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RAF Preference for in line engines vs radials in WW2

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RAF Preference for in line engines vs radials in WW2

Old 23rd Nov 2022, 21:01
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Minimizing the frontal area (and resultant drag) of a big radial engine was non-trivial, but there were several highly successful designs that did that and gave the resultant fighter aircraft impressive top speed.
To name just a few:
F6F Hellcat
F4U Corsair
P-47 Thunderbolt
Japanese A6M Zero
Not forgetting the FW 190A, B, C, F & G
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Old 23rd Nov 2022, 22:50
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I concur with Jheiminga, in that available or proposed engines would be called up in a new design and the airframe is built around it and the other key requirements of the contract. Significant modifications could then be assessed for a different engine type but would ultimately proven by flight test.
As stated above, some engine/airframe combinations were much more successful: the Hercules powered Halifax and Beaufighter than the Merlin-powered versions, for instance. The Hawker Tempest flew with both the Napier Sabre (Mk.V) and Bristol Centaurus(II) and both were extremely capable.

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Old 24th Nov 2022, 00:25
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Originally Posted by Bing View Post
The Secret Horsepower Race by Calum Douglas answers all the possible questions about the engine configuration choice of the British, American, and German manufacturer and is well worth a read. BMW for instance were basically told by the RLM to start building radials despite having no experience and started off by licence building some P&W models.
Interesting to think how the ‘political’ goings on may have been a decider of just what engine brand got to be used.

“..the company, acting under pressure from the Air Ministry, bought the aero-engine division of the bankrupt Cosmos Engineering Company based in the Bristol suburb of Fishponds, to form the nucleus of a new aero-engine operation..”


https://en.m.wikipedia.org/wiki/Bris...oplane_Company
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Old 24th Nov 2022, 02:28
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I seem to remember reading about RR scuppering work on other engine types on the basis that all resources should go towards improving the Merlin. I believe one casualty was an engine by Napier.

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Old 24th Nov 2022, 06:23
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In-line engines were water cooled - they had less frontal area, but had complex cooling systems and could be taken out by a single hit on the cooling system. Some of that drag advantage was given back due to the need for big radiators, plus you couldn't run them very long on the ground prior to takeoff without overheating.
Radial engines had more frontal area, but were air cooled so no plumbing, radiator, etc. and were far more tolerant of battle damage. Radial engined aircraft were preferred for ground attack due to the better tolerance to damage from ground fire, and since cooling airflow was provided by the turning prop, they didn't generally overheat sitting on the ground waiting to takeoff.
One of the major problems with the B-29 during WWII was overheating during the taxi for take off, such that there was a CHT limit laid down for commencing the take off run, reach that and it was back to dispersal, take off was commenced with the cooling flaps fully open and the flight engineer with an eye on the CHT's progressively closed them while barreling down the runway. They learnt that maintenance of the cooling baffles around the cylinders was critical. Certainly could take battle damage though, reports of radials making it home with cylinders missing, as in shot off, not misfiring.
A quick search brought up the Hurricane Mk VII Radial Hurricane with a Bristol Hercules installed

It says it flew with 320 squadron, but I can't find any other information
I think someone is having a leg pull with that site. 320 never flew fighters, they flew Fokker T.VIII, Anson, Hudson and B-25.
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Old 24th Nov 2022, 08:54
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Originally Posted by megan View Post
One of the major problems with the B-29 during WWII was overheating during the taxi for take off, such that there was a CHT limit laid down for commencing the take off run, reach that and it was back to dispersal, take off was commenced with the cooling flaps fully open and the flight engineer with an eye on the CHT's progressively closed them while barreling down the runway. They learnt that maintenance of the cooling baffles around the cylinders was critical. Certainly could take battle damage though, reports of radials making it home with cylinders missing, as in shot off, not misfiring.I think someone is having a leg pull with that site. 320 never flew fighters, they flew Fokker T.VIII, Anson, Hudson and B-25.
Serial NH342 was allocated to a Spitfire. So could not have been a radial Hurricane
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Old 24th Nov 2022, 09:11
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Originally Posted by Anilv View Post
I seem to remember reading about RR scuppering work on other engine types on the basis that all resources should go towards improving the Merlin. I believe one casualty was an engine by Napier.
I don't think history supports that. RR were developing the Merlin and Griffon side-by-side and only abandoned the Peregrine because there was little point in developing the last stretch of a 21Litre Kestrel V12 when they had a better 27LitreV12 (the merlin) which had both more power and more growth potential - it might have made 1200bhp but there was no demand for such an engine. They also abandoned the Vulture and other X-configuration engines because they never did find a solution to the problem of lubricating 4 big ends on the same crank pin at very high power outputs. This didn't stop them following Napier's lead and producing the H-configuration Eagle II, whose twin crankshafts (and thus only two big-ends per crankpin) circumvented the lubrication problem, alongside the Merlin and Griffon.

The engine they DID abandon despite it's evident potential was the 26litre V12 2-stroke Crecy - an engine that could have delivered a Spitfire limited only by Mcrit (~500mph) with 3,000bhp plus 1200lbs of direct exhaust thrust being a very realistic possibility. But the Crecy was mainly abandoned simply because jets were already in development that offered even better performance.

Of course Napier spent pretty well 100% of their effort tinkering with the potentially superb but flawed Sabre. The shear amount of time and effort dissipated in their tinkering was a serious issue at a time when war-survival needs demanded efficient use of all resources. There was a point when they were threatened with compulsory nationalisation and being handed to Rolls Royce (who were a "trusted pair of hands") if they didn't get their act together. But I digress.

On the point of the original question - in the limit a liquid-cooled engine can always be run at higher specific power outputs than an air-cooled one because of the finer control over the thermal dynamics of the internals. High-power air-cooled engines need to have wider fits and clearances when cold to allow for the poorer control of running temperatures, which is one of the reasons why they often belch so much smoke (burned oil) during start-up and warm-up compared to liquid-cooled ones

£0.02 supplied,

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Old 24th Nov 2022, 15:20
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I think also it was due to industrial history. Rolls-Royce built inline engines. Bristol built radials. In the US, it was really only Allison that built inlines. Wright and Pratt & Whitney built radials and were good at it. Wartime exigency, you use the engines and technology you have, rather than trying to re-invent the wheel. Witness the RR Vulture and Napier Sabre developmental problems. You just develop the ones you have as much as you can, while concentrating on the next gen ie turbojet.
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Old 24th Nov 2022, 16:00
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I'm surprised nobody mentioned the Sea Fury. It had a radial.
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Old 24th Nov 2022, 20:24
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Maybe the U.S. found Rolls Royce's license fees to high?
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Old 24th Nov 2022, 20:45
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Originally Posted by Chu Chu View Post
Maybe the U.S. found Rolls Royce's license fees to high?
Packard seemed to have no problems with the licence fee building 55,000 odd merlins
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Old 24th Nov 2022, 21:34
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Originally Posted by megan View Post
One of the major problems with the B-29 during WWII was overheating during the taxi for take off, such that there was a CHT limit laid down for commencing the take off run, reach that and it was back to dispersal, take off was commenced with the cooling flaps fully open and the flight engineer with an eye on the CHT's progressively closed them while barreling down the runway. They learnt that maintenance of the cooling baffles around the cylinders was critical. Certainly could take battle damage though, reports of radials making it home with cylinders missing, as in shot off, not misfiring.
The big problem with the B-29 engine was that it was a four-row radial (I believe the first ever attempt at a four-row radial) and cooling the back row was very problematic - especially as you note when there wasn't a lot of airflow through the engine.
Having race air-cooled engines, one trick to keep air-cooled engines from overheating was to run them intentionally fuel rich - the extra fuel that can't burn acts to keep the peak temps down (rather counter-intuitive to run more fuel to cool the engine but it works). Of course that has an adverse effect on range...
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Old 24th Nov 2022, 22:13
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Originally Posted by tdracer View Post
The big problem with the B-29 engine was that it was a four-row radial (I believe the first ever attempt at a four-row radial) and cooling the back row was very problematic - especially as you note when there wasn't a lot of airflow through the engine.
Having race air-cooled engines, one trick to keep air-cooled engines from overheating was to run them intentionally fuel rich - the extra fuel that can't burn acts to keep the peak temps down (rather counter-intuitive to run more fuel to cool the engine but it works). Of course that has an adverse effect on range...
The B-29 used the Wright R3350 which is a two row 18 cylinder engine; as you say the rear row initially proved hard to cool. The B-29 was then developed into the B-50 and C-97, both of which used the P&W R4360 with four rows and 28 cylinders - the "corncob". Also problematic in keeping the backmost rows cool. Did any other four row radial engines see production?

Fading memories of seeing/hearing a couple of AFRES KC-97s passing over the UK in the mid-70s, fantastic sound!
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Old 25th Nov 2022, 00:16
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Originally Posted by vegassun View Post
I'm surprised nobody mentioned the Sea Fury. It had a radial.
The Sea Fury was developed from the Tempest, which was originally designed to have the liquid-cooled Napier Sabre (with its Tornado stablemate that was intended to have the liquid-cooled Rolls Royce Vulture). Development issues and delays with the Sabre led to evaluation with the air-cooled Centaurus radial (Tempest II) as the Centaurus was the only other engines of similar power. A perceived demand for a very high performance interceptor for V1s led to the specification for a "fast light fighter" and Hawker responded with a design based on a tempest wing on a cut-down centre section to reduce the span, mounted on a new monocoque fuselage just big enough to carry the driver, a Centaurus engine (lower complexity due to lack of radiators to reduce development risk) and enough fuel for (I think) 60 minutes. They called it the Fury, but A&AEE Martlesham Heath pronounced it "dangerously overpowered" so the RAF didn't want it. The Fleet Air Arm didn't accept that it was possible for an aeroplane to be overpowered, so Hawker navalised it (folding wings, slightly bigger fuel tanks, higher cockpit seat position etc) and the result was the Sea Fury.

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Old 25th Nov 2022, 02:46
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Originally Posted by treadigraph View Post
The B-29 used the Wright R3350 which is a two row 18 cylinder engine; as you say the rear row initially proved hard to cool. The B-29 was then developed into the B-50 and C-97, both of which used the P&W R4360 with four rows and 28 cylinders - the "corncob". Also problematic in keeping the backmost rows cool. Did any other four row radial engines see production?

Fading memories of seeing/hearing a couple of AFRES KC-97s passing over the UK in the mid-70s, fantastic sound!

I’m guessing these engine cooling issues were also a factor in the B29’s noticeable shallow, almost flat climb angle after takeoff

Best forward speed for cooling ?
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Old 25th Nov 2022, 13:40
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B36's featured production, 4-row P&W R4360.

It was also used in Boeing 377, B50, C97, and the Flying Boxcar.

It was terribly maintenance intensive in all of them. But, yes, production 4-row radials existed, even, briefly, in civilian service.
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Old 25th Nov 2022, 14:00
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Originally Posted by tdracer View Post
Having race air-cooled engines, one trick to keep air-cooled engines from overheating was to run them intentionally fuel rich - the extra fuel that can't burn acts to keep the peak temps down (rather counter-intuitive to run more fuel to cool the engine but it works). Of course that has an adverse effect on range...
Funnily enough, many rocket engines use the same trick to stop the nozzles melting. Nothing new under the sun!
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Old 25th Nov 2022, 20:39
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Originally Posted by tdracer View Post
The big problem with ... cooling the back row was very problematic - especially as you note when there wasn't a lot of airflow through the engine.
Sounds like my father's Ariel Square Four Mk II...
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Old 25th Nov 2022, 21:11
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Originally Posted by treadigraph View Post
The B-29 used the Wright R3350 which is a two row 18 cylinder engine; as you say the rear row initially proved hard to cool. The B-29 was then developed into the B-50 and C-97, both of which used the P&W R4360 with four rows and 28 cylinders - the "corncob". Also problematic in keeping the backmost rows cool. Did any other four row radial engines see production?
Yea, I stand corrected - apparently most B-29s (and all earlier build) had the two row R3350 engine, with the Pratt R4360 coming along later.
I visited an air museum last year that had a B-29 on display (sort of the crown jewel of their collection). It had the four row R4360 engines (one engine was sitting on a display stand next to the aircraft) - I mistakenly assumed that was the typical configuration.
At any rate, cooling the back row(s) on a big radial engine proved problematic, a problem big, water-cooled engines don't have.
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Old 25th Nov 2022, 22:48
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Originally Posted by tdracer View Post
Yea, I stand corrected - apparently most B-29s (and all earlier build) had the two row R3350 engine, with the Pratt R4360 coming along later.
I visited an air museum last year that had a B-29 on display (sort of the crown jewel of their collection). It had the four row R4360 engines (one engine was sitting on a display stand next to the aircraft) - I mistakenly assumed that was the typical configuration.
At any rate, cooling the back row(s) on a big radial engine proved problematic, a problem big, water-cooled engines don't have.
Think you will find that your "B-29" was actually a B-50. No B-29 was ever fitted with the R-4360 engines, and I suspect the aircraft you viewed had the extremely tall fin/rudder assembly which was vital to all B-50s. Perhaps the person at the museum was not too well informed on American military aircraft of the 1950s, and just got it wrong with their caption. A B-50 is still rather B-29 (ish) after all!
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