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Foil burble/stall speed going upwind

 
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moondog

Since 15 Aug 2007
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white salmon
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PostThu Sep 27, 18 5:46 pm    Foil burble/stall speed going upwind Reply with quote

Now that I'm an intermediate foiler I have been noticing a phenomenon that was taught when I was a flight instructor 38 years ago. A 180 lb. pilot weighed 180 lbs. when he was straight and level =1 G. In a 60 degree bank his weight doubled to 360Lbs.= 2 G's. The airplane's stall speed increased in this bank. So when I am foiling very close to the wind my angle of my foil in the water is in a 30-60 degree bank and I'm getting the stall warning (Burble).

There is lots of new math that disagrees with this hypotheses (Nak) but it plays a huge part of me stalling my wing out there when I'm leaned over close to the wind. I'm interested to see if other foilers have noticed this aerodynamic phenomenon and their opinions.

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Nak

Since 19 May 2005
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Camas
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PostThu Sep 27, 18 8:57 pm    Re: Foil burble/stall speed going upwind Reply with quote

moondog wrote:

There is lots of new math that disagrees with this hypotheses (Nak)

???? Can't say I'd disagree with that. Stall happens when the critical angle of attack is exceeded. Increase the angle of bank in level flight and you increase the speed that angle of attack is reached. (The stall speed.) Pretty basic stuff. I must have been less than clear in a post for to get the idea I'd argue against that.

What rig are you riding that you can feel a pre-stall burble? I get zip for pre-stall warning.

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wylieflyote

Since 30 Jun 2006
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Puget Sound & Wa. Coast
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PostFri Sep 28, 18 4:54 am     Reply with quote

Anyone here remember "slotted fins" for windsurfing? These were needed if you drove your back foot too aggressively. The fin would do a similar thing that you've explained.
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Kip Wylie

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bwd

Since 04 Aug 2007
368 Posts

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PostFri Sep 28, 18 6:44 am     Reply with quote

I get this pre-stall warning on zeeko carver wing (710cm2). Weighing 83kg so that gives loading of 117g/cm2. I think that’s a relationship
to explore, as some riders report the experience and some don’t.

There’s probably a number out there that gives a threshold for this but to be universal it would need to have an expression including the lift curve of the foil in question, at least. Weight to area is probably a decent easy to find proxy within wing types though...

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moondog

Since 15 Aug 2007
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white salmon
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PostFri Sep 28, 18 7:31 am     Reply with quote

Nak the reason I brought your name up is that you had an updated version of Bernoulli's Principle or Equation which was the principle basis for flight or fluid dynamics back in 1738. Obviously we didn't have airplanes back then. You posted an updated version of his principle on NW Kite a year or 2 ago. There are some mathematicians who don't agree with the principle I stated in the initial post.

I ride the Moses/SS 633 on a 91/36 mast. It is a huge wing and very forgiving, hence the stall warning/burble. Consequently I always recover from this warning and keep on foiling with a small adjustment of front foot pressure. My Lift 110 (much smaller surface area) wing would also burble and 50% I would stall and crash. In aviation terms I would compare my SS wing to a Super Cub wing and my Lift to a 727 wing. Also the SS wing does everything at a slower speed which reduces high speed face plants.

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ldhr

Since 21 Jul 2009
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Hood River
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PostFri Sep 28, 18 7:42 am     Reply with quote

I ride the same wing and mast and I used to experience the burble when I went upwind at speed.
I spent some time sanding and fixing the scratches and dings on my wings and that helped.
I still get a warble/vibration when going hard upwind at high speed.
One thought - your burble could be caused by the wingtip breaching the surface.
I've seen you on the water and your riding skills are getting radical!

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dwaynej

Since 09 Sep 2013
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PostFri Sep 28, 18 1:38 pm     Reply with quote

The only times I have induced cavitation from wing loading is when using a a small wing. AlpineFoil made a wing for long distance straight line racing, and at high speed I could make it "burp" when stomping on it. Lower speeds, it was "burping" frequently during different maneuvers.

I say if you are having cavitation, it's a bad wing profile/design or the wing is too small for you - MikesLab, Banga, Spotz Shark, Spotz Tuna, Lift have never been a problem for me and the AlpineFoil wing was expected since it was tiny.

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ldhr

Since 21 Jul 2009
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PostFri Sep 28, 18 2:09 pm     Reply with quote

It's not full cavitation where you spin out. I'm not sure it's any kind of cavitation at all.
It's a slight vibration or warble that lasts for a split second at high speed only when going upwind.
It's happening on a large surf wing - so wing size is not the issue.
This is also a hugely popular wingset - so the design is solid.

IMO - It's either pilot error - wing tip breaching when leaned over - or the wings need to be sanded to remove scratches and dings.

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Chipotle

Since 26 Mar 2011
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PostFri Sep 28, 18 6:58 pm    mast Reply with quote

Are you sure it's not the mast? Ventilation and cavitation can occur at high angles of attack as the mast is leaned over.

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ldhr

Since 21 Jul 2009
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PostSat Sep 29, 18 8:58 am    Re: mast Reply with quote

Chipotle wrote:
Are you sure it's not the mast? Ventilation and cavitation can occur at high angles of attack as the mast is leaned over.


It might be - but the mast is a standard Ghost Whisperer and is in perfect condition.
The mast does not need to be leaned over to extreme for the shimmy to occur.

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moondog

Since 15 Aug 2007
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PostSat Sep 29, 18 4:18 pm     Reply with quote

I did some fluid dynamic testing on the mighty Columbia today with my SS/Moses 633 wing. I'm finally good enough that I can watch the wing in various configurations. I did the 45-60 degree bank in 25 mph winds to observe the dynamics. As I approached 45 degrees I would see bubbles forming on the wingtip, at 60 degrees I would get the official stall warning burble with wing tip vortices and if I didn't reduce my bank or lower the nose full stall would occur. So everything I learned in aviation seems to hold true in foiling. There is no way the mast is twisting.
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dwaynej

Since 09 Sep 2013
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PostSat Sep 29, 18 8:33 pm     Reply with quote

moondog wrote:
So everything I learned in aviation seems to hold true in foiling. There is no way the mast is twisting.


I have an aviation background as well including my second favorite sport - Gliding. What is unique to foiling is the angle of attack combined with the weight of the rider, and the how the power of the kite is being transferred. IMO, kite position low in the sky vs high in the sky is also going to be a factor.

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