# What does M Ratio and S Ratio mean in the RAF pilot's notes?



## NatanielHewelt (Feb 21, 2021)

In the pilot's notes for both the Hurricane (AP1564B) and Spitfire (AP1565E) there is no mention of what M and S ratio actually pertains to. There is no explanation I could find in pilot's notes general (AP2095 ) either. 

Any help in understanding exactly what this setting is would be greatly appreciated! 

Facsimile of Page 31, AP1564B


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## pbehn (Feb 21, 2021)

The supercharger had two ratios though why they use the letters "M" and "S" is not clear to me. Quote "For low speed operation, the supercharger turns at 6.391 times crankshaft speed, for high speed the blower spins at 8.095 times crankshaft speed. The supercharger is driven through a gear train coupled to the spring drive through the supercharger driving gear in the wheelcase" from here Inside The Merlin Engine

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## wuzak (Feb 21, 2021)

pbehn said:


> The supercharger had two ratios though why they use the letters "M" and "S" is not clear to me.



M = Medium supercharged - that is, for medium altitudes.
S = fully Supercharged - for high altitudes.
L = Low supercharged - for low latitudes.

Not many engines had the L designation, but the 100 series Griffons had a 3 speed supercharger, with Low Medium and Fully Supercharged gears.

The reason that S was used was because the first supercharged engines were given that designation.

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## NatanielHewelt (Feb 22, 2021)

Thank you so much for your replies! I can sleep easy now.


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## wuzak (Feb 22, 2021)

I got to thinking about single speed engines and whether they were rated as Medium or fully supercharged, so I checked some ratings in Lumsden, _British Piston Aero-Engines and their Aircraft_.

It could be the case that several variants of a particular engine would have the same rating, even though they had different mark numbers.

The mark numbers would change for different reduction gear ratios, maybe different accessories, or a number of other reasons.

The Merlin 21 used in the Mosquito used reverse flow cooling, but it had the same rating as the Merlin XX - RM.3SM

R = Rolls-Royce
M = Merlin
3 = engine rating
S = fully supercharged
M = medium supercharged

This was a 2 speed engine.

For early Merlins, the Merlin III was rated RM.1S and the Merlin VIII RM.3M.

The Merlin X, a 2 speed engine, was rated RM.1SM and the Merlin XII (single speed) RM.3S.

The Merlin 45 was rated RM.5S and the Merlin 45 RM.5M.

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## SaparotRob (Feb 22, 2021)

wuzak said:


> I got to thinking about single speed engines and whether they were rated as Medium or fully supercharged, so I checked some ratings in Lumsden, _British Piston Aero-Engines and their Aircraft_.
> 
> It could be the case that several variants of a particular engine would have the same rating, even though they had different mark numbers.
> 
> ...


Impressive.


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