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  1. #976

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

    Hey Malcolm,

    I was thinking that as the blade is narrowed, more of the load is transfered to the inner 50% of the blade, and here the pitch is slightly larger, which might partially compensate for the reduction in pitch towards the tip.

    Brenner ...

  2. #977
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Brenner,

    I'm no expert on this but certainly at smaller dias narrowing and washing out the tips always gave an increase in rpm both on the ground and in the air.

    Brian, get that prop on the drive and give us the numbers!

    Malcolm

  3. #978

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

    Hey Malcolm,

    I agree completely. I'm just saying that maybe the increase won't be as large.

    Brenner ...

  4. #979

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

    Hi Malcolm,
    Like you I'm no expert on this - I'm trying to interpret M Hepperle's guidance.
    The thinking is to reduce substantially the work right out at the tip.
    If this is done by modifying a current prop it will spin a bit faster but the work will shift inboard - perhaps considerably.
    This should happen for two reasons ; the pitch load is substantially reduced at the tip as is drag ,friction drag and in particular induced.
    These are primary noise sources too.
    It seems ,to me anyway, that high pitch at the tip results in a high work load a lot of which goes directly to loss as it spills out off the tip.
    This 'spillage' may be adding to the noise level.
    Martin is saying extra diameter and or wash out is better and more practical than tiplets/fences etc.
    We can't modify existing props to extra diameter - that is a redesign.

    Brenner;
    To what extent do you think I should wash out the tip ??

    That quick job I did to post a photo for Malcolm actually looks quite good. Done properly or more meticulously it should look the part.
    I reduced the tip chord from 10.25 to 7mm at the TE - so not a lot of material removed at all.
    I did this over 60mm of span. This has the effect of making the TE thicker as gets out to the tip.
    Sanding this back ,on the back side , to the original TE thickness takes out pitch as does sanding down the front side forward 30% of chord.
    It is surprisingly stiff twist wise - smaller moments !.
    I have worked out a method to twist with heat if needed - but it will involve making an fairly elaborate jig.

    Brian

  5. #980
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Brian,

    I agree with your methodology right up to the part where you sand the front of the prop.

    This was alway a no no in the old days as you are modifying the aerofoil shape and as Martin says at this scale its difficult to do accurately and you don't really know what you are trying to achieve with a mod. Work on the back and keep it flat and all you are modifying is pitch and to a all extent thickness.

    Malcolm

  6. #981
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Sorry working on my phone over 3G that last sentence should read "lesser extent thickness"

    M

  7. #982

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

    Hi Malcolm,
    I was just trying it out to see how much pitch I could 'wash' out.
    However I think I can get some there safely enough - microns maybe .
    A better perspective in this pic,, .

    Brian
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  8. #983

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

    Sorry to interrupt a great discussion.....

    Brenner, back to the new Neu adaptor: do you have an estimate as to when it will be available?

    I took apart my drive tonight and found that 4 of the 8 teeth are almost gone. (1 season of flying, and it should be noted that my motor is one of the ones with the swapped spacers resulting in less engagement distance)
    From Neu's website, an estimated repair cost to the motor is $126 + 2x shipping. So, I'm not excited about putting a newly repaired motor on the existing adaptor.

    The good news it that it seems like I'll be able to get additional life from the current pinion by carefully assembling it so that the unworn teeth now engage the drive.

    Another reason I'm looking forward to the new adaptor, is that I've always suspected that the clatter in that interface contributes significantly to the overall noise.
    ( it sounds really bad at startup)

    Thanks, Dan
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  9. #984

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

    Hi underdw, my motor had the exact same problem and I have had the repair done plus bearings. I would also be interested in the new adapter.
    The noise at startup could also be settings on your ESC. If the timing is set too low (I think) there may be oscillation or rough start.. Worth experimenting with maybe?
    Paul Houlihan

  10. #985

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

    ORIGINAL: Malcolm H

    Brian/Brenner,

    About 100 years ago when I was actively into control line teamrace we habitually used to washout the tips of our 7 x 8'' Bartels glass props by removing some material from the last inch of trailing edge towards the tip in a triangle as viewed in plan and then using a flat sanding block thin back the trailing edge keeping the back of the prop as flat as possible and washing out the tip.

    The result was more revs than the standard prop without losing dia and h99opefully an increase in efficiency. The problem I see with modifying a standard pair of props in this way will be an increase in revs and hence noise both static and airborne. I believe the only way this approach will bear fruit is with an increase in dia which isn't really desirable or a redistribution of blade area which might be worth experimenting with.

    Malcolm
    Hi Malcolm,
    One thing worth remembering is that ;
    'In the old days' ;
    Engine noise and exhaust noise were significant factors for a noise test.
    So a relatively small rpm increase made what seemed to be a lot of extra noise due to tone and pitch chances in the engine sound.
    Our entire focus can be on prop noise because our engines are so quiet.

    Brian

  11. #986

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


    ORIGINAL: underdw

    Sorry to interrupt a great discussion.....

    Brenner, back to the new Neu adaptor: do you have an estimate as to when it will be available?

    I took apart my drive tonight and found that 4 of the 8 teeth are almost gone. (1 season of flying, and it should be noted that my motor is one of the ones with the swapped spacers resulting in less engagement distance)
    From Neu's website, an estimated repair cost to the motor is $126 + 2x shipping. So, I'm not excited about putting a newly repaired motor on the existing adaptor.

    The good news it that it seems like I'll be able to get additional life from the current pinion by carefully assembling it so that the unworn teeth now engage the drive.

    Another reason I'm looking forward to the new adaptor, is that I've always suspected that the clatter in that interface contributes significantly to the overall noise.
    ( it sounds really bad at startup)

    Thanks, Dan
    One would think that if it's a manufacturing defect then the repair would be free....

    Cheers,
    Jason.

  12. #987

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


    ORIGINAL: Jason Arnold



    One would think that if it's a manufacturing defect then the repair would be free....

    Cheers,
    Jason.
    Thanks guys,

    I will of course make the case for reduced repair cost, but given the severity of the wear, it's hard to argue that no significant wear would have occurred had the extra 3mm tooth engagement existed.

    Paul,
    I will verify my ESC settings, which I think I got from Dave Lockhart.

  13. #988

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

    Hey Dan,

    We have a redesigned pinion gear interface available right now. We've just started shipping new Drives with it. We are going to a bronze interface that won't cause the motor rotor shaft to wear. We are doubling the length of engagement, and going to eight spline contact. I'll post pictures when I get home.

    If you have time, drop your gearcase in the mail to me, and I'll update your system at no charge.

    Brenner ...

  14. #989

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

    Hey Brian,

    Your modified props look wicked! Have you tried them yet?

    Brenner ...

  15. #990

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

    Hi Brenner,
    I did that really quickly to show Malcolm etc. - 1 picture = 1000 words !
    I will do a pair or 3 (2 front sizes) properly in the next few days.
    Then need a calm day to do noise tests and inflight evaluations - days to suit are few and far between here now.

    How much wash out do you think I should go with.
    There is no going back once I cut.

    Brian

  16. #991

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

    Let me think about the washout for a bit. I've got to do some figuring.

  17. #992

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

    Hi Brenner,
    Ok - that I can understand
    I'm a bit bamboozled myself.
    However in this article ,to which you put the link a few pages ago, Paul Lipps talks of tapering to zero chord.
    The degree of twist and the small tips are amongst the striking features of his props.
    His interest in noise is for efficiency only - no noise test there.

    http://www.eaa.org/experimenter/arti...02_elippse.asp
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  18. #993

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

    Hey Brian,

    Yes, his design makes more and more sense in light of the previous discussion here.

    Brenner ...

  19. #994

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

    Hi Brenner,
    Yes indeed.

    If one was to ignore the aerodynamics for just a moment and examine a prop from a pitch or pitch speed point of view only.
    I mean to work out the pitch speed at all the points from spinner to prop.
    To get all of the point values to the same 'pitch speed' will require a huge twist - like above or more even.
    Also if our props (I mean all aero props) were operating in a medium, say like ballistics gel, they would just get stuck due to the pitch to travel mismatch along the length of the blade. To travel through the medium the twist from tip to root would have to be almost 90 degrees. It would have to align to the shaft as it reaches the center line of the shaft.

    Paul Lipps degree of twist seems very close to what I'm trying to describe.
    He is focused on air entry angles , ie ;as a function of rotational and forward velocities.
    I know that the aerodynamics really complicate things due to the velocity changes along the length of the blade and the resulting effect on lift and drag.
    He seems to be trying to strike a new balance between the physics of driving a screw through a medium and the aerodynamics involved.
    It seems that he is trying to get to where all ,or most anyway, of the blade is working efficiently at the desired cruise speed.
    That is our aim too.
    I'm just not qualified to comment on whether he has got it right or not but he makes some great points.

    Brian


  20. #995
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    RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

    Hi Brenner and Brian,

    Like you guys I think Paul Lipps' ideas are worth pursuing but its worth making the point that your existing props have a good area to span proportion and work very well as is. As consumers we are just very lucky that the Contra manufacturers are so development concious!

    One other point worth mentioning is that props both model and full size approach or even exceed a "square" pitch to dia ratio. I think that rather than increasing the dia of our props there is scope for increasing the pitch some and if necessary increasing the gear ratio to stay within a chosen overall motor to prop gearing. There may be static noise issues with this but I'm unsure whether the potential for increased "slip" would harm or help the situation.

    Malcolm

  21. #996

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

    There are also a couple of things to note about the current Contra props, that might be of use to consider:

    1/.. The "disk power loading" for each prop (W/mm^2) is close to half the disk power loading that a single prop setup has. This improves efficiency, and it also changes the angle of attack that each prop runs at in a steady state condition. For instance. my calculations show that at maximum horizontal speed a 22x22 setup is running at a theoretical pitch speed of -11 mph, and a 22x18 setup is running at a theoretical pitch speed of -3 mph, which in and of itself is perhaps a good argument for higher pitch props, but it also suggests to me that additional washout isn't going to improve noise or efficiency much during high speed flight. However, static noise testing on the ground is a different matter.

    2/.. When the Contra is run at full throttle on the ground, (like during noise testing..) the front prop is stalled, and the rear prop is probably partially stalled, so under these conditions, I think that anything that can unstall the blades is probably going to help with efficieny and noise. Here additional washout near the tip could perhaps help.

    Brenner ...

  22. #997

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

    Here are some pictures that show our new pinion gear interface. We have separate inserts for the Neu motor and the Hacker C50 motors. However, the Neu motor insert now only works with the new version of the Neu motor.

    We are willing to update anyone's drive for no charge, if they can send their gearcase back to me. At the same time, I will replace any bearings that need replacing as well.

    Brenner ...
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  23. #998

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

    Hi Brenner,
    Getting more bullet proof all the time.
    That gold looks great.

    Brian

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

    Brian, I would also call his/their service and follow-up first class and worth it's weight in gold! Regards, Everette

  25. #1000

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

    ORIGINAL: Brenner

    There are also a couple of things to note about the current Contra props, that might be of use to consider:

    1/.. The ''disk power loading'' for each prop (W/mm^2) is close to half the disk power loading that a single prop setup has. This improves efficiency, and it also changes the angle of attack that each prop runs at in a steady state condition. For instance. my calculations show that at maximum horizontal speed a 22x22 setup is running at a theoretical pitch speed of -11 mph, and a 22x18 setup is running at a theoretical pitch speed of -3 mph, which in and of itself is perhaps a good argument for higher pitch props, but it also suggests to me that additional washout isn't going to improve noise or efficiency much during high speed flight. However, static noise testing on the ground is a different matter.

    2/.. When the Contra is run at full throttle on the ground, (like during noise testing..) the front prop is stalled, and the rear prop is probably partially stalled, so under these conditions, I think that anything that can unstall the blades is probably going to help with efficieny and noise. Here additional washout near the tip could perhaps help.

    Brenner ...
    Everette,
    It's so good it's almost ott. The guys have been great since the get go and as Malcolm said they have a great interest in development/ quality improvement.

    Brenner,
    That's the dilemma , noise test V cruise.
    In lay man's terms it would seem that if a lot of wash out is put in for the noise test then this would effectively be negative pitch at cruise/top speed.
    However we need to be successful in both areas.

    I'm leaning towards less and less tip chord as the best route to compromise.

    Re the two power discs V one and efficiencies therein - this is something you said a long time age and it has remained in the back of my mind since.
    It may have created an undue expectation ,in my mind, that we would see a reduction in battery consumption with the contra.
    I hasten to add that we have not seen an increase - yet we get all the contra benefits.
    My own experience is that the contra helps with consistency in my flying thus more consistency in battery consumption

    Brian



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