The Mystery of the F-arc

Kitesurfing/Kiteboarding on Liquid H2O

The Mystery of the F-arc

Postby Snake » January 14th, 2013, 6:10 am

The F-arc, one of the best awesome low wind engines that exists. They were made over 10 years ago yet they have the record for fastest depower kite and go up wind better than almost any kite. Crazy lifty and amazing hangtime. Some call it a death trap, others a thing of beauty. What makes these kites so high preformance?

Aspect Ratio and profile. The combination of crazy high AR and a profile similar to a paraglider makes the F-arc have all it's crazy goodness. The profile is an important part of what makes a F-arc so high preformance, but the AR is what makes them what they are.

Twinskins lack of bridles makes them have unique characteristics. The 16m F-arc , with an AR of 7, does something strange when is gets moving.
http://www.youtube.com/watch?v=3cT3EEeRnU8
Notice how the kite seems to open up once Hardwater starts moving. Using multiple spots in multiple videos to take measurements. I have calculated the AR of the kite in the arc shape when moving to be 5.625 with a projected area of just under 13m. Peter lynn himself states the S-arc flys with a near perfect arc. This is true for all arc except for the f-arc and maybe the phantom. If the f-arc flew with a perfect arc then it would have a projected area of 9.5m and an 4.43 AR when in the arc shape. My theory of why this happens is that the amount of shoulder the kite creates is proportional to the cord of the kite, not the area.

All of this means that the f-arc, when moveing, is equivalent to a lifty 13m kite without bridles. Bridles add alot of drag to a kite. One round bridle line creates as much drag as an airfoil 10x as thick. Being bridle-less the f-arc has less drag than a bridled foil. Less drag mean more prefomance and light wind power.
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Re: The Mystery of the F-arc

Postby Timothy Grossnickle » January 15th, 2013, 11:39 am

Had a F-arc back in the day. Happy the day I sold it. Found it a complete bear to launch often going to a bow tie due to its long narrow shape. Super fast across the water once you got it going with precious little grunt. Certainly a kite better suited to land/ice use generating a lot of speed, but little low end power. Brian Holbrook, the USA rep for VO told me F-arcs still available as a special order. He used one to set a speed record on a cart in Nevada recently. Got a lot of comments with the F-arc on water when I used it. After F-arc PLk in NZ produced the highly successful wider winged Venom water kite. Then they went back to the high aspect F-arc design with the Scorpion which Peter Lynn designed to pull his kite sled to South Pole. This limited use kite with little grunt and tendency to bow tie on launch proved the undoing of PLK in New Zealand leading to the transfer of design operation to VO in Holland. In this regard, narrow high aspect wings as seen on fighter planes and land racing kites fly fatter than the fatter wings on passenger planes and water kites. The relative lack of success of the F-arc and Scorpion as commercial water kites illustrate their lack of value on the water....
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Re: The Mystery of the F-arc

Postby Timothy Grossnickle » January 15th, 2013, 11:43 am

Correction to post above.... "fly fatter" should read "fly faster than fatter wings"
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Re: The Mystery of the F-arc

Postby Snake » January 16th, 2013, 3:46 am

I just found some vintage 2002 F-arc footage. The airtime is insane!!! I'd say the 16m has as much airtime as a speed 3. There is even some footage of the rare 9m version.
http://www.awindofchange.com/photo.html
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Re: The Mystery of the F-arc

Postby Hardwater Kiter » January 16th, 2013, 12:41 pm

I was hot and bothered for a 9m F for a while. Thought it would be a wicked speed machine. I spoke to Chris Brent about it and he said they took some 12m's and removed some cells. So yes it's a 9m but with a reduced AR compared to it's siblings. I'm sure the 9m is still wicked fun to ride but I suspect that it lacks some of what I like about the 12 and 16m. I don't doubt that a Phantom 2 or Charge 1 can give me the the same results that I would have gotten on a 9m F.
"The rider makes the kite."
Owner/instructor @ http://www.hardwaterkiter.com
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Re: The Mystery of the F-arc

Postby Snake » January 17th, 2013, 12:16 am

I kind of want to make a 9m. I think I read that all that was done was cutting out 3 cells per side. I also read on the old arcusers that there was a 14m. They said "modded 14m" so it may have been 16m with some cells removed. Either way it sounded like it tamed it out a bit but still had all the float and speed.
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Re: The Mystery of the F-arc

Postby Snake » January 22nd, 2013, 5:28 am

I think I figured out why the F-arc opens up when flying. This weekend I noticed the kite opens up slightly when on the decent a of a jump. I after some research on aerodynamics I found the solution.

When you are falling, your decent is perpendicular to the true wind, createing apparent wind diagonally across the shoulder of the kite. Since the kite is in an arc shape, the curve of the kite slightly deflects the wind createing more lift in the shoulders than in the center, which causes the kite to change shape and and become more open. This can be applied to rideing horizontally also as you create apperent wind when you begin to move.

I will post some pictures tomarrow since this is really confuzing to understand.
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Re: The Mystery of the F-arc

Postby Snake » January 23rd, 2013, 3:15 am

An arc will always fly in a near perfect semi-circle when it is stationary. When the wind speed increases, the lift everywhere on the kite increases, so the kites shape stays the same.

The reason the kite opens up when the apparent wind starts to develop is because there is another force acting on the shoulders of the kite. The apparent wind moves over shoulders diagonally at the angle of the lift to drag ratio. This causes the the curve of the arc shape to deflect some air, which, according to newtons 3rd law, causes the kite to react in an opposite direction, which is outwards. This causes there to be more lift in the wingtips, changing the shape of the kite to have more projected area.

My original observation in the first post is backed up by theory. I used a lift to drag ratio of 8 to do my calculations. This gave me an additional 3.55 degrees of deflection which increases the lift 35.5% in the shoulders. This in tern causes the kite to open up to a projected area of 12.8725m, not far off from my inital guess of 13m.

This picture shows how the extra lift is created. The air (black arrows) curves around the skin of the kite (blue arc), which pulls the wingtips outwards.
Image

This picture shows the kite (orange box) from the side. The red arrow is the true wind, the blue arrow is the apparent wind, and the black arrow is the the wind created from forward movement. The blue line shows how the wind flows over the shoulder of the kite. The angle of the apparent wind is drawn much steeper than what it would be to help illustrate how the wind is flowing.
Image
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