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 Post subject: stalling issue
PostPosted: Tue Jun 17, 2014 1:23 am 
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On my home made foilboard I stall out often when I try to head a bit more upwind. Is it simply because I'm pushing too hard upwind or due to design issue ? Would it help to create a small vertical fin in the back, right in front if the rear fin? There's no anhedral or dihedral on either fins


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 1:29 am 
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When you don't have any anhedral on the front wing, it will be easier to roll to a stall. By adding two twintip fins on the wing tip will solve the problem. The wing is upside down to show the wingtip fins.


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 1:36 am 
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Hawaiis wrote:
When you don't have any anhedral on the front wing, it will be easier to roll to a stall. By adding two twintip fins on the wing tip will solve the problem. The wing is upside down to show the wingtip fins.

Great idea thank you. You recommend the anhedral on the front wing as opposed to a rear vertical stabilizing fin, or in addition?


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 1:45 am 
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The two twin tip fins will help the foil to "bite" into the water. Making it easier to ride and perform better. The worst commercially available foil that I have tried is the Moses Vorace, it has no anhedral at all and it has a rear vertical fin.


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 1:57 am 
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Awesome I'll give it a go and report back. My front wing is high aspect so the fin will be longer than the length of the wing , should be fine think. Thanks again


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 10:59 am 
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Location: Fuerteventura
darippah wrote:
On my home made foilboard I stall out often when I try to head a bit more upwind. Is it simply because I'm pushing too hard upwind or due to design issue ? Would it help to create a small vertical fin in the back, right in front if the rear fin? There's no anhedral or dihedral on either fins


There are many factors that could be causing this.

However you really sure that it's a stall and not a ventilation?

Stalls typically happen at low speeds with the wing parallel to the surface.

I found that most of the time when the Foil "breaks away" on upwind riding it's a problem with the mast or fuselage being too flexible. Especially torsion can lead to a lot of ventilation problems.

Best would be to attach a Go Pro to your lines and film it happening. It can be a real eye opener.


What could also be is that your wings are interfering with each other. I have seen quite a few home builds with the rear wing on the same plane or too close as the front wing. This will destabilise the foil at higher speeds, eg. the faster you go, the more the foil will want to wobble or nose down.

If we are talking about a real stall, the biggest reason will be with your profiles and Angle of attack.

If your profiles are too flat with very little camber it can be very susceptible to stall. Also check that your root AoT is not too high as this could also be a culprit to stalling.

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Gunnar


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 11:19 am 
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Fully agree with that, especially the last bit about Insufficient profile causing the flow to detach over a large range of angle of attack

Really not sure about Hawaiis and the theory that tail fins are evil, alpine, tarroa zeeko spotz radical etc maybe magma too all have them, what none of them have are two twin tip fins bolted to the ends of a flat plate with virtually no shape


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 11:43 am 
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I use Tailfins as an option to make the foil more roll stable and to slow it down for beginners (fat TT fin)

I take don't use one if I want to reach top speeds.

I have not found that it has any bearing on a foil being more or less susceptible to stalling.

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Gunnar


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 2:06 pm 
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As Gunnar says, you have to be sure you are stalling and not ventilating.

When going upwind, you tilt the foil a lot, so the main wing are closer to the surface and might easily suck air down, causing a sometimes very severe stall.

But if it is a clean stall with foil 100% under the water away from the surface, there are many reasons, but the principle is the same:

The AOA of your given foil is too high, relative to your speed and weight.

Having a bigger foil (area) will make you able to ride with lower AOA (go slower) and have the same total lift.

So you can either use a bigger foil wing, or a more cambered one.

OR, use your existing one and go faster !
More speed, and you will avoid the stall :thumb:

More speed means bearing a bit lower, but as you go faster too, your VMG is not necessarily bad.

Are you having imperfections in your foil, it will also create local turbulence which can give separation and it will stall easier.

If your leading edge is very sharp, you will get a violent stall - where a round one will be softer.
But if you dont have sufficient camber (curve) in your foil, it WILL stall easier, even with a round LE.

It will also be faster though, if thin and flat, so always a balance.


So go faster, meaning a bit lower, and/or use a slightly bigger kite :naughty:

8) PF


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 Post subject: Re: stalling issue
PostPosted: Tue Jun 17, 2014 7:44 pm 
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Manufacturers designs include a combination of profile "washout" and sweep to vertical to either/both provide progressive feedback to impending stall, or control the adverse effects of stall. Untilmately, based on the weight of the rider, size of wing, and wing design, stall and the effects of a stall are going to vary. Given its happening to you, without changing any other aspect of the foil itself, all you can do is lose weight/ride faster...

Changes include... increasing area of rear wing and moving the keel back to reduce front wing loading... Reviewing Front Wing airfoil profile (H105 or similar??) Adding area to front wing, Adding area/washout to front wing... Adding vertical sweep/washout to minimize effects of stall...


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