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Small stabilizers

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lederhosen
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Re: Small stabilizers

Postby lederhosen » Tue Jul 30, 2019 3:03 am

Peter_Frank wrote:
Mon Jul 29, 2019 12:59 pm

Then the opposite also exists, namely that really short fuselages with big elevator can give you a super narrow "loop" radius, so for aerobatics this can be extremely fun.

For a hydrofoil, we dont use the control surface(s) the same way, as we steer by weightshift and yaw input instead, so the agility gain from a short fuselage and big stab would probably not exist the same way.

BUT, I think a superlong fuselage and small stab will at some point give you more drag, as the fuselage will not always stay in the waterstream, as you ride with different angle of attacks going upwind, versus going fast downwind or halfwind.
And the stab might stall in tight turns too, as the aoa gets pressed too high :(

In turns it is evident, the fuselage is turned quite extreme, and you can easily see this on photos from above if you got a bit of bubbles also.
The fuselage should be bent both horisontally and vertically to follow the flow.

So too long might not be performing better, at some point.

Apart from this, the practical issue, a long fuselage will increase the risk of hitting the bottom when starting in low winds, or you hitting the foil with your feet when in the water.

This is why I think there is a limit, where a longer fuselage becomes a downside both structurally but also in terms of aligning with the flow (curve) when carving, see the pic here that shows when you do a tight donut (360), the fuselage can not follow the "curve" and will be very draggy.
On this pic I get some air into the wing tips (happens often if you do a lean back 360 instead of the upright ones), but that is not my piont - it is simply that the picture shows how much curve you have in a tight turn or donut, and why a shorter fuselage, or no fuselage, would work well :rollgrin:

D2.jpg

Having said that, it is all down to how it feels, and nothing but that - and this is most likely the reason we got the current length of most fuselages, somer shorter like the early Spotz racefoils, and some longer, but still about the same range.

I just think the stab sizes are WAY too big for todays way of riding, thus this thread :thumb:

8) Peter
How about a flat rear part of the fuselage instead of a stabilizer? If its stiff enough to not bend under most of the riding but has just enough flex to deform under tight turns like on the picture it could give some nice release coming out of the turn. Or do you think it will always wobble no matter how stiff it is?

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Re: Small stabilizers

Postby Peter_Frank » Tue Jul 30, 2019 6:37 pm

Kamikuza wrote:
Tue Jul 30, 2019 12:43 am
Amazing that a 1% difference could make so much of a ... difference :lol:

So this is exactly what I said about big wings -- drag increases sure but the LD coefficient isn't hugely different because more lift.

At 20 knots, the difference in drag between those two stabs is ... 30%.

Perhaps the drag we feel and complain about is just the 1% difference...

I dont think so really :roll:

When you decrease the stab 30 %, the foil becomes these 30 % more lively, which is a HUGE difference in feel :rollgrin:

It gets so light and agile when you turn the foil now, and still easy to ride :thumb:

The overall 1 % less drag, noone can feel I am pretty sure :wink:

Yes, the big wings got the same lift/drag coefficient, so they can go upwind in the same angle, great :rollgrin:

In fact, just made some comparisons now, the bigger wings if same AR, can go more than 6 % higher to the wind than one half the size, because of the "fixed" drag, and because in lighter winds slower foils can go at a higher angle although usually still slower VMG.

But the half sized small wing will only have 47 % of the big wing foils drag, thus being able to ride 30-35 % faster upwind in VMG. Depends on windstrength both are used in, thus not that easy to compare.
At max speed, the small one no matter if high or low AR, will have 41 % less drag, meaning able to ride 30 % faster (drag and lift increases squared with the speed, thus not 41 % faster speed).

But they are not same AR, so if we have a 1200 cm2 wing 3.6 in AR, and we change to a 600 cm2 wing same span, meaning 7.2 in AR, then it changes a lot upwind.
Now the 600 cm2 foil only got one third the drag compared to the 1200, thus able to ride 55-65 % faster upwind in VMG.
And, the angle to the wind can be 36 % higher.

That is a huge difference, and the reason racers use smaller and higher AR wings of course, except for superlight winds where bigger sizes, still high AR, are chosen.


Luckily it does not matter much (apart from in marginal winds) for us freestyle/freeride/waveride hydrofoilers, where medium or big wings are awesome.

Would just put some numbers on this, now some of you asked, and I had fun joggling with this :D

8) Peter

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Re: Small stabilizers

Postby Peter_Frank » Tue Jul 30, 2019 6:48 pm

lederhosen, interesting idea yes, having a "birds tail" fuselage, but two things are in my mind not that good with this.

It will have extremely low AR, a lot longer than wide, so if it does not follow the flow (being "soft" like you say), it might be so inefficient that it can not deliver much lift, thus not much stabilisation, or more drag if too big to compensate.

If it follows too much, too soft, it will not have any stablilisation purpose and you are much better off riding without fuselage and stab.

So something just in between should be the goal, not stiff nor too soft.

Experiences from windsurf and other surfboards with soft tails to accommodate for changing speed, has been quite bad in history though...

Never say never, I like the idea, especially because we unlike all aircraft dont use the stab(s) as control surfaces, but as stabilisers only :thumb:

Although, on our current hydrofoils where we can put a small stab on, or a big one, and even adjust the aoa on the stab, I think it is hard to make something better.

8) Peter

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Re: Small stabilizers

Postby Kamikuza » Wed Jul 31, 2019 12:29 am

What is the unit is measurement for liveliness? :D


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