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Wing and stab design

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TomW
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Re: Wing and stab design

Postby TomW » Sun Jun 18, 2017 11:57 am

Guess I'm on a monologue thread. Did some online study about aerodynamics. Note: there's very little/ no online literature on kite hydrofoil design.

In aerodynamics and airplane design there's lots of information about center of gravity and anhedral on heavy freight planes for roll stability and on fighter jets so they are inherently instabile but agile. And why plans have dihedral. It's all about roll stability and tendency to fly flat. But none of this explains why anhedral would do anything in hydrofoil for kiting.

From my reading, and please feel free to correct / inform me if I have this wrong. Statements below refer to wing design for kite hydrofoil.
Anhedral will increase yaw stability, but also make foil more agile (?). It will decrease lift efficiency.
Wing sweep has same effect as anhedral. Perhaps this explains why non anhedral designs generally have a lot of sweep.

Looking at hydrofoil wing design, I wonder how much is science / engineering and how much is just " this looks cool, so lets do it. "

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Re: Wing and stab design

Postby BWD » Sun Jun 18, 2017 2:53 pm

There are two man things about anhedral for kitefoils that appear especially significant to me:

Preventing ventilation when flying high. People can keep foiling with one wingtip above the water but quite often they crash, especially if wingtip pops out of water through a wave in uncontrolled way. Anhedral keeps the tip a few cm deeper when riding heeled over, helpful if riding high in chop.

Reduction in variability of projected area at different roll/heel angles and lower angle needed to generate lateral resistance. A flat wing has max PA and efficiency but loses PA faster as it heels. Anhedral wing benefits in chop for these reasons also.

There is another aspect of anhedral that may be a factor in big low AR wings with fatter tips (though theses are few) - the tip areas lifting "away" from each other may give the foil more inertia and therefore more yaw stability. This may (or not) be a factor with surf foils more than kite foils. I started thinking of this looking at gofoil and takuma foil.

Despite all this theoretical speculation, flat wings can work great, of course...

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Re: Wing and stab design

Postby plummet » Sun Jun 18, 2017 10:28 pm

I do note that the front wing on the Americas cup yachts are running dihedral. The year looks pretty flat!

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downunder
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Re: Wing and stab design

Postby downunder » Mon Jun 19, 2017 5:26 am

TomW wrote:
Sun Jun 18, 2017 11:57 am

Looking at hydrofoil wing design, I wonder how much is science / engineering and how much is just " this looks cool, so lets do it. "

That.

I'm impressed with all the science and effort in crouching numbers, yet my friend is 'killing it' with the foil he built with some plywood.

For me, as with everything, it's all about the weight and will always fetch the lighter board no matter of performance.

Re your wing, I forgotten about this but the bolts should not be parallel (difficult to make).

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Re: Wing and stab design

Postby TomW » Mon Jun 19, 2017 11:31 am

Thanks guys.

BWD- i think i follow you. the anhedral is not about the roll stability as main benefit-( but there may be some roll stability due to opposing lift-) but more about forgiving behavior in a turbulent and variable angles situation.

Look at Mr Stringfellows wing- He added these little winglets to a totally flat bottom front wing. He says they are for safety and durability- but really ? perhaps he needs them for Yaw stability and to allow quicker and more agile turning ? That they are a compromise solution because flat CNC machining process wont allow anhedral ?

I want to use my existing Moses Fuse that has in-line bolts.

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Re: Wing and stab design

Postby max » Mon Jun 19, 2017 11:52 am

TomW wrote:
Mon Jun 19, 2017 11:31 am

Look at Mr Stringfellows wing- He added these little winglets to a totally flat bottom front wing. He says they are for safety and durability- but really ? perhaps he needs them for Yaw stability and to allow quicker and more agile turning ? That they are a compromise solution because flat CNC machining process wont allow anhedral ?
hey Tom. Just a bit more info . . . not sure if it is useful but anyway . . .

I use the Zeeko Carver (710cm2 and 61cm span) which is their light wind/carving wing and it has neither swept back wings nor anhedral and yet I find that it is very stable. Zeeko give it a rating of 2/5 for Yaw control and 5/5 for carving for which it is so much fun.

The smaller freeride blue wing (535 cm2 and 54cm span)with lots of anhedral gets 4/5 for yaw control and 4/5 for carving.

I remember the blue wing as great but I only really used it while I was learning and then changed to the carver once i could go from heel to toe and continue riding toeside. That was at Nico's suggestion as he said the carver would be a bit too powerful in the turns until I could already do them.

My current setup is the zeeko alloy with the carver and the pocket size 118 board that i built. A true fun setup.

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Re: Wing and stab design

Postby NYKiter » Mon Jun 19, 2017 3:58 pm

Looks like the spitfire wing is flat....

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Re: Wing and stab design

Postby BWD » Mon Jun 19, 2017 5:40 pm

another thing the downcurved foil tips/anhedral may do for you is function somewhat like washout in delaying stall at tips, as it will lower loading....
so why am I still learning on my carver wing? I don't know, because it's still fun I guess!

also, BTW I have found out why it is worth it to get the bolts parallel imo:
when you crash/run aground, if bolts are parallel you can unscrew them even when they get bent a little, if you keep them "in sync." Likely to be be harder if they weren't parallel.
This happened to my stab attachment...

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Re: Wing and stab design

Postby TomW » Tue Jun 20, 2017 9:00 pm

Interesting input guys. BTW had great session on my Fluente with 548 wing today- i´m really getting the hang of it.

so Max- it seems that with the carver you have less Yaw control but better carving- the Zeeko has that vertical fin on stab to give some yaw control . Correct ?
That would be similar to Stringy approach.
then the Blue wing has a lot of anhedral giving more yaw control that reduces carving performance.
This is what i would expect from my reading.

But yes, the anhedral would seem to provide more forgiving ventilation characteristics. I am noticing that on my moses- i can ventilate and recover now. Actually, i am feeling surface before ventilation too.

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Re: Wing and stab design

Postby Anthonyshopguy » Wed Jun 28, 2017 4:58 pm

Does anybody have plans out there with some basic dimensions. I would like to make a foil using Baltic birch. High end Plywood and Laminate with simple fiberglass. I am a very experienced wood worker and shop teacher. Have built and flown foam airplanes from scratch using home made foam cutter. These worked well.


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