Merlin 45M boost control

[gecko]

Hi all,

I am running into what appears to be some contradicting data on the Merlin 45M. The pilot's notes for the Spitfire Mk.V state:

"+18 lb/sq.in. boost is obtained, only on "M" type engines, by moving the throttle lever through the gate. On other engines +16 lb/sq.in. boost is obtained by operating the boost control cut-out."

My understanding of the "through the gate" system on other Merlins, such as the Merlin XII and 45, is that it is mainly a feature for takeoff, and functions as an override to the boost controller, by opening the throttle valve farther than the boost controller could compensate for at low level. Thus the maximum boost achievable with the system is obtained at sea level only, and the boost controller gradually reduces the excess boost until it reaches the same level as it would maintain at the gate, and maintains it there to full throttle height.

The performance charts (granted this is for a 50M, but it would seem from the pilot's notes that there is no difference on this point) don't seem to show that this is how the system worked on the Merlin 45M:
http://www.spitfireperformance.com/w3228speed.jpg

Instead +18 lbs is maintained up to almost 6,000ft. So, does the "through the gate" system work completely different on the Merlin 45M? What about the boost control cut-out - is it still fitted, or does the "through the gate" movement of the throttle completely replace it?

Thanks for any help.

 

[Greyman]

Replaced it. Basically the same result, just a different way for the pilot to engage it on later engines (M series, 60 series, etc.)

 

[gecko]

Great, thanks! That makes a lot of sense. So I take it that on these engines +12 lbs is obtained at the gate, as opposed to +9lbs on the Merlin 45? I assume no mixture control for the M series - or is that specific to the new carbs on the 50 series and the 45M would still have one?

 

[Greyman]

+12 at the gate, yes.

I haven't looked into the specifics on the mixture control, but later Mk.V aircraft had none, or at least if the lever was there it was nonfunctional. I'm not sure if this is specifically related to particular engines or if it just coincided with them in the timeframe. I suspect the latter.

 

[gecko]

Thanks! Yeah, I think I agree. My pilot's notes for the Mk.V have very similar wording to what you say about the mixture control, but it covers Merlin 45, 45M, 46, 50, 50A, 50M, 55, and 55M, so there's very little to differentiate small differences between engines. However, my pilot's notes for the Mk.VI also mention removing the mixture control on later aircraft, and it only covers the Merlin 47. So it seems 40 series engines could have their mixture controls removed, and it is likely that the 50 series never had it at all.

BTW, does anyone know what a Merlin 50A was?

 

[Greyman]

Merlin 45 with the enlarged rotor and circular-arc rotating guide vanes of the Merlin 46. Only about 100 built.

 

[gecko]

So, effectively a Merlin 46 (performance wise) with the Bendix-Stromberg carb of the 50 series?