Originally Posted by
Centaurus
Interesting observation. The Pilots Notes for aircraft equipped with Rolls Royce Merlins (Mustang and Lincoln for example, which I flew) in the Limitations section mentioned a minimum of 6o degrees C for coolant. I was never aware of the term "shock cooling" when I flew these types in the Fifties. The term is relatively more modern I believe.
From recollection (always a bit suss with age
) coolant temperatures would rise rapidly after engine start probably within one minute and so the 60C lower limit was not an issue. Probably because all of my experience on these aircraft was in the Australian tropics.
I am not sure if the point about shock cooling of RR Merlin cylinders is valid. My reading of WW2 history where RAF Spitfires and Hurricanes did "scramble" take-offs in Britain mid-winter soon after engine start, would suggest that shock cooling was not an issue for Rolls Royce Merlin liquid cooled engines.
The liquid-cooled WW2 fighters are an interesting case. There was a clear operational requirement for rapid start-up for scrambles, and this requirement was addressed in design features of both the oil & glycol cooling systems and the engines themselves. I'm told it was very, very easy to overheat a spitfire (especially the early ones up to mkV) because the cooling systems were barely adequate at zero airspeed - this was a design feature to speed the warm-up. Similarly the engines had wide clearances to allow them to operate with colder oil than might be ideal. A consequence of this was higher oil consumption* when cold. This was partly mitigated with operational procedures in which aircraft on "standby" would be initially warmed up and then given a few minutes running every 20-30 mins to keep the oil warm (the oil was the bigger concern) and ready for "scrambles".
Of course the main driver for proper warm-up procedures would be the reduced wear and longer engine life. This was not really a consideration in 1940 when few spitfire or hurricane engines ever reached their TBO (a huge number of these aircraft never even got to the 100 hour checks).
* Anyone who ever owned a Coventry Climax engine (whether in an Imp, an Elite or any of the range of racing cars which used them) would be thoroughly familiar with this. The Climax was originally a firepump engine designed to be dragged next to a nearby pond to suck water for fire hozes. So it was very light and also designed to be started and immediately run at a governed 9,000rpm from stone cold. To achieve this reliably required very wide clearances, so these engines were all prodigious oil-burners. It wasn't a fault - it was a feature.