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Forces on a twintip

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rynhardt
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Re: Forces on a twintip

Postby rynhardt » Thu Jan 18, 2018 2:07 pm

We've looked at two load scenarios so far.

Next up is the load distribution on the board while we are edging.

I've take some clips from one of my old videos, where I'm just nicely powered. In general the stagnation line (water line) is at an angle between the footpads.
Attachments
Capture2.PNG
Capture2.PNG (601.84 KiB) Viewed 2007 times
Capture.PNG
Capture.PNG (380.16 KiB) Viewed 2007 times

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Re: Forces on a twintip

Postby fluidity » Sun Jan 21, 2018 10:05 am

Not sure how these math generated pressure distribution charts relate to the determinism of real-world?
For a harder impact, the downward forces are centred more on the heels as they are more rigidly connected to the body than the rest of the foot.
Most boards aren't a perfect rectangle.
Most board's edging surface comprises 3 fins and a rough triangle from the back of the board to the front.
Water escapes most easily from the rear and sides of the boards, it has less distance to travel.
What are you trying to achieve?

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Re: Forces on a twintip

Postby rynhardt » Sun Jan 21, 2018 6:44 pm

fluidity wrote:
Sun Jan 21, 2018 10:05 am
Not sure how these math generated pressure distribution charts relate to the determinism of real-world?
For a harder impact, the downward forces are centred more on the heels as they are more rigidly connected to the body than the rest of the foot.
Most boards aren't a perfect rectangle.
Most board's edging surface comprises 3 fins and a rough triangle from the back of the board to the front.
Water escapes most easily from the rear and sides of the boards, it has less distance to travel.
What are you trying to achieve?
Well the modelling only provides qualitative results and are only useful if they match empirical results. I'm comfortable that they reflect my personal experiences, but if someone else has a different opinion then I would love to hear about it. This is after all a public forum.

The model I used has tapered tips (see start of thread for pictures), but I don't think it has a significant impact on the results in any case. Maybe 10% less surface area.

For those that still ride with fins, I don't think the fins have much impact on the board in terms of water pressure. Fins provide lift orthogonal to the board surface.. There might be a small local moment where the fins are mounted, but that's it.

I'm relying on previous research to determine the pressure distribution. Whether that distribution is along the long axis of the board, or at a say 30 deg angle, will certainly affect the way the board responds. I still need to model the angled scenario.

Lastly the initial reason for this thread was another discussion where it was asserted that the top of the board doesn't need a lot of reinforcement. My assertion is that it does.

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Re: Forces on a twintip

Postby fluidity » Sun Jan 21, 2018 7:34 pm

rynhardt wrote:
Sun Jan 21, 2018 6:44 pm
I'm relying on previous research to determine the pressure distribution. Whether that distribution is along the long axis of the board, or at a say 30 deg angle, will certainly affect the way the board responds. I still need to model the angled scenario.

Lastly the initial reason for this thread was another discussion where it was asserted that the top of the board doesn't need a lot of reinforcement. My assertion is that it does.
I absolutely agree with your last paragraph. I think most people would be very surprised by where the forces are greatest. I did a very light weight thin version of my semi-eliptical high concave to see if I could still get flex with the concave. It turned out to fail on top with compressive forces on the front edge of the frontstrap footpad(paulonia strip built 6mm with top and bottom basalt and polyester print cloth.) My other boards I usually put some unidirectional carbon over the top 3/4 in a strip about 150mm wide and some carbon strip under the outside edges. I think most people judge layups by aesthetics but the true loadings would benefit carbon fibre on top around the foot pad area and under the board between to resist compression, all the other areas get a lot less compression and suit something more like basalt or glass. But then if you localise reinforcement, you also have to overdo it or blend the edges with some zigzags etc to stop fractures at material boundaries.

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Re: Forces on a twintip

Postby rynhardt » Sun Jan 21, 2018 8:18 pm

fluidity wrote:
Sun Jan 21, 2018 7:34 pm
But then if you localise reinforcement, you also have to overdo it or blend the edges with some zigzags etc to stop fractures at material boundaries.
100% agree with this. I think the footpad area is a prime candidate for extra reinforcement but you need to be careful with the boundaries.

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Re: Forces on a twintip

Postby rynhardt » Mon Jan 29, 2018 8:42 pm

Okay, finally got around to doing the edging scenario, with the stagnation line running at 30 deg across the centre of the board.

First picture shows the load distribution. I used the same Savitsky load distribution magnitudes as before, just angled across the board.

Second picture shows the deformation. As expected, the board twists as it goes over the stagnation line. This is why we put reinforcement at +45/-45 deg.

Last picture shows that when edging, the rear footpad area carries the most stress, but also, a significant amount of stress in the middle of the board.
Attachments
30 deg loads.PNG
Load distribution
30 deg - end view.PNG
End view, twist visible
30 deg stress front.PNG
Stress at rear footpad

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Re: Forces on a twintip

Postby board dude » Sun Mar 11, 2018 8:10 pm

Great topic gentleman , looks like we are all thinking along the same lines.
A kiteboard has a static rocker curve which we see when the board sits on a table, in the water with the weight of the rider pushing down and the force of the water pushing up the kiteboard has a dynamic shape , the trick to good kiteboard design is that dynamic curve is an efficient quick shape offering great grip and minimal resistance.

D1 - a simple beam with forces along its entire length and the riders feet pushing down from the top.
D1 - Non tapered beam bending.png
D2 - a tapered beam with forces along its entire length and once again forces pushing down from the riders feet.
D2 - Uniform foces on the entire beam.png
D3 - a tapered beam with the water pushing up from its first contact point and to the rear of the kiteboard, riders feet pushing down.
D3 - Tapered beam bending.png
D4 - Simulated kiteboard moving to the left with water pushing up from contact point and constant to the rear of the board , riders weight pushing downwards through the footstrap positions.
D4  - Movement forces on hull.png
The trick is to design the taper in the kiteboard to bend with a constant curve , this will give the board the most efficient pass through the water. A non constant bend will cause drag and can effect the very important water flow over the fins.

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rynhardt
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Re: Forces on a twintip

Postby rynhardt » Mon Mar 12, 2018 9:47 am

board dude wrote:
Sun Mar 11, 2018 8:10 pm
Great topic gentleman , looks like we are all thinking along the same lines.
A kiteboard has a static rocker curve which we see when the board sits on a table, in the water with the weight of the rider pushing down and the force of the water pushing up the kiteboard has a dynamic shape , the trick to good kiteboard design is that dynamic curve is an efficient quick shape offering great grip and minimal resistance.

D1 - a simple beam with forces along its entire length and the riders feet pushing down from the top.
D1 - Non tapered beam bending.png
D2 - a tapered beam with forces along its entire length and once again forces pushing down from the riders feet.
D2 - Uniform foces on the entire beam.png
D3 - a tapered beam with the water pushing up from its first contact point and to the rear of the kiteboard, riders feet pushing down.
D3 - Tapered beam bending.png
D4 - Simulated kiteboard moving to the left with water pushing up from contact point and constant to the rear of the board , riders weight pushing downwards through the footstrap positions.
D4 - Movement forces on hull.png
The trick is to design the taper in the kiteboard to bend with a constant curve , this will give the board the most efficient pass through the water. A non constant bend will cause drag and can effect the very important water flow over the fins.
Nice! :thumb:
Two points I'd like to raise:
1. As soon as the board has a velocity > 0, the load distribution is non-uniform and will approximate the Savitsky curve. It would be very cool if you could update your D3 and D4 with this load distribution.
2. If you are designing only for efficiency, then a constant foil profile would be great. But if you add control surfaces (channels, fins, edges, edge profiles) and flex, then compromises have to be made.

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Re: Forces on a twintip

Postby Grkite » Mon Mar 26, 2018 1:12 pm

Great topic!! And the points raised. !!

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Re: Forces on a twintip

Postby Grkite » Mon Mar 26, 2018 1:57 pm

Grkite wrote:
Mon Mar 26, 2018 1:12 pm
Great topic!! And the points raised. !!
Just a point....
"And when I see yout transparent board ...I remember through my initial calculations/analysis...that a transparent board would make my life easier through photoelasticity ...that was taught during uni time.."


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