Author Topic: Can We Talk Stabilizer Geometry?  (Read 3091 times)

SanoSlatchSup

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Can We Talk Stabilizer Geometry?
« on: August 12, 2020, 08:11:56 AM »
Flat stabs, anhedral down stabs, stab tips curved up, stab tips curved down, stabs mounted on the bottom of the fuse, stabs mounted on the top of the fuse...wth?  Can someone please help make up my mind as to which shaped stab I want to use when and where.

I get the size and AOA thing and all, but all these different shapes and mounting location reasons has me scratching my head as to what it is they're all specifically designed to do differently, and maybe even more importantly for my Axis selections in the future (I currently have accumulated the 390, 400, and 440)...why that is. TIA.
Me: 6'1"/185...6'0" Kings Foil Board...6'0" Chelu Foil Board...9'6" Bob Pearson "Laird Noserider"...9'6" Costa Azul Wide Body...14' Lahui Kai "Manta"...8'0" WaveStorm for when proning urges still hit, and 7'3" Chuck Glynn foil board backup.

jondrums

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Re: Can We Talk Stabilizer Geometry?
« Reply #1 on: August 12, 2020, 09:00:10 AM »
Going from gofoil flat stab (18N) to the new 14.5 upturned tips made a noticeable difference in railing turns.

It is my view that directional stability will come from all vertical surfaces - and related to both the projected area of the vertical surface and the distance from the turning center.  Super hard to know where the turning center is, so that's a bit of a mystery.  But I do think part of the confusion around tail profiles is because the mast is placed differently on different foil brands/setups.  If the mast is further forward with respect to the turning center, then I'm guessing people are going to really want vertical tips on the tail and vice-versa.

We have to have a mast, so it would be nice to get the directional stability from the mast we already have to have rather than rely on upturned tail tips which add drag.  That means the mast would need to be a bit further back on the fuse.  But then, directional stability will vary with depth in the water as the mast area changes dramatically.  So, hard to know if that's a good solution.

PonoBill

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Re: Can We Talk Stabilizer Geometry?
« Reply #2 on: August 12, 2020, 09:10:24 AM »
I'm equally confused. I know how they are supposed to work, but the reality and the current designs don't match up to theory. Wings aren't making much more sense either. There's at least a hundred years of research into hydrofoils but getting my hands on real data and designs is not easy. I've found one practical book on hydrofoil design. Everything else I've dug up is either irrelevant or covers issues we'll never have. I don't plan on going over 60mph with my foil.

My take is that there isn't any science behind current designs for our use--it's just cut and try. The only thing I've seen recently that made sense to me is some of the Moses wing designs, but they look like they would amputate rather than cut if you fell on them.

Screwing around with stabilizer incidence angle has made my foils a lot more stable in pitch, but my front foot is now on the handle, well behind the footstrap mounts.

I'm not good enough yet to really feel any difference in turning ability with various stabilizer shapes. My wing sizes are going down fast though. Yesterday I was on my new 860 and I convinced myself that I really should have bought the 760. My 440 stabilizer looks almost as big as the wing. The GoFoil Kai used to look impossibly small to me, it's 1200cm*2. The same approximate size of the Axis 900 and 860, and the Kai is much thicker, with a higher foil coefficient. That's just weird to me, though I know velocity is much more important than area, and I'm generally going a lot faster than I used to be.

Every time I try to convince myself that i know what I'm doing with this stuff I look at the variety of current designs and realize I'm clueless.
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.

bigmtn

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Re: Can We Talk Stabilizer Geometry?
« Reply #3 on: August 12, 2020, 10:04:00 AM »
Sano, I have no idea. But I can tell you I didn't notice a huge change between the 400 and 370 stab. But the 460 flat stab, gave me a whole different feeling. So I'd pick that one up next. 390 pumps great, but feels really slow compared to the 460. My general feeling with the 900 is 370 surf, 390 winging, 460 downwind. But haven't tried the 460 with the 900 yet. Only downwinding with the 1000.

PonoBill

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Re: Can We Talk Stabilizer Geometry?
« Reply #4 on: August 12, 2020, 10:08:43 AM »
The biggest change in stabilizer I've felt is the 340 shimmed to two degrees. Feels like no drag but it's still stable. I don't know if it helps turning or not, but it feels very loose and skatey, especially with the short fuselage. I should try it with the ultra short.
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.

tarquin

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Re: Can We Talk Stabilizer Geometry?
« Reply #5 on: August 12, 2020, 10:21:21 AM »
Not much help but foils are very speed and weight related. A foil that works at 5 kts doesn't work as well at 15 kts. Then you change the angle as well, then you break the surface with it and re enter causing airation and just expect to get perfect laminar flow again. It's amazing they work at all.
 The Americas cup teams and IMCOA 60s are having to develop new computer programs to design foils as existing programs just dont work. Some have turned to F1 teams to use there programs and simulator tech. Mercedes excels in this domain.
 You can see some very different foil shapes and placement in the IMOCA boats. So there is still no right answer. Some will perform better in light air and some better in heavy airs.
 So I guess for the stabilizer some will be better for turning and some will be better for going fast.
 What you want in a foil wing is a soft outer that compresses as speed and pressure increases making the foil thinner. This gives you good lift and low speed and a thinner foil for higher speed. Not easy to do though. You want a foil to flex at the right spot and speed as well. Again giving better shape for high speed. If a curved foil straightens it becomes longer, mainly for avoiding box rules. Also a highly cambered foil that flattens as speed and pressure increases.
 I very much doubt the companies making the foils are using the sort of tech the americas cup or IMOCA boats are using. So I think there is a long way to go for surf/sup foiling.
 No I dont foil.
 But i think my next build may just have foils.

Admin

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Re: Can We Talk Stabilizer Geometry?
« Reply #6 on: August 12, 2020, 10:29:35 AM »
Within Axis there is a drop below the 400 from chord at 90 to 80 so the 390 is actually quite a bit smaller than the 400.  The 390 feels a lot faster to me with the wing than the 400 or 370.  Why I am not sure.  I had heard that the 390 had zero incidence which was supposed to have been significantly lower incidence than on the other stabilizers.   The 390 appears to have an incidence (angle in relation to the fuselage and in our use also the mast plate and in many instances board base and deck) of -2.5 degrees so still negative incidence.  I don't have any of the other tails any longer to check those.  Those may have even more negative incidence which might be the explanation for what I am feeling.  It may also be that 390/80 is just a really nice size for me on a certain group of front wings. 

I would be very hesitant to apply a blanket "this design/angle/size works" to anything other than a specific wing and stabilizer.  If something feels off in that closed system then adjust.  Fuselage length is definitely a factor as well. 

« Last Edit: August 12, 2020, 10:39:52 AM by Admin »

SanoSlatchSup

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Re: Can We Talk Stabilizer Geometry?
« Reply #7 on: August 12, 2020, 09:55:08 PM »
Thanks all, some really interesting and great responses...much appreciated.

The reason I asked was that having been a GF guy up until a few sessions ago, it was pretty easy to figure out what they were going to do, as it was pretty much just a choice in size because the two shapes of each profile (Maliko/Kai & 18W/N) were basically the same...and having so many to choose from with Axis (one reason I switched) I suddenly found myself with analysis paralysis in trying to figure out which different stab (400, 440) I wanted to try with the 900 instead of the 390 I'd been running with it.

Yes, I could just throw one on and try it, but that's not my personality, I like to do something with a reason and purpose, so having a little idea of what was what with the various designs was intriguing to me. So since I'm really liking the 900/390 combo compared to any of my old GF setups, on occasion when I really stomp on it, I'll sometimes feel the stab sort of fall off or "break through the ice" as a friend of mine described feeling with the GF GL180/18N instead of his normal Iwa/Maliko he just loves.

Also at times I'd get a high speed bobble just setting up a turn, or a pretty good slip/wiggle coming out of those higher speeds turns. When things were slowed down a little bit, it was solid as a rock. But because I kinda like trying to stand on the gas at times, I felt that I needed a little more wing in then rear, but then didn't want to change the overall straight line flight too much adding more lift, so went on the wth do all these different wings do search.

So I've come to the conclusion that with the AOI differences, the foiling from middle to tips, and chord differences to the three, and for what I'm looking for...I'm going to just throw on the 440 first, try it, see what it does, paddle in, and throw the 400 on, rinse and repeat. Cuz I'm still lost on all of it, and just gonna go do the scientific "try and fly" model to discovery. :D


Me: 6'1"/185...6'0" Kings Foil Board...6'0" Chelu Foil Board...9'6" Bob Pearson "Laird Noserider"...9'6" Costa Azul Wide Body...14' Lahui Kai "Manta"...8'0" WaveStorm for when proning urges still hit, and 7'3" Chuck Glynn foil board backup.

PonoBill

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Re: Can We Talk Stabilizer Geometry?
« Reply #8 on: August 12, 2020, 11:45:38 PM »
Well, while you're at it, start farting around with shimming the stabilizer. I have a couple of different angle measuring devices, and occasionally walk around measuring everyone's foils/stabilizer incidence angle. A lot of the combos I've checked are in the 5-degree range. The 390 is about -4.7 degrees to the wing. Admin insists on checking to the fuselage and to the baseplate, neither of which makes any sense to me, but he's nuts, so I don't argue anymore, because he'll insist on explaining it to me. I think that's called Adminsplaining.

Absent some active system for controlling the stabilizer, 5 degrees seems like a hell of a lot. I've settled on shimming to two degrees. This makes for some very radical changes that take a while to get used to, but I like the end result. I've tried 0 degrees. Not great, though I might be able to get used to controlling the foil pitch with my back foot.
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.

Admin

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Re: Can We Talk Stabilizer Geometry?
« Reply #9 on: August 13, 2020, 12:46:38 AM »
Well, while you're at it, start farting around with shimming the stabilizer. I have a couple of different angle measuring devices, and occasionally walk around measuring everyone's foils/stabilizer incidence angle. A lot of the combos I've checked are in the 5-degree range. The 390 is about -4.7 degrees to the wing. Admin insists on checking to the fuselage and to the baseplate, neither of which makes any sense to me, but he's nuts, so I don't argue anymore, because he'll insist on explaining it to me. I think that's called Adminsplaining.

Absent some active system for controlling the stabilizer, 5 degrees seems like a hell of a lot. I've settled on shimming to two degrees. This makes for some very radical changes that take a while to get used to, but I like the end result. I've tried 0 degrees. Not great, though I might be able to get used to controlling the foil pitch with my back foot.

Here comes some Adminsplaining :)

The method of measuring angle mentioned in the vid below (Bill does something similar) is useful provided a few things. 

1.  Nothing moves at all between the foil and the stabilizer measurements.  Your caliper is rock solid (no flex) or wiggle. That means no  measuring on a beach with the board moving or on a floor unsecured.  It is way too easy to slip a degree or four in a game where you are counting on fractions of a degree.  That is why we were unable to reproduce a measurement.  In the vid below he has it really well clamped down and has a flex free caliper and accurate measuring tool. 

2.  Most importantly, for this method to be useful you need to have a target number (difference between front and rear wing angle) that is known good for a given system (specific front wing and and specific stabilizer).  Trying to apply a generic number (2 degree difference for instance) to any foil combination would be silly.  In the vid below George mentions one system that he knows is correct at a 2 degree difference and another that is correct at a 4 degree difference.  He says that the range for that one is 3.8 to 4.2 degrees with 4 being the sweet spot.  That is a very tight range which requires very specific tools and previous knowledge of that desired range. 

My point to Bill was that we were meeting neither of the above points.  :)

George also mentions that what you are really trying to do is to adjust based on what you are feeling for a given system.  That makes sense to me.  What is the specific issue that an individual is trying to address?  It is likely going to be very different for each of us even on the same foil set.  Since we are only adjusting stabilizer angle (incidence) in this scenario all you really need is an accurate measure of that single angle as a start point and again as a measure of the change that you have made.  It is easier and more accurate to measure that single angle then to try to get a relative angle between two foils.  Check out two posts down.  This is a very easy way to measyre shim angle with only a cell phone camera.  Washers may also be too coarse of an adjustment in many instances.  A single thin washer can make a 2 degree change on some foil systems.  Some angle markings on the fuselage at the stabilizer would be a nice idea. 

https://www.youtube.com/watch?v=jTCyag5N_vE
« Last Edit: August 13, 2020, 02:22:18 AM by Admin »

Admin

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Re: Can We Talk Stabilizer Geometry?
« Reply #10 on: August 13, 2020, 01:11:16 AM »
PS:

Here are the two printable shims that Axis offers (+/- 1 degree).  I do wish that they would sell these.

https://axisfoils.com/collections/foil-parts-accessories/shim





« Last Edit: August 13, 2020, 01:13:00 AM by Admin »

Admin

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Re: Can We Talk Stabilizer Geometry?
« Reply #11 on: August 13, 2020, 01:33:07 AM »
This free online tool is an easy way to measure shim angle:

Please note that Axis is measuring these shim angles in relation to the horizontal axis of the fuselage.

https://www.ginifab.com/feeds/angle_measurement/

Below is an example using the Axis shim as an example.  I also showed the same online tool being used with a cell phone image of a mounted 390 tail with no shim.  You just need to level and weight the fuselage to get an accurate and very replicable baseline reading.  This is the incidence of the stabilizer.  In this example it is -2.5 degrees.


« Last Edit: August 13, 2020, 02:10:31 AM by Admin »

Dwight (DW)

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Re: Can We Talk Stabilizer Geometry?
« Reply #12 on: August 13, 2020, 04:43:39 AM »
We (and a local buddy) bought into the Delta video a few years ago and bought those tools.

Tried it on original Naish with tilting tail. All we learned was we didnít know shit. The factory setting was best.

My buddy, whoís heavy, kept trying to find more lift on every foil he owned with the delta method.  All he got was confused and never was successful in making anything better than the factory setting.

Fast forward to my first foil with factory shims, Armstrong, and everyone says just use the +1 shim for max speed. Donít use the 0 shim, itís too much drag and lift. Sounds backwards, but WTF, I have already proven to myself, I cannot do better than the manufacturer, so +1 it is and it is wonderful.


Admin

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Re: Can We Talk Stabilizer Geometry?
« Reply #13 on: August 13, 2020, 06:16:59 AM »
Fast forward to my first foil with factory shims, Armstrong, and everyone says just use the +1 shim for max speed. Donít use the 0 shim, itís too much drag and lift. Sounds backwards, but WTF, I have already proven to myself, I cannot do better than the manufacturer, so +1 it is and it is wonderful.

The language can be confusing.  It sounds like Arstrong labels their shims opposite to Axis with -2 for instance providing the most downforce.  With Axis the -1 shim has less downforce than the + 1.   

If so, what you wrote above would make sense if a particular combo was feeling draggy.  Then adding a degree to the tail would reduce its negative angle and it would provide less down pressure and less drag. 
« Last Edit: August 13, 2020, 06:25:59 AM by Admin »

PonoBill

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Re: Can We Talk Stabilizer Geometry?
« Reply #14 on: August 13, 2020, 10:17:04 AM »
I modified one set of my calipers to do this, but it's only 6" jaws, so it only works on a limited number of wings. I could buy some cheap 12" calipers, or make some other carrier for the angle detector, but the model airplane incidence measuring tool showed up at the UPS store while I was in the middle of making a carrier--I don't remember ordering it, but obviously I did (if you know me, you aren't surprised by this). It's the same tool Mark Ribkoff uses. He's the one that got me started on this kick.

The purpose of the incidence angle is to push the front wing into an angle of attack that increases the lift. That's completely a function of the angle between the stabilizer and the wing. The angle to the fuselage or the baseplate might be interesting in some fashion, but it's nothing I really care about.
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.

 


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