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Importance of wing AR?

Posted: Tue Sep 26, 2017 9:07 pm
by Pablo
Hello,
Having bought one of these chinese hydrofoils (aspect ratio=6) i was planning to build a more begginer friendly set of wings . But before starting the construction, i did some quick math and found that the difference in the lift slope between a high and low (3.5?) AR wing is at most 20%. in other words, when you change the angle 1 degree, the lift of the high ar would go up 20% more (at most!) than the low AR one.

I was expecting a really big difference. My guess was that low ar wings were less sensitive to changes in the angle of attack and hence easier to control, but <20% doesnt seem like a huge improvement. So, my question is, what am i missing? What makes learning foils begginer-friendly?

Thanks!
Pablo

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 7:22 am
by Blackrat
High lift does not make a wing easier or less easier to ride

It's more about stability , speed and stall

A slow stable foil is much easier to learn to ride than a fast one

What makes a foil stable
Flat wings , longer and thicker chord slows it down and doesn't stall easily

What makes a foil fast
Thin profile to reduce drag , you can only have a shorter chord (thinner wing) by going with longer wings to keep enough surface area
But now the stall speed increases

There are a million other things that also influence the foil , distance of front to back wing , size of back wing and so the list goes on


If you want a super easy foil blade , go with 3:1 to 4:1 aspect ratio , with a 800-1000cm2 from 25-30mm thick chord, can't go wrong , it's super slow and great fun in waves

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 8:01 am
by Peter_Frank
Pablo wrote:
Tue Sep 26, 2017 9:07 pm
Hello,
Having bought one of these chinese hydrofoils (aspect ratio=6) i was planning to build a more begginer friendly set of wings . But before starting the construction, i did some quick math and found that the difference in the lift slope between a high and low (3.5?) AR wing is at most 20%. in other words, when you change the angle 1 degree, the lift of the high ar would go up 20% more (at most!) than the low AR one.

I was expecting a really big difference. My guess was that low ar wings were less sensitive to changes in the angle of attack and hence easier to control, but <20% doesnt seem like a huge improvement. So, my question is, what am i missing? What makes learning foils begginer-friendly?

Thanks!
Pablo

The low AR wings has a little bit less lift yes, not much as you have found.

But the drag is much higher at lift so both your lift/drag is a lot lower than 20 %, and your topspeed when lift is lower, so it is slower.

Both of above means the world, in terms of riding in low wind - where you need speed for apparent wind, and l/d for good angles to get apparent wind to start foiling :thumb:

A high aspect wing is not more or less stable than a low AR as such, when riding at speed - but the low AR wings will have a much less sensitive pitch when you ride at high angles of attack, and it will stall a lot more gentle sometimes even warning you beforehand.
They will also be able to turn faster, more lively and fun.

But the price is low end and speed, and it is very significant so when it is below 10 knots I often avoid the low AR ones for this very reason, they are simply not as useful IMO, many not even rideable at all.

If you want a beginnerfriendly wing, that is not too ridiculous in performance (speed/angles thus low end), I would recommend an AR between 4 and 4.5 which is often used for wavewings :D
I would not go lower though, as even around these values you can feel the drag and low performance and sometimes want higher AR wings.

My experiences, having more than a handful of wings.

If you have loads of kitepower and just want it easy, you can go to even lower AR of course, but then you might outgrow the foil instead of having a fun foil for freestyle and waves :naughty:

8) PF

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 4:20 pm
by diogovilla
Peter_Frank wrote:
Wed Sep 27, 2017 8:01 am
If you have loads of kitepower and just want it easy, you can go to even lower AR of course, but then you might outgrow the foil instead of having a fun foil for freestyle and waves :naughty:

8) PF
Hi Peter,

I have an alpine foil with a huge high AR wing. I am lately considering buying a takuma foil for waves (and keep my alpine foil for cruising). Do you think I would outgrow the Takuma foil for waves only? What should I do Instead?

Thanks,
Diogo

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 9:19 pm
by Peter_Frank
Sorry Diego, I have no idea...

I dont think the Takuma and short mast is for me though, also because our waves are not long and with big distance so makes no sense here.

Personally when up in 8-900 cm2 sizes, it works awesome for wind at or below 10 knots and riding waves, really fun - but as soon as the wind starts to pick up, they becomes too slow really fast sometimes, at other times they are great riding with a 6 or 7 m2 kite in 12 knots, as they dont outrun even our slow waves here.

So the Takuma style is most likely not for me for these reasons, but as I havent ridden that type, and others like Gunnar love this type, I am not qualified to answer :wink:

I can see Alpine got really high AR wings, but hard to find info about any lower AR bigger sized wave wings ?

So understand your question as should you buy a totally new setup board + foil Takuma style, or ?
If Alpine dont have a big low AR wave wing, it is not that easy...

You could get a low cost full new Alufoil, but it might be hard if used to light good gear like you have now.
A full new good foil + board also an option, the most expensive but good :thumb:
Or you could also get a Ketosfoil without board, t.ex with the WaveXL wing, as the mast will fit your board with one screw, great.
But relatively costly (worth it though ha haa, my friend just bought one after having tried it)

Here we have had a handful of riders having had high AR wings first, and getting a lower AR freeride wing for fun and waves also, or changed the full foil to a freeride type - the opposite has never happened.

8) PF

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 9:33 pm
by Kykeon
http://www.alpinefoil.com/pages-hydrofo ... ort-1.html

This one has an AR of 4.32 which is perfectly within the range you recommended and with 700cm2 it's bigger than average

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 9:39 pm
by Peter_Frank
Kykeon wrote:
Wed Sep 27, 2017 9:33 pm
http://www.alpinefoil.com/pages-hydrofo ... ort-1.html

This one has an AR of 4.32 which is perfectly within the range you recommended and with 700cm2 it's bigger than average

Spot on, it would be my choice Diego, that very wing with your normal mast, awesome :thumb:

But can not tell whether you would prefer the short mast and Takuma wing style, or the Alpine Wave wing :wink:

8) PF

Re: Importance of wing AR?

Posted: Wed Sep 27, 2017 11:58 pm
by Pablo
Thanks everyone for the replies.
I did search a bit more and found the missing piece in the puzzle: sweep!
https://aviation.stackexchange.com/ques ... back-wings
Just like lowering the AR, increasing the sweep decreases the sensibility of the foil to a variation in the Angle of Attack and also helps with the stall. The link to the alpine foil is quite illustrative in this sense, not so low AR but high sweep.
I will follow your advice and aim to 4-4.5; the alpine planform (but a bit bigger to get to 800 cm^2 ) looks like a good starting point. I hope the tail volume will be enough with this wing, otherwise I'll have to make also a custom stabilizer...

Pablo

Re: Importance of wing AR?

Posted: Thu Sep 28, 2017 4:08 am
by slowboat
Peter_Frank wrote:
Wed Sep 27, 2017 9:19 pm
.... as soon as the wind starts to pick up, they becomes too slow really fast ......
Can you explain what you mean by this?

Re: Importance of wing AR?

Posted: Thu Sep 28, 2017 4:44 am
by toddsphresh
God I thought I wanted a fool but after reading this thread I'm having second thoughts I need an interupter for this thread