Page 2 of 2 FirstFirst 12
Results 11 to 13 of 13

Thread: How do they do it?

  1. #11
    rwanttaja's Avatar
    Join Date
    Jul 2011
    Location
    Seattle
    Posts
    2,948
    Quote Originally Posted by Siggypoo72 View Post
    A semi-supine position might be much more comfortable. As for the pilot above weight shift prob, I was trying to wrap my head around what you said, Ron. Do you think greater dihedral would help counter that tendency?
    Thanks for any feedback. I want to get this right, and am rather ignorant.
    Like I said, I'm not an expert...perhaps someone else here will chime in, who knows better.

    A useful analogy would be a child's swing vs. a see-saw. The swing is like the conventional hang-below weight-shift design. You can sit in the swing, and via weight shift only, get the swing to oscillate. But if you stop pumping, the swing will return to the stable position, hanging directly below its attach point. If Peter Perturbation comes along and shoves you (the cad!), it'll displace you from the stable position but you'll quickly return to it without further action.

    The pilot-on-top weight shift is like the see-saw. Imagine standing on it and straddling the pivot point. It doesn't take much to hold position. You can even lean over a bit (a bit!) and hold the plank in a slightly tilted position. But too far, and that end will come crashing to the ground without you being able to stop it. The same thing will happen if Gary Gustfront sneaks up and pushes one side up. If your reflexes are good, and you see him coming, and he doesn't push TOO hard, you might be able to recover. Odds are, no.

    And recall, your design uses weight shift for both roll and pitch. The see-saw in this case doesn't have just a hinge across the middle, the whole plank is resting atop a cone. Any motion...fore and aft, left or right...is going to displace your position and will require a countering action to keep things from going goofy.

    Yes, increased dihedral will help. If you run a computer program to determine how MUCH dihedral would be necessary for stability, it'll probably give you a value that puts the center of lift ABOVE the pilot. In other words, it becomes a classic weight-shift design.

    There's a lot of argument about the need/desirability for stability. Some pilots believe that neutral stability produces more responsive aircraft. However, one must not forget that the Pitts Special is a fully Part 23-certified airplane...no one complains about ITS stability.

    The problem is, it's my believe that this "inverted" weight shift arrangement produces negative stability. It'd require a very good pilot to keep it under control, and it'll probably require their full-time focused attention. Not fun.

    I do like your design, though, Arthur, and look forward to hearing how your RC tests come out...with whatever configuration you end up with.

    Ron Wanttaja

  2. #12

    Join Date
    Oct 2011
    Location
    WA
    Posts
    1,205
    I don't think weight shift works well in zero g maneuvers. Or upside down.
    Weight shift does work at slow speed, that is good.
    After Otto Lilienthal died, the Wright's set out to improve upon the limitations of weight shift.

    I don't like weight shift.... Too much muscle work for me.

  3. #13
    So sorry for the long wait between replies. I just returned from Cuba where I had my wedding.
    My design has been has been edited to include significantly more dihedral and a four-degree washout at the wing tips. I've yet to finish construction, but the clamshells have been broadened to provide more drag when opened. The pilot no longer lays above the fuselage, but now directly on top. I expect with that there will be no exacerbation in pilot CG changes.
    as I mentioned before, for the sake of construction simplicity and reliability there still are no control surfaces beyond the clamshells. I now have them opening from high-torque servos instead of a physical cable or push-rod connection.
    with the advances in motor and fan design and efficiency I'm now also exploring electric ducted fan options instead of RC turbines. Less thrust but much less expensive and maybe more relatable. I'm still not decided on which airfoils to use, but the Marske 7 is still looking good. This isn't a high-speed design, and the high-lift/low-drag Marske 7 seems ideal. There's a much higher sink rate with it over other reflexed airfoil designs, so I'm thinking it bleeds airspeed faster than others too. With my models, as the velocity of the plane diminishes, I've had to add throttle to counter much earlier and to a higher degree than I did with other wings. But at speed it's so super stable, and forgiving.
    My mindset is so old-school, I've taken the full size glider out and learned what changes to make that way.
    here in Oshawa, there's one spot on Rossland Road where the road descends over 100 feet over a bridges over a creek, then back up hill towards Oshawa Airport (CYOO).
    Around 2am I've taken the plane out there, laid down on it and had friends push me down the hill. Yah, I'm silly I know. But I'm a tactile kinda guy, lol.
    Last edited by Siggypoo72; 03-27-2016 at 01:33 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •