The Newtonian explanation often falls short in explaining how lift is created when the foil is at low or zero angle of attack. What I have read lately seems to explain things well. A properly shaped foil will impart a downwash even at zero angle of attack. There is some "suction" created by the low pressure. This has been measured in studies with sensors above a wing. However, one misconception is that the fluid MUST join the stream at the same speed aft of the foil. Many studies show that the flow continues to move downward or toward the HIGH pressure side for some time/distance past the aft edge of the foil.

This effect manifests itself as wake turbulence (not the vortices at the tip, but downwash), helicopter downwash, "ground effect" and even in sailing in the form of a "safe leeward" position which actually backwinds the boat behind and a bit upwind of the lead boat while sailing upwind.

Now that the experts are assembled, perhaps a conundrum can be resolved. My fancy, and expensive, new sails have thick threads running through them. They're really quite rough. What ever happened to smooth?

Don't we exalt smooth, seek smooth, lust for smooth? Is the world gone mad?

Regards
Chet

A smooth golf ball doesn't fly as far or as striaght. A tennis ball with out fuzz won't go as fast with the same amount of force. Turbulant boundry layer? I think is what it is called. A small amount of air "clings" to the surface acting like a bearing for the rest of the air to move over. At least that's what I remember from physics.

Karl is basically right - there are two basic types of boundary layers, laminar and turbulent.

Flows across sails start as laminar and transition to turbulent at some point:



Laminar flow has less friction, but is unstable. Turbulent boundary layers are more stable and less likely to separate from the sail.

The transition from laminar to turbulent depends on the viscosity of the fluid, the speed of the fluid and the roughness of the surface.

The bottom line is that the stitching and seams on a sail are within that viscous sublayer that's at the bottom of the sail's turbulent boundary layer that starts on the mast in all but the lightest windspeeds.

This laminar flow sounds great. Why don't I promote laminar flow on my hull and stop waxing the damn thing?

Regards
Chet

Sooo...what about that Teflon Spray, (McLube?) does it help reduce surface friction over the sails, hulls and foils to spray that stuff all over your boat and sails? I've heard some of the mono guys do this. In Frank Bethwaite's High Performance Sailing, he tow tests two hulls, one polished to a high sheen and one not, and the polished hulls had less drag.

I know the guys who race airplanes at Reno wax the snot out of their wings to reduce drag, but the boundry layer is supposed to be about 1/16th of an inch, which would be more than the layer of wax, so does it really help reduce friction?

Talk amongst yourselves... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

When I last fueled, it was around $5.50/ Gallon, plus prist (Jet A). Our company has various accounts at different airports we frequent and can get as much as $1.50 off.

Back to lift. Is there not a difference between 'flying' and 'projectile'?

Sure, you may throw a rock and it 'flies' thru the air, but with no control. A missile also flies thru the air, with very little control. Rockets the same. Your comparison to the 'missile with a man in it' F104 is accurate, with enough thrust, you do not need a pressure differential. Aircraft of old, that had very little excess thrust/ power had very large, 'airfoil' shaped wings.

I am no engineer, but I think it is a blend of both principles that achieve flight. With smaller 'GA' aircraft, it is pressure difference as a result of wing shape, with newer business jets, its excess thrust, more efficient wings/ airfoils.

Ok, I think that is my 2 cents. BTW, check out this plane, radical design, excellent speed/ range, and very fuel efficient.


http://www.piaggioaero.com/en/products/aircraft/p180/showcase/showcase.php

Here's a good read on bottom smoothness:

Slip Sliding Away

You need it.

In order to fly, a plane needs a force to counteract gravity. It also has a surface that you can't get rid of. As a consequence, a pressure differential will exist whenever the plane is flying in equilibrium.

In the case of thrust equal to (or greater than) weight, the thrust exerts a positive pression downwards that creates a pressure gradient in relation to the atmospheric pressure.

The correct expression would be something like this:

"A pressure gradient is required to counteract weight and achieve unaccelerated flight. The gradient may be provided by any onboard device, including an airfoil flying at an angle of attack, a rotating airfoil at an angle of attack, an internal combustion engine with its propeler, a turbine, a rocket, etc."

I thought your link would send us to Victoria's Secret website...

Hi Greg and all,

actually there is no controvery between Newtons and Bernoullies laws. The formulas which describe the behaviour of fluids are called Navier Stokes equations. One part of them are Newton laws. You may look in Wikipedia to see these equations. With some assumptions, you can simplify the Navier Stokes equations to Bernoullies law.
I guess that the speaker in the TV just wanted to explain, that you need more formulas then just Bernouilles law.


It is true, that the upper surface is more sensitive than the lower surface. Hence it is better to have the spi halyard on the lower (pressure) side of the sail and not on the suction side.


In fact the boundary layer of our mast and sails is turbulent due to roughness, contamination and oscillation,
but it may be usefull to fix the separation point, i.e. have a mast with roughness at the leading edge to prevent early separation.

Cheers,

Klaus

Hello All,

While researching information on Hydrofoils for my project I came accross and interesting web site that forever supper explains wing lift theory in a simplified manner just when I thought I knew most of it. Check out " A Physical Description of Flight" by David Anderson & Scott Eberhardt or web site http://home.comcast.net/~clipper-108/lift.htm

Let me know what you think!

Delane

Why are we talking about flying on a sailing web site anyway?? There are tons of flying web sites for that. Can we at least talk about hydrodynamics or something, hull shape, lifting hull shapes, foils, etc. Calling Pete Melvin, HELP!

I think the "next big thing" speed wise, in the world of sailing is going to be foils, not wings, (sorry Ben) and if we have nothing better to discuss (it being the dead of winter up here in the Northern Hemi, even here in Florida it is only 30 degrees out right now!) how about we discuss that?

I am really impressed with what Rohan Veal has done with the Moths. I think the same could be done with cats and it would yeild a more stable, faster craft than the Moth. I liked Dave Carlsons inverted A foils on his Jackalope. Dave, more pictures! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

What about canting rigs? If the mast can pivot it can also lean...

A continuous line stay to stay, decent amount of purchase, change it on each tack. Reduce heeling moment significantly.

I think some of the high tech tri's and mono's are doing that now, helps lift the boat up out of the water a little bit, kind of like a windsurfer, tilted upwind (towards the high side of the boat).

I don't see why we couldn't do it on our cats, just need to work out the details. Have any of the A cat guys tried this or is it outlawed?

Something else to break! <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />

Here's what I'm thinking: Take the daggerboards, move them forward near the main beam, and mount the slot on a 45 degree angle, tilted inwards, so the boards slide in towards under the boat, not outwards, put some T's on the rudders, Voila, lifting boat! Not so much to add, just move the daggers and add T's to the rudders, which has already been done on the Moths and 18'skiffs. All we need is someone with the time and money to work out the details. No, I don't have any spare time, or money!

How about a new keel design The Loop?

http://www.boatdesign.net/forums/showthread.php?t=16265

And a canting rig cat design-

http://www.wingo.com/chriswhite/tiltrig.html

If you mean something like this...

...my test sails will start in a few days. Those "details" are not as simple to work out as it would seem at first sight.

Congrat Luiz!
I know it's been a long, hard struggle!

Get more photos and tell us how she goes...

Mike.