Educate me on countra rotating props.

Discussion in 'Aviation' started by Rufus123, Oct 19, 2013.

  1. Rufus123

    Rufus123 Member

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    What were the advantages and disadvantages?

    There are some that by looking I could guess at but guessing my yield poor results on my part.

    Advantages:


    No more prop torque. I want to be careful on this guess as I wonder if it makes a difference if the one in back torques different than the one in front.


    Disadvantages:


    More weight? I don't know if it is that much heavier but I think it would have to be?

    More complicated? I am guessing a lot more complicated but don't know.

    Mechanical efficiency, I don't mean the blades slicing the air but the work to deal with the gearing and such. I don't know if there is a mechanical inefficiency but I am guessing there is a little?


    Things I don't even want to guess at:


    Prop efficiency (blades moving air) I don't know if one prop creates turbulence or something that affects the other prop?

    Is there a horsepower threshold for this to be worth trying.


    Why didn't we see these in WW2: Nothing but guesses on my part.


    Development time?

    Cost?

    Development not started early enough?

    Negatives out weight positives?

    When development was moving along the Jet Age killed it off?
     
  2. DerAdlerIstGelandet

    DerAdlerIstGelandet Der Crew Chief
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    I could be wrong here, so please correct me if I am.

    Typically contra rotating propellers are more efficient (up to approx. 15%). It should have no or less rotational airflow which results in higher performance. It also reduces the effect of asymmetrical torque, which will help counter the yawing of the aircraft to one side caused by the propeller. Also known as P-Factor (Aircraft Ground School does wonders...:lol:).

    Some disadvantages are that they are typically noisier, and more heavier which causes a slight loss in the gained performance.
     
  3. GregP

    GregP Well-Known Member

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    #3 GregP, Oct 20, 2013
    Last edited: Oct 20, 2013
    Contra-props are a two sided animal. They CAN and DO reduce torque, but are much heavier than a single prop. Also, the gear train can be a really difficult item. The early Allison T-40 that powered the two VTOL planes from Convair and Lockheed were failures not because of the engines, but because of the gearboxes.

    The Douglas Skyshark (a turboprop Skyraider variant) was a failure due to the gearbox, Everything else worked quite well.

    Contraprops also usually have a lower g-rating than single props due to the distance of the shaft from the origin. You can break one at too high a g-load easily.

    The Soviets have some very notable contra-props, especially the Bear bomber / patrol plane. It is the fastest propeller-driven plane on the planet at over 575 mph. It rivals jets and definitely out-accelerates them over the short-run. A Bear can out-accelerate an F-15 for a considerable distance.
     
  4. pattle

    pattle Member

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    I seem to recollect that the Fairey Gannet would switch between running either both sets of props or a single set of props depending on the power required, the Gannet was quite a good airplane.
     
  5. stona

    stona Well-Known Member

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    You would be correct. It had a double Armstrong Siddeley Mamba power plant and could also run either or both these too. That sort of redundancy is useful in an aircraft designed for long, over water, patrols.
    Cheers
    Steve
     
  6. vinnye

    vinnye Member

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  7. GregP

    GregP Well-Known Member

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    I believe the Gannet used a double mamba and the props were not contra-props, but two independent props, each driven by one mamba engine with no interconnect between them.
     
  8. Waynos

    Waynos Active Member

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    As I understand it, there was a common gearbox so either or both props could be driven by either or both engines. Sounds a bit complicated to me though :)

    Before the Gannet there was also a one off Griffon powered Fairey Firefly fitted with contra props.
     
  9. GregP

    GregP Well-Known Member

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    #9 GregP, Oct 20, 2013
    Last edited: Oct 20, 2013
    If that is the case, it sounds VERY complicated to me.

    I have seen pics of a Gannet in flight with one prop running and one prop stopped. You can't do that with a mechanlically geared contra-prop, so this makes me want to look into it a bit further. Now I'm curious about the powertrain of the Gannet ...
     
  10. Waynos

    Waynos Active Member

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    That's exactly what I've been thinking too.
     
  11. wuzak

    wuzak Well-Known Member

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    The DB606/610 had the two engines driving a single rotation prop through a single gearbox, but I believe that either engine could be disconnected from the prop using a clutch.

    If the Gannet's pair of prop could be driven by either engine then it may have used a similar system.
     
  12. R Pope

    R Pope Member

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    I believe the Gannet used what was termed "co-axial" props rather than contraprops, since each engine drove one prop independently of the other.
     
  13. Shortround6

    Shortround6 Well-Known Member

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    description in 1954/55 Jane's says each engine drove "one of the two co-axial airscrews. Each engine could be stopped, started, cruised or feathered under conditions entirely separate from the other."

    No mention is made of both props being driven by one engine in either the aircraft description or the engine description.

    Jane's can be wrong but I want a good source :)
     
  14. GregP

    GregP Well-Known Member

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    If you can feather a coaxial prop, it is independent from the other one with no mechanical interconnection. Hence, it is NOT a cotra-prop, just a co-axial prop.
     
  15. HBPencil

    HBPencil Member

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    I can't speak for other manufacturers but having read Jeffry Quill's autobiography Supermarine started testing contra-rotating props mid war (on a Mk IX I think) and it was apparent that the negated prop torque was of great value (especially with the introduction of the Griffon engine) however they had problems getting it reliable enough; the main problem was that part of the pitch control would brake; resulting in the pilot only being able to control the pitch of the rear-most props while those forward would be locked into what ever they happened to be in at the time of the failure. So only one production type (the Seafire F.47) had contra-rotating props and that was post war.
     
  16. R Pope

    R Pope Member

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    That locking of the pitch thing is what caused Howard Hughes' crash in his F12, one half of one prop went to reverse pitch.
     
  17. Balljoint

    Balljoint Member

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    Contra rotating props should pick up some weight and safety advantage in that they seemingly would not be subject to whirl mode oscillation. Whirl mode occurs when the propeller shaft moves axially. Such movement sets up a gyroscopic force that results in movement 90° to the first which in turn…

    This phenomenon became evident with the early long turboprop engines with the greater leverage between the prop and motor mount. Wing failure in the Lockheed Electra turboprop was due to whirl mode.
     
  18. nuuumannn

    nuuumannn Well-Known Member

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    #18 nuuumannn, Oct 21, 2013
    Last edited: Oct 21, 2013
    Yep. The apparent weakness was in the engine mounts where they attach to the wing, and the supporting structure; the torque from the Allison 501s and those big HS props was literally tearing the entire ESU (Engine Support Unit) from the wing.

    We are getting ahead of ourselves here. According to The Aviation Dictionary For Pilots and Aviation Maintenance Technicians published by Jeppesen Maintenance, the definition of contra-rotating propellers is, "Two propellers mounted on concentric shafts that turn in opposite directions. This type of rotation cancels the torque forces caused by the rotation of the propellers."

    The deifinition applies to the opposite direction of rotation of each propeller, not necessarily to the workings of the mechanism.

    Co-axial propellers are two props mounted in line with each other (co-axial - on a common axis) on concentric shafts; from the same source:

    "Propellers with concentric shafts that allow them to rotate in opposite directions, cancelling torque and p-factors inherent in single propellers. Co-axial propellers can be geared for power by a single engine or multiple engines."

    So, by nature of their installation, co-axial propellers are contra-rotating.
     
  19. SHOOTER

    SHOOTER Member

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    While many people outside of the Soviet Union thing the Bear Bomber is the fastest prop plane on the planet, it is actually the Tu-114 Passenger Liner that holds the speed record.
    Secondly, and much more important, there are only two planes that might, under some conditions be able to out accelerate the F-15! from any speed. They are; the Su-27 family of planes under some conditions and the F/A-22. There are no others that even come close under the best possible circumstances. None!
    While the Big Tupolev might be very efficient at cruising speed, at high speeds it takes forever to accelerate from say 500 to 545 MPH. At much lower speeds say 350 to 450 MPH, it is very much quicker, but not even in the same league as the F-15/16 with their F-100/229 engines with AB/Re-heat. The F-15 with eight up and the CL 600 Gallon tank can out accelerate the Bear by over two minutes M0.8-1.2 Vs 450-500 MPH. Furthermore, the F-199/229 has a throttle response that is second to none! Even possibly better than the F-119 in the F/A-22? Please do not equate Specific Excess Thrust >F/A-22 <F-15C with Throttle Response >F-15C/F-16 <F/A-22.
    The other thing is that the air liner is both smoother and has fewer protuberances and better Whitcomb curve.
     
  20. SHOOTER

    SHOOTER Member

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    You are confusing Contra-props with the system used to drive them.
    Two props spinning on one access are Contra-Props, regardless of how they are twisted.
     
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