Does it suck?

I read somewhere that "low pressure actually sucks you forward, you are not being pushed upwind... but being sucked"

is this true? how can it be?
feel free to get as technical as you wish....

You're kidding. Right?

It is true.

The suction forces on your sails are larger then the overpressure forces and therefore you more "sucked" then "pushed" forward.

Wouter

Same concept as an airplane wing air has to move faster over the curved leward side of your sail than the windward side causing a low pressure on the leward side.

From Wikipedia:

Real-world application

Condensation visible over the upper surface of a wing caused by a pressure drop due to air accelerationIn modern everyday life there are many observations that can be successfully explained by application of Bernoulli's principle, even though no real fluid is entirely inviscid [19] and a small viscosity often has a large effect on the flow.

Bernoulli's Principle can be used to calculate the lift force on an airfoil if you know the behavior of the fluid flow in the vicinity of the foil. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lift force.[nb 1][20] Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations[21] – established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the underside. To understand why, it is helpful to understand circulation, the Kutta condition, and the Kutta–Joukowski theorem.

So...in our world, ie. racing sailboats, if you are going upwind, you are being Sucked. If you are going downwind, you are being Blown.

Sucked or Blown, that is the question. And here's some light math for you to figure it out:

value of effort
The plots of air strike sail on both sides so:

On the windward side efforts are atmospheric pressure, wind pressure, and virtually no depression due to wind.
On the leeward side efforts are atmospheric pressure, a bit of depression and almost no wind pressure.
To simplify the manipulation of these forces, the forces are summed into a single force and that the entire surface of the profile (sailing) in a simple formula (valid for airplane wings like a rudder, a sail, an anti Plan -drift) [7]:

[8]

with

E = effort that can give up the wind (see Max Q);
C = coefficient Aerodynamic
According to the Bernoulli, the maximum stress of wind or density of kinetic energy maximum for the entire surface of the sail:



The full expression of the force is:



with

F = lift, expressed in Newton
ρ (rho) = density air (ρ varies with the temperature and the pressure) ;
S = typical surface, for sail, it is the sail area in m²
C = coefficient Aerodynamic. Aerodynamic coefficient is unit-less, it is the sum of two percentages: the percentage of recovered energy leeward side + percentage of the recovered energy into the wind. For this reason, the coefficient aerodynamics can be greater than 1. It depends on the angle of upwind sailing.
V = Speed is the speed of the wind relative to the sail (Apparent wind) in m / s.
The sail is deformed by the wind and takes a form named Airfoil. When the flow of air around the profile is Laminar [9], the factor against depression in the wind becomes crucial. This effect is then called lift. Studies and theory to draw a sail [10] that:

Depression on the upper (leeward side) represents two thirds of the lift,
The pressure on the lower surface (facing the wind) represents one third of the lift.
[edit] Lift effect on sail
The study effect of lift can compare cases with and without lift [11]. A typical example is a gaff sail. The sail is rectangular and is approximately vertical. The sail has an area of 10 sqm, with 2.5m of foot by 4m of leech. The apparent wind is 8.3 m / s (about 30 km / h). The boat is supposed to uniform velocity, no wave. It does not heel, does not pitch. The density of air is set at: ρ = 1.2kg / m3

[edit] turbulent flow or downwind
The boat is running downwind. The shape of the sail is approximated by a plane perpendicular to the apparent wind.

The depression effect on the sail is second order, and therefore negligible, it remains:

On the windward side, efforts are atmospheric pressure and wind pressure
On the leeward side, there remains only the atmospheric pressure
Efforts to atmospheric pressure cancel out. There remains only pressure generated by the wind.

Roughly speaking, shock of parcel on the sail forward all their energy from wind in 90% of the surface of the sail. This means that the Cz or aerodynamic lift coefficient is equal to 0.9.



[edit] laminar flow
The boat is Close hauled. The wind has an angle of about 15 degrees with chord of the sail.

Because the setting of the sail at 15 ° relative to the apparent wind, the camber of the sail creates a lift. In other words, the effect of depression on the leeward side is not neglected. As air pressure forces cancel out, efforts remain are:

On the windward side, wind pressure,
On the leeward side, wind depression.
The only unknown is the drag coefficient to be estimated. Curve takes a good adjustment of sail is close to upper shape NACA 0012[12][13]. A sail less well adjusted or older technology (old rig), will be more hollow, more camber. The coefficient of aerodynamic lift will be higher but the sail will be less efficient (lower finesse). The profiles would be more suitable profiles as NACA 0015, NACA 0018 [14].

For a given profile, there are tables which giving the lift coefficient of the profile. The lift coefficient (Cz) depends on several variables:

Incidence (angle: apparent wind / Profile)
The lift hill of the sail, which depends on its extension,
The surface roughness and Reynolds number, which affect the flow of fluid (laminar, turbulent).
The coefficient is determined for a fluid stable and uniform, and a profile of infinite extension.

The Reynolds number is:

with

U - fluid velocity or apparent wind [m / s]
L- characteristic length or foot of the sail [m]
ν - kinematic viscosity fluid: ν = η / ρ [m / s]
ρ - density air [kg / m³]
μ - dynamic viscosity air [Pa] or Poiseuille [pl]
so for this sail about Re = 106

Under an incidence of 15 ° and a Reynolds number to one million, reached a NACA0012 Profile Cz 1.5 instead of 0.9 for 90 ° incidence.



The lift has increased by 50%. This also corresponds on sheet an increase of 50% effort for the same apparent wind.[15][16]

Aww crap, the formulas didn't copy/paste for some reason. Well, you get the general idea.

where

L is lift force,
ρ is air density
v is true airspeed,
A is planform area, and
CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number

Also,

where:

L is the lift,
A is the wing surface area
p is the value of the pressure,
n is the normal unit vector pointing into the wing, and
k is the vertical unit vector, normal to the freestream direction.

Should I be worried that airline pilots get their information from wikipedia?
(I'm flying next week).

Yes, be very affraid, and drive to your next destination.

We don't build them, we just fly them. Pull back, the houses get smaller, push foreward, the houses get bigger.

That's about all there is to it. Just ask those Quantas boys on that A380 with the exploding engine...

rofl! Clearly you need more time on the water.

~~ I don't care if I'am gettin "BLOWN" or "SUCKED" on my boat...... If it's "WET, It's "FUN" ~~~~ ;)~

LOL, yes a transatlantic tunnel! Wouldnt that be awesome!

You might want to ask the people who got stuck in the Chunnel first. And that's only a short tunnel (26mi?) under the English Channel.

I was sitting in a pub in Brighton the day the Chunnel opened many years ago, watching all the hoopla on the TV over the bar. There was a reporter asking the Britts on site what they thought about now being connected by road directly to France.

One guy said, "Well I don't think it's a good idea." The reporter asked him, "Why, don't you like France?" He said, "I LOVE France, it's the French I hate!"

Classic!

I think the term "suck" is misleading. How can an air molecule pull on anything? It's like the old egg in a bottle experiment. The egg is pushed into the bottle, not sucked or pulled in.

Define Suction.

Using your logic, the term should not exist.

In training they told us a jet engine works like this:

Suck, Squeeze, Bang, Blow. I think a recip works the same way, but with more moving parts...

Tim, I take issue with the "no substance is invicid". Look up Bose-Einstein Condensate. This material has ZERO viscosity

Even better, scientists have actually used this super-cold substance to slow the speed of light to roughly the speed of your average bicycle (without being diffracted). Now you can actually say you're moving "faster than light" (under certain conditions, of course)

So I should rub it on my sails?

interesting thought. maybe wear gloves

It would be hard to time your finish, however, with all that quantum flux going on... But your finish time could be sooner than your start time... How do you handicap that?

Which side?

.....depends which tack you're on.

I love messin' with Jay!

I can't envision an area of low pressure sucking. I can put a dime on my desk and push it with my finger, but no matter how hard I try the dime just won't follow my finger when I move my finger away.

See if you can envision this.

http://www.snopes.com/risque/penile/pool.asp

Boy, that must have...Sucked!

I was in the UK when Parliament decided not to improve the rail system between London and the Chunnel. Reporters were asking people for their reaction. A pastor responded, "I am against the Chunnel because the high speed trains will force the smell of garlic through the tunnel and envelop England."

That's one more fool I can ignore. As my list gets longer, this site becomes more enjoyable.

awesome! thanks guys!

what i got out of this:
1. i read correctly, they do get "sucked" forward
2. don't fly with Timbo
3. math hurts my head

Timbo, you said 'downwind you're being blown'

On our cats downwind, especially with a spin up, it's still being drawn along by lift on the various sails.

Yeah I know, I was just trying to get the word Blown into the discussion somehow...

We still have not answered the question, Would you rather be sucked or blown? And can there be any Sucking, without it also Blowing? How fast do you go on a zero wind day? I submit that in order to be sucked, you must first be blown.

Thanks again for the input...

ok, here is why i posted this... i mentioned that we are being sucked upwind... and was met with this as resistance:

Sorry Andrew, you have it mixed up. Nothing is sucked, or pulled. We don't actually "pull" a wagon, it is pushed. If you analyze the loads involved you realize that the palm of your hand is pushing on the rear edge of the wagon handle. This increased pressure forces the wagon forward.
Our shrouds are under tension, follow the loads from the hound to the pin at the hull & you find the same thing happens. There is no sucking that shroud downwards.
A sail is much the same as an aircraft wing, which operates on Bernoullis Therom. Airflow meeting the leading edge of a wing, or sail, is forced to split. To simplify greatly, lets think of two air "packets", "A" is forced to travel around the front side, "B" the rear. The two packets want to meet at the same time at the trailing edge.
A wing generally has a degree of camber,(again simplifying, the greater the camber, the greater the potential lift) this camber results in a greater distance along the top edge. "A" is forced to speed up to go the greater distance in the same time. We never added any energy to packet "A" or "B", so the total energy must not change. To achieve this, the pressure of faster moving packet A decreases. We now have a low pressure on top of the wing, & a relatively higher pressure underneath. You have heard since grade 4 science that Nature abhors a vacumn. The higher pressure area tries to get into the lower pressure & equalize it. Result, the wing is forced upwards,(lift) by this higher pressure. We need the higher pressure to obtain results.
Our cars engine does not "suck" air, it is pushed in by higher outside pressure, hence the decrease in power at higher altitudes as atmospheric pressure decreases. To regain that power we turbo charge, or super charge. TC or SC do not create more "suction", they create more pressure on the intake side. It is important to understand this principle, it is not just a matter of semantics.
This differential of pressure is responsible for quite an amazing repertoire in our everyday lives, carburetors, forcing solvent up the tube (notice I didn't say "suck") of the gizmo we attach to our air compressor for cleaning things, & most importantly breathing. Take a breath, our diaphragm drops creating a lower pressure in the lung cavity. Higher atmospheric pressure pushes air into our lungs. Jump off your Cat with a piece of hose in your mouth, then use it as a snorkel. Floating on the surface it is easy to breathe, about 16" down, you die. At that point the water pressure squeezing your chest cavity has equaled the air pressure trying to push into your lungs. No amount of "sucking" can refute the laws of physics and allow you to draw breath. If you could increase the atmospheric pressure it could overcome water pressure to a greater depth & allow you to breathe. This is one reason a SCUBA tank must be pressurized.
Back to sails, now that we understand it is a higher pressure driving the sail,(and skegs or boards converting this push into forward motion). To get more lift(power)we need greater total pressure,which can be obtained by more area, higher wind velocity, denser air, co efficient of lift that is determined by wing shape (CL), or some combination of these. Air velocity has a "squared" function in the equation, & so has the greatest effect. We can't do much with air density, but we can alter the CL by changing sail shape.
A wing,(aircraft or sail) can increase/decrease camber several ways, flaps being the most common way to increase. As they move rearwards & down, they increase both chord & camber. Same is true for leading edge devices,(slats). Increasing chord results in an increased wing area. If you couple the larger area with an increase in CL ie greater camber, the result is greater lift, (power). Of course nothing comes free, greater lift results in greater induced drag. As long as we increase lift more than the resultant increase in drag, we are ahead of the game. A tall narrow sail results in what is called a high aspect ratio,(chord vs span) very efficient...think of a gliders wing, or any soaring bird. This is where the sail trimmer comes in. Modern rigs allow us to change the sail shape to suit the conditions. A hard wing sail will be more tunable, & powerful, but the cost & complexity will also be greater.
And you thought college physics was a waste of time

Yeah,

I figured you knew that. I also agree it must blow before you can be sucked.

With all the talk of getting s..ked or getting bl.wn it sounds as though we are in JW's words "Circling the drain"

As mentioned above, it's really just a question of differential pressure. What you label it is pointless. It's just as correct to say "pushed" as "sucked" because you have the high pressure on one side and the low pressure on the other. Going downwind, the same principles are in play.

As an aside, I made the mistake of using the word "sucked" in an explanation in a high school science class, the teacher nearly ripped my head off (not because the word was objectionable, but because I didn't use the correct explanation regarding the pressures).

Mike

Kudos to you for discussing it at a HS class!

I mean discussing the forces at work for the boat to 'go'

Keep in mind, this was a very long time ago...

The discussion involving sucking had more to do with how a vacuum worked, or something like that.

Honestly, the guy just liked to ridicule us. He cracked himself up asking questions that should have been aswered with "boron" and when no one spoke up, he would drop "helpful hints" like, "Rhymes with MORON..."

We did discuss airfoils, wings, sails, etc. in a later high school physics class, with a much better teacher.

All things being equal, my high school education was probably the best money that I ever spent. Well, that and my Hobie Cats.

Mike