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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Sun Nov 17, 2013 4:59 pm 
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don pitcher wrote:
zfennell wrote:

I want to get rid of my frame and add a streamlined riser. I might have to give the extrusion above a try.

Image


For a twin tip, wouldn't it be easier just to use a V-shaped, surface-piercing hydrofoil? Put one at each end and it regulates the height on it's own. You can use that aluminum extrusion you show as the foil and two or three flat plates for a support


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Sun Nov 17, 2013 5:24 pm 
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don,
whats the material list and dimensions for your twin-tip prototype?

i am surprised that its so difficult to find a suitable extrusion for your next mod.
perhaps the next size up in the bcawning catalog?

Bille has a point about the potential drag penality, but i believe the cost will be mostly a function of the total surface area for the strut that you choose. ( a quick check in Abbott's book for airfoils will give you relative drag numbers to play with)

the one thing i have discovered is that the principal consumer for streamlined alum extrusions appears to be louver blades for shade and ventilation systems. (maybe enquire with "NAILOR.com" for replacement parts . they have 4" x 1/2" louver blades)

the science and techmology guys still seem to be doing this one at a time... and all the flying applications appear more structural than aerodynamic (struts for wings and landing gear)
good luck, somebody must have one OTS.

my appologies for a severe hijack of Hawaiis' wood approach.
and many thanks for your impressive 'mix and match" of the many ways to make this work.
-bill


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Sun Nov 17, 2013 8:06 pm 
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zfennell wrote:
...

Bille has a point about the potential drag penality, but i believe the cost will be mostly a function of the total surface area for the strut that you choose. ( a quick check in Abbott's book for airfoils will give you relative drag numbers to play with)


...
-bill


Ironic -- that now I'm the one pushing the Wood for my first foil .

On the Drag thing due to thickness :

Two equally thick airfoil profiles that have the thickness distribution at the same
cord percentage, but with different cord Lengths ; the Longer cord length
will produce LESS drag , even though it has MORE surface aria.

Example here , and pretend both high-points are at 35% back from the Leading Edge :
1) cord thickness = 15mm, cord length = 120mm -------------- 12.5% cord-thickness.
2) cord thickness = 15mm, cord length = 100mm ------------ 15%% cord thickness.

# 1 will be FASTER, even though it has MORE surface aria !!!!

The percent thickness of an airfoil is one of the Most important factors
regarding the ultimate speed the airfoil will be allowed to go. As a beginner
on a foil-board, i kinda could care less though ; i just don't wanna be able
to break it.


don pitcher wrote:
...

Billie,

If using aluminum extrusions, an aluminum hydrofoil can be about the same weight as a Carafino. Also, the concern about aluminum not being safe is not true. The ski boat world has been using aluminum hydrofoils for years.
...



Most foil-chairs that are towed behind a boat ; yes -- your strapped-in, so
there's little chance you'll be hit in the head when you fall.

On the Safety part ; who should i believe More :

* You, that has never made a decent foil-board in his life, and who's work
looks like a 3rd grader built it .
OR
** Carafino , who has made and sold more hydrofoils in his life, than most
any person on the planet .

I'll go with Carafino Thank-you !!

Plenty of HG pilots with aluminum down-tubes, have had broken arms
over the years, (including me) ; the guys that use the carbon ones say
that the carbon down-tubes break before the arm does, so there Safer in that respect.


C A R A F I N O 2014 wrote:
Metal kills people, no matter how light you can make it. No matter how fast it will go. Metal is out.
Plus the cost for titanium is horrendous. keep up the good work.


Bille


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Mon Nov 18, 2013 1:31 am 
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You are funny Billie. Rush Randle has been selling aluminum hydrofoils longer than Carafino. I believe Hawaiis forum picture shows him riding his Rush Randle.

When I first wanted to buy a foil kite in 2004, the dealer told me he would not sell me a foil because "they were dangerous", but he was more than happy to sell what he had in stock. Ironic.




davesails7,

That is an idea I have not considered. Thanks.

I imagine the foil would need a fin to control side loads when it is surfaced.




Thanks Bill. I imagine you are on to something about surface area vs. frontal area. I wonder at what point the trade-off is and how significant it is? Probably need some empirical data/testing to get drag coefficients?


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Mon Nov 18, 2013 2:03 am 
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i'd rather be waked in the head by a lightweight carbon one for sure (they are actaully not the light in my opinion some carbon ones) - still have weight behind them and i doubt they will break before your body does..., flipside is my friends have been towing with aluminium ones for years. i might be preferring a pvc core carbon wing to th head than a g10, aluminium fin or 3d printed abs wing tho..

The sharpness of the foils is a huge factor is whether it can chop your leg head or whatever rater than bruise it - same problem with raceboard fins and surfboard fins, i sand my surfboard fins down but u make your choices..


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Mon Nov 18, 2013 2:27 am 
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zfennell wrote:
don,
whats the material list and dimensions for your twin-tip prototype?


The fuselage is 1" 6061-T6 square stock. I made it very strong because of the cantilever with the wings at the end but also wider than I like because the moment on the weld gets ridiculous with a small width (this is a weakness with this design). If the wing is placed with little to no cantilever, then a lighter square tube could be used.

The wings are 1/8" 7075-T6. They are way too big. 6" x 24"

The frame is made of 1/8" x 3" 5086. 24" tall. I think closer to 30" would be better except the frame would likely buckle at that height unless more draggy bracing was added. Also, if welding aluminum and not heat treating afterword, 5086 is nicer than 6061 because it has a much higher annealed strength.

I really think an extrusion riser, square tube fuselage, a wing under the riser, and bolted together would be better and easy.

Image

Edit: originally wrote 5068 but meant 5086.


Last edited by don pitcher on Tue Nov 19, 2013 4:24 pm, edited 1 time in total.

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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Mon Nov 18, 2013 11:26 pm 
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Location: rhode island
thanks don,
nice drawing.
you're gizmo appears to be considerably stiffer than the numbers posted by Europ2.
(the center brace is where most of the stress/deflections occur)
I found a mistake in my first estimates for the 3" bcawning extrusion.
i dont think it's too far off the mark.
i want to check a few more numbers, but if a 3' section was cheap, it would be worth a try.

regarding drag coefficient.
i'm as guilty as anyone when it comes to generalisations. This is certainly a case where the 'devil is in the details'
so let me give you some numbers to compare:
Abbott lists lift and drag coefficients for numerous NACA profiles.
all drag and lift estimates are scaled against: plan area, velocity^2, density.
(D= 1/2cd*A*rho*v^2)
for reynolds numbers approx. = 1,000,000 and 0 deg Angle of Attack:
NACA 0006: cd=.005
NACA 0012: cd=.006
NACA 0024: cd = .008
if you compare the above with
drag coefficient of infinitely thin flat plate: cd=.003 (all skin friction)
or the drag coefficient of a cyllinder in cross flow cd = .45 (mostly form drag)
you get a feel for any penalty you may pay for the section profile you choose.

at 20 ft/sec the drag of a 3ft section of NACA 0024 (3"chord) is about 2-1/5 lbs
the cyllinder would be150 lbs
the flat plate woud be 1 lb

there is always more to the story:
lift vs drag with increasing AOA , surface piercing wakes, aspect ratio,...etc)
but all the numers above are pretty much apples to apples.

the good news about the fat profiles is less sensitivity to AOA . You get to increase AOA and lift with less drag penalty than the narrow, more efficient profiles.

online guys like
http://airfoiltools.com/
should let you calc your own lift and drag data from any std profile.

no worries,
-bill


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Tue Nov 19, 2013 4:29 pm 
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You are quite the nerd Bill. Keep up the good work!

I noticed the BC awnings extrusion in made of 6063-T5. This only has a yield strength of 22,000 psi. I will have to check some calculations to make sure this won't be a problem.


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Tue Nov 19, 2013 8:20 pm 
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Don:
Your wing area is twice that of a directional foilboard. The V strut and two braces also contribute to lifting force.
I think by reducing the size of the two big wings will reduce drag and reduce the lift that you are experiencing. nice work.


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 Post subject: Re: DIY Home Made wooden Hydrofoil fins
PostPosted: Thu Nov 21, 2013 1:41 am 
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Thanks. I imagine just using one wing in the center would be better, but I need to improve pitch control.

I think Bill has convinced me to start over and try the awning extrusion. Thanks Bill. I will give them a call tomorrow.


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