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  1. #926

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Tony has been flying Contras pretty much since we started, and his experiences have been very helpful to us.

    It's because of guys like Tony, Dave Snow, Dave Lockhart, Bill Ahrens, and Chip, (These are just the North Americans...) that that we are able to continuously improve the Contra to increase reliability and improve performance.

    Brenner ...

  2. #927

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    Do you think there may be any merit in a tip mod,, per the picture below ;

    Brian
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  3. #928
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    I believe that this tip is 'patented' by plettenberg

    not 100% sure but can remember Manfred Greve mentioned that on a F3A worldcup contest

  4. #929

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    ORIGINAL: wvr

    I believe that this tip is 'patented' by plettenberg

    not 100% sure but can remember Manfred Greve mentioned that on a F3A worldcup contest
    They seem to modified Rassa paddles.
    They could hardly complain about being copied - the APC size embossing is still on 'their' normal props.
    I'm not sure who did the mod .

  5. #930

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hey Brian,

    Very interesting. Have you seen them fly? Are they quiet? My first thought is that the winglets at the tip are pretty small, so it's hard to understand how they could have an outsize effect on performance or noise. Also, they look awfully delicate. I'd hate to fly off of a grass field with them.

    However, if they do have positive effects, then there may be more practical ways to acomplish a similar thing.

    Brenner ...

  6. #931
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes


    ORIGINAL: bobwalden

    Fascinating discussion guys, I made a Bugatti 100P from info on http://www.bugattiaircraft.com/plane.htm at just 32 inch span. The original 1937 design hadcoaxial contra rotating props driven by a pair of 450+ BHP Bugatti Racecar engines. The intent was to use this aircraft in European Air-Racing, but it did not fly before war broke out, and the plane was stored. It was found inthe 1960's I think, and after restoration is now in the EAA museum at Oshkosh. While the typical contemporary racers were doing a respectable 300 to 350 MPH, recent computer analysis suggested that this amazing plane would do a good 500MPH.
    The designer found he could reduce the tail sizes, for less drag and more speed.
    I tried flying my model with a single prop/motor but it rolled in left, so eventually tracked down and fitted a small twin contra motor.(Axi make a larger one if someone fancies making a bigger model). This was a little tricky, since the front motor drives the rear prop, but only had a stubby bit of the motor rotating can to fit that prop. I had to make a sleeve nut for this, then bodged a pair of spinners tocomplete the front end.
    Hope to get it airborne very soon. I made an identically sized Depron model that flew OK on a single motor, so should be a good un!
    Neat.
    This airplane was referred to be me and my teen buddies in the 70's as the "Lost Bugatti". It was actually designed to compete at speeds of 500 KPH, which is around 330 MPH.
    Chris...


  7. #932

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes


    ORIGINAL: Brenner

    Hey Brian,

    Very interesting. Have you seen them fly? Are they quiet? My first thought is that the winglets at the tip are pretty small, so it's hard to understand how they could have an outsize effect on performance or noise. Also, they look awfully delicate. I'd hate to fly off of a grass field with them.

    However, if they do have positive effects, then there may be more practical ways to acomplish a similar thing.

    Brenner ...
    Hi Brenner,
    I have not seen them but I have spoken to people that have.
    There are a few guys in Germany that are using these on different props including a contra ,see post 837 here.
    I am reliably informed (multiple sources) that they are super quiet and they go very well.
    The guys using them are claiming a 15% efficiency increase. Apparently some of these guys have access to a wind tunnel.
    Whatever about the actual figure it does make sense that if noise = loss then super quiet = efficient.
    Obviously this depends on the prop performance prior to the mod.
    If there were large radial outflows then there might be a big gain to be had.
    I agree with your comments including re the apparent fragility.
    However if there is a noise and performance benefit I'd like to hear your views on a better/more practical way to get the same outcome.
    This is a good time in the season to play around with stuff.

    Brian

  8. #933

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    I will be modifying my carbon contra props shortly (reduce diameter, reshape tip etc) how are most of you rebalancing your modified props after these changes. Mainly the rear prop as the size of the hub will not work with my current balancer.

  9. #934

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Peter,
    I got a friend to turn a set of cones to suit it.
    He turned them from a black plastic of some sort.
    He made a cone to enter from the front of the prop and a flat plate to square it from the rear. These need to be tight on the spindle.
    The repeatability is great.
    Prior to that I used the hub itself.
    To do this you have to pull out the rear bearing and remove the spacer tube (otherwise it will move about the place changing the reading). Then clean it up and mount it on the spindle.
    With mine I had to balance the hub a little before putting the prop on it - I did this with a dab of grease.

    Brian

  10. #935

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hey Brian,

    If there is definite beneft from using winglets at the tips, then I would be tempted to try forming the entire tip over so that it droops down maybe 5mm to 10mm or so. Something like this should retain all of the strength and stiffness, and at the same time limit spanwise flow.

    Hey Peter,

    One more thing to remember is that the blades of the Contra props are hollow, which means you can inject small amounts of epoxy inside the blades through holes in the hub to adjust the balance. What we do is fling the props by hand to distribute the epoxy inside until the prop balances. Then we set the prop aside to let the epoxy set.

    Brenner ...

  11. #936

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    Yes ,if we/you were absolutely sure of a benefit. This would be a lot of work and the benefit would have to be clear in advance of considering the undertaking.
    I was thinking more along the line of a mod,, to an existing pair or pairs to conduct some trials.
    The various sizes would need testing as it is likely that the higher pitch's will have more outflow.
    The winglet in the picture does look small ,cordwise anyway, but I assume they believe this is 'the narrow neck at the lower end of the valley' so to speak.

    Brian

  12. #937

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes



    Prop used at the European Nats.

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  13. #938

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi,
    These props are for sale here ; http://www.f3alorenz.homepage.t-onli...g-zubehoer.htm
    It does claim copyright ??
    A daisy cutter for sure.

    Brian
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  14. #939

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Seems likea lot of trouble to go to for something that is going to snap off on the first grass blade strike!
    Ed Alt
    Tech-Aero Designs LLC

  15. #940
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    When I saw the first pic posted, I'd thought the tip had delaminated.

  16. #941

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    I think robustness is the biggest challenge here.

    I wonder if the same effect could be aproximated with the use of fences outboard towards the tip? If a miniature wing fence was placed about 10mm inboard from the tip, it would be protected from tip strikes, and might even be replaceable in case of damage.

    Brenner ...

  17. #942

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    It does look as if it was split down the mold seems and then each half folded in/out to almost 90 degrees.
    By this I mean the pitch angle at and after the fold appears to have been preserved and that they 'fold' it both ways to try to balance centrifugal loads, thus they can fold over quite a lot spanwise and stay light construction wise - speculation of course.
    I just don't know if the efficiency claim is for the entire prop or for the tip - probably the prop.

    Yes Brenner something like what you are suggesting seems like it is worth a trial at least.

    Brian

  18. #943

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Here's a Phd thesis that used Genetic algorithms to optimize UAV propellers for efficiency and noise.

    http://etd.auburn.edu/etd/bitstream/...pdf?sequence=1

    I can't speak to the completeness of this work, but it is interesting that in the results section towards the back there are a lot or propeller profiles with tips that are either upturned or downturned.

    What this suggests to me is that one of the keys to optimizing propeller shape is so find a way to manage the loading at the tip. Winglets is obviously one way to do this, as are up and downturned tips. I also think that tapering the prop section so that it narrows significantly at the tip has promise as well.

    Brenner ...

  19. #944

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    I have not done this much reading since I finished in college.
    Some interesting stuff there.
    Obviously the scope of the study is limited.
    However the efficiency levels predicted are getting there at the cruise speed examined.
    Where he says ;
    ' The geometries of the optimized propeller blades, illustrated in Figures 7.21-7.23 share
    similar shapes. The root chord is small and increases to the propeller mid section with a
    decrease in chord length towards the tip. The propeller blade geometries have a smooth
    shape with no geometric perturbation visible. '
    , reminded me somewhat of the prop on the racing bipe.

    They though want to maximize speed for a fixed amount of power.
    We want a sort of constant pull/consistent speed for least energy consumption.

    Turned tips are certainly a feature in a lot of his resolutions.
    However some parameters , somewhat significant to our case, are fixed or assumed.
    Noise tests aside props that are optimized for a reasonably narrow cruise range are what we need.
    There is no end to this stuff though.

    Brian

  20. #945

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hey Brian,

    I agree that the scope of the study is limited. I'm also suspicious that the author didn't draw any general conclusions about fundamental design principles for optimum prop design.

    My thinking is that he would have done this if he had taken the work far enough, but all he did was show different configurations that resulted from optimizations that used different simplifying assumptions, and since these configurations were all substantially different, this suggests to me that none of his simplifying assumptions were appropriate, or at the very least two of the three sets of assumptions were inappropriate, but he didn't take the work far enough to be able to show which two.

    Brenner ...

  21. #946

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    I agree.
    However taper ,to the tip or both to tip and root, and tip-lets/wing-lets are appearing in his optimized geometries.
    This might be helpful or at least interesting reading and not too heavy going (there is a lot of stuff on the main web site - also interesting reading) ; http://www.mh-aerotools.de/airfoils/winglets.htm

    Brian

  22. #947

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hey Brian,

    Good information...

    Looking at Martin Hepperle's results suggests another possibility to me. Instead of a winglet, why not add a slight negative twist at the wing tip so that the lift tapers to zero at the tip in a controllable fashion?

    I suggest this because in the discussion of the results Martin mentions that winglets only provide a benefit at higher lift coefficients due to the added induced drag, so if we reduce the lift coefficient at the wing tip, we potentially reduce the need for a winglet.

    Obviously this would reduce the lift of the prop, but since the prop loading would also reduce, I'm thinking that the prop would just spin faster to compensate.

    Brenner ...

  23. #948

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    Yes, in my view more taper to a smaller ,rounded and washed out tip would do a lot for noise.
    The 'lost' pitch could be fed back the rest of the blade to maintain the same load to the motor.
    It is a redesign though , but having said that it may be progress.

    However if radial flow/ radial flow losses is/are significant due to our wing spinning the tiplets may still be the favorable method to getting an improvement.

    Brian

  24. #949

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hey Brian,

    Based on Martin Hepperle's work, I would think that a small wing fence near the tip would have a similar enough effect to allow for proof of concept testing.

    Brenner ...

  25. #950

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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner,
    I agree, it's a concept that is worth at least some evaluation and the fence is the simplest/easiest way to start some tests.
    The tip is traveling quite fast at approx 370 kph rotational + 100kph forward velocities.
    So the fence may need a fairing and a downstream shift or extension.

    Brian


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