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

Discussion in 'Aviation' started by tomo pauk, Sep 22, 2012.

  1. tomo pauk

    tomo pauk Creator of Interesting Threads

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    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).
     
  2. RCAFson

    RCAFson Well-Known Member

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    The Merlin 45 always had 12lb overboost.
     
  3. tomo pauk

    tomo pauk Creator of Interesting Threads

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    Care to point me to an easily accessible source?
     
  4. RCAFson

    RCAFson Well-Known Member

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    see:
    Spitfire Mk V AA.878 Report

    Section 2.2. part 2, (iii).
     
  5. wuzak

    wuzak Well-Known Member

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

    tomo pauk Creator of Interesting Threads

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    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.
     
  7. Hop

    Hop Member

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    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.
     
  8. RCAFson

    RCAFson Well-Known Member

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    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.
     
  9. Vincenzo

    Vincenzo Active Member

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    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
     
  10. Hop

    Hop Member

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

    RCAFson Well-Known Member

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

    tomo pauk Creator of Interesting Threads

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    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?
     
  13. RCAFson

    RCAFson Well-Known Member

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    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?
     
  14. Shortround6

    Shortround6 Well-Known Member

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    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?
     
  15. tomo pauk

    tomo pauk Creator of Interesting Threads

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    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 :)
     
  16. RCAFson

    RCAFson Well-Known Member

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    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.
     
  17. Shortround6

    Shortround6 Well-Known Member

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    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.
     
  18. RCAFson

    RCAFson Well-Known Member

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    #18 RCAFson, Sep 26, 2012
    Last edited: Sep 26, 2012
    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).
     
  19. tomo pauk

    tomo pauk Creator of Interesting Threads

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    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).
     
  20. RCAFson

    RCAFson Well-Known Member

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    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.
     
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