Duncan's got a Flea

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Aug 3, 2017.

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  1. Nov 6, 2017 #521

    rtfm

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    Question:
    The HM293 (and other models as well, I suspect) have a terribly complicated locking mechanism on the underside of the wing fold. The top is a simple hinge, but the bottom locking mechanism seems overly complex to me.

    Why not have a simple slot fitting between two guides with a pin going through all three - a bit like a dead bolt on a gate? One could make it with reasonably good tolerances, and it is as secure as anything else.

    Duncan
     
  2. Nov 6, 2017 #522

    pictsidhe

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    I'd rather have one simple and one complex hinge, than 4 simple ones. I can't find a pic, but many Pous with long wings simply have a long hinge that sticks up into the air on one side, giving the right geometry to fold that wing over the one from the other side. I'm the kind of person who'd build a complex hidden hinge to do that, though.
     
  3. Nov 6, 2017 #523

    cluttonfred

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    FWIW, it's my understanding that the hinges on the HM.293 are completely separate from the fittings that carry the flight loads, and yes, the hinge on one side projects high enough for one wing to fold over the other one. I am not sure what you mean about a complex fitting, perhaps you could post a pic?
     
  4. Nov 6, 2017 #524

    Victor Bravo

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    +1 on having the wing fold hinges NOT be the same parts that carry flight loads !
     
  5. Nov 6, 2017 #525

    rotax618

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    The 293 Flea wing folding hinge and locking pin are about as simple as you could design. The main problem I found was that trailing over bumpy roads eventually cracked the rear hinge which is as narrow as the top of the rear spar. The claw-like fitting on the bottom of the front connectors is a simple device to retain the the removeable pin.
     
  6. Nov 6, 2017 #526

    pictsidhe

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    Didn't you use Aluminium hinges?
     
  7. Nov 6, 2017 #527

    rotax618

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    Most certainly not! Mignet was most emphatic that only steel should be used for all fittings, I used 4130. The hinge probably wouldnt have cracked if the folded wing was cushioned on foam to support it while in transit. In any case it is not a solution to trailer aircraft over long distances, even on a well sprung trailer a ā€œGā€ meter can read more than 10Gs in shock load.
    I used to tow my Flea 60km to an ex military airfield so I could do short hops without having to commit to circuits, the local field was too short.
     
  8. Nov 8, 2017 #528

    rtfm

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    Hi,
    I have a question which I'm hoping one of you can answer...

    In order to balance a rudder, some designs call for a counter-weight in front of the hinge line. Others rely on an aerodynamic surface in front of the hinge line. I wish to opt for the latter. So my question is: how does one estimate how much area needs to be in front of the hinge line? Is it a percentage of the rudder area? If so, what percentage?

    Thanks,
    Duncan
     
    Last edited: Nov 9, 2017
  9. Nov 9, 2017 #529

    rotax618

    rotax618

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    Are you balancing the rudder for mass (flutter) or balncing it for aerodynamic load (make it easier to deflect)?
     
  10. Nov 9, 2017 #530

    rtfm

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    Hi,
    To be honest, I wasn't aware of the difference. But having thought about it, with the speeds flown by Fleas, neither is really necessary I guess...

    Duncan
     
  11. Nov 9, 2017 #531

    cluttonfred

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    There is some interaction, but basicslly mass balance is for flutter, aerodynamic balance for control forces. Traditional Pou-du-Ciel types had aerodynamic balance to lighten the control forces of a large, all-flying rudder moved with a hand not two feet but no mass balance. Mass balance certainly never hurts and the aerodynamic balance surface provides the lever arm to do so with little additional weight.
     
  12. Nov 18, 2017 #532

    rtfm

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    Hi,
    Another question:
    How much rudder deflection is sufficient (generally speaking)? The reason I ask is because it would be really neat to extend one of the rudder ribs (the one which just clears to top of the aft fuse). The idea is to extend it forwards into the aft fuse itself (ie just under the rear wing). Sort of like a tiller in a boat (see attached sketch). Then all I need do is move the tiller arm back and forth to move the rudder. No external wires. And the tiller arm can double as an internal balance weight for the rudder. Quite a neat way to do things, I think. But I would only have about 20 degrees left and right movement if the tiller arm is to remain within the confines of the aft fuse. Is this enough? Bearing in mind that the rear wing will have quite significant ailerons (ie full 3-axis control) so the rudder (even though it is a Pou) will not be soley responsible for turning the plane.

    Duncan

    Tiller arm for rudder.jpg
     
    Last edited: Nov 18, 2017
  13. Nov 19, 2017 #533

    rtfm

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    Hi again,
    Well, I've run out of materials and money to buy more, so I have been whiling away my hours playing with a Kookie-cutter POU design in Sketchup.

    There has been a lot of discussion on the Youtube videos of Peter Sripol's electric foam plane. I'm not ready to entertain electric, but I loved the simplicity of his design. And a lot of the discussion was about how easy and affordable the build was. So I set about to design a Flea which could be built in a month for as little money as possible. Here's what I came up with.

    Of course, building the prototype will take much longer than the (projected) 21 days - because I would have to work out HOW to build it and make the build repeatable. I will also have to make all the jigs, CNC hotwire the foam (and I foresee many iterations before I'm satisfied with the results), CNC cut the hardware, and so on. But once all that has been done, and a flying prototype has been (hopefully) shown to behave itself, building subsequent examples would be relatively straightforward.

    I'm calling this the KC-Flea (Kookie Cutter Flea) for now, but I may change the name if I get tired of calling it that.

    I have tried to save weight and build complexity by (for example) making the gas tank double as the nose section rather than be someting which sits under the nose section. The tank could be transparent (or maybe just translucent) for a visual capacity check.

    I have a CNC hotwire (not assembled yet, but it's all sitting in a box in the workshop) which I will use to cut the foam blocks. I'll also use it to cut the wings. I plan to get a local company to make the nose gear for me and am still thinking about the best way to build the main gear. All controls will be push-pull rods. Ailerons on rear wing. I plan to use two Android tablets for engine instruments and flight instruments. Pretty much plug and play.

    I have the Valley Engineering Big Bad Twin (50hp, 116lbs ready to fly with redrive and prop fitted), but that might be a bit heavy. Still thinking about engine options.

    What do you think?

    Duncan

    View attachment 67303
     

    Attached Files:

  14. Nov 20, 2017 #534

    TFF

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    Why not scale your plane to fit your engine?
     
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  15. Nov 20, 2017 #535

    rotax618

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    Why put the curve in the rear lower strongback stringer, would be stronger if it was straight, and look better.
    Just my opinion.
     
  16. Nov 20, 2017 #536

    Mcmark

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    And for those of us who think it should be a taildragger......
     
  17. Nov 20, 2017 #537

    Victor Bravo

    Victor Bravo

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    Almost everything he has ever drawn and posted on this forum has some version of that curve in it. The version in this last post is the straightest that line has ever been :)
     
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  18. Nov 20, 2017 #538

    nestofdragons

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    Duncan, seeing your sideview i noticed that rear wing has other chord than front wing. Why? Most Flying Fleas have same chords.
     
  19. Nov 20, 2017 #539

    rotax618

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    If you lower the Reynolds number for the rear wing its lift will be less and the forward wing will have to be loaded to a greater amount to prevent catastrophic rear wing stall. It is best to stay within th Mignet formula unless you have access to a wind tunnel.
     
  20. Nov 20, 2017 #540

    radfordc

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    And...they have all performed so well!
     
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