Merlin 45: was overboosting feasible before second half of 1942?

[tomo pauk]

Hello, folks,

As noted in another thread, the Melrlin 45 was over-boosted in 1942 (+16 lbs/sq in, previously only 9,3 lbs/sq in). Flight tests with over-boosted engines were conducted from July to September of 1942. So, my questions are: was it feasible before that date? Does anybody know what major/minor changes on the engine were undertaken?

The benefits of the over-boosting were noticeable mostly at lower altitudes, where that improvement was feasible anyway. The full throttle height for that rating (noted as 'combat rating') was, on climb, 8800 ft, and, at max speed, 13000 ft. The speed gain was, on 13000 ft, 35 mph, and the RoC was increased, at 8800 ft, some 1050 ft/min (now being 3710 ft/min at that altitude).

 

[RCAFson]

Hello, folks,

As noted in another thread, the Melrlin 45 was over-boosted in 1942 (+16 lbs/sq in, previously only 9,3 lbs/sq in). Flight tests with over-boosted engines were conducted from July to September of 1942. So, my questions are: was it feasible before that date? Does anybody know what major/minor changes on the engine were undertaken?

The benefits of the over-boosting were noticeable mostly at lower altitudes, where that improvement was feasible anyway. The full throttle height for that rating (noted as 'combat rating') was, on climb, 8800 ft, and, at max speed, 13000 ft. The speed gain was, on 13000 ft, 35 mph, and the RoC was increased, at 8800 ft, some 1050 ft/min (now being 3710 ft/min at that altitude).

The Merlin 45 always had 12lb overboost.

 

[tomo pauk]

Care to point me to an easily accessible source?

 

[RCAFson]

Care to point me to an easily accessible source?

see:
Spitfire Mk V AA.878 Report

Section 2.2. part 2, (iii).

 

[wuzak]

+12 boost was authorised for the Merlin II/III in March 1940, when using 100 octane fuel.

 

[tomo pauk]

Thanks, I'm aware of that. However, I'm after the Merlin 45 data

see:
Spitfire Mk V AA.878 Report

Section 2.2. part 2, (iii).

That is not in agreement with 'The Merlin 45 always had 12lb overboost' statement; the report covers the results of the test being done between July and Sept of 1942, while the M 45 was in use a full year (or more?) before that.

 

[Hop]

Letter to the British Air Commission from the Ministry of Aircraft Production, dated 3 January 1942, setting out requirements for 100 octane fuel:

Merlin 45, 46, 47, covering large proportion of 1942 production fighters introduced in 100 O.N. fuel with emergency boost of 54.5 inch now increased to 60.5 inch.

 

[RCAFson]

Thanks, I'm aware of that. However, I'm after the Merlin 45 data



That is not in agreement with 'The Merlin 45 always had 12lb overboost' statement; the report covers the results of the test being done between July and Sept of 1942, while the M 45 was in use a full year (or more?) before that.

The report states: (iii) Max for all out level in special emergency (prior to increase to +16) +12

It pretty clearly states that +12 was in effect prior to +16.

 

[Vincenzo]

but this is not a pre 2nd half 1942 situation. the report give us info that in july (2nd half) the 12 lbs was in effect

 

[Hop]

The Ministry of Aircraft Production letter says the Merlin 45 was "introduced" with 12lbs boost, and that's dated 3 January 1942.

 

[RCAFson]

The Ministry of Aircraft Production letter says the Merlin 45 was "introduced" with 12lbs boost, and that's dated 3 January 1942.

Yes, and even the Merlin XII (Spitfire II), which was the predecessor of the 45 had a 12lb 5min combat rating.

 

[tomo pauk]

Thanks for the feedback. Is there some info about the steps taken to ensure that the M 45 was working properly at +12 lb boost, and later at +16 lb boost?

 

[RCAFson]

Thanks for the feedback. Is there some info about the steps taken to ensure that the M 45 was working properly at +12 lb boost, and later at +16 lb boost?

I don't quite understand what you are trying to ask. Are you asking what modifications were made to ensure that the engine could run reliably at 12 and 16lb boost?

 

[Shortround6]

The Merlin III was even rated at 16lb boost for Hurricane CAM fighters but then they didn't really expect those engines to last long anyway

Boost limits may have been "adjusted" as more experience was gained without much, if any modifications to the engines. Please remember that the ACTUAL limit on the engine is the "gross horsepower" and not the "net" or propeller HP. Friction and pumping losses PLUS power needed to drive the supercharger added to the propeller hp. Also temperature of the intake charge plays a part. In some cases the Merlin was allowed to use on boost rating in one gear but another limit (2lbs different?) in another gear or in some cases engines using different supercharger drive gears were allowed to use different boost limits?

 

[tomo pauk]

Yep, the data can be easily found at the Mike Williams' site (Hurricane II can use higher boost for the low gear, but not tat high for the high gear).

I don't quite understand what you are trying to ask. Are you asking what modifications were made to ensure that the engine could run reliably at 12 and 16lb boost?

That's what I'm trying to find out

 

[RCAFson]

Yep, the data can be easily found at the Mike Williams' site (Hurricane II can use higher boost for the low gear, but not tat high for the high gear).



That's what I'm trying to find out

There was nothing preventing pilots from using the Merlin XX with 12lb boost, even in high gear. I'm fairly certain that advisory warnings not to do so, went out the window in life and death situations.

The same site has this article:
http://www.wwiiaircraftperformance.org/merlin-lovesey.pdf
which is an excellent summary of Merlin development and mods required for higher boost.

 

[Shortround6]

The Merlin XX seems to be a cross between the Merlin XII and the the 2 speed Merlin X with the new supercharger thrown in. There are a number of differences between these engines and the Merlin III. Longer cylinder spigots, strengthened side panels, deeper main bearing caps and wider piston rings are some of the differences from a Merlin II or III.

Merlin XIIs were rated for 12lb boost for take-off as were the Merlin XX from the start. The Merlin X was rated for 10lb boost for take-off on 100 octane.

There seems to be little reason to doubt that the Merlin XX could use 12lbs boost as a combat rating from the start.

 

[RCAFson]

BTW, the chart (fig 24) on page 223 of Lovesey states that the Merlin 45 was approved for 16lb in late 1941 (same time as Merlin II/III).

 

[tomo pauk]

There was nothing preventing pilots from using the Merlin XX with 12lb boost, even in high gear. I'm fairly certain that advisory warnings not to do so, went out the window in life and death situations.

The same site has this article:
http://www.wwiiaircraftperformance.org/merlin-lovesey.pdf
which is an excellent summary of Merlin development and mods required for higher boost.

BTW, the chart (fig 24) on page 223 of Lovesey states that the Merlin 45 was approved for 16lb in late 1941 (same time as Merlin II/III).

Thanks for pointing me to read again the Lovesey. If I'm reading the available data right, it took from late 1941 to July of 1942 to start flight tests of the M 45 at +16 psi, while the results were issued on 25th Nov 1942?

As for the limitation for the Merlin 20 series, it took until late 1942 to cancel the limitation (allowing the + 14 and +16 psi for high gear).

 

[RCAFson]

Thanks for pointing me to read again the Lovesey. If I'm reading the available data right, it took from late 1941 to July of 1942 to start flight tests of the M 45 at +16 psi, while the results were issued on 25th Nov 1942?

As for the limitation for the Merlin 20 series, it took until late 1942 to cancel the limitation (allowing the + 14 and +16 psi for high gear).

The RAF was using 12lb boost in combat from March or earlier of 1940, yet I have never seen a set of comprehensive performance tests for the Merlin II/III using 12lb boost. It seems that it simply wasn't RAF policy to conduct comprehensive tests using the combat rating of their various engines until 1942.