Author Topic: Cross-brand tail force comparison?  (Read 3358 times)

MikeLima

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Cross-brand tail force comparison?
« on: April 01, 2022, 10:31:56 AM »
During my most recent prone session I had a repeat of my most common failure: getting repeatedly launched at take off. I had forgotten the tail Iíve been using(with 1į less downforce shim): a chopped 220 kujira which is probably ~200cm2.  So instead I used a 178 with no shim, with the 1095. It was a pretty stark difference, particularly since my technique isnít to the point where I can adapt well. Kujira foils are supposed to be front foot heavy, Armstrong not so much(Iím on an Armstrong board). So to get them to work together Iíve got the mast all the way back, the mast shimmed for less AOA, and had usually been shimming the tails, with some success.

But this experience got me to thinking about tails in a new-to-me way. From the time I first looked at a foil setup, Iíd taken it as a given that the tail creates some amount of downforce. Then, while discussing his Armstrong setup with another member of our crew, he said something about a neutral tail. I was confused at the time, but now itís dawning on me that there is a greater diversity of tail dynamics than I had previously known. So how do you quantify what a tail is doing in a standard way, since shimming a tail is only relative to the standard setting for that tail and fuselage?

My best guess so far has been to attach a fuse to a mast in such a way that the fuse is free to pivot up and down, and put it over the side of a boat with a tail and no front foil. If the fuse remains horizontal* then its a neutral tail, with varying angles showing varying behaviors of the tail. Would such a test show Armstrong staying level*, and Takuma riding tail down? It would also be neat to change tails and shims and see how all those change thingsÖÖ.

*actually, horizontal meaning: at the same angle that the fuse is at with a given foil, given rider weight, I guess in the middle of the foilís speed range**? Maybe a second mast with a digital level(phone) for accurately measuring itís angle.

**A bit of a tangent here: if a tail is neutral in the middle of a foilís speed range, would that mean that at higher speeds, it would start to create downforce, but lift at lower speeds, because of the changing angle of the fuse? Iíve noticed kujiraís angle of incidence changes as you move outward. Would a similar shape be helpful for a tail meant to be neutral, in order to smooth out the transition from - to + lift?

Anyhoo, food for thought I guess.
77kg rider
Armstrong 39L wing/surf
Armstrong 88 wing/SUP

Kujira foils
NoLimitz masts

Duotone unit 2022 3/4/5;dlab6

PonoBill

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Re: Cross-brand tail force comparison?
« Reply #1 on: April 01, 2022, 11:43:54 AM »
It's good thinking, but there are a number of forces at play that make it challenging to do much more than try stuff and see how it works. Both wings have both lift and deflection (forces caused by changing the direction of water flow, like an elevator or a rudder does when it's moved) forces. When you change the angle of attack to any amount less than the point of flow separation and turbulence, both the lift and deflection forces change. How much they change is a function of speed, surface area, deflection angle, foil profile, the moment arm (fuselage length), and probably some other stuff I haven't thought of.

When you catch a wave you're going through a dynamic wad of variables--speed, the angle relative to flow, weighting, foil position relative to the surface. You might try to simulate that with a supercomputer (nah) or watch the foil while you drag it with a boat, but it's really not going to give you much information relevant to that dynamic environment and all those variables. 3D print a bunch of shims and spend a few days tuning, or tweak gently over the summer until you find a combination that works best for you. That's probably as good as it gets. I'm still amazed this stuff works at all.
« Last Edit: April 01, 2022, 11:54:08 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

MikeLima

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Re: Cross-brand tail force comparison?
« Reply #2 on: April 01, 2022, 06:22:38 PM »
Realistically Iíll never try the whole thing, but maybe running it through the water holding it shimmed and not might be interesting to feelÖ.
77kg rider
Armstrong 39L wing/surf
Armstrong 88 wing/SUP

Kujira foils
NoLimitz masts

Duotone unit 2022 3/4/5;dlab6

PonoBill

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Re: Cross-brand tail force comparison?
« Reply #3 on: April 01, 2022, 10:01:43 PM »
When I was testing paddle designs I built a simple water tunnel, but the volume was small and it ran the water from an elevated tank, through a test chamber, into a lower tank. About six seconds worth, then I had to run a pump for ten minutes or so to move the water back up. Boring. But then I realized I had easy access to a stretch of the Hood River that had a section with fairly straight flow--the entrance flume for an old Pelton wheel powerhouse. A gopro on a stick and a frame to hold the paddles, and Presto--water tunnel.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Dontsink

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Re: Cross-brand tail force comparison?
« Reply #4 on: April 01, 2022, 10:17:20 PM »
https://www.seabreeze.com.au/forums/Kitesurfing/General/Some-Foil-theory?page=1#:~:text=A%20positive%20stabilizer%20allows%20a,diagram%20below%20(top%20left)%3A

Some  old info on positive and neutral lift stabilizer kitefoil
setups.Written by the ZEEKO foil designer guy.

MikeLima

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Re: Cross-brand tail force comparison?
« Reply #5 on: April 02, 2022, 05:52:18 AM »
https://www.seabreeze.com.au/forums/Kitesurfing/General/Some-Foil-theory?page=1#:~:text=A%20positive%20stabilizer%20allows%20a,diagram%20below%20(top%20left)%3A

Some  old info on positive and neutral lift stabilizer kitefoil
setups.Written by the ZEEKO foil designer guy.

Thanks for that! A very interesting read indeedÖÖ.
77kg rider
Armstrong 39L wing/surf
Armstrong 88 wing/SUP

Kujira foils
NoLimitz masts

Duotone unit 2022 3/4/5;dlab6

surfcowboy

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Re: Cross-brand tail force comparison?
« Reply #6 on: April 02, 2022, 08:35:48 AM »
You can learn a lot, non-scientifically, by playing with a foil in waist deep still water. Fly it with your hands, etc.

 


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