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How to turn a hydrofoil - and what is YAW ?

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BWD
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Re: How to turn a hydrofoil - and what is YAW ?

Postby BWD » Tue Dec 12, 2017 4:03 pm

Mossy i think the dihedral or anhedral on the tail of kite foils mainly to give some lateral projected area and avoid needing a vertical tail, since there is little net lift from the tail I wouldn't say it affects the handling in the way you propose.

Flat or lower anhredral main wings in kite foils give LESS resistance to roll and yaw.
That's why zeeko, horue, et al. use flat main wings for responsiveness in wave wings, and tips of ketos wave are flattened, etc.
(SUP wings that have more significant anhedral are made that way because there's no kite to give stability.}
Wings with anhedral are easier to handle overpowered and in messy water because they are more stable, the difference is palpable switching from for example, zeeko carver to zeeko anhedral wing, requires some technique adjustment from the rider to make anhedral wing turn if used to flat wing!

Nobody uses dihedral on main foil wing because it's too unstable (except reportedly one French guy who builds his own foils, of course). The effect of anhedral/dihedral in foils vs. airplanes is opposite since CG of kitefoil/rider is high above the foil, rather than inside the fuselage.

To the original topic, for you pilot/aero guys, isn't it correct to describe yaw effect in turning as:

Rider applies yaw inducing moment to foil, causing one side wing to advance (this will become the outside wing in the turn).
Center of lift is advanced by doing so, and shifted toward the advancing wing, as the CG is now relatively more aft of it.
Lift from the trailing (or "inside") wing decreases, partly due to turbulence from the fuse, partly due to effective AR shift slightly lower since wing is not "flying straight" along its chord.
Since more lift has shifted slightly to one side, and the rider has shifted CG to the other side slightly while starting the yaw, roll is induced.
Having achieved complementary yaw and roll, the foil now carves in an arc.
With the CG inside the CL, and foil carving in an arc, the rider can now add more roll and tune the AOA to stay foiling in stability and control the radius and length of the turn.
Applying a little yaw the other way helps end the turn....

Is that about right?

jash999
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Re: How to turn a hydrofoil - and what is YAW ?

Postby jash999 » Tue Dec 12, 2017 4:26 pm

Interesting topic. Coming from a different background of ski and snowboard, maybe some common elements are there even if having the kite and foil in HF makes it somewhat different. So for my 2 cents, which you can shoot down later:

Teaching a beginner to turn on a snowboard usually starts with him making slow, long radius, deep turns on a beginner slope. These involve a lot of shoulder/ hip/ knee rotation with the snowboard going from one edge to flat to other edge going in nearly opposite direction with noticeable body rotation. This looks like yawing the HF. a slow long radius turn on a flat board.

https://www.youtube.com/watch?time_cont ... 9af352o5Ok

After progress to carving fast, short radius linked turns (similar to S-turns on HF) the upper body is mostly stable, and it is weight shifting using the knees and feet that switch from edge to edge quickly with minimal flat board in between that make the turns. This is similar to rolling the HF in S- turns, and it needs speed and shallower turns.

https://www.youtube.com/watch?v=76N7S9NdaT8

To make deeper carving snowboard turns with any speed you need to add some combination of hip/ shoulder rotation to the carving from edge to edge, and mostly this is from the hips.

Similarly on HF, a deep turn with some speed feels like some degree of roll and yaw combined, the amounts depending on speed and depth of turn.

Mossy 757
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Re: How to turn a hydrofoil - and what is YAW ?

Postby Mossy 757 » Tue Dec 12, 2017 5:06 pm

BWD wrote:
Tue Dec 12, 2017 4:03 pm
Mossy i think the dihedral or anhedral on the tail of kite foils mainly to give some lateral projected area and avoid needing a vertical tail, since there is little net lift from the tail I wouldn't say it affects the handling in the way you propose.
Since the stabilizer is creating an opposite moment to the front wing (to counter-balance the forward pitching from the tow point being so high up on the rider's waist) then the dihedral angle will be opposite but complementary to the front wing. I do not think this is intended to replace vertical control surfaces previously built into older fuselage designs. I think the turned wingtips we see on a lot of stabilizers (Mike's Lab, Delta, Enata) are there for wingtip efficiency, not providing a control surface.
Flat or lower anhredral main wings in kite foils give LESS resistance to roll and yaw.
Right, less resistance is akin to saying "more unstable" when you use the word "stable" as a term to define the forces in play, not necessarily how the rider perceives the quality of the ride. If a foil were over-stabilized in any one important direction, it would be very difficult to ride despite being technically "stable," using the word as a physicist might use it.
That's why zeeko, horue, et al. use flat main wings for responsiveness in wave wings, and tips of ketos wave are flattened, etc.
(SUP wings that have more significant anhedral are made that way because there's no kite to give stability.}
I'm not sure about that. I'd want to absolutely eliminate the variable of "rapidly designing prototypes to see what works using easily CNC'ed materials" before immediately deciding that those brands went with flat wings based on hydrodynamic considerations. I think it's much more likely that their R&D process is more conducive to machining a bunch of easy and cheap to make wings and then going to the beach and finding the setup that works. If they had the ability to deliver a product to market that was a complex polyhedral wing shape, it's likely that's what we'd see. Modern manufacturing restrictions and the price point those brands target prohibit such extensive development so I think we're seeing a lot more flat shapes as they're faster and easier to rapidly iterate. Plus as soon as your customers think flat = waves then you've automatically created a marketing niche to exploit with your messaging.
Wings with anhedral are easier to handle overpowered and in messy water because they are more stable, the difference is palpable switching from for example, zeeko carver to zeeko anhedral wing, requires some technique adjustment from the rider to make anhedral wing turn if used to flat wing!
Technically anhedral wings are less stable than flat wings because there's not the same righting moment trying to restore the foil to a wings-level condition of flight. "Easier to ride" is not the same thing as "more stable" from a physics standpoint.
Nobody uses dihedral on main foil wing because it's too unstable (except reportedly one French guy who builds his own foils, of course). The effect of anhedral/dihedral in foils vs. airplanes is opposite since CG of kitefoil/rider is high above the foil, rather than inside the fuselage.
I think nobody uses a dihedral because you can't easily roll a dihedral wing into a turn if you're also pulling against the kite because they're naturally stable. Since the kite is resisting the roll by pulling orthogonally at the harness hook, the wing can be "less stable" in the roll axis as some of the stability comes back from your front lines. It's this delicate balance of rolling away from the kite and the kite pulling you back that creates upwind stability and it's why going downwind is damned scary when you're slow because you're not creating enough apparent wind to hike out against your kite and regain that stability from you front lines. This is why the best advice for how to get downwind is to go as fast as possible; once you're up to speed the apparent wind shifts so far forward that you're in essentially the same body position as riding upwind.
To the original topic, for you pilot/aero guys, isn't it correct to describe yaw effect in turning as:

Rider applies yaw inducing moment to foil, causing one side wing to advance (this will become the outside wing in the turn).
This is the thing Peter Said with which I'm disagreeing. I don't think riders are inducing a yawing moment on a hydrofoil, there's not a control surface against which to push or pull that would make this possible. If you just picture a perfectly vertical foil riding through the water, what we perceive as "yaw" or rather the rate of change of the nose angle of the board is really a product of rolling the wings. Yes, I guess from an external observer's standpoint you could see a guy riding along on a hydrofoil wagging his nose back and forth in the "yaw" plane but it's not because he's applying yawing forces to the hydrofoil to do so, it's because it's basically just a rolling turn in that direction with a commensurate change in kite power/angle or pitch to maintain ride height. There's no rudder on a hydrofoil.

Center of lift is advanced by doing so, and shifted toward the advancing wing, as the CG is now relatively more aft of it.
Lift from the trailing (or "inside") wing decreases, partly due to turbulence from the fuse, partly due to effective AR shift slightly lower since wing is not "flying straight" along its chord.
Since more lift has shifted slightly to one side, and the rider has shifted CG to the other side slightly while starting the yaw, roll is induced.
Having achieved complementary yaw and roll, the foil now carves in an arc.
With the CG inside the CL, and foil carving in an arc, the rider can now add more roll and tune the AOA to stay foiling in stability and control the radius and length of the turn.
Applying a little yaw the other way helps end the turn....

Is that about right?
That's all correct, the advancing wing has more lift and therefore creates the rolling moment in the direction of the turn, but that's assuming you advanced the wing with rudder pedals, something you can't do on a hydrofoil.

Kevin Brooker
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Re: How to turn a hydrofoil - and what is YAW ?

Postby Kevin Brooker » Tue Dec 12, 2017 7:01 pm

From the aero guy: positive stability is when a craft is displaced and returns to the original pre-displaced orientation. Neutral stability is the craft remains in the new displaced orientation. Negative stability ( unstable) is when the displacement amplifies itself. The stability definitions can be applied to all three axis; roll, pitch, and yaw. A craft can be pitch stable and roll-unstable and this is where the HF we use is and allows for turning. Yaw instability is unridable as the board will want to constantly swap ends around the mast. To be stable the CG of the craft must be ahead of the CP ( center of pressure) or CE ( center of effort) Keeping the angles the same and mount the foil setup just ahead of he stab and try to ride it. You will not t be able to unless you reconfigure the wings to move the CP/CE.

The CO/CE is a combination of the lift generated by the foil and the lift generated by the kite. Going downwind is scary as the CP/CE is near or ahead of the CG of the entire rig the board feels twitchy as the CG is trying to get ahead of the CP.

Kites are tough as the lift vector from the kite moves around a lot and on traditional sailboats with a fixed CE or an aircraft with fixed CE these centers do not move and are easy to figure out and work with. They are easier to explain. With a kite the vectors all work but are constantly in flux and this dynamic situation takes a lot of skill to operate.

I think what is happening when we apply a yaw force we also induce a rolling moment by the nature of what we are trying to do. In order to purely yaw a flying HF we will be rotating it around the verticals axis without changing by the pitch or the roll. The nose of the fuse will move which will impart a bit of roll as explained in a previous post. Yaw stability will be ant to return the fuse to e original heading and apply a righting moment inte same way it imparted the rolling moment. The craft returns to the pre-yaw orientation.

What I believe happens is when we try to apply a yaw moment we also apply a rolling input. Given the roll axis is relatively neautral to slightly unstable the foil rolls and the resulting lift vector turns the craft. I also believe when the craft rolls the lift component relative to the water goes down so we apply a bit of pitch so the f Not lines transfer so of the lifti g to the kite. The higher pitch also pulls the turn tighter transferring more to the kite.

Mossy 757
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Re: How to turn a hydrofoil - and what is YAW ?

Postby Mossy 757 » Tue Dec 12, 2017 7:48 pm

Kevin Brooker wrote:
Tue Dec 12, 2017 7:01 pm
when we try to apply a yaw moment
I agree with everything you wrote but want to be clear about this point; how would a rider standing on a board apply a yaw moment without a dynamic control surface in the water OR something against which to push? Can the rider induce yaw simply by resisting the front lines of the kite?

I'm just struggling to understand how one might make the strut pivot about its axis while standing atop the board...

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Re: How to turn a hydrofoil - and what is YAW ?

Postby joyrider1 » Tue Dec 12, 2017 8:01 pm

Mossy 757 wrote:
Tue Dec 12, 2017 7:48 pm
Kevin Brooker wrote:
Tue Dec 12, 2017 7:01 pm
when we try to apply a yaw moment
I agree with everything you wrote but want to be clear about this point; how would a rider standing on a board apply a yaw moment without a dynamic control surface in the water OR something against which to push? Can the rider induce yaw simply by resisting the front lines of the kite?

I'm just struggling to understand how one might make the strut pivot about its axis while standing atop the board...
Very easy: you twist your body. if your upper body is yawed/twisted clockwise, then your lower body is twisted anticlockwise.....

BWD
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Re: How to turn a hydrofoil - and what is YAW ?

Postby BWD » Tue Dec 12, 2017 8:04 pm

Since the stabilizer is creating an opposite moment to the front wing (to counter-balance the forward pitching from the tow point being so high up on the rider's waist) then the dihedral angle will be opposite but complementary to the front wing. I do not think this is intended to replace vertical control surfaces previously built into older fuselage designs. I think the turned wingtips we see on a lot of stabilizers (Mike's Lab, Delta, Enata) are there for wingtip efficiency, not providing a control surface.
gofoil -stab vee, tips down, works great, super stable.
most everything else -stab tips up, works, varying stability by design/use
Inconclusive.
I'd want to absolutely eliminate the variable of "rapidly designing prototypes to see what works using easily CNC'ed materials" before immediately deciding that those brands went with flat wings based on hydrodynamic considerations. I think it's much more likely that their R&D process is more conducive to machining a bunch of easy and cheap to make wings and then going to the beach and finding the setup that works. If they had the ability to deliver a product to market that was a complex polyhedral wing shape, it's likely that's what we'd see. Modern manufacturing restrictions and the price point those brands target prohibit such extensive development so I think we're seeing a lot more flat shapes as they're faster and easier to rapidly iterate. Plus as soon as your customers think flat = waves then you've automatically created a marketing niche to exploit with your messaging.
Granted shaping G10 protos and producing CNC copies is an efficient method, keep in mind: Zeeko and Ketos both have been around a long time and have expertise with molded carbon, as well as offering flat g10 wings for some purposes. Zeeko prior to 2015 only made molded, anhedrally curved carbon foils to my knowledge. These are still made, and updated.
I think the flat carver wing, 710cm^2 came out in '15 for wave/light wind/higher lift use, not just to be cheap.
Same year, they offered a molded anhedral glass wing to update the original flat G10 wing on their aluminum/glass foil. They still offer the carver wing for waves/carving, it is still flat. So the rationale of g10 to be cheap is out imo. Being able to slam G10 into a sandbar many times with minimal damage matters, though.
Ketos is a subsidiary of a decades old composite company. Except the new flat g10 stab ("G10, program, maneuverability, reserved for confirmed or expert rider" on website), everything they make is curvy carbon. Their foils aren't cheap and they seem to be always putting in development effort. Nothing looks banged out in a hurry.
here's the wave one I referenced, head on:
Image
Clearly, it is designed with anhedral, but not a lot. Looks like it was made flatter for a reason.
I think nobody uses a dihedral because you can't easily roll a dihedral wing into a turn if you're also pulling against the kite because they're naturally stable.
I doubt they are stable anymore when you have a CG 2-3 wingspans vertically displaced above the wing, and the CG moves farther outboard than the effective CL. The dynamics are not simple, but we have both a mobile and very high CG. Seems like a dihedral kitefoil could be prone to induce hinging at the feet and nasty crashes. But I've never tried one, naturally.
I don't think riders are inducing a yawing moment on a hydrofoil, there's not a control surface against which to push or pull that would make this possible.

With a kitefoil, no moving control surfaces, just a moving CG. Moment is applied the same way as with this:
Image
If you just picture a perfectly vertical foil riding through the water, what we perceive as "yaw" or rather the rate of change of the nose angle of the board is really a product of rolling the wings. Yes, I guess from an external observer's standpoint you could see a guy riding along on a hydrofoil wagging his nose back and forth in the "yaw" plane but it's not because he's applying yawing forces to the hydrofoil to do so, it's because it's basically just a rolling turn in that direction with a commensurate change in kite power/angle or pitch to maintain ride height. There's no rudder on a hydrofoil.
I most often ride the large carver wing with low to moderate kite power and find yaw readily perceptible.
Visually, with the board vertical, kite high, yaw is the nose changing heading relative to the direction I'm moving. This easily and often happens flying high but at moderate speed without a lot of power, riding over top of small swells without trying to carve any, for example. There is some tolerance of yaw, if greater than this tolerance, you have to respond to it with a little matching roll to avoid falling, and voila, you have started carving S turns downwind on little waves. My favorite thing to do foiling, so far.
Last edited by BWD on Tue Dec 12, 2017 8:11 pm, edited 2 times in total.

Mossy 757
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Re: How to turn a hydrofoil - and what is YAW ?

Postby Mossy 757 » Tue Dec 12, 2017 8:05 pm

joyrider1 wrote:
Tue Dec 12, 2017 8:01 pm
Mossy 757 wrote:
Tue Dec 12, 2017 7:48 pm
Kevin Brooker wrote:
Tue Dec 12, 2017 7:01 pm
when we try to apply a yaw moment
I agree with everything you wrote but want to be clear about this point; how would a rider standing on a board apply a yaw moment without a dynamic control surface in the water OR something against which to push? Can the rider induce yaw simply by resisting the front lines of the kite?

I'm just struggling to understand how one might make the strut pivot about its axis while standing atop the board...
Very easy: you twist your body. if your upper body is yawed/twisted clockwise, then your lower body is twisted anticlockwise.....
Twisting one's body (i.e. pushing against air) doesn't create a yaw moment about the vertical axis of the strut. Leaning into the turn by rotating hips and shoulders is just weight transfer inducing a roll.

joyrider1
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Re: How to turn a hydrofoil - and what is YAW ?

Postby joyrider1 » Tue Dec 12, 2017 8:11 pm

Mossy 757 wrote:
Tue Dec 12, 2017 8:05 pm
joyrider1 wrote:
Tue Dec 12, 2017 8:01 pm
Mossy 757 wrote:
Tue Dec 12, 2017 7:48 pm


I agree with everything you wrote but want to be clear about this point; how would a rider standing on a board apply a yaw moment without a dynamic control surface in the water OR something against which to push? Can the rider induce yaw simply by resisting the front lines of the kite?

I'm just struggling to understand how one might make the strut pivot about its axis while standing atop the board...
Very easy: you twist your body. if your upper body is yawed/twisted clockwise, then your lower body is twisted anticlockwise.....
Twisting one's body (i.e. pushing against air) doesn't create a yaw moment about the vertical axis of the strut. Leaning into the turn by rotating hips and shoulders is just weight transfer inducing a roll.
Think of an astronaut in space. Your body/muscles is the control surface itself! Jump into the air and twist your shoulders to one direction. Experience how your feet will twist the other way......

joyrider1
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Re: How to turn a hydrofoil - and what is YAW ?

Postby joyrider1 » Tue Dec 12, 2017 8:15 pm

joyrider1 wrote:
Tue Dec 12, 2017 8:11 pm
Mossy 757 wrote:
Tue Dec 12, 2017 8:05 pm
joyrider1 wrote:
Tue Dec 12, 2017 8:01 pm


Very easy: you twist your body. if your upper body is yawed/twisted clockwise, then your lower body is twisted anticlockwise.....
Twisting one's body (i.e. pushing against air) doesn't create a yaw moment about the vertical axis of the strut. Leaning into the turn by rotating hips and shoulders is just weight transfer inducing a roll.
Think of an astronaut in space. Your body/muscles is the control surface itself! Jump into the air and twist your shoulders to one direction. Experience how your feet will twist the other way......
Of course you have to jump straight into the air and only twist in the air ;-)


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