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Small stabilizers

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lederhosen
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Re: Small stabilizers

Postby lederhosen » Thu Jul 25, 2019 9:03 am

Peter_Frank wrote:
Thu Jul 25, 2019 8:28 am
Kamikuza wrote:
Thu Jul 25, 2019 12:36 am
Peter_Frank wrote:
Wed Jul 24, 2019 11:52 pm
Less drag?
Yes a tad, but not really worth mentioning, so shouldnt be a deciding factor.

8) Peter
30% smaller, I'd have thought more than a tad...

Did some fast calculations, and a 30 % smaller stab with say a 1200 cm2 wing, would only decrease the overall drag about 2 % even in the most ideal scenario foiling at speed only 20 cm mastlenght in the water.
And this drops to below 1 % if riding upwind or slow or turning, where induced drag more than doubles the main wing drag.

Unless you race, I wouldn't care at all about 1-2 percent drag difference, compared to the huge agility change :wink:

That was my point.

8) Peter
Thats really interesting, how did you calculate that?

How much do you think will a smaller stabilizer increase the front foot pressure?

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Re: Small stabilizers

Postby jumptheshark » Thu Jul 25, 2019 12:47 pm

Am also curious about the interplay between fuse length and stab size.

Does shortening the fuse change the ride in a different way than decreasing the size of the stab?

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Re: Small stabilizers

Postby Peter_Frank » Mon Jul 29, 2019 12:59 pm

lederhosen, a smaller stabilizer will not increase, but decrease the front foot pressure, as the stab lifts a tad downwards in straight "flight", so a smaller stab will feel like it pushes "up" (less down) on your rear foot.

It is IMO not really noticeable, very little, but using a stab say half the size only, you will need to stand an inch or more furher back yes.

Can be compared to removing the stab fully, then you need to step back on the board.


jumptheshark, a good question, I have two thoughts, and knowledge/experience from aviation:

For gliders, as long a fuselage as possible is used.
This will let you have the rudder and elevator in more free non turbulent wind, and also use smaller control surfaces with less drag.
Last but not least, you get a more responsive rudder, if used for competition soaring fast turns.

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
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Re: Small stabilizers

Postby jumptheshark » Mon Jul 29, 2019 1:35 pm

Just picked up a lift 170 classic so I won't be changing my stab!

The stab is a little bit bigger than what was on my zeeko but the fuse is more than three inches shorter putting it much closer to the main wing.

So far after a good few sessions on it, it feels like the added surface area does a few things at low speeds. Additional lift on take off for easier up and go on the light end, and likewise, way lower overall stall speed with very forgiving stall warning. The main wing is actually likely a touch smaller or at least the same size as the Stringy wing I was on, but the Lift has way more overall lift and can cruise comfortably at much lower speeds without the drastic quick stall. Very smooth foil overall that has made pretty much everything feel quite a bit easier.

Counts me out of the small stab experiment :(

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Re: Small stabilizers

Postby Peter_Frank » Mon Jul 29, 2019 1:49 pm

Sorry, but this is wrong.

A bigger stab will make the initial speed higher, as less total lift, thus just opposite.

Whatever difference you feel in this regard, is not because of the stab size, but the main wing solely IMO.
As area of the wing is only part of the overall lift, so many other parameters determine lift and stall speed.

Of course, a bigger stab giving more stability, might give new foilers a lower practical stall speed, this is pretty normal and will dissapear over time.

Only canards works opposite, here bigger stabs or main wings both give more overall lift thus lower stall speed.

On a "normal" hydrofoil, the stab lifts downward, so a bigger stab, aoa unchanged, will result in less overall lift and higher stall speed.

8) Peter

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Re: Small stabilizers

Postby Kamikuza » Mon Jul 29, 2019 2:28 pm

lederhosen wrote:
Thu Jul 25, 2019 9:03 am
Thats really interesting, how did you calculate that?

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Re: Small stabilizers

Postby jumptheshark » Mon Jul 29, 2019 3:28 pm

I understand that the stab is both angled and foiled to pull down, but cant help picture that at the initial stages during take off or riding really slowly and approaching stall, the AOA can get high enough that the the stab protrudes down deep enough to overpower the set angle and lift from it shape that the sheer effacement of the surface area is effectively adding to overall lift. You are likely right and it may all be the difference in main wing, but this foil has such a soft stall warning that feels like the stab beginning to stall as it protrudes down below the main wing where the main bigger wing is still flying. The main wing has more profile than stab. It's a nice warning that I can generally just pump out of.

So, There may well also be implications for pumping when changing stab size. The lift 170 is a pumping machine.... Very easy to feel and maximize thrust. There may also be an impact on how the foil surfs in a wave where there is upward movement of the water against the broad surface of the stab compared to when riding along normally in flat water.

There are implications for stab size and effect in really hard carves too, with the fuse length factoring in as well..

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Re: Small stabilizers

Postby Peter_Frank » Mon Jul 29, 2019 10:26 pm

Kamikuza wrote:
Mon Jul 29, 2019 2:28 pm
lederhosen wrote:
Thu Jul 25, 2019 9:03 am
Thats really interesting, how did you calculate that?

Okay, have been 14 days on vacation so not possible to show till now when home.

My first calculation was a quick one on my mobile phone calculator, and said 1-2 % difference in drag as I postulated, if you decreased the stab by 30 %

Just made a quick calculation again right now, still pretty simple but with more variables, and it gives about the same, around 1 percent using a typical low AR bigger wing, and will be around 2 % if using a smaller really high AR wing, when riding at high aoa.

So my point is, 30 % decrease in stabilizer area, only decreases drag by not much more than 1 % :rollgrin:

Even at high speeds almost zero lift and hardly any induced drag, a 30 % decrease in stab area would only decrease the overall drag about 3 % in below case


Assume words arent necessary:

Image

8) PF
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Re: Small stabilizers

Postby Kamikuza » 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...

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Re: Small stabilizers

Postby jumptheshark » Tue Jul 30, 2019 2:00 am

BTW Peter, sweet pic.


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