Allison and Merlin in a P-51

[ColFord]

Nice post. I wonder if 430 squadron R.C.A.F. doesn't deserve some mention here. They were still equipped with Mustang I when they started converting to Spitfire XIV in Nov 44.

Oh, I think if you read the Operations Record Books of 2 and 430 Squadrons you'll find that the Spitfire XIV equipped units acquitted themselves quite admirably in the Tac/R role. They could also engage enemy aircraft successfully when opportunities presented themselves such as here and here and here and here and here

No.430 (RCAF) Squadron ceased operating the North American Mustang Mk.I, rather abruptly on the morning of 1 January 1945. So they were not operating the Mustang by VE-Day, which was the context in which the other units were listed. Their final sortie using Mustang Mk.I was on 31 December 1944 - see below.

I've read all the ORBs and all the combat reports of all the Army Co-operation and Tactical Reconnaissance Squadrons, No.II(AC) RAF and No.430 (RCAF) Squadrons included. I've also read the original trials reports and assessments on the various aircraft put forward from 1940 to 1945 for the AC-Tac/R role, plus had the priviledge to know a number of the AC-Tac/R pilots who flew from Hawker Hectors and Westlands Lysanders through Tomahawks, Hawker Hurricanes, Mustangs 1 thru IA and II (and Merlin engined types as well), Hawker Typhoon FR.1b to Spitfire FR.XIVe, and been able to get their personal appreciation of the types. To a number of them the Spitfire FR.XIVe was a nice new type, had some particular performance advantages, but compared to the Mustang Mk.II that was the staple of the remaining Allison Mustangs at the time, was a step backwards. Didn't have the same range, didn't have a vertical camera (some of the Mustang Mk.II by early 1945 were equipped with 5 cameras - 2 oblique each side and a vertical to permit horizon to horizon full coverage on a single pass), was trickier to take off and land (both a consequence of its narrower undercarriage and the torque of the prop on take-off - and don't throw the power on to do a go around on landing), and visibility over the nose of the Griffon was worse than over the nose of the Allison. Didn't stop the Allison Mustang boys remaining having a go when the opportunity presented itself, Lyke and Mercer on Mustang Mk.IA on 1 January 1945 (same early morning sorties as Packwood and Young of Shiney Two out of Gilze), or Perkins in a Mustang Mk.II in February 1945 on a Me-262 (damaged) and a FW-190 (claimed damaged upgraded to destroyed). The Spitfire FR.XIVe were good at what they did, but could not do all the required tasking that could be done by the Mustang Mk.IA and particularly the Mustang Mk.II. When the Wing Ops staff were making decisions on the tasks to be accepted from the Army demands, and then handing out the tasks to the Squadrons, they had to take account of the capabilities of the aircraft types they had, and the Mustang Mk.IA and Mk.II could - according to those who were actually there at the time - do more of the potential demands than the Spitfire FR.XIVe.

 

[BarnOwlLover]

Just reading though this thread, and I found it interesting that the RAF seemed to want an advanced Allison P-51 for the low altitude tactical fighter/recon role. I do wonder if a slightly reduced range Mustang with a low altitude Merlin (like maybe the 32) with the cannon armament and cameras could've worked?

 

[Shortround6]

Just reading though this thread, and I found it interesting that the RAF seemed to want an advanced Allison P-51 for the low altitude tactical fighter/recon role. I do wonder if a slightly reduced range Mustang with a low altitude Merlin (like maybe the 32) with the cannon armament and cameras could've worked?

The Allison and the Merlin weren't quite as interchangeable as it might seem. It wasn't particular hard but they needed different radiators and oil coolers (look at P-40F radiator cores vs Allison) and the weights were off a bi, for some reason Packard Merlin's were heavier than British Merlin's (as least the single stage ones). The two stage Merlins needed a 3rd radiator and oil cooler set up.

What you may not have is the engineering support and draftsmen to work on the "simple" Merlin conversion without delaying the two stage project. Lashing together a prototype might be one thing. doing production drawings maybe something else.

I am also not sure it gets you much of anything. The Merlin 32 was good for 1640hp at 18lbs of boost at 2000ft. Late model Allison's were allowed to run 1500hp 52in (11lbs?) in A-36s at 2000ft and 1580hp in P-40Ks at 2500ft using 60in (15lbs?) (different gear ratios for the supercharger).
Allisons were famous for being over boosted but switching engines just to get an 40-120hp at 2000ft over what the Allisons would give (if you used the right Allison) doesn't seem to be worth the trouble.

 

[BarnOwlLover]

What I'm getting at is the comment about the British wanting more Allison Mustangs (a somewhat advanced variant) with a more powerful engine, heavier armament and either a bulged, sliding hood or maybe even a bubble canopy. However, by the time that was determined, the Allison Mustangs were out of production for several months and NA was only making Merlin Mustangs.

What I'm suggesting is using a Merlin Mustang as the basis for such a project with the Merlin 32, or a boosted variant optimized for the low alt. role?

 

[Shortround6]

A Merlin 32 was about 1430lbs, the engines in the P-51B-D went about 1675-1715lbs.
You don't need the same size prop, you don't need the size radiator and radiator duct (you don't have the intercooler for one thing, Trying to cool a 1600hp engine at 2000ft is a lot easier than trying to cool a 1500hp engine at 19-20,000ft is another)

BTY a P-51D will make 1720hp at 18 1/4lbs boost at 6200ft so the Merlin 32 was around 100hp lower in power.
The Merlin 32 used a cropped impeller much like the Cropped Merlin 45s ( 9.75in impeller vs a 9.50 in impeller but using the 8.588 supercharger gear instead of the 9.089 supercharge gear.)
Maybe you could do it but why??
You have to take a lot of stuff back out or ballast the heck out the thing to get the CG right. It will only be useful at low altitude and it won't be any better than an unmodifed P-51D.
You have to take the vertical splice back out to get the fuselage size/cross section back to the size of the Allison version and fit the smaller air scoop radiator tunnel to get the lower drag of the Allison version.

The -7 engine in the P-51D was a "low altitude" version of the -7 engine used in the P-51Bs.

 

[tomo pauk]

The -7 engine in the P-51D was a "low altitude" version of the -7 engine used in the P-51Bs.

(my bold)
A typo - the bolded part should be 'of the -3'

 

[BarnOwlLover]

Maybe you could do it but why??

I wonder that myself, given that the Merlin Mustangs were more than competent performers at low altitude. The B/C/D/K (Mustang III/IV) based on what I read at World War II aircraft performance could do around 380 mph at sea level (few prop planes were faster at the time, such as the Hawker Tempest and the lightweight Mustang prototypes), and the P-51H could do almost 415 at sea level (but wasn't available until 1945, and not until after VE day, and there was never a FR version of it).

The RAF did consider using Mustang IIIs and IVs for the tactical fighter/recon role, but Fighter Command got priority so only a handful were ever used for any recon role (mostly high altitude), and it seems they couldn't source any F-6C/D/K's, either.

 

[ThomasP]

I ran across an interesting bit of info re P-51D used as FR. It is from Wiki but seems to be accurate. It seems CAC (Commonwealth Aircraft Corporation) in Australia was contracted to build ~120x P-51D airframes post-war. The first of the batch were designated Mk 21:

"the first 14 Mark 21s were converted to fighter-reconnaissance aircraft, with two F24 cameras in both vertical and oblique positions in the rear fuselage, above and behind the radiator fairing; the designation of these modified Mustangs was changed from Mark 21 to Mark 22. An additional 14 purpose-built Mark 22s, built after the Mark 23s, and powered by either Packard V-1650-7s or Merlin 68s, completed the production run."

 

[pbehn]

I wonder that myself, given that the Merlin Mustangs were more than competent performers at low altitude. The B/C/D/K (Mustang III/IV) based on what I read at World War II aircraft performance could do around 380 mph at sea level (few prop planes were faster at the time, such as the Hawker Tempest and the lightweight Mustang prototypes), and the P-51H could do almost 415 at sea level (but wasn't available until 1945, and not until after VE day, and there was never a FR version of it).

The RAF did consider using Mustang IIIs and IVs for the tactical fighter/recon role, but Fighter Command got priority so only a handful were ever used for any recon role (mostly high altitude), and it seems they couldn't source any F-6C/D/K's, either.

Not everything about an Alisson Mustang was about top end performance. In a piece about their operation it said that the Alisson ran smoother at very low revs @ low altitude which was an advantage on operations. I presume that is a part of an engine working better at low altitudes.

 

[BarnOwlLover]

Which is why the RAF study in 1944 - after D-Day - concluded that the best replacement for the Allison Mustang for the Tac/R role with the RAF, was another Allison Mustang. In particular they wanted what was essentially a Mustang Mk.II, with the 4 x 20mm cannon armament of the Mk.IA, fitted with a Malcolm Hood, and fitted with twin oblique and one vertical camera mount. Given that they could not get what they wanted as NAA had closed the Allison engine Mustang production line when Merlin engine Mustang production commenced they had to look at alternatives.

Is there a place where this info came from, namely if there's more detail info and specs? And what of the Mustang IIIs the RAF modified for high altitude PR duties?

 

[GregP]

Regards the Allison in the P-51

Could the Merlin 2-Stage have been mated to the Allison without much fanfare.?

If that WAS possible, would the P-51 have the same gas mileage with the Allison as the Merlin.

Could Allison have kept up with the demand for Allison motors, in a P-51 from 1944 onward.?
Thank You

You would not need a redesigned accessory housing, you would need an adapter plate with Allison holes on one side and Merlin holes on the other side. Joe Yancey made one to adapt an Allison G-series power section to an F-series accessory section. It worked and the engine runs great. He ran it at Reno in a Yak called "Full Noise" in 2017. The engine still runs.

Joe also has two VERY rare Allison tidbits.

He has one two-speed supercharger section and one intake with the connection to a top-mounted intercooler, like the Merlin with the intercooler box on top. They didn't make many, but they DID make at least ONE each since Joe has a set.

 

[MIflyer]

I recall reading a letter to Air and Space Magazine back in the early 90's that said the writer had worked on a project to enable the USAF to stop paying the $1500 royalty for each Merlin to RR and replace the engines on F-51's with R-2800's.

What an abortion that would have been!

 

[ColFord]

Is there a place where this info came from, namely if there's more detail info and specs? And what of the Mustang IIIs the RAF modified for high altitude PR duties?

Sorry for not replying earlier, but I was away overseas when this thread started up again and am only now catching up with all the various aviation themed forums I am involved with. And finishing up a book project as well.

All that information came from many years of research in the UK Archives, across various collections within the Archives related to the RAF's acquisition and use of the Mustang in both its original Allison engine version and the later Merlin engine versions. Documentation spread across many files. That was then overlaid with letters, faxes (yes that's how far back the research goes), then emails, interviews, discussions with RAF and Commonwealth Air Force aircrew and ground crew who flew and maintained Mustangs operating with the RAF during WW2. Access to a lot of original documentation from them as well.

The specifics of the specifications for an aircraft for the low to medium Tactical Reconnaissance role, was in a number of staff papers and reports done by the staff of 2TAF, with input from senior personnel and pilots in the Tactical Reconnaissance Wings and Squadrons. Two pilots I knew were involved in the trials of Merlin engine Mustang III & IV aircraft in comparison with other types for their suitability for the Tac/R role. They looked at things such as potential for most likely a Mustang III fitted with a low altitude rated Merlin, but the Merlins power characteristics, the fuel consumption rate and the revs the engine would need to run at to fit the role didn't stack up. There was much back and forth between 2TAF and Air Ministry, with input from ADGB and others, and the issue of availability of Mustang III airframes, either new or 'used' for the role - given they were receiving the Mustang IV by then and the priority was for the long range fighter squadrons, it was a no go. There were also other factors in relation to the comparative performance of the Allison engine Mustang airframe versus the Merlin engined Mustang airframes at low altitude, that made the relatively lighter and 'cleaner' Allison Mustang more suited for the job.

One of the things that often doesn't get appreciated in the consideration of Allison vs Merlin is the different characteristics of the two engines, at what revs they produced particular horsepower to attain certain airspeeds at various altitudes and at those rev ranges what vibration the engine may be producing that can be transmitted to the airframe and on to the reconnaissance cameras - camera damping using rubber bushings and other methods can only go so far and then introduces other issues. Just happened that at the best airspeed ranges for a low level Tac/R run, the Allison engine ran smoother compared to the Merlin, and was burning less fuel to do it.

Continuation of Allison engine Mustang production when the Merlin engine Mustangs was introduced was not an option due to a number of factors. Key to that was the amount of Allison engine Mustang tooling that was used in producing the the Merlin engine Mustangs, especially the P-51B/C. If you go through the parts listings for the later P-51D/K you will still find lots of parts with the part serial number prefixes from the earlier Allison engine Mustang variants. Not enough tooling to keep both lines going given the priority given to the Merlin engine Mustangs. There was then also the matter of capacity in the factory, raw materials, etc, etc.

There was only a handful of Mustang IIIs modified for use in the PR role, and it was no means for high altitude use. They were primarily used in the low to medium altitude range, on days where the cloud cover in the target areas made the use of high altitude PR aircraft unsuitable. They were mainly tasked with medium altitude coverage of initially V1 sites, and then tasked with finding V2 launch and support sites in the Netherlands. Again it was largely a range issue why they were used, operating purely on internal fuel, they could operate from the UK over the target areas in northern France, Belgium and the Netherlands. Depending on the cloud cover found in the target area, if it was high enough they would use the vertical camera at medium altitude, if the cloud base was lower, they could use the oblique camera from a lower altitude. That was also taking into consideration the flak types usually found around those sites and keeping yourself out of the worst of it, but close enough for the cameras and lenses to do their job properly. And they retained their armament. Only used for about 5-6 months.

 

[BarnOwlLover]

Not to risk derailing this thread, but how would you think that the P-51H and the other lightweight Mustangs may've fared in the FR role? Granted, history shows that there was no FR developments of the LW Mustangs and looking at cutaways of the P-51H, camera installation would've been difficult for the H, especially a vertical camera because of the radiator exit fairing position.

 

[MIflyer]

You would not need a redesigned accessory housing, you would need an adapter plate with Allison holes on one side and Merlin holes on the other si

Allsion did take a two stage Merlin and couple it remotely to a V-1710, and it worked fine on the test bench - for a short period of time. Vibe situation apparently was bad.

 

[BarnOwlLover]

There was the V-1710-45, which had Allison's take on a two stage supercharger, per P-51B Mustang: NAA's Bastard Stepchild. However, due to how long the supercharger was, the engine would've needed to be mounted at least 2 inches further forward, or the firewall (and maybe the cockpit) moved that much further back. You do have to remember, in fairness, that the two stage Merlin weighed about 350 lbs more than a single stage Merlin or Allison, as did the 4 bladed prop. Hence, ballast had to be moved around to maintain trim and CG.

 

[GregP]

Regards the Allison in the P-51

Could the Merlin 2-Stage have been mated to the Allison without much fanfare.?

If that WAS possible, would the P-51 have the same gas mileage with the Allison as the Merlin.

Could Allison have kept up with the demand for Allison motors, in a P-51 from 1944 onward.?
Thank You

Back in 2016, Joe Yancey (a gifted Allison overhauler) decided to race at Reno. He took a V-1710G engine and decided to mate an F-series accessory housing to it and run it at high MAP. To do that, he fabricated a mating plate and built up the engine. It was installed into Grahame Frew's Yak-9. In 2018, they won Bronze, won Silver, and transferred into Gold. The engine was then removed and a stock F-Series Allison was installed for the Gold Race. They finished last. Had they left the custom-built Allison in it, they likely would not have won Gold, but they would have finished quite a bit farther up in the heat; likely as far forward as second. I won't argue the theoretical placement because it is a "what if" with no answer. Suffice to say the "hot" Allison wasn't run in Gold.

However, the Allison engine was capable of a LOT more power than they ever put to it in stock form, especially if you crank up the manifold pressure. At Reno, in Bronze, they were running more than 80 inches. In Silver, they were running more than 90 inches, and they were also running higher than 3,000 rpm. I could guess the rpm from conversations I heard, but it's not important here.

I am of the opinion that the second-stage Merlin accessory housing, or at minimum a very similar unit, could have been adapted to fit the Allison. Would there have been more to do? Yes, but the only thing stopping them was money and resources. Late-war Allisons were putting out a LOT of horsepower on test stands. Only the promise of jet technology prevented them from getting into production. Of course, it ALSO prevented the Merin / Griffon and almost all other piston engines from being materially improved at the time, too, since the main ingredient for development is money, and the money was going into jet engines.

But the Allison is a conventional V-12. All the standard improvements to it could have been fitted. If they worked on the Marlin, they would have worked on the Allison. The only thing needed was the desire to do it, the parts, and the money to develop it into a running unit.

In fact, it never materialized until 2018 at Reno, and then in single-stage form, but the potential was always there and still is. There is simply no real reason to do it now. There is no racing at Reno anymore and the market for the engine Joe built is about zero at this point, so I seriously doubt anything will ever come of it going forward, especially with parts being so hard to get.

Joe is near to closing his Allison business since the people who fly warbirds are retiring, the expertise to overhaul these engines and accessories is also retiring, the younger generation hasn't really taken up the gauntlet, at least as far as innovation goes, and without people who want to innovate, nothing much gets developed. I'm guessing that the current set of warbirds is near to the last of the WWII warbirds to be restored. Soon, the capability to overhaul these engines and props will simply disappear. It's already down to a trickle. We are very close to the point where, if you want an Allison or a Merlin part, you might have to make it yourself.

I can tell you from experience that people who make parts run into the same sort of issues the original developers ran into. Only the capability to recover from catastrophic failures and continue just isn't really there due to ever-dwindling supplies of engine cores.

 

[Trilisser]

A few comments:

1. Where are the stories of "bad Typhoon wing" coming from? The Tempest has only marginally higher Critical Mach Number than the Typhoon (figures in Eric Brown's Testing for Combat) and according to Mason, a late production Typhoon with a 4-blade prop is only 15 mph slower at 20,000 ft (427 Cs. 442). Their service ceilings differ by only some 1500 ft.

As for handling, there is an American pilot report on the Typhoon and it is very positive on its handling from "ideal" stall to high speeds.

2. If one interpretes the problems (as listed in Whitney) with earlier Lightnings in Europe, the key is poor piloting. For example , they cruised the absolute wrong way of low boost/high rpm when they should done the opposite (high boost/low rpm).

3. According to American tests (in Whitney), the Merlin tolerated much less coolant loss before vapour locking than the Allison.

 

[GregP]

It could be done, the question would be whether it could have been got into production in numbers to make a difference, and if it wasnt better than the Merlin why do it? The P-82 twin Mustang first flew as the war was coming to an end.

"Economy" depends on many things, I have read that the RAF found the the Mustang MkI to be more economical in low altitude low speed conditions because the Allisson engine ran more smoothly at low revs.

This was NOT a question in WWII. If e could make a U.S.-built alternativer, then the decision would have been to do it. At the time, we were nothing if not nationalistic.

I don't think we would have done it simply because we were already making Merlins and there was no desire to spend development dollars to make up an Allison-designed 2-stage when the Merlin was already coming off the lines. I've seen a letter from Allison to the USAAF asking for funds to develop a 2-stage supercharger. It was declined ... the USAAF got exactly what it ordered. About the time the Allison was making really great horsepower, all the development money was going into jets anyway.

 

[tomo pauk]

I don't think we would have done it simply because we were already making Merlins and there was no desire to spend development dollars to make up an Allison-designed 2-stage when the Merlin was already coming off the lines. I've seen a letter from Allison to the USAAF asking for funds to develop a 2-stage supercharger. It was declined ... the USAAF got exactly what it ordered.

That's very interesting. What date is on the letter?