Colin,

O.K., I wasn't kidding about Bernoulli but i'm cautious slipping into the theoretical deep end with my flaccid biology degree for bouncy. Lemme give you and example; watch what happens when I foolishly address-

Nick , Ron and ,(gulp) Wouter,

A few corrections if I may:

Lift is a reaction force. The sail pushes air down and so the air pushes the sail up (forward). The key though ,for you and I, isn't Bernoulli - it's the Coanda effect (ya know the tendency of a fluid [air] to stick to a curved surface [sail] over which it is flowing). Coanda is key because as air flows over the curved lee of a sail it gets bent downward and pulls on the air above it creating a low pressure bubble. To bend air the sail has to exert a force (Isaac's 1st) which is opposed equally and oppositely( Isaac's 3rd), and lift happens. The magnitude of the lift is equal to the mass of the air diverted downward (Wouter does this clarify the "wash" question ?)...
Now, quit fooling around and get going on those T-Foil rudders !


Paul


P.S.

Doc , I vote for the Orange sails as by far the coolest !!

Lift generated on a foil is directional, and when applied to sails one can say that the lift moments generated at any one point of the surface of the sail will be at 90 degrees to that surface (although the amount of lift generated will depend on the velocity that the sail is travelling through the wind and the displacement of the wind at that point) therefore one can calculate the amount of lift at any given point of the sail AND ITS DIRECTION. In the case of the working sails the direction that the lift can be adjusted (in the vertical plane) while sailing is relatively limited, but in the case of a spinnaker there is quite a large degree of “vertical” directional adjustment available for the use of that lift generated I.E. by letting the foot of the spinnaker be set further forward the lift generated by the spinnaker can be directed more “upwards”. In so doing reducing downward pressure on the bows. The best comparative example is to a “parachute” or better still a “Para glider” where lift generated is DIRECTED to give both lift upwards and forwards.

That was the way I always understood it.

Wouter

Hobienick,

I'm actually a mechanical engineer myself and I told did my share of fluid mechanic courses. You won't loose me any time soon with vector diagrams and math or whatever. In addition and as a hobby I continued getting educated on these subjects because of my interest in sailing.

My question still stands though, you say that the 3rd law of newton explains the larger portion of lift created by an airplane wing than how exactly is this downwash set into existance and how does it excerts its force on the wing if not by a pressure difference over the same wing section ?

I wish to underline ones more that I'm familiar with the different theories of creation of lift. I should rather say models of creating lift. You can condens your explanations without loosing me.

Wouter

I can only speak for myself but I'm never offended in that way.

With respect to Bernoulli, Isn't this a bit of a yes-no issue. Bernoulli also stated that a change of speed in an airflow will always be accompanied by a pressure gradient. And this is very much part of the processes around a lift creating element. This is the yes-part of the issue. However, if we take out viscous friction in the models (all of them) but keep the model satisfy Bernoulli's law then no lift is ever created. This means that what is commonly refered to as "Bernoulli" is not the complete working core of the creation of lift. Although with Bernoulli the creation of lift will become very difficult as well, although not impossible because the 3rd law of newton is still there. But the last topic is a different matter.

Wouter

okay lets change the way we approach this issue.

You say :



Please give us the quotes were you feel that Gentry is saying that. Thanks to a poster we have Gentry's (1982) article available.

Wouter

About Bernoulli. It has been a number of years since I took the meassurements in a wind tunnel. There is a pressure difference due to the differences in speed on airfoils. But the delta p is not significant. I can't remember the exact number but I think it is down around 5% of the total force the airfoil creates. We also have to keep in mind that Bernoulli worked on experiments in enclosed systems like pipes. In open systems, like airfoils, very different things happen to the fluid flow. Carburetors, venturis, and subsonic jet engines all work on Bernoulli.

Two more things happen on an airfoil. One, it is very rare that an airfoil is heading into the apparent wind such that the lower surface (if it were flat) is perpendicular to the apparent wind. Since it usually has a positive angle of attack, the airfoils redirect the wind downward. These changing angles of attack are where the force vector diagrams come into effect. You are comparing drag force to lifting force. As the shapre presented to the apparent wind changes the magnitude of the drage force and the magnitude of th lifting force caused by the redirection of the wind chage drastically. I am not sure of the amount of lift this contributes to the total number as we had a hard time isolating just the underside of the airfoil in the wind tunnel.

Two, the coanda effect on the top of the wing. This is where the airfoil needs to have that curved geometry. We are all familiar with the water on the back of the spoon effect. The top of the wing is the back of the spoon and the air is the water. This being said, the air is then accelerated downward causing a force (F=ma). Again, I am not sure of the portion of the total lifting force this contributes, but I believe it is the majority of it.

Here's why... Wings stall. We know this happens when the air separates from the top surface of a wing. When tis happends, the coanda effect is lost and this component of the lift force is lost. If we look at Bernoulli here, we see there still is a delta p present. All of the separated air is trying to fill the void on the top of the wing. If you keep the airspeed the same, but increase the angle of attack until the airfoil stalls the lift will drastically drop off. By keeping the airspeed the same in a wind tunnel you remove the issue of too much drag for the engine to pull the wing through the air at sufficent speed, therefore removing one of the variable (velocity of the airfoil) from the equation.

Also, look at hydrofoils versus airfoils. Why do hydrofoils have to be much smaller in chord and thickness to create the same amount of lifting force? I'm not sure here, but wouldn't the delta p be less on the hydrofoil than on the airfoil? If this is true then the larger force must be produced by water having more mass than air and when you redirect it (accelerate it) downward it will produce a larger force according to F=ma.

Again. Wouter, thanks for having this discussion with me (and the bow lifting discussion). I am learning how to better explain my side. I hope everyone following is gleaning something as well about how sails work.

Paul,

Just to recap our real life conversation on the beach. I think it went something like this:

Paul: Look a my new ultra slippery silicone-impregnated spinnaker with the perfect luff length, flatter cut and set up perfectly for my patented in-board sheeting angle. This thing's gunna exploit the Coanda effect like nothing before.

Eric: Uh...yeah...whatever. It's bright orange, though.

[I still need to get you copies of the video Jill took of you at Huntington.]

Doc,

That you insist upon raising the essential issue of COLOR in this highly technical and nuanced thread, reminds me of all of those reasons I so often finish behind USA #297.You've no doubt read that the Caifornia Prison System will begin painting all prison interiors pink and you've gleened the reports natural applications to sailboat racing ! Seems exposure to a certain flat shade, causes otherwise aggressive , focused ,potentially violent individuals (remember that skippers meeting last summer where the RC announced we'd share a start
with the H-14's - " ...aggressive,focused,potentially violent" applies to sailors in some circumstances...), to become passive ,disinterested individuals divorced from their fate entirely ! Imagine what a weapon a racing sailboat, painted entirely pink would be ; the visual equivalent of powdered Valium in the orange juice !

I'll be at the Alter Cup qualifier barring a Beau bomb... Love to see that vid....

PK

Powdered Valium in orange juice...hmmm...tastes bitter but is there an application for Beau?

On the subject of spinnaker color--all that great video Jill took of you was because she never spotted my inconspicuously-colored two-tone sail; only yours.

On a side note, Paul, if you (or Beau) have the photoshopping ability (that I lack) to cut and paste a certain sunglass manufacturer's logo onto an action photo of your spinnaker I can finish my proposal to the company to get us both new spinnakers for next season (have you seen the holes in mine lately?).

The way I understand the issue now is that the Bernoulli law itself can not fully explain the pressure differences found over the wingsection that load to a certain lift. And this is very much corrected in my opinion. Only problem I see here is that I don't remember anybody in this thread expressing the opposite believe. At least we agree on this part of the discussion.

This makes me think about what started of this tangient. It was Gentry writing that adding a jib to a mainsail hurts pointing less than what would inititally be expected and the reason for this was that the upwash of the mainsail will make the jib work at a better angle of attack then it would be without the vicinity of the mainsail. I think we found ourselfs in a tangient that doesn;t help us on this initial topic.

Wouter

Completely off the topic... were those naked kayakers in that picture?

-Rob V.
Panama City
Naked... I mean Nacra 5.2

Hobienick,

Some of what is said here is contradictory. For instance, in the first paragraph above, you state that "As the shapre presented to the apparent wind changes the magnitude of the drage force and the magnitude of th lifting force caused by the redirection of the wind chage drastically." This is demonstrably true and occurs mainly at the area immediately behind the stagnation point on any airfoil. That is the point where the maximum air mass acceleration happens. At the stagnation point, the air mass actually comes to a *stop*, then is accelerated in a new direction over the top of the wing. It continues to accelerate over the balance of the airfoil, but nowhere near as violently as at that small strip just behind the stagnation point.

You state later WRT the balance of the wing chord well behind the stagnation point: "This being said, the air is then accelerated downward causing a force (F=ma). Again, I am not sure of the portion of the total lifting force this contributes, but I believe it is the majority of it."

But relatively little acceleration, and hence reaction (lift) is actually generated here. Collectively, the balance of the wing chord does contribute significantly to lift, to wit the center of effort of the foil is said to act somewhere near the point of maximum thickness. But this is usually well forward, implying that most lift is really being created well forward of that.

I don't know if anyone in this discussion is familiar with this paper, but it is very instructive and supported with experimantal data:

http://www.tspeer.com/Wingmasts/teardropPaper.htm

Quting from that paper: "When the pressure changes over some distance, any small volume of fluid will have an unbalanced pressure on each side, and this net force accelerates the fluid, either changing its speed or its direction of motion in accordance with the conservation of momentum. This links the pressure gradients, the bending of the flow direction, and the local flow velocities so as to satisfy the three conservation laws: mass, momentum and energy."

So we would expect that wherever the maximum acceleration occurs, maximum pressure gradients exist. Look at the graphs of distributed velocity and pressure contained in the paper and you will find this confirmed.

Something I found really interesting is exactly where on a typical sail the line of maximum lift is. It is actually only inches from the tip of the mast! For some boats with longer mast sections, this point may actually be located on the mast itself!

This does NOT imply in any way that the balance of the airfoil under discussion is any less important WRT shape and function simply because most of the 'action' is happening farther forward. It does tend to clarify, however what the function of the balance of the airfoil actually is; it smoothly reunites (deccelerates) the air mass on the lift side with the other side. This is essential to retaining adhesion of the airmass to the lift side of the airfoil, and thus essential for creating lift.

Great discussion, guys!

Jimbo

Nope--teenage daughters of a Prindle 19 sailor in the fleet.

Just to keep this off topic--notice the drifter, less than 5 knot conditions. The RC held not one but two races in those conditions! It was the toughest conditions I've ever "raced" in. Nobody griped, no redress, no juries, no excuses. We still competed and the best guys somehow still came out on top. Great camping, great raffle, great people; fun regatta nonetheless.

I think Wouter is right about us getting really off topic in this thread. When I get home tonight, I will start another thread about lift generation over an airfoil so us "geeks" can have at it. If anyone else wants to beat me to it, please do.

This thread HAS been completely hijacked,
As I understand it the theme should not have been a debate about how "lift" is generated, but more to the applicable "direction" that the lift that is generated by a spinnaker acts on the performance of a cat, or more directly whether the direction of that lift can be used to relieve downward pressure on the bows, increase it, or remain neutral to the horizontal direction of the hulls through the water?

I think one of the aspects of this discussion that has not been mentioned to this point is that the direction of the center of effort from the sails when flying the spinnaker is different than when not flying a spinnaker. Because the speed increases with the spinnaker, the apparent wind is changed (coming from further forward), the sails are trimmed to this apparent wind causing the center of effort to move from a more forward direction to a more lateral direction. The force on the platform is more lateral, allowing the crew to put that force to good use to propel the boat. The appearance of lifting the bows may be more a function of changing the direction of the force than the vector of force around the center of resistance actually lifting the front of the boat. Just thinking about the force diagram and relating it to sailing the F17 for several years, this is at least part of what is going on.

i can see where the inclined luff of a spi. would lift the bows, but how are the jibes? we sail overpowered with seas alot and the idea of adding sail area just seems wrong. maybe a shorter mast/kite combo and just rely on the foresail like a keelboat is the ticket. magic rudders would be great to keep the bows up if possible.
so, anyone ever capsize jibing the spi? we seem to be right on the edge of a capsize on every jibe with just a jib. talk about threading the needle...head up too much after the jibe-you go over side ways. bear off too soon after the jibe and the new windward bow auggers in and it's a pitchpole. not that we actually ate it on a jibe recently, just that the potential is there when high winds hit the top part of the sail.

How funny it may seem but adding the spi will make all that more controlled as well. At least that has been my experience on both the Prindle 18 and the Taipan 4.9

Both in the sloop and cat-rig setups.

This is the reason why I (we = crew and I) always pull the spi no matter what the conditions.

Truly,

Wouter

It has been my experience as well that jibes with a kite up are much more behaved than without. I remember jibing my H17 and feeling like the mast would pop off the ball in a blow.