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how small can a front wing get.

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edt
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Re: how small can a front wing get.

Postby edt » Sat Feb 14, 2015 12:03 am

davesails7 wrote:
edt wrote:if you have cavitation or ventilation problems it is a flaw in the design. For instance Hering propellers can go above 100 knots without cavitation problems.
Cavitaion isn't caused by speed, it's caused by low pressure. If you are getting the same lift out of a smaller wing, you're going to have lower pressure on the top of the foil. So you can't just go smaller and smaller, at some point you're asking too much of too small of an area, pressure will be too low and you get cavitation.
Sure. But I have seen an 8,000 pound boat with about 1,000 horse power engine being pushed around up to 100 knots by a prop that is only a few inches long. So . . . yeah sure the principle is there but just looking at technology they have in propellers today it doesn't seem like we are even close to maxing it out.

"at some point"

I don't think we are close to theoretical maximum.

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Re: how small can a front wing get.

Postby davesails7 » Sat Feb 14, 2015 3:31 am

edt wrote:Sure. But I have seen an 8,000 pound boat with about 1,000 horse power engine being pushed around up to 100 knots by a prop that is only a few inches long.
Those are super-cavitating props though right? They go past the point of cavitation but have a different shape (wedge shaped) to continue lifting after cavitation.

You're right though, it's not a hard limit for kite hydrofoils either. You could make a kite hydrofoil that's super cavitating, but it would have higher drag in sub-cavitating speeds. Also, not sure how hard it would be to balance going through the point where you start cavitating.

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Re: how small can a front wing get.

Postby edt » Sat Feb 14, 2015 3:49 am

they might be supercavitating not sure. Remember the sailrocket has a record of 65 knots with non-cavitating style hydrofoils and that whole boat was mostly built in a garage.


from the sailrocket page:

"Cavitation is the main problem faced in hydrofoil design. This is the phenomenon in which bubbles of water vapour spontaneously form on the foil surface due to the pressure reduction. Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation, although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots."

So we should get our foils small enough to cruise around at 50 knots before we have to worry about cavitation. Let's do it.

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Re: how small can a front wing get.

Postby cglazier » Sat Feb 14, 2015 6:33 am

How small can a front wing get? I have ridden this Moses one which has a span of 34 cm and an area of about 270 square cm. It certainly takes more speed to get foiling.

:wink: CG
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Re: how small can a front wing get.

Postby windfreak74 » Sat Feb 14, 2015 12:20 pm

Hello CG,
the vorace sure looks small! very sleek too!
On normal wing 600 cm2 you get up with 10knots no problem.
compared to a 600 cm2 wing setup, what speed or wind speed do you need to get up on a area like vorace?

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Re: how small can a front wing get.

Postby revhed » Sat Feb 14, 2015 12:28 pm

Nice!
Now I can use my biggest R wing for a F wing test next summer!
R H

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Re: how small can a front wing get.

Postby cglazier » Sat Feb 14, 2015 3:22 pm

windfreak74 wrote:.. what speed or wind speed do you need to get up on a area like vorace?
I only had one short session on it so I can't really comment further. My point is only that small wings can get you up hydrofoiling.
:wink: CG

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Re: how small can a front wing get.

Postby ozchrisb » Sat Feb 14, 2015 7:48 pm

edt wrote: "Cavitation is the main problem faced in hydrofoil design. This is the phenomenon in which bubbles of water vapour spontaneously form on the foil surface due to the pressure reduction. Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation, although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots."
.
Sailrocket II was a lot more complicated than that. It had low speed foils that got it started and then super duper fast ones once it got up to speed. If you look on youtube at the footage of it you'll see it slipping sideways a lot when it's starting, all of a sudden it foils and takes off lie a rocket. It's awesome!

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Re: how small can a front wing get.

Postby zfennell » Sat Feb 14, 2015 9:54 pm

edt wrote:they might be supercavitating not sure. Remember the sailrocket has a record of 65 knots with non-cavitating style hydrofoils and that whole boat was mostly built in a garage.


from the sailrocket page:

"Cavitation is the main problem faced in hydrofoil design. This is the phenomenon in which bubbles of water vapour spontaneously form on the foil surface due to the pressure reduction. Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation, although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots."

So we should get our foils small enough to cruise around at 50 knots before we have to worry about cavitation. Let's do it.
edt
In many respects, you are probably right .
A traditional, non cavitating foil should be able to approach 50 kts w/o cavitation.
But there are no guarantees your small foil will have enough lift to get you up to speed.
Or that your kite has enough thrust to accelerate you up to speed.
Unlike the example of the 1000hp ocean racer going 100 kts, there is little chance he can do it with only 50 hp.
It becomes a question of efficiency.
Makin the foil smaller, does nothing for the strut requirements.
The strut is still a significant chunk of wetted surface in a KBHF
All that surface area is still generating increased drag proportional to velocity squared.
Ultimately, you may need to get all that stuff out of the water.
James has already said it, but there is a reason that surface piercing foils are chosen for their ability to reduce wetted area as lift increases.

just to beat this to death for a bit:
assuming the basic equation for lift of a foil section : L = Cl * .5 * density*Area* Velocity^2

Coefficient of lift (Cl) can be changed with flaps or other camber adjusting techniques. But KBHFs don't do that yet.
You can change Area ( topic at hand) , but still no 'real time' adjustments happen here, either.
The rider can certainly change the pitch angle to increase Cl.
Typical low drag design Cl (ranging .1-.3) happen at small angles. if you increase AOA to increase Cl, the drag goes up and stall becomes a possibility and the inception of cavitation gets closer.
a practical limit for Cl is probably Cl=.5
so if you pick an area and low drag Cl to allow you to fly at 50 kts, what is the likely take-off speed for a small foil with no rider adjustments other than AOA?
if you can increase Cl by 5x you can maintain the same lift capacity @ sqrt (50*50/5) = 22kts takeoff speed and 50 kts top end.
... if true that would seem like a really specialized application and not appealing to most folks
(similar to what sail-rocket does)


It will be interesting to see where the design for fast foils ends up.
Regards,
Bill

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Re: how small can a front wing get.

Postby Zeeko » Wed Feb 18, 2015 12:50 pm

Hi

As Peter wrote, the smallest wing to be ridable depends on what you want to loose in term of speed of take off and upwind ability.
Using small wings need more speed to lift the hydrofoil but have an incredible sensation of riding (no drags, accelaration). And they are suitable for jumps as the smaller surface makes less stress on the fuselage during landing.
For info with the freeride and race wings my speed of take off is around 6knots
With the speed wing, my take off speed is approx 12knots.
To achieve 12knots of speed, you need to use thick board to start by riding the board on the water to create speed, or stronger winds.
My max speed is around 35knots for the speed wing and 30-32knots for the freeride wing,
But the feeling is awesome with the speed wing.
My speed wing is almost 40% smaller than the freeride wing,
Ride easy
Nicolas


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