Hi
I heard this. Is it true? I knew the downtilt of the rear wing gives more lift in light winds but does it provide more stability?
Cheers

Gives you something to lean against so you can be stable.

It's true. The best explanation I've seen is here:

www.av8n.com/how/htm/aoastab.html

The more angle, the more stability can be achieved.
However, approaching around 3-4 degrees, you get too much lift too quickly, as speed increases, and control get's compromised.
The exact #'s are different for different sailors.

I actually think it provides LESS stability if you increase the down angle. The reason is that the foil is now very sensitive to speed, and small speed changes mean big changes in lift. Too easy to correct too late and foil out. Of course, if you are foiling in fairly steady conditions, this is not a problem.

Yesterday, I changed the down angle of my old AFS-2 foil with shims to tune this. In gusty/holey conditions, the best foiling was with LESS down angle. My board speed could go and up and down without me having to make big corrections in stance and foot weighting. I call this MORE stable. And more relaxing.

The problem is that we are using "stability" to describe something which isn't stability.
Yes in theory more stab down force coupled with a further forward center of gravity will be more stable and also slower.
In practice however it results in greater trim change with speed as Segler points out. But also needs more forward weight in gybes and results in a more aggressive foil out or stall.

Everything is a trade off.

When you use Naish '18-19, you can set stab angles.
Set neutral, very slippery but touchy up and down.
Set with slight download, almost stable, as long as boardspeed is controlled.
Set more down, actually very stable, until boardspeed increases even a tiny bit. No good.

Thanks. In case of an slow big front wing, the effect of downtilt is the same?

Yes. But less difference in boardspeed affect your ability to control pitch.

Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

With Naish, zero is very UNstable thru out the whole speed range.
Maybe 1.2-2 has a stable sweetspot, and more get's weird in stronger gusts that force higher speeds.
Of course, heading upwind solves the speed problem.
Some of you can foil sheeting in and out, but my skills haven't nearly gotten there yet.

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marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

I just couldn't get I76 in B on the wizard 125 to dial in, not so much the rear wing angle the foil is too far forward and bring a tuttle box you cannot make micro adjustments

Now that I have the wizard 130 and twin tracks you can move the foil back further and it's night and day difference, far more controllable at speed.

Yes, let's define it. My definition of "stable" is that the foil maintains the same (or nearly the same) level of lift with different board speeds.

Since I foil in almost-always gusty conditions (Columbia River), this meaning of "stable" is important.

Tuning the down angle of the stab helps a lot. Rather than tuning on paper, you have to tune on the water until you find what works.

At Roosevelt on Friday I tuned my AFS-2 with F800 wing. With a shim that increased the down angle, the foil was actually scary. In any kind of gust it would rise up faster than I could react, and I crashed a lot. Removing the shim reduced the down angle, and the foil was much more manageable in the gusts. I could ride it all the way across the river with no drama. Just fun.

We will never achieve perfect stability. These foils are lifting bodies, and lifting bodies always generate more lift with increasing flow. All we can do is mitigate the change somewhat with (1) stab shims, (2) sail mast base position and sail sheet, and (3) body trim/foot stance and pressure.

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marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

The i76 which I measured up had 4.1 degrees of stabilizer angle.
My copy has 3.7 degrees and like you, I felt that I had more lift than I needed on B position.
I'm now running 3 degrees and it definitely feels more consistent and stable ;-) lift wise.
It seems that it may be possible to run Slingshots with less stab angle.
Interestingly, the i76 foil which I measured also has negative on the main wing and as I measure it has 0.6 degrees of front wing nose down.
If the wing flies flat and level (re Le and Te) that would mean that the stabilizer is running at 2.4 degrees.
Also if the wing is flying flat, the board is running at 0.6 degrees nose up and the whole system has 'rake' which 'White of Heart' reckons is a big benefit for touchdowns.
In the past I measured a NP pinky and it also ran front wing nose down but in the order of 2 degrees.
All of these measurements are referenced to the (vertical) mast.

It's very good you referenced "relative to mast".
Also, of importantance, is reference to board, which is difficult to measure.

Simon, Jesper and me have been stoked with the standard Infinity 76 setup for the past 2.5 years - B position, no shim.
It can generate a lot of lift but that's useful in the ocean where you're dealing with the variability of swell energy when gybing, riding waves or carving downwind in lighter winds.

We all initially found the lift tricky to suppress especially riding waves but perseverance paid off - preemptive forward bodyweight transfers become subconscious. We're all 82-85 kgs.

Might be worth experimenting with a shim to decrease rear wing angle to see if top speed can be boosted past 24 knots although I don't expect much improvement. Probably quite a reasonable speed already for a 1500 cm2 wing in ocean chop.

Good stuff here--thanks all.

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CoreAS said..

marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

I just couldn't get I76 in B on the wizard 125 to dial in, not so much the rear wing angle the foil is too far forward and bring a tuttle box you cannot make micro adjustments

Now that I have the wizard 130 and twin tracks you can move the foil back further and it's night and day difference, far more controllable at speed.

Dean, did you ever try the 76 in A position on the Wiz 125?

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marc5 said..
Good stuff here--thanks all.

CoreAS said..

marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

I just couldn't get I76 in B on the wizard 125 to dial in, not so much the rear wing angle the foil is too far forward and bring a tuttle box you cannot make micro adjustments

Now that I have the wizard 130 and twin tracks you can move the foil back further and it's night and day difference, far more controllable at speed.

Dean, did you ever try the 76 in A position on the Wiz 125?

Yes, position A was not good, way too pitchy.

When I tried the i76 in other tuttle box boards (JP 135, Horue Tiny) it was no problem, but when using on the wizard 125 it stronger winds it would lift and foil out no matter how much front foot pressure and leaning forward I done.

On the wizard 130 you set the i76 mast base all the way back on #1 in the twin tracks and its perfect.

Select to expand quote

CoreAS said..

marc5 said..
Good stuff here--thanks all.

CoreAS said..

marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

I just couldn't get I76 in B on the wizard 125 to dial in, not so much the rear wing angle the foil is too far forward and bring a tuttle box you cannot make micro adjustments

Now that I have the wizard 130 and twin tracks you can move the foil back further and it's night and day difference, far more controllable at speed.

Dean, did you ever try the 76 in A position on the Wiz 125?

Yes, position A was not good, way too pitchy.

When I tried the i76 in other tuttle box boards (JP 135, Horue Tiny) it was no problem, but when using on the wizard 125 it stronger winds it would lift and foil out no matter how much front foot pressure and leaning forward I done.

On the wizard 130 you set the i76 mast base all the way back on #1 in the twin tracks and its perfect.

When I had the Wizard 125 I had the same problem with the i76 foil -- too much lift in strong winds even with small sails. With my Isonic 130 the i76 was much more controllable. I now only use boards with foil mast tracks -- way more flexibility to control lift depending on conditions. I have absolutely no interest in going back to the Tuttle.

I'm an airline pilot so deal with stab trim every day at work. I hope the following is useful and explains what is going on:

On an aircraft the centre of mass is determined by the passenger and cargo distribution, and calculated before every flight. The stab angle is then set for takeoff based on that calculation. In flight the stab angle is adjusted for the speed we are travelling at for level flight. Just like when foiling, the faster you go the more the aircraft wants to pitch up, so we adjust the stab angle of attack to make the overall rotation zero.

When foiling, the stab is fixed, which is why we need to move the centre of mass around as we speed up and slow down. However, it might already be so far out of trim that neutral flight requires weight to be way forward or way back, which is uncomfortable and harder to make adjustments. There are two solutions to this - either move the foil in the track, or adjust the stabiliser angle (or trim as we would call it).

Stability is a different concept. Commercial aircraft (and the foils we use) are positively stable, thanks to the stabiliser. If you are flying along with the setup neutral and hit some turbulence, either in the water or the air, then the angle of attack changes on both the main wing and stailiser. Because they are aligned in different directions, the resulting change in lift on both makes the aircraft/foil pitch to counteract any upset. So if you suddenly get knocked 2 degrees nose up, the lift on the main wind increases, but on the stab it decreases, meaning a nose down rotation back to where you started.


To answer the question about stab angle and stability, the answer is yes, but we're not only talking about upsets, but also the pitch effect of shifting our weight. If you have a foil that has massive pitch changes for tiny body movements it also feels unstable.

The reason is as follows...

If you have a bigger distance between the centre of mass and the centre of lift, you need a bigger downforce from the stab. Or, looking at it the other way around, if you set the stab for more downforce, you need to have your weight further away from the center of lift when you stand on your foilboard. This greater distance means more stablity.

In the top image the centre of lift is closer to the center of mass, so little downforce is required. In the bottom one the distance is further so more is required. These forces are exactly the same as when foiling, except in foiling we are shifting our weight to keep the board level.

The top image is less stable in pitch than the bottom. If in the top picture imagine shifting the weight back 1 metre, when the distance from CofM to CofL is 2 metres. This halves the pitch down rotational force (Force*Distance)

However if you shift your weight in the bottom picture by 1 metre, where the distance from CofM to CofL is 4 metres, then the rotational pitch down force only changes by 25%.

So bodyweight shifts have less effect when there is a greater distance between the centre of mass and the centre of lift, meaning bigger bodyweight shifts are required to control pitch. But it's not the stab that is providing more stability, it's the distance between CofL and CofM.

Very good explanation Brit. I'm really wanting to crack my stability and control books and do the numbers on my foil and see what the loads actually are...And how things like rake/shim all relate to this.

Sounds like....move my butt forward and add a brick to the nose of my board.
Gee, that was my 4th day thoughts.

thanks brit for the illustration. for winging, some have advocated lining up the CoL and the CoM, which would result in a setup very pitch unstable (maybe good for pumping, but maybe this balance is counterintuitive for ease of use.

Since I'm totally UNstable, I also went long fuze, besides moving feet forward and keeping foil mast as is.
Heavy sails are coming next.

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sunsetsailboards said..
thanks brit for the illustration. for winging, some have advocated lining up the CoL and the CoM, which would result in a setup very pitch unstable (maybe good for pumping, but maybe this balance is counterintuitive for ease of use.

Monofoils require the CofM to be aligned over CofL. They don't seem to have caught on as I'm guessing they're too difficult to ride to be fun. You could have the CofM and CofL close together, with a very small stab and this will be looser in pitch/less stable.

Modern jet fighters are similar, in that they are a lot less stable in pitch than a normal aircraft. They are also unflyable by humans without the aid of fly-by-wire controls assisting them.

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CoreAS said..

marc5 said..
Good stuff here--thanks all.

CoreAS said..

marc5 said..
Thanks for the report, segler. I've always felt that my i76 in B position on my Wizard 125 develops too much lift as my board speed increases. I've measured the stock angle on the stabilizer and it's about -3.5 degrees. Offhand that seems extreme. The next time I go out on it I plan to shim the front of the stabilizer with a plastic bread tag to make that angle closer to zero by about a degree. Should be interesting. Maybe more stable as speed increases and induce less drag?

I just couldn't get I76 in B on the wizard 125 to dial in, not so much the rear wing angle the foil is too far forward and bring a tuttle box you cannot make micro adjustments

Now that I have the wizard 130 and twin tracks you can move the foil back further and it's night and day difference, far more controllable at speed.

Dean, did you ever try the 76 in A position on the Wiz 125?

Yes, position A was not good, way too pitchy.

When I tried the i76 in other tuttle box boards (JP 135, Horue Tiny) it was no problem, but when using on the wizard 125 it stronger winds it would lift and foil out no matter how much front foot pressure and leaning forward I done.

On the wizard 130 you set the i76 mast base all the way back on #1 in the twin tracks and its perfect.

Dean, now I understand why you mostly used the 84 and the 99 on the 125 Wizard. They don't have the "neck" like the 76 does. If I can't trade my 2018 Tuttlex box Wiz on a new track model, I guess I will have to go with a shimmed stabilizer.

Remember.....ice cream is good. More ice cream can go both ways.....even MORE ice cream can become detrimental.

the position of the stabilizer affects the stability of the flight. Also, the larger the stabilizer area, the greater the stability. for example, I have a horueh10 wingspan of 50cm and an area of 400cm2. Adjustment of the angle by oblong holes. conveniently made in the stabilizer holes and you can change the angle even on the water.