-jet the carbs?
Did they take the backfire screens out?
Did they change the valve clearance?
Was there something else?
The British did find that in order to use higher boost they needed colder spark plugs than normal and there were only about 3 types of spark plugs that were suitable. It was not a matter of just changing the boost limiter of disconnecting it. Some notes say that higher boost should not be used unless the proper spark plugs are fitted (with brand and type listed).

In the later P-40 charts the Military power has a limit of 15 minutes (early ones may be 5 minutes) while take-off is limited to 5 minutes and War Emergency was limited to "Emergency" only. Exact manual may vary.

 

[Wild_Bill_Kelso]

1942 was also a time of great change in the available fuel supply. For most (all?) of 1941 the US was using US 100 octane fuel which was around 100/98-104 in extreme, most of the time it was 100/100. The British were already using 100/115-120 during the BoB. Trouble is nobody really knew it in 1940 and for most of 1941. The rating scale had not been invented yet. The US came out with a very short lived 100/120 fuel followed by a bit longer lasting 100/125 fuel spec which was superseded by the joint British/American 100/130 fuel specification ( the first one with 3.0 CCs of lead). Tomahawks In Britain were given the BoB fuel when they arrived and in 1941 they were getting much better fuel than the US was giving their fighters. When the British got 100/130 fuel to Africa I don't know. Could very well have been before Hawaii and the Philippines got it. The Flying Tigers were supplied with British fuel. It may have been dirty or contaminated but was more Knock resistant than US fuel.
The British were running 54in (or close) in the BoB and were moving towards 60in in 1941. The Merlin used lower compression in the cylinder and could use a bit more boost than the Allison. So the US was running too low a boost limit in 1941 and most of 1942, however it took a while for the US Forces to catch up and evaluate the new fuel/s. This was not helped by changing the fuel specification several times (100/130 with 4.0 CCs of lead and then 100/130 with 4.6 CCs of lead, these are not the only differences but are shorthand for specifications that allowed for a number of changes). Changing the specifications for a fuel type does not change the fuel already in shipment/storage.

All quite interesting, I had been under the impression that the British had invented the higher octane fuel

Units in NA supplied by the British in 1942 probably had little trouble with boost, They may have had trouble with other things. Units in the South West Pacific may or may not have had trouble? Who were they getting their fuel from? By the summer of 1942 most of the old US stuff should have been gone or stayed in the US for training.

The high boost settings (over the WEP range) were only useful at very low altitudes and in certain conditions. If you are flying around at 0-2000ft you aren't escorting (or intercepting) bombers at 12,000ft.

Well, for better or worse, British policy often had the escorts flying cover for fighter bombers and light bombers at a round 5,000-7,000 feet or even less, and the fighter bombers were flying even lower than that. The Germans commented on this that they thought it was crazy, which was part of an evolving set of policies as the British were figuring out how to do proper close air support / interdiction. Later they were flying at least the high cover around 10-12,000 ft and up to 16,000, and once they got the merlin engined ones at 20-22,000.

But (and here is the important part) even from 10,000 or 20,000 ft they could be down at Sea Level very quickly as part of an escape maneuver, and that is when that extra power came in very handy for disengagement. And sometimes for turning the tables.

In one of the Osprey books there is another passage from a letter home by a 57th FG P-40 pilot who mentions his boost settings as he was rather easily outrunning pursuing Bf 109s all the way back to his base, where they ended up shooting down some of their pursuers. There is also a passage in MAW where one of the British pilot does that, dives out of a battle, escapes, goes back and kills the guy who was chasing him. I'll try to run those down and post them.

For the P-40s with 8.80 gears you had 44.6in at 11,700ft (not including RAM), you had 56in at 4300ft (not including RAM)and you had under 70in at sea level (not including RAM).

By the time those arrived in NA they were not really militarily relevant in the fighter role, though they were to some extent in China and a bit less, in the Pacific, the speed variance wasn't so severe for Allied planes in those zones.

Mustangs were around 30-40mph faster (?) and had more RAM at any given altitude. If you know the Pressures at two altitudes you can plot it on graph for all altitudes.

Mustangs were a good bit faster than most planes.

Now for the P-40s and Mustangs you had 3 and 2 engines (not counting the A-36) and once the US figured that 100/130 was here to stay and not a passing fad the P-40s use a 56in limit on the -39 engines, a 60in limit on the -73 engines (which were sturdier) but the 60in was only good at 2500ft or lower without RAM and then with the 9.60 gears they got the 44.5in at 15,500ft and the 57in at 9,500ft. Because the higher gear ratio (faster spinning impeller heated the intake charge more) ran a higher risk of detonation they had to lower the boost limit back down from the -73 engines.
I would also be a little careful of applying a boost limit you could use over the English Channel and NW Europe to a boost limit you could use in North Africa or New Guinea/Solomon's. Having your carb intake air running 30-40 degrees hotter is pushing you closer to limit. Also note that often the cooling system was not designed for the increased heat output. Level flight may be OK, using the extra power in a climb may not be.

Well Shortround6, the Allison memo I quoted just above, and which you read many times, notes that they were apparently not only doing this on an informal basis, they were rewiring their throttles for the 66" Hg boost already in 1941. In both North Africa and Australia / New Guinea. So I say that makes it pretty clear they were in fact able to operate them at the higher boost settings in those tropical zones.

To take our P-40K as an example, the engine would make over 1600hp just sitting on the runway at something close to 64-65in IF you opened the throttle all the way. So the throttle, was partially closed to block part of the airflow. Depending on how the throttle and linkage were set up the Throttle lever would not break the wire at or near take off, Please note that this may change from model to model or even within a model. Allison Mustangs had either no boost limiters, Claudel-Hobson boost limiters or Delco boost limiters. The last two did not operate exactly the same, one of them had wire inside the throttle lever housing that had to be replaced by a ground crewman.

All very interesting, and maybe I missed something, but I don't see here any reason to assume they had the boost limit wire set to military power as you originally stated. Clearly since the takeoff power setting is around 50" they would have to be set for at least that high or else it would break on every flight.

I would also note that while P-40Ns with engines rated at 1200hp for take-off used up to 52in off boost to get their 1200hp they were not officially allowed to use anymore what ever the load. What they did in combat may be another story but even with a pair of 225 gallon ferry tanks take-off was supposed to have been done at 52in, despite a 4500ft ground run.

52" Hg is a good bit more than military power isn't it?

This does not tell us anything.
Did they change the spark plugs?
Did they re-jet the carbs?
Did they take the backfire screens out?
Did they change the valve clearance?
Was there something else?

That is fair, and it's a bit of conjecture on my part. But I have about 4 dozen pilot interviews from that area in 1942, and they almost all mention this issue of having problems with the Kittyhawks initially (P-40D or E Kittyhawk type, flown with the boost limits and weights indicated in the original manual, performed considerably worse than a P-40B/C Tomahawk type. Like as in, half the initial climb rate).

The British did find that in order to use higher boost they needed colder spark plugs than normal and there were only about 3 types of spark plugs that were suitable. It was not a matter of just changing the boost limiter of disconnecting it. Some notes say that higher boost should not be used unless the proper spark plugs are fitted (with brand and type listed).

I think it's quite likely they made a range of changes, but this time period also coincides with the first mentions of using higher boost, and with the change in the P-40 manual to show the 56 or 57" WEP rating.

In the later P-40 charts the Military power has a limit of 15 minutes (early ones may be 5 minutes) while take-off is limited to 5 minutes and War Emergency was limited to "Emergency" only. Exact manual may vary.

Yes they do vary a good bit, i have a bunch of them. Always looking for more

 

[Shortround6]

All quite interesting, I had been under the impression that the British had invented the higher octane fuel

There were two different 100 octane fuels. Anybody could make small batches of 100 octane fuel, you just used 100% iso-octane. Howard used this in 1935 to set a world speed record. Problem was it cost about 10 times what regular aviation fuel did. So the British and the Americans (and to a much lesser extent other people) were working on how to get 100 octane performance from cheaper fuel. The Americans went straight for a 100 octane fuel equivalent with no difference in rich or weak mixture and the British knew they wanted better rich mixture response but they didn't know how to get it. They both knew that while you could use a crap load of lead for testing it didn't work as service fuel as you had too much trouble with lead deposits (valve and spark plug problems). The US came around to the British thinking but it took as while as the British fuel tended to dissolve American rubber fuel parts, components, including early self sealing tanks. So who invented it??

Later they were flying at least the high cover around 10-12,000 ft and up to 16,000, and once they got the merlin engined ones at 20-22,000.

The P-40D/E may have done OK at 10-12,000ft but there was darn little over boosting at that altitude.

In one of the Osprey books there is another passage from a letter home by a 57th FG P-40 pilot

57th FG used P-40Fs with Merlins and have little bearing on Allison story.

By the time those arrived in NA they were not really militarily relevant in the fighter role

?????
The Tomahawks had 8.77 gears and the P-40D/E had the 8.80 gears as did the P-40Ks.
The P-40Ms and NS had the 9.60 gears and as you say, the P-40s were were being phased out of fighter work at that time. Which was early 1943 by the time they made it overseas.

So I say that makes it pretty clear they were in fact able to operate them at the higher boost settings in those tropical zones.

Squadrons may have thought so. The guys who had to arrange for replacement aircraft and engines may have had other ideas. The 66in boost level never made it to official levels.

All very interesting, and maybe I missed something, but I don't see here any reason to assume they had the boost limit wire set to military power as you originally stated. Clearly since the takeoff power setting is around 50" they would have to be set for at least that high or else it would break on every flight.

The thing you are missing is that they had figured out how to vary the actual power level a bit from a straight line on the throttle.
50in on a -33 engine with a wide open throttle was just under 1400hp at about 5000ft. You could get about 1250hp at sea level from 50in while running at 2400rpm if you could get the throttle and propeller controls to cooperate.

Kittyhawks initially (P-40D or E Kittyhawk type, flown with the boost limits and weights indicated in the original manual, performed considerably worse than a P-40B/C Tomahawk type

Well, part of that was the increased gross weight for the D/E.

 

[Wild_Bill_Kelso]

There were two different 100 octane fuels. Anybody could make small batches of 100 octane fuel, you just used 100% iso-octane. Howard used this in 1935 to set a world speed record. Problem was it cost about 10 times what regular aviation fuel did. So the British and the Americans (and to a much lesser extent other people) were working on how to get 100 octane performance from cheaper fuel. The Americans went straight for a 100 octane fuel equivalent with no difference in rich or weak mixture and the British knew they wanted better rich mixture response but they didn't know how to get it. They both knew that while you could use a crap load of lead for testing it didn't work as service fuel as you had too much trouble with lead deposits (valve and spark plug problems). The US came around to the British thinking but it took as while as the British fuel tended to dissolve American rubber fuel parts, components, including early self sealing tanks. So who invented it??

The P-40D/E may have done OK at 10-12,000ft but there was darn little over boosting at that altitude.

Right but could drop down to say, 4,000 feet in about 40 seconds.

57th FG used P-40Fs with Merlins and have little bearing on Allison story.

Actually they were flying at least one squadron of P-40K at the time, due to a shortage of the Fs, and the one in the story was probably a P-40K though it's hard to be certain

?????
The Tomahawks had 8.77 gears and the P-40D/E had the 8.80 gears as did the P-40Ks.
The P-40Ms and NS had the 9.60 gears and as you say, the P-40s were were being phased out of fighter work at that time. Which was early 1943 by the time they made it overseas.

Sorry I was referring to the M and later

Squadrons may have thought so. The guys who had to arrange for replacement aircraft and engines may have had other ideas. The 66in boost level never made it to official levels.

Well it seems to have been part of an official discussion in that memo. Anyway, it's clear it was being done. I would not at all be surprised if the complaint by Allison led to some pressure to modify that downward a bit (maybe to 60") but the US military had limited influence on RAAF units on the front line.

The thing you are missing is that they had figured out how to vary the actual power level a bit from a straight line on the throttle.
50in on a -33 engine with a wide open throttle was just under 1400hp at about 5000ft. You could get about 1250hp at sea level from 50in while running at 2400rpm if you could get the throttle and propeller controls to cooperate.

The Allison memo mentions flying at 66" by over-revving. Some of the Soviet pilots like Golodnikov mentioned doing exactly that. Of course, we know they also burned through engines very quickly.

Well, part of that was the increased gross weight for the D/E.

Without a doubt - the gross weight was a problem, until they increased all of the boost ratings - military power, takeoff, and WEP, then it was much more manageable. At 57" Hg initial climb rate is about as good as the P-40C, (which is to say, average for the time rather than abyssmal, though it got worse as they gained altitude) and speed is fairly good for 1942 up to about 12,000 ft.

 

[Greg Boeser]

View attachment 723324

Actually they were flying at least one squadron of P-40K at the time, due to a shortage of the Fs, and the one in the story was probably a P-40K though it's hard to be certain

57th FG began receiving P-40K-1 in November, 1942. 64th FS fully equipped with K by end of December. 66th partially equipped. Ks were phased out following the end of the North African campaign during spring and summer 1943 as long tailed P-40F and L models arrived.

 

[Shortround6]

Ks were phased out following the end of the North African campaign during spring and summer 1943 as long tailed P-40F and L models arrived.

And here is part of the problem when looking back at History.
The P-40L was built from Jan through April of 1943, it just took a while ( months at times) to get them to combat zones. This goes for just about all US aircraft, not just P-40s.

 

[Greg Boeser]

And here is part of the problem when looking back at History.
The P-40L was built from Jan through April of 1943, it just took a while ( months at times) to get them to combat zones. This goes for just about all US aircraft, not just P-40s.

And here is another problem. Many of those P-40Ks weren't retired. They were transferred the the CBI.

 

[Wild_Bill_Kelso]

Ok this is the account I mentioned a few posts back, found it in my Osprey "Aircraft of the Aces #44 - P-40 Warhawk Aces of the MTO", by Carl Molesworth. This is on page 16 in the book.

The following is from a diary entry from 9 October, 1942 by a pilot named George D. Mobbs, of the 64th FS / 57th FG. Molesworth uncharacteristically doesn't give us the specific aircraft flown initially, but he later says it's a P-40F. But 64th FS did fly both P-40K and F (and later L), and doing a search I did find what appears to be his aircraft: P-40K-10-CU, "white 11" , although that is a 1943 photo, probably from a few months later after this incident. But it does sound like it was a P-40K.

I found the entry here: 57th_FG

This is apparently the man himself, George D. Mobbs.

Here is the passage:

"We got mixed up and got to the landing ground ahead of the bombers, but went in to strafe anyway, that is, most of us did. I was on the outside, and just as we started to go down, four or five '109s started to attack me. I turned into them and got a short burst at one, but it was a 90 degree deflection shot. Three of them kept attacking me, and I kept evading them, and occasionally getting a shot. Meanwhile, the rest of our aeroplanes had gone in to strafe and then flown out to sea, but I couldn't join them because the three German fighters kept attacking me.

I was running the engine at 55 to 65 inches of mercury and 3000 rpm, so I could pretty well stay with them. They would keep alternating the attacks between them. After a few minutes I got one one of their tails and was overtaking him. I didn't open fire until i was about 100 yards from him. I gave him a squirt and nothing happened. I moved over a little and changed my sighting, and on about the third burst his aeroplane burst into flames and fell off to one side. I was going to watch him go down so i would have a chance of getting credit for one destroyed, but one of the other jokers attacked so i was busy evading him. However I spotted the first one moments later a few thousand feet below me, still spiralling down, but I never got another look at him after that.

I was still in a hole. The other two kept attacking, one after the other. Later, I got a few shots at one from directly behind and slightly above as we were diving. I could see the aeroplane jerk each time. I pulled the trigger but saw no debris or fire from it, and I was drawn away by the other one attacking. I must have hit the Jerry because I never saw him again.

Now I just had one to worry about, but on his next attack i finished my ammunition. He kept following and attacking, but with just him to worry about I was making good time back toward our lines. On another attack we met head-on, and I don't think he fired his guns. I didn't see them anyway, and I was already out of ammunition.

We were down pretty low by then, 1000 ft, and the German ack-ack had opened up at me. But I was going so fast that they were shooting behind me. I had everything forward. I was running awfully hard, and the ack-ack was getting pretty close to the Jerry pilot behind me. It was kind of amusing, because it looked as if I was going to make it back if my engine didn't quit. We were so low that I could seee the ack-ack gun emplacements below."

Molesworth goes on to note: "In fact, the engine on lt Mobb's P-40F did hold together, and the Bf 109 gave up the chase. The American returned safely to base , where he was awarded one Bf 109 probably destroyed for the mission. Four days later, Mobbs recorded his first of four confirmed victories during a scrap with 20 Bf 109s over El Alamein."

I looked this day up in Shores just for fun. This was a very busy day because it was during the lead up to El Alamein. The British claimed 11 Bf 109 killed and multiple damaged or probable, the US claimed one Bf 109 killed and two damaged (one of these was Mobbs claim, listed as near LG. 104, no time given). The Germans claimed 14 x P-40s, 3 x P-39s, 5 x Spitfires, 3 x Hurricanes, and a Boston. The Italians (all flying MC 202s) claimed 12 x P-40, 2 x P-46, 5 x Spitfire, and 1 Hurricane.

Actual losses were:
Axis - (4 fighters) 4 x Bf 109F-4 shot down (two of these say 'shot down by P-40'), with multiple others destroyed or damaged by bombs and strafing on the ground, 4 x MC 202 'Damaged'
Allied- (16 fighters and one bomber) - 9 x Hurricane IIc, 4 x Kittyhawk III, 1 x Spitfire V, 1 x Kittyhawk I, 2 x Tomahawk, and 1 Baltimore. All of these were British, no US casualties.

 

[Wild_Bill_Kelso]

So my interpretation:

This shows what it really means to have the extra power down low, which is something Shortround6 and I have often discussed. At that altitude, the P-40K almost certainly had a speed and overall performance advantage, something he was able to use to keep turning with what was originally three enemy fighters. Not only could he hold his own against the Bf 109F-4 in this incident, he seems to have been able to turn the tables and may have even shot one down (very hard to know if any of the German losses were due to his attacks, given all the chaos that day). This is the key point and the reason for posting the anecdote.

Also of course, we know he was specifically using high boost from 55" - 65" Hg. He was probably alternating to manage his engine temperature which was likely near the redline.

It's interesting that Mobbs was largely using British terminology ('Jerry', 'ack-ack' etc.), this is probably because 57th FG trained with the British and adopted their tactics.

The power of the P-40K (or it could have been an F, it's not all that different) was sufficient to keep turning without sort of gradually spiraling downward in an literal energy-sink death-spiral as is often claimed vis a vis P-40s. (and this is how they are modeled in most of the popular airplane video games these days). Here you can see how maneuverability did indeed matter, and this is described in pilot anecdote after anecdote in that book. So long as the pilot had sufficient situational awareness to see attacks coming, they could out turn their opponents and then re-engage, and at that altitude they had the power to keep turning. The longer the German pilot stayed engaged with them below 5,000 ft or so, the more likely they were to die.

So in other words, these aircraft, at the higher boost settings down low, not only had the ability to disengage in a dive, they could also fight and turn the tables because they had more power and an advantage in roll and turning. Mobbs wasn't able to escape because he was attacked by three opponents and didn't have the altitude needed to do an escape maneuver and dive to pick up enough speed to pull safely ahead, but he did utilize his aircraft's capabilities to the maximum, gaining benefit from the hard won lessons of the British, Australians and South Africans before him, and he was also quite lucky. And skilled, most likely, considering that he went on to get four confirmed victories.

It's also worth noting that he was able to keep shooting his guns until he ran out of ammunition, which was very helpful in his desperate situation. Earlier marks of P-40s probably would have had some jammed guns by then, amidst all that tight turning and pulling Gs. They made some kind of changes to the ammunition storage late in the P-40E run which was standard for P-40K. If you read accounts of British pilots from 5 or 6 months earlier they were often experiencing stoppages with the guns. This change alone made the K a much more formidable opponent.

It's also worth noting how tough the Bf 109s were -they fairly often took hits from .50 cal machine guns and kept flying. The P-40s were also able to absorb many 20mm cannon hits and keep going as well.

 

[Wild_Bill_Kelso]

Also the German and and Italian claims for the "P-46" and "P-39" were confusion regarding late model P-40s, and that was routine in late 1942 and all through 1943.

 

[Escuadrilla Azul]

For what it's worth Wikipedia says:


Kawanishi N1K - Wikipedia

Sources seems a bit old.

In this book the 343 Ku history is told and claims, IIRC, are balanced against allied loses.

I wil try to found out when arrive home, some hours from now:

Amazon.com