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Light wind/low end - span or area?

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Kamikuza
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Light wind/low end - span or area?

Postby Kamikuza » Fri Aug 14, 2020 10:58 am

Another thread got me thinking -- if two wings are pretty much the same area and volume, but one has greater span (and higher AR) will the greater span wing have better low end? How much better?

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Re: Light wind/low end -- span or area?

Postby Peter_Frank » Fri Aug 14, 2020 11:15 am

Kamikuza wrote:
Fri Aug 14, 2020 10:58 am
Another thread got me thinking -- if two wings are pretty much the same area and volume, but one has greater span (and higher AR) will the greater span wing have better low end? How much better?

A lot better, I have made some calculations on this, but dont have any numbers right here.

The low AR one will of course be easier to ride as it can not stall that easy, and less sensitive to erratic aoa changes for the less experienced.

But you get a lot of loss in overall lift because of tip vortices and localized lowering in aoa thus loss of quite some lift, but even worse, a huge increase in drag, which reduces the "sink rate" quite a lot.

Increase in AR, a lot more lift and lower drag at high aoa, because we operate at such low AR values sometimes 3 to 4 only.

The very reason whey I use a smaller wing in area, higher AR, for riding in the lowest possible wind (and still fun, dont want it too big).

Not as lively in turns no, but can ride in less wind :D

8) Peter

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Re: Light wind/low end -- span or area?

Postby Kamikuza » Fri Aug 14, 2020 12:24 pm

Peter_Frank wrote:
Fri Aug 14, 2020 11:15 am
Kamikuza wrote:
Fri Aug 14, 2020 10:58 am
Another thread got me thinking -- if two wings are pretty much the same area and volume, but one has greater span (and higher AR) will the greater span wing have better low end? How much better?

A lot better, I have made some calculations on this, but dont have any numbers right here.

The low AR one will of course be easier to ride as it can not stall that easy, and less sensitive to erratic aoa changes for the less experienced.

But you get a lot of loss in overall lift because of tip vortices and localized lowering in aoa thus loss of quite some lift, but even worse, a huge increase in drag, which reduces the "sink rate" quite a lot.

Increase in AR, a lot more lift and lower drag at high aoa, because we operate at such low AR values sometimes 3 to 4 only.

The very reason whey I use a smaller wing in area, higher AR, for riding in the lowest possible wind (and still fun, dont want it too big).

Not as lively in turns no, but can ride in less wind :D

8) Peter
I'm thinking for those days when it just won't get over 15 knots and I feel like cruising around gently and sedately...!

So the converse -- if you have two wings of same span, but higher and lower AR (and volume) the higher AR will perform higher in lower wind?

Volume doesn't factor at all?

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Re: Light wind/low end -- span or area?

Postby Kiterpep » Fri Aug 14, 2020 12:56 pm

Kamikuza wrote:
Fri Aug 14, 2020 12:24 pm
Peter_Frank wrote:
Fri Aug 14, 2020 11:15 am
Kamikuza wrote:
Fri Aug 14, 2020 10:58 am
Another thread got me thinking -- if two wings are pretty much the same area and volume, but one has greater span (and higher AR) will the greater span wing have better low end? How much better?

A lot better, I have made some calculations on this, but dont have any numbers right here.

The low AR one will of course be easier to ride as it can not stall that easy, and less sensitive to erratic aoa changes for the less experienced.

But you get a lot of loss in overall lift because of tip vortices and localized lowering in aoa thus loss of quite some lift, but even worse, a huge increase in drag, which reduces the "sink rate" quite a lot.

Increase in AR, a lot more lift and lower drag at high aoa, because we operate at such low AR values sometimes 3 to 4 only.

The very reason whey I use a smaller wing in area, higher AR, for riding in the lowest possible wind (and still fun, dont want it too big).

Not as lively in turns no, but can ride in less wind :D

8) Peter
I'm thinking for those days when it just won't get over 15 knots and I feel like cruising around gently and sedately...!

So the converse -- if you have two wings of same span, but higher and lower AR (and volume) the higher AR will perform higher in lower wind?

Volume doesn't factor at all?
Peter is right. Volume of the wings will give negligible lift, since the wing buoyancy should be a magnitude smaller than the lift created by the foil. The difference in volume between the two wings will not matter, but the difference in aspect ratio has a huge effect on induced wing drag: https://www.grc.nasa.gov/www/k-12/airplane/induced.html. Can make a back-of-the-envelope calculation this weekend if you're interested.

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Re: Light wind/low end - span or area?

Postby Jyoder » Fri Aug 14, 2020 1:42 pm

Buoyancy is negligible, but doesn’t increased volume mean thicker airfoil and therefore increased lift, decreased stall speed, but increased drag?
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Re: Light wind/low end - span or area?

Postby Greenturtle » Fri Aug 14, 2020 1:54 pm





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Re: Light wind/low end - span or area?

Postby Peter_Frank » Fri Aug 14, 2020 2:17 pm

Jyoder wrote:
Fri Aug 14, 2020 1:42 pm
Buoyancy is negligible, but doesn’t increased volume mean thicker airfoil and therefore increased lift, decreased stall speed, but increased drag?

Usually yes, volume means thicker foil and "maybe" more lift, but not necessarily.

Yes, decreased stall speed most often, and increased drag.

But the lift depends on the mean camber line, combined with the thickness.
All wing profiles are like fingerprints, almost unique each and everyone, so you can not use the mean camberline and thickness alone, but it most often gives you a polar close to the reality.

You can not use area/span/volume as parameters alone, you need to know the profile (mean camber at least).

The Cl/Cd polars, will give you a comparison that is useable, but very few got these.

And then you still has to add the loss in lift and increase in drag because of the low AR wing.

8) Peter

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Re: Light wind/low end - span or area?

Postby Kamikuza » Fri Aug 14, 2020 2:45 pm

Jyoder wrote:
Fri Aug 14, 2020 1:42 pm
Buoyancy is negligible, but doesn’t increased volume mean thicker airfoil and therefore increased lift, decreased stall speed, but increased drag?
In my thinking I skipped a public step there -- I'm thinking about my bigger wing when I mentioned volume, which has more chord...

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Re: Light wind/low end - span or area?

Postby Kamikuza » Fri Aug 14, 2020 2:46 pm

Thanks all, doing some more googling now.

Tempted to look into building a water tunnel too. Could get some amazing data and footage of hydrofoils...
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Re: Light wind/low end - span or area?

Postby Whattheflock » Fri Aug 14, 2020 3:05 pm

633 <3 :D


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