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What force turns a twintip?

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sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Sat Jun 08, 2019 12:50 pm

Herman wrote:
Fri Jun 07, 2019 6:32 pm
Sergie. In my experience when a novice is told to edge harder they naturally weight the back heel and this makes them turn upwind. Similarly when they are told to flatten the board off they naturally put more weight on the front toes and this makes them bear away. This can take you to a good level. Eventually when you can ride well you will be able to revisit board trim to get more out of the rail and use it more optimally to get upwind.
When you come to do your first turns just fly the kite up, lower your bum into the water and water start in the new direction. Once you have mastered that turn by putting loads of pressure on the backfoot as you fly the kite up. Firstly you will feel the board carve into wind as you increase its angle of attack to the water, as your velocity decreases water flow across the board slows and the back end of the board will slide out pointing to a broad reach in the new direction. You can then dive the kite and ride off on the new tack. This will be a real "EUREKA" moment for you. It is well worth persaveverring!
Thanks Herman.
I think I really need to practice 1-2m runs left and right a lot. I think I can do your lying back drill reasonably already. Watched a video on YouTube where an instructor suggests doing lots of short runs and immediately stop by bringing kite back to 12 and pushing bar away. I really need to practice pushing bar away too, so seems like a great drill for me. We have 12 mph tomorrow on our Troon (Scotland) beach so might try my 17 Slingshot Turbine. Should have lots of power for my 145x48 Allround CF.
What do you think - good idea? :
around 3:19

Herman
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Re: What force turns a twintip?

Postby Herman » Sat Jun 08, 2019 2:58 pm

I think this is an excellent video and is good advice

17m is fine if you are a big bloke.

Your homework before going to the beach, promise me you will do it, sit on the living floor in what you think is the water start position holding your bar. Now without hurting her back get the Mrs to pull you up to your feet, she probably won't be able to. Now really bend those knees and scrunch up pushing head over your knees. If you are in the right position the Mrs should be able to pull you up quite easily, maybe even one handed.

Nobody ever seems to do enough land based kite control practice before they hit the water - it is just the nature of the beast!
When it is safe for your 17m LEI on the water imho it is just as safe if not safer to be flying a 6m LEI on land assuming you have clear space.

You can mess about body dragging, particularly if you have flat water but don't worry if you can not get upwind. Usually, unless you have ideal conditions you have to go back to body dragging once you have developed your kite control skills.

Land practice and visualisation is great but temper your expectations. Once you grab a big kite in a new environment the brain will light up and say *TILT*, but it will all come together as you start to get comfortable.

Take heart in the fact that once you are riding and a comfortable with proper power all this tricky stuff of getting in position to water start etc is a complete non issue (until you start hydrofoiling ha ha!). Good luck!
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Herman
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Re: What force turns a twintip?

Postby Herman » Sat Jun 08, 2019 4:45 pm

Sergie, last input was heat this is ice after a bit of rehab.
I may be teaching you to suck eggs and so if it's old hat just ignore.

The so called depower strap is, imho, better referred to as the trimmer or trim strap. Pulling in the trim strap effectively shortens the front lines which reduces the kites angle of attack. This indeed reduces the power generated in most modes of flight but not all. I will give you an example of where pulling in the strap will actually vastly increase kite power later.

Generally speaking, novices forced to ride rather underpowered so when they lose control of the kite they do not slammed too hard. In these conditions it is particularly easy to get the kite to backstall. This happens when the kite is over sheeted. The kite will fly backwards into the power zone usually with moderate or less power. If you let the kite get well back into the power zone before you sheet out you could be in for a yarding as the kite powers up and it fly's forward. Unfortunately the novice is most unlikely to have the skills to prevent being dragged in this situation, if they go for the qr they let the bar out and get dragged while reaching for the qr. The only real solution is do not let it happen in the first place. Practice for this is flying a small LEI on the beach where you can handle the power generated from a dead downwind launch. With these conditions it's easy to set up for backstall. In light conditions sheeting out may not be enough to recover the kite. In this case grab the frontlines and pull; release as the kite flys back up. If you don't release the front lines the kite will fly to the edge of the window and Hindenburg - to be explained in another therapy session if you want?

If you understand the above you can work out your own example of when pulling in the strap will actually increase power!
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sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Mon Jun 10, 2019 10:59 am

Herman wrote:
Sat Jun 08, 2019 4:45 pm
Sergie, last input was heat this is ice after a bit of rehab.
I may be teaching you to suck eggs and so if it's old hat just ignore.

The so called depower strap is, imho, better referred to as the trimmer or trim strap. Pulling in the trim strap effectively shortens the front lines which reduces the kites angle of attack. This indeed reduces the power generated in most modes of flight but not all. I will give you an example of where pulling in the strap will actually vastly increase kite power later.

Generally speaking, novices forced to ride rather underpowered so when they lose control of the kite they do not slammed too hard. In these conditions it is particularly easy to get the kite to backstall. This happens when the kite is over sheeted. The kite will fly backwards into the power zone usually with moderate or less power. If you let the kite get well back into the power zone before you sheet out you could be in for a yarding as the kite powers up and it fly's forward. Unfortunately the novice is most unlikely to have the skills to prevent being dragged in this situation, if they go for the qr they let the bar out and get dragged while reaching for the qr. The only real solution is do not let it happen in the first place. Practice for this is flying a small LEI on the beach where you can handle the power generated from a dead downwind launch. With these conditions it's easy to set up for backstall. In light conditions sheeting out may not be enough to recover the kite. In this case grab the frontlines and pull; release as the kite flys back up. If you don't release the front lines the kite will fly to the edge of the window and Hindenburg - to be explained in another therapy session if you want?

If you understand the above you can work out your own example of when pulling in the strap will actually increase power!
Thanks Herman. I understand backstall and generally are watching out for kite moving backwards.
Made some progress yesterday doing lots of short runs as from video above.
This is a great drill with little space lost on every try. So I think is much more productive. Had a couple of longer runs going left (my trouble side) where I managed to sign kite twice before sinking!
There was not enough wind for my weight and skills with 12m. 15mph.
Did not try 17m as appeared to have knots on the middle lines...
Thanks

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Re: What force turns a twintip?

Postby Herman » Mon Jun 10, 2019 12:09 pm

Sergie, glad to hear you had a good and productive session. I been back in a buggy now, 20knts with tiny 4m Lei, lovely just cruising about session. Hopefully landboard next week and on the water after that. Injured myself in a stupid splits fall in the skate park, maybe getting too old for that. Thanks for helping me pass the time while I been laid up icing or heating. I will leave you alone now. Once you have built your kite and board skills you will probably be amazed how easy all the beginner issues become!
Good luck and stay safe.

sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Fri Jan 17, 2020 8:15 pm

Now I can actually ride and turn the board :D I decided I might answer my own question. I think I have a very simple explanation (at least it works for me as "improving beginner" :lol: )...

Fisrt I imagined a flat piece of light plywood (a board) falling through the air with a weight on top of it to one side as below. Obviously the total force of air resistance ( from speed of board falling down) will be in the centre of the board. But centre of mass of the board+weight is to the right of the picture, so the force of the air resistance will rotate the board.
If weight was to the left of the board rotation direction reverses.
BoardWeightBack.png
Now we could:

1)Substitute gravity force with force of the kite which constantly moves the board downwind (towards the kite really). I think of it as board constantly "falls" downwind/to the kite all the time (it falls horizontally).

2)Substitute dead weight - with rider capable of moving his weight forward and backward. Or rather total force of rider's legs on the board.
The part of the force of the legs we are interested in is directed horizontally and pushes board downwind.

Makes sense to anyone?

Obviously board is not 90 degrees to the kite pull but if board in the picture above was already turned 45 degrees the forces would still keep rotating it, so it does not have to be 90 degrees.

Also moving board is partially out of water at the front. So centre of water resistance is not in the centre of the board - slightly to the back of the board I guess.
But principle is the same - when weight of the rider (or rather total force rider applies to the board through the legs) is behind this "centre of water resistance" board turns upwind - by force of water pressure alone.
Weight forward of the centre of water resistance - board turns downwind. Basically board "weathervanes" in water.

This picture also incidentally shows that edging of the board does not really matter that much, only weight/leg force distribution relative to the centre of the pressure.
The centre of pressure can probably be in different points depending on how much front of the board is out of water though.
When board is flat on the water with all 4 fins fully submerged centre of pressure can probably be right in the centre as only fins are perpendicular to direction of the "fall downwind".

Also the board will stop rotating upwind or downwind when both forces are effectively applied to the same spot on the board, i.e. say centre of pressure is 1 foot behind middle of the board and (vector) sum of rider's legs's push is applied to the same spot. IMHO this is what we do when we want to move in a straight line :)

Just my two cents :lol: Might even help a complete beginner to get riding quicker :thumb:


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