Reduction gear box

Discussion in 'Engines' started by Jenisch, Dec 4, 2012.

  1. Jenisch

    Jenisch Active Member

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    It was used in WWII aircraft? I can understand it's employment in turboprop engines, but not in piston ones. Someone can explain me?
     
  2. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    It will regulate propeller speed to an optium range and prevent the prop (prop tips) from reaching super sonic speeds.
     
  3. gumbyk

    gumbyk Well-Known Member

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    It served the same purpose ina piston engine as it does on a turbo-prop, i.e. allows the engine and propeller to both be at their optimum speeds during cruise.


    The reduction in a piston engine is much lower than in a turbine. I'm not aware of any that are more than 2:1
     
  4. Jenisch

    Jenisch Active Member

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    Yes. And for that the piston engines were generally of low rotation and higher piston displacement. From what I have read however, there were some piston engines (and turboprops, of course), which have high rotation and thus have reduction gears. The factors which most influence the engine power were piston displacement, thermal efficiency and rotation speed. From what I understand, such high rotation engines provided extra power at low speed, I'm correct?
     
  5. Jenisch

    Jenisch Active Member

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    Yes. And for that the piston engines were generally of low rotation and higher piston displacement. From what I have read however, there were some piston engines (and turboprops, of course), which have high rotation and thus have reduction gears. The factors which most influence the engine power were piston displacement, thermal efficiency and rotation speed. From what I understand, such high rotation engines provided extra power at low speed, I'm correct?
     
  6. dobbie

    dobbie Member

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    #6 dobbie, Dec 5, 2012
    Last edited: Dec 5, 2012
    Not sure what you mean concerning "high rotation engines providing extra power". Factors influencing would indeed be the size of the engine in displacement, thermal efficiency, and propeller efficiency. Whether or not the engine is capable of high rpms doesnt figure in all that much, because to spin the prop too fast is going to cause a lot of issues, up to and including the propeller blades departing the aircraft. The larger the prop diameter, the slower you want the propeller rpm because of the tip speed. Look at the B29 for example. 4 big engines turning 4 really big 4 bladed propellers, yet at cruise speed, the prop rpm is about 400.

    A good recip engine is going to be designed to get the max out of the engine at somewhere around 2500-3500 rpm and the reduction gears will depend on the diameter of the propeller. The pitch of the propeller also has a big role to play. A fine pitch setting allows you to run up to takeoff, then as the aircraft heads to and reaches its cruising altitude, the pitch is progressively changed to a more coarse setting, taking a bigger bite and allowing slower engine speed, fuel economy ard range. Advances in propeller technology, such as variable pitch and constant speed propellers have also helped.
     
  7. davebender

    davebender Well-Known Member

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    Used in some WWI era aircraft and airships. Should have been mature technology by WWII.
     
  8. wuzak

    wuzak Well-Known Member

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    One of the reduction ratios used on Merlins was 0.42:1 (0.42 prop rpm to 1 engine rpm ~ 2.38:1).

    The Napier Sabre IIA had a reduction ratio of 0.274:1 (3.65:1).

    Most Hercules, as well as the Taurus, seem to have had a ratio of 0.444:1 (2.25:1). Some Hercules had 0.444:1 and others 0.4:1 (2.5:1).
     
  9. gumbyk

    gumbyk Well-Known Member

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    O.K. I'll consider myself educated....... :)
     
  10. Edgar Brooks

    Edgar Brooks Active Member

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    The vast majority of Spitfire Merlins had a ratio of .477; .42 was used, initially, on the Merlin 61, but, in trials, the .477 ratio airframe was markedly superior at height. I'll leave Wuzak to tell you what .477 equates to, since my maths learning is 55-60 years old (in other words I don't know.)
    The Griffon seems to have been all over the place, at .451, .51, .4423, and .442.
     
  11. wuzak

    wuzak Well-Known Member

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    .51 was used on the Griffon on Spitfires XIV, XVIII and XIX.

    That is less than 2:1 (actually 1/0.51 = 1.96). This lower reduction was allowed by the lower speed of the Griffon vs the Merlin (2750 max vs 3000 max) and the smaller diameter prop used on the XIV/XVIII/XIX.

    0.477 = 2.096:1
     
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