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At 19:00 16 January 2005, Greg Arnold wrote:
'Apply full rudder opposite to the direction of the yaw string' -- what does that mean? What is the direction of the yaw string? If the loose end of the yaw string is on the right side of the canopy, is the direction of the yaw string to the right, or is it to the left? Seems like there is some potentially confusing terminology being used here that I've never heard before. I've always been taught that the the 'direction' of the yaw string is the side of the glider it leans to, and so you correct by pressing the rudder on the opposite side. Here I think people are saying that if the yaw string is displaced to the right side of the glider it is 'pointing' left. While I can understand how you might naturally want the front end to be the tip of the 'pointer', I think it's confusing to refer to 'pointing' at all because of this left/right confusion. I prefer to say the yaw string is 'to the left' or 'to the right' as position is less ambiguous that the 'pointing' direction. To be honest, I've never looked at the yaw string in a spin as it has never been ambiguous to me which way the world was turning - if the world is going round and round counter-clockwise how can this not be spinning to the right? I guess I presumed that for a spin to persist the glider would have to stay skidding, but in a fully established spin you might wonder, if the yaw string was far enough forward (say in a two-seater), whether the rotation overcomes the skid in terms of the local flow across the canopy - apparently not given the comments here. Boy I'd hate for that to be wrong though. Years ago when I was flying a Ventus A 16.6 (easy to spin unintentionally in my experience) I taught myself that if the inside wing in a turn ever dropped, to push the stick forward and into the turn and to hit top rudder. You try to make it as instinctive as possibe, but it takes practice. The top rudder is the easiest part - the stick movement is against most people's instincts. 9B |
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Andy,
Remember my spin entry tests in the V2b a year and a half ago. One of the departures was so violent, I lost reference, and even though I knew which way I entered, I can see how a surprise stall with the same sort of departure could cause disorientation and application of pro spin rudder. At low altitudes (during a save, for instance), the delay in recovery could be very bad news. As for spin dynamics, it might be interesting to look at airfoil tufts throughout to see what's going on. Anyone have a link? I think most of us envision a spin as a straight line down, the aircraft rotating about this axis. That seems too simple. At any rate, the yaw string should always be displaced into the direction of spin (or average there if oscillating). Staying in the spin requires that the inside wing be producing higher drag (as a result of AOA), and thus the pro-rotation displacement. I won't be able to fly until April, but maybe one of you southwestern types could provide some video of the yaw string through 2 or three full rotations. We can discuss this ad nauseum, but a few pictures would make the discussion much more interesting and possibly fruitfull. Andy, you up for it? |
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Some very interesting thoughts there - especially about
whether or not the glider spins about an axis in a vertically downwards line. The airflows - and especially the behaviour of the yaw string - are very much easier to understand if the glider c of g is thought of descending in a spiral and the centre of the whole spin rotation is inside the rotation circle of the yaw string. From the pilot/glider frame of reference in a steep spin the nearest point on the central axis of rotation of the spin may be a short distance above the pilot's head - or looking down the nose the point of interception of the central axis may be a long way in front of the nose (the first view being like looking down a radius to the centre of an inverted verically descending cone cone and the second being like looking down a side of the cone to the vertically descending point) There is a natural tendency to imagine the spinning glider as rotating a point somewhere near its C of G with the yaw string on the opposite side of the axis of rotation from the tail. But, if the tail and the yaw string were both to turn out to be on the same side of the central axis of rotation of the spin, as described above, then the loose end of the string would, very obviously to everyone, be pointing to the inside spin/downgoing wing side of the canopy - irrespective of the other factors causing the same deviation of the string to the inside that have been described previously in this thread. John Galloway At 17:00 19 January 2005, wrote: Andy, Remember my spin entry tests in the V2b a year and a half ago. One of the departures was so violent, I lost reference, and even though I knew which way I entered, I can see how a surprise stall with the same sort of departure could cause disorientation and application of pro spin rudder. At low altitudes (during a save, for instance), the delay in recovery could be very bad news. As for spin dynamics, it might be interesting to look at airfoil tufts throughout to see what's going on. Anyone have a link? I think most of us envision a spin as a straight line down, the aircraft rotating about this axis. That seems too simple. At any rate, the yaw string should always be displaced into the direction of spin (or average there if oscillating). Staying in the spin requires that the inside wing be producing higher drag (as a result of AOA), and thus the pro-rotation displacement. I won't be able to fly until April, but maybe one of you southwestern types could provide some video of the yaw string through 2 or three full rotations. We can discuss this ad nauseum, but a few pictures would make the discussion much more interesting and possibly fruitfull. Andy, you up for it? |
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I won't be able to fly until April, but maybe one of
you southwestern types could provide some video of the yaw string through 2 or three full rotations. We can discuss this ad nauseum, but a few pictures would make the discussion much more interesting and possibly fruitfull. Andy, you up for it? Good idea. I've just looked at some video I took in 2003 which includes a 1 turn spin in each direction. Initially, as the nose drops, the yaw string points inside the spin, i.e. a spin to the right, the string says use more right rudder. Once the glider starts rotating, the string points outside the spin, i.e. spinning right, string says use left rudder. Can't post it, nowhere to put it. Ed. (Pilatus) |
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