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  1. #26

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    RE: Shaft driven props

    Another idea as regards cooling might be to develop a fan-which would sit on the front of the engine-and perhaps have a bit of flywheel incorporated as well-much like some of the early R/C helicopter engines of the 1980s used. The flywheel element would assist starting-and if a pulley groove was fitted, permit belt starting-but this is all getting rather complicated for a .4cc diesel......or even a .75cc if you do go ahead with the full sized model..............

    'ffkiwi'

  2. #27

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    RE: Shaft driven props

    Thanks for the kind words Max Power. The set-up was just run on the bench so there was no extra flight air moving across the fins. In our cool Canadian climate, excessive engine heating may not be a big problem but I want to fly this model in FAC competition in the USA where temperatures can be hot indeed.

    The engine is arranged so that the cylinder head protrudes out of the cockpit of the Caproni and will make it look like a fat-headed pilot is flying it!

    A very astute observation Trisquire. This plan-form in true to scale makes a very bad and unstable free flight model. It has no dihedral, a very short tail moment, too small a stab and small fins. It probably cannot be trimmed to fly as is... but I have a plan... and besides if this wasn't challenging, what fun would it be?

    I am now building a foam sheet prototype to house the power unit for testing out some stability concepts.

    1. I have done this approach successfully in the past with a model of similar plan-form, A Burnelli RB-1 Transport. I will trim the model like you would a flying wing.
    I will make separated moveable ailerons which I can set to give the wings excessive washout, up to 15 degrees. This will usually compensate the model for weak tail moment forces.

    2. To compensate for the lack of dihedral, I plan on adding some clear plastic film sheets between the wing bracing wires at the tips.
    If you look at the plan front view you can see bracing wires on the tip two bays of the wing that have a 45 degree slant. I will put clear film between the wires that slant up so that the resulting planes will have a dihedral effect.

    3. The fact that this will be a large span, very draggy model also provides some added stability especially in yaw. I just hope I have enough power to overcome the drag. In fact, I don't want a hot model, just one that will stooge around in the air for 2 minutes.

    This project is slow going at this time. I had a motorcycle crash a month ago and am still hobbling around on crutches[].

    I will add more details and photos as I progress with this project.

    Orv.


  3. #28

    Join Date
    Jan 2005
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    Columbus, OH
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    RE: Shaft driven props

    Looks like you've thought it all through Orv. They'll award you all kinds of bonus points for such a challenging project. Sorry about your motorcycle crash.

  4. #29

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    RE: Shaft driven props

    Hi Chris,
    I think your idea of a small fan mounted directly to the engine might be a good one. It could pull air in through the cockpit past the fins. I would have to leave some of the cockpit pod open at the rear for an air exit but it might work. I will give it a try.

    Now, when I had the initial running problems with the system, I machined two flywheels of different mass and they sorted out the running problem but created another one. They were both 1.25" in diameter and the lightest weighed 40 grams! Believe it or not but this 40 gram flywheel has the same moment of inertia as a 5 gram 6 X 3 plastic prop! This heavy flywheel would make the model way too heavy to fly on the available power. This is why I had to get the props-only system working.

    A pull-string on the flywheel worked pretty go but would put too much stress on the already light weight model. The electric starter on the pusher prop works real slick.

    Orv.

  5. #30

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    RE: Shaft driven props

    Orv-a fan need not be heavy though-it could be quite light and contribute very little to inertia. I merely proposed the combination as an answer to several conflicting problems-cooling, airflow, starting. You indicated that both of these aspects are relevant. Given the concept-you're going to be up against it regardless-and a long stroke diesel may not be the most efficient option. We all know what the simplest one is-but we won't mention it here..............! Other engine styles have successfully been used in FF scale in the past-including ED Racers and Super Fury's-the antithesis of a Mills..........
    For all its many good attributes, a Mills 75 only produces 0.05 BHP, and if the model needs 0.07 BHP to fly, all the fiddling and cleverness won't get this out of a Mills, no matter which way you slice it......the only option is a more powerful engine.

    ChrisM
    'ffkiwi'

  6. #31

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    RE: Shaft driven props

    Dunno IF this applies:
    But Electric Motors when fitted with driveshafts for Air propellers,
    be they solid, tube or even CF tube, develop Serious Vibration / resonance issues... Airframe damaging typically.
    Band aid finally found was have a flexi coupling at the engine end; Rubber Tube.. Spring etc?
    But the real 'fix' was to gimbal mount the engine itself (yup 2 axis) so the generated harmonics have nowhere to go
    Sounds odd indeed, But having seen it in action.. it's a V effective solution.

  7. #32
    ZAGNUT's Avatar
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    Feb 2003
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    tel-aviv, ISRAEL
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    RE: Shaft driven props

    20 years ago i built a four prop plane using a single .12 sized pull start car engine in the fuselage and small timing belts running to the nacelles. ran the props at half the engine RPM. crunched a wing on the third or fourth flight and decided that multis weren't worth the hassle.

    edit: sorry, forgot to look at how old the thread was[:@]...don't even know how i stumbled upon it...
    \"I\'ll take the money- and you can have the rope\"


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