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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 2:40 am 
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Thanks Peter! Good stuff.

On cavitation, it is caused by low pressure, not change in pressure. The smaller the foil, the lower the pressure on top of the foil has to be to support the same weight. This will be a problem for going fast with small foils.

I think it should be possible to get past this though with a different shaped foil. Super-cavitating propeller blades are wedge shaped instead of foil shaped, pointy leading edge and a straight cut off at the back like an axe blade.

Not to be confused with aeration which is sucking air from the surface due to the low pressure. In cavitation the water boils because the pressure is too low.


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 7:28 am 
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Yes, it is low pressure that causes cavitation (boiling water), but it can sometimes be quite locally where there are rapid "drops" (changes) in pressure - so you try to make the profile so it has more distributed pressure so to speak, that was my point.

But a different topic.

I still dont agree about the high AR foils being "just as easy" as the low AR ones, not at all :roll:

One reason many medium or advanced riders has got that impression, is because the low AR ones typically ALSO has more camber, maybe a thicker profile, and more area - so it will start ealier for beginners.

This is much easier for the very first learning yes - but agree with Christoff, that when you can ride more, you will feel a very big or lifting foil is harder to control when going fast.

But if you compared similar lifting foils, one low AR and one high AR, I will say the low AR will be much easier to handle, with a much more gentle stall (and recovery) - but of course also more drag, thats the trade off.

I think our impressions are, or at least often can be, mixed up, because low and high AR foils are not only having the AR as the only typical difference :naughty:


Regarding the stall, it is true that high AR ones can be recovered indeed, just not as easy as it happens over the full surface simultaneously.
I should have mentioned that I was talking about both stalls (too low speed for sufficient lift) AND "aeriation" (I did not know that word till now, thanks) will be easier handled by low AR wings IMO.

So when I say "Stall" in this regard I mean both the aeriation and the stall when fully submerged.
Aeriation is often much more violent - but it is somewhat the same that happens - the water flow separates particulary on the rear top surface and gets turbulent, and a lot or all lift is lost :o

8) Peter


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 8:12 am 
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The air that follow the main strut down to the foil is called "Ventilation"


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 11:06 am 
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Hi Everyone,

It's been a while since I have posted, so that to the OP for this post.

First off I would like to address a few things:

1) High AR vs Low AR argument:

In my experience the High AR (8+) wings are not as easy to use as low AR (5 or less) wings. That being said, there are huge differences between wing shapes and more importantly size so it is difficult to compare the different models. However in general a Low AR wing will need less speed to foil stably. They will however reach a point where they do not go any faster, and this can get boring for some. This is why I designed big medium AR (6.6) Wings for our first production run.

Why are high AR wings generally faster? In theory a high AR will have less induced drag e.g.. more speed. This is talking in very simple terms. There are a hell of a lot of other variables that come into play. eg. Profile, Wingshape, angle of incidence, washout etc etc. The list is long.

High AR wings are typically not as roll stable as low AR wings. But here again, this is an oversimplification. If you compare two Wings that have the exact same profiles, sweep, AoA etc. but two different ARs then the Low AR will always be slower and more pitch and roll stable. But when you design wings you will not just scale on design to a different AR and leave it at that. So the argument is moot.

Stability is also controlled by things like the use of vertical stabilisers, an/dihedral.

Another big factor for roll stability is the span of the back wing.


2) Stiffness of the strut( main mast) is all-important to Torsion is death.

One of the most important factors that a lot of people overlook is the flexibility of the strut. You can have the best wings and fuselage, but if your mast is twisting too much this will ruin everything. If your plane (wings and fuselage) is moving around you will have major problems controlling it as speeds.

3) What is really faster?

All theory aside, the simple truth is: You are only as fast as your ability to control the foil. My fastest speeds in a straight line are all with an AR 7.8 Wing instead of my AR 11 or 9. The potential for speed of the High AR wings is there, but I am not good enough to push them hard and still control them at 60km/h+. The AR 7.8 wing is where my level is now and I can push that wing to the limit. That is the biggest factor for me when it comes to speed. If you are in a crossing or race the most important thing is to have consistent speed and not fall. So you should always use the equipment that you can control the best, over what is the fastest and newest.

4) High AR vs Low AR on the Racecourse.

A real high AR wing will have a much higher lift potential and will go higher to the wind (upwind) then at lower AR Wing. So on an up and down course you would want a high AR wing to get you to the upwind mark fast.

However foil racing is not always on up and down courses and also long crossings are usually on reaches. There you want a slightly lower AR wing with fast profiles that is trimmed for reaching.

5) DO YOUR MATH, but spend time experimenting.

I am not a aeronautical engineer. Everything I learned about designing foils has come though trial and error and reading up on the subject. There are a lot of great calculation tables which can give you a good reference on where to place your wings or what profiles you should use.

6) SIZE is important.

The more surface area your wing has, the more drag you will have. Smaller wings are faster, but this comes with a drawback. You will need more power to get up and foiling stably. A bigger wing (HighAR or Low AR) will always get you up and foiling earlier than a small wing.

If you are a heavy guy, build or buy a bigger wing. If you are lighter, you will want something smaller. We all typically have 3 kites in our quiver to cover the wind range we need. This is no different for Hydrofoil wings. If you really want to get going in the lightest of winds (sub 7 knots) you will want a big wing. and for super fast foiling in strong winds you want something smaller.

7) Pitch Stability:

I have been reading and hearing from a bunch or riders that their foils become very pitch sensitive at high speeds. e.g.. The faster you go the more twitchy the foil gets with respect for height control. I for one cannot confirm this. I find, the faster I go on my foil, the more pitch stable the thing becomes. If your foil actually becomes more pitch unstable at speed, then it's not balanced. The rear wing should be stabilising your pitch and fighting against the pitch moment more the faster you go.

So if you foil is dolphining up and down when you are going fast, you should check your rear wings. There is something wrong with them and they are not doing their job.

Another factor on pitch stability is obviously the profiles used for the front wings. Some profiles are more pitch stable than others.

8) Take all I say with a pinch of salt.

All I am telling you here is from my experience of developing my own foils for the last 4 years. I am not all-knowing and there I learn new things everyday. That is the fun of working on foils.

I may also be totally wrong about a few things and will retract what I say in a few months, but for now this is how things work for me.

--
Gunnar


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 11:23 am 
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thanks Gunnar for pitching in.. you have help out a lot in the last few months..

terrie


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 1:06 pm 
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Gunnar - great summary. Thanks for your time to share your present knowledge.
I am taking all what you stated above as granted including that stability of a foil at high speed.
Faster I was I had impresion to go smoother.
Please share more!

@ PF - you are on the same advice team as Gunnar. Keep posting...

gmb13 wrote:
There are a lot of great calculation tables which can give you a good reference on where to place your wings or what profiles you should use.

Would you link any in English please? I tried to go through the work in French and I failed :oops:

BTW. Would anybody provide more links to the hydrodynamic of foils?


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 7:43 pm 
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BraCuru wrote:
Gunnar - great summary. Thanks for your time to share your present knowledge.
I am taking all what you stated above as granted including that stability of a foil at high speed.
Faster I was I had impresion to go smoother.
Please share more!

@ PF - you are on the same advice team as Gunnar. Keep posting...

gmb13 wrote:
There are a lot of great calculation tables which can give you a good reference on where to place your wings or what profiles you should use.

Would you link any in English please? I tried to go through the work in French and I failed :oops:

BTW. Would anybody provide more links to the hydrodynamic of foils?


Hi,

Have a look here: http://www.tailwindgliders.com/Files.html

--
Gunnar


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 Post subject: Re: Foil wing basics
PostPosted: Fri Apr 04, 2014 9:48 pm 
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gmb13 wrote:
IHigh AR wings are typically not as roll stable as low AR wings. Gunnar


I feel the same. Additionally, riding on short, steep waves i.e. on a shallow bay, roll stability is seriously decreased by low mast bending stiffness connected with high AR front foil. You'll feel gel under your feets in such case.


Quote:
What is really faster?


I would say the smaller the foil, and the later it will start to sing (cavitate), the higher max speed potential.

Quote:
High AR vs Low AR on the Racecourse.


Of course, high AR foil will go upwind like crazy, providing it has enough area for a speed you ride. But we shuld keep in mind, that AR is just the proportion. If we scale the AR=7 foil by factor 0,5 we will get the foil with AR=7, just 4 times less area. To keep satisfactory upwind ability(VMG), you probably have to go twice as fast.


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