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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 10:10 pm 
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Hawaiis wrote:
......I tried to bend the wood foil and with 16mm thickness, it was definitely more stiff than my 2.5mm titanium high AR foil.


is the 16mm wood foil the high AR version with vibration problems?
if so, does the more flexible titanium foil have any noticable vibration modes ?
...perhaps at speeds lower than the stiffer wood foil?


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 10:45 pm 
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Yes, the high AR 25 inch wood had vibrations, that is why I used fiberglass to reinforce it.
16mm is after the fiberglass. The fiberglass probably made the vibration even worst due to the thickness.

Check out this video,
No vibrations on the titanium foil
https://www.vimeo.com/78373843


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 12:59 am 
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Hawaiis wrote:
Yes, the high AR 25 inch wood had vibrations, that is why I used fiberglass to reinforce it.
16mm is after the fiberglass. The fiberglass probably made the vibration even worst due to the thickness.

...


I have a Theory ...
Maybe someone else could confirm or deny ?

See the round trailing edge ; it's possible the water is sticking
to the TE and causing a resonate frequency because of it's roundness.
Kinda like the way a cylinder makes the air flow behind it.

http://www.youtube.com/watch?v=K79RrZTtRoc

Easy test is to place some epoxy and filler on the TE and make it Flat
or tapered it to a point ; then go try it again. Sharp exit for the water, with
no "Round" edges for it to stick on is the theory. Cost ya 3 bucks and an hour to find out.

Bille

Ps :
& --Hawaiis--
i come off rather Sharp and cold to most people
just know i got a LOT of respect for your intellect and perseverance !!


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 1:34 am 
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i will probably never ride a hydrofoil but i´m really liking this tread


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 2:46 am 
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in line with Bille's comment,
The hydro gurus have something called 'Strouhal number'.

Its a way to describe the excitation frequency that objects in some flow flield may be subjected to.

Its calculated from the velocity of the flow divided by some nominal reference length (i.e chord length of a wing or diameter of a cyllinder)

in this case :
if V= 15 kts = 25 ft/sec
and
L= 4 in = .25 ft

then frequency = V / L = (25 ft/sec) / (.25 ft) = 100 hz

the implication is that if the 100hz excitation is close to any resonant mode of the foil, then vibration amplitues would become excessive.
the resonant mode could easily be a low order bending or twisting.

if you consider the foil to be a cantilevered spring with a certain mass:
its natural frequency will increase with increasing stiffness and
decrease with increasing mass.

if you believe any of this, then adding weight to the end of the offending foil would reduce the natural frequency and avoid resonance at 15 kts.
.....not necessarily a good thing, but possibly a way to see if any of this is true.

shit happens.


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 1:35 pm 
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zfennell wrote:
in line with Bille's comment,
The hydro gurus have something called 'Strouhal number'.

Its a way to describe the excitation frequency that objects in some flow flield may be subjected to.

Its calculated from the velocity of the flow divided by some nominal reference length (i.e chord length of a wing or diameter of a cyllinder)

in this case :
if V= 15 kts = 25 ft/sec
and
L= 4 in = .25 ft

then frequency = V / L = (25 ft/sec) / (.25 ft) = 100 hz

the implication is that if the 100hz excitation is close to any resonant mode of the foil, then vibration amplitues would become excessive.
the resonant mode could easily be a low order bending or twisting.

if you consider the foil to be a cantilevered spring with a certain mass:
its natural frequency will increase with increasing stiffness and
decrease with increasing mass.

if you believe any of this, then adding weight to the end of the offending foil would reduce the natural frequency and avoid resonance at 15 kts.
.....not necessarily a good thing, but possibly a way to see if any of this is true.

shit happens.


:thumb:
Great advce, I am completely Ignorant to the "strouhal number" but certain if VIBRATION is your problem "singing" rather then FLEX "wobbling" you can most often times tune it out using mass in addition to changing stiffness.

I've got a big bag of magnetic weights in my vibration kit when called out to do vib analysis in my day job.....also have a modal hammer which basically tells you the resonate frequency of an object by hitting it...super useful but you won't likely ever get your hands on one of these.


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 6:09 pm 
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Location: rhode island
Hawaiis wrote:
Yes, the high AR 25 inch wood had vibrations, that is why I used fiberglass to reinforce it.
16mm is after the fiberglass. The fiberglass probably made the vibration even worst due to the thickness.



the video is cool
music, glassy water, ..almost hypnotic

one point i failed to make and pehaps you have not realised.

Despite the fact that titanium is stiffer and stronger than wood (material properties).
Both of your wood foils appear to be stiffer than the titanium foil.
i say this only because you can bend the titanium foil with less force than the wood foils
all due to differences in geometry ( differences in thickness and/or length)

the titanium, being heavier and less stiiff than a comparably sized wood foil would have a LOWER resonant frequency
(This is the one that works, right?)
Adding glass to the wood foil will make it stronger and likely stiffer. But, i'm not sure how much it would shift the natural frequency of the wing if this is your problem.
Regardless, you may be trying to move it in the wrong direction.

-bill


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 6:58 pm 
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Location: Denmark
Can only agree.

Normally one wants the self oscillation frequency to be as high as possible.

As otherwise the wing will suddenly start to do "odd" things and wobble and maybe break :(

Have experienced this with model sailplanes, when going past 200km/h and the resonance is hit, a carbon wing simply explodes to "smitherines" - a view that would please the Mythbuster group to watch I am sure :rollgrin:

There are two ways of increasing the resonance:

Weight - keep the weight as low as possible.
Also, the distibution of mass is important, actually more important than the absolute mass.
This is in order to keep the moment of inertia down - which raises the oscillation frequency.
The mass (moment of inertia) should be as low as possible just at the C.O.E (Center Of Effort = where all forces act if hang up in a string)

And the second way, is to increase stiffness.


These two properties are a bit counterwise, meaning, you can design a very very light and good weight distribution wing, which is not that stiff, just "adequate" - and it will work without oscillations.
This is what we often do in the aircraft design, as we get a win-win then.
Especially, as IF a high frequency oscillation starts - then it can be easier recovered, in particulary with low mass - instead of ending in a catastrophe...

The other method, is to increase stiffness A LOT, often on cost of weight (and price/materials/production methods).
This gives another benefit - namely that our hydrofoils are stronger during handling (riding into something, rolling over a sandbank, etc etc)

I can show you a photo of the design principles in aircraft later, as it is really amazing how efficient a carefully balanced light weight design can work much better, than one based on pure "strength" :naughty:

8) Peter

PS: There can be oscillations both in regards of


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 7:40 pm 
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Some very technical discussions going on here!. I like it.

I agree with Bille. If you know how to work with glass. Carbon is an easy and relatively cheap method of manufacture. Most of the cost when building a carbon part is still that labour cost to lay it up.

Sure your not going top make a part as strong as the amerca's cup boys. But who cares. You can over engineer it and it will be sweat.


Now I have a basic question that I hope doesn't bring the technicality of this discussion down too much.

Why is the tip dihedral on the front and back wings apposing?


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 Post subject: Re: Building a hydrofoil
PostPosted: Sat Nov 02, 2013 8:35 pm 
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plummet wrote:
Some very technical discussions going on here!. I like it.

I agree with Bille. If you know how to work with glass. Carbon is an easy and relatively cheap method of manufacture. Most of the cost when building a carbon part is still that labour cost to lay it up.

Sure your not going top make a part as strong as the amerca's cup boys. But who cares. You can over engineer it and it will be sweat.


Now I have a basic question that I hope doesn't bring the technicality of this discussion down too much.

Why is the tip dihedral on the front and back wings apposing?


I am pretty sure it is because you want the fuselage to be "level" with the waterflow for the least resistance.

So any turbulence from the front lift wing, might hit the rear control wing simultaneously, thus could cause control problems when the front wing flow is disturbed.

But are you only talking about the local tip dihedral apposing, or the full dihedral ?

A lot of what we see is also "fashion" and thoughts IMO, as this area is still not discovered very well (GREAT, more invention to come) it seems.

Some are fully flat though, both front and rear wings.

8) Peter


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