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How to turn a hydrofoil - and what is YAW ?

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

Postby TomW » Mon Dec 11, 2017 8:41 pm

Thanks Peter. Much appreciated!

I'm just getting the gybe thing. Some click and I carve around, others I am ventilating due to poor altitude control, I roll off or mis time the kite.

I'm using a sort of counter rotation to initiate the turn. A short upwind tweek, a slight roll/lean up wind, followed by bringing kite and body up into up turn, and counter rotation yawing / then leaning into turn to follow the kite.
I find it helps to visualize the boards tip sweeping in a downward 180 degree arc towards the water, looking towards the imagined water surface where the nose should touch down.
Usually the tip doesn't dive down in arc, but stays at altitude.
Basically I think I am yawing and rolling at the same time.

But I'm still learning..

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

Postby Peter_Frank » Mon Dec 11, 2017 9:12 pm

I still say you are wrong Mossy, and can in fact show that planes can turn fully around with wings perfectly level because of horisontal COE coming from the fuselage can be well in front of the rudder so the resulting sideways force acts just like banking and pulling the elevator, but no reason to waste time arguing about this...

Hydrofoiling, leaning into the turn while using yaw movement as input is the way to get tight turns - is the short version - let riders try themselves and find out how much it means :D

RH got a really good point indeed :thumb:

My message with this thread was to make it easy and be helpful, and SHOW what is meant with the term yaw, visually, so everybody can understand it and hopefully use it right away.

(and keep all the nerd talk out for simplicity - not to quarrel about this and that)

8) PF

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

Postby Mossy 757 » Mon Dec 11, 2017 10:33 pm

Peter_Frank wrote:
Mon Dec 11, 2017 9:12 pm
I still say you are wrong Mossy, and can in fact show that planes can turn fully around with wings perfectly level because of horisontal COE coming from the fuselage can be well in front of the rudder so the resulting sideways force acts just like banking and pulling the elevator, but no reason to waste time arguing about this...

Hydrofoiling, leaning into the turn while using yaw movement as input is the way to get tight turns - is the short version - let riders try themselves and find out how much it means :D

RH got a really good point indeed :thumb:

My message with this thread was to make it easy and be helpful, and SHOW what is meant with the term yaw, visually, so everybody can understand it and hopefully use it right away.

(and keep all the nerd talk out for simplicity - not to quarrel about this and that)

8) PF
I get what you're trying to do, but I think explaining it to novice riders using the wrong terms isn't helpful. It's not nerd talk, it's correctly attributing the physical principles that underlie hydrofoil behavior.

All turning on a hydrofoil is a product of rolling the foil in the direction of the turn so that the lift vector propels you towards the center of the turn radius. There are a lot of forces to control when doing this, so I can understand that you need to shift your hips, lead with your shoulders, etc. but just because your body twists about your waist doesn't mean the foil is yawing, it just means you're balancing the board as you conduct a maneuver that destabilizes the foil-lift/kite-power vectors.

Unless you're making the specific claim that one can twist the strut with one's body weight to induce a left/right yaw moment that initiates turns, I think you've used the wrong term. Having done something like that on my Sword2 (hard upwind it felt like you could pull on the strut hard enough to get it to twist a bit when fully hiked out) I can understand that not every strut 100% resists deflection, but I don't think that's your claim, especially not when talking to riders on aluminum struts that do NOT deform in this plane of motion.

I guess it doesn't really matter all that much, except that if someone said what you wrote to me in person on the beach, I'd have no qualms correcting their misuse of terms.

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

Postby Peter_Frank » Tue Dec 12, 2017 12:05 am

I don't see any misuse of terms.

If many with knowledge says I do, that I am wrong and misleading, I would of course reconsider.

8) PF

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

Postby BWD » Tue Dec 12, 2017 12:34 am

How I see it, it’s a little like riding a bike, you’re constantly starting to fall and correcting to get a reaction force that keeps you from falling, both when riding “straight” and in a curve or turn.
Twisting to start yaw on a foil, are you not simply starting a “fall” toward the turn apex, such that your next reaction, and the foil’s, will have you upright, and exiting the turn in the right direction?

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

Postby jeromeL » Tue Dec 12, 2017 4:25 am

I also thought it was similar to biking. When heading far downwind at low speed yaw is pretty apparent but for fast carving in gybe I feel it's more like counter steering, you acrually yaw slightly upwind in order to roll and lean into the turn when doing carving turn to toesfide.
But very quickly it's muscle memory, I had those feelings first couple of session but by the time you learn to balance it's hard to tell, at least on bicycle you can push handle bar with one hand to feel countersteering 😂
I guess that explains why the wipe out are so severe when you first get up on foil.
As far as twintip comparaison when you are edging hard upwind fairly powered only way to lean into in more is to first push the front toe slightly to push board away form you and make room to lean into it more.

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

Postby jakemoore » Tue Dec 12, 2017 5:29 am

Watching cloudfoiling videos. Roll or yaw? It's still beyond me but thank you for everybody who is helping to teach.

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

Postby Kevin Brooker » Tue Dec 12, 2017 12:50 pm

With dihedral in the wing, instigating a yaw vector will cause differential lift from the change in AoA of each panel. This will cause the entire setup to roll thus changing the lift vector to turn the craft. When adding rudder only to an aircraft the dihedral does the same thing. Keeping the wings level with ailerons negates the AoA change caused by the input of yaw via the rudder. Keeping the wings level with ailerons whilestomping on the rudder will yaw the aircraft but not turn it. The exposed fuselage is now just drag and the sink rate goes way up.

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

Postby revhed » Tue Dec 12, 2017 2:30 pm

Kevin Brooker wrote:
Tue Dec 12, 2017 12:50 pm
With dihedral in the wing
And what would you say as almost all of our KBHF F wings have anhedral or are flat?
Same thoughts?
R H

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

Postby Mossy 757 » Tue Dec 12, 2017 3:22 pm

revhed wrote:
Tue Dec 12, 2017 2:30 pm
Kevin Brooker wrote:
Tue Dec 12, 2017 12:50 pm
With dihedral in the wing
And what would you say as almost all of our KBHF F wings have anhedral or are flat?
Same thoughts?
R H
"Anhedral" wings, aka those with a negative dihedral angle like a AV-8B Harrier, destabilize the foil about the roll axis making it easier to initiate the turn compared to "dihedral" wings, aka those with a positive dihedral angle like most passenger airliners.

Image

I think Kevin was making a point about the effect of rudder inputs when flying a plane more so than he was commenting on hydrofoil performance dynamics as most of his language seemed to resemble what I learned in flight school rather than at the beach.

Either way, the fact that most foils have gone in the direction of a downward swept wing proves that designers are optimizing for roll instability to improve maneuvering characteristics.


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