6 ms·

I don't know. If you look at the swan song of big aviation piston engines just before turbines took over (immediate post-WWII era), it was all air-cooled radial

by jabl 4y ago

I don't know. If you look at the swan song of big aviation piston engines just before turbines took over (immediate post-WWII era), it was all air-cooled radials. If the WWII experience would have proved liquid cooled engines to be superior, I guess all those Convairs, Constellations, DC-6's etc. would be using Griffons, Sabres or such.

VLM 4y ago
Classic scalability issue, those engines were HUGE compared to general aviation so all the rules about shock cooling and surface area to volume ratios don't apply unchanged to engines a thirtieth the size.
- jabl 4y ago
  I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as well (correspondingly huger than fins on smaller engines in order to cool the much higher mass) to cool them under normal operations. So those same fins would equally rapidly cool the engine when descending quickly at idle power. So shock cooling is certainly an issue with big radials as well. Now I don't know the exact scaling relation, but I'd guess it's more similar to a smaller engine than a straightforward surface/volume scaling would imply. Or to put it another way, the surface area of the cooling fins is not proportional to the surface area of the cylinders without any fins, but to the power output of the engine.
consp 4y ago
That's survivor bias and sheer production numbers of us bombers, only the us models mainly used air cooled radial engines (bombers and carrier fighters), neither the german nor the UK ones did (mostly, exceptions always exist).
- jabl 4y ago
  > That's survivor bias and sheer production numbers of us bombers To an extent yes, but the US certainly produced liquid cooled aircraft engines as well, like the Allison's and Packard Merlins powering many famous WWII aircraft. If they would have considered liquid cooled engines obviously superior, I'm sure they would have preferred those in post-WWII piston aircraft. > neither the german nor the UK ones did (mostly, exceptions always exist). The UK produced nearly 60000 Bristol Hercules radials, practically all used in multi engined aircraft (Beaufighter, Wellington, Stirling, Halifax being the major ones AFAIK). Sure, a much lower production numbers than the famous Merlin at 150000 engines, but far from insignificant. Similarly, Germany produced over 60000 BMW 801 radials, mostly used in the FW190A fighters but some were also used in some Ju88 variants. In comparison the DB 600 engines powering mostly Bf 109 was about 60000 as well, and about 70000 Jumo 21X series used mostly in bombers. So very broadly speaking, of total aircraft engine production, both UK and Germany produced about 1/3 radials and 2/3 liquid cooled.
  - p_l 4y ago
    Allisons and Packard Merlins happened IIRC due to RAF orders and were at least partially related to British Merlin engine. Briston Hercules was fitted to sever plane types due to lack of production capability for the more complex liquid cooled engines, this was specifically a case with Beaufighter which suffered badly for it.
    - jabl 4y ago
      The Packard Merlin was a license produced version of the Rolls-Royce Merlin, famous for powering the P-51. The Allison, OTOH, was AFAIK an entirely US design.
      - deleted 4y ago
        [deleted]
      - p_l 4y ago
        Allison was produced for P-51, which was designed to specification done by RAF and designed around Merlin engine. The reasons for existence of Allison engine were iirc part political part practical (second sourcing, low spin up of license production as Packard team learnt Merlins, low amount of British Merlins available)
        jabl 4y ago
        The Allison was much older than the P-51 airframe, it was used earlier e.g. for the P-39 and P-40. My memory of Mustang history is a bit hazy, but IIRC the Mustang was originally meant to use the Allison and later they switched to the Merlin to get better high altitude performance as the Allison lacked a two-stage supercharger. But maybe you're right that it was originally meant for the Merlin and the Allison was a stopgap until Merlin production ramped up in the states.
        p_l 4y ago
        Ah, I forgot about P-39 and P-40. Pretty sure Merlin was present in the design from start, or at least consideredin contract if not manufactured from start with it (I somehow doubt the RAF would have accepted the high altitude performance of Allison engine)
- jimnotgym 4y ago
  Indeed the liquid cooled Rolls Royce Griffon engined Avro Shackleton was in service until 1991 with the Royal Air Force https://en.m.wikipedia.org/wiki/Avro_Shackleton https://en.m.wikipedia.org/wiki/Avro_Shackleton
samatman 4y ago
I worry about people who use a word like 'superior' without any dimensions. I can think of plenty of reasons the simpler design with one less failure mode might dominate production during the war, without being the obviously better choice given peacetime applications and seventy years of technical advances.
- Gibbon1 4y ago
  I worry when there is a hidden assumption that people were dummies for the choices they made. That tends to be more true for policy than engineering. Distilled impression. Water cooled inline engines. Small frontal area means lower drag. Water cooled provides a forgiving flexible operating envelope. What you want in a fighter. Downside, more complicated. Upside better performance means kill enemy better. Air cooled radials. The drag penalty is less as the engine size increases. More reliable as long as you stay inside the design envelop. Scales up better than inline water cooled engines. Better suited for multi-engined bombers with a well defined mission profile. At the end of the war though it was obvious that turbojets were the future for fighters. Which is why water cooled engines went away. Radials though are more efficient at lower speeds than early turbojets which is why they persisted.
  - jabl 4y ago
    I don't think it's that simple. Inline engines have less frontal area, yes, but then you need a radiator which will add back quite a bit of drag. Further, as Kurt Tank demonstrated with the, at the time, revolutionary FW 190, it's possible to make a very low drag radial engine installation. These ideas were then used in the Tempest II and Sea Fury, which were among the fastest piston engined planes ever made. Further, I'd say in history there's a lot of path dependence here (in aviation engine development as well as elsewhere, obviously) as well rather than 'pure' arguments based on the best option for a from scratch design. As it wasn't clear at the time which engine type would 'win', major powers developed both lines in parallel, if for no other reason but to not be so much behind their competitors in case some development would cause one of the engine types to clearly pull ahead. WWII showed that both air cooled radial and water cooled inlines could be used for top of the line fighters for the entire conflict. After the war, it was clear that for speed jets were going to be the future and there wasn't the wartime pressure to keep any and all production lines going at maximum capacity, so aircraft designers had more leeway in choosing powerplants. Perhaps it was because US multi-engine aircraft designers were familiar with radials and it was clear to them they were plenty good enough, perhaps it was better reliability, but radials they chose (with some exceptions, obviously).
dTal 4y ago
A) the highest performance aircraft were all liquid cooled B) combat aircraft have to deal with bullet holes
- jabl 4y ago
  > A) the highest performance aircraft were all liquid cooled Well, how do you define high performance? Judging by speed at least, Tempest Mk II, Sea Fury, Bearcat, and P-47M are among the fastest piston engined planes ever made. > B) combat aircraft have to deal with bullet holes Sure. But an engine that lacks a critical subsystem by design (say, a liquid cooling system with pumps, pipes, radiators etc.), all else being equal, is less failure prone than an engine that has it. Whether that failure happens due to shells or mechanical failure.

by jabl 4y ago

VLM 4y ago

Classic scalability issue, those engines were HUGE compared to general aviation so all the rules about shock cooling and surface area to volume ratios don't apply unchanged to engines a thirtieth the size.

jabl 4y ago
I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as well (correspondingly huger than fins on smaller engines in order to cool the much higher mass) to cool them under normal operations. So those same fins would equally rapidly cool the engine when descending quickly at idle power. So shock cooling is certainly an issue with big radials as well. Now I don't know the exact scaling relation, but I'd guess it's more similar to a smaller engine than a straightforward surface/volume scaling would imply. Or to put it another way, the surface area of the cooling fins is not proportional to the surface area of the cylinders without any fins, but to the power output of the engine.

consp 4y ago

That's survivor bias and sheer production numbers of us bombers, only the us models mainly used air cooled radial engines (bombers and carrier fighters), neither the german nor the UK ones did (mostly, exceptions always exist).

jabl 4y ago
> That's survivor bias and sheer production numbers of us bombers To an extent yes, but the US certainly produced liquid cooled aircraft engines as well, like the Allison's and Packard Merlins powering many famous WWII aircraft. If they would have considered liquid cooled engines obviously superior, I'm sure they would have preferred those in post-WWII piston aircraft. > neither the german nor the UK ones did (mostly, exceptions always exist). The UK produced nearly 60000 Bristol Hercules radials, practically all used in multi engined aircraft (Beaufighter, Wellington, Stirling, Halifax being the major ones AFAIK). Sure, a much lower production numbers than the famous Merlin at 150000 engines, but far from insignificant. Similarly, Germany produced over 60000 BMW 801 radials, mostly used in the FW190A fighters but some were also used in some Ju88 variants. In comparison the DB 600 engines powering mostly Bf 109 was about 60000 as well, and about 70000 Jumo 21X series used mostly in bombers. So very broadly speaking, of total aircraft engine production, both UK and Germany produced about 1/3 radials and 2/3 liquid cooled.
- p_l 4y ago
  Allisons and Packard Merlins happened IIRC due to RAF orders and were at least partially related to British Merlin engine. Briston Hercules was fitted to sever plane types due to lack of production capability for the more complex liquid cooled engines, this was specifically a case with Beaufighter which suffered badly for it.
  - jabl 4y ago
    The Packard Merlin was a license produced version of the Rolls-Royce Merlin, famous for powering the P-51. The Allison, OTOH, was AFAIK an entirely US design.
    - deleted 4y ago
      [deleted]
    - p_l 4y ago
      Allison was produced for P-51, which was designed to specification done by RAF and designed around Merlin engine. The reasons for existence of Allison engine were iirc part political part practical (second sourcing, low spin up of license production as Packard team learnt Merlins, low amount of British Merlins available)
      - jabl 4y ago
        The Allison was much older than the P-51 airframe, it was used earlier e.g. for the P-39 and P-40. My memory of Mustang history is a bit hazy, but IIRC the Mustang was originally meant to use the Allison and later they switched to the Merlin to get better high altitude performance as the Allison lacked a two-stage supercharger. But maybe you're right that it was originally meant for the Merlin and the Allison was a stopgap until Merlin production ramped up in the states.
        p_l 4y ago
        Ah, I forgot about P-39 and P-40. Pretty sure Merlin was present in the design from start, or at least consideredin contract if not manufactured from start with it (I somehow doubt the RAF would have accepted the high altitude performance of Allison engine)
jimnotgym 4y ago
Indeed the liquid cooled Rolls Royce Griffon engined Avro Shackleton was in service until 1991 with the Royal Air Force https://en.m.wikipedia.org/wiki/Avro_Shackleton https://en.m.wikipedia.org/wiki/Avro_Shackleton

samatman 4y ago

I worry about people who use a word like 'superior' without any dimensions. I can think of plenty of reasons the simpler design with one less failure mode might dominate production during the war, without being the obviously better choice given peacetime applications and seventy years of technical advances.

Gibbon1 4y ago
I worry when there is a hidden assumption that people were dummies for the choices they made. That tends to be more true for policy than engineering. Distilled impression. Water cooled inline engines. Small frontal area means lower drag. Water cooled provides a forgiving flexible operating envelope. What you want in a fighter. Downside, more complicated. Upside better performance means kill enemy better. Air cooled radials. The drag penalty is less as the engine size increases. More reliable as long as you stay inside the design envelop. Scales up better than inline water cooled engines. Better suited for multi-engined bombers with a well defined mission profile. At the end of the war though it was obvious that turbojets were the future for fighters. Which is why water cooled engines went away. Radials though are more efficient at lower speeds than early turbojets which is why they persisted.
- jabl 4y ago
  I don't think it's that simple. Inline engines have less frontal area, yes, but then you need a radiator which will add back quite a bit of drag. Further, as Kurt Tank demonstrated with the, at the time, revolutionary FW 190, it's possible to make a very low drag radial engine installation. These ideas were then used in the Tempest II and Sea Fury, which were among the fastest piston engined planes ever made. Further, I'd say in history there's a lot of path dependence here (in aviation engine development as well as elsewhere, obviously) as well rather than 'pure' arguments based on the best option for a from scratch design. As it wasn't clear at the time which engine type would 'win', major powers developed both lines in parallel, if for no other reason but to not be so much behind their competitors in case some development would cause one of the engine types to clearly pull ahead. WWII showed that both air cooled radial and water cooled inlines could be used for top of the line fighters for the entire conflict. After the war, it was clear that for speed jets were going to be the future and there wasn't the wartime pressure to keep any and all production lines going at maximum capacity, so aircraft designers had more leeway in choosing powerplants. Perhaps it was because US multi-engine aircraft designers were familiar with radials and it was clear to them they were plenty good enough, perhaps it was better reliability, but radials they chose (with some exceptions, obviously).

dTal 4y ago

A) the highest performance aircraft were all liquid cooled B) combat aircraft have to deal with bullet holes

jabl 4y ago
> A) the highest performance aircraft were all liquid cooled Well, how do you define high performance? Judging by speed at least, Tempest Mk II, Sea Fury, Bearcat, and P-47M are among the fastest piston engined planes ever made. > B) combat aircraft have to deal with bullet holes Sure. But an engine that lacks a critical subsystem by design (say, a liquid cooling system with pumps, pipes, radiators etc.), all else being equal, is less failure prone than an engine that has it. Whether that failure happens due to shells or mechanical failure.