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Rave HG.jpg
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12 Comments

  • Tony  Castillo
    by Tony Castillo 11 months ago
    In the subject of twist, washout, billow ... I always liked this photo as illustration. Launching from Lookout Mountain, Wills Win Raven in a shallow right turn. Check out the twist (what we called billow back then - well at least I did call it billow) in the right wing ... also the kill pocket & keel tube shift and take a look at the washout difference in the wingtips... all this flexing, reshaping and shifting helps our wings turn.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Fantastic example Tony! I don't think we could find a better photo illustrating Billow shift
  • Tony  Castillo
    by Tony Castillo 11 months ago
    The WW Raven had a fixed cross bar (not floating cross bar as most all WS wings do have nowadays) and that required a large keel pocket to allow proper shift on the trailing edge, overall sail twist and enhance the roll.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Again tony this is the best shot I have seen that shows wing twist, washout, billow shift what ever you want to call it. If I remember correctly this was before the WW HP, the first production wings WW HP (modern flex wing with no floating cross bar), that really scared me. Move your weight over to turn and not much happens. Second most scariest was the more modern Trike SST wing. Third was the Kiss Hang glider.

    The problem with hang glider wings is that unless you have a allot of altitude you are going to get near the ground soon unless you can gain altitude in lift which is not often guaranteed.

    It least with trikes you can power out and get plenty of altitude and choose your landing time/place/areas.

    My most significant and I feel my important quote "The air is soft and the ground is hard."

    When I first started flying ultralights (power), everyone was asking me: why do you fly so high when you could fly lower?

    How would you answer this question?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    I'm guessing there is around 20 degrees difference in AOI between the wings. Assuming that's a 30 foot wing span how fast would he have to be rolling for the AOA to be equal if he is going 35 MPH?

    It sure looks like the keel is pulling the billow out of the left wing in this photo. I guess it's all leading edge flex.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Fast... real fast! Great shot, Tony!

    The Raven is an interesting case. The first attempt at this design, if I remember correctly, had a higher span, lower twist, shorter chord, and less area than it's predecessor. On paper it should have had a significantly greater L/D. In practice it didn't. It was a flop.

    The next attempt was to produce an easy handling, modest performing beginner's glider. It turned out to be a sparkling performer. That's the Raven.

    The point being that the best of us don't know everything.

    Paul, I think you meant to say that the HP had no keel pocket - it has a floating crossbar. Between the Raven and the HP was the WW Duck, and Ive always loved that name. The competition version with stiffer leading edges was called the Attack Duck. Gotta love their humour... :-)
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Very very roughly, as I don't have a calculator, the roll rate in the picture above is 7 degrees per second. I'm guessing conversions from mph to fps and what the tangent of 20 degrees is...

    And I keep changing my mind about that conversion of mph to fps. I'm sitting on a boat on a hot day with beersies at hand, and can't decide. Can we go metric, please??? :-)
  • Vince  Morson
    by Vince Morson 11 months ago
    Hey Paul- I used to own a Pacific Airwave Kiss back in the day- it was a 154 sq footer if my memory serves. It was amazingly easy to thermal, apparently because it had more anhedral than most wings. Actually, it had a little too much I think, because it liked to increase its bank angle automatically- otherwise known as spiral instability. This is probably why it makes your "scariest glider" list. Overall, it wasn't a bad wing though.

    Funny thing is I now fly a Kiss 13 on my trike- something about Kiss on a wing that just does it for me I guess!
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Vince,
    That is awesome with a Kiss on a trike. Thanks for not being sensitive and offended by me saying the Kiss was one of my scariest wings. After I got used to controlling the yaw it was fine. Just like MOST wings, if you have the time to get the time and/or transition training to figure out all the flight characteristics you will be allot safer.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes Bryan the MPH and FPS is a pretty barbaric system. Divide things by 12 and multiply by 1.47. I agree. When we had the civil war with England and broke away to form our own country we somehow took this with us and have been stuck with it ever since.... Perhaps the WW HP did have a floating cross bar but it did not feel like it.

    Amazing number of hangies here who went to a motor.......
  • Bill Chance
    by Bill Chance 11 months ago
    HA HA Paul the split wasn't that far back. Although they talked about it for a hundred years before, the UK didn't plan on going full metric until the 50's and didn't fully start planning until the late 60's. They hung on to the Imperial measurement system (and still do). Imperial Measurements for spirits was phased put in 1988 but its still illegal to sell beer/cider in anything other than pints. Didn't mean to high jack Tony's thread.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Paul, I think your comment on the HP nails it. The HP was the first modern glider to do away with the keel pocket, the theory being that it could operate exactly as Larry describes with his wing: a lateral weight shift by the pilot pulls the keel across while yaw stability steadied the wing, and the keel movement puts billow into the downgoing wing.

    In the case of the HP, it didn't work. There is sufficent aerodynamic damping that the effect you describe on your thread of the pilot pulling the wing down is strongly resisted, and on the HP the pilot must pull themselves in an arc, with an upward component, to get the roll initiated before billow shift develops and comes to the pilot's aid. HP was supposed to mean High Performance, the joke was that it stood for Handling Problems,and weight shift became 'shift, wait'.

    A keel pocket in conjunction with floating crossbars isolates the lateral movement of the pilot from a fair bit of the damping of the wing, meaning that the keel can move sideways without having to make billow shift happen. This may slow the roll rate but it means the pilot is moving more sideways, less up. So the roll pressure is reduced.

    I believe Larry gets a sequence of lateral pilot shift with keel movement and billow shift occuring almost simultaneously on his wings. This runs contrary to what I've seen on keel pocketed hang gliders. I've not got solid evidence of this, but both modes make total sense and it seems fair to suggest different modes are in operation on different wings.

    Roll rate and roll pressure are different beasts and can be addressed differently. Another factor is coordination, as Vince says. The Kiss sacrifices coordination and needs that high siding to balance, as the manufacturer added a heap of anhedral in order to get good roll rate coupled in from yaw. One of those embuggerances in aircraft is that there's no such thing as a free lunch. :-/

    Is anyone going to correct my maths above? My mental arithmetic is off...
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