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C.L. Chennault
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Re: How Models as High-Speed Kites Fly
Reply #7 - May 13th, 2012 at 4:58pm
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This sounds very interesting!  You may have posted it and I missed it, but do you have a video of you flying one ?
  

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Dave L
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CG on Models flown as kites
Reply #6 - May 12th, 2012 at 11:58pm
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I forgot to mention CG.  It doesn't matter much, particularly when the lines are attached near the leading edge of the wing and there's 5 or 6 degrees of incidence in the wing relative to the stabilizer.  This is a good thing because you don't have to add nose weight!  You can build it however you want.  I find that with my foam-filled nose pieces, the CG  comes out about 50% of the wing chord. 

If you tune a plane for high performance, with relatively rearward line placement and 4-5 degrees incidence, the CG has a more noticeable effect on the flight performance.  Rearward CG makes the tail whip around more in turns, while forward CG, like on a free flight plane, seems to make the kite more stable and smooth tracking.
  
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What to do if you're going to crash
Reply #5 - May 12th, 2012 at 11:46pm
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If it looks like you're going to crash, and you have time and it looks safe, let go of the handle!  The kite will usually pull out because it has a lot of up elevator/wing incidence, so it will float gently on the breeze until it settles on the grass somewhere--really!
  
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Launching
Reply #4 - May 12th, 2012 at 11:42pm
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I'll try to get some video of launching, but it may take a while.  Launching is one of the trickier problems in flying these.  I think the difficulty comes from trying to throw the kite like a glider.  Think about what you want the kite to do:  start lifting and pulling itself against the lines and forward using the wind power.  If you throw it, you supply the power, and that power combined with a lot of incidence in the wing or up elevator causes the plane to pull up and bounce off the lines and into the grass.  The trick is to gently let the plane get going on its own with the lines taut.  Luckily, the planes seem to survive launch problems pretty well.
  
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Attaching lines
Reply #3 - May 12th, 2012 at 10:48pm
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I attach lines using lark's-head knots.  This is standard for kite flying with Spectra lines. Here's a nice video from one of the kite companies:

http://www.youtube.com/watch?v=cm50iRZBJQc
  
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The handle
Reply #2 - May 12th, 2012 at 10:41pm
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The handle should be about as wide as the distance between the line attachment points on the bottom of the wing.  I have a 9" handle and an 11" handle.  One's a 3/4" dowel and one's  a rectangular stick.  Sometimes the bigger one is an advantage when a plane won't turn easily. 

I hold the handle vertically, as in control-line.  I'm curious as to how a flier without control-line experience would hold it.  Most kite fliers who use a bar hold it horizontally with two hands.  That would probably work, but it seems like one-handed flying allows for faster reaction time.  I wouldn't try it with those straps that kite fliers use, one in each hand, because the control movements are too small and over-controlling could be a problem.
  
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Dave L
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Flying lines
Reply #1 - May 12th, 2012 at 10:36pm
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Until recently, I had always flown 30" wingspan kites on 100 foot long steel piano wire lines, but recently, since I'm flying these slightly smaller planes with kite flyers in a big park, I've tried Spectra lines, and they're a lot easier to handle while still offering good performance.  The problem I've found is that commercial lines in the 100 foot length are too heavy-duty.  I bought one pair of 80 lb test, they work well in wind over 15 mph, but under that, they make for sluggish performance.  So I read about sleeving and stretching Spectra lines, ordered a 300 yard role of 20 lb test line for $15 or so, and made my own 100' lines from that. Here's the source I found:

http://www.wfilament.com/rec/tuf.html

I found the performance in light wind was much improved, but the plane buffeted from line stretch as the wind got above 15 mph or so.  I would definitely recommend making your own lines, since it's pretty easy and the performance is a lot better.  30 or 40 lb line might be a good size.  Green isn't the best color in grass.  Get yellow.  The sleeving was harder to find.  You can buy sleeving kits from kite stores online, but I found some spider line at West Marine that has a removable core.  You want the smallest diameter dacron sailing line that has a removable core.  You take out the core and use the outside.  Here's the description:

http://www.windwizard.com/knots.html


  
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How Models as High-Speed Kites Fly
May 12th, 2012 at 1:56pm
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Models can be flown as high-speed kites.  Why do this?  It's fun.  It's different.  It may be a better alternative for some kinds of models, such as models of airplanes that are streamlined and fly fast.  And actually, kites are very simple, with no moving parts such as motors, control surfaces, wheels, etc.  They arenít big, because when they fly fast, they pull hard.

How do they work?  Think of a kite line as replacing gravity as one of the forces acting on the model.  Wherever you attach the line is the equivalent of the CG on a free flight plane.  On small, hand-flown models, two lines attach to tabs on the bottom of the wing  between the leading edge and the high-point of the rib camber.

Another force acting on the model as it flies is the stabilizer pushing the tail down so that the wing flies at a high, but not stalling, angle of attack.  Without this down-force on the tail, wings like those on airplanes are hard to stabilize.  Of course, having a tail with an elevator and rudder looks normal on airplanes,, but kites such as deltas or parafoils donít usually have this aerodynamic stabilizer.  They rely on bridles that hold their wings at a stable angle of attack relative to the line.  Sailboats use hydrodynamic forces to keep the sail moving at the best angle to the wind. For built-up models, though, an airplane-like tail works very well.

Now we get to the wing.  Itís going through the air at the highest angle of attack that it can maintain without stalling.  Sounds like a freeflight when gliding, doesnít it?  Thatís right, except that the lift that it generates isnít just enough to hold the plane in the air, itís being used to move the plane forward, like a sail moves a sailboat.  And the amount of lift created increases as the square of the wind speed.  This means that when the wind blows a little faster, the wing creates more lift.  This extra lift pulls the plane forward, increasing the speed of the wind hitting the model, or apparent wind.  This extra wind speed then creates more lift, and on and on. 

Of course, there is another force acting on the model, drag.  Most drag comes as a byproduct of lift, so the wing is the chief culprit.  The better the lift to drag ratio of the model, the faster it can go.  This is why airplane-shaped kites can go much faster than conventional deltas or parafoil kites.  An airplane, even a small model such as a 2-3 foot span P51, has a lift to drag ratio perhaps twice that of a parafoil or delta kite, so the airplane-shaped kite will go much faster, since for the same amount of lift generated, it has only about half the drag.  Since the lift increases as the square of the wind speed, and the lift to drag ratio remains constant, small improvements in L/D result in big increases in speed. 
  
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