Performance modifications done at Squadron level.

[andy1967]

Recently watched 2 YouTube videos that had modifications done on aircraft at Squadron level. Pitch stops on Hurricane mk1 was changed by 3 degrees to increase top speed at 15000ft up to 12mph.Boost levels was increased by 2lbs on Mosquito aircraft to increase top speed by up to 10mph.Does anybody know any more that was unofficial done.

 

[MIflyer]

From Iwo Jima P-51's escorted B-29 raids but found they had a frustrating problem. Over Japan the Japanese fighter pilots were not stupid enough to come fight the B-29's and their escorts up above 20,000 ft, where the Mustang performance was excellent and Japanese fighters were sagging off. The Mustang pilots really hated making that very long flight with nothing to show for it except bombers not being shot down. Descending down to 15K meant that they were operating at the upper range of their fighters' low speed supercharger setting. The performance decayed until 18,000 ft, where an aneroid device automatically switched the supercharger to high speed, with an instantaneous huge increase in performance. One P-51 Squadron commander had a solution. There was a switch in the cockpit that enabled the supercharger to be put in "Low Speed", bypassing the automatic feature, or, flipped the other way, engage the "High speed" setting, also bypassing the automatic feature. But the High Speed setting was spring loaded to go back to the Automatic setting; you had to hold it in High Speed manually below 18,000 ft.

The fix he suggested was to replace the spring loaded switch with one that was not spring loaded, enabling him to manually engage the high speed setting. The Packard rep nonconcurred, saying it would damage the engine to run it on high speed supercharger at altitudes below 18,000 ft. They made the mod anyway (maybe by just taking out the spring, which I have done with WW2 vintage switches) and the squadron commander was well pleased at his ability to put on a burst of speed and catch Japanese fighters at 15,000 ft. After the war the supercharger system was changed to operate based on air ram pressure rather than altitude. That way, if you were at lower latitudes but going slow you could get up to speed faster.

Reportedly, when the Hawker Typhoon was being used to intercept low flying FW-190 fighter bombers the ground crews fiddled with the supercharger boost controls to get more power down low and that was an action that in part led to the Napier Sabre having a bad reputation when it came to reliability.

There are a number of Youtube videos about field mods to WW2 aircraft, and I have some serious reservations about the accuracy of at least some of them.

 

[Steamed_Banana]

Recently watched 2 YouTube videos that had modifications done on aircraft at Squadron level. Pitch stops on Hurricane mk1 was changed by 3 degrees to increase top speed at 15000ft up to 12mph.Boost levels was increased by 2lbs on Mosquito aircraft to increase top speed by up to 10mph.Does anybody know any more that was unofficial done.

This kind of thing, especially increasing emergency boost, was done with P-40 Kittyhawks both in North Africa and in the South Pacific (Darwin and New Guinea originally) in 1942 and there is a famous letter from Allison engine company complaining about it. It appears the AVG may have also done this in Burma though less is known about it.

 

[Shortround6]

This may sound like nit-picking and to some extent it is.
The Americans, unlike some other air forces, did not use boost limiters in the early part of the war. And many (most) of the planes sold to or given to allied air forces did not have boost limiters. The pilot was expected to keep an eye on the boost gauge and adjust the throttle accordingly.
There was nothing in the throttle linkage or even a screw or knob on the throttle body to keep a ham fisted movement of the throttle lever from going into over-boost.
Boost limits were doctrine and training (follow the manual).
Now if a pilot in a P-40 is using 44in of boost at around 13,000ft (altitude and ram dependent) and he dives without pulling back on the throttle he is going to be getting 50in of more pretty quickly. If he doesn't pull back on the throttle he could be hitting over 55in by the time he gets to around 7000ft and around 66in at 2000ft and this does not include RAM.
It was one more thing for the American (and allied pilots of American aircraft) to keep track of in combat and we can all guess that more than a few engines were over-boosted by accident.
At some points in 1942-43 American aircraft did get boost limiters although not all planes got them at the same time. American planes often just got a thin wire across the throttle to limit the movement to a 'safe' area. Push the throttle lever hard enough and wire broke and then pilot was then responsible for watching the boost gauge. By the end of the war a few American planes might have gotten a bit more sophisticated.
1940 Hurricanes and Spitfires had a knob or button that moved a spring to keep the boost within limits. 6lbs of boost 'normal' and 12lbs when the control was activated. Pilot did not have to watch the boost gauge at low altitudes to keep from going over the 12lb limit. The control device would take care of it.
When later Merlins were allowed to use more boost the mechanics could adjust the control to allow for more boost. The Americans had nothing to adjust.
Americans may have trashed more engines by accident
Fortunately the Allison was pretty rugged and would tolerate a fair amount of abuse.

What really scared the crap out of the Allison company was the USAAC deciding to OK the use of higher boost as an official policy in late fall of 1942 at about the same time that they were introducing the engines with the 9.60 supercharger gears instead of the 8.80 gears. To hit 68-72in of boost with the 8.80 gears you needed a cool day, to be flying at around 2000ft or lower and be flying pretty much straight and level at high speed to maximize RAM. With the 9.60 gears they not only could hit closer to 80in they were also operating closer to the detonation limits at all times. At 44in the engine with the higher gears had a higher manifold temperature than the engine with the 8.80 gears and when over boosted it would hit the detonation limit sooner while the higher gear made it easier to hit higher boost in many flight conditions.
Official limits for the P-40s were supposed to be
P-40D/E was 56in.
P-40K was 60in
P-40M/N was 57in
but K/M/N had different crankshafts and blocks than the early D/E. At some point in E production they started to switch over but not at the same point in time so identifying which engine in which planes had both new components was tough.
Complicating this was the fact that the engines didn't quite make the same power at the same boost pressures.

 

[Steamed_Banana]

This may sound like nit-picking and to some extent it is.
The Americans, unlike some other air forces, did not use boost limiters in the early part of the war. And many (most) of the planes sold to or given to allied air forces did not have boost limiters. The pilot was expected to keep an eye on the boost gauge and adjust the throttle accordingly.
There was nothing in the throttle linkage or even a screw or knob on the throttle body to keep a ham fisted movement of the throttle lever from going into over-boost.
Boost limits were doctrine and training (follow the manual).
Now if a pilot in a P-40 is using 44in of boost at around 13,000ft (altitude and ram dependent) and he dives without pulling back on the throttle he is going to be getting 50in of more pretty quickly. If he doesn't pull back on the throttle he could be hitting over 55in by the time he gets to around 7000ft and around 66in at 2000ft and this does not include RAM.
It was one more thing for the American (and allied pilots of American aircraft) to keep track of in combat and we can all guess that more than a few engines were over-boosted by accident.
At some points in 1942-43 American aircraft did get boost limiters although not all planes got them at the same time. American planes often just got a thin wire across the throttle to limit the movement to a 'safe' area. Push the throttle lever hard enough and wire broke and then pilot was then responsible for watching the boost gauge. By the end of the war a few American planes might have gotten a bit more sophisticated.
1940 Hurricanes and Spitfires had a knob or button that moved a spring to keep the boost within limits. 6lbs of boost 'normal' and 12lbs when the control was activated. Pilot did not have to watch the boost gauge at low altitudes to keep from going over the 12lb limit. The control device would take care of it.
When later Merlins were allowed to use more boost the mechanics could adjust the control to allow for more boost. The Americans had nothing to adjust.
Americans may have trashed more engines by accident
Fortunately the Allison was pretty rugged and would tolerate a fair amount of abuse.

What really scared the crap out of the Allison company was the USAAC deciding to OK the use of higher boost as an official policy in late fall of 1942 at about the same time that they were introducing the engines with the 9.60 supercharger gears instead of the 8.80 gears. To hit 68-72in of boost with the 8.80 gears you needed a cool day, to be flying at around 2000ft or lower and be flying pretty much straight and level at high speed to maximize RAM. With the 9.60 gears they not only could hit closer to 80in they were also operating closer to the detonation limits at all times. At 44in the engine with the higher gears had a higher manifold temperature than the engine with the 8.80 gears and when over boosted it would hit the detonation limit sooner while the higher gear made it easier to hit higher boost in many flight conditions.
Official limits for the P-40s were supposed to be
P-40D/E was 56in.
P-40K was 60in
P-40M/N was 57in
but K/M/N had different crankshafts and blocks than the early D/E. At some point in E production they started to switch over but not at the same point in time so identifying which engine in which planes had both new components was tough.
Complicating this was the fact that the engines didn't quite make the same power at the same boost pressures.

You might be interested to know, that in two memoirs I just read by RAF / DAF Kittyhawk pilots, they both mentioned serious and virtually identical problems with the Allison engines, due they said to the dust, in particular with the bearings which would begin to disintegrate showing telltale 'silver fish' which needed to be rigorously checked for. In both cases these were pilots flying "Kittyhawk III" which could mean either P-40K or M. One pilot (Bert Horden) was in 112 Sqn RAF and one (DH Clarke) was in 450 Sqn RAAF, both operating from roughly mid 1943.

According to both pilots, if careful checking wasn't done after every mission, there was risk of a catastrophic emergency engine failure sometimes including a rupture of the crank case and a fire, according to Clarke. He also quoted some songs from 450 Sqn which bemoaned the problem. He also routinely mentions flying at 54" and 55" boost basically as soon as he saw German aircraft.

By this time (mid 1943) 112 Sqn was pretty much being relegated to fighter-bomber missions and 450 was mostly fighter-bomber strikes from the get-go I think, so it's possible they were getting clapped out planes from other squadrons like 260 RAF and 3 RAAF. But it could just mean that Allisons were not as reliable in these kinds of conditions as we have been led to believe.

The problems may be due to using high boost or may just be due to the dust, but I would guess probably both. They do also mention a filter but I guess it wasn't enough.

 

[GregP]

Silver particles in the oil mean you have main bearing bits in the oil. Actually, could be cam beareings, too. Both have silver plating over lead, over steel. It certainly wasn't boost. The Allison is capable of 75" easily, and was actually designed for 4,000 rpm while the normal max was either 3,000 or 3,200, depending on dash number. Of course, that assume s the engines were operated correctly and allowed to warm up before getting high boost.

If you give it 75" before it gets into the green temperature range, that will cause bearing issues with a Merlin, too.

I'd lean toward dust mixed with operator error, but each engine willl have some telltales inside. Operate it properly and the higher boost is not an issue. Today, too.

Joe Yancey put together a racing Allison back in 2017 and we ran it at Reno in a Yak-3. Won Bronze, won Silver, and then changed that engine out for a stocker for the Gold race because of an issue unrelated to the engine. It easily ran 90" in the Yak-3 called "Full Noise" with no ill effects after several qualifying runs and two 8-lap, full power races. Still runs good to this day. The only real issue we had was an undersized radiator on the Yak, so it ran a bit warm.

Edit: The "We" above was a group of people from New Zealand including owner Graham Frew and crew, Joe Yancey (Allison guy). I am a friend of Joe's and offered minor help. I saw the issues first hand and heard Joe Yancey's thoughts, but was not an official "crew member," so perhaps it would ne more accurate to say "they" and not "we."

 

[ColFord]

Joe Yancey put together a racing Allison back in 2017 and we ran it at Reno in a Yak-3. Won Bronze, won Silver, and then changed that engine out for a stocker for the Gold race because of an issue unrelated to the engine. It easily ran 90" in the Yak-3 called "Full Noise" with no ill effects after several qualifying runs and two 8-lap, full power races. Still runs good to this day. The only real issue we had was an undersized radiator on the Yak, so it ran a bit warm.

Full Noise ran with a water spray bar in front of the radiator in 2018 which helped with the overheating, also used the water spray rig when it set the NZ propellor/piston air speed record. The 2017/2018 engine was replaced after the take off accident at Omaka in 2023, a new build engine being sourced from Vintage V-12s. During initial flight trials with the new engine fitted after the rebuild, temperatures were way higher than expected. A radiator core flow test showed the old radiator core was not performing as expected, so a new radiator core was fabriacted and installed fixing the overheating problem - may have been an issue back in 2017 not identified at the time. Now back in the air and performing brilliantly.

But, back to wartime Allison V-1710 engines. The RAF with the Allison V-1710s fitted to their early Allison engine Mustangs, experimented with a number of engine modifications, both to boost settings and modifications to the supercharger gearing and impellor design. As delivered to the RAF, the engines fitted to the NA-91 P-51(no suffix) Mustang IA came fitted with a Delco Remy automatic boost control unit, the engine fitted to the earlier NA-73 and NA-83 Mustang I did not. In service, the automatic boost control unit was soon disconnected. In addition, a change was made to the gearing ratio of the supercharger and the impellor rotor was 'cropped' to achieve higher levels of sustainable boost and power at lower altitudes for longer duration. When the RAF fed back details of the modifications made and the levels of boost and duration of boost was being sustained to GM-Allison, the response was one of "don't keep doing that" - which the RAF didn't follow, followed by a request for more details of the nature of the modifications and details on how the engines were performing with the modifications in terms of durability and any ill affects. A number of instances of RAF Allison engine Mustang pilots utilising 60 inches of boost maintained below 5,000ft for over 15 minutes, over 400mph TAS ASL on the clock.

 

[andy1967]

This may sound like nit-picking and to some extent it is.
The Americans, unlike some other air forces, did not use boost limiters in the early part of the war. And many (most) of the planes sold to or given to allied air forces did not have boost limiters. The pilot was expected to keep an eye on the boost gauge and adjust the throttle accordingly.
There was nothing in the throttle linkage or even a screw or knob on the throttle body to keep a ham fisted movement of the throttle lever from going into over-boost.
Boost limits were doctrine and training (follow the manual).
Now if a pilot in a P-40 is using 44in of boost at around 13,000ft (altitude and ram dependent) and he dives without pulling back on the throttle he is going to be getting 50in of more pretty quickly. If he doesn't pull back on the throttle he could be hitting over 55in by the time he gets to around 7000ft and around 66in at 2000ft and this does not include RAM.
It was one more thing for the American (and allied pilots of American aircraft) to keep track of in combat and we can all guess that more than a few engines were over-boosted by accident.
At some points in 1942-43 American aircraft did get boost limiters although not all planes got them at the same time. American planes often just got a thin wire across the throttle to limit the movement to a 'safe' area. Push the throttle lever hard enough and wire broke and then pilot was then responsible for watching the boost gauge. By the end of the war a few American planes might have gotten a bit more sophisticated.
1940 Hurricanes and Spitfires had a knob or button that moved a spring to keep the boost within limits. 6lbs of boost 'normal' and 12lbs when the control was activated. Pilot did not have to watch the boost gauge at low altitudes to keep from going over the 12lb limit. The control device would take care of it.
When later Merlins were allowed to use more boost the mechanics could adjust the control to allow for more boost. The Americans had nothing to adjust.
Americans may have trashed more engines by accident
Fortunately the Allison was pretty rugged and would tolerate a fair amount of abuse.

What really scared the crap out of the Allison company was the USAAC deciding to OK the use of higher boost as an official policy in late fall of 1942 at about the same time that they were introducing the engines with the 9.60 supercharger gears instead of the 8.80 gears. To hit 68-72in of boost with the 8.80 gears you needed a cool day, to be flying at around 2000ft or lower and be flying pretty much straight and level at high speed to maximize RAM. With the 9.60 gears they not only could hit closer to 80in they were also operating closer to the detonation limits at all times. At 44in the engine with the higher gears had a higher manifold temperature than the engine with the 8.80 gears and when over boosted it would hit the detonation limit sooner while the higher gear made it easier to hit higher boost in many flight conditions.
Official limits for the P-40s were supposed to be
P-40D/E was 56in.
P-40K was 60in
P-40M/N was 57in
but K/M/N had different crankshafts and blocks than the early D/E. At some point in E production they started to switch over but not at the same point in time so identifying which engine in which planes had both new components was tough.
Complicating this was the fact that the engines didn't quite make the same power at the same boost pressures.

Interesting point about no boost limits on US aircraft. Why did the US introduce limits in 42/43 was it to reduce workload on the pilot in combat situations or to prolong maintenance schedules. Your thoughts on this would be greatly appreciated.

 

[Shortround6]

Not all Allisons used the same supercharger gear ratio. They didn't vary a lot but there are some differences.
A higher ratio means it take more power to drive and it heats the air more for a given manifold pressure. Lower ratio means less power to drive (more power to the prop) the air is cooler. This is relative as even the air from the 7.48 gear set is going to be hundreds of degrees F.

I have no idea what anybody building a race Allison was using in the last 20 years (or earlier) for supercharger gears or exactly which engines they were using or what parts they were using.
From 1940 through 1945 Allison made 4 different crankshafts and even the last ones built would fit in the early engines. The last version had the counterweights and had a huge margin of fatigue life over the early versions. These counter weighted cranks went into the the last P-38s built, perhaps some P-63s and in the Allison powered F-82s. These late model engines were allowed to use much high boost pressure than the earlier Allisons. Most had 8.10 gears and in some cases used water injection and/or 100/150 fuel.

7.48 gears were used in the A-36 and in P-38F & G.
8.10 gears were used in the P-38H, J and L and the P-63s. These are all two stage supercharged and this ratio is only on the main supercharger.
8.80 (or 8.77) was standard supercharger gear for most P-40s and most P-39s.
9.60 was the gear set for the P-40M and N and the P-39M, N and Q.

 

[tomo pauk]

In addition, a change was made to the gearing ratio of the supercharger and the impellor rotor was 'cropped' to achieve higher levels of sustainable boost and power at lower altitudes for longer duration.

This is very interesting. Can you share some details, like what was the, now changed, gearing ratio, as well as what was the diameter of the impeller after it was cropped?

 

[Shortround6]

Interesting point about no boost limits on US aircraft. Why did the US introduce limits in 42/43 was it to reduce workload on the pilot in combat situations or to prolong maintenance schedules. Your thoughts on this would be greatly appreciated.

When the US introduce the limits it was done by publishing limits in memo's and pilots manuals. There was no new hardware fitted to existing planes or on planes being produced at the time. Fitting boost limiting hardware did come in time.
So no, it was not to reduce work load on the pilot.
It may have helped prolong maintenance schedules but this was sort of secondary. By using the War Emergency ratings they could get better performance out of existing aircraft or aircraft in the pipe line. But since the pilots were already doing it without any sort of guideline except "Sgt Joe in the 576th says they use 60in of boost all the time with no problem" something better was needed to to both reduce maintenance problems and to keep from blowing engines up in combat and loosing pilots.

Any War Emergence rating was not approved until a test engine on a stand had completed 7 1/2 hours of run time on the stand at that rating. This could be done using 5 minute intervals with 5 minute cool down periods (engine ran at a low cruise setting) or it could be done all at once or some variation between the two. This was at the engine manufactures recommendation/choice. This is done at the standard engine test temperature (59 degrees F/15 degrees C) and pressure in the test house/cell.
The idea was to provide a safety margin for both weather and engines that already run for dozens of hours.
Sgt Joe at the 576th may have gotten away with running his engines at 60in over Holland in March. Does not mean that Sgt Willie can tell his pilots to do the same thing over North Africa in July.

Use of WEP required notes in the logs every time it was used. More frequent spark plug changes and more frequent oil inspections. There may have been other procedures that had to followed depending on the number of hours spent at WEP over time.

 

[Steamed_Banana]

Silver particles in the oil mean you have main bearing bits in the oil. Actually, could be cam beareings, too. Both have silver plating over lead, over steel. It certainly wasn't boost. The Allison is capable of 75" easily, and was actually designed for 4,000 rpm while the normal max was either 3,000 or 3,200, depending on dash number. Of course, that assume s the engines were operated correctly and allowed to warm up before getting high boost.

If you give it 75" before it gets into the green temperature range, that will cause bearing issues with a Merlin, too.

I'd lean toward dust mixed with operator error, but each engine willl have some telltales inside. Operate it properly and the higher boost is not an issue. Today, too.

Joe Yancey put together a racing Allison back in 2017 and we ran it at Reno in a Yak-3. Won Bronze, won Silver, and then changed that engine out for a stocker for the Gold race because of an issue unrelated to the engine. It easily ran 90" in the Yak-3 called "Full Noise" with no ill effects after several qualifying runs and two 8-lap, full power races. Still runs good to this day. The only real issue we had was an undersized radiator on the Yak, so it ran a bit warm.

Yes you are right, both of these guys mentioned bearing failure as the cause, I should have noted that as well. So the assumption is dust, operator error is not unlikely either, the more I learn about the early use of the Kittyhawk it seems that training on type was tragically neglected though that may have been unavoidable due to circumstances.

 

[andy1967]

Full Noise ran with a water spray bar in front of the radiator in 2018 which helped with the overheating, also used the water spray rig when it set the NZ propellor/piston air speed record. The 2017/2018 engine was replaced after the take off accident at Omaka in 2023, a new build engine being sourced from Vintage V-12s. During initial flight trials with the new engine fitted after the rebuild, temperatures were way higher than expected. A radiator core flow test showed the old radiator core was not performing as expected, so a new radiator core was fabriacted and installed fixing the overheating problem - may have been an issue back in 2017 not identified at the time. Now back in the air and performing brilliantly.

But, back to wartime Allison V-1710 engines. The RAF with the Allison V-1710s fitted to their early Allison engine Mustangs, experimented with a number of engine modifications, both to boost settings and modifications to the supercharger gearing and impellor design. As delivered to the RAF, the engines fitted to the NA-91 P-51(no suffix) Mustang IA came fitted with a Delco Remy automatic boost control unit, the engine fitted to the earlier NA-73 and NA-83 Mustang I did not. In service, the automatic boost control unit was soon disconnected. In addition, a change was made to the gearing ratio of the supercharger and the impellor rotor was 'cropped' to achieve higher levels of sustainable boost and power at lower altitudes for longer duration. When the RAF fed back details of the modifications made and the levels of boost and duration of boost was being sustained to GM-Allison, the response was one of "don't keep doing that" - which the RAF didn't follow, followed by a request for more details of the nature of the modifications and details on how the engines were performing with the modifications in terms of durability and any ill affects. A number of instances of RAF Allison engine Mustang pilots utilising 60 inches of boost maintained below 5,000ft for over 15 minutes, over 400mph TAS ASL on the clock.

Some Spitfire mkv were also cropped to improve low level performance, extra boost and sometimes clipped wings aswell.

 

[Steamed_Banana]

I think clipped wings were very common for all marks of Spitfires (except the HF variants) in North Africa and Italy by late 1942. It was generally an improvement across the board except for highest altitude bands.

 

[andy1967]

I think clipped wings were very common for all marks of Spitfires (except the HF variants) in North Africa and Italy by late 1942. It was generally an improvement across the board except for highest altitude bands.

Yes you are correct but the MkV was the first Spitfire to have clipped wings to improve roll rate and low altitude performance because of the superior Fw190A.The cropped propeller and extra boost closed the gap in performance between the two aircraft but the Fw190 was still superior until the introduction of the Spitfire mkix.

Edit ... should have been cropped impellor.

 

[Steamed_Banana]

Yes you are correct but the MkV was the first Spitfire to have clipped wings to improve roll rate and low altitude performance because of the superior Fw190A.The cropped propeller and extra boost closed the gap in performance between the two aircraft but the Fw190 was still superior until the introduction of the Spitfire mkix.

Edit ... should have been cropped impellor.

There was a lot of variation with Spit Vs, with lots of little improvements available, especially many arriving toward the end of 1942. Cropped wings were an almost entirely beneficial change, with some limitations to ceiling and tiny hit on climb rate more than offset by the roll rate improvement which took the Spitfire from one of the slower rolling fighters of it's moment at higher speed, to almost the fastest. The metal ailerons also helped with this.

Cropped impeller was good for a lot of speed and power down low but at the expense of a much bigger hit to performance at medium and higher altitudes. I don't think they made too many of those.

But other good improvements included higher boost rating up from +12 up to +16, the Aboukir instead of Vokes filter, Malcolm hoods, bullet proof glass (especially on the inside of the canopy which I think came a bit later), improved radios, and improved VP propeller nose-cones. With all those additional things, (but especially cropped wings, Aboukir filter and +16 boost) the Spit VC was a significantly more lethal aircraft and much more competitive with the FW 190 than most assume, depending on the variant of the FW of course.

 

[ColFord]

This is very interesting. Can you share some details, like what was the, now changed, gearing ratio, as well as what was the diameter of the impeller after it was cropped?

Based off the documentation, they went on the ratio from the 8.80:1 on the original supercharger gearing ratio to around 7.80:1 - which was not as 'extreme' as the 7.48:1 adopted for the V-1710-87 on the A-36A. The exact degree of 'cropping' is not recorded in the surviving documentation that I have seen, but a number of different degrees of cropping were trialled. The trials were assisted in part with representatives from the Air Ministry, MAP, Rolls-Royce and GM-Allison providing input. The end result was an impellor that best I can determine from the reports was cropped down from 9.5 inches to 9.0 inches. Modifications to the superchargers for the engines from the documentation were conducted by AST, Roots and R-R. Priority was given to first line operational squadrons equipped with the Mustang I, then second line units - those not actively engaged in operations. The Mustang IA were all basically modified before they were issued to units, ditto the Mustang II - part of RAF modification works conducted before issue to MU/GSU then to Squadrons. As late as December 1943, some Mustang I on issue to units in 'rear' areas had still to be modified, and a priority was then given to having those aircraft modified so that they could be a part of the reserve for the planned invasion mid-year.

 

[ColFord]

Cropped impeller was good for a lot of speed and power down low but at the expense of a much bigger hit to performance at medium and higher altitudes. I don't think they made too many of those.

Actually quite a few were modified.

The majority of the remaining Spitfire V squadrons based in the UK by the beginning of 1944 were flying Spitfire Vs that had been "clipped, cropped, clapped". This was in part considering the planned role for those squadrons in the lead up to the planned invasion mid-1944 and the type of flying they were performing operationally. Many of the remaining Spitfire V Squadrons were located outside 11 Group, and the types of operations they were flying was focussed on the medium to low altitude ranges. Where these squadrons were flying bomber escort, it was usually for the medium bombers of the RAF and USAAF, flying at medium altitudes, flying the close escort to the bombers. In those cases, the high cover would be provided by Spitfire IX equipped units. They also did a lot of the penetration escort for US heavy bombers - as they cllimbed from bases in the UK to cross the enemy coast; and then the exit cover from the enemy coast back to the UK as the bombers were descending back to their bases in the UK. That also included low level cover for the SAR launches and RAF SAR aircraft. A very large percentage of their activity in the immediate lead up to D-Day was flying cover over convoys around the UK coastline and covering the ports where the vessels were congregating before the invasion. There the focus was on preventing hit and run raids by low level German fighter bombers on the shipping, or attacks by E-Boats. A number of the RAF's Spitfire V squadrons in the UK in the lead up to D-Day also became heavily involved in the large scale Army exercises that took place, in part providing cover to the exercises but also familiarising the pilots with air-ground support methods and practices. Four of the RAF Spitfire V squadrons also undertook naval gunnery direction courses in Scotland so they could be ready to be part of the Fleet Spotting Pool for D-Day.

By the time D-Day actually arrived, there were still 12 RAF Squadrons - including RCAF, Polish and FF, operating the Spitfire V. A number of Squadrons that had been operating the Spitfire V in the period January to May 1944, converted over to the Spitfire IX with 'hand me downs' from other units converting to other aircraft - often as a result of the squadron flying the Spitfire IX converting over to the Mustang III, or directly from the Spitfire V to the Mustang III. By 1 June 1944 37 RAF Squadrons were listed as being equipped with the Spitfire IX, with 24 listed as flying the LF.IX variant - most of those were units destined for 2TAF use and to move to ALGs in the bridgehead after D-Day where their primary use would tend to be as fighter bombers. Added to that were two Spitfire XII Squadrons, the first couple of Spitfire XIV squadrons working up, and four Squadrons with Spitfire VII spread across the UK primarily focussed on very high altitude fighter interception against high altitude German reconnaissance aircraft.

 

[Steamed_Banana]

Mustang IA and II were being used super low, often flying right over the sea. A-36 had to fly over mountains sometimes. So i can see a difference. Generally the British cropped impellers seemed to go a little too far, they'd end up with a lot of power down low but it tapered off very quickly and they were anemic at even marginally medium altitude. This was a big problem for the Barracuda for example when they needed to fly over mountains in the Pacific.

 

[Steamed_Banana]

Actually quite a few were modified.

The majority of the remaining Spitfire V squadrons based in the UK by the beginning of 1944 were flying Spitfire Vs that had been "clipped, cropped, clapped". This was in part considering the planned role for those squadrons in the lead up to the planned invasion mid-1944 and the type of flying they were performing operationally. Many of the remaining Spitfire V Squadrons were located outside 11 Group, and the types of operations they were flying was focussed on the medium to low altitude ranges. Where these squadrons were flying bomber escort, it was usually for the medium bombers of the RAF and USAAF, flying at medium altitudes, flying the close escort to the bombers. In those cases, the high cover would be provided by Spitfire IX equipped units. They also did a lot of the penetration escort for US heavy bombers - as they cllimbed from bases in the UK to cross the enemy coast; and then the exit cover from the enemy coast back to the UK as the bombers were descending back to their bases in the UK. That also included low level cover for the SAR launches and RAF SAR aircraft. A very large percentage of their activity in the immediate lead up to D-Day was flying cover over convoys around the UK coastline and covering the ports where the vessels were congregating before the invasion. There the focus was on preventing hit and run raids by low level German fighter bombers on the shipping, or attacks by E-Boats. A number of the RAF's Spitfire V squadrons in the UK in the lead up to D-Day also became heavily involved in the large scale Army exercises that took place, in part providing cover to the exercises but also familiarising the pilots with air-ground support methods and practices. Four of the RAF Spitfire V squadrons also undertook naval gunnery direction courses in Scotland so they could be ready to be part of the Fleet Spotting Pool for D-Day.

By the time D-Day actually arrived, there were still 12 RAF Squadrons - including RCAF, Polish and FF, operating the Spitfire V. A number of Squadrons that had been operating the Spitfire V in the period January to May 1944, converted over to the Spitfire IX with 'hand me downs' from other units converting to other aircraft - often as a result of the squadron flying the Spitfire IX converting over to the Mustang III, or directly from the Spitfire V to the Mustang III. By 1 June 1944 37 RAF Squadrons were listed as being equipped with the Spitfire IX, with 24 listed as flying the LF.IX variant - most of those were units destined for 2TAF use and to move to ALGs in the bridgehead after D-Day where their primary use would tend to be as fighter bombers. Added to that were two Spitfire XII Squadrons, the first couple of Spitfire XIV squadrons working up, and four Squadrons with Spitfire VII spread across the UK primarily focussed on very high altitude fighter interception against high altitude German reconnaissance aircraft.

That's interesting thanks, I'm familiar with the Mediterranean / North Africa / Italy theaters in some detail but I do not really know the history of the units flying out of UK, that helps fill in some gaps for me.