Contra rotating Propellers

Discussion in 'Technical' started by Seawitch, Jul 21, 2007.

  1. Seawitch

    Seawitch Member

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    When I first noticed these on aircraft like the later Seafires I didn't think thats what they were, I rather thought that it was a way of mounting that many blades without weakening the mount/shaft.
    Now i know different and wonder was this very complicated to do, why do it?
    Did this produce more power? Neutralise Torque?
    Over to the experts!
     
  2. mkloby

    mkloby Active Member

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    I would imagine that it would produce more thrust than a single prop. However, it will add more weight, and no doubt complicate design and maintenance. It should also serve to aid in offseting torque-effect, p-factor, and gyro precession.
     
  3. ToughOmbre

    ToughOmbre Active Member

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    Technical question...

    Contra rotating...that the same as counter rotating? I've seen the terms used inter-changeably. When talking about the P-38 I say counter. :?:
     
  4. k9kiwi

    k9kiwi Member

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    Contra rotating is what the US Government did around a South American guerilla force. :p

    They are interchangeable expressions, however the correct usage is Contra Rotating.
     
  5. Graeme

    Graeme Well-Known Member

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    Certainly reduced torque. Many of the powerful single prop Schneider Trophy racers had great difficulty taking off due to engine torque. The Macchi-Castoldi MC.72 solved its torque problem with the counter-rotating propeller.
     
  6. pasoleati

    pasoleati Banned

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    They are not interchangeable terms!!! Contrarotating means a propeller that has two props rotating in different directions on the same axis while counterrotating described two separate props like the P-38 case.

    Their primary benefit is elimination of asymmetric torque. Read the Sptfire 21 handling reports with both prop types. The contrarotating prop made flight possible without even touching the rudder.

    Had the F4U been fitted with one, accident rates would have plummeted.
     
  7. Graeme

    Graeme Well-Known Member

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    Maybe..But not according to Derek James-The Schneider Trophy Aircraft.

    [​IMG]
     
  8. 130fe

    130fe New Member

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    Mr James misspoke, pasoleati did not.
     
  9. Graeme

    Graeme Well-Known Member

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    Disappointing, coming from the 'esteemed' Putnam range. Oddly, he corrects himself in the captioning.

    [​IMG]
     
  10. pasoleati

    pasoleati Banned

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    The topic was discussed in Torque Meter, the journal of Aircraft Engine Historical Society and indeed the terms are defined as I previously wrote.
     
  11. HoHun

    HoHun Active Member

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    Hi Pasoleati,

    >The topic was discussed in Torque Meter, the journal of Aircraft Engine Historical Society and indeed the terms are defined as I previously wrote.

    Years ago, I read in the context of a Fairey Gannet discussion that "contra-rotating" not only included co-axial propellers, but actually one common drive train for both.

    I'm not sure about the Macchi, but it might be that each of the two coupled engines has its own independend drive train. If we follow the above definition, it would have to be called "counter" rather than "contra rotating".

    (From an linguistic point of view, I disapprove of the attempt to make a distinction by using two different variants of a synonym. We don't have a thread "The Greatest Etymological Blunders of WW2", but if we had, I'd nominate this one ;)

    Regards,

    Henning (HoHun)
     
  12. SoD Stitch

    SoD Stitch Banned

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    My understanding as to the main reason for contra-rotating props is that they are more efficient; the self-cancelling torque feature of contra-rotating props is a side-benefit, not the main benefit. Contra-rotating props eliminate the rotational airflow around the airframe which causes energy loss, making the propellers more efficient.

    Quote:

    "When airspeed is low the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow.

    If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the asymmetrical torque effect of a conventional propeller. Some contra-rotating systems were designed to be used at take off for maximum power and efficiency, and allowing one of the propellers to be disabled during cruise to extend flight time.

    The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and rotors are moderately common in military aircraft and naval applications, such as torpedoes, where the added maintenance is not a concern to government budgets."

    From: contra-rotating propellers: Information from Answers.com
     
  13. ccheese

    ccheese Member In Perpetuity
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    Lemme see..... If I were sitting in a P-38, the left prop spinning counter-
    clockwise and the right prop spinning clockwise, I would be witnessing
    counter-rotating props. Now.... the Seafire (and there was a Reno
    racing P-51 so configured) has two props on two shafts (one within the
    other) turning in opposite directions. Aha .....Contra-rotating.....

    And of course, you have the Dornier 335.....push-pull.

    Don't you just love it when a plan comes together ??

    Charles
     
  14. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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  15. Graeme

    Graeme Well-Known Member

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    Very true!

    And you're right about the Macchi. A unique engine installation that has 'fused' the definitions.

    [​IMG]
     
  16. ccheese

    ccheese Member In Perpetuity
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    Joe: That's a very good explanation of the "critical engine" I had heard
    the term before but Wikipedia explains it so even I can understand it.

    Is that why they say, "never turn into a dead engine" ?

    Charles
     
  17. Parmigiano

    Parmigiano Member

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    Counter (or contra) rotating prop on the same axis, either driven by 1 or 2 engines, also have the advantage of allowing shorter blades and so to reduce the tip speed (vs a single prop configuration designed to handle the same power)
     
  18. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    Yep - and as in WW2, today the biggest killer of (GA) pilots is lack of proficiency when performing emergency engine out procedures on twin engine aircraft - I've ranted about this on earlier threads.
     
  19. mkloby

    mkloby Active Member

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    Losing your critical engine sucks. I would always hate it when they failed my left engine (critical) in the C-12B... standing on the damn rudder gets old REAL quick.
     
  20. chuter

    chuter Member

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    Well, the P38 didn't have a critical engine because it didn't have a better performing engine (inboard turning) of the two. Pfactor is the result of an an effective AOA change of a tilted prop disc. Tip a prop disc back . . . say, ten degrees . . . and maintain its horizontal direction and the down going blade has a higher angle of attack (pitch) than the up going blade. This has the effect of moving the thrust line from the prop axis horizontally towards the down going blade. Whether this is towards or away from aircraft centerline determines how much rudder (drag) is required to maintain neutral yaw. With both engines running the inboard wing sections would have a higher effective AOA than the outboard sections (spiral slipstream affect) which would help stalls start inboard where they should. With the surplus of power the P38 had as well as reasonable yaw control this was seen as a good compromise. The P82 adopted the same approach initially but stall performance was so benign North American switched the engines so both engines gave superior single engine performance.



    :D
     
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