# Understanding Wing Technology

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Sep 18, 2010.

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### ErwanSenior Member

Hi Peter,

As far as I understand Bernoulli equation relates the pressure and the fluid velocity, it is a kind of " energy conservation" relationship, high pressure means low speed and conversly.

In a perfect fluid with no viscosity, air flow would be symetric at the leading edge and trailing edge, if you make integration of all pressures around your "body" you will get 0 therefore no lift.

Classic approach is 1-Bernoulli 2-D'Alembert paradox 3-Magnus effect 4-Prandlt boudary layer theory.

As mentionned in Marchaj book:

Since Prandlt "The flow round a foil in a real fluid can be treated as consisting of the 2 distinct parts:
One part, that very close to the surface of the foil, is entirely affected by viscosity but its effect are limited to a thin layer immediatly adjacent to the wetted surface. This restricted layer in which viscosity dominates was called by Prandtl the " boundary layer".
The second part consists of the flow outside this boundary layer, where effects of viscosity are negligible and therefore the flow may be regarded as that of an ideal frictionless flow.

Marchaj Aerodynamic of sailing page 182

Regarding circulation, I would suggest you to dig into Kutta-Joukowsky hypothesis,
the starting vortex and mechanical analogy with a gear-train is very informative.
I think it is the best physical analogy of circulation, but not only an analogy:

In the middle of the 90' a friend of mine who was convoying a 60 feet Tri (may be Fudjicolor or Banque Populaire, don't remenber which one) between 2 harbours, with a typical Bretagne wheather (very thin rain, almost a spray), told me that he was able to observe behind the leech of the main sail, the starting vortex as described by Kutta-Joukowsky.

Dont forget I am more a benchwarmer than a star for fluid mechanics.

Cheers Mate

Erwan

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### high on carbonWing Nut

I have

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### peterengSenior Member

Hi Erwan - I've been doing more reading and wikipedia expressed it best. The total lift is the sum of the translational lift and the rotational lift. The Translational lift is the airstream around the foil, the rotational lift is the circulation in the BL. Once flow is established a starting vortex will shed then there will be a stream of small vortices after that. If the conditions change (eg change in angle of attack) then another starting vortex will shed and then it will stabilise again. This is all I need to know. It clarifies my thoughts. Better get back to the 250 tonne dump truck tub I'm analysing at the moment, it would have a huge von karman vortice street! Ta Peter S

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### Mikko BrummerSenior Member

I thought that rotational and translational lift are what makes a helicopter fly? ;-)

I would be cautious about Bernoulli when explaining lift - Bernoulli is valid for closed systems, like a pipe flow, not for open flows.

I like to think that sails create lift by simply bending the wind. When the sail bends the airflow (mass of air), it puts a force on it - by action-reaction, the air puts a force on the sail.

On the windward side, it's obvious how the airflow is bent - wind cannot go through the sails and has to leave the leech in its direction. On the leeward side it's not that straightforward - maybe flow just has a tendency to try to follow a surface, if it can...

Also you might like to think that the leeward side flow wants to "join" the windward one, to remain "coherent"... maybe not such a good explanation, as when started from rest, the leeward side flow reaches the leech before the windward side flow. Maybe the starting vortex has a role here, to keep the flow attached until the windward side comes along. But the flow has a tendency to follow a curved surface, even if there is no "windward side", like in the usual spoon under watertap-test. This is another example where Bernoulli is often wrongly quoted, it's not the Bernoulli pressure dropping that is sucking the spoon into the flow, but the tendency of the flow to follow the surface. The spoon puts a force on the water flow curving it, and the water flow reciprocally sucks in the spoon. No Bernoulli there, definitely not a closed system.

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### tspeerSenior Member

I didn't design the structure, I didn't build it, I didn't sail it. But I designed the cross-sections for the wing and it did win a couple of races.

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Imagine that.

FYI, it is more than just a little bit awesome that we get to talk tech stuff with an "insider" associated with such an over the top accomplishment (the BOR 90 aka Dogzilla aka USA 17 build effort - not the Lawyer junk).

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### high on carbonWing Nut

It only needed to win two though

Good times those were in Valencia Tom

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Now that I think about it, the biggest shame of the last AC is that they basically put the boats out to pasture after the event.

I understand that there is a huge cost to operate them, but it still seems like such a shame that both of the boats are probably the fastest lake sailing boats currently built, yet "on the water" and under way is just not part of the equation.

Tom, just wondering if you got much opportunity for first hand viewing of the wing in action.

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### tspeerSenior Member

I never sailed on USA 17, although I did spend a number of late nights aboard her at anchor in Valencia, serving as unskilled labor on a project. I went out on the chase boats in San Diego quite a bit, starting with the second sail with the wing. It was a big relief to see the telltales laying flat and only lifting where they were predicted to lift! I was able to watch both races from the Oracle Racing VIP boat.

The tradition of limited use boats to win the AC goes way back. Reliance, the largest boat ever to race for the America's Cup (144 ft LOA), was broken up for scrap within months after her successful defense in 1903.

It required something like 40 people to get USA 17 sailing, when you include the sailors, shore team, performance (instrumentation) team, etc. Many of the people with specialized knowledge of her systems are no longer with the team; some are on the other AC teams. I don't know the status of her systems - some of her equipment may have been cannibalized. She reached her maximum design loads in 5 kt of wind, and the load alarms were going off almost continuously around the race course. In fact, the first time she sailed the entire 40 nm AC race course without breaking something or having to stop for other reasons, was Race 1. She's just too fragile and high-strung to use for casual sailing, and it would be a big effort to reconstitute a team that could sail her.

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I kinda knew the "limited use" design and logistics stuff, but it still seems like a shame.

I guess I was really just whining about not get more "spectator" videos showcasing that wing.

We get to see relatively regular spectacular stuff from the French (BP-V, Hydroptere, et. al.). However, I am a huge fan of wings and it was just great when a US based "super tri" was out showing off "top of the class" performance.

On the other hand it is great to hear that you got out on the chase boats and that you made it to the big event. I am betting that some of the shake down / training runs were real shows (I would be green with envy except that I am sure you deserved it where I would probably only qualify as a team janitor). I expect that you will remember your involvement as one of the pinnacles of your involvement in both sailing and wings.

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### giulio.iJunior Member

some good news and update on www.herusails.it

ciao
giulio

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### DCockeySenior Member

Bernoulli's equation works for incompressible "open flows" along a streamline as long as the viscous effects are negligable.

Bernoulli's equation is valid for water flowing past the spoon along streamlines outside the boundary layer of the spoon. Put in the velocity and the pressure can be obtained as long as the pressure and velocity at some other point along the streamline are known.

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### ErwanSenior Member

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Ben Halls A-Class Wing

from the A Class wing thread in Multihulls on SA: ( http://forums.sailinganarchy.com/index.php?showtopic=134280&st=0 )

captain horizon

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16-February 08 Posted Today, 06:47 PM

Great looking wing Thilo!I just want to clear up some misinformation on the front page story. Firstly the wing was by no means overweight. I was told by all the expert wing nuts that I would be hard pressed to build a wing that was less than 34 kg. my original weight goal was 27.2 kg or less. In fact the final all up weight was 24 kg. this compares to a mast, sail, battens, boom, Mainsheet system and traveler at 21.2 kg. The big difference is that the CG of the wing is about 2 meters higher than the soft rig. As reported in this thread, this dramatically increased the pitching and really hurts upwind performance even in a moderate chop. It will be interesting to see if Thilos wing is lighter. I think he will experience the same thing I did especially in a venue like Islamorada if the wind is out of the NE to SEE directions. The C class also suffers from pitching due to the wing weight but it not nearly as dramatic as the A Class. And no the stiffness of the rig was also not a problem . The real problem was time in the boat learning the rig.I built my rig in 2007 but I doubt that other than maybe design improvements that the rig was built with new or higher technology. The majority of the components in my rig were autoclave cured using high modulus carbon. I noticed in Thilos blog that all his components were wet layup. Lots of ways to build things but certainly wet layup is not higher tech than autoclaved parts.As far as rigging the boat with the wing, not that big a deal. My wife Nan (wing Sherpa) helped me each morning at the Worlds install the rig. It took an average of 25 minutes from trailer storage to ready to launch. Heck at my age it almost takes that long to suit myself up for a days sailing. I did make a special trailer to transport and store the wing but judging from the cool trailers I have seen in Europe and the US lately this is a minor problem. I suppose if everyone in the boat part had wings it could be a logistic problem. Cost of the wings certainly is a big factor. Even with doing a lot of the work myself the cost of my wing was close to that of a complete new boat. To achieve the weight that will be competive expensive materials need to be used. I was was right on minimum weight at the Worlds at 75kg.It will interesting to see how Thilo does at Lake Garda in the big breeze. Good luck and hope all the great design and hard work pays off.Ben HallUSA 99

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