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  1. #1

    COAX-P: Counter Rotating Propeller Gearbox

    Thought this might interest the likes of Jeremy and other Techies

    This unique counter rotating propeller gearbox is designed to fit for Rotax 503 and Rotax 582 engines, not for 912 or any other brands engine.
    The COAX-P eliminates the torque effect, creates much less noise (low frequency noise), and improves the thrust for about 15-20% more, compared to various single propeller types.


    for Hang-glider Trikes and Paraplanes - Powered Parachutes

    eliminates torque effect,considerably more power,reduced noise,gear reduction ratio i=3,7


    special propellers for HG-Trike or Paraplane - Powered

    weight: 17kg (Rotax E-box 11,2kg)

    total length of gearbox incl. second propeller: 20cm more than
    Rotax box

    Hofbauer GmbH.
    Zimmeterweg 4
    6020 Innsbruck

    Phone: +43 664 5403660
    Fax: +43 512 546445

    (Please replace XX with @. This is for SPAM-security reasons. Thank you for your understanding.)
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  2. #2
    Founding Member - See my blog entries for help using the forums VinceG's Avatar

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    Feb 2009
    Centre of the Universe, Morley, Leeds
    Interesting. That's what my son has on his cheap made in china model helicopter. Never thought I would see one made for a real plane...
    Happy Safe Flying
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    Vince Gledhill
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  3. #3
    Senior Member xair582's Avatar

    Join Date
    May 2009
    letterkenny, Co.Donegal, ireland
    interesting set up wonder if it willl fix and xair? and if it is approved in the uk ?
    Xair 582 power to the people

  4. #4
    Quote Originally Posted by xair582 View Post
    interesting set up wonder if it willl fix and xair? and if it is approved in the uk ?
    Fits on 503 / 582 but no other Rotax Engines, I would imagine there is no reason why it shouldn't be approved in the UK if someone was to apply for a mod to fit this....... Maybe Jeremy can comment..... this is something he has better knowledge of the mod system than the rest of us.

  5. #5
    It's been around a while, but doesn't really give anything like a 15 to 20% improvement in "thrust". The key here is really efficiency, not thrust. You can size and pitch the prop (and gearbox ratio) to deliver any value of thrust you want. As an extreme example, look at a helicopter, which will generate more thrust than it's own weight, just with a large diameter, fine pitch, "prop" (and helicopters have flown on 503s and 582s).

    As Russell rightly says, one of the main reasons for using a contra rotating set up is because you can't physically fit a big enough prop to absorb the power from the engine, which isn't often a problem with two stroke powered microlights. Another reason is to get rid of the torque effect that a single prop gives. Again, this isn't really an issue with low powered microlights, although it does tend to cause a very slight trike twist on a flexwing. Torque effect is a bit of a pain with powered parachutes, though, which is why this thing came into being in the first place. As there is no rigid couple between the "trike" and the wing on a PPC, prop torque on a big, heavy machine can be a bit of a problem. It even causes a noticeable turn on my little 20hp PPG, and is compensated out by offset hang points on the arms.

    Prop efficiency is usually around 70% or so, maybe a bit more for a good prop like an Arplast or Powerfin, probably a bit less for a rubbish one like an Ivo. Adding a second, contrarotating, prop very marginally increases efficiency, by maybe a couple of percent, but this will be more than offset by the added losses from the extra gears in the gearbox and the critical pitch sensitivity induced by having the second prop. The pair of props will have a narrower efficient operating range than one single prop, in effect there's only any measurable gain at one airspeed.

    Finally, there's the added weight. The extra prop and heavier gearbox add to the all up weight, which increases induced drag, which then needs more thrust, and hence power, in order to fly at the same speed as before.

    All told, it might be a worthwhile option for a PPC, where prop torque is an issue, but it doesn't make much sense for any other sort of microlight. Nice bit of engineering though.


  6. #6
    Senior Member andy dixon's Avatar

    Join Date
    Feb 2009
    west yorkshire
    thanks jeremy,a superb answer.
    i got all giddy when i first saw the post,my thoughts of a more fuel eficient 582 sprang to mind.......but looks like i will have to carry on saving up for a 912. per 2010 European libel act....these are my thoughts only and may not represent the thoughts or actions of any person /company/group/manufacturer named in this article/Internet posting

  7. #7
    Interestingly the Americans claim 30% increase !!!!!
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  8. #8
    Well it is America !!!

  9. #9
    I think we're a bit too used to the way that laws over here restrict manufacturer's ability to make claims about their products that aren't supported by evidence so get surprised by the different approach to advertising in the US. It's always surprised me that, although the US is pretty litigious and tends to sue for the most bizarre reasons, they seem to accept product claims that have little or no factual basis.

    The "thrust" claims for this product are a classic example. A good prop, well matched to the engine and airframe, may have an efficiency of around 70 to 75%. The very best prop efficiency figure achievable, over a very narrow operating range, is about 85%. There are good reasons for this being the limit, the most significant of which are viscous (skin friction) and tip losses (from spanwise flow causing energy-sapping tip vortices), that cannot be eliminated for a prop, no matter what you do to it (short of making it into a ducted fan).

    The implication is that there might, possibly, be a theoretically possible gain in "thrust" of maybe 13% or so, if a system could be designed that managed to get a prop to work as efficiently as is theoretically possible. These people claim an improvement of 15 to 20%, which means they've done much, much better than anyone at NACA Langley or RAE Farnborough ever managed to do in many tens of years of propeller research, backed by US and UK government funding and the pressures of building faster and more powerful aeroplanes to fight in two World wars.


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