An Allison V-1710 with a two-speed supercharger in 1942 (1 Viewer)

[ColFord]

Again one of those older publications that have no clue.

RAF retired the last of its operational/front line use Allison engine Mustangs in mid-August 1945. Still being used up until then in the Tac/R role, doing tasks that their 'replacements' - primarily Spitfire FR.XIVes couldn't do. Last RAF Allison engine Mustangs retired - formally - from service late 1947.

MOST RAF Mustang Is were fitted with the oblique F.24 recce camera behind the cockpit, and the great number, probably also qualifies as MOST also received the vertical F.24 camera installation in the tail. All done without any of the wing guns being removed, or nose guns. It was a mandatory RAF operational modification to have the oblique camera mounts and wiring, etc fitted to all RAF Mustang I, IA and II aircraft - camera may not always be fitted depending on nature of sortie being flown, but aircraft could always be quickly fitted with a camera if the task required it.

The first Mustang I was supplied to No.26 Squadron on 9 January 1942.

The fourth and tenth early build NA-73s from the NAA production line were diverted to the USAAC to become the XP-51s, but the RAF still received and accounted for the full 320 NA-73 Mustang Is they paid and ordered = 322 built, plus the 300 NA-83 Mustang I = 622. Aligns to NAA allocated production numbers on aircraft data plates.

 

[Steamed_Banana]

How substantial of a change was needed to put in A) bomb racks and B) 'wet' bomb racks tied into the fuel system?

 

[wuzak]

So, is the P-40 the only realistic/sensible possible recipient for a 2 speed supercharged V-1710.

I'm assuming the Mustang/P-51 proceeds from 1 speed V-1710 to 2 speed/2 stage V-1650.

Bombers tended to receive the 2 speed supercharged versions first in Britain.

But were there any production bombers that used the V-1710?
The only one that I can think of was the XB-42, but that wasn't production model, and was later in the war, using the 2 stage V-1710.

 

[tomo pauk]

So, is the P-40 the only realistic/sensible possible recipient for a 2 speed supercharged V-1710.

I'm assuming the Mustang/P-51 proceeds from 1 speed V-1710 to 2 speed/2 stage V-1650.

P-39, plus P-51, IMO.
Might also look good on the Mosquito.

 

[Steamed_Banana]

So, is the P-40 the only realistic/sensible possible recipient for a 2 speed supercharged V-1710.

I'm assuming the Mustang/P-51 proceeds from 1 speed V-1710 to 2 speed/2 stage V-1650.

Bombers tended to receive the 2 speed supercharged versions first in Britain.

But were there any production bombers that used the V-1710?
The only one that I can think of was the XB-42, but that wasn't production model, and was later in the war, using the 2 stage V-1710.

Good question.

Aside from the P-40, I think we have established that some early P-51 conversions could have been possible, and probably Spitfire too as we've been discussing

A sort of budget version of a Spit III or just a Spit V hack, a Spit III/V hybrid let's say. IMO a Merlin XX powered Spitfire would have been very helpful in the Middle East and in Burma / Pacific Theater if they had been available prior to the arrival of the Spit IX and VIII.

It seems like a P-39 conversion is ruled out due to the need for extensive fuselage changes including probably lengthening the fuselage like they did with the P-63, though that arguably might have been worth doing. Thousands of P-39s produced that ended up being deployed by the US didn't really help the war effort that much.

Maybe some P-38 conversions are feasible, as a stop-gap to get around turbo problems for a year or so. Seems like a Merlin XX / V1710 equivalent powered P-38 might actually have some advantages over a P-38F or G. I'm not sure about the amount of work fitting a different engine would be, but I imagine if you remove the turbos you get a lot more space and some weight savings as well.

Would a Fulmar have benefited from a Merlin XX equivalent? Fulmar Mk I had a Merlin VIII, according to Wikipedia, and Mk II had a Merlin XXX which I gather was similar, with a critical altitude of around 7,000 ft / 2,200 m. A Merlin XX would possibly give the aircraft the low altitude power they wanted for carrier ops but with more power overall and a good amount of extra juice at altitude, which could have come in handy. They were still making Fulmars into December 1942, and the Firefly took a long time to arrive in combat units yet. Martlets / Wildcats were good but never in enough numbers, and the Hellcat and Corsair came very late. It seems like change to Merlin XX might not be a huge engineering problem and could possibly improve the viability of the type. I am not sure if Merlin XX uses more fuel than Merlin VIII or XXX?

This is only about 350 aircraft in the Mk II, though perhaps Mk I could be retrofitted as well? And FAA fighter numbers were always pretty low for most of the war. Fulmars didn't get into that much fighter combat but when they did it was often at higher altitudes where more power and higher speed might have helped a great deal.

All this also makes me think about Seafires, for similar reasons. Good low altitude power but keeping good medium-high altitude capability as well.

We mentioned the Beaufighter but unless they could make contra-rotating versions perhaps, I'm not sure Merlin or any inline engine is the right fit for that type. Possibly Beaufort?

US designed bombers were mostly radial-engine powered of course, and after the earliest these were big ones ala R-2600. Though the A-36 demonstrates the potential of a P-51 airframe as a dive bomber, it seems like these weren't going to come together soon enough. I was a bit surprised to learn that the early Mustangs didn't even have bomb racks.

P-40s did make fairly good fighter-bombers though, as we know, but something with better range and more potential for speed (or say, internal bomb racks), and / or a dedicated dive bomber type with dive brakes might have been very helpful.

 

[Steamed_Banana]

P-39, plus P-51, IMO.
Might also look good on the Mosquito.

I was thinking about Mosquitoes but weren't those getting Merlin XX from pretty much the get-go?

 

[Steamed_Banana]

(To be clear, when I say FAA fighters were "low", that was kind of poorly worded - what I mean is there were low numbers of combat aircraft, especially carrier capable fighters. There is a separate issue of FAA military aircraft usually being given engines tuned for low-altitude)

 

[Howard Gibson]

I was thinking about Mosquitoes but weren't those getting Merlin XX from pretty much the get-go?

All Mosquitos had two speed superchargers. Some Mosquitos at two stage superchargers.

 

[Steamed_Banana]

And maybe that is a partial explanation for the spectacular success of the Mosquito

 

[ThomasP]

I wrote the following up for my own edification to sort out the timeline of the Merlin XX and Spitfire Mk III, but figure it might be worthwhile posting here.

re better use and availability of Merlin XX for Spitfire

A list for when the first squadrons to get the Merlin XX began building up, plus the first of the Spitfire Mk V and Mk IX squadrons:

Jun '40_____Beafighter NF/F Mk II
Aug'40_____Hurricane Mk II
Mar'41_____Spitfire Mk VA/B
[ Aug'41_____Defiant Mk II ]
Aug'41_____Mossi PR Mk I
Mar'42_____Lancaster Mk I
Apr'42_____Halifax Mk II
Jul '42_____Spitfire Mk IX

This helps indicate (I think) the earliest that a Spitfire Mk III or similar could get into squadron size service.

Spitfire Mk III timeline
Mar'40_____Spitfire Mk III prototype #1 with RM2SM, DH prop, & new 'A' wing, first flight. Subsequently fitted with Merlin XX & standard 'B' wing.
Jun'41_____Spitfire Mk III prototype #2 with Merlin XX, Rotol prop, & standard 'B' wing.
Mar'41_____Spitfire Mk III prototype #1 sent to RR, for fitting of Merlin 61 & 4-blade prop, plus Mods as prototype for Mk IX.

The Beaufighter NF Mk II may have needed the additional altitude & speed provided by the Merlin XX for its night fighter duties? Beaufighter F Mk II Vmax was 317 / 337 mph TAS at 13,500 / 22,000 ft, while for the Mk I it was 311 / 330 mph at 4,000 / 16,000 ft.

The Defiant could probably be done without?

The Mosquito is too important to delay and needed the Merlin XX series for performance. I think only a few of the very early production Mosquito used the Merlin XX model, the rest used the Merlin 21 or later Marks.

The Lancaster Mk I & Halifax Mk II are a separate issue. The Merlin XX was a bit underpowered for the Halifax Mk II aerodynamics and the weights it was operating at during this period, but the Hercules powered variant was not ready yet and there were not enough Hercules engines being produced?

To me, the reasonable potential time frame for production of the Spitfire Mk III seems to be from Jun'40 - when the Merlin XX becomes available in squadron size numbers - to Jul'42 - when the Spitfire Mk IX enters service. Delays and loss of production due to switching over would be the main draw back - how bad this would be is up for question?

Bleh!

[edited]

 

[ThomasP]

re Other airframes that could benefit from fitting of the Merlin XX.

The Fulmar would have benefited quite a bit from the Merlin XX.

Fulmar Mk I with Merlin VIII had a WEP of 1280 BHP at FT(+9 lbs) at SL with RAM, but only had 720 BHP available at 18,000 ft.

Fulmar Mk II with Merlin 30 had a WEP of 1360 BHP with +12 lbs at 6,000 ft, but was only putting out about 900 BHP at 18,000 ft.

Merlin XX was putting out 1360 BHP with +12 lbs at 8,600 ft, 1145 BHP with +9 lbs at 18,250 ft, 900 BHP at 24,000 ft, and 730 BHP still available at 29,000 ft. With the Merlin XX the operational ceiling would be about 9,000 ft higher than with the Merlin VIII, and about 5,000 ft higher than with the Merlin 30. Everything else being equal, Vmax with the Merlin XX would have been about 265 mph TAS at 20,000 ft, vs 235 mph TAS with the Merlin 30 or 220 mph TAS with the Merlin VIII.

Since the primary complaints concerning the Fulmar were speed, ROC/TTH, snd too low an operational ceiling, the Merlin XX would have been a good fit.

 

[Howard Gibson]

And maybe that is a partial explanation for the spectacular success of the Mosquito

Mosquitos powered by Merlin 32s, 45Ms, and 55Ms would run out of supercharging at 6,000ft. This was a problem for "clipped, clapped and cropped" Spitfire_Vs, and low altitude Seafires. If the Mosquitos are bombing stuff at low altitude, these engines would have worked fine.

 

[ColFord]

How substantial of a change was needed to put in A) bomb racks and B) 'wet' bomb racks tied into the fuel system?

Though the A-36 demonstrates the potential of a P-51 airframe as a dive bomber, it seems like these weren't going to come together soon enough. I was a bit surprised to learn that the early Mustangs didn't even have bomb racks.

It was a not unsubstantial change to put bomb racks with the associated plumbing into the wings of the Mustang. In addition to the wing strengthening to carry the loads on the hardpoints under the wings, you have all the associated changes to the fuel system to add the selection of the external tanks and feeds, the wiring and controls for the operation of the hardpoints, all with decision points along the way about what range of stores the hardpoints will carry, the maximum load on the hardpoints, to what flight parameters and associated restrictions to flight operations with the full load in place on the hardpoints, etc.

The early Allison engine Mustangs ordered by the United Kingdom were ordered as fighters. They were not ordered as fighter bombers or anything other than fighters. At the time they were ordered, none of the other in service or in development RAF fighter types were designed for or had provision for stores carriers/bomb racks. The RAF operational doctrine of the time had fighters, primarily as originally specified and conceived, intended to intercept and destroy unescorted enemy bombers approaching the UK from distant bases - they were interceptor fighters. At that time, the RAF did not have an operational doctrine or an operational requirement for an aircraft type that could be identified as a fighter bomber.

The RAF doctrine and policy at the time was that bombs were carried on Army Co-operation aircraft, then in light bombers, medium bombers and the proposed heavy bombers. Also in maritime patrol aircraft. The identification for the requirement for dive bombers, then fighter bombers - as it would for high performance fighter reconnaissance aircraft, would not come until after the Norwegian Campaign, then the German attacks on the Low Countries and France, and the initial campaigns against the Italians in the Middle East and East Africa. Even then, the main drive for the adaption of fighters to initially carry light bombs came from the Middle East - they had less aircraft, less 'modern' aircraft and had to do more with less. Even at the height of the potential invasion of the UK by German forces in 1940 and 1941, the Air Ministry was slow to permit any of their then front line interceptor fighters being considered for modification into fighter bombers. The initial choices for modification were fighters that were considered to be heading towards perceived obsolesence or due to be replaced by newer, higher performance types in development in the near future. Or as we saw with the 'Banquet' Plan, putting bomb carriers on training and second line aircraft as an emergency measure - which did work to a degree in the defence of 4FTS at Habbinaya in Iraq in 1941.

 

[MiTasol]

I agree fully and would add that the bomb/tank shackles would have also had mechanical release systems so there is the requirement to add the controls to the cockpit, cables from the controls to the hard points, and then a mechanical connection to the release mechanism itself. The cables must be routed in such a way that they do not rub on any electrical, fuel or structural components, and they must have clearance from all other components as they will almost certainly vibrate in flight as they are not under high tension like flight control cables.

Furthermore the structural changes must not interfere with existing components of the flight control system, fuel system, armament, landing gear, etc, etc and must not remove any maintenance access points. Easy in theory but not so in practice as the additional strength must be where required and discharge the loads out through other structure without producing any stress or fatigue points.

 

[Shortround6]

Most anything can be rebuilt/modified if you spend enough time and money. Whether it is cost effective is another story. It is an awful lot easier to change things while they are being manufactured that to pull them apart on completed aircraft, like pull wing skins off the bottom, replace certain ribs/braces (assuming spars are good enough) install fuel lines and control cables for the bomb racks ( you may need both drop release cables and bomb arming cables) and then you need to reskin the wing where the modifications were made and do so to about the same standard of fit/finish as the factory could do. Which was of a higher standard that most other factories could do.
We are not talking about one or two prototypes. We are talking about dozens or aircraft or even a few hundred? Once the A-36 was in production and the 1200 P-51As are ordered in June of 1942 for planed delivery starting in Jan/Feb 1943 with racks we have to ask, how much would a large (dozens or hundreds) refit program delay the P-51A project (or the A-36 production)? All of the stronger wing parts have to be manufactured, all of the extra fuel line parts and bomb rack parts/controls have to manufactured and early production parts that are sent to the refit/modification center will not be available for the A-36/P-51 program until production of such parts can be ramped up even more. This does not really sound like an in-service squadron level modification.

 

[Steamed_Banana]

re Other airframes that could benefit from fitting of the Merlin XX.

The Fulmar would have benefited quite a bit from the Merlin XX.

Fulmar Mk I with Merlin VIII had a WEP of 1280 BHP at FT(+9 lbs) at SL with RAM, but only had 720 BHP available at 18,000 ft.

Fulmar Mk II with Merlin 30 had a WEP of 1360 BHP with +12 lbs at 6,000 ft, but was only putting out about 900 BHP at 18,000 ft.

Merlin XX was putting out 1360 BHP with +12 lbs at 8,600 ft, 1145 BHP with +9 lbs at 18,250 ft, 900 BHP at 24,000 ft, and 730 BHP still available at 29,000 ft. With the Merlin XX the operational ceiling would be about 9,000 ft higher than with the Merlin VIII, and about 5,000 ft higher than with the Merlin 30. Everything else being equal, Vmax with the Merlin XX would have been about 265 mph TAS at 20,000 ft, vs 235 mph TAS with the Merlin 30 or 220 mph TAS with the Merlin VIII.

Since the primary complaints concerning the Fulmar were speed, ROC/TTH, snd too low an operational ceiling, the Merlin XX would have been a good fit.

Yeah that is about what I was thinking, only you have the actual facts and details to flesh out the idea! I didn't realize that the Fulmar was quite that slow at altitude.

 

[Steamed_Banana]

Most anything can be rebuilt/modified if you spend enough time and money. Whether it is cost effective is another story. It is an awful lot easier to change things while they are being manufactured that to pull them apart on completed aircraft, like pull wing skins off the bottom, replace certain ribs/braces (assuming spars are good enough) install fuel lines and control cables for the bomb racks ( you may need both drop release cables and bomb arming cables) and then you need to reskin the wing where the modifications were made and do so to about the same standard of fit/finish as the factory could do. Which was of a higher standard that most other factories could do.
We are not talking about one or two prototypes. We are talking about dozens or aircraft or even a few hundred? Once the A-36 was in production and the 1200 P-51As are ordered in June of 1942 for planed delivery starting in Jan/Feb 1943 with racks we have to ask, how much would a large (dozens or hundreds) refit program delay the P-51A project (or the A-36 production)? All of the stronger wing parts have to be manufactured, all of the extra fuel line parts and bomb rack parts/controls have to manufactured and early production parts that are sent to the refit/modification center will not be available for the A-36/P-51 program until production of such parts can be ramped up even more. This does not really sound like an in-service squadron level modification.

I realize that it's much trickier to add in "wet" hard points on the wings, but hard points just for bombs should be a bit easier, no? I'm envisioning something before the A-36 and P-51A obviously which did have these built in.

 

[Shortround6]

re Other airframes that could benefit from fitting of the Merlin XX.

The Fulmar would have benefited quite a bit from the Merlin XX.

Fulmar Mk I with Merlin VIII had a WEP of 1280 BHP at FT(+9 lbs) at SL with RAM, but only had 720 BHP available at 18,000 ft.

Fulmar Mk II with Merlin 30 had a WEP of 1360 BHP with +12 lbs at 6,000 ft, but was only putting out about 900 BHP at 18,000 ft.

Merlin XX was putting out 1360 BHP with +12 lbs at 8,600 ft, 1145 BHP with +9 lbs at 18,250 ft, 900 BHP at 24,000 ft, and 730 BHP still available at 29,000 ft. With the Merlin XX the operational ceiling would be about 9,000 ft higher than with the Merlin VIII, and about 5,000 ft higher than with the Merlin 30. Everything else being equal, Vmax with the Merlin XX would have been about 265 mph TAS at 20,000 ft, vs 235 mph TAS with the Merlin 30 or 220 mph TAS with the Merlin VIII.

Since the primary complaints concerning the Fulmar were speed, ROC/TTH, snd too low an operational ceiling, the Merlin XX would have been a good fit.

The Merlin VIII was pretty much a Merlin III (some parts were Merlin XII) with a 6.313 supercharger gear. Very close to the gear ratio used in the Merlin X low gear. Yes, performance at height sucked. One the other hand this shows the slow pace of British construction of the time. The first production order was placed in mid 1938 (?) and first production plane was delivered in Jan 1940. Perhaps the Air Ministry could have requested Merlin X engines? But such a request should have been done either at the initial order or shortly after. Switching engines only a few weeks or two months before expected delivery was going to cause a lot a waves in production of the aircraft you are swiping the better engines from.
Merlin 30 was the original 'cropped' Merlin. It used a 9.75in impeller and the original 8.588 gear ratio of the Merlin III. Used a lot more parts in common with the Merlin XII and the stronger parts allowed for higher boost.
Fulmar II started leaving the factory in Jan 1941. Unfortunately, the Fulmar was rather low on the list for better engines. A two speed Fulmar would have been better, but in Jan-June of 1941 there were a number of other aircraft that needed the two speed engines more, although swiping the engines from the Defiant IIs would have been a plus. Unfortunately the Defiant II production started in the summer of 1941, not 1940 so they aren't much help.
A high altitude (over 10,000ft?) Fulmar would have been interesting. Actual need might not have been that great. Fulmars really needed to stop torpedo bombers and low altitude bombers. With hindsight we know that high altitude (over 10,000ft) level bombers didn't hit much anyway
chasing off snoopers was another important job and here they were aided by the Italians using their own low altitude planes (float and flying boat?) to bulk out numbers.
One wonders if a Merlin 45 with a small change in gear ratio and 9.75in prop instead of the 9.5in 45M impeller could have added 3000+ft plus the Fulmars operational height without going to the two speed supercharger? The Merin 32 showed up late.
The successors to the Fulmar and Swordfish showed up very late.
Maybe the British would have been much better served by a Fulmar III with a two speed Merlin rather than waiting for the Firefly until 1944?

 

[Shortround6]

I realize that it's much trickier to add in "wet" hard points on the wings, but hard points just for bombs should be a bit easier, no? I'm envisioning something before the A-36 and P-51A obviously which did have these built in.

We may be being confused by the P-40s.
Most P-40s had some short of bomb rack/s and control/cables in the wings. These were often for a something like three 20lb bombs or other light weight weapons in addition to the 500lb bomb that could be fitted under the fuselage. The P-40D & E had both manual release (salvo?) L-21A and electrical release on the B-4 trigger switch and setting the selector switch to "bomb".
Having squadrons overload/modify the racks for larger bombs (250lb or multiples) might mean they were using the exiting cables/controls and not trying to snake brand new cables though the existing wing structure. Or add switches to existing electrical circuits.
The whole P-40 bomb saga seems to be poorly documented.
One manual for the P-40F & L says that the L-21A control quadrant can arm and release the wing bombs from the Q-2 wing racks. Size of bombs are not given but the L-21A can release bombs in pairs up to three times or in salvo (all six at once) and for emergency drops all 6 can released at once in unarmed condition. Information in one P-40N manual is much less informative.
Having such equipment already in the aircraft makes modifying the aircraft to handle larger bombs much easier.
Not having the drop handles and cabling and the safety/arming cables/wires running from the cockpit to the wing racks and not having existing control boxes makes things a lot harder.

Dropping an 'armed' 250-500lb bomb on the home airfield by accident is generally frowned upon. Engine trouble on take-off often calls for jettisoning under wing/under fuselage bombs in 'safe' condition before trying to either land normally or doing a belly landing. Flying around with armed bombs over friendly troops with jury rigged bomb racks is also usually frowned upon by senior officers.
Not saying it cannot be done at all. Saying that jumping through hoops to beat the introduction of the A-36 and P-51A by a few months with a Rube Goldberg bomb system may not be a good use of resources.

 

[MiTasol]

I realize that it's much trickier to add in "wet" hard points on the wings, but hard points just for bombs should be a bit easier, no? I'm envisioning something before the A-36 and P-51A obviously which did have these built in.

No. The plumbing for the fuel tanks is a very minor part of the changes required. You have to modify the ribs and skins and, most likely, spars to carry the weight and drag of the drop tanks.

75 usg of fuel weighs 451lb. Add the tank and you have the same weight as a 500lb bomb but far more frontal area which means more drag which means that the structure must be stronger to carry a 75 usg drop than a 500lb bomb. As a rough rule of thumb drag increases with the square of the frontal area so the increase in drag for a drop tank produces a twisting moment that is very significant.