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General question about contra-rotating propellers.

Discussion in 'Information Requests' started by Shadow Master, Oct 15, 2010.

  1. Shadow Master

    Shadow Master Member

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    Ok, so I am just interested in some basic info here.

    Contra rotating props give more thrust than a single prop as I understand it, but how much more? If on a twined engine aircraft, would two counter rotating beat one contra rotating?

    Basically, what I am wondering is could a fighter be built along the lines of the He119 (twined engines in fuselage turning one paired contra rotating prop) as a better competitor than the FW187 (two engines in the wings-turning two omni-directional props) to the Me109?
    Thanks.
     
  2. Spartanroller

    Spartanroller Ace

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    Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers (Wiki)

    certainly the 2 engines driving contra rotating set in the fuselage could be done - it was essentially done in each nacelle on the Bristol Brabazon.

    sorry don't have any thrust tables yet to say if it would be certainly better, but the weight distribution has to be an advantage, the roll would be potentially faster, although the pitch would probably be slower (assuming the engines were one forward one behind the pilot approx), wings could be lighter.

    basing the idea on the wikipedia efficiency figures, for the same 2 engines there almost has to be an advantage to having them inboard if you can eliminate shaft problems.

    Maybe another solution is the pusher puller config, although that has it's own problems especially with ejection and perhaps need for twin tails.

    Maybe the long shaft problems are too much to deal with, or maybe one bigger engine driving both props gives better thrust to weight ratio.

    I'd say it could be done and it could be good, but it would be hard and troublesome. Sorry nothing more scientific :)
     
  3. Stitchy

    Stitchy Member

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    Contra-rotating props don't really give you "more thrust", it just converts the engine torque into thrust more efficiently; some of the power from the propeller ends up getting wasted in moving the air laterally instead of perpendicularly. A contra-rotating propeller (for the most part) counteracts this tendency, and is therefore more efficient than a single propeller. It also has the side benefit of eliminating the thrust torque from a single propeller, which could be a problem for inexperienced pilots when they applied too much thrust too soon (the aircraft would tend to roll, sometimes rather suddenly, in the opposite direction of the propeller). The downside is the gearing to get two propellers on a common shaft turning in opposite directions is fairly complicated, so you've increased some of the parasitic power loss in the drivetrain, though this is usually more than made up for by the recovery of power from the twin propellers.
     
  4. Spartanroller

    Spartanroller Ace

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    cancel
     
  5. Spartanroller

    Spartanroller Ace

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    there was a French prototype made that was to be a contra rotator - the Sud-Est 580. I don't think it flew in the end because of the invasion, but they at least thought they had some problems dealt with and a design worth following.

    Secret Projects of Aviation: Dewoitine /Sud Est SE 580

    And the P-39 Airacobra had the engine behind the pilot, so the shaft problems could be solved.
     
  6. Shadow Master

    Shadow Master Member

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    Hmmmm.

    Thanks for the replies and interesting links and thoughts. I have to say I wasn't thinking in terms of something along the lines of the Dornier Do 335 here, but the in-line configuration would definitely seem to be one that would have less of a cross section and therefor be faster.

    Interesting indeed.


    [​IMG]
    Uploaded with ImageShack.us

    Please excuse my lack of auto-cad like software!:eek::rolleyes:

    They say a picture is worth a thousand words, so the above crudity should be worth a paragraph or two.

    The FW 187 had it's engines in the wings as approximated in the top tow drawings.

    The Me 109 had it's engine something like in the middle drawings.

    The He 119 (badly) represented by the lower two drawings shows the two engines in a side-by-side configuration within the fuselage.

    I forget from where I got the impression that there was some concern about using the engines in line (one behind the other) on the same shaft, because if one engine sized up that could cause the plane to crash.

    Allegedly, the side by side configuration was supposed to offset this, but as Dornier Do 335 shows, those problems must have been solved. That or I don't remember what I think I do, lol.

    Either way, the FW 187 was supposedly about a match for the Me 109 in manoeuvrability and had twice the range, but at the cost of twice the price and two engines rather than one.

    So if that were the case, and then another design were to place the engines within the fuselage (lets go ahead and discuss both in line and side by side configurations), wouldn't such a design prove to be more maneuverable and/or faster than the FW 187?

    If so, then we have:
    FW 187 => Me 109 as a fighter (and therefor > Bf110)
    New design > FW 187, and roughly the same cost.

    So, two props turned by two engines for either of the 2 'new' designs and the FW 187. Differences are each separate prop creates it's own drag and wastes energy in airflow torque, vs one set of contra rotating props (with less total drag than the two separate props) and greater (but only just a little) thrust as the rear prop is taking advantage of the front props airflow torque.

    If I am grasping these concepts correctly, then the contra rotating design would outperform the traditional design not only because of the small but important increased efficiency but because it would have considerably less drag as well.

    Any thoughts?
     
  7. Spartanroller

    Spartanroller Ace

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    I believe you could be right, but unfortunately I don't have the wind tunnel experience to confirm anything.

    The thing to perhaps consider is that the Germans especially, but in general all combatants, weren't shy about trying new designs and almost certainly someone would have conceived these arrangements, and yet they never went into prototype as far as I can tell. Perhaps there was another major problem discovered that we haven't considered yet?
     
  8. Fred Wilson

    Fred Wilson "The" Rogue of Rogues

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    Do you mean contra rotating propellers like the:[​IMG]
    1. V-12 "Homer"

    2. or like the Hiller - x18

    or more modern versions like the heavy lifters such as:
    3. The fabulous Sikorsky X2 type variants of which now own the logging and heavy lifting markets.

    Such as the (best of the best) Kamov 31A12 HB_ZFX
     
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  9. Shadow Master

    Shadow Master Member

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    Heh heh, I wasn't really looking into helicopters, but I have to admit that the link you provided to the Russian air museum took a bit of my time!:cool:

    Nick links, and thanks.
     
  10. mcoffee

    mcoffee Son-of-a-Gun(ner)

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    There is an axiom that states “there is no free lunch in aviation”, and it can be well applied to contra-rotating props. If contra-rotating props were the end-all, every propeller driven aircraft would now have them.
    A propeller’s job is to convert engine torque into thrust. As engines are introduced with more power, more propeller is need to convert that power to thrust. “More” propeller can be achieved by can be achieved by several methods or combinations; larger diameter, wider blades, more blades, etc.
    As a general rule, increasing diameter is the most efficient way to gain “more” propeller, as it accelerates a larger column of air than a smaller diameter with all else being equal. In the real world of aircraft design, limitations are soon reached on prop diameter – the prop has to clear the ground when the aircraft is on its landing gear, and the propeller tip speed needs to stay below the transonic regime. Increasing diameter increases tip speed for a given RPM. As the prop tip approaches the transonic zone, drag increases rapidly and becomes a real limitation on propellers.
    When the designer runs out of diameter to absorb power, other means must be used. Increased blade pitch can be used to a point, but soon you will reach an angle of attack where the blades stall. More and wider blades can be used, but taken to extreme, you wind up with a solid disk that passes no air, and there is a practical point of diminishing return.
    The contra-rotating design is one method of harnessing engine power in a limited diameter and it has its advantages as others have stated. The main one being the reduction of spiral slipstream and the resultant inefficiency found in a conventional propeller. It also has disadvantages as others have stated. The drive system is complicated, expensive, has mechanical losses of its own, and adds weight. Weight added to the nose of a fighter increases pitch stability and you don’t want a fighter overly stable in pitch, so other changes are required to compensate.
    Contra-rotating props were implemented on some existing designs such as late Mark Spitfires on a test basis. The concept was overtaken by jets for fighters before it came to much. There is a Mustang fitted with a Griffon engine turning contra-rotating props that has campaigned as an unlimited Reno air racer, but without much success.
    The idea of side-by-side coupled engines might work for a bomber, but for a fighter there is too much penalty to pay for the increased fuselage cross section and resultant frontal area. Then you have to cool the beast and pay the cooling drag penalty. You wind up with a big heavy fighter that would be hard pressed to compete with conventional designs. The coupled engines will have to be mounted near the aircraft center of gravity which interferes with single pilot cockpit placement. The coupled engines and complex prop drives have ‘maintenance nightmare’ written all over them. I would want my single seat fighters available for quick turnaround.
    If the FW 187 was nearly a match for the Bf 109, why was it withdrawn from service in 1942?
     
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  11. Shadow Master

    Shadow Master Member

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    Hey, some good stuff here.:D


    Now we're talking, this is the kind of information I was really looking for when I started this thread.:cool:

    I wonder what kind of research was conducted into the possibility/feasibility of non-identical sets of props? Specifically, did the possibility of having a smaller diameter front propeller and a second, larger diameter contra-rotating propeller in the back get tested out? I wonder then what about the other possibilities of mixing and matching different propellers with each other might prove worth while.

    That or the plane crashes.:rolleyes:;)

    "Pitch stability"?

    Hmmm. Now the FW 187 had not just both engines but a separate fuselage in addition. Wouldn't the "new" design actually have less frontal area total?


    You beat me to this one! I was pondering what the effect would be on the cooling needs for two engines. Is the term "parasitic heat" correct for the effect of each engine helping to overheat the other by their proximity?

    I wonder if the possibility of a 'quick change' nacelle could compensate for this? I have to agree that the side-by-side engine placement would be more of a problem for center-of-gravity (and cockpit placements) than the other idea of placing the engines one behind the other - possibly with the cockpit between them.

    I had not realised that it actually went into production as more than a prototype program. The problems I do remember had to due with the fuselage being to narrow, the cost of the plane being twice that of the Me 109, and using up two of the scarce engines (DB 601's IIRC) with the only benefit being that it also had twice the range of the Me 109.

    Basically, why build one FW 187 when we can build two Me 109's?
     
  12. mcoffee

    mcoffee Son-of-a-Gun(ner)

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    Pitch = nose up/nose down. Aircraft maneuver by rolling their lift vector, then pitching to increase lift (pulling G). An aircraft that is stable in pitch can be cruised "hands off". Pitch stability is resistance to change in pitch and for a fighter a compromise between stability and maneuverability is needed. A major factor in how stable an aircraft is in pitch is the relationship between the center of gravity and the center of lift (for a simple rectangular wing planform, halfway back on the wing or 50% of Mean Aerodynamic Cord [MAC]). Center of gravity ranges are typically 25 -33% of MAC, i.e. ahead of the center of lift. Forward CG is more stable, rearward CG is less stable. Weight on the nose moves the CG forward. Modern fighters have the CG so far back that they are aerodynamically unstable in pitch, relying on the flight control system to provide pitch stability. But, that allows them to go from 1G to 9G nearly instantaneously.

    Maybe a bit less frontal area, but maybe not. Look at a twin like the F7F Tigercat - very narrow fuselage just wide enough for the cockpit and tightly cowled engines. Same with the P-38. The side by side fuselage will have to be sized to accomodate both the engine installation and the cockpit - two different shape requirements. Not sure if you'd really gain in frontal area. Total "wetted" surface area is certain to be greater. The P-38 example has a short cockpit pod and short engine nacelles. The side by side is going to have a long fat fuselage unless it is built like a snake that swallowed a chipmunk.

    Yep, the best twins of WWII were only competitive with single engined fighters when they enjoyed a significant speed advantage (P-38 vs A6M) or started combat from a favorable position.
     

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