Forums > Kitesurfing Foiling

Stable vs unstable foils - Front wing shape?

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Created by Lambie A week ago, 4 Jun 2018
Plummet
4201 posts
8 Jun 2018 11:15AM
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Kamikuza said..

Plummet said..

Yes, increasing stab aoa will increase drag and stability. It will give more front foot pressure as well. You may need to move straps or foil position to compensate for that. But it will be more stable for learning. I cranked mine up to 3.5 degrees and it made the learning process easier as I had a definite front foot pressure to fight against. Stability was higher so I could be more course on my weight distribution without ridiculous crashes. It made trying flying foot switches heaps easier. As I progressed I trimmed back the stab aoa to 2.75 deg. That made my foil more sensative, better at speed and more maneuverable. Things I didn't want while learning I now wanted with a bit of experience.

I've managed to take my foil from a sedate learning foil to a snappy responsive wave riding foil with 0.75 Deg stab aoa adjustment.



You should add that you made your own foil from your own designs and hadn't ridden a foil before and can't move the mast or straps IIRC.

With a good production foil, you'll probably find out that they've ironed out all the idiosyncrasies of the design and the problem is then between the kite and the board...


Actually, I have had my foil on 2 boards with multiple different foot strap and foil mounting positions.

I've also run straps, strapless and foot hooks.

I've also ridden production foils too.

Tested and compared.

Your turn man. Get out there and do some science. See if your "physics" understanding meets reality.

Kamikuza
QLD, 3187 posts
8 Jun 2018 2:25PM
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Plummet said..

Kamikuza said..


Plummet said..

Yes, increasing stab aoa will increase drag and stability. It will give more front foot pressure as well. You may need to move straps or foil position to compensate for that. But it will be more stable for learning. I cranked mine up to 3.5 degrees and it made the learning process easier as I had a definite front foot pressure to fight against. Stability was higher so I could be more course on my weight distribution without ridiculous crashes. It made trying flying foot switches heaps easier. As I progressed I trimmed back the stab aoa to 2.75 deg. That made my foil more sensative, better at speed and more maneuverable. Things I didn't want while learning I now wanted with a bit of experience.

I've managed to take my foil from a sedate learning foil to a snappy responsive wave riding foil with 0.75 Deg stab aoa adjustment.




You should add that you made your own foil from your own designs and hadn't ridden a foil before and can't move the mast or straps IIRC.

With a good production foil, you'll probably find out that they've ironed out all the idiosyncrasies of the design and the problem is then between the kite and the board...



Actually, I have had my foil on 2 boards with multiple different foot strap and foil mounting positions.

I've also run straps, strapless and foot hooks.

I've also ridden production foils too.

Tested and compared.

Your turn man. Get out there and do some science. See if your "physics" understanding meets reality.


Been there done that. And whaddya know? The predictions were accurate. You saw the expected results yourself -- see the bolded text in the quote.

Incredible, who'd've thunk it?

straddiepaul
QLD, 136 posts
8 Jun 2018 5:11PM
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my sshg mast is 11.5cm deep.. its like a rudder.. this must affect forward stability and side current yaw.. narrower mast affects?

Lambie
VIC, 693 posts
8 Jun 2018 6:33PM
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Thanks for the great conversations guys !! Im finally thinking I have enough experience to start appreciating the feel of different front wings and stabiliser AOA - so if the wind gods co -operate this long weekend I think Ill have a play with changing the front wings of my Zeeko Bullet and mucking around with a shim - Im interested to see if I can pick it at my stage of foiling.

As mentioned by others - its seriously addictive!! But the upside is - days that I would normally wipe as a non starter are now very much on my radar !!!!

AquaPlow
QLD, 624 posts
9 Jun 2018 12:10AM
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RAL INN said..
Lambie, I have a feeling that you are now going through one of the learning phases and maybe are in the process of finding out what brain input goes into roll balance.
something that muscle memory will take over for you.
For now maybe work on keeping more than 50% of mast in water to just help that bit.


My 102 cents worth - RAL INN is on the money.
You can argue every which way about roll stability and the design of a particular wing - and make comparisons to aerodynamics.
And yes roll stability in a static wing aero dynamic situation it comes from Dihedral.
But we do not build aircraft shapes with the bulk of the weight centered way off the fuselage as a foil operating in two totally different fluids.
For foils I would guestimate the foil shape once you get a realistic wing area effects roll stability by under 5%.
The totally dominant force is the pilots body weight measured from essentially the point of rotation which for simplicity is the water surface.
Here is my thinking...
So make everything equal and just change one element in this case the length of the mast.
Assume Flat water = lake, constant wind, same size kite and board same speed. and foil to surface height stays the same like being on an underwater rail track. the stability envelope afforded by the foil is constant.
10 cm mast if it would work on a foil would be very stable.
As we all know increase the length of the mast increases the sensitivity of input causing roll to become a problem.
If you are roll stable on a 30cm mast you have mastered the range of movement tolerance.
If you map that range of movement as an area to visualise it say 3 cms from the vertical either way this stays a constant.
Now If you double the mast length to 60 cms you still only have that 3cm tolerances but your movements are amplified by the height change so and it requires you to learn the correct way to absorb roll movement and stay balanced.
So getting your body into the best position and applying force in a totally different plane = pitch = AOA although counter intuitive is also likely giving you the ability to learn to absorb roll. The biggest help in this instance is to have to apply positive constant pressure for pitch. Think of this as balancing on a log than having to balance on a ball. It makes it more two dimensional instability as opposed to three.
Nailed
Cheers
AP



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"Stable vs unstable foils - Front wing shape?" started by Lambie