Reduction Gear Ratio

Discussion in 'Engines' started by billswagger, Nov 12, 2010.

  1. billswagger

    billswagger Member

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    This is a more technical question.

    I know what the reduction gear is for, but when reading about some aircraft engines i sometimes see the ratio published as 2:1 or some other figure.
    So i wondered if anyone knew how the ratio is factored into the rotations of the engine and propeller.
    Is it the propeller that spins twice for every time the engine revolves once, or is it the other way around?


    While i have your attention, i've also seen blower gear ratios for the supercharger that read something like 8.8:1 or 9.6:1. So i also wondered how and what those ratios correspond to.

    I've tried looking it up briefly, but nothing really touches on the ratios instead they focus on the engineering aspect of the engine.

    thanks

    Bill
     
  2. Colin1

    Colin1 Active Member

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    The reduction gearbox will reduce the rotational speed of the propeller in relation to the powerplant. This is to prevent the tips of the prop from exceeding a mach number rotational speed, as at that point it will become significantly less efficient.

    Compression ratios define the maximum amount of air you can stuff into a powerplant before the intercooler can no longer cool the charge air sufficiently. This is important as the presence of excess heat in the charge air will produce less power and more of what is produced, will be wasted. Hot charge air will also cause detonation.
     
  3. Shortround6

    Shortround6 Well-Known Member

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    The propeller is almost always turning slower than the engine (never say always:)
    Somewhere, somebody, probably made a set up where the prop turns faster if only to say they did it.

    Engines make more power at higher rpm (moving more air through the engine) but small fast propellers are inefficient and with big props you don't want the tips going supersonic.

    With superchargers, at least centrifugal ones, lots of rpm are needed. The work the supercharger does is in relation to the square of the rpm, double the impeller speed and get 4 times the work out of the unit. Depending on need the impeller in an aircraft supercharger will always turn at least crankshaft speed (some mid 1920s engines) and usually much faster, like the numbers you gave. Physical limits of the supercharger are the strength of the impeller and again not wanting the tips of the impeller to go supersonic. Please note that the speed of sound in the compressed medium inside the supercharger is higher than in the open air.
     
  4. Milosh

    Milosh Well-Known Member

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    A reduction gear reduces whatever the input turns at. The engine is the input and the prop is the output.
     
  5. billswagger

    billswagger Member

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    thanks for the replies.

    I found something that explains the ratios better.

    The first number is the input number, while the second number is the output.

    So a 2:1 reduction gear ratio means the engine (input) turns over twice for every one propeller (output) rotation.
    This doesn't mean that the engine will go through two cycles, but that the gear on end of the crank case will turn over twice.

    Actually the A6m2 is said to have had a reduction gear ratio closer to 0.7:1
    There might other examples on early war aircraft.

    Bill
     
  6. Shortround6

    Shortround6 Well-Known Member

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    I am afraid that explanation isn't quite right or that some writers may have swapped numbers around.

    Supercharger gears were not 8.8:1 if the first number is the input number. They would have been 1:8.8

    Reduction gears were used in WW I on a number of engines.

    A little math can also help check things out. A 10 foot propeller turning 2500rpm would have tip speeds of 1308 fps, well above supersonic. Gearing a propeller to turn even faster would do no good at all. However the same 10ft propeller when attached to a 2700rpm engine with 0.7 reduction gear would have tip speed of 989fps. Well within bounds and even some to spare for overspeeding in a dive. For comparison a 13ft prop on a 2700rpm engine with a 2:1 reduction gear has a tip speed of 918 fps.
     
  7. billswagger

    billswagger Member

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    thanks for explaining that.

    I was a little puzzled over the supercharger blower ratio, but i figured it must've been backward.

    It would seem reduction ratio is related to the size of the propeller too.
    I've seen reduction gearing for the R-2800 from 1.49:1 to 2:1

    A longer blade must necessitate a higher ratio.

    I've also seen reduction ratios for the Landcaster as low as 0.42:1

    So there must be some benefit for lower ratios depending on the air screw

    Bill
     
  8. billswagger

    billswagger Member

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    Another thing,

    You can see reduction gear ratio stated as a number on some historical tests:

    Spitfire Mk IX versus Me 109 G - Flight Testing

    You see it shown for the Merlin as a 0.477 under the engine data.

    This would equate to 0.477:1

    i checked wiki for various engines and it also gives reduction gear information.

    It also shows a reduction gear ratio of 0.594:1 for the DB605

    I wonder if there were advantages to having a lower ratio.

    Keep in mind the merlin pushed an 11' prop. So are these ratios backward, too?

    Bill
     
  9. Shortround6

    Shortround6 Well-Known Member

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    It is all part of a propulsion system.

    A very good propeller converts about 80-82% of the input power into thrust.
    But there are very different conditions. Taking off at low speed (100mph?), high speed dash at low altitude, high speed dash at 20,000ft where the air has half the density of sea level, etc.

    A bad analogy to cars might be the gear ratio of the airplane engine is like the gear ratio in a car or truck and the propeller is like the tires getting traction. A small light car that goes fast can use less gearing down and smaller tires to transmit the power while a heavy car/truck needs a bigger gear reduction to move the weight and needs bigger tires to keep from just spinning the wheels and going nowhere.
    I know it is not a good analogy and maybe full of holes.:oops:

    Some German engines and at least one Russian engine came with different gear ratios depending on wither they were intended for fighters or bombers.
     
  10. Ivan1GFP

    Ivan1GFP Member

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    Hi Shortround6,
    The Mitsubishi Kasei engine by the Japanese came in several different reduction gear ratios. The engine in a H8K Emily was driving an awfully big propeller while a much smaller prop was used in a J2M Raiden.

    Also, your earlier message about propeller tip speed doesn't take into account forward speed of the aircraft which adds significantly.

    Hi billswagger,
    The Propeller on a Me 109G is much smaller diameter (I believe 9.8 feet or so) than that of the Spitfire which may explain the gear ratios. Also, the max RPM on the DB 605 is lower than that of the Merlins (2800 versus 3000).

    - Ivan.
     
  11. billswagger

    billswagger Member

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    #11 billswagger, Nov 15, 2010
    Last edited: Nov 15, 2010
    In search of a more specific answer about ratios I've learned that the first number is divided into the shaft RPM speed of the engine to give RPMs of the propeller.

    So a 1.49:1 reduction means the engine rpms are devided by 1.49 to give prop speed.
    2800 rpms / 1.49 = 1879 prop speed.

    On a spitfire with a 0.477 gear reduction, you would take 3000 rpms / 0.477 = 6289 prop speed??
    That doesn't seem right.

    I think there is some confusion with the labeling of gear ratios depending on the country or manufacturer.
    A 0.477 gear reduction might only refer to the propeller rotations for every 1 engine turn.
    It should be listed as 1: 0.477
    or approximately 2.10:1

    A ratio listed as a single digit usually refers to the larger gear rotation which is good to know.

    Bill
     
  12. Milosh

    Milosh Well-Known Member

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    Bill that is 3000rpm x 0.477 for 1431 rpm prop speed.

    A 2:1 reduction is 0.5 ratio. It is arrived at by divided the number of teeth on one gear by the number of teeth on the other gear.
     
  13. riacrato

    riacrato Member

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    Shouldn't a reduction gear always reduce the output revs no matter which number in front of the ":"?
     
  14. Colin1

    Colin1 Active Member

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    :shock:
     
  15. Ivan1GFP

    Ivan1GFP Member

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    Here are some details on the Mitsubishi Kasei I uploaded earlier. The J2M Raiden has a 3.3 meter diameter propeller.

    - Ivan.

    [​IMG]
     
  16. billswagger

    billswagger Member

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    sort of hints at why they call it a REDUCTION gear.:lol:

    I came across those numbers quite often and to confuse matters you have publishers saying stuff like 0.477:1

    A single digit ratio always refers to the engine shaft rotation for every one propeller turn.

    thanks for the replies in helping me clear that up.


    Bill
     
  17. renrich

    renrich Active Member

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    A nice side effect of the reduction gear was to allow a gun to fire through the prop hub.
     
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